Remote sensing (RS) for vegetation monitoring can involve mixed pixels with contributions from vegetation and background surfaces, causing biases in signals and their interpretations, especially in low-density forests. In a case study in the semi-arid Yatir forest in Israel, we observed a mismatch between satellite (Landsat 8 surface product) and tower-based (Skye sensor) multispectral data and contrasting seasonal cycles in near-infrared (NIR) reflectance. We tested the hypothesis that this mismatch was due to the different fractional contributions of the various surface components and their unique reflectance. Employing an unmanned aerial vehicle (UAV), we obtained high-resolution multispectral images over selected forest plots and estimated the fraction, reflectance, and seasonal cycle of the three main surface components (canopy, shade, and sunlit soil). We determined that the Landsat 8 data were dominated by soil signals (70%), while the tower-based data were dominated by canopy signals (95%). We then developed a procedure to resolve the canopy (i.e., tree foliage) normalized difference vegetation index (NDVI) from the mixed satellite data. The retrieved and corrected canopy-only data resolved the original mismatch and indicated that the spatial variations in Landsat 8 NDVI were due to differences in stand density, while the canopy-only NDVI was spatially uniform, providing confidence in the local flux tower measurements.
Responses of terrestrial ecosystems to climate change have been explored in many regions worldwide. While continued drying and warming may alter process rates and deteriorate the state and performance of ecosystems, it could also lead to more fundamental changes in the mechanisms governing ecosystem functioning. Here we argue that climate change will induce unprecedented shifts in these mechanisms in historically wetter climatic zones, towards mechanisms currently prevalent in dry regions, which we refer to as ג€˜dryland mechanismsג€™. We discuss 12 dryland mechanisms affecting multiple processes of ecosystem functioning, including vegetation development, water flow, energy budget, carbon and nutrient cycling, plant production and organic matter decomposition. We then examine mostly rare examples of the operation of these mechanisms in non-dryland regions where they have been considered irrelevant at present. Current and future climate trends could force microclimatic conditions across thresholds and lead to the emergence of dryland mechanisms and their increasing control over ecosystem functioning in many biomes on Earth.
Carbonyl sulfide (COS) has emerged as a multi-scale tracer for terrestrial photosynthesis. To infer ecosystem-scale photosynthesis from COS fluxes often requires knowledge of leaf relative uptake (LRU), the concentration-normalized ratio between leaf COS uptake and photosynthesis. However, current mechanistic understanding of LRU variability remains inadequate for deriving robust COS-based estimates of photosynthesis. We derive a set of closed-form equations to describe LRU responses to light, humidity and CO2 based on the Ballג€“Berry stomatal conductance model and the biochemical model of photosynthesis. This framework reproduces observed LRU responses: decreasing LRU with increasing light or decreasing humidity; it also predicts that LRU increases with ambient CO2. By fitting the LRU equations to flux measurements on a C3 reed (Typha latifolia), we obtain physiological parameters that control LRU variability, including an estimate of the Ballג€“Berry slope of 7.1 without using transpiration measurements. Sensitivity tests reveal that LRU is more sensitive to photosynthetic capacity than to the Ballג€“Berry slope, indicating stomatal response to photosynthesis. This study presents a simple framework for interpreting observed LRU variability and upscaling LRU. The stoma-regulated LRU response to CO2 suggests that COS may offer a unique window into long-term stomatal acclimation to elevated CO2.
The ongoing global warming and associated drying are shaping the fate of forests worldwide. While processes of tree mortality are visible and studied, a decrease in forest regeneration is mostly overlooked, although equally deleterious. Populations at the edge of tree species distribution areas are at higher risk and are hence hotspots for species extinctions. Here we use a semi-arid pine forest growing at the timberline edge of forest existence as a model for forest survival under warming and drying conditions. Seedling recruitment, including seed germination, seedling survivorship, and multiyear seedling growth, were measured along six consecutive years. To pinpoint the role of drought, we designed a field experiment, manipulating stand density at three levels and grazing regimes. Seed germination was high across all studied plots, but seedling survivorship and multiyear seedling growth were near-zero. Stand density and grazing exclusion positively affected germination. Seedling survivorship was higher in wetter years. Multiyear seedling growth was stunted by grazing, and seedling height was distributed differently across different stand densities. Our data indicate that seedling survivorship during the first dry season acts as a bottleneck for forest existence at the dry and hot edge of current forest distribution. We also quantified the roles of other stressors such as shading, and highlighted the eliminating role of grazing on multiyear seedling growth. Forest regeneration should be more closely monitored in sensitive populations, as climate change-driven forest loss can happen even without mature tree mortality.
The seasonality and interannual variability of terrestrial carbonyl sulfide (COS) fluxes are poorly constrained. We present the first easy-to-use parameterization for net COS forest sink based on the longest eddy covariance record from a boreal pine forest, covering 32 months over 5 years. Fluxes from hourly to yearly scales are reported, with the aim of revealing controlling factors and the level of interannual variability. The parameterization is based on the photosynthetically active radiation, vapor pressure deficit, air temperature, and leaf area index. The spring recovery of the flux after the winter dormancy period was mostly governed by air temperature, and the onset of the uptake varied by 2 weeks. For the first time, we report a significant reduction of ecosystem-scale COS flux under large water vapor pressure deficit in summer. The maximum monthly and weekly median COS uptake varied 26 and 20ג€‰% between years, respectively. The timing of the latter varied by 6 weeks. The fraction of the nocturnal uptake remained below 21ג€‰% of the total COS uptake. We observed the growing season (Aprilג€“August) average net flux of COS totaling גˆ’58.0ג€‰gSג€‰haגˆ’1 with 37ג€‰% interannual variability. The long-term flux observations were scaled up to evergreen needleleaf forests (ENFs) in the whole boreal region by the Simple Biosphere Model Version 4 (SiB4). The observations were closely simulated by using SiB4 meteorological drivers and phenology. The total COS uptake by boreal ENF was in line with a missing COS sink at high latitudes pointed out in earlier studies.
Drylands cover more than 40% of Earth's land surface and occur at the margin of forest distributions due to the limited availability of water for tree growth. Recent elevated temperature and low precipitation have driven greater forest declines and pulses of tree mortality on dryland sites compared to humid sites, particularly in temperate Eurasia and North America. Afforestation of dryland areas has been widely implemented and is expected to increase in many drylands globally to enhance carbon sequestration and benefits to the human environment, but the interplay of sometimes conflicting afforestation outcomes has not been formally evaluated yet. Most previous studies point to conflicts between additional forest area and water consumption, in particular water yield and soil conservation/desalinization in drylands, but were generally confined to local and regional scales. Our global synthesis demonstrates that additional tree cover can amplify water consumption through a nonlinear increase in evapotranspiration-depending on tree species, age, and structure-which will be further intensified by future climate change. In this review we identify substantial knowledge gaps in addressing the dryland afforestation dilemma, where there are trade-offs with planted forests between increased availability of some resources and benefits to human habitats versus the depletion of other resources that are required for sustainable development of drylands. Here we propose a method of addressing comprehensive vegetation carrying capacity, based on regulating the distribution and structure of forest plantations to better deal with these trade-offs in forest multifunctionality. We also recommend new priority research topics for dryland afforestation, including: responses and feedbacks of dryland forests to climate change; shifts in the ratio of ecosystem ET to tree cover; assessing the role of scale of afforestation in influencing the trade-offs of dryland afforestation; and comprehensive modeling of the multifunctionality of dryland forests, including both ecophysiological and socioeconomic aspects, under a changing climate.
Drought can cause tree mortality through hydraulic failure and carbon starvation. To prevent excess water loss, plants typically close their stomata before massive embolism formation occurs. However, unregulated water loss through leaf cuticles and bark continues after stomatal closure. Here, we studied the diurnal and seasonal dynamics of bark transpiration and how it is affected by tree water availability. We measured continuously for six months water loss and CO2 efflux from branch segments and needle-bearing shoots in Pinus halepensis growing in a control and an irrigation plot in a semi-arid forest in Israel. Our aim was to find out how much passive bark transpiration is affected by tree water status in comparison with shoot transpiration and bark CO2 emission that involve active plant processes, and what is the role of bark transpiration in total tree water use during dry summer conditions. Maximum daily water loss rate per bark area was 0.03-0.14 mmol m(-2) s(-1), which was typically ~76% of the shoot transpiration rate (on leaf area basis) but could even surpass the shoot transpiration rate during the highest evaporative demand in the control plot. Irrigation did not affect bark transpiration rate. Bark transpiration was estimated to account for 64-78% of total water loss in drought-stressed trees, but only for 6-11% of the irrigated trees, due to differences in stomatal control between the treatments. Water uptake through bark was observed during most nights, but it was not high enough to replenish the lost water during the day. Unlike bark transpiration, branch CO2 efflux decreased during drought due to decreased metabolic activity. Our results demonstrate that although bark transpiration represents a small fraction of the total water loss through transpiration from foliage in non-stressed trees, it may have a large impact during drought.
Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, highג€�precision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remoteג€�sensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c.ג€‰2ֲ°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation Lbg.A novel ג€˜dualג€�referenceג€™ method was developed to increase the accuracy of infrared needleג€�leaf surface temperature measurements in the field. It accounts for variations in Lbg and corrects for the systematic camera offset using two reference plates.We accurately captured surface temperature and leafג€�toג€�air temperature differences of needleג€�leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ֲ±ג€‰0.23ֲ°C and ֲ±ג€‰0.28ֲ°C, respectively.Routine highג€�precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leafג€�scale processes and their dynamics.
Summary. The drier climates predicted for many regions will result in reduced evaporative cooling, leading to leaf heat stress and enhanced mortality. The extent to which nonevaporative cooling can contribute to plant resilience under these increasingly stressful conditions is not well known at present.Using a novel, high accuracy infrared system for the continuous measurement of leaf temperature in mature trees under field conditions, we assessed leafג€�toג€�air temperature differences (־”Tleafג€“air) of pine needles during drought.On midג€�summer days, ־”Tleafג€“air remained
Global warming and drying trends, as well as the increase in frequency and intensity of droughts, may have unprecedented impacts on various forest ecosystems. We assessed the role of internal water storage (WS) in drought resistance of mature pine trees in the semi-arid Yatir forest. Transpiration (T), soil moisture, and sap flow (SF) were measured continuously, accompanied by periodical measurements of leaf and branch water potential (־¨leaf) and water content (WC). The data were used to parameterize a tree hydraulics model to examine the impact of WS capacitance on the tree water-relations. The results of the continuous measurements showed a 5-hour time lag between T and SF in the dry season, which peaked in the early morning and early afternoon, respectively. A good fit between model results and observations was only obtained when the empirically estimated WS capacitance was included in the model. Without WS during the dry season, ־¨leaf would drop below a threshold known to cause hydraulic failure and cessation of gas exchange in the studied tree species. Our results indicate that tree WS capacitance is a key drought resistance trait that could enhance tree survival in a drying climate contributing up to 45% of the total daily transpiration during the dry season.
ג€¢Quantifying the carbon and water economy of non-commercial urban citrus orchards can help recognize the value of such orchards in the urban settings.ג€¢A 40 years old citrus orchard within urban setting and sustained on minimal irrigation showed high water use efficiency and significant carbon storage capacity.ג€¢The water and carbon use efficiency of the urban orchard were higher than that of a similar commercial ones, in spite of 2x difference in water supply.ג€¢Low-density row-crop type orchard help minimize water and carbon losses in evaporation and respiration in the low activity open spaces.Characterizing the carbon and water economy of non-commercial urban citrus orchards can help determine their value in urban settings. This includes provisions of urban ecology and ecosystem services, such as shade, conservation of biodiversity, and carbon sequestration, under space and water limitations that may be particularly suitable in the semi-arid Mediterranean regions. We carried out canopy-scale eddy covariance (EC) measurements of net ecosystem CO2 exchange (NEE) and evapotranspiration (ET), partitioned these fluxes to their components using chamber-based soil fluxes, and combined them with carbon stock to provide a first approximation of the apparent ecosystem carbon turnover rate (ֿ„eco). The urban orchard switched from a carbon sink of 0.6 ־¼mol mגˆ’2 sגˆ’1 to a carbon source of 0.2 ־¼mol mגˆ’2 sגˆ’1 between winter and summer, with a first approximation of the annual carbon storage capacity of גˆ¼75.3 g mגˆ’2 and a total carbon accumulation over its 40 year life span of גˆ¼3014 g mגˆ’2. Carbon accumulated predominantly below ground (67 % of total), and soil CO2 effluxes showed low sensitivity to temperature (Q10 גˆ¼1.6) and therefore also to climate warming, but also a fast C turnover rate (גˆ¼5.4 y), and therefore sensitivity to disturbances. The rates of ET increased from 0.40 to 1.25 mmol mגˆ’2 sגˆ’1 between the wet and dry seasons, and was גˆ¼50 % of a similar nearby commercial orchard. Partitioning of the ecosystem carbon and water fluxes indicated high canopy water use efficiency (11.7 ־¼mol CO2/mmol H2O; during the peak activity period). We demonstrate the potential of urban citrus orchards with low supplemental irrigation (50 % compared with commercial orchards in the area) to store significant amounts of carbon with high transpiration efficiencies. The results will help decision making in regard to urban tree planting and the ecological management of urban green spaces and community use in water-limited environments.
AbstractThe leaf economics spectrum1,2 and the global spectrum of plant forms and functions3 revealed fundamental axes of variation inֲ plant traits, which represent different ecological strategies that are shaped by the evolutionary development of plant species2. Ecosystem functions depend on environmental conditions and the traits of species that comprise the ecological communities4. However, the axes of variation of ecosystem functions are largely unknown, which limits our understanding of how ecosystems respond as a whole to anthropogenic drivers, climate and environmental variability4,5. Here we derive a set of ecosystem functions6 from a dataset of surface gas exchange measurements across major terrestrial biomes. We find that most of the variability within ecosystem functions (71.8%) is captured by three key axes. The first axis reflects maximum ecosystem productivity and is mostly explained by vegetation structure. The second axis reflects ecosystem water-use strategies and is jointly explained by variation in vegetation height and climate. The third axis, which represents ecosystem carbon-use efficiency, features a gradient related to aridity, and is explained primarily by variation in vegetation structure. We show that two state-of-the-art land surface models reproduce the first and most important axis of ecosystem functions. However, the models tend to simulate more strongly correlated functions than those observed, which limits their ability to accurately predict the full range of responsesֲ to environmental changes in carbon, water and energy cycling in terrestrial ecosystems 7,8.
Forestation actions are a major tool for both climate-change mitigation and biodiversity conservation. We address two weaknesses in this approach: the little attention given to the negative effects of reduced albedo associated with forestation in many regions, and ignoring the potential of drylands that account for 40% of the global potential land area for forestation. We propose an approach to identify suitable land for forestation and quantify its 'net equivalent carbon stock change' over 80 years of forest lifetime (NESC), accounting for both carbon sequestration and albedo changes. We combined remote-sensing tools with data-based estimates of surface parameters and with published climate matrices, to identify suitable land for forestation actions. We then calculated the cumulative (over 80 years) 'net sequestration potential' (־”SP), the 'emission equivalent of shortwave radiation forcing' (EESF) due to changes in surface albedo, and, in turn, the combined NESC = ־”SPגˆ’EESF, of planting forests with >40% tree-cover. Demonstrating our approach in a large climatically diverse state (Queensland), we identified 14.5 million hectares of potential forestation land in its semi-arid land and show that accounting for the EESF, reduces the climatic benefits of the ־”SP by almost 50%. Nevertheless, it results in a total NESC of 0.72 Gt C accumulated by the end of the century, and 80 years of forestation cycle. This estimated NESC is equivalent to 15% of the projected carbon emissions for the same period in Queensland, for a scenario of no change in emission rates during that period. Our approach extends restoration efforts by identifying new land for forestation and carbon sequestration but also demonstrates the importance of quantifying the climatic value of forestation in drylands.
The chapter describes a long-term (2000ג€“2019) perspective of carbon and energy fluxes from the leaf to the whole ecosystem scale in a semi-arid Aleppo pine forest plantation (the Yatir Forest) in the northern edge of the Negev desert (Israel), using the eddy covariance approach combined with meteorological and supplemental small-scale measurements. The site is characterized by a long dry season (over 7ֲ months) with mean annual precipitation of 285 mm. The forest was a carbon sink during the wet season from December to April, reaching a maximum Net Ecosystem Production (NEP) rate of 4ֲ gC mגˆ’2 dגˆ’1 in March, and a carbon source in Juneג€“October, with a mean summer value of ~ גˆ’1ֲ gC mגˆ’2 dגˆ’1. The carbon inventory showed that during 2001ג€“2016 Yatir Forest accumulated 145ֲ ֲ±ֲ 26ֲ gC mגˆ’2 yearגˆ’1, ~71% of which was stored in the soil. By sequestering carbon, the forest has a cooling effect on the earthג€™s surface. The solar radiation burden over the forest was high, with an annual average flux of 240ֲ Wmגˆ’2, while the forest albedo (the reflected solar radiation) was 0.12 and significantly lower than that of the surrounding desert, which was 0.25. Heat exchange with the atmosphere was characterized by high sensible heat flux (H) representing 50ג€“90% of the net absorbed radiation flux (Rn). The net absorbed Rn by the forest ecosystem was 67% higher than in the surrounding desert; the additional absorbed radiation has a warming effect on the surface. Our campaign-based measurements in another Aleppo pine forest in northern Israel with 755 mm annual precipitation (the Birya Forest, 190ֲ Km north of Yatir) showed that this forest was a carbon sink both in the wet and dry seasons with mean NEP ~5 and ~3ֲ gC mגˆ’2 dגˆ’1, respectively. The vegetation in the ecosystems around Birya Forest was much more developed than the desert vegetation in Yatir, thus the albedo effect and the thermal radiation suppression were lower. The net radiation load difference between Birya Forest and its surroundings was one-third lower than between Yatir and its surrounding desert. This case study show that pine forests can adjust to conditions at the dry, semi-arid, timberline and store significant amounts of carbon below ground with long residence time. Its large effects on the surface energy budget can result with local warming, but this can change considerably across relatively small geographical scale.
The impact of extreme climate episodes such as heatwaves on plants physiological functioning and survival may depend on the event intensity, which requires quantification. We unraveled the distinct impacts of intense (HW) and intermediate (INT) heatwave days on carbon uptake, and the underlying changes in the photosynthetic system, in a Mediterranean citrus orchard using leaf active (pulse amplitude modulation; PAM) and canopy level passive (sun-induced; SIF) fluorescence measurements, together with CO2, water vapor, and carbonyl sulfide (COS) exchange measurements. Compared to normal (N) days, gross CO2 uptake fluxes (gross primary production, GPP) were significantly reduced during HW days, but only slightly decreased during INT days. By contrast, COS uptake flux and SIFA (at 760ֲ nm) decreased during both HW and INT days, which was reflected in leaf internal CO2 concentrations and in nonphotochemical quenching, respectively. Intense (HW) heatwave conditions also resulted in a substantial decrease in electron transport rates, measured using leaf-scale fluorescence, and an increase in the fractional energy consumption in photorespiration. Using the combined proxy approach, we demonstrate a differential ecosystem response to different heatwave intensities, which allows the trees to preserve carbon assimilation during INT days but not during HW days.
Droughtג€�related tree mortality is increasing globally, but the sequence of events leading to it remains poorly understood. To identify this sequence, we used a 2016 tree mortality event in a semiג€�arid pine forest where dendrometry and sap flow measurements were carried out in 31 trees, of which seven died. A comparative analysis revealed three stages leading to mortality. First, a decrease in tree diameter in all dying trees, but not in the surviving trees, 8ג€‰months ג€�prior to the visual signs of mortalityג€� (PVSM; e.g., near complete canopy browning). Second, a decay to near zero in the diurnal stem swelling/shrinkage dynamics, reflecting the loss of stem radial water flow in the dying trees, 6ג€‰months PVSM. Third, cessation of stem sap flow 3ג€‰months PVSM. Eventual mortality could therefore be detected long before visual signs were observed, and the three stages identified here demonstrated the differential effects of drought on stem growth, water storage capacity and soil water uptake. The results indicated that breakdown of stem radial water flow and phloem function is a critical element in defining the ג€�point of no returnג€� in the sequence of events leading to mortality of mature trees.
Climate change will impact forest productivity worldwide. Forecasting the magnitude of such impact, with multiple environmental stressors changing simultaneously, is only possible with the help of process-based models. In order to assess their performance, such models require careful evaluation against measurements. However, direct comparison of model outputs against observational data is often not reliable, as models may provide the right answers due to the wrong reasons. This would severely hinder forecasting abilities under unprecedented climate conditions. Here, we present a methodology for model assessment, which supplements the traditional output-to-observation model validation. It evaluates model performance through its ability to reproduce observed seasonal changes of the most limiting environmental driver (MLED) for a given process, here daily gross primary productivity (GPP). We analyzed seasonal changes of the MLED for GPP in two contrasting pine forests, the Mediterranean Pinus halepensis Mill. Yatir (Israel) and the boreal Pinus sylvestris L. Hyytiֳ₪lֳ₪ (Finland) from three years of eddy-covariance flux data. Then, we simulated the same period with a state-of-the-art process-based simulation model (LandscapeDNDC). Finally, we assessed if the model was able to reproduce both GPP observations and MLED seasonality. We found that the model reproduced the seasonality of GPP in both stands, but it was slightly overestimated without site-specific fine-tuning. Interestingly, although LandscapeDNDC properly captured the main MLED in Hyytiֳ₪lֳ₪ (temperature) and in Yatir (soil water availability), it failed to reproduce high-temperature and high-vapor pressure limitations of GPP in Yatir during spring and summer. We deduced that the most likely reason for this divergence is an incomplete description of stomatal behavior. In summary, this study validates the MLED approach as a model evaluation tool, and opens up new possibilities for model improvement.
Climate change can impose large offsets between the seasonal cycle of photosynthesis and that in solar radiation and temperature which drive it. Ecophysiological adjustments to such offsets in forests growing under hot and dry conditions are critical for maintaining carbon uptake and survival. Here, we investigate the adjustments that underlie the unusually short and intense early spring productive season, under suboptimal radiation and temperature conditions in a semi-arid pine forest. We used eddy covariance flux, meteorological, and close-range sensing measurements, together with leaf chlorophyll content over four years in a semi-arid pine forest to identify the canopy-scale ecophysiological adjustments to the short active season, and long seasonal drought. The results reveal a range of processes that intricately converge to support the early spring peak (March) in photosynthetic activity, including peaks in light use efficiency, leaf chlorophyll content, increase in the absorption of solar radiation, and high leaf scattering properties (indicating optimizing leaf orientation). These canopy-scale adjustments exploit the tradeoffs between the yet increasing temperature and solar radiation, but the concurrently rapidly diminishing soil moisture. In contrast, during the long dry stressful period with rapidly declining photosynthesis under high and potentially damaging solar radiation, physiological photoprotection was conferred by strongly relaxing the early spring adjustments. The results provide evidence for canopy-scale ecophysiological adjustments, detectable by spectral measurements, that support the survival and productivity of a pine forest under the hot and dry conditions, which may apply to large areas in the Mediterranean and other regions in the next few decades due to the current warming and drying trends.
The rate of change in atmospheric CO2 is significantly affected by the terrestrial carbon sink, but the size and spatial distribution of this sink, and the extent to which it can be enhanced to mitigate climate change are highly uncertain. We combined carbon stock (CS) and eddy covariance (EC) flux measurements that were collected over a period of 15 years (2001-2016) in a 55-year-old 30 km2 pine forest growing at the semi-arid timberline (with no irrigating or fertilization). The objective was to constrain estimates of the carbon (C) storage potential in forest plantations in such semi-arid lands, which cover ~ 18% of the global land area. The forest accumulated 145-160 g C m-2 y-1 over the study period based on the EC and CS approaches, with a mean value of 152.5 ֲ± 30.1 g C m-2 y-1 indicating 20% uncertainty in carbon uptake estimates. Current total stocks are estimated at 7,943 ֲ± 323 g carbon m-2 and 372 g nitrogen m-2 . Carbon accumulated mostly in the soil (~71% and 29% for soil and standing biomass carbon, respectively) with long soil carbon turnover time (59 y). Regardless of un-expected disturbances beyond those already observed at the study site, the results support a considerable carbon sink potential in semi-arid soils and forest plantations, and imply that afforestation of even 10% of semi-arid land area under conditions similar to that of the study site, could sequester ~ 0.4 Pg C y-1 over several decades.
Background: Net primary productivity (NPP) in forests plays an important role in the global carbon cycle. However, it is not well known about the increase rate of Chinaג€™s forest NPP, and there are different opinions about the key factors controlling the variability of forest NPP. Methods: This paper established a statistics-based multiple regression model to estimate forest NPP, using the observed NPP, meteorological and remote sensing data in five major forest ecosystems. The fluctuation values of NPP and environment variables were extracted to identify the key variables influencing the variation of forest NPP by correlation analysis. Results: The long-term trends and annual fluctuations of forest NPP between 2000 and 2018 were examined. The results showed a significant increase in forest NPP for all five forest ecosystems, with an average rise of 5.2 gCֲ·m גˆ’ 2ֲ·year גˆ’ 1 over China. Over 90% of the forest area had an increasing NPP range of 0ג€“161 gCֲ·m גˆ’ 2ֲ·year גˆ’ 1. Forest NPP had an interannual fluctuation of 50ג€“269 gCֲ·m גˆ’ 2ֲ·year גˆ’ 1 for the five major forest ecosystems. The evergreen broadleaf forest had the largest fluctuation. The variability in forest NPP was caused mainly by variations in precipitation, then by temperature fluctuations. Conclusions: All five forest ecosystems in China exhibited a significant increasing NPP along with annual fluctuations evidently during 2000ג€“2018. The variations in Chinaג€™s forest NPP were controlled mainly by changes in precipitation.
The future of forests and their productivity in dry environments will depend on both water availability through precipitation and ecosystem and plant water use characteristics. It is increasingly recognized that better understanding water use patterns and their response to change depends on our ability to partition evapotranspiration (ET). Here, we use chamber-based direct measurements of soil evaporation (Es) in a semi-arid Pinus halepensis forest to partition ET to Es and tree transpiration (Et), to assess the daily and seasonal changes and to compare annual-scale values with measurements carried out at the same site ten years earlier. The ecosystem is characterized by a high annual Es/ET ratio of 0.26, and an Et/ET of 0.63. Es diminished in the long dry season, but as much as 74 ֲ± 5% of the residual flux was due to the re-evaporation of nighttime moisture adsorption, which may provide critical protection from soil drying. Over the 10 years observation period concurrent increase in the transpiration ratio (TR=Et/ET; +29%) and in leaf area index (LAI; +44%) were observed, with the ratio of TR/LAI remaining constant at ~0.31, and with persistently closed hydrological balance (ET/P of 0.94ג€“1.07). The observed Et/ET values are similar to the estimated global mean values, but are attained at a much higher aridity index (5.5) than the mean one, demonstrating the potential for expanding forestation into dry regions.
Partitioning carbon fluxes is key to understanding the process underlying ecosystem response to change. This study used soil and canopy fluxes with stable isotopes (13C) and radiocarbon (14C) measurements in an 18 km 2, 50-year-old, dry (287 mm mean annual precipitation; nonirrigated) Pinus halepensis forest plantation in Israel to partition the net ecosystem's CO 2 flux into gross primary productivity (GPP) and ecosystem respiration (Re) and (with the aid of isotopic measurements) soil respiration flux (Rs) into autotrophic (Rsa), heterotrophic (Rh), and inorganic (Ri) components. On an annual scale, GPP and Re were 655 and 488 g C m -2, respectively, with a net primary productivity (NPP) of 282 g C m -2 and carbon-use efficiency (CUE D NPP = GPP) of 0.43. Rs made up 60 % of the Re and comprised 24 ֲ± 4 %Rsa, 23 ֲ± 4 %Rh, and 13 ֲ± 1 %Ri. The contribution of root and microbial respiration to Re increased during high productivity periods, and inorganic sources were more significant components when the soil water content was low. Comparing the ratio of the respiration components to Re of our mean 2016 values to those of 2003 (mean for 2001 2006) at the same site indicated a decrease in the autotrophic components (roots, foliage, and wood) by about-13 % and an increase in the heterotrophic component (Rh=Re) by about C18 %, with similar trends for soil respiration (Rsa=Rs decreasing by-19 % and Rh=Rs increasing by C8 %, respectively). The soil respiration sensitivity to temperature (Q10) decreased across the same observation period by 36 % and 9 % in the wet and dry periods, respectively. Low rates of soil carbon loss combined with relatively high belowground carbon allocation (i.e., 38 % of canopy CO2 uptake) and low sensitivity to temperature help explain the high soil organic carbon accumulation and the relatively high ecosystem CUE of the dry forest.
Drought-induced productivity reductions and tree mortality have been increasing in recent decades in forests around the globe. Prescribed reduction in stand density, i.e. thinning, has been proposed as a management tool to improve forest sustainability in face of a warmer, drier future. Thinning should potentially reduce net stand water use and improve water-availability for remaining trees, thus reducing their subsequent drought vulnerability. However, few studies have directly measured these effects.In 2009 we established a large-scale thinning experiment in a semi-arid, 40-years-old pine afforestation. Study plots (70 x 70 m) were thinned to 100,200, and 300 trees ha(-1), and compared with unthinned control plots (210-400 trees ha(-1)), each at five replications. Stem and needle growth, and needle gas exchange were measured along 3-9 consecutive years at seasonal to annual temporal resolution. Measurements at the tree-scale were further up-scaled using both simple upscaling relationships and using an ecosystem model of coupled carbon, energy and hydrology (Regional Hydro Ecologic Simulation System, RHESSys).At the needle scale, photosynthesis was 70% higher at the 100 trees ha(-1) than at 300 trees ha(-1), whereas transpiration was merely 10% higher. Consequently, stem and needle growth increased by 100% and 20%, respectively. For most parameters, there was little change between 200 and 100 trees ha(-1). Applying RHESSys at the stand-scale, these effects on tree physiology translated into 35% reduction in CO2 uptake and a 47% reduction in tree water-use, which was compensated for by increased evaporation from exposed soil.Our long-term measurements at the dry timberline highlight the role of thinning in enhancing the activity and growth of remaining trees, with increased water-use efficiency. Unexpectedly, this density reduction was associated with a relatively small decrease in forest carbon uptake. Light availability was a limiting factor in the higher density plots, even in our light-abundant forest.
Photoprotection strategies in a Pinus halepensis Mill. forest at the dry timberline that shows sustained photosynthetic activity during 6-7 month summer drought were characterized and quantified under field conditions. Measurements of chlorophyll fluorescence, leaf-level gas exchange and pigment concentrations were made in both control and summer-irrigated plots, providing the opportunity to separate the effects of atmospheric from soil water stress on the photoprotection responses. The proportion of light energy incident on the leaf surface ultimately being used for carbon assimilation was 18% under stress-free conditions (irrigated, winter), declining to 4% under maximal stress (control, summer). Allocation of absorbed light energy to photochemistry decreased from 25 to 15% (control) and from 50% to 30% (irrigated) between winter and summer, highlighting the important role of pigment-mediated energy dissipation processes. Photorespiration or other non-assimilatory electron flow accounted for 15-20% and ~10% of incident light energy during periods of high and low carbon fixation, respectively, representing a proportional increase in photochemical energy going to photorespiration in summer but a decrease in the absolute amount of photorespiratory CO loss. Resilience of the leaf photochemical apparatus was expressed in the complete recovery of photosystem II (PSII) efficiency (ֿ†PSII) and relaxation of the xanthophyll de-epoxidation state on the diurnal cycle throughout the year, and no seasonal decrease in pre-dawn maximal PSII efficiency (Fv/Fm). The response of CO assimilation and photoprotection strategies to stomatal conductance and leaf water potential appeared independent of whether stress was due to atmospheric or soil water deficits across seasons and treatments. The range of protection characteristics identified provides insights into the relatively high carbon economy under these dry conditions, conditions that are predicted for extended areas in the Mediterranean and other regions due to global climate change.
Aim The mechanisms of plant trait adaptation and acclimation are still poorly understood and, consequently, lack a consistent representation in terrestrial biosphere models (TBMs). Despite the increasing availability of geo-referenced trait observations, current databases are still insufficient to cover all vegetation types and environmental conditions. In parallel, the growing number of continuous eddy-covariance observations of energy and CO2 fluxes has enabled modellers to optimize TBMs with these data. Past attempts to optimize TBM parameters mostly focused on model performance, overlooking the ecological properties of ecosystems. The aim of this study was to assess the ecological consistency of optimized trait-related parameters while improving the model performances for gross primary productivity (GPP) at sites. Location Worldwide. Time period 1992-2012. Major taxa studied Trees and C-3 grasses. Methods We optimized parameters of the ORCHIDEE model against 371 site-years of GPP estimates from the FLUXNET network, and we looked at global covariation among parameters and with climate. Results The optimized parameter values were shown to be consistent with leaf-scale traits, in particular, with well-known trade-offs observed at the leaf level, echoing the leaf economic spectrum theory. Results showed a marked sensitivity of trait-related parameters to local bioclimatic variables and reproduced the observed relationships between traits and climate. Main conclusions Our approach validates some biological processes implemented in the model and enables us to study ecological properties of vegetation at the canopy level, in addition to some traits that are difficult to observe experimentally. This study stresses the need for: (a) implementing explicit trade-offs and acclimation processes in TBMs; (b) improving the representation of processes to avoid model-specific parameterization; and (c) performing systematic measurements of traits at FLUXNET sites in order to gather information on plant ecophysiology and plant diversity, together with micro-meteorological conditions.
Accurate determination of infrared (IR) emissivity is important for non-contact temperature measurement and for energy balance evaluation in systems that exchange radiation. A method for accurate measurement is proposed based on active modulation of the background radiation. The hemispherical directional reflectance is measured as a proxy for directional emissivity using an IR camera and an integrating sphere, while the background radiation is modulated using an IR emitter and a mechanical shutter. Measurement of the apparent temperature observed by the camera under two different illumination conditions allows the extraction of reflectance and emissivity. The accuracy of the measurement and its sensitivity to surface properties are analyzed, showing uncertainty values as low as 0.004 in some cases. Example measurements of natural and artificial surfaces are presented. (C) 2019 Optical Society of America
Drought-related tree mortality had become a widespread phenomenon in forests around the globe. This process leading to these events and its complexity is not fully understood. Trees in the dry timberline are exposed to ongoing drought, and the available water for transpiration in the soil can determine their survival chances. Recent drought years led to 5%ג€“10% mortality in the semi-arid pine forest of Yatir (Israel). The distribution of dead trees was, however, highly heterogeneous with parts of the forest showing >80% dead trees (D plots) and others with mostly live trees (L plots). At the tree level, visible stress was associated with low pre-dawn leaf water potential at the dry season (גˆ’2.8ֲ MPa vs. גˆ’2.3ֲ MPa in non-stressed trees), shorter needles (5.5 vs. 7.7ֲ mm) and lower chlorophyll content (0.6 vs. 1ֲ mg/g dw). Trends in tree-ring widths reflected differences in stress intensity (30% narrower rings in stressed compared with unstressed trees), which could be identified 15ג€“20ֲ years prior to mortality. At the plot scale, no differences in topography, soil type, tree age or stand density could explain the mortality difference between the D and L plots. It could only be explained by the higher surface rock cover and in stoniness across the soil profile in the L plots. Simple bucket model simulations using the siteג€™s long-term hydrological data supported the idea that these differences could result in higher soil water concentration (m 3 /m 3 ) in the L plots and extend the time above wilting point by several months across the long dry season. Accounting for subsurface heterogeneity may therefore critical to assessing stand-level response to drought and projecting tree survival, and can be used in management strategies in regions undergoing drying climate trends.ֲ
Efficiency and energetics of the turbulent transport in the canopy sublayer of a semi-arid pine forest and the atmospheric surface layer of a sparse desert-like shrubland are investigated from ultrasonic anemometer measurements during the summer dry season. The results show that an increased sensible heat flux over the forest canopy is generated by more energetic turbulence at small and large scales, but the transport process itself is equally efficient compared to the neighbouring shrubland. In contrast, the turbulent momentum flux over the forest canopy is caused by more efficient and energetic momentum transport at large scales. The more energetic turbulence appears to reduce the aerodynamic resistance to heat transfer of the semi-arid forest, which enables the increased sensible heat flux at a lower surface temperature. The results help explain the observed differences in diurnal variation of statistical moments of turbulent quantities that are caused by the interaction between radiative forcing, the background wind and the different turbulence production regimes of the forest and the shrubland. Lastly, the results also explain the observed cooler nighttime temperatures and quicker formation of a residual layer at the forest site.
Carbonyl sulfide (COS) is used as a tracer of CO
2 exchange at the ecosystem and larger scales. The robustness of this approach depends on knowledge of the soil contribution to the ecosystem fluxes, which is uncertain at present. We assessed the spatial and temporal variations in soil COS and CO
2 fluxes in a Mediterranean citrus orchard combining surface flux chambers and soil concentration gradients. The spatial heterogeneity in soil COS exchange indicated net uptake below and between trees of up to 4.6pmolm
-1 and net emission in sun-exposed soil between rows of up to 2.6pmolm
-1 , with an overall mean uptake value of 1.1ֲ±0.1pmolm
-1 . Soil COS concentrations decreased with soil depth from atmospheric levels of גˆ¼450 to גˆ¼100ppt at 20cm depth, while CO
2 concentrations increased from גˆ¼400 to גˆ¼5000ppm. COS flux estimates from the soil concentration gradients were, on average,-1:0 ֲ± 0:3 pmol m
-1 , consistent with the chamber measurements. A soil COS flux algorithm driven by soil moisture and temperature (5 cm depth) and distance from the nearest tree, could explain 75 % of variance in soil COS flux. Soil relative uptake, the normalized ratio of COS to CO
2 fluxes was, on average,-0:4 ֲ± 0:3 and showed a general exponential response to soil temperature. The results indicated that soil COS fluxes at our study site were dominated by uptake, with relatively small net fluxes compared to both soil respiration and reported canopy COS fluxes. Such a result should facilitate the application of COS as a powerful tracer of ecosystem CO2 exchange.
Dry deposition of ozone (O-3) to vegetation is an important removal pathway for tropospheric O-3, while O-3 uptake through plant stomata negatively affects vegetation and leads to climate change. Both processes are controlled by vegetation characteristics and ambient conditions via complex mechanisms. Recent studies have revealed that these processes can be fundamentally impacted by coastal effects, and by dry and warm conditions in ways that have nut been fully characterized, largely due to lack of measurements under such conditions. Hence, we hypothesized that measuring dry deposition of O-3 to vegetation along a sharp spatial climate gradient, and at different distances from the coast, can offer new insights into the characterization of these effects on O-3 deposition to vegetation and stomatal uptake, providing important information for afforestation management and for climate and air-quality model improvement. To address these hypotheses, several measurement campaigns were performed at different sites, including pine, oak, and mixed Mediterranean forests, at distances of 20-59 km from the Eastern Mediterranean coast, under semiarid, Mediterranean and humid Mediterranean climate conditions. The eddy covariance technique was used to quantify vertical O-3 flux (F-tot) and its partitioning to stomatal flux (F-st) and non-stomatal flux (F-ns). Whereas F-st tended to peak around noon under humid Mediterranean and Mediterranean conditions in summer, it was strongly limited by drought under semiarid conditions from spring to early winter, with minimum average F-st/F-tot, of 8-11% during the summer. F-ns in the area was predominantly controlled by relative humidity (RH), whereas increasing F-ns with RH for RH 70% indicated enhancement of F-ns by aerosols, via surface wetness stimulation. At night, efficient turbulence clue to sea and land breezes, together with increased RH, resulted in strong enhancement of F-tot Extreme dry surface events, some induced by dry intrusion from the upper troposphere, resulted in positive F-ns events. (C) 2018 Elsevier B.V. All rights reserved.
We investigate the effects of an isolated meso-־³-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: 28km2, forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (600Wm-2 in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.
Afforestation in semi-arid regions can potentially enhance the global carbon sink by increasing the terrestrial biomass. However, the survival of planted forests under such extreme environmental conditions is not guaranteed a priori, and critically depends on the surface-atmosphere exchange of energy. We investigate the pine forest Yatir in Israel, an example of a man-made semi-arid ecosystem, by means of large-eddy simulations. We focus on the interaction between surface-atmosphere exchange and secondary circulations that couple the isolated forest to the surrounding shrubland. The large-eddy simulations feature a grid resolution that resolves the forest canopy in several layers, and are initialized by satellite data and Doppler lidar, eddy-covariance and radiosonde measurements. We perform three large-eddy simulations with different geostrophic wind speeds to investigate the influence of those wind speeds on the surface-atmosphere exchange. We reproduce the measured mean updrafts above the forest and mean downdrafts above the shrubland, which increase in strength with decreasing geostrophic wind speed. The largest updrafts emerge above the older, denser part of the forest, triggering secondary circulations. The spatial extent of these circulations does not cover the entire forest area, although we observe a reduced aerodynamic resistance in the regions of updraft. Our simulations indicate that the enhanced surface-atmosphere exchange of the Yatir forest is not sufficient to compensate for the increased net radiation, due to the lower albedo of the forest with respect to the surroundings, resulting in higher air temperatures inside the forest. However, the difference between the forest and shrubland temperatures decreases with increasing geostrophic wind speed due to reduction in the aerodynamic resistance.
Remote sensing of sun-induced chlorophyll fluorescence (SIF) has been suggested as a promising approach for probing changes in global terrestrial gross primary productivity (GPP). To date, however, most studies were conducted in situations when/where changes in both SIF and GPP were driven by large changes in the absorbed photosynthetically active radiation (APAR) and phenology. Here we quantified SIF and GPP during a short-term intense heat wave at a Mediterranean pine forest, during which changes in APAR were negligible. GPP decreased linearly during the course of the heat wave, while SIF declined slightly initially and then dropped dramatically during the peak of the heat wave, temporally coinciding with a biochemical impairment of photosynthesis inferred from the increase in the uptake ratio of carbonyl sulfide to carbon dioxide. SIF thus accounted for less than 35% of the variability in GPP and, even though it responded to the impairment of photosynthesis, appears to offer limited potential for quantitatively monitoring GPP during heat waves in the absence of large changes in APAR.
Ecosystem evapotranspiration (ET) can approach annual precipitation (P) often leaving a residual [P-ET], referred to as an ecosystem water yield (WYe). Using a mobile lab, we estimate ET and WYe, in paired forest and nonforest (shrub or grassland) sites along the precipitation gradient (285ג€“755 mmֲ a
גˆ’1) in Israel. WYe was 69 mm in the dry sites and was further reduced by גˆ¼51 mm by forestation. Both WYe and the impact of forestation increased in the wetter sites, with forestation reducing WYe by >200 mm, equivalent to גˆ¼30% of the local P. This was associated with increase in ET by a factor of 2.2 and 1.8 in the forest and nonforest sites, respectively, along the rainfall gradient. Losses in WYe due to forestation approached a maximum of גˆ¼200 mm above P גˆ¼ 500 mm, but the forest WYe could vary between גˆ¼300 mm at P = 900 mm and גˆ¼100 mm at P = 500 mm (with equivalent change in WYe between 500 and 300 mm in the nonforest sites), reflecting the increasing ג€�hydrological costג€� associated with vegetation ET and the expected climate change in these regions. The results quantify the interactions of land use and climate on ecosystem ET, indicating that in dry climates, afforestation impact on WYe varies significantly across small spatial scales and can reduce WYe with significant impacts on local hydrology. Such impact may be diminished by management (e.g., plant species, thinning, and grazing) but should also consider the trade-offs with other ecosystem services (e.g., carbon sequestration, soil protection, and surface cooling).
Carbonyl sulfide (COS) is a tracer of ecosystem photosynthesis that can advance carbon cycle research from leaf to global scales; however, a range of newly reported caveats related to sink/source strength of various ecosystem components hinder its application. Using comprehensive eddy-covariance and chamber measurements, we systematically measure ecosystem contributions from leaf, stem, soil, and litter and were able to close the ecosystem COS budget. The relative contributions of nonphotosynthetic components to the overall canopy-scale flux are relatively small (~4% during peak activity season) and can be independently estimated based on their responses to temperature and humidity. Converting COS to photosynthetic CO2 fluxes based on the leaf relative uptake of COS/CO2, faces challenges due to observed daily and seasonal changes. Yet, this ratio converges around a constant value (~1.6), and the variations, dominated by light intensity, were found unimportant on a flux-weighted daily time-scale, indicating a mean ratio of daytime gross-to-net primary productivity of ~2 in our ecosystem. The seasonal changes in the leaf relative uptake ratio may indicate a reduction in mesophyll conductance in winter, and COS-derived canopy conductance permitted canopy temperature estimate consistent with radiative skin temperature. These results support the feasibility of using COS as a powerful and much-needed means of assessing ecosystem function and its response to change.
The role of secondary circulations has recently been studied in the context of well-defined surface heterogeneity in a semiarid ecosystem where it was found that energy balance closure over a desert-forest system and the structure of the boundary layer was impacted by advection and flux divergence. As a part of the CliFF ("Climate feedbacks and benefits of semi-arid forests", a collaboration between KIT, Germany, and the Weizmann Institute, Israel) campaign, we studied the boundary layer dynamics and turbulent transport of energy corresponding to this effect in Yatir Forest situated in the Negev Desert in Israel. The forest surrounded by small shrubs presents a distinct feature of surface heterogeneity, allowing us to study the differences between their interactions with the atmosphere above by conducting measurements with two eddy covariance (EC) stations and two Doppler lidars. As expected, the turbulence intensity and vertical fluxes of momentum and sensible heat are found to be higher above the forest compared to the shrubland. Turbulent statistics indicative of nonlocal motions are also found to differ over the forest and shrubland and also display a strong diurnal cycle. The production of turbulent kinetic energy (TKE) over the forest is strongly mechanical, while buoyancy effects generate most of the TKE over the shrubland. Overall TKE production is much higher above the forest compared to the shrubland. The forest is also found to be more efficient in dissipating TKE. The TKE budget appears to be balanced on average both for the forest and shrubland, although the imbalance of the TKE budget, which includes the role of TKE transport, is found to be quite different in terms of diurnal cycles for the forest and shrubland. The difference in turbulent quantities and the relationships between the components of TKE budget are used to infer the characteristics of the turbulent transport of energy between the desert and the forest.
For the past decade, observations of carbonyl sulfide (OCS or COS) have been investigated as a proxy for carbon uptake by plants. OCS is destroyed by enzymes that interact with CO2 during photosynthesis, namely carbonic anhydrase (CA) and RuBisCO, where CA is the more important one. The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink, whereas the sources and sinks of CO2 are co-located in ecosystems. The drawdown of OCS can therefore be related to the uptake of CO2 without the added complication of co-located emissions comparable in magnitude. Here we review the state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. OCS uptake is correlated well to plant carbon uptake, especially at the regional scale. OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. More work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain.
Afforestation is an important approach to mitigate global warming. Its complex interactions with the climate system, however, makes it controversial. Afforestation is expected to be effective in the tropics where biogeochemical and biogeophysical effects act in concert; however, its potential in the large semi-arid regions remains insufficiently explored. Here, we use a Global Climate Model to provide a process-based demonstration that implementing measured characteristics of a successful semi-arid afforestation system (2000 ha, ~300 mm mean annual precipitation) over large areas (~200 million ha) of similar precipitation levels in the Sahel and North Australia leads to the weakening and shifting of regional low-level jets, enhancing moisture penetration and precipitation (+0.8 ֲ± 0.1 mm d
-1 over the Sahel and +0.4 ֲ± 0.1 mm d
-1 over North Australia), influencing areas larger than the original afforestation. These effects are associated with increasing root depth and surface roughness and with decreasing albedo. This results in enhanced evapotranspiration, surface cooling and the modification of the latitudinal temperature gradient. It is estimated that the carbon sequestration potential of such large-scale semi-arid afforestation can be on the order of ~10% of the global carbon sink of the land biosphere and would overwhelm any biogeophysical warming effects within ~6 years.
Aims: This study investigated the impact of canopy cover and seasonality on litter decay in Mediterranean pine forests to enhance climate predictions. Methods: We conducted litterbag experiments in plots of different tree densities in two Mediterranean pine forests differing in precipitation amounts. In each plot, local litter was placed in forest gaps and under tree canopies for 613ֲ days, starting in the dry season. Results: Litter mass loss was greater in forest gaps than under tree canopies across forests and tree densities. Similarly, a reduction in tree density tended to increase mass loss. Additionally, while the decay rate slowed down from the first to the second wet season, the decay rate remained constant during the first and the second dry season, and the dry seasons contributed 30% to the overall mass loss. Conclusions: Reduction in canopy cover enhances litter decay, and the stability and magnitude of the dry season contribution to annual mass loss have the potential to control litter mass loss when accounting also for the dry periods in the wet season. Combined, the ongoing tree mortality and the predicted prolongation of dry periods due to climate change may enhance litter decay, possibly reducing ecosystem carbon stocks in drylands.
Estimations of ecosystem-level evapotranspiration (ET) and CO 2 uptake in water-limited environments are scarce and scaling up ground-level measurements is not straightforward. A biophysical approach was previously proposed for ecosystem-level assessment relying on vegetation index and meteorological data (RS-Met) in temperate Mediterranean ecosystems. However, these RS-Met models have not been tested yet in extreme high-energy water-limited ecosystems that suffer from continuous stress conditions. Owing to the lack of ET and CO 2 flux estimations in the Eastern Mediterranean, we examined the RS-Met approach using a newly developed mobile lab system and the single active Fluxnet station operating in this region, in seven forest and non-forest sites across a climatic transect in Israel (280-770 mm y גˆ’1 ). The RS-Met models were used with and without the addition of a seasonal drought stress factor (f DS ), which was based on daily rainfall, temperature and radiation data.Results show that the RS-Met models with the inclusion of the f DS were significantly improved compared to the non-f DS models (r=0.64-0.91 compared to 0.05-0.80; P=0.06 and r=0.72-0.92 compared to r=0.56-0.90; P
The authors suggest an approach to analyze the effects of small-scale afforestation on the surrounding climate of a large heterogenic area. While simple statistics have difficulty identifying the effect, here a well-known eigenvector technique is used to overcome several specific challenges that result from a limited research region, complex topography, and multiple atmospheric circulation patterns. This approach is applied to investigate the influence of the isolated Yatir forest, at the north edge of Israel's Negev Desert. It was found that this forest does influence the daily climate, primarily seen in the main pattern of the empirical orthogonal function (EOF) of temperature and humidity. The EOF explains 93% and 80 respectively, of the total variance in the data. Although the Yatir forest is small, it is significant in regulating the climate in the nearby surroundings, as it is located in a sharp transition area toward an arid climate. The results are presented as maps of correlation and regression between the normalized principal component time series of each pattern as well as other time series of the raw data and spatially interpolated data stations. Analysis of short-term campaign measurements around the Yatir forest supports the EOF results, and shows the forest's influence to the south, mainly during nighttime when the forest becomes cooler than its surroundings. Overall, results suggest that in areas of transition to semiarid climates, forests regulate the surrounding surface air temperature and humidity fields. Wind analysis based on a complex EOF technique reveals the pattern of the daily cycle of surface wind over the region.
More frequent and intense droughts are projected during the next century, potentially changing the hydrological balances in many forested catchments. Although the impacts of droughts on forest functionality have been vastly studied, little attention has been given to studying the effect of droughts on forest hydrology. Here, we use the Budyko framework and two recently introduced Budyko metrics (deviation and elasticity) to study the changes in the water yields (rainfall minus evapotranspiration) of forested catchments following a climatic drought (2006-2010) in pine forests distributed along a rainfall gradient (P = 280-820 mm yr(-1)) in the Eastern Mediterranean (aridity factor = 0.17-0.56). We use a satellite-based model and meteorological information to calculate the Budyko metrics. The relative water yield ranged from 48% to 8% (from the rainfall) in humid to dry forests and was mainly associated with rainfall amount (increasing with increased rainfall amount) and bedrock type (higher on hard bedrocks). Forest elasticity was larger in forests growing under drier conditions, implying that drier forests have more predictable responses to drought, according to the Budyko framework, compared to forests growing under more humid conditions. In this context, younger forests were shown more elastic than older forests. Dynamic deviation, which is defined as the water yield departure from the Budyko curve, was positive in all forests (i.e., less-than-expected water yields according to Budyko's curve), increasing with drought severity, suggesting lower hydrological resistance to drought in forests suffering from larger rainfall reductions. However, the dynamic deviation significantly decreased in forests that experienced relatively cooler conditions during the drought period. Our results suggest that forests growing under permanent dry conditions might develop a range of hydrological and eco-physiological adjustments to drought leading to higher hydrological resilience
Current climate models disagree on how much carbon dioxide land ecosystems take up for photosynthesis. Tracking the stronger carbonyl sulfide signal could help.
We quantified springtime ecosystem-scale monoterpene fluxes from two similar Aleppo pine (Pinus halepensis Mill.) forests, located in Israel, that differed in the amount of received precipitation: Yatir in the arid south and Birya in the northern part of Israel (291 and 755 mm annual average rainfall, respectively). In addition to the lower water availability, during our measurement campaign the Yatir site suffered from a heat wave with temperatures up to 35 degrees C, which made the campaign-average net CO2 assimilation to occur in the morning (1 mu mol m(-2) S-1), with the rest of the daytime hours mainly dominated by net release of CO2. The milder conditions at Birya favored a higher net CO2 assimilation during all daytime hours (with average peaks higher than 10 pmol m(-2) s(-1)). Despite these large differences in ambient conditions and CO2 net assimilation, daytime monoterpene emission capacities at both sites were comparable. While observed monoterpene fluxes were lower at Yatir than at Birya (hourly averages up to 0.4 and 1 mg m(-2) h(-1), respectively), the standardized hourly fluxes, after accounting for the differences in light, temperature and stand density between both sites, were comparable (0-1.3 mg m(-2) h(-1)). The approach typically used by biogenic emission models overestimated monoterpene fluxes at Yatir when temperatures rose during the heat wave. This result, together with complementary leaf-level measurements showing that summertime monoterpene fluxes almost completely ceased at Yatir while being enhanced at Birya, highlight the interaction of water scarcity and high temperatures that drive monoterpene emissions from vegetation in such extreme climate zones and the need to further improve model performance. (C) 2017 Elsevier B.V. All rights reserved.
This is a protocol to evaluate gross primary productivity (GPP) of a forest stand based on the measurements of treeג€™s sap flow (SF), 13C derived water use efficiency (WUE), and meteorological (met) data. GPP was calculated from WUE and stomatal conductance (gs), the later obtained from SF up-scaled from sampled trees to stand level on a daily time-scale and met data. WUE is obtained from 13C measurements in dated tree-ring wood and/or foliage samples. This protocol is based on the recently published study of Klein et al., 2016.
Climate and topography have both strong effects on forest water and carbon cycles. However, little is known about their combined effects on the long-term water use and productivity of forests that have different water-use strategies. Here, we used structural equation modelling (SEM) to test direct and indirect influences of climate and topography on the long-term (mean over 2000-2014) evapotranspiration (ET) and gross ecosystem productivity (GEP), as assessed from satellite-based models, across two major co-occurring Mediterranean forest types with different water-use strategies (oak woodlands and pine forests). The estimated GEP and ET were higher by c. 6% and 15%, respectively, in the oak woodlands (Quercus calliprinos) than in pine forests (Pinus halepensis). As a result, the water use efficiency was higher in the pine forests (by 9%), consistent with P. halepensis conservative behaviour. Using the SEM, we found that the mean annual surface skin temperatures had the largest influence on the productivity and ET, with a similar net adverse effect across both forest types. In contrast, the mean annual precipitation was not related to GEP across both forest types but positively affected the ET in the oak woodlands. Slope and aspect had both significant but secondary influence on the forests fluxes, with higher GEP and ET found on the steeper slopes across the oak woodlands and higher ET found on the steeper slopes across the pine forests, associated with north-facing aspects. Applying the SEMs for the pine and oak forests, we predicted reductions of 16% and 31% in the productivity of both forests for projected increases in temperatures of 1(circle)C and 2(circle)C, respectively. Our results suggest that projected warming may have a strong impact on the productivity of Mediterranean forests, severely decreasing the CO2 uptake of the trees, independent of their water -use strategy. (C) 2016 Elsevier B.V. All rights reserved.
The temporal dynamics of water transport and storage in plants have major implications for plant functioning and survival. In trees, stress on the conductive tissue can be moderated by water storage. Yet, trees can survive high percent loss of conductivity (PLC, up to 80%), suggesting efficient recovery. We assess the role of tree water storage and PLC recovery based on simultaneous measurements of leaf transpiration, branch hydraulic conductivity, and stem sap-flow from different seasons in three study years in mature Pinus halepensis (Miller) trees in a semi-arid forest. During the wet season the rates of transpiration (T) and sap flow (SF) peaked at high morning and through the midday. During the dry season T peaked at similar to 9: 00 and then decreased, whereas SF lagged T and fully compensated for it only in the evening, resulting in a midday water deficit of similar to 5 kg tree(-1), and with up to 33% of daily T derived from storage. PLC of 30-40% developed during mid-day and subsequently recovered to near zero within 2-3 hr in the dry season (May, June, and September), but not in the wet season (January). The observed temporal decoupling between leaf water loss and soil water recharge is consistent with optimization of the trees' water and gas exchange economy, while apparently facilitating their survival in the semi-arid conditions.
Water stress results in a reduction of the metabolism of plants and in a reorganization of their use of resources geared to survival. In the Mediterranean region, periods of drought accompanied by high temperatures and high irradiance occur in summer. Plants have developed various mechanisms to survive in these conditions by resisting, tolerating or preventing stress. We used three typical Mediterranean tree species in Israel, Pinus halepensis L., Quercus calliprinos and Quercus ithaburensis Webb, as models for studying some of these adaptive mechanisms. We measured their photosynthetic rates (A), stomatal conductance (g s), and terpene emission rates during spring and summer in a geophysical gradient from extremely dry to mesic from Yatir (south, arid) to Birya (north, moist) with intermediate conditions in Solelim. A and g s of P. halepensis were threefold higher in Birya than in Yatir where they remained very low both seasons. Quercus species presented 2ג€“3-fold higher A and g s but with much more variability between seasons, especially for Q. ithaburensis with A and g s that decreased 10ג€“30-fold from spring to summer. Terpene emission rates for pine were not different regionally in spring but they were 5ג€“8-fold higher in Birya than in Yatir in summer (P
Short-term, intense heat waves (hamsins) are common in the eastern Mediterranean region and provide an opportunity to study the resilience of forests to such events that are predicted to increase in frequency and intensity. The response of a 50-yr-old Aleppo pine (Pinus halepensis) forest to hamsin events lasting 1-7d was studied using 10yr of eddy covariance and sap flow measurements. The highest frequency of heat waves was c. four per month, coinciding with the peak productivity period (March-April). During these events, net ecosystem carbon exchange (NEE) and canopy conductance (gc ) decreased by c. 60%, but evapotranspiration (ET) showed little change. Fast recovery was also observed with fluxes reaching pre-stress values within a day following the event. NEE and gc showed a strong response to vapor pressure deficit that weakened as soil moisture decreased, while sap flow was primarily responding to changes in soil moisture. On an annual scale, heat waves reduced NEE and gross primary productivity by c. 15% and 4%, respectively. Forest resilience to short-term extreme events such as heat waves is probably a key to its survival and must be accounted for to better predict the increasing impact on productivity and survival of such events in future climates.
Semi arid regions are transition areas between the desert and temperate climates, and are sensitive to climate change. Several semi-arid regions, such as the Sahel and North Australia, are under the influence of monsoon regimes, low-level jets, and mesoscale convective activity, which influence on the amount of annual precipitation. In these regions the summer solar heating leads to migration of the equatorial trough and the tropical convergence zones (ITCZ), leading to monsoon rains in some parts of the regions and to a meridional surface temperature gradient that generates low-level easterly jet. This jet acts as a barrier to the penetration of moisture into other parts of semi arid areas. These processes are strongly influenced by the surface energy budget, which, in turn, is highly sensitive to land cover and vegetation type. This study, tested the hypothesis that large-scale afforestation in these semi-arid regions can result in changes in local and regional atmospheric circulation and, in turn, in the amount of precipitation. We tested our hypothesis using the Ocean-Land-Atmosphere Model (OLAM), General Circulation Model (GCM) by simulating the effects of the afforestation in the Sahel and North Australia. We carried out 17 years simulations (1996-2012) with 200 km horizontal grid scale and 6 years simulations (2000-2006) with 50 km horizontal grid scale of afforestation scenarios. The afforested areas parameterization (Sahel 2.6 E6 km2 and North Australia 2.1 E6 km2) was based on the extensive data from a 15 years study of the semi-arid Yatir forest in Israel. The regional effect of the afforestation focused on the summer rainy season (Sahel: Jul-Sep., and N-Aust: Jan-Mar). The results showed that changing land cover from shrubland to forest increases root depth, soil water content and ET that leads to lower surface temperature over the forest and decrease in the meridional temperature gradient. Changes in temperature gradient leads to increased precipitation due to weakening and displacement of the low level thermal wind, responsible for the generation the easterly jet. Ocean contribution to the precipitation over the Sahel has positive feedback due to higher sea-surface-temperature (SST) of the Northern hemisphere during summertime. This study results demonstrates the significant effect of Monsoon rains and oceans presence nearby large-scale afforestation in semi-arid regions, on the local and regional hydrological cycle. This effect is expressed in an increase of latent heat leading to land surface cooling and to positive feedbacks that evolve in precipitation increase in the afforested areas surroundings.
PhD Thesis at the Weizmann Institute of Science
Plant phenological development is orchestrated through subtle changes in photoperiod, temperature, soil moisture and nutrient availability. Presently, the exact timing of plant development stages and their response to climate and management practices are crudely represented in land surface models. As visual observations of phenology are laborious, there is a need to supplement long-term observations with automated techniques such as those provided by digital repeat photography at high temporal and spatial resolution. We present the first synthesis from a growing observational network of digital cameras installed on towers across Europe above deciduous and evergreen forests, grasslands and croplands, where vegetation and atmosphere CO2 fluxes are measured continuously. Using colour indices from digital images and using piecewise regression analysis of time series, we explored whether key changes in canopy phenology could be detected automatically across different land use types in the network. The piecewise regression approach could capture the start and end of the growing season, in addition to identifying striking changes in colour signals caused by flowering and management practices such as mowing. Exploring the dates of green-up and senescence of deciduous forests extracted by the piecewise regression approach against dates estimated from visual observations, we found that these phenological events could be detected adequately (RMSE
The Yatir forest in Israel is a solitary forest at the dry timberline that is surrounded by semi-arid shrubland. Due to its low albedo and its high surface roughness, the forest has a strong impact on the surface energy budget, and is supposed to induce a secondary circulation, which was assessed using eddy-covariance (EC) and Doppler lidar measurements and large-eddy simulation (LES). The buoyancy fluxes were 220-290 W m(-2) higher above the forest, and the scale of the forest relative to the boundary-layer height is ideal for generating a secondary circulation, as confirmed by a LES run without background wind. However, usually a relatively high background wind (6 m s(-1)) prevails at the site. Thus, with the Doppler lidar a persistent updraft above the forest was detected only on 5 of the 16 measurement days. Nevertheless, the secondary circulation and convective coherent structures caused low-frequency flux contributions in the mixed-layer w spectra, the surface layer u and w spectra and in the surface-layer momentum fluxes. According to the ogive functions from the tower data and a control volume approach using the LES, such low-frequency contributions with timescales >30 min were a major reason the non-closure of the energy balance at the desert site. In the roughness sublayer above the forest, these large structures were broken up into smaller eddies and the energy balance was closed. (C) 2015 Elsevier B.V. All rights reserved.
Soil respiration is the sum of respiration processes in the soil and is a major flux in the global carbon cycle. It is usually assumed that the CO2 efflux is equal to the soil respiration rate. Here we challenge this assumption by combining measurements of CO2 with high-precision measurements of O-2. These measurements were conducted on different ecosystems and soil types and included measurements of air samples taken from the soil profile of three Mediterranean sites: a temperate forest and two alpine forests. Root-free soils from the alpine sites were also incubated in the lab. We found that the ratio between the CO2 efflux and the O-2 influx (defined as apparent respiratory quotient, ARQ) was in the range of 0.14 to 1.23 and considerably deviated from the value of 0.9 +/- 0.1 expected from the elemental composition of average plants and soil organic matter. At the Mediterranean sites, these deviations are explained as a result of CO2 dissolution in the soil water and transformation to bicarbonate ions in these high-pH soils, as well as by carbonate mineral dissolution and precipitation processes. Thus, a correct estimate of the short-term, chamber-based biological respiratory flux in such soils can only be made by dividing the measured soil CO2 efflux by the average (efflux-weighted) soil profile ARQ. Applying this approach to a semiarid pine forest resulted in an estimated short-term biological respiration rate that is 3.8 times higher than the chamber-measured surface CO2. The ARQ values often observed in the more acidic soils were unexpectedly low (
A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its delta O-18 and tritium content, dissolved inorganic carbon (DIC) and its delta C-13 (parts per thousand) and Delta C-14 (parts per thousand) content, and delta C-13 (parts per thousand) and Delta C-14 (parts per thousand) in the solid sediment. The profiles of moisture and delta O-18 in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr(-1)). The source of carbon in the DIC is CO2 released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The delta C-13 (parts per thousand) profiles show clearly the chemical transition from dissolved CO2 (delta C-13 = -22 parts per thousand) to bicarbonate (delta C-13 = -14 parts per thousand). At greater depth (-11.3 parts per thousand), the delta C-13 becomes similar to the delta C-13 in the aquifer below (-12.5 parts per thousand). The effect of secondary processes is evident in the profile of Delta C-14 in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10 parts per thousand yr(-1). Precipitation of carbon from the DIC on the sediment is 1.1 mg C L-sed(-1) yr(-1), negligible compared to the 28 g C in 1 L-sed. In the solid sediment, there is a gradient in Delta C-14(carb) at the top meter. The net precipitation of C-14 from the DIC on the sediment (0.25 to 1.1 parts per thousand yr(-1)), corrected for decay, cannot be observed in the deeper sediment. The presence of C-14 in the top 1 m of the sediment is explained by two possible processes: accumulation of C-14-tagged dust (similar to 0.05 mm yr(-1)) and/or long-term cumulative precipitation from the DIC.
In trees exposed to prolonged drought, both carbon uptake (C source) and growth (C sink) typically decrease. This correlation raises two important questions: (i) to what degree is tree growth limited by C availability; and (ii) is growth limited by concurrent C storage (e.g., as nonstructural carbohydrates, NSC)? To test the relationships between drought, growth and C reserves, we monitored the changes in NSC levels and constructed stem growth chronologies of mature Pinus halepensis Miller trees of three drought stress levels growing in Yatir forest, Israel, at the dry distribution limit of forests. Moderately stressed and stressed trees showed 34 and 14% of the stem growth, 71 and 31% of the sap flux density, and 79 and 66% of the final needle length of healthy trees in 2012. In spite of these large reductions in growth and sap flow, both starch and soluble sugar concentrations in the branches of these trees were similar in all trees throughout the dry season (2-4% dry mass). At the same time, the root starch concentrations of moderately stressed and stressed trees were 47 and 58% of those of healthy trees, but never
Knowledge of the relationship between soil water dynamics and tree water use is critical to understanding forest response to environmental change in water-limited ecosystems. However, the dynamics in soil water availability for tree transpiration (T-t) cannot be easily deduced from conventional measurements of soil water content (SWC), notably because T-t is influenced by soil water potential ((s)) that, in turn, depends on soil characteristics. Using tree sap flow and water potential and deriving depth-dependent soil water retention curves, we quantified the transpirable soil water content' (tSWC) and its seasonal and inter-annual variations in a semi-arid Pinus halepensis forest. The results indicated that tSWC varied in time and with soil depth. Over one growing season T-t was 57% of rain and 72% of the infiltrated SWC. In early winter, T-t was exclusively supported by soil moisture at the top 10cm (tSWC=11mm), whereas in spring (tSWC>18mm) and throughout the dry season, source water for T-t shifted to 20-40cm, where the maximum fine root density occurs. Simulation with the soil-plant-atmosphere water and energy transport model MuSICA supported the idea that consistent tSWC at the 20-40cm soil layer critically depended on limited water infiltration below 40cm, because of high water retention below this depth. Quantifying tSWC is critical to the precise estimation of the onset and termination of the growing season (when tSWC>0) in this semi-arid ecosystem. Copyright (c) 2013 John Wiley & Sons, Ltd.
Understanding the processes that control the terrestrial exchange of carbon is critical for assessing atmospheric CO2 budgets. Carbonyl sulfide (COS) is taken up by vegetation during photosynthesis following a pathway that mirrors CO2 but has a small or nonexistent emission component, providing a possible tracer for gross primary production. Field measurements of COS and CO2 mixing ratios were made in forest, senescent grassland, and riparian ecosystems using a laser absorption spectrometer installed in a mobile trailer. Measurements of leaf fluxes with a branch-bag gas-exchange system were made across species from 10 genera of trees, and soil fluxes were measured with a flow-through chamber. These data show (1) the existence of a narrow normalized daytime uptake ratio of COS to CO2 across vascular plant species of 1.7, providing critical information for the application of COS to estimate photosynthetic CO2 fluxes and (2) a temperature-dependent normalized uptake ratio of COS to CO2 from soils. Significant nighttime uptake of COS was observed in broad-leafed species and revealed active stomatal opening prior to sunrise. Continuous high-resolution joint measurements of COS and CO2 concentrations in the boundary layer are used here alongside the flux measurements to partition the influence that leaf and soil fluxes and entrainment of air from above have on the surface carbon budget. The results provide a number of critical constraints on the processes that control surface COS exchange, which can be used to diagnose the robustness of global models that are beginning to use COS to constrain terrestrial carbon exchange.
This study aims to test the hypothesis that as leaf water potential decreases, stomata! conductance (g(s)) and total water use decrease faster in trees tending toward isohydric behavior than in coexisting anisohydric trees. We measured leaf gas exchange rates in two key Mediterranean species: Pious halepensis (isohydric) and Quercus calliprinos (anisohydric) growing together in two different sites during seven field campaigns over 14 months. Intrinsic water-use efficiency (WUEi) was calculated from gas exchange ratios, and independently from carbon isotopic composition, delta C-13, of annual tree-ring sub-sections in four representative growth years. As expected, g(s) was greatly restricted already at VPD 5 kPa. Consequently, mean transpiration rates were 0.2-2.2 and 0.5-3.9 mmol m(2) s(-1) in coexisting pines and oaks, respectively. Mean delta C-13 values were 1.5 parts per thousand higher in tree- rings of the pine compared to the oak trees, consistent with the differences in WOE; between 75 and 64 mu mol CO2 mol(-1) H2O in pines and oaks, respectively, based on the short-term gas exchange measurements. A preliminary attempt to upscale the results to typical forest stands of the two species, on annual time-scales, demonstrated that the differences in stomatal regulation and water-use could imply similar to 30% higher water-use (or similar to 70% lower water yield) in oak stand compared to pine stand, related to its tendency toward anisohydric behavior. This sets the limit for typical 300 trees ha(-1) oak and pine stands at the 460 and 360 mm iso-precipitation lines, respectively, consistent with their current distribution along the precipitation gradient in our region. The results can help predict or manage changes in species composition in the face of increasing water limitations in Mediterranean regions. (C) 2013 Elsevier B.V. All rights reserved.
Laboratory studies indicate that plant respiratory efficiency may decrease in response to low nutrient availability due to increased partitioning of electrons to the energy-wasteful alternative oxidase (AOX); however, field confirmation of this hypothesis is lacking. We therefore investigated plant respiratory changes associated with succession and retrogression in soils aged from 10 to 120000 years along the Franz Josef soil chronosequence, New Zealand. Respiration rates and electron partitioning were determined based on oxygen isotopic fractionation. Leaf structural traits, foliar nutrient status, carbohydrates and species composition were measured as explanatory variables. Although soil nutrient levels and species composition varied by site along the chronosequence, foliar respiration across all sites and species corresponded strongly with leaf nitrogen concentration (r2=0.8). In contrast, electron partitioning declined with increasing nitrogen/phosphorus (r2=0.23) and AOX activity correlated with phosphorus (r2=0.64). Independently, total respiration was further associated with foliar Cu, possibly linked to its effect on AOX. Independent control of AOX and cytochrome pathway activities is also discussed. These responses of plant terminal respiratory oxidases and therefore respiratory carbon efficiency to multiple nutrient deficiencies demonstrate that modulation of respiratory metabolism may play an important role in plant responses to nutrient gradients.
Carbonyl sulfide (COS) is an atmospheric trace gas that participates in some key reactions of the carbon cycle and thus holds great promise for studies of carbon cycle processes. Global monitoring networks and atmospheric sampling programs provide concurrent data on COS and CO2 concentrations in the free troposphere and atmospheric boundary layer over vegetated areas. Here we present a modeling framework for interpreting these data and illustrate what COS measurements might tell us about carbon cycle processes. We implemented mechanistic and empirical descriptions of leaf and soil COS uptake into a global carbon cycle model (SiB 3) to obtain new estimates of the COS land flux. We then introduced these revised boundary conditions to an atmospheric transport model (Parameterized Chemical Transport Model) to simulate the variations in the concentration of COS and CO2 in the global atmosphere. To balance the threefold increase in the global vegetation sink relative to the previous baseline estimate, we propose a new ocean COS source. Using a simple inversion approach, we optimized the latitudinal distribution of this ocean source and found that it is concentrated in the tropics. The new model is capable of reproducing the seasonal variation in atmospheric concentration at most background atmospheric sites. The model also reproduces the observed large vertical gradients in COS between the boundary layer and free troposphere. Using a simulation experiment, we demonstrate that comparing drawdown of CO2 with COS could provide additional constraints on differential responses of photosynthesis and respiration to environmental forcing. The separation of these two distinct processes is essential to understand the carbon cycle components for improved prediction of future responses of the terrestrial biosphere to changing environmental conditions.
Transpiration is a fundamental datum in understanding the ecophysiology of planted forests in dry regions and is central to the construction of an ecosystem-level water balance. The present aims were: (i) to measure daily transpiration in a dryland Pinus halepensis Mill. (Aleppo pine) forest and to examine its relationship to environmental conditions such as soil water content and evaporative demand; (ii) to determine the seasonal and annual water balances of the ecosystem; and (iii) to explore management implications in the context of a climate-change scenario of increasing aridity. The study was conducted in the Yatir forest (300 trees ha(-1)) in Israel's semiarid northern Negev, during three consecutive years (starting 2003/4) in which rainfall (R) was 231, 334 and 224 mm, the last designated a drought because of relatively long dry spells between major rain events. Tree transpiration was measured by the heat-pulse method and values were upscaled to the forest canopy level. Independent estimates were obtained for daily ecosystem-level evapotranspiration (ET), and soil and understory vegetation evapotranspiration (E). Daily canopy-level transpiration rate (7) ranged from 0.1 to 1.6 mm d(-1) and showed a highly dynamic and irregular pattern during the rainy season. For the two non-drought years a large part of the variation in T could be attributed to the following simple relationships. When soil water content (SWC) 0.15 m(3) m(-3), potential evapotranspiration (PET) was of paramount importance; quadratic regression of T on PET yielded highly significant relationships and explained a high proportion of the variance in T. During the wet season (215 d), on average, cumulative ET (201 mm) accounted for 0.76 R (R = 263 mm) and
Limited understanding of carbon dioxide sinks and sources on land is often linked to the inability to distinguish between the carbon dioxide taken up by photosynthesis, and that released by respiration(1,2). Carbonyl sulphide, a sulphur-containing analogue of carbon dioxide, is also taken up by plants, and could potentially serve as a powerful proxy for photosynthetic carbon dioxide uptake, which cannot be directly measured above the leaf scale. Indeed, variations in atmospheric concentrations of carbonyl sulphide are closely related to those of carbon dioxide at regional, local and leaf scales(3-9). Here, we use eddy covariance and laser spectroscopy(10) to estimate the net exchange of carbon dioxide and carbonyl sulphide across three pine forests, a cotton field and a wheat field in Israel. We estimate gross primary productivity-a measure of ecosystem photosynthesis-directly from the carbonyl sulphide fluxes, and indirectly from carbon dioxide fluxes. The two estimates agree within an error of +/- 15%. The ratio of carbonyl sulphide to carbon dioxide flux at the ecosystem scale was consistent with the variability in mixing ratios observed on seasonal timescales in the background atmosphere. We suggest that atmospheric measurements of carbonyl sulphide flux could provide an independent constraint on estimates of gross primary productivity, key to projecting the response of the land biosphere to climate change.
CO2 exchange between terrestrial ecosystems and the atmosphere is key to understanding the feedbacks between climate change and the land surface. In regions with carbonaceous parent material, CO2 exchange patterns occur that cannot be explained by biological processes, such as disproportionate outgassing during the daytime or night-time CO2 uptake during periods when all vegetation is senescent. Neither of these phenomena can be attributed to carbonate weathering reactions, since their CO2 exchange rates are too small. Soil ventilation induced by high atmospheric turbulence is found to explain atypical CO2 exchange between carbonaceous systems and the atmosphere. However, by strongly altering subsurface CO2 concentrations, ventilation can be expected to influence carbonate weathering rates. By imposing ventilation-driven CO2 outgassing in a carbonate weathering model, we show here that carbonate geochemistry is accelerated and does play a surprisingly large role in the observed CO2 exchange pattern of a semi-arid ecosystem. We found that by rapidly depleting soil CO2 during the daytime, ventilation disturbs soil carbonate equilibria and therefore strongly magnifies daytime carbonate precipitation and associated CO2 production. At night, ventilation ceases and the depleted CO2 concentrations increase steadily. Dissolution of carbonate is now enhanced, which consumes CO2 and largely compensates for the enhanced daytime carbonate precipitation. This is why only a relatively small effect on global carbonate weathering rates is to be expected. On the short term, however, ventilation has a drastic effect on synoptic carbonate weathering rates, resulting in a pronounced diel pattern that exacerbates the non-biological behavior of soil-atmosphere CO2 exchanges in dry regions with carbonate soils.
The rate of migration and in situ genetic variation in forest trees may not be sufficient to compete with the current rapid rate of climate change. Ecophysiological adjustments of key traits, however, could complement these processes and allow sustained survival and growth across a wide range of climatic conditions. This was tested in Pinus halepensis Miller by examining seven physiological and phenological parameters in five provenances growing in three common garden plots along a climatic transect from meso-Mediterranean (MM) to thermo-Mediterranean (TM) and semi-arid (SA) climates. Differential responses to variations in ambient climatic conditions were observed in three key traits: (i) growing season length decreased with drying in all provenances examined (from 165 under TM climate to 100 days under SA climate, on average); (ii) water use efficiency (WUE) increased with drying, but to a different extent in different provenances, and on average from 80, to 95, to 110 mu mol CO2 mol(-1) H2O under MM, TM and SA climates, respectively; (iii) xylem native embolism was stable across climates, but varied markedly among different provenances (percent loss of conductivity, was below 5% in two provenances and above 35% in others). The results indicated that changes in growing season length and WUE were important contributors to tree growth across climates, whereas xylem native embolism negatively correlated with tree survival. The results indicated that irrespective of slow processes (e.g., migration, genetic adaptation), the capacity for ecophysiological adjustments combined with existing variations among provenances could help sustain P. halepensis, a major Mediterranean tree species, under relatively extreme warming and drying climatic trends.
A study of CO2 in soil gas was conducted in a bare plot in the unsaturated zone (USZ) of Yatir Forest, northern Negev, Israel. In 2006, 6 tubes for sampling of soil gas were inserted into the USZ to depths of 30, 60, 90, 120, 200, and 240 cm. Profiles of soil gas in the USZ were collected from the tubes 5 times between October 2007 and September 2008. Measurements of the collected profiles of soil gas were of CO2 (ppm), delta C-13 (parts per thousand), and Delta C-14 (parts per thousand). At all times, the concentration of CO2 in the soil gas was higher than in the air at the surface (CO2 similar to 400 ppm; delta C-13 similar to -9 parts per thousand). The main source of the CO2 in soil gas is from biotic activity released through roots of trees and of seasonal plants close to the surface. In the winter, the CO2 concentrations were lowest (6000 ppm) and the delta C-13 was -20 parts per thousand. In the spring and through the summer, the CO2 concentration increased. It was estimated that the major source of CO2 is at similar to 240 cm depth (delta C-13 similar to -22 parts per thousand; CO2 similar to 9000 ppm) or below. Above this level, the concentrations decrease and the delta C-13 (parts per thousand) become more positive. The C-14 values in the measured profile are all less than atmospheric and biotic C-14. It was deduced that biotic CO2 dissolves in porewater to form carbonic acid, which then dissolves secondary carbonate (delta C-13 similar to -8 parts per thousand; C-14 similar to -900 parts per thousand) from the sediments of the USZ. With the C-14 data, the subsequent release of CO2 into the soil gas was then estimated. The C-14 data, supported by the C-13 and CO2 data, also indicate a biotic source at the root zone, at about 90 cm depth.
Plants can alter rates of electron transport through the alternative oxidase (AOX) pathway in response to environmental cues, thus modulating respiratory efficiency, but the 18O discrimination method necessary for measuring electron partitioning in vivo has been restricted to laboratory settings. To overcome this limitation, we developed a field-compatible analytical method. Series of plant tissue subsamples were incubated in 12 mL septum-capped vials for 0.54 h before aliquots of incubation air were injected into 3.7 mL evacuated storage vials. Vials were stored for up to 10 months before analysis by mass spectrometry. Measurements were corrected for unavoidable contamination. Additional mathematical tools were developed for detecting and addressing non-linearity (whether intrinsic or due to contamination) in the data used to estimate discrimination values. Initial contamination in the storage vials was 0.03 +/- 0.01 atm; storing the gas samples at -17 degrees C eliminated further contamination effects over 10 months. Discrimination values obtained using our offline incubation and computation method replicated previously reported results over a range of 1031 parts per thousand, with precision generally better than +/- 0.5 parts per thousand. Our method enables large-scale investigations of plant alternative respiration along natural environmental gradients under field conditions.
We extend our recent study of the effects of tree density on evapotranspiration (ET) partitioning in a semi-arid pine forest by examining the influence of the temporal patterns in rainfall (P) on the dynamic contributions of tree transpiration (T-t), soil evaporation (E-s) and rainfall interception (I-p) to total ET. Soil evaporation accounted for 39% of average annual ET over the four-year period, and was associated with soil moisture content in the upper 5 cm and solar radiation, therefore peaking during the wetting and drying seasons (up to 0.75 mm day(-1)). In the dry summer, E-s diminished and as much as 50% of the residual flux was due to re-evaporation of moisture condensed at night (adsorption). Tree tra:nspiration accounted for 49% of average annual ET, and was associated with soil moisture at a depth of 10-20 cm. Transpiration peaked only in late spring (1.5 mm day(-1)), after the accumulation of large storms allowing infiltration below the topsoil. Moisture at these depths was maintained for longer periods and was even carried over between rain seasons following a high precipitation year. Interception was 12% of annual ET but was larger than 20% during the rainy period. The results indicated that both T-t/ET and E-s/ET could vary between 30% and 60% due to their differential response to seasonal environmental drivers. Annual T-t/ET, a major parameter indicating forest productivity and survival, was more influenced by the occurrence of large storms (>30 mm; P-30/P ratio) than by P itself. In an assessment of the potential warming and drying trends predicted for the Mediterranean region in the next century, changes in both total precipitation and in its temporal patterns must be considered. (c) 2012 Elsevier B.V. All rights reserved.
Urbanization is accelerating across the globe, elevating the importance of studying urban ecology. Urban environments exhibit several factors affecting plant growth and function, including high temperatures (particularly at night), CO2 concentrations and atmospheric nitrogen deposition. We investigated the effects of urban environments on growth in Quercus rubra L. seedlings. We grew seedlings from acorns for one season at four sites along an urban-rural transect from Central Park in New York City to the Catskill Mountains in upstate New York (difference in average maximum temperatures of 2.4 degrees C; difference in minimum temperatures of 4.6 degrees C). In addition, we grew Q. rubra seedlings in growth cabinets (GCs) mimicking the seasonal differential between the city and rural sites (based on a 5-year average). In the field experiment, we found an eightfold increase in biomass in urban-grown seedlings relative to those grown at rural sites. This difference was primarily related to changes in growth allocation. Urban-grown seedlings and seedlings grown at urban temperatures in the GCs exhibited a lower root: shoot ratio (urban similar to 0.8, rural/remote similar to 1.5), reducing below-ground carbon costs associated with construction and maintenance. These urban seedlings instead allocated more growth to leaves than did rural-grown seedlings, resulting in 10-fold greater photosynthetic area but no difference in photosynthetic capacity of foliage per unit area. Seedlings grown at urban temperatures in both the field and GC experiments had higher leaf nitrogen concentrations per unit area than those grown at cooler temperatures (increases of 23% in field, 32% in GC). Lastly, we measured threefold greater C-13 enrichment of respired CO2 (relative to substrate) in urban-grown leaves than at other sites, which may suggest greater allocation of respiratory function to growth over maintenance. It also shows that lack of differences in total R flux in response to en
RATIONALE: Environmental and biological investigations may require samples that vary over a wide range of concentrations and isotope ratios, making measurements using continuous flow isotope ratio mass spectrometry (CF-IRMS) problematic due to nonlinear signal response. We therefore developed a mathematical approach for correcting nonlinearities over a wide range of sample concentrations and actual d values. METHODS: Dilution series for two standards were prepared in septum-capped vials and introduced into the mass spectrometer via the standard sampling pathway. Parameters for a nonlinear signal correction were determined by regression on measured isotope ratio vs. both signal strength and actual isotope ratio. We further extended the dynamic range by adjusting the position of an open split based on analyte concentration. Effects of the open split setting required additional mathematical correction. RESULTS: The nonlinearities were corrected over a 100-fold range of sample concentrations and across a 600 parts per thousand change in isotope ratios (for delta O-2/N-2 values). The precision, measured as standard deviation, across the upper 90% of the concentration range was +/- 0.08 parts per thousand, +/- 0.05%, and +/- 2.6 parts per thousand for delta O-18, delta N-15, and delta O-2/N-2 values, respectively; the precision across the lower 10% of the range was +/- 0.22 parts per thousand, +/- 0.07 parts per thousand, and +/- 7.6 parts per thousand, respectively. In all cases the linearity correction represented only a small fraction of these precision values. CONCLUSIONS: The empirical correction described here provides a relatively simple yet effective way to increase the usable signal range for CF-IRMS applications. This improvement in dynamic range should be especially helpful for environmental and biological field studies, where sampling methods may be constrained by external factors. Copyright (C) 2012 John Wiley & Sons, Ltd.
Nitrogen (N) and water availability are important factors affecting ecosystem productivity that can be influenced by land-use change. We hypothesized that the observed increase in carbon (C) sequestration associated with afforestation of semi-arid sparse shrubland must also be associated with an increase in N input. We tested this hypothesis by reconstructing the ecosystem N budget of two ecosystems, a semi-arid shrubland and a nearby planted pine forest, using measurements augmented with literature-based estimates. Our findings demonstrate that, contrary to our hypothesis, massive C sequestration by the pine forest could be accounted for without a change in the net N budget (i.e., neither elevated N inputs nor reduced N losses). However, in comparison to the shrubland, the forest showed an almost tripling in aboveground N use efficiency (NUE; 235 vs. 83 kg dry mass kg(-1) N) and a doubling in ecosystem level C/N ratio (16 vs. 8, for the forest and shrubland, respectively). Nitrogen cycling slowed in the forest compared to the shrubland: net N mineralization rates in soils decreased by approximately 50%, decomposition rates decreased by approximately 20%, and NOx loss decreased by approximately 64%. These adjustments in N cycling provide a possible basis for increased NUE and subsequent C sequestration without net change in the overall N budget, which should be addressed in future investigations.
The potential use of carbonyl sulfide (COS) as tracer of CO2 flux into the land biosphere stimulated research on COS interactions with leaves during gas exchange. We carried out leaf gas-exchange measurements of COS and CO2 in 22 plant species representing deciduous and evergreen trees, grasses, and shrubs, under a range of light intensities, using mid-infrared laser spectroscopy. A narrow range in the normalized ratio of the net uptake rates of COS (A(s)) and CO2 (A(c)), leaf relative uptake (A(s)/A(c) x [CO2]/[COS]), was observed, with a mean value of 1.61 +/- 0.26, which is advantageous to the use of COS in photosynthesis research. Notably, increasing COS concentrations between 250 and 2,800 pmol mol(-1) (enveloping atmospheric levels) enhanced stomatal conductance (g(s)) to a variable extent in most plants examined (up to a normalized enhancement factor [f(e) = (g(s-max) 2 g(s-min))/g(s-min)] of 1). This enhancement was completely abolished in carbonic anhydrase (CA)-deficient antisense lines of both C3 and C4 plants. We suggest that the stomatal response is mediated by CA and may involve hydrogen sulfide formed in the reaction of COS and water with CA. In all species examined, the uptake rates of COS and CO2 were highly correlated, but there was no relationship between the sensitivity of stomata to COS and the rate of COS uptake (or, by inference, hydrogen sulfide production). The basis for the observed stomatal sensitivity and its variations is still to be determined.
Seasonal dynamics of atmospheric carbonyl sulfide (OCS) at regional and continental scales and plant OCS exchange at the leaf level have shown a close relationship with those for CO(2). CO(2) has both sinks and sources within terrestrial ecosystems, but the primary terrestrial exchange for OCS is thought to be leaf uptake, suggesting potential for OCS uptake as a proxy for gross primary production (GPP). We explored the utility of OCS uptake as a GPP proxy in micrometeorological studies of biosphere-atmosphere CO(2) exchange by applying theoretical concepts from earlier OCS studies to estimate GPP. We partitioned measured net ecosystem exchange (NEE) using the ratio of measured vertical mole fraction gradients of OCS and CO(2). At the Harvard Forest AmeriFlux site, measured CO(2) and OCS vertical gradients were correlated and were related to NEE and GPP, respectively. Estimates of GPP from OCS-based NEE partitioning were similar to those from established environmental regression techniques, providing evidence that OCS uptake can potentially serve as a GPP proxy. Measured vertical CO(2) mole fraction gradients at five other AmeriFlux sites were used to project anticipated vertical OCS mole fraction gradients to provide indication of potential OCS signal magnitudes at sites where no OCS measurements were made. Projected OCS gradients at sites with short canopies were greater than those in forests, including measured OCS gradients at Harvard Forest, indicating greater potential for OCS uptake as a GPP proxy at these sites. This exploratory study suggests that continued investigation of linkages between OCS and GPP is warranted.
Motivated by persistent predictions of warming and drying in the entire Mediterranean and other regions, we have examined the interactions of intrinsic water-use efficiency (W(i)) with environmental conditions in Pinus halepensis. We used 30-year (1974-2003) tree-ring records of basal area increment (BAI) and cellulose (13)C and (18)O composition, complemented by short-term physiological measurements, from three sites across a precipitation (P) gradient (280-700 mm) in Israel. The results show a clear trend of increasing W(i) in both the earlywood (EW) and latewood (LW) that varied in magnitude depending on site and season, with the increase ranging from ca. 5 to 20% over the study period. These W(i) trends were better correlated with the increase in atmospheric CO(2) concentration, C(a), than with the local increase in temperature (similar to 0.04 degrees C year(-1)), whereas age, height and density variations had minor effects on the long-term isotope record. There were no trends in P over time, but W(i) from EW and BAI were dependent on the interannual variations in P. From reconstructed C(i) values, we demonstrate that contrasting gas-exchange responses at opposing ends of the hydrologic gradient underlie the variation in W(i) sensitivity to C(a) between sites and seasons. Under the mild water limitations typical of the main seasonal growth period, regulation was directed at increasing C(i)/C(a) towards a homeostatic set-point observed at the most mesic site, with a decrease in the W(i) response to C(i) with increasing aridity. With more extreme drought stress, as seen in the late season at the drier sites, the response was W(i) driven, and there was an increase in the W(i) sensitivity to C(a) with aridity and a decreasing sensitivity of C(i) to C(a). The apparent C(a)-driven increases in W(i) can help to identify the adjustments to drying conditions that forest ecosystems can make in the face of predicted atmospheric change.
Carbonyl sulfide (COS) and (COO)-O-18 exchange by leaves provide potentially powerful tracers of biosphere-atmosphere CO2 exchange, and both are assumed to depend on carbonic anhydrase (CA) activity and conductance along the diffusive pathway in leaves. We investigated these links using C-3 and C-4 plants, hypothesizing that the rates of COS and (COO)-O-18 exchange by leaves respond in parallel to environmental and biological drivers. Using CA-deficient antisense lines of C-4 and C-3 plants, COS uptake was essentially eliminated and discrimination against (COO)-O-18 exchange ((18)Delta) greatly reduced, demonstrating CA's key role in both processes. (18)Delta showed a positive linear correlation with leaf relative uptake (LRU; ratio of COS to CO2 assimilation rates, A(s)/A(c), normalized to their respective ambient concentrations), which reflected the effects of stomatal conductance on both COS and (COO)-O-18 exchange. Unexpectedly, a decoupling between A(s) and (18)Delta was observed in comparing C-4 and C-3 plants, with a large decrease in (18)Delta but no parallel reduction in A(s) in the former. This could be explained by C-4 plants having higher COS concentrations at the CA site (maintaining high A(s) with reduced CA) and a high phosphoenolpyruvate carboxylase/CA activity ratio (reducing O-18 exchange efficiency between CO2 and water, but not A(s)). Similar A(s) but higher A(c) in C-4 versus C-3 plants resulted in lower LRU values in the former (1.16 +/- 0.20 and 1.82 +/- 0.18 for C-4 and C-3, respectively). LRU was, however, relatively constant in both plant types across a wide range of conditions, except low light (
The C-13 concentration in atmospheric CO2 has been declining over the past 150 years as large quantities of C-13-depleted CO2 from fossil fuel burning are added to the atmosphere. Deforestation and other land use changes have also contributed to the trend. Looking at the C-13 variations in the atmosphere and in annual growth rings of trees allows us to estimate CO2 uptake by land plants and the ocean, and assess the response of plants to climate. Here I show that the effects of the declining C-13 trend in atmospheric CO2 are recorded in the isotopic composition of paper used in the printing industry, which provides a well-organized archive and integrated material derived from trees' cellulose. C-13 analyses of paper from two European and two American publications showed, on average, a -1.65 +/- 1.00 parts per thousand trend between 1880 and 2000, compared with -1.45 and -1.57 parts per thousand for air and tree-ring analyses, respectively. The greater decrease in plant-derived C-13 in the paper we tested than in the air is consistent with predicted global-scale increases in plant intrinsic water-use efficiency over the 20th century. Distinct deviations from the atmospheric trend were observed in both European and American publications immediately following the World War II period.
Drought-induced tree mortality has increased over the last decades in forests around the globe. Our objective was to investigate under controlled conditions the hydraulic adjustments underlying the observed ability of Pinus halepensis to survive seasonal drought under semi-arid conditions. One hundred 18-month saplings were exposed in the greenhouse to 10 different drought treatments, simulating combinations of intensities (fraction of water supply relative to control) and durations (period with no water supply) for 30 weeks. Stomata closed at a leaf water potential (psi(l)) of -2.8 MPa, suggesting isohydric stomatal regulation. In trees under extreme drought treatments, stomatal closure reduced CO(2) uptake to -1 mu mol m(-2) s(-1), indicating the development of carbon starvation. A narrow hydraulic safety margin of 0.3 MPa (from stomatal closure to 50% loss of hydraulic conductivity) was observed, indicating a strategy of maximization of CO(2) uptake in trees otherwise adapted to water stress. A differential effect of drought intensity and duration was observed, and was explained by a strong dependence of the water stress effect on the ratio of transpiration to evapotranspiration T/ET and the larger partitioning to transpiration associated with larger irrigation doses. Under intense or prolonged drought, the root system became the main target for biomass accumulation, taking up to 100% of the added biomass, while the stem tissue biomass decreased, associated with up to 60% reduction in xylem volume.
Effective leaf area index (LAI(e)) in the semi-arid Pinus halepensis plantation, located between arid and semi-arid climatic zones at the edge of the Negev and Judean deserts, was measured bi-annually during four years (2001-2004) and more intensively (monthly) during the following two years (2004-2006) by a number of non-contact optical devices. The measurements showed a gradual increase in LAI(e) from similar to 1 (+/- 0.25) to similar to 1.8 (+/- 0.11) during these years. All instruments, when used properly, gave similar results that were also comparable with actual leaf area index measured by litter collection and destructive sampling and allometric estimates. Because of the constraint of clear sky conditions, which limited the use of the fisheye type sensors to times of twilight, the fisheye techniques were less useful. The tracing radiation and architecture of canopies system, which includes specific treatment of two levels of clumpiness of the sparse forest stand, was used successfully for the intensive monitoring. The mean clumpiness index, 0.61, is considered representative for the specific environment. Finally, the LAI(e) measurements at the start of each season were used to constrain phenology-based estimates of annual LAI(e) development, resulting in a continuous course of LAI(e) in the forest over the five-year period. Intra-seasonal LAI(e) variation in the order of 10% of total LAI(e) predicted by the model was also observed in the intensive TRAC measurements, giving confidence in the TRAC system and indicating its sensitivity and applicability in woodlands even with low LAI(e) values. The results can be important for forest management decision support as well as for use in evaluation of remote sensing techniques for forests at the lowest range of LAI(e) values. (C) 2011 Elsevier B.V. All rights reserved.
Land use and land cover changes greatly influence surface energy balance and consequently climate, and are likely to be associated with the persistent predictions of warming and drying throughout the Mediterranean and other regions. We specifically address the question of how the high radiation load and suppressed latent heat flux, intrinsic to dry regions, interact with land use changes and climate in these environments. We use for this purpose a detailed 6-year (2003-2008) study of the redistribution of the radiation load in an open-canopy pine forest. The results show that compared with the background shrubland, there was a 23.8 W m-2 increase in shortwave radiation load on the forest (to a mean annual net solar radiation of 211 W m-2) associated with a decrease in albedo of 0.1. Surface (skin) temperature in the forest was lower than in the shrubland (by similar to 5 degrees C on average) due to an efficient 'convector effect' and the production of a large sensible heat flux (up to 926 W m-2 in summer), which effectively shifted heat from the canopy to the overlying boundary layer. The cooler forest skin temperature resulted in suppression of upwelling longwave radiation (by 25 W m-2, annual average), further increasing the forest radiation load (mean annual net radiation of 116 and 67 W m-2 for forest and shrubland, respectively). This suppression also resulted in a local 'canopy greenhouse effect', where upwelling longwave radiation from the ground to the canopy was larger than from the canopy to the atmosphere (by up to 150 W m-2 in summer) and was associated with similar to 3 degrees C warming below the canopy. The ability of the dry productive forest to deal with the high radiation load indicates the potential for afforestation in dry areas.
PhD Thesis at the Weizmann Institute of Science
We report the first investigation of changes in electron partitioning via the alternative respiratory pathway (AP) and alternative oxidase (AOX) protein abundance in field-grown plants and their role in seasonal acclimation of respiration. We sampled two alpine grasses native to New Zealand, Chionochloa rubra and Chionochloa pallens, from field sites of different altitudes, over 1 yr and also intensively over a 2-wk period. In both species, respiration acclimated to seasonal changes in temperature through changes in basal capacity (R(10)) but not temperature sensitivity (E(0)). In C. pallens, acclimation of respiration may be associated with a higher AOX : cytochrome c oxidase (COX) protein abundance ratio. Oxygen isotope discrimination (D), which reflects relative changes in AP electron partitioning, correlated positively with daily integrated photosynthetically active radiation (PAR) in both species over seasonal timescales. Respiratory parameters, the AOX : COX protein ratio and D were stable over a 2-wk period, during which significant temperature changes were experienced in the field. We conclude that respiration in Chionochloa spp. acclimates strongly to seasonal, but not to short-term, temperature variation. Alternative oxidase appears to be involved in the plant response to both seasonal changes in temperature and daily changes in light, highlighting the complexity of the function of AOX in the field.
Linking measurements of carbonyl sulfide (COS) with those of carbon dioxide (CO(2)) has a potential in providing a powerful tracer of gross CO(2) fluxes between the atmosphere and land plants, a critical element in understanding the response of the land biosphere to global change. A new application of online COS, CO(2) and water vapor measurements based on newly designed mid-infrared (IR) dual quantum cascade laser (QCL) spectrometer measures COS and CO(2) (at 2056 cm-1) and water vapor (at 2190 cm-1), with detectors cooled thermoelectrically (at -43 degrees C) or with liquid nitrogen (-197 degrees C) for improved precision. Using the cryogenic detectors with averaging time of 1 s, precision was 50 pmol mol-1, 0.4 mu mol mol-1 and 0.01 mmol mol-1 for COS, CO(2) and water vapor, respectively (14, 0.2 and 0.003, respectively, for 60 s averaging time). We measured COS concentrations in ambient air, and changes in the rates of COS, CO(2) and water vapor exchange of attached leaves in response to changes in light intensity and ambient COS concentrations. The results were consistent with those of nononline gas chromatography-mass spectrometry for COS and IR gas analyzer for CO(2) and water vapor, with a high linear correlation for a broad range of concentrations (R2 = 0.85 for COS and R2 = 1.00 for CO(2) and water vapor). The new methodology opens the way for lab and field explorations of COS fluxes as a powerful new tracer for CO(2) exchange in the land biosphere.
The distribution of precipitation inputs into different hydrological components of water-limited forest ecosystems determines water availability to trees and consequently forest productivity. We constructed a complete hydrological budget of a semi-arid pine forest (285 mm annual precipitation) by directly measuring its main components: precipitation (P), soil water content, evapotranspiration (ET, eddy covariance), tree transpiration (sap flux), soil evaporation (soil chambers), and intercepted precipitation (calculated). Our results indicated that on average for the 4-year study period. ET accounted for 94% of P, varying between 100% when P 300 mm (with indications for losses to subsurface flow and soil moisture storage in wetter years). Direct measurements of the components of the ET flux demonstrated that both transpiration and soil evaporation were significant in this dry forest (45% and 36% of ET, respectively). Comparison between ecosystem ET (eddy covariance measurements) and the sum of its measured components showed good agreement on annual scales, but up to 30% discrepancies (in both directions) on shorter timescales. The pulsed storm pattern, characteristics of semi-arid climates, was sufficient to maintain the topsoil layer wet during the whole wet season. Only less often and intensive storms resulted in infiltration to the root zone, increasing water availability for uptake by deeper roots. Our results indicate that climate change predictions that link reduced precipitation with increased storm intensity may have a smaller effect on water availability to forest ecosystems than reduced precipitation alone, which could help forests' survival and maintain productivity even under drier conditions. (C) Copyright 2009 John Wiley & Sons, Ltd.
While there is currently intense effort to examine the 13C signal of CO2 evolved in the dark, less is known on the isotope composition of day-respired CO2. This lack of knowledge stems from technical difficulties to measure the pure respiratory isotopic signal: day respiration is mixed up with photorespiration, and there is no obvious way to separate photosynthetic fractionation (pure c(i)/c(a) effect) from respiratory effect (production of CO2 with a different delta 13C value from that of net-fixed CO2) at the ecosystem level. Here, we took advantage of new simple equations, and applied them to sunflower canopies grown under low and high [CO2]. We show that whole mesocosm-respired CO2 is slightly 13C depleted in the light at the mesocosm level (by 0.2-0.8 parts per thousand), while it is slightly 13C enriched in darkness (by 1.5-3.2 parts per thousand). The turnover of the respiratory carbon pool after labelling appears similar in the light and in the dark, and accordingly, a hierarchical clustering analysis shows a close correlation between the 13C abundance in day- and night-evolved CO2. We conclude that the carbon source for respiration is similar in the dark and in the light, but the metabolic pathways associated with CO2 production may change, thereby explaining the different 12C/13C respiratory fractionations in the light and in the dark.
In dry environments, water availability is a major limitation to forest productivity while losses to soil evaporation (E) are a significant component in ecosystem hydrology. We report on a 3-year study (2004-2007) in a semi-arid pine forest in Southern Israel (40-year-old Pinus halepensis; leaf area index = 1.5; mean precipitation 285 mm year(-1)) that estimated soil E, assessed its spatial variability and identified the factors influencing it. We used a modified and specially calibrated soil respiration chamber to directly measure E on a weekly basis at 14 permanently installed soil collars across the range of soil surface conditions. Results showed that spatial variability in E was large, with SD of +/- 47% between measurement sites. E fluxes measured in sun-exposed areas were on average double those in shaded areas (0.11 mm h(-1) vs. 0.06 mm h(-1)). The spatial variability in E correlated with radiation (measured in the photosynthetically active range), which was up to 92% higher in exposed compared to shaded sites, and with soil water content, which was higher in exposed areas during the wetting season but higher in shaded areas during the drying season. The fraction of shaded forest floor area was described as a function of canopy geometry (mean tree height, crown width and stand density) and the daily variation in solar altitude. Simple simulations based on the relationship between E and the shaded fraction indicated that E/P(precipitation) for the Yatir forest could decrease from 0.53 (undeveloped canopy of 10% cover) to 0.30 (full canopy closure). However, according to our analysis, increasing canopy cover will also increase intercepted precipitation and transpiration such that current precipitation inputs will not be able to support forest growth above a canopy cover of 65%. Combining direct measurements of environmental conditions and canopy characteristics with such simulations can provide a simple predictive and management tool to optimize tree water
Analysis of the stable isotopic composition of atmospheric moisture is widely applied in the environmental sciences. Traditional methods for obtaining isotopic compositional data from ambient moisture have required complicated sampling procedures, expensive and sophisticated distillation lines, hazardous consumables, and lengthy treatments prior to analysis. Newer laser-based techniques are expensive and usually not suitable for large-scale field campaigns, especially in cases where access to mains power is not feasible or high spatial coverage is required. Here we outline the construction and usage of a novel vapour-sampling system based on a battery-operated Stirling cycle cooler, which is simple to operate, does not require any consumables, or post-collection distillation, and is light-weight and highly portable. We demonstrate the ability of this system to reproduce delta(18)O isotopic compositions of ambient water vapour, with samples taken simultaneously by a traditional cryogenic collection technique. Samples were collected over 1 h directly into autosampler vials and were analysed by mass spectrometry after pyrolysis of I mu L aliquots to CO. This yielded an average error of
Forests both take up CO(2) and enhance absorption of solar radiation, with contrasting effects on global temperature. Based on a 9-year study in the forests' dry timberline, we show that substantial carbon sequestration (cooling effect) is maintained in the large dry transition zone (precipitation from 200 to 600 millimeters) by shifts in peak photosynthetic activities from summer to early spring, and this is counteracted by longwave radiation (L) suppression (warming effect), doubling the forestation shortwave (S) albedo effect. Several decades of carbon accumulation are required to balance the twofold S + L effect. Desertification over the past several decades, however, contributed negative forcing at Earth's surface equivalent to similar to 20% of the global anthropogenic CO(2) effect over the same period, moderating warming trends.
P>Carbonyl sulfide (COS) exchange in C-3 leaves is linked to that of CO2, providing a basis for the use of COS as a powerful tracer of gross CO2 fluxes between plants and the atmosphere, a critical element in understanding the response of the land biosphere to global change. Here, we carried out controlled leaf-scale gas-exchange measurements of COS and CO2 in representative C-3 plants under a range of light intensities, relative humidities and temperatures, CO2 and COS concentrations, and following abscisic acid treatments. No 'respiration-like' emission of COS or detectable compensation point, and no cross-inhibition effects between COS and CO2 were observed. The mean ratio of COS to CO2 assimilation flux rates, As/Ac, was c. 1.4 pmol mu mol-1 and the leaf relative uptake (assimilation normalized to ambient concentrations, (As/Ac)(C(a)c/C(a)s)) was 1.6-1.7 across species and conditions, with significant deviations under certain conditions. Stomatal conductance was enhanced by increasing COS, which was possibly mediated by hydrogen sulfide (H2S) produced from COS hydrolysis, and a correlation was observed between As and leaf discrimination against C18OO. The results provide systematic and quantitative information necessary for the use of COS in photosynthesis and carbon-cycle research on the physiological to global scales.
Improved global estimates of terrestrial photosynthesis and respiration are critical for predicting the rate of change in atmospheric CO2. The oxygen isotopic composition of atmospheric CO2 can be used to estimate these fluxes because oxygen isotopic exchange between CO2 and water creates distinct isotopic flux signatures. The enzyme carbonic anhydrase (CA) is known to accelerate this exchange in leaves, but the possibility of CA activity in soils is commonly neglected. Here, we report widespread accelerated soil CO2 hydration. Exchange was 10-300 times faster than the un-catalyzed rate, consistent with typical population sizes for CA-containing soil microorganisms. Including accelerated soil hydration in global model simulations modifies contributions from soil and foliage to the global (COO)-O-18 budget and eliminates persistent discrepancies existing between model and atmospheric observations. This enhanced soil hydration also increases the differences between the isotopic signatures of photosynthesis and respiration, particularly in the tropics, increasing the precision of CO2 gross fluxes obtained by using the delta O-18 of atmospheric CO2 by 50%.
This paper presents a study of the isotopic composition of dissolved inorganic carbon (DIC) stable and, radioactive, in pore water of the unsaturated zone (USZ) above the coastal aquifer of Israel The carbon. content and its isotopic composition in the gas and solid phases of the USZ are also presented. In the soil gas, large quantities of CO2 with quite modern C-14 activity were measured along the section (0.15 to 2.7% and 97 to 109 pMC respectively). In the inorganic fraction of the sediments, the C-14 activity between 2.5 and 13 m was 33 to 1.5 pMC. In the organic fraction the 14C activity between 7 and 9 m was 40 to 33 pMC. In the pore water, the high values of tritium and C-14 at depths of similar to 15-18 m have been attributed to the thermonuclear contamination of the 1960s. A significant decrease with depth in the DIC of pore water, from 23 to 4 mmol C/L, along with a decrease with depth in dissolved inorganic C-14, from 100 pMC near the surface to 72 pMC at depth of 20 m were observed. The delta C-13 values in the DIC are similar to the values in the inorganic sediment (similar to -10 parts per thousand). A first order reaction was applied to estimate the yearly rates of DIC loss by net precipitation (3.2%/year) and of C-14 activity (dpm/L) loss from the DIC by gross precipitation from the DIC on the sediment, (4.4%/year). From the gradient of dissolved inorganic C-14, the initial level at the bottom of the USZ, which is the groundwater table of the coastal aquifer of Israel, was estimated to be 0.54 of contemporaneous atmospheric value of C-14 when the rain fell on the ground. This value can be used for improved dating of groundwater with C-14 in the coastal aquifer of Israel. (C) 2009 Elsevier B.V. All rights reserved.
Warming and drying is predicted for most of the Mediterranean and other regions, and knowledge of this effect on forest carbon dynamics cannot be easily extrapolated from temperate climates. Instead, we provide quantitative information from a 6-year study in a 40-year old pine forest at the dry "timberline" (280 mm annual rainfall) on soil CO2 efflux (F-s) and some of its controlling factors. Annual Fs was relatively low (405.9 +/- 23.8 g C m(-2) a(-1)), but within one standard deviation of a global nonlinear relationship we derived between mean annual precipitation and F-s in forests. Seasonal variations in F-s were dominated by soil temperature (with Q(10) = 2.45) in the wet season, and by soil moisture in the water-limited seasons, but not by pulse responses to precipitation. No temperature sensitivity was observed in the dry season, but there was clear evidence for down regulation of sensitivity to Q(10) = 1.18 when soil moisture was kept high by a supplement summer irrigation treatment. Interannual variations in F-s correlated linearly with cumulative rainwater availability, indicating the combined importance of both precipitation amount and its temporal distribution between the wet (and cool) season and the transitional periods characterized by high evaporative demand. Low rates of soil carbon loss combined with high belowground carbon allocation (41% of canopy CO2 uptake) might explain the high soil organic carbon accumulation and net ecosystem productivity in this dry forest. Our results indicate that F-s in pine forests may adjust to dry conditions with better carbon economy than estimated from response to episodic drought in more temperate climate.
Nitric oxide (NO) plays a central role in the formation of tropospheric ozone, hydroxyl radicals, as well as nitrous and nitric acids. There are, however, large uncertainties around estimates of global NO emissions due to the paucity of data. In particular, there is little information on the rate of NO emission and its sensitivity to processes such as land use changes in dry environments. Here we report on a two-year study on the influence of afforestation on soil NO fluxes in the semi-arid afforestation system in Southern Israel (Yatir forest, mean annual precipitation similar to 280 mm). Laboratory incubations were carried out under seasonally defined conditions of soil moisture and temperature using soils sampled in different seasons from the native shrubland (taken both under shrub canopy and in the inter-shrub areas), and from the adjacent similar to 2800 ha, 40-year-old pine afforestation site. Combining laboratory results with field measurements of soil moisture and temperature, we up-scaled soil-atmosphere NO fluxes to the ecosystem level. The different microsites differed in their annual mean NO release rates (0.04, 0.14 and 0.03 mg m(-2) d(-1) for the shrubland under and between shrubs and for the forest, respectively), and exhibited high inter-seasonal variability in NO emission rates (ranging from zero up to 0.25 mg m(-2) d(-1) in the wet and dry-rewetting seasons, respectively), as well as in temperature responses. Up-scaling results to annual and ecosystem scales indicated that afforestation of the semi-arid shrubland could reduce soil NO emission by up to 65%. (C) 2009 Elsevier Ltd. All rights reserved.
Drought is the major factor limiting wheat productivity worldwide. The gene pool of wild emmer wheat, Triticum turgidum ssp. dicoccoides, harbours a rich allelic repertoire for morpho-physiological traits conferring drought resistance. The genetic and physiological bases of drought responses were studied here in a tetraploid wheat population of 152 recombinant inbreed lines (RILs), derived from a cross between durum wheat (cv. Langdon) and wild emmer (acc# G18-16), under contrasting water availabilities. Wide genetic variation was found among RILs for all studied traits. A total of 110 quantitative trait loci (QTLs) were mapped for 11 traits, with LOD score range of 3.0-35.4. Several QTLs showed environmental specificity, accounting for productivity and related traits under water-limited (20 QTLs) or well-watered conditions (15 QTLs), and in terms of drought susceptibility index (22 QTLs). Major genomic regions controlling productivity and related traits were identified on chromosomes 2B, 4A, 5A and 7B. QTLs for productivity were associated with QTLs for drought-adaptive traits, suggesting the involvement of several strategies in wheat adaptation to drought stress. Fifteen pairs of QTLs for the same trait were mapped to seemingly homoeologous positions, reflecting synteny between the A and B genomes. The identified QTLs may facilitate the use of wild alleles for improvement of drought resistance in elite wheat cultivars.
Quantitative trait loci (QTLs) for yield and drought related physiological traits, osmotic potential (OP), carbon isotope ratio (delta(13)C, an indicator of water use efficiency), and leaf chlorophyll content (Chl), were exchanged via marker-assisted selection (MAS) between elite cultivars of the two cotton species Gossypium barbadense cv. F-177 and G. hirsutum cv. Siv'on. The resulting near isogenic lines (NILs) were examined in two field trials, each with two irrigation regimes, in order to (1) evaluate the potential to improve cotton drought resistance by MAS and (2) test the role of physiological traits in plant productivity. NILs introgressed with QTLs for high yield rarely exhibited an advantage in yield relative to the recipient parent, whereas a considerable number of NILs exhibited the expected phenotype in terms of lower OP (5 out of 9), higher delta(13)C (4 out of 6) or high Chl (2 out of 3). Several NILs exhibited considerable modifications in non-targeted traits including leaf morphology, stomatal conductance and specific leaf weight (SLW). In G. barbadense genotypes, yield was correlated negatively with delta(13)C and OP and positively with stomatal conductance, SLW and Chl, whereas in G. hirsutum yield was negatively correlated with delta(13)C, SLW and Chl. This dissimilarity suggests that each of the respective species has evolved different mechanisms underlying plant productivity. We conclude that the improvement of drought related traits in cotton NILs may lead to improved drought resistance via MAS, but that conventional breeding may be necessary to combine the introduced QTL(s) with high yield potential.
The flux (Rs) and carbon isotopic composition (delta C-13(Rs)) of soil respired CO2 was measured every 2 h over the course of three diel cycles in a Mediterranean oak woodland, together with measurements of the delta C-13 composition of leaf, root and soil organic matter (delta C-13(SOM)) and metabolites. Simulations of R-s and delta C-13(Rs) were also made using a numerical model parameterised with the SOM data and assuming short-term production rates were driven mainly by temperature. Average values of delta C-13(Rs) over the study period were within the range of root metabolite and average delta C-13(SOM) values, but enriched in C-13 relative to the bulk delta C-13 of leaf, litter, and roots and the upper soil organic layers. There was good agreement between model output and observed CO2 fluxes and the underlying features of delta C-13(Rs). Observed diel variations of 0.5% in delta C-13(Rs) were predicted by the model in response to temperature-related shifts in production rates along a similar to 3 parts per thousand gradient observed in the profile of delta C-13(SOM). However, observed delta C-13(Rs) varied by over 2 parts per thousand, indicating that both dynamics in soil respiratory metabolism and physical processes can influence short-term variability of delta C-13(Rs).
The launch of the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra and Aqua satellites improved the ability to evaluate several surface biophysical parameters, including Leaf Area Index (LAI), which is provided as an operational MODIS product, available at 1-km spatial resolution and at 8-day intervals. However, for heterogeneous and sparse planted forests that are common to the semi-arid eastern Mediterranean region, the data at low spatial resolution may be significantly biased by the contribution of different background elements to the total surface reflectance received by the sensor and cannot therefore correctly reflect the real forest phenology. In the current paper the performance of the MODIS LAI product was examined over a dryland Mediterranean forest in southern Israel. The study found a significant discrepancy between ground-based and MODIS LAI datasets. In general, MODIS LAI values were c. 51% of the ground-based LAI measurements. In addition ground based LAI peaked in the summer due to the natural growth cycle of the pine trees, while MODIS values peaked in the winter. The MODIS seasonal course could be explained by the development of annuals and crypto- and micro-phytes in the understorey and the clearing areas during the mid winter months that are included in the MODIS LAI product but not in the ground based measurements. However, for that period MODIS estimates should have exceeded ground-based estimates while in fact they were still lower. The relationship between MOD12C1 Land Cover Type 3 and MOD15A2 products is discussed.
Understanding of isotopic variations in leaf water is important for reconstruction of paleoclimate and assessment of global biochemical processes. We report here a study of isotopic distributions within a single needle of two pine species, Pinus resinosa Ait and Pinus strobes L., with the objective of understanding how isotopic compositions of leaf water are controlled by environmental and physiological variables. A 2D model was developed to simulate along-leaf isotopic variations and bulk leaf water isotopic compositions. In addition to variables common to all leaf water isotopic models, this 2D model also takes into account the specific geometry and dimensions of pine needles and the isotopic transport in xylem and mesophyll. The model can successfully simulate oxygen isotopic variations along a single needle and averaged over a leaf (bulk leaf water). The simulations suggest that isotopic composition of the bulk leaf water does not always depend only upon the average transpiration rate, which in turn raises questions about using leaf water isotopic values to estimate transpiration rates. An unsuccessful attempt to simulate along-needle hydrogen isotopic variations suggests that certain unknown biological process(es) may not have been incorporated into our 2D model, and if so, it calls for a reevaluation of all other models for hydrogen isotopic simulations of leaf water since they too lack these processes. Existing leaf water isotopic models are reviewed in this work. In particular, we evaluate the most frequently used model, the stomatal boundary layer model (also referred to as the Craig-Gordon model). We point out that discrepancy between the boundary layer model and the measured bulk leaf water seems to depend upon relative humidity. Using our 2D model, we show that this humidity dependency is a result of an interplay between environmental and physiological conditions: if the transpiration rate of plant leaves decreases with increasing relative humidity, o
Isotopic measurements of leaf water have provided insights into a range of ecophysiological and biogeochemical processes, but require an extraction step which often constitutes the major analytical bottleneck in large-scale studies. Current standard procedures for leaf water analysis are based on cryogenic vacuum or azeotrophic distillation, and are laborious, require sophisticated distillation lines and the use of toxic materials. We report a rapid technique based on centrifugation/filtration of leaf samples pulverised in their original sampling tubes, using a specifically adapted, simple apparatus. The leaf water extracts produced are suitable for isotopic analysis via pyrolysis gas chromatography isotope ratio mass spectrometry (PYR/GC/IRMS). The new method was validated against cryogenic vacuum distillation and showed an overall accuracy of +/- 0.5 parts per thousand (nine grouped comparisons, n = 110) over a range of 21 parts per thousand. Effects due to the presence of soluble carbohydrates were near the detection limits for most samples analysed, and these effects could be corrected for (the extracted soluble organics could also be used for isotopic analysis). The extraction time for a routine eight-sample subset was reduced from 4 h (cryogenic distillation) to 45 min, limited only by the size of the centrifuge(s) used. This method provides a rapid, low-cost and reliable alternative to conventional vacuum and other distillation methods that can alleviate current restrictions on ecosystem- and global-scale studies that require high-throughput leaf water isotopic analysis. Copyright 2008 John Wiley & Sons, Ltd.
Although the isotopic composition of precipitation is widely used in global climate change studies, use of water vapour isotopes is considerably more limited. Here we present the results from 9 yr of atmospheric vapour measurements in the Eastern Mediterranean, at a site in Israel. The measurements show a strong mean seasonal cycle of about 4 parts per thousand in 180 (peaking around July). This seasonality could not be adequately explained by changes in surface interactions or in air mass trajectories, as usually invoked for variations in local precipitation. We could explain this cycle only as a combination of three components: (1) rainout effects; (2) temperature and relative humidity control of the initial vapour and (3) seasonal variations in the vertical mixing across the top of the planetary boundary layer. This last component is emphasized in the Current Study, and it was shown to be a significant factor in the seasonal cycle features. The measurements were also compared with an isotope-enabled GCM (CAM2) run, which exhibited a markedly different seasonal cycle. Such comparisons with vapour isotopes data could help in constraining models better.
The oxygen stable isotope composition (delta O-18) of CO2 is a valuable tool for studying the gas exchange between terrestrial ecosystems and the atmosphere. In the soil, it records the isotopic signal of water pools subjected to precipitation and evaporation events. The delta O-18 of the surface soil net CO2 flux is dominated by the physical processes of diffusion of CO2 into and out of the soil and the chemical reactions during CO2-H2O equilibration. Catalytic reactions by the enzyme carbonic anhydrase, reducing CO2 hydration times, have been proposed recently to explain field observations of the delta O-18 signatures of net soil CO2 fluxes. How important these catalytic reactions are for accurately predicting large-scale biosphere fluxes and partitioning net ecosystem fluxes is currently uncertain because of the lack of field data. In this study, we determined the delta O-18 signatures of net soil CO2 fluxes from soil chamber measurements in a Mediterranean forest. Over the 3 days of measurements, the observed delta O-18 signatures of net soil CO2 fluxes became progressively enriched with a well-characterized diurnal cycle. Model simulations indicated that the delta O-18 signatures recorded the interplay of two effects: (1) progressive enrichment of water in the upper soil by evaporation, and (2) catalytic acceleration of the isotopic exchange between CO2 and soil water, amplifying the contributions of 'atmospheric invasion' to net signatures. We conclude that there is a need for better understanding of the role of enzymatic reactions, and hence soil biology, in determining the contributions of soil fluxes to oxygen isotope signals in atmospheric CO2.
Predictions of warming and drying in the Mediterranean and other regions require quantifying of such effects on ecosystem carbon dynamics and respiration. Long-term effects can only be obtained from forests in which seasonal drought is a regular feature. We carried out measurements in a semiarid Pinus halepensis (Aleppo pine) forest of aboveground respiration rates of foliage, R(f), and stem, R(t) over 3 years. Component respiration combined with ongoing biometric, net CO(2) flux [net ecosystem productivity (NEP)] and soil respiration measurements were scaled to the ecosystem level to estimate gross and net primary productivity (GPP, NPP) and carbon-use efficiency (CUE=NPP/GPP) using 6 years data. GPP, NPP and NEP were, on average, 880, 350 and 211 g C m(-2) yr(-1), respectively. The above ground respiration made up half of total ecosystem respiration but CUE remained high at 0.4. Large seasonal variations in both R(f) and R(t) were not consistently correlated with seasonal temperature trends. Seasonal adjustments of respiration were observed in both the normalized rate (R(20)) and short-term temperature sensitivity (Q(10)), resulting in low respiration rates during the hot, dry period. R(f) in fully developed needles was highest over winter-spring, and foliage R(20) was correlated with photosynthesis over the year. Needle growth occurred over summer, with respiration rates in developing needles higher than the fully developed foliage at most times. R(t) showed a distinct seasonal maximum in May irrespective of year, which was not correlated to the winter stem growth, but could be associated with phenological drivers such as carbohydrate re-mobilization and cambial activity. We show that in a semiarid pine forest photosynthesis and stem growth peak in (wet) winter and leaf growth in (dry) summer, and associated adjustments of component respiration, dominated by those in R(20), minimize annual respiratory losses. This is likely a key for maintaining high CUE and e
This study explored possible advantages conferred by the phase shift between leaf phenology and photosynthesis seasonality in a semi-arid Pinus halepensis forest system, not seen in temperate sites. Leaf-scale measurements of gas exchange, nitrogen and phenology were used on daily, seasonal and annual time-scales. Peak photosynthesis was in late winter, when high soil moisture, mild temperatures and low leaf vapour pressure deficit (D-L) allowed high rates associated with high water- and nitrogen-use efficiencies. Self-sustained new needle growth through the dry and hot summer maximized photosynthesis in the following wet season, without straining carbon storage. Low rates of water loss were associated with increasing sensitivity of stomatal conductance (g(s)) to soil moisture below a relative extractable water (REW) of 0.4, and decreased g(s) sensitivity to D-L below REW of approx. 0.2. This response was captured by the modified Ball-Berry (Leuning) model. While most physiological parameters and responses measured were typical of temperate pines, the photosynthesis-phenological phasing contributed to high productivity under warm-dry conditions. This contrasts with reported effects of short-term periodical droughts and could lead to different predictions of the effect of warming and drying climate on pine forest productivity.
To identify factors that influence the relatively high productivity of a semi-arid pine afforestation system in southern Israel, we investigated inorganic nitrogen deposition and mineralization for more than 2 years. To this end, we measured bulk and dry deposition, in situ N-mineralization over the seasonal cycle, and the potential activity of nitrifying microorganisms by soil slurry incubations. There was a small increase in bulk N deposition in the forest, compared with shrubland, but no change in dry deposition. Ali unexpected rapid increase in nitrite concentration in the forest soil was observed after soil rewetting by the first winter rains, which could not be explained by deposition. This was accompanied by a decrease in ammonium and only a slight increase in nitrate concentrations. Only a small increase in nitrite and a rapid increase in nitrate concentration in the mineral soil were observed in the surrounding shrubland. Soil slurry incubations from the forest sites exhibited significant delay in nitrite, compared with nitrate accumulation (up to 50h under lab conditions) in samples taken in the dry season, but not in the wet season. This indicated different rates of ammonium and nitrite oxidation that are most likely linked to differential activation of different microbial populations after the summer stress. The initial oxidation process of ammonia to nitrate, upon soil rewetting in semi-arid environments, appears to occur as a partially uncoupled two-step process, as opposed to a rapid continuous one in wetter environments. This may have implications for the synchronization of nitrate availability to plants and therefore for high forest productivity and nitrogen use efficiency. Forest productivity in the semi-arid regions, in turn, is becoming increasingly more important with persistent predictions of warming and drying trends over the entire Mediterranean basin and other regions. (c) 2007 Elsevier Ltd. All rights reserved.
Biological soil crusts (BSC) contribute significantly to the soil surface cover in many dryland ecosystems. A mixed type of BSC, which consists of cyanobacteria, mosses and cyanolichens, constitutes more than 60% of ground cover in the semiarid grass-shrub steppe at Sayeret Shaked in the northern Negev Desert, Israel. This study aimed at parameterizing the carbon sink capacity of well-developed BSC in undisturbed steppe systems. Mobile enclosures on permanent soil borne collars were used to investigate BSC-related CO(2) fluxes in situ and with natural moisture supply during 10 two-day field campaigns within seven months from fall 2001 to summer 2002. Highest BSC-related CO(2) deposition between -11.31 and -17.56 mmol m(-2) per 15 h was found with BSC activated from rain and dew during the peak of the winter rain season. Net CO(2) deposition by BSC was calculated to compensate 120%, -26%, and less than 3% of the concurrent soil CO(2) efflux from November-January, February-May and November-May, respectively. Thus, BSC effectively compensated soil CO(2) effluxes when CO(2) uptake by vascular vegetation was probably at its low point. Nighttime respiratory emission reduced daily BSC-related CO(2) deposition within the period November-January
A survey of the stable isotope content of tissue waters of plants from the Negev desert was conducted. Large differences were observed in the extent of enrichment of the heavy isotopes in leaf water relative to local precipitation among different plants. This is apparently caused by the species-dependent stratagems adopted by the plants to cope with water stress, primarily by differences in the depth of water uptake in the soil and through the timing of stomatal openings during the daily cycle. Salt stressed plants showed extreme variability in the isotopic composition of leaf-water. The results show that plants with adaptation to arid conditions can avoid the transpiration regime, which would lead to the strong isotopic enrichment in their leaf water expected under arid conditions. This has implications for the use of stable isotopes in plants as indicators of either plant ecophysiology or paleoclimate.
An approach is presented for determining leaf area index (LAI) of a forest located at the desert fringe by using high spatial resolution imagery and by implementing values from a moderate spatial but high temporal resolution sensor. A 4-m spatial resolution multi-spectral IKONOS image was acquired under clear sky conditions on March 25, 2004. Normalized differences vegetation index (NDVI) and a linear mixture model were applied to calculate fractional vegetation cover (FVC). LAI was calculated using a non-linear relationship to FVC and then compared with ground truth measurements made in ten 1000 m(2) plots Using the tracing radiation and architecture of canopies (TRAC) canopy analyzer under bright and clear sky conditions during March and April, 2004. Calculated LAI, corrected with a measured clumping index, was highly correlated with measured LAI (R-2=0.79, p
The O-18/O-16 ratio of CO2 is a potentially powerful tracer of carbon dioxide fluxes from the soil to the atmosphere, which is influenced by complex interactions involving both biotic and abiotic soil processes. We use a simplified experimental approach and numerical simulations to examine in isolation the O-18 exchange between CO2 and soil water associated with the abiotic invasion of atmospheric CO2 into soil. This allowed us to verify, in particular, whether the O-18 of the retro-diffusion flux of CO2 from the soil reflects O-18 equilibration with water at the soil surface, or at some depth. Sterile soil samples with known water isotopic composition were placed in a closed box attached to a specially designed flow chamber and the changes in delta O-18 of CO2 between the chamber inlet and outlet, due only to invasion effects, were determined. Numerical simulations constrained by the laboratory gas exchange measurements indicated that between the two commonly used diffusion models [Penman, H.L. (1940). Gas and vapor movements in soil, l: the diffusion of vapors through porous solids. Int. J. Agric. Sci. 30, 437-462; Moldrup, P., Olesen, T., Yamaguchi, T., Schjonning, P., Rolston, D.E. (1999). Modeling diffusion and reaction in soils, IX, the Backingham-Burdine-Campbell equation for gas diffusivity in undisturbed soil. Soil Sci. 164, 542-551], only the former provided good agreement with the measurements over a wide range of soil water contents. Based on the model calculations constrained by experimental data, and on comparison of characteristic diffusion/reaction times, we conclude that the depth required for full CO2-water O-18 equilibration ranges between 2 and 8.5 cm. The depth depends, in order of importance, on (1) soil moisture content; (2) temperature, which dominates the rate of hydration isotopic exchange; (3) CO2 residence time, which is determined by the time of replacement of the column air above the soil; and (4) soil structure, including porosity,
Vacuum distillation is shown to be useful for the quantitative extraction of dissolved inorganic carbon (as CO2) and water from sediments of the unsaturated zone in the Coastal Aquifer of Israel. Several tests of vacuum extractions from tap water and sediments are presented, including standard addition, which show that the distillation procedure is quantitative, with minimal or no carbon isotope fractionation. The optimal temperature of the sediment during the extraction was also defined. Examples of vacuum extractions of sediments are shown.
Ecosystems in dry regions are generally low in productivity and carbon (C) storage. We report, however, large increases in C sequestration following afforestation of a semi-arid shrubland with Pinus halepensis trees. Using C and nitrogen (N) inventories, based in part on site-specific allometric equations, we measured an increase in the standing ecosystem C stock from 2380 g C m(-2) in the shrubland to 5840 g C m(-2) in the forest after 35 years, with no significant change in N stocks. Carbon sequestration following afforestation was associated with increased N use efficiency as reflected by an overall increase in C/N ratio from 7.6 in the shrubland to 16.6 in the forest. The C accumulation rate in the forest was particularly high for soil organic C (SOC; increase of 1760 g C m(-2) or 50 g C m(-2) yr(-1)), which was associated with the following factors: 1) Analysis of a small (13)C signal within this pure C(3) system combined with size fractionation of soil organic matter indicated a significant addition of new SOC derived from forest vegetation (68% of total forest SOC) and a considerable portion of the old original shrubland SOC (53%) still remaining in the forest. 2) A large part of both new and old SOC appeared to be protected from decomposition as about 60% of SOC under both land-use types were in mineral-associated fractions. 3) A short-term decomposition study indicated decreased decomposition of lower-quality litter and SOC in the forest, based on reduced decay rates of up to 90% for forest compared to shrubland litter. 4) Forest soil included a significant component of live and dead roots (12% of total SOC). Our results suggest a role for increased N use efficiency, enhanced SOC protection and reduced decomposition rates in the large C sequestration potential following afforestation in semi-arid regions. These results are particularly relevant in light of persistent predictions of drying trends in the Mediterranean and other regions.
Studies of forest biomass dynamics typically use long-term forest inventory data, available in only a few places around the world. We present a method that uses photogrammetric measurements from aerial photographs as an alternative to time-series field measurements. We used photogrammetric methods to measure tree height and crown diameter, using four aerial photographs of Yatir Forest, a semi-arid forest in southern Israel, taken between 1978 and 2003. Height and crown-diameter measurements were transformed to biomass using an allometric equation generated from 28 harvested Aleppo pine (Pinus halepensis Mill.) trees. Mean tree biomass increased from 6.37 kg in 1978 to 97.01 kg in 2003. Mean plot biomass in 2003 was 2.48 kg/m(2) and aboveground primary productivity over the study period ranged between 0.14 and 0.21 kg/m(2) per year. There was systematic overestimation of tree height and systematic underestimation of crown diameter, which was corrected for at all time points between 1978 and 2003. The estimated biomass was significantly related to field-measured biomass, with an R-2 value of 0.78. This method may serve as an alternative to field sampling for studies of forest biomass dynamics, assuming that there is sufficient spatial and temporal coverage of the investigated area using high-quality aerial photography, and that the tree tops are distinguishable in the photographs.
The isotopic composition of atmospheric O-2 depends on the rates of oxygen cycling in photosynthesis, respiration, photochemical reactions in the stratosphere and on delta O-17 and delta O-18 of ocean and leaf water. While most of the factors affecting delta O-17 and delta O-18 of air O-2 have been studied extensively in recent years, delta O-17 of leaf water-the substrate for all terrestrial photosynthesis-remained unknown. In order to understand the isotopic composition of atmospheric O-2 at present and in fossil air in ice cores, we studied leaf water in field experiments in Israel and in a European survey. We measured the difference in delta O-17 and delta O-18 between stem and leaf water, which is the result of isotope enrichment during transpiration. We calculated the slopes of the lines linking the isotopic compositions of stem and leaf water. The obtained slopes in ln(delta O-17 + 1) VS. ln(delta O-18 + 1) plots are characterized by very high precision (similar to 0.001) despite of relatively large differences between duplicates in both delta O-17 and delta O-18 (0.02-0.05 parts per thousand). This is so because the errors in 6180 and 617 0 are mass-dependent. The slope of the leaf transpiration process varied between 0.5111 +/- 0.0013 and 0.5204 +/- 0.0005, which is considerably smaller than the slope linking liquid water and vapor at equilibrium (0.529). We further found that the slope of the transpiration process decreases with atmospheric relative humidity (h) as 0.522-0.008 x h, for h in the range 0.3-1. This slope is neither influenced by the plant species, nor by the environmental conditions where plants grow nor does it show strong variations along long leaves. (c) 2006 Elsevier Inc. All rights reserved.
A study has been conducted into pertinent physiological responses underlying survival and
productivity of a 40 yr old Pinus halepensis Mill. forest growing in an arid-Mediterranean
environment in Israel. Rates of carbon fixation and water loss were controlled mainly by
stomatal conductance and were high in winter and spring and low in summer. Soil water
content limited stomatal conductance in the summer months but the effect of vapour pressure
deficit was apparent at any time of the year. Water use efficiency of photosynthesis was
determined primarily by vapour pressure deficit and was therefore higher in the cool wet
months than in the water limited period. The low rates of photosynthesis maintained through
the summer were, however, seen to be sufficient to sustain the growth of new foliage. A
highly constrained feature, dry summer leaf phenology also decoupled growth from nitrogen
availability and relied on the reallocation of nitrogen from the mature and senescing needles.
This phenological trait indicates off-season physiological performance is critical to tree
survival. The capacity for photosynthetic activity through the summer was facilitated by
effective photoprotection mechanisms. Both pre- and post-photochemical electron transport
energy dissipation was employed to prevent any chronic reduction of photosystem efficiency
under the conditions of high light and low stomatal conductance. The relative importance of
the various protection mechanisms depended on the nature and extent of the drought stress,
but pigment bed thermal dissipation was seen to be more important than sustaining electron
flow through photorespiration. A down-regulation of foliage and stem dark respiration in
summer minimized temperature induced respiratory CO2 losses. Both stem and foliage
displayed a continual acclimation of their respiration temperature response, but different
metabolic drivers resulted in dissimilar seasonal patterns of respiration between the
components. Foliage respiratory capacity was highly correlated with that of photosynthesis
while stem respiration had a strong phenological influence. Evidence from the 13C
composition of respired CO2 indicates above-ground respiration also remained supported by
current photosynthate during the dry summer. While the dry summer photosynthetic activity
is a crucial feature of productivity in this ecosystem, reliance on off-season photosynthesis for
growth and metabolism may also increase ecosystem vulnerability to more extreme
PhD Thesis at the Weizmann Institute of Science
This paper reports oil ranges of carbon dioxide (CO,) activity in biological soil crusts (BSC) correlated with different ranges of the BSC's spectral reflectance throughout the Phenological cycle of the year. Methodology is based oil surface CO2 exchange measurements, ground spectral measurements, and satellite images interpretation. Thirty-nine field campaigns, each of duration of 3 days, were conducted over the Course of 2 years at a sand duties and a loess environment of the northwestern Negev desert in Israel, in order to relate the CO2 fluxes and the spectral signals to the seasonal phenology. The Normalized Difference Vegetation Index (NDVI) was derived from ground measurements of the BSC's reflectance and correlated with their CO2 exchange data. A linear mixture model, incorporating the different contributions of the sites' ground features, was calculated and compared with SPOT-HRV data. From the ground measurements, fairly good correlations were found between the NDVI and the CO, fluxes oil a seasonal scale. Hence, the NDVI successfully indicates the potential magnitude and capacity of the BSC's assimilation activity. The linear mixture model Successfully describes the phenological cycles of the BSC, annual, and perennial plants and corresponds well to the satellite data. Moreover, the model enables annual changes of the phenology cycle and the growing season length to be distinguished. Both the linear mixture model and the derived NDVI values recorded the recovery of the BSC at the beginning of the wet season before annuals had germinated. Finally, it is concluded that a combination of CO, exchange measurements, linear mixture model, and NDVI values is suitable for monitoring BSC's productivity in and regions. (c) 2005 Elsevier Inc. All rights reserved.
The aim of this research was to study the relationships between the biological soil crusts (BSC), spectral reflectance and photosynthetic activity. Twenty field campaigns, each lasting several days, were conducted during the 2002-2003 rainy season at sand dune and loess environments in the north-western Negev desert of Israel. SiluldtaneOUS measurements Of CO2 net exchange and spectral reflectance were carried out for several types of BSC. The Normalized Difference Vegetation Index (NDVI) was derived from the BSC reflectance and correlated with their CO2 exchange data. The relationship between NDVI and CO2 exchange is discussed in detail with respect to environmental factors, such as soil water content, air temperature, and light intensity. Fairly good correlations were found in the rainy season. The NDVI was useful in indicating the potential magnitude and capacity of the BSC assimilation activity. Furthermore, the index corresponded well with different rates of photosynthetic activity of the different types of microphytes. The results demonstrate that spectral reflectances of the BSC can be related to photosynthetic activities and posseses the potential to assess the amount of carbon sequestration by these microphytes on an areal scale using satellite images. (c) 2005 Elsevier Ltd. All rights reserved.
Variation in the (COO)-O-18 content of atmospheric CO2 (delta(18)O(a)) can be used to distinguish photosynthesis from soil respiration, which is based on carbonic anhydrase (CA)-catalyzed O-18 exchange between CO2 and O-18-enriched leaf water (delta(18)O(w)). Here we tested the hypothesis that mean leaf delta(18)O(w) and assimilation rates can used to estimate whole-leaf (COO)-O-18 flux (isoflux), ignoring intraleaf variations in CA activity and gas exchange parameters. We observed variations in CA activity along the leaf (>30% decline from the leaf center toward the leaf ends), which were only partially correlated to those in delta(18)O(w) (7 to 21 parts per thousand), delta(18)O and delta(13)C of leaf organic matter (25 to 30 parts per thousand and -12.8 to -13.2 parts per thousand, respectively), and substomatal CO2 concentrations (intercellular CO2 concentrations, c(i), at the leaf center were similar to 40% of those at the leaf tip). The combined effect of these variations produced a leaf-integrated isoflux that was different from that predicted based on bulk leaf values. However, because of canceling effects among the influencing parameters, isoflux overestimations were only similar to 10%. Conversely, use of measured parameters from a leaf segment could produce large errors in predicting leaf-integrated (COO)-O-18 fluxes.
Eddy covariance technique to measure CO2, water and energy fluxes between biosphere and atmosphere is widely spread and used in various regional networks. Currently more than 250 eddy covariance sites are active around the world measuring carbon exchange at high temporal resolution for different biomes and climatic conditions. In this paper a new standardized set of corrections is introduced and the uncertainties associated with these corrections are assessed for eight different forest sites in Europe with a total of 12 yearly datasets. The uncertainties introduced on the two components GPP (Gross Primary Production) and TER (Terrestrial Ecosystem Respiration) are also discussed and a quantitative analysis presented. Through a factorial analysis we find that generally, uncertainties by different corrections are additive without interactions and that the heuristic u(*)-correction introduces the largest uncertainty. The results show that a standardized data processing is needed for an effective comparison across biomes and for underpinning interannual variability. The methodology presented in this paper has also been integrated in the European database of the eddy covariance measurements.
(.) The use of the C-13 : C-12 isotopic ratio ( delta C-13) of leaf- respired CO2 to trace carbon fluxes in plants and ecosystems is limited by little information on temporal variations in delta C-13 of leaf dark- respired CO2 ( delta C-13(r)) under field conditions. (.) Here, we explored variability in delta C-13(r) and its relationship to key respiratory substrates from collections of leaf dark- respired CO2, carbohydrate extractions and gas exchange measurements over 24- h periods in two Quercus canopies. (.) Throughout both canopies, delta C-13(r) became progressively C-13- enriched during the photoperiod, by up to 7%, then C-13- depleted at night relative to the photoperiod. This cycle could not be reconciled with delta C-13 of soluble sugars ( delta C-13(55)), starch ( delta C-13(st)), lipids ( delta C-13(l)), cellulose ( delta C-13(c)) or with calculated photosynthetic discrimination (Delta). However, photoperiod progressive enrichment in delta C-13(r) was correlated with cumulative carbon assimilation ( r(2) = 0.91). (.) We concluded that there is considerable short- term variation in delta C-13(r) in forest canopies, that it is consistent with current hypotheses for C-13 fractionation during leaf respiration, that leaf carbohydrates cannot be used as surrogates for delta C-13(r), and that diel changes in leaf carbohydrate status could be used to predict changes in delta C-13(r) empirically. (c) New Phytologist ( 2005).
We present carbon stable isotope, delta C-13, results from air and organic matter samples collected during 98 individual field campaigns across a network of Carboeuroflux forest sites in 2001 (14 sites) and 2002 (16 sites). Using these data, we tested the hypothesis that delta C-13 values derived from large-scale atmospheric measurements and models, which are routinely used to partition carbon fluxes between land and ocean, and potentially between respiration and photosynthesis on land, are consistent with directly measured ecosystem-scale delta C-13 values. In this framework, we also tested the potential of delta C-13 in canopy air and plant organic matter to record regional-scale ecophysiological patterns. Our network estimates for the mean delta C-13 of ecosystem respired CO2 and the related 'discrimination' of ecosystem respiration, delta(er) and Delta(er), respectively, were -25.6 +/- 1.9 parts per thousand and 17.8 +/- 2.0 parts per thousand in 2001 and -26.6 +/- 1.5 parts per thousand and 19.0 +/- 1.6 parts per thousand in 2002. The results were in close agreement with delta C-13 values derived from regional-scale atmospheric measurement programs for 2001, but less so in 2002, which had an unusual precipitation pattern. This suggests that regional-scale atmospheric sampling programs generally capture ecosystem delta C-13 signals over Europe, but may be limited in capturing some of the interannual variations. In 2001, but less so in 2002, there were discernable longitudinal and seasonal trends in delta(er). From west to east, across the network, there was a general enrichment in C-13 (similar to 3 parts per thousand and similar to 1 parts per thousand for the 2 years, respectively) consistent with increasing Gorczynski continentality index for warmer and drier conditions. In 2001 only, seasonal C-13 enrichment between July and September, followed by depletion in November (from about -26.0 parts per thousand to -24.5 parts per thousand to -30.0 parts per tho
Associations between delta C-13 values and leaf gas exchanges and tree-ring or needle growth, used in ecophysiological compositions, can be complex depending on the relative timing of CO2 uptake and subsequent redistribution and allocation of carbon to needle and stem components. For palaeoenvironmental and dendroecological studies it is often interpreted in terms of a simple model of delta C-13 fractionation in C-3 plants. However, in spite of potential complicating factors, few studies have actually examined these relationships in mature trees over inter- and intra-annual time-scales. Here, we present results from a 4 years study that investigated the links between variations in leaf gas-exchange properties, growth, and dated delta C-13 values along the needles and across tree rings of Aleppo pine trees growing in a semi-arid region under natural conditions or with supplemental summer irrigation. Sub-sections of tissue across annual rings and along needles, for which time of formation was resolved from growth rate analyses, showed rapid growth and 6 C responses to changing environmental conditions. Seasonal cycles of growth and delta C-13 (Up to similar to 4%.) significantly correlated (P
Wild emmer wheat (Triticum turgidum spp. dicoccoides (Korn.) Thell.), the tetraploid progenitor of cultivated wheat, is a potential source for various agronomical traits, including drought resistance. The objectives of this study were to characterize (1) the genetic diversity for drought resistance in wild emmer wheat, and (2) the relationship between drought responses of the wild emmer germplasm and the ecogeographical parameters of its collection sites. A total of 110 wild emmer accessions consisting of 25 populations and three control durum wheat cultivars were examined under two irrigation regimes, well-watered ('wet') and water-limited ('dry'). Wide genetic diversity was found both between and within the wild emmer populations in most variables under each treatment. A considerable number of the wild emmer accessions exhibited an advantage in productivity (spike and total dry matter) over their cultivated counterparts. Most wild emmer wheat accessions exhibited a greater carbon isotope ratio (delta(13)C, indicating higher water-use efficiency) under the dry treatment and higher plasticity of delta(13)C relative to the cultivated controls, which may have contributed to the drought adaptations in the former. The most outstanding drought-tolerance capacity (in term of productivity under the dry treatment and susceptibility indices) was detected in wild emmer populations originated from hot dry locations. The results suggest that wild emmer has the potential to improve drought resistance in cultivated wheat.
Resins of Hymenaea, an angiosperm tree genus known to be a copious resin producer and a major source of amber since the Oligo-Miocene, were collected from a wide range of tropical environments from Latin America and Africa, and analyzed for their carbon, hydrogen, and oxygen stable isotope composition. The average value for delta(13)C in the resins was found to be -27.0+/-1.3%, which is very similar to the values reported for resins in other studies. delta(18)O values for the Hymenaea resins averaged +11.2+/-1.6%, or about 20% more depleted than normal plant cellulose. deltaD values of the resins ranged from -196 to -319%, with an average of -243+/-30%. Rough estimates suggest a fractionation of -200 to -210% between the resins and the environmental water. This value is similar to the -200% value observed for the fractionation between other plant lipids and environmental water. The present study suggests that the stable isotope composition of fossil resins (amber) has the potential to provide information on ancient environmental waters.
Higher plants represent a significant sink for atmospheric carbonyl sulfide (COS) and a potential source of dimethyl sulfide (DMS). In the present work, COS uptake was investigated on various plant species (Quercus robur, Juniperus excelso, Hibiscus spec., Sorghum bicolor) differing in the activities of carbonic anhydrase (CA), the enzyme recognized responsible for COS consumption. COS uptake was observed for all plant species, and the range of COS consumption was 1.5-25 pmol m(-2) s(-1) (deposition velocity 1.2-10.6 mm s(-1)). The COS uptake was found to be light-independent, but was strongly under stomatal control. For the C-3 plant species the uptake rates were well correlated with the inherent capacity of CA, a fact that may confer a comfortable tool to model COS uptake by plants, and ultimately may help to decrease the uncertainty in estimates of the global COS sink strength of vegetation. S. bicolor, owing a C-4 metabolism and respective low CA activity, exhibited a relatively high COS uptake rate as compared to the C-3 plants. Potential reasons for this deviation are discussed. Emission of DMS was species-specific and was only observed in case of Hibiscus spec. under light conditions.
This study investigated the relationship between delta(13)C of ecosystem components, soluble plant carbohydrates and the isotopic signature of ecosystem respired CO2 (delta(13)C(R)) during seasonal changes in soil and atmospheric moisture in a beech (Fagus sylvatica L.) forest in the central Apennine mountains, Italy. Decrease in soil moisture and increase in air vapour pressure deficit during summer correlated with substantial increase in delta(13)C of leaf and phloem sap soluble sugars. Increases in delta(13)C of ecosystem respired CO2 were linearly related to increases in phloem sugar delta(13)C (r(2)=0.99, P
A widespread conversion of sagebrush-dominated plant communities to post-fire communities dominated by herbaceous annual species has occurred in the Great Basin of North America's Intermountain West during the last century, mainly driven by invasions of alien species and a subsequent increase in fire frequency. Little experimental data, however, are available on the potential consequences of wildfire and post-fire plant community alteration on key ecosystem functions such as ecosystem hydrology. The objectives of our study were to (1) quantify the effects of this vegetation transformation on the spatial and temporal distribution of soil water in the rooting zone; (2) quantify the effects of wild-fire on surface soil water distribution (upper 20 cm) to infer about potential consequences on re-establishment of the native plant community; and (3) examine which processes (e.g., soil water recharge or replenishment, plant water uptake, and soil evaporation) may be responsible for observed changes soil water distribution. Continuous measurement of soil water content using a segmented time domain reflectrometry system in sixteen locations in an intact sagebrush ecosystem (eight shrub and eight inter-shrub locations) and in eight locations in an adjacent post-fire ecosystem from March 2001 to October 2002 indicated that soil water contents in the upper 75 cm of the soil were similar and very low during summers, but that substantially lower water recharge in post-fire ecosystems after large snowfalls - possibly due to decreased snow deposition and higher sublimation - caused large differences in water storage between the two ecosystems in winter. These differences declined with the onset of the vegetation period in March, probably due to higher plant water uptake in the more densely vegetated intact sagebrush ecosystem. The presence of patchiness of near-surface soil water (0-20 cm), quantified among 80 points of nested grid systems placed in each ecosystem, appears to be
Scaling is a naturally iterative and bi-directional component of problem solving in ecology and in climate science. Ecosystems and climate systems are unquestionably the sum of all their parts, to the smallest imaginable scale, in genomic processes or in the laws of fluid dynamics. However, in the process of scaling-up, for practical purposes thewhole usually has to be construed as a good deal less than this. This essay demonstrates how controlled large-scale experiments can be used to deduce key mechanisms and thereby reduce much of the detail needed for the process of scaling-up. Collection of the relevant experimental evidence depends on controlling the environment and complexity of experiments, and on applications of technologies that report on, and integrate, small-scale processes. As the role of biological feedbacks in the behavior of climate systems is better appreciated, so the need grows for experimentally based understanding of ecosystem processes. We argue that we cannot continue as we are doing, simply observing the progress of the greenhouse gas-driven experiment in global change, and modeling its future outcomes. We have to change the way we think about climate system and ecosystem science, and in the process move to experimental modes at larger scales than previously thought achievable.
Climate warming is most pronounced at high latitudes, which could result in the intensification of the extensively cultivated areas in the boreal zone and could further enhance rates of forest clearing in the coming decades. Using paired forest-field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Chronosequences showed large soil C losses during the first two decades following deforestation, with mean C stocks in agricultural soils being 44% or 8.3 kg m(-2) lower than C stocks in original forest soils. This suggests that soil C losses from land-use change in the boreal region may be greater than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences indicated an increase in C storage during later decades after forest clearing, with 60-year-old grassland showing net ecosystem C gain of 2.1 kg m(-2) over the original forest. This increase in C stock resulted probably from a combination of large C inputs from belowground biomass and low C losses due to a small original forest soil C stock and low tillage frequency. Reductions in soil N stocks caused by land-use change were smaller than reductions in C stocks (34% or 0.31 kg m(-2)), resulting in lower C/N ratios in field compared with forest mineral soils, despite the occasional incorporation of high-C forest-floor material into field soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which could indicate that decomposition rates are more sensitive in forest soils than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes more widespread in the boreal region, t
Testing of the extent to which different complex traits share common genetic control provides a means to distinguish associations that are truly diagnostic of genetic potential for improved adaptation to abiotic stress, from incidental phenotypic correlations. In two generations of progeny from a cross between Gossypium hirsutum and Gossypium barbadense, quantitative trait loci (QTL) mapping was used to evaluate correspondence in genetic control of selected physiological measures and productivity under water-limited and well-watered environments, respectively. A total of 33 QTLs were detected for five physiological variables [osmotic potential (OP), carbon isotope ratio (delta(13)C; indicator of water use efficiency), canopy temperature, chlorophyll a and b], and 46 QTLs for five measures of crop productivity [dry matter, seed cotton yield (SC), harvest index, boll weight, and boll number]. QTL likelihood intervals for high SC and low OP corresponded in three genomic regions, two of which mapped to homoeologous locations on the two subgenomes of tetraploid cotton. QTLs for delta(13)C showed only incidental association with productivity, indicating that high water use efficiency can be associated with either high or low productivity. Different cotton species have evolved different alleles related to physiological responses and productivity under water deficit, which may permit the development of genotypes that are better-adapted to arid conditions.
 The O-18 content of atmospheric O-2 is an important tracer for past changes in the biosphere. Its quantitative use depends on knowledge of the discrimination against O-18 associated with the various O-2 consumption processes. Here we evaluated, for the first time, the in situ O-18 discrimination associated with soil respiration in natural ecosystems. The discrimination was estimated from the measured [O-2] and delta(18)O of O-2 in the soil-air. The discriminations that were found are 10.1 +/- 1.5parts per thousand, 17.8 +/- 1.0parts per thousand, and 22.5 +/- 3.6parts per thousand, for tropical, temperate, and boreal forests, respectively, 17.9 +/- 2.5parts per thousand for Mediterranean woodland, and 15.4 +/- 1.6parts per thousand for tropical shrub land. Current understanding of the isotopic composition of atmospheric O-2 is based on the assumption that the magnitude of the fractionation in soil respiration is identical to that of dark respiration through the cytochrome pathway alone (similar to18parts per thousand). The discrimination we found in the tropical sites is significantly lower, and is explained by slow diffusion in soil aggregates and root tissues that limits the O-2 concentration in the consumption sites. The high discrimination in the boreal sites may be the result of high engagement of the alternative oxidase pathway (AOX), which has high discrimination associated with it (similar to27parts per thousand). The intermediate discrimination (similar to18parts per thousand) in the temperate and Mediterranean sites can be explained by the opposing effects of AOX and diffusion limitation that cancel out. Since soil respiration is a major component of the global oxygen uptake, the contribution of large variations in the discrimination, observed here, to the global Dole Effect should be considered in global scale studies.
Rising atmospheric CO2 concentrations may lead to increased water availability because the water use efficiency of photosynthesis (WUE) increases with CO2 in most plant species. This should allow the extension of afforestation activities into drier regions. Using eddy flux, physiological and inventory measurements we provide the first quantitative information on such potential from a 35-year old afforestation system of Aleppo pine (Pinus halepensis Mill.) at the edge of the Negev desert. This 2800 ha arid-land forest contains 6.5 +/- 1.2 kg C m(-2) , and continues to accumulate 0.13-0.24 kg C m(-2) yr(-1) . The CO2 uptake is highest during the winter, out of phase with most northern hemispheric forest activity. This seasonal offset offers low latitude forests similar to10 ppm higher CO2 concentrations than that available to higher latitude forests during the productive season, in addition to the 30% increase in mean atmospheric CO2 concentrations since the 1850s. Expanding afforestation efforts into drier regions may be significant for C sequestration and associated benefits (restoration of degraded land, reducing runoff, erosion and soil compaction, improving wildlife) because of the large spatial scale of the regions potentially involved (ca. 2 x 10(9) ha of global shrub-land and C4 grassland). Quantitative information on forest activities under dry conditions may also become relevant to regions predicted to undergo increasing aridity.
Accurate knowledge of surface emissivity is essential for applications in remote sensing (remote temperature measurement), radiative transport, and modeling of environmental energy balances. Direct measurements of surface emissivity are difficult when there is considerable background radiation at the same wavelength as the emitted radiation. This occurs, for example, when objects at temperatures near room temperature are measured in a terrestrial environment by use of the infrared 8-14-mum band. This problem is usually treated by assumption of a perfectly diffuse surface or of diffuse background radiation. However, real surfaces and actual background radiation are not diffuse; therefore there will be a systematic measurement error. It is demonstrated that, in some cases, the deviations from a diffuse behavior lead to large errors in the measured emissivity. Past measurements made with simplifying assumptions should therefore be reevaluated and corrected. Recommendations are presented for improving experimental procedures in emissivity measurement. (C) 2003 Optical Society of America.
Isoprene emission from leaves is dynamically coupled to photosynthesis through the use of primary and recent photosynthate in the chloroplast. However, natural abundance carbon isotope composition (delta(13)C) measurements in myrtle (Myrtus communis), buckthorn (Rhamnus alaternus), and velvet bean (Mucuna pruriens) showed that only 72% to 91% of the variations in the delta(13)C values of fixed carbon were reflected in the delta(13)C values of concurrently emitted isoprene. The results indicated that 9% to 28% carbon was contributed from alternative, slow turnover, carbon source(s). This contribution increased when photosynthesis was inhibited by CO2-free air. The observed variations in the delta(13)C of isoprene under ambient and CO2-free air were consistent with contributions to isoprene synthesis in the chloroplast from pyruvate associated with cytosolic Glc metabolism. Irrespective of alternative carbon source(s), isoprene was depleted in C-13 relative to mean photosynthetically fixed carbon by 4%. to 11%.. Variable C-13 discrimination, its increase by partially inhibiting isoprene synthesis with fosmidomicin, and the associated accumulation of pyruvate suggested that the main isotopic discrimination step was the deoxyxylulose-5-phosphate synthase reaction.
 Photosynthesis and respiration impart distinct isotopic signatures to the atmosphere that are used to constrain global carbon source/sink estimates and partition ecosystem fluxes. Increasingly, the "Keeling plot'' method is being used to determine the carbon isotope composition of ecosystem respiration (delta(13)C(R)) in order to better understand the processes controlling ecosystem isotope discrimination. In this paper we synthesize emergent patterns in delta(13)C(R) by analyzing 146 Keeling plots constructed at 33 sites across North and South America. In order to interpret results from disparate studies, we discuss the assumptions underlying the Keeling plot method and recommend standardized methods for determining delta(13)C(R). These include the use of regression calculations that account for error in the x variable, and constraining estimates of delta(13)C(R) to nighttime periods. We then recalculate delta(13)C(R) uniformly for all sites. We found a high degree of temporal and spatial variability in C-3 ecosystems, with individual observations ranging from -19.0 to -32.6parts per thousand. Mean C-3 ecosystem discrimination was 18.3parts per thousand. Precipitation was a major driver of both temporal and spatial variability of delta(13)C(R), suggesting (1) a large influence of recently fixed carbon on ecosystem respiration and (2) a significant effect of previous climatic effects on delta(13)CR. These results illustrate the importance of water availability as a key control on atmospheric (CO2)-C-13 and highlight the potential of delta(13)C(R) as a useful tool for integrating environmental effects on dynamic canopy and ecosystem processes.
This study investigated the effects of radiation heat-load reduction by shading on the growth and development of citrus trees in a warm subtropical region. The experiment was conducted from mid-June until late October when daily maximal air temperature averaged 29.3 degreesC. Two-year-old de-fruited Murcott tangor (Citrus reticulata Blanco x Citrus sinensis (L.) Osb.) trees were grown under 30% or 60% shade tunnels, or 60% flat shade (providing midday shade only), using highly reflective aluminized nets. Nonshaded trees were used as the control. Shading reduced direct more than diffuse radiation. Daily radiation was reduced by 35% for the 30% Tunnel and 60% Flat treatments, and by 55% for the 60% Tunnel. Two days of intensive measurement showed that shading increased average sunlit leaf conductance by 44% and photosynthesis by 29%. Shading did not significantly influence root and stem dry weight growth, but it increased the increment in leaf dry weight during the three month period by an average of 28% relative to the control, while final tree height in the 30% Tunnel treatment exceeded the control by 35%. Shoot to root and shoot mass ratios increased and root mass ratio decreased due to shading because of the increase in leaf dry weight. Shading increased starch concentration in leaves while the shadiest treatment, 60% Tunnel, decreased starch concentration in the roots. Carbon isotope ratio (delta(13)C) of exposed leaves that developed under shading was significantly reduced by 1.9parts per thousand in the 60% Tunnel, indicating that shading increased CO2 concentrations at the chloroplasts (C-c), as would be expected from increased conductance. Substomatal CO2 concentrations, C-i, computed from leaf net CO2 assimilation rate and conductance values, also indicate that shading increases internal CO2 concentrations. Based on tree dry mass, tree height, and total carbohydrates fractions, the 30% Tunnel and the 60% Flat were the optimal shade treatments.
Our objective in compiling the Special Issue on Environmental Chemistry was to offer scientists from different disciplines some exposure to the breadth and depth of this multifaceted field. Environmental chemistry is indeed not a coherent field. Even the seemingly more precise term chemistry encompasses today an exceedingly wide range of research areas, traditionally consigned to such diverse fields as biology, material science, or physics. Accordingly, the natural sciences become ever more multi-, cross- and inter-disciplinary. We also wished to expose some misconceptions. For example, Environmental Chemistry is not the ג€�pollution chemistryג€� it is sometimes perceived as, and it is not merely part of an effort to ג€�save the Earthג€�. Scientists cannot and need not save the planet or cure all environmentג€™s ills, but rather should direct their professional zeal to unraveling the scientific basis needed to maintain environmental conditions that are favorable to mankind and for the recovery of environmental compartments already deteriorated.[first paragraph]
Isoprene (2-methyl-1,3-butadiene) protection against effects of singlet oxygen was investigated in Myrtus communis and Rhamnus alaternus. In M. communis, singlet oxygen produced in the leaves by Rose Bengal (RB) led to a 65% decrease in net assimilation rates within 3 h, whereas isoprene emission rates showed either a 30% decrease at ambient CO(2) concentrations or a 70% increase under high CO(2). In both cases, these changes led to an increase in calculated internal isoprene concentrations. The isoprene protection effect was directly demonstrated by fumigation of young (non-emitting) leaves, treated with RB or bromoxynil (simulating photoinhibition). There was 42% and 29% reduction in the damage to net assimilation compared with non-fumigated leaves for RB or bromoxynil, respectively. In R. alaternus, similar effects of RB on net assimilation were observed, and additional fluorescence measurements showed a significantly smaller decrease in F(v)/F(m) in isoprene-fumigated young leaves treated with RB (from 0.78 to 0.52), compared with non-fumigated leaves (from 0.77 to 0.27). The internal isoprene concentrations used in this study and possible rate of (1)O(2) production in leaves indicate that the protective effects observed should be beneficial also under natural conditions.
The O-18 content of atmospheric O-2 is an important tracer for past changes in the biosphere and has been used to estimate changes in the balance between terrestrial and marine productivity. Its quantitative use depends on knowledge of the isotopic fractionations associated with the various O-2 production and consumption processes. Here we monitored oxygen concentration and delta O-18 of O-2 in sandy and clayey soils to evaluate in situ O-18 fractionation associated with soil respiration. In the clayey soil, O-2 concentrations decreased as low as 1% at 150 cm depth, and delta O-18 values ranged from 0 parts per thousand to -1.6 parts per thousand relative to atmospheric O-2. In the sandy soil the O-2 concentration was 20.38-20.53%, and delta O-18 values were -0.06 +/- 0.015 parts per thousand to 0.06 +/- 0.015 parts per thousand relative to atmospheric O-2. Using the observed [O-2] and delta O-18 profiles and their change with time, together with a one-box analytical model and a five-box numerical model, a mean discrimination of 12 +/- 1 parts per thousand was estimated for the two sites (including effects of concentration and temperature gradients). This low discrimination was consistent with that determined in closed-system soil incubation experiments (8.4-16.9 parts per thousand). The current understanding of the composition of air O-2 attributes the magnitude of the fractionation in soil respiration to biochemical mechanisms alone (about 18 parts per thousand and 25-30 parts per thousand in cyanide-sensitive and cyanide-resistant respiration, respectively). The low discrimination we report is significantly less than in dark respiration and is explained by diffusion limitation in soil aggregates and root tissues that results in low O-2 concentration in the consumption site. Soil respiration is a major component of the global oxygen uptake, and the potential contribution of low discrimination, such as observed here, to the global Dole effect should be considere
The interaction of genotype with environment is of primary importance in many aspects of genomic research and is a special priority in the study of major crops grown in a wide range of environments. Water deficit, the major factor limiting plant growth and crop productivity worldwide, is expected to increase with the spread of arid lands. In genetically equivalent cotton populations grown under well-watered arid water-limited conditions (the latter is responsible for yield reduction of similar to 50% relative to well-watered conditions), productivity arid quality were shown to be partly accounted for by different quantitative trait loci (QTLs), indicating that adaptation to both arid and favorable conditions can be combined in the same genotype. QTL mapping was also used to test the association between productivity and quality under water deficit with a Suite of traits often found to differ between genotypes adapted to arid versus well-watered conditions. In this study, only reduced plant osmotic potential was clearly implicated in improved cotton productivity under arid conditions. Genomic tools and approaches may expedite breeding of genotypes that respond favorably to specific environments, help test roles of additional physiological factors, and guide the isolation of genes that protect crop performance under arid conditions toward improved adaptation of crops to arid cultivation.
The oxygen-18 (O-18) content of atmospheric carbon dioxide (CO2) is an important indicator of CO2 uptake on land. It has generally been assumed that during photosynthesis, oxygen in CO2 reaches isotopic equilibrium with oxygen in O-18-enriched water in leaves. We show, however, large differences in the activity of carbonic anhydrase (which catalyzes CO2 hydration and O-18 exchange in leaves) among major plant groups that cause variations in the extent of O-18 equilibrium (theta (eq)). A clear distinction in theta (eq) between C-3 trees and shrubs, and C-4 grasses makes atmospheric (COO)-O-18 a potentially sensitive indicator to changes in C-3 and C-4 productivity. We estimate a global mean theta (eq) value of similar to0.8, which reasonably reconciles inconsistencies between O-18 budgets of atmospheric O-2 (Dole effect) and CO2.
The O-18 content of CO2 is a powerful tracer of photosynthetic activity at the ecosystem and global scale. Due to oxygen exchange between CO2 and O-18-enriched leaf water and retrodiffusion of most of this CO2 back to the atmosphere, leaves effectively discriminate against O-18 during photosynthesis. Discrimination against O-18 (Delta O-18) is expected to be lower in C-4 plants because of low ci and hence low retrodiffusing CO2 flux. C-4 plants also generally show lower levels of carbonic anhydrase (CA) activities than C-3 plants. Low CA may limit the extent of O-18 exchange and further reduce Delta O-18. We investigated CO2-H2O isotopic equilibrium in plants with naturally low CA activity, including two C-4 (Zea mays, Sorghum bicolor) and one C-3 (Phragmites australis)species. The results confirmed experimentally the occurrence of low Delta O-18 in C-4, as well as in some C-3, plants. Variations in CA activity and in the extent of CO2-H2O isotopic equilibrium (theta (eq)) estimated from on-line measurements of Delta O-18 showed large range of 0-100% isotopic equilibrium (theta (eq) = 0-1). This was consistent with direct estimates based on assays of CA activity and measurements of CO2 concentrations and residence times in the leaves. The results demonstrate the potential usefulness of Delta O-18 as indicator of CA activity in vivo. Sensitivity tests indicated also that the impact of theta (eq) O-18 in leaf water.
The use of stable isotopes to estimate evapotranspiration (ET) fluxes from vegetated areas is increasing. By complementing conventional net flux measurements (gradient or eddy correlation techniques), isotope analyses can allow partitioning ET between its gross components, soil evaporation and leaf transpiration. Isotopic analyses of atmospheric water vapour above canopies can also constrain, or provide alternatives for estimating ET. A brief discussion of the isotope approach is aimed at highlighting some of the uncertainties that require further research. We also demonstrate first, the application of combined concentration and isotopic gradient analysis of atmospheric water vapour above crop fields in order to estimate ET fluxes and its gross components (soil evaporation was estimated at 1.5-3.5% of mid-day ET flux in a mature wheat field). Second, we demonstrate the potential in monitoring delta(ss)- delta(L), the difference between predicted and measured leaf water delta(18)O values, as an indicator of seasonal variations in canopy-scale transpiration in a desert ecosystem (linear correlation between this indicator and conventional ET measurements was observed). Improving our analytical capabilities for high-precision isotopic analysis of very small water vapour samples was a limiting factor in the above applications and a method for pyrolysis and on-line O-18 analysis of 0.2-2 mu L water samples is described. Copyright (C) 2000 John Wiley & Sons, Ltd.
Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing. In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with concentration and flux measurements of CO2 and water vapor are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem components such as the isotopic composition of CO2, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant-atmosphere system. Combining stable isotopes and concentration measurements is very effective through the use of "Keeling plots." This approach allows the identification of the isotopic composition and the contribution of ecosystem, or ecosystem components, to the exchange fluxes with the atmosphere. It also allows the estimation of net ecosystem discrimination and soil disequilibrium effects. Recent modifications of the Keeling plot approach permit examination of CO2 recycling in ecosystems. Combining stable isotopes with dynamic flux measurements requires precision in isotopic sampling and analysis, which is currently at the limit of detection. Combined with the micrometeorological gradient approach (applicable mostly in grasslands and crop fields), stable isotope measurements allow separation of net CO2 exchange into photosynthetic and soil respiration components, and the evapotranspiration flux into soil evaporation and leaf transpiration. Similar applications in conjunction with eddy correlation techniques (applicable to forests, in addition to grasslands and crop fields) are more demanding, but can potentially be applied in combination with the Keeling plot relationship. The advance and potential in using stable isotope measurements should make their use a standard com
O-18 discrimination in CO2 stems from the oxygen exchange between O-18-enriched water and CO2 in the chloroplast, a process catalyzed by carbonic anhydrase (CA). A proportion of this O-18-labeled CO2 escapes back to the atmosphere, resulting in an effective discrimination against (COO)-O-18 during photosynthesis (Delta(18)O). By constraining the delta(18)O of chloroplast water (delta(e)) by analysis of transpired water and the extent of CO2-H2O isotopic equilibrium (theta(eq)) by measurements of CA activity (theta(eq) = 0.75-1.0 for tobacco, soybean, and oak), we could apply measured Delta(18)O in a leaf cuvette attached to a mass spectrometer to derive the CO2 concentration at the physical limit of CA activity, i.e. the chloroplast surface (c(cs)). From the CO2 drawdown sequence between stomatal cavities from gas exchange (c(i)), from Delta(18)O (c(cs)), and at Rubisco sites from Delta(13)C (c(c)), the internal CO2 conductance (g(i)) was partitioned into cell wall (g(w)) and chloroplast (g(ch)) components. The results indicated that g(ch) is variable (0.42-1.13 mol m(-2) s(-1)) and proportional to CA activity. We suggest that the influence of CA activity on the CO2 assimilation rate should be important mainly in plants with low internal conductances.
Measurements of O-18 in atmospheric CO2 can be used to trace gross photosynthetic and respiratory CO2 fluxes between the atmosphere and the terrestrial biosphere. However, this requires knowledge of the O-18 signatures attributable to the fluxes from soil and leaves. Newly developed methods were employed to measure the O-18 of soil-respired CO2 and depth profiles of near-surface soil CO2 in order to evaluate the factors influencing isotopic soil-atmosphere CO2 exchange. The O-18 soil-respired CO2 varied predominantly as a function of the O-18 of soil water which, in turn, changed with soil drying and with seasonal variations in source water. The O-18 of soil-respired CO2 corresponds to full isotopic equilibrium with soil water at a depth ranging between 5 and 15 cm. The O-18 of respired CO2, in reality, results from a weighted average of partial equilibria over a range of depths. Soil water isotopic enrichment of up to 10 parts per thousand in the top 5 cm did not appear to strongly influence the isotopic composition of the respired CO2. We demonstrate that during measurements "invasion" of atmospheric CO2 (the diffusion of ambient CO2 into the soil, followed by partial equilibration and retrodiffusion) must be considered to accurately calculate the O-18 of the soil-respired CO2. The impact of invasion in natural settings is also considered. We also have determined the effective kinetic fractionation of CO2 diffusion out of the soil to be 7.2 +/- 0.3 parts per thousand. High-resolution (1 cm) depth profiles of O-18 of near-surface (top 10 cm) soil CO2 were carried out by gas chromatography-isotope ratio mass spectrometry (IRMS). This novel technique allowed us to observe the competitive diffusion-equilibration process near the soil surface and to test simulations by a diffusion and equilibration model of the soil CO2 O-18 content.
Carbon isotope ratio (C-13/C-12, expressed with a differential notation as delta(13)C) has been proposed as an indirect selection criterion for plant water-use efficiency (WUE=total dry matter produced or yield harvested/water used). For efficient modification of WUE in breeding programs, it is essential to determine a sampling strategy, which will provide consistent genotypic ranking for delta(13)C and maximum differentiation between genotypes. The effects of growth stages and plant organs on delta(13)C values and their genotypic variations were studied in cotton cultivars grown in the field under two irrigation regimes. Values of delta(13)C in leaf remained stable during peak flowering and boil development stages and significantly increased at boil ripening stage, which could result from the effect of late-season water stress on WUE. delta(13)C varied significantly between plant organs, with lower values obtained in assimilating organs, leaf and bur, and higher values in non-assimilating organs, stem and fiber. This could possibly have resulted from carbon discrimination during secondary metabolism. A non-crossover interaction was found between growth stage and cultivar, whereas plant-organ effect did not interact with cultivars, indicating that a similar ranking of cultivars can be expected with delta(13)C at various growth stages and plant organs. Late season leaf samples and bur of the last boil revealed maximal differentiation among genotypes and water regimes, both among other growth stages as well as among other plant organs. These samples have also revealed the highest correlations between delta(13)C and WUE estimates. It is concluded that leaf sampling during boil ripening stage may be most effective for the assessment of delta(13)C as an indicator of WUE. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.
The O-18 content of leaf water strongly influences the O-18 contents of atmospheric CO2 and O-2 The O-18 signatures of these atmospheric gases, in turn, emerge as important indicators of large-scale gas exchange processes. Better understanding of the factors that influence the isotopic composition of leaf water is still required, however, for the quantitative utilization of these tracers. The O-18 enrichment of leaf water relative to local meteoric water, is known to reflect climatic conditions. Less is known about the extent variations in the O-18 content of leaf water are influenced by nonclimatic, species-specific characteristics. In a collection of 90 plant species from all continents grown under the same climatic conditions in the Jerusalem Botanical Garden we observed variations of about 9 parts per thousand in the delta(18)O values of stem water, delta(s), and of about 14 parts per thousand in the mid-day delta(18)O enrichment of bulk leaf water, delta(LW) - delta(s). Differences between delta(18)O values predicted by a conventional evaporation model, delta(M), and delta(LW) ranged between - 3.3 parts per thousand and + 11.8 parts per thousand. The delta(18)O values of water in the chloroplasts (delta(ch)) in leaves of 10 selected plants were estimated from on-line CO2 discrimination measurements. Although much uncertainty is still involved in these estimates, the results indicated that delta(ch) can significantly deviate from delta(M) in species with high leaf peclet number. The delta(18)O values of bulk leaf water significantly correlated with delta(18)O values of leaf cellulose (directly) and with instantaneous water use efficiency (A/E, inversely). Differences in isotopic characteristics among conventionally defined vegetation types were not significant, except for conifers that significantly differed from shrubs in delta(18)O and delta(13)C values of cellulose and in their peclet numbers, and from deciduous woodland species in their delta(18)O and del
The effects of sugar concentration in the medium on the autotrophic/heterotrophic carbon metabolism were studied in tissue cultured potato plants. The weight of plants grown in the light on media containing 3 % sucrose was significantly higher than that of plants grown on 8 % sucrose media. Similarly, the weight increased when plants were transferred from 8 % to 3 % sucrose and decreased in the reverse case. In contrast, under dark conditions, a higher weight was observed in plants grown on higher sucrose concentrat ion. The uptake of C-14-sucrose from the medium was higher when the sugar's concentration was 8% as compared to 3 %, indicating a higher proportion of heterotrophic carbon in the former. These results were further supported by the natural abundance of C-13/C-12 ratios in the plant tissue. Analyses of the C-13/C-12 in the plants, sugars and the air CO2, indicated that 90 % of the tissue carbon was of heterotrophic origin in light-grown plants on 8 % sucrose. Only 50 % of the tissue carbon was of heterotrophic origin when the plants were grown on 2 % glucose medium. These results provide evidence that a high sugar concentration in the medium significantly reduced the contribution of autotrophic CO2 assimilation to the accumulation of dry weight. The possible mechanisms inhibiting CO2 assimilation in potato plants grown in vitro, under conditions of high sugar concentrations (or low osmotic potential) are discussed.
CO2 profiles obtained along a 30-m-thick unsaturated zone under land irrigated with sewage effluents show two production regions: a seasonal one in the root zone and another, at steady state, near the water table (29 m). On an annual basis the CO2 flux from the deep source toward the atmosphere (6.3 g C m(-2) yr(-1)) is balanced by a similar influx of soluble organic carbon (SOC) from sewage effluents. The delta(13)C values of soil CO2 indicate that CO2 is produced from plant material in the root zone and from biodegradation of total sedimentary organic carbon in the capillary fringe. High CO2 concentration in the capillary fringe (up to 2%) is likely to reflect a decrease in diffusivity relative to the unsaturated zone due to increase in both water content and tortuosity induced by the capillary fringe structure. The long residence time of SOC and CO2 in the unsaturated zone (29 years at the study site) suggests that the unsaturated zone of deep aquifers may have a significant storage capacity for carbon and may act as a temporary carbon sink.
Cotton (Gossypium spp.) is often exposed to drought, which adversely affects both yield and quality. Improved water-use efficiency (WUE = total dry matter produced or yield harvested / water used) is expected to reduce these adverse effects. Genetic variability in WUE and its association with photosynthetic rate and carbon isotope ratio (C-13/C-12) in cotton are reported in this paper. WUE of six cotton cultivars-G. hirsutum L., G. barnbadense L., and an interspecific F-1 hybrid (G. hirsutum X G. barbadense, ISH), was examined under two irrigation regimes in two field trials. The greatest WUE was obtained by two G. hirsutum cultivars (2.55 g dry matter or 1.12 g seed-cotton L-1 H2O); the ISH obtained similar or somewhat lower values, and two G. barbadense cultivars and one G. hirsutum cultivar exhibited the lowest values (2.1 g dry matter or 0.8 to 0.85 g seed-cotton L-1 H2O). These results indicate that different cotton cultivars may have evolved different environmental adaptations that affect their WUE. Photosynthetic rate was correlated with WUE in only a few cases, emphasizing the limitation of this parameter as a basis for estimating crop WUE. Under both trials, WUE was positively correlated with carbon isotope ratio, indicating the potential of this technique as a selection criterion for improving cotton WUE.
Opaline mineralized bodies are produced by many terrestrial plants and accumulate in certain soils and archaeological sites. Analyses of the oxygen isotopic compositions of these so-called phytoliths from stems and leaves of wheat plants grown in a greenhouse showed a linear relationship with stem and leaf water isotopic compositions and hence, indirectly, rain water isotopic composition. Analyses of wheat plants grown in fields showed that stem phytoliths isotopic composition directly reflects the seasonal air temperature change, whereas leaf phytoliths isotopic composition reflects both temperature and relative humidity. Temperature and the oxygen isotopic composition of stem phytoliths were related by an equation similar to that proposed for marine opal. Oxygen isotopic compositions of fossil phytoliths, and in particular those from stems, could be valuable for reconstructing past terrestrial climate change.
The C-13/C-12 and O-18/O-16 ratios of stem cellulose of Tamarix jordanis (a tree common in wadis of arid regions) increased with decreasing relative humidity (RH) in individual trees growing along a climatic gradient in Israel. The response to RH observed in the delta(18)O of the wood cellulose was strongly similar to that observed in leaf water over a diurnal cycle. Most of the data for delta(13)C and all of the data for delta(18)O could be fitted to two independent linear equations that, combined, allowed the reconstruction of RH and the delta(18)O of source water from the isotopic composition of ancient T. jordanis wood previously reported from the ancient fortress of Masada. Since the Roman period, RH at Masada decreased by about 17%, while the delta(18)O value of local groundwater remained similar to present-day values, suggesting that changing atmospheric circulation has played a role in climate change in the Middle East over the past two millennia.
A strong correlation is observed between an El Nino index (anomalies in tropical Pacific sea surface temperature) and rainfall in the Judean foothills near Jerusalem over the past 20 years. These relationships clearly influenced the growth of local pine trees, as reflected in the width of their annual tree rings. The ability to predict El Nino events about a year in advance lend a special significance to relationships reported here for ecology, agriculture and water management in this climatic transition zone. To help explain the observed, long-range teleconnection we propose a possible mechanism based on a newly identified direct cloud connection between equatorial Africa (more directly affected by El Nino) and the Southeastern Mediterranean shoreland. The penetration and contribution of the moisture current from equatorial Africa to this region may depend on a shift in the usual rain generating moisture currents to southwesterly trajectories (passing over north Africa). The occurrence of such shifts is supported by the observed decrease in the mean O-18 content of the local precipitation during El Nino winters.
THE atmospheric budget of carbon compounds can be balanced only by invoking a significant 'missing sink' for carbon dioxide(1-3). Identifying this sink requires a knowledge of CO2 fluxes at global and local scales. The former can be estimated from global averages of CO2 concentration and isotope composition(4-9); local-scale measurements have been made by analysing individual eddies of air(10,11). In both cases, the net CO2 exchange is the sum of two opposing fluxes: uptake by gross primary productivity and release by respiration. Here we show that these two components can be estimated separately at the local scale from small vertical gradients in C-13 and O-18 in atmospheric CO2 above vegetation. By also analysing the O-18 content of moisture in the air samples, we can estimate evapotranspiration rates, providing information on water exchange between the biosphere and atmosphere(12). We suggest that this approach can be extended to the regional scale.
Banana plants (Musa sp,, Musaceae) were grown for 2 years in the field in 12x20 m plots under irradiance levels incident upon the canopy of 100, 81, 62 and 32% of sunlight, Time-integrated parameters such as leaf delta(13)C, yield and leaf mass to area ratio were linearly correlated with irradiance level (R(2)>0.9). In contrast, midday CO2 assimilation and transpiration efficiency (A/g) decreased significantly and substomatal CO2 concentrations (c(st)) increased significantly only at the lowest irradiance levels (below 81% irradiance), Diurnal gas exchange measurements indicated that the linear response of the long-term parameters may be associated with the significant variations in photosynthetic activity among the irradiance treatments observed only in the early morning hours, The linear fit between yield and irradiance level (per cent of control) had a slope of 0.82 (with apparently constant yield to biomass ratio), These results directly demonstrate the significance of variations in incident light, such as may be associated with increasing cloudiness, on productivity of tropical plants such as the banana, The importance of using time-integrated indicators in general, and the reliability of using C-13 discrimination in particular, in evaluating the responses of plants to changes in incident irradiance is demonstrated.
Keywords: Geosciences, Multidisciplinary; Water Resources
Temporal variations in the degrees delta(18) oxygen (delta(18)O) content of water transpired by leaves during a simulated diurnal cycle fluctuated around the delta(18)O content of the source water, Reconstructed variations in the delta(18)O values of leaf water differed markedly from those predicted by conventional models, Even when transpiring leaves were maintain;ed under constant conditions for at least 3 h, strict isotopic steady-state conditions of leaf water (equality of the O-18/O-16 ratios in the input and transpired water) were rarely attained in a variety of plant species (Citrus reticulata, Citrus paradisi, Gossypium hirsutum, Helianthus annuus, Musa musaceae and Nicotinia tabacum), Isotopic analysis of water transpired by leaves indicated that leaves approach the isotopic steady state in two stages, The first stage takes 10 to 35 min (with a rate of change of about 3.3 parts per thousand h(-1)), while in the second stage further approach to the isotopic steady state is asymptotic (with a rate of change of about 0.4 parts per thousand h(-1)), and under conditions of low transpiration leaves can last for many hours, Substantial spatial isotopic heterogeneity was maintained even when leaves were at or near isotopic steady state, An underlying pattern in this isotopic heterogeneity is often discerned with increasing O-18/O-16 ratios from base to tip, and from the centre to the edges of the leaves, It is also shown that tissue water along these spatial isotopic gradients, as well as the average leaf water, can have O-18/O-16 ratios both lower and higher than those predicted by the conventional Craig and Gordon model, We concluded, first, that at any given time during the diurnal cycle of relative humidity the attainment of an isotopic steady state in leaf water cannot be assumed a priori and, secondly, that the isotopic enrichment pattern of leaf water reflects gradual enrichment along the water-flow pathway (e.g. as in a string of pools), rather than a si
The contribution of the cyanide-resistant, alternative pathway to plant mitochondrial electron transport has been studied using a modified aqueous phase on-line mass spectrometry-gas chromatography system. This technique permits direct measurement of the partitioning of electrons between the cytochrome and alternative pathways in the absence of added inhibitors. We demonstrate that in mitochondria isolated from soybean (Glycine max L. cv Ransom) cotyledons, the alternative pathway contributes significantly to oxygen uptake under state 4 conditions, when succinate is used as a substrate. However, when NADH is the substrate, addition of pyruvate, an allosteric activator of the alternative pathway, is required to achieve the same level of alternative pathway activity. Under state 3 conditions, when the reduction state of the ubiquinone pool is low, the addition of pyruvate allows the alternative pathway to compete with the cytochrome pathway for electrons from the ubiquinone pool when the cytochrome pathway is not saturated. These results provide direct experimental verification of the kinetics consequences of pyruvate addition on the partitioning of electron flow between the two respiratory pathways. This distribution of electrons between the two unsaturated pathways could not be measured using;conventional oxygen electrode methods and illustrates a dear advantage of the mass spectrometry technique. These results have significant ramifications for studies of plant respiration using the oxygen electrode, particularly those studies involving intact tissues.
Biomass accumulation (annual net primary productivity, NPP) in a plantation of Musa sp., Musaceae is linearly correlated with solar irradiance (IR) between 100 and 32% of ambient levels (NPP = 0.82 IR + 20.42, p
THE level of the Dead Sea, the lowest (about -400 m) and one of the most salty (salinity about 340 g l(-1))lakes on earth, is lowering at a rate of approximately 0.5 m annually owing to extensive exploitation of its main perennial tributary (the Jordan River) and the extreme aridity of the region (annual precipitation is about 60 mm)(1). Consequently, new hypersaline sea shores are exposed, forming a unique, originally sterile ecosystem. The first plants invade these newly exposed shores after several years while soil water salinity is still extremely high, Here we use stable isotopes of oxygen and hydrogen to show that a variety of such perennial pioneer plants are able to make use of occasional floodwater which is distinct from the bulk of the hypersaline soil water found in their root zone. Our results provide new insight into the ways in which plants can invade extremely hostile environments and extend their ecological limits of distribution.
Discrimination against O-18 during dark respiration forms the basis of a new technique for measuring flux through the alternative pathway during plant respiration. This technique, first reported by Guy and coworkers, is the first to allow measurements of the alternative oxidase in vivo under steady-state conditions. Improvements to the technique have produced a gas-phase system which allows measurements of alternative pathway flux in intact tissues in less than an hour. The development and application of these techniques and the potential for future experiments are discussed in this review.
Amber, a fossil tree resin, is known in the geological record from at least the Triassic period, 220-230 million years ago, to the sub-Recent. GC/MS analyses have shown that amber is composed mainly of macromolecular structures retaining the chemical fingerprints of the original resin. Diagenetic reactions seem to have relatively minor effects on the resin, suggesting that the amber can preserve most of the isotopic signature of the original resin. Recent pine (P. halepensis) and araucaria resins from Israel gave stable isotope ratio values of: delta(13) = -24 to -25.7 parts per thousand, delta D = -172 to 188 parts per thousand, and delta(18)O of +14.5 to +16 parts per thousand. Philippine copal, 400 years old, gave values of delta(13)C = -24 parts per thousand, delta D = -236 parts per thousand, and delta(18)O of +14.5 parts per thousand. Fossil amber, mostly belonging to diterpenoids based polymers (Anderson's class I amber), gave delta(13)C values ranging from -19 to -25 parts per thousand, delta D from -160 to 270 parts per thousand, and delta(18)O of +16 to +19 parts per thousand. Because the isotopic composition of the original resin is influenced by the metabolic pathway of carbon fixation (for carbon) and the composition of environmental water (for oxygen and hydrogen) and presumably it is probably not strongly modified by diagenetic effects it is probable that paleobotanical, paleoclimatological and paleoenvironmental information can be potentially obtained from the stable isotope composition of the amber. In addition, the relatively small spread of the isotopic values of amber within a defined deposit, relative to the variance among isotopic values of amber from different sources, indicate the potential of amber as a stratigraphic marker as well as in identifying sources of amber in archaeological artifacts and gemology.
The mixing patterns of water from the water-storage multiple-epidermis (ME) and the spongy parenchyma (SP) of leaves of Peperomia congesta HBK were studied by following variations in the natural abundance of deuterium. The H-2/H-1 ratios of water from the two tissues were different throughout the diel cycle and were always lower in the ME. Withholding water from the potted plants resulted in complete isotopic homogeneity of leaf water within 3 days. Isotopic homogeneity and the extent of H-2 enrichment of leaf water were constant for the duration of the S-week stress period while the weight of the ME differentially and continuously decreased during the same period. Reinstating irrigation resulted in the rapid restoration of pre-stress ME weight, and a simultaneous sharp drop in the H-2/H-1 ratio of water in both the ME and SP. The proportion of recharge by soil water was similar whether calculated based on changes in tissue weight or on H-2/H-1 ratios. In contrast to the rapid recovery in water content, isotopic enrichment and restoration of isotopic heterogeneity were gradual over several days. The results indicate that under favorable conditions water, and not only solutes, stored in the ME does not mix well with water in other tissues, but becomes a more integral part of total leaf water under water stress conditions. In recovery from water stress, recharge with soil water is rapid (hours) while communication with the external environment, as reflected by isotopic enrichment and restoration of isotopic heterogeneity in leaf water, is gradual (days). The results demonstrate the potential of using stable isotopes at the natural abundance level to study the dynamics of water movement within plants.
The isotopic ratios C-13/C-12 and O-18/O-16 of cellulose from tamarix trees which were used by the Roman army as a groundwork of the siege-rampart of Masada (AD 70-73) were compared with ratios measured in present-day tamarix trees growing in the Masada region and in central Israel. The ancient tamarix cellulose is depleted in both C-13 and O-18 compared to cellulose from trees growing in the Masada region today. Similar trends were observed on comparing modern tamarix trees growing in the Negev Desert with those growing in the temperate climate of central Israel. Considering the factors that can contribute to the observed changes in isotopic composition, we conclude that the ancient trees enjoyed less arid environmental conditions during their growth compared to contemporary trees in this desert region. This report demonstrates the potential in using combined O-18 and C-13 analyses of archeological plant material as independent indication of regional climatic change in desert areas (where conventional isotopic analyses, such as in tree rings, are impractical).
Two direct but independent approaches were developed to identify the average delta(18)O value of the water fraction in the chloroplasts of transpiring leaves. In the first approach, we used the delta(18)O value of CO2 in isotopic equilibrium with leaf water to reconstruct the delta(18)O value of water in the chloroplasts. This method was based on the idea that the enzyme carbonic anhydrase facilitates isotopic equilibrium between CO2 and H2O predominantly in the chloroplasts, at a rate that is several orders of magnitude faster than the non-catalysed exchange in other leaf water fractions. In the second approach, we measured the delta(18)O value of O-2 from photosynthetic water oxidation in the chloroplasts of intact leaves. Since O-2 is produced from chloroplast water irreversibly and without discrimination, the delta(18)O value of the O-2 Should be identical to that of chloroplast water. In intact, transpiring leaves of sunflower (Helianthus annuus cv. giant mammoth) under the experimental conditions used, the average delta(18)O value of chloroplasts water was displaced by 3-10 parts per thousand (depending on relative humidity and atmospheric composition) below the value predicted by the conventional Craig and Gordon model. Furthermore, this delta(18)O value was always lower than the delta(18)O value that was measured for bulk leaf water. Our results have implications for a variety of environmental studies since it is the delta(18)O value of water in the chloroplasts that is the relevant quantity in considering terrestrial plants influence on the delta(18)O values of atmospheric CO2 and O-2, as well as in influencing the delta(18)O of plant organic matter.
Variations in the natural abundance of O-18 and H-2 in plant cellulose are influenced by the isotopic composition of the water directly involved in metabolism-the metabolic water fraction. The isotopic distinction between the metabolic source water and total tissue water must reflect the formation of isotopic gradients within the tissue that are influenced by the rate of water turnover, by properties of the water conducting system and by environmental conditions. It seems that the O-18 abundance in the metabolic water is conserved in cellulose with a relatively constant isotope effect. The relationship of the H-2 abundance between metabolic water and cellulose is more complex. Hydrogen incorporated into photosynthetic products during primary reduction steps is highly depleted in H-2. However, a large proportion of these hydrogens are subsequently replaced by exchange with water, leading to H-2 enrichment during heterotrophic metabolism. Deciphering the oxygen isotope ratio of cellulose could help in providing insights into the carbon and oxygen fluxes exchanged between plants and the atmosphere. This is because the O-18 abundance in cellulose records the O-18 abundance in the metabolic water, which in turn, controls the oxygen isotopic signatures of the CO2 and O2 released by plants into the atmosphere. The hydrogen isotope effects associated with carbohydrate metabolism provide insights into the autotrophic state of a plant tissue. This is because the hydrogen isotope ratio of carbohydrates must reflect the net effects of the two opposing isotope effects associated with photosynthesis and heterotrophic metabolism.
Discrimination against O-18 during dark respiration in tissues of Kalanchoe daigremontiana, Medicago sativa, and Glycine max was measured using an on-line system that enabled direct measurements of the oxygen fractionation of samples in a gas-phase leaf disk electrode unit. Discrimination factors for cytochrome pathway respiration were 18.6 to 19.8 parts per thousand for all tissues. However, discrimination in cyanide-resistant respiration was significantly higher in green tissues (30.4-31.2 parts per thousand) compared with nongreen tissues (25.3-25.9 parts per thousand). Using these discrimination factors, the partitioning of electron transport to these pathways was calculated from measurements of discrimination in the absence of inhibitors. Changes in flux through the alternative pathway were measured during the light and dark phases of Crassulacean acid metabolism in leaf disks of K. daigremontiana. The flux of electrons through the alternative pathway was higher during deacidification than during the other phases of Crassulacean acid metabolism. The increase in alternative pathway electron flux accounted for all of the increased respiration in the light phase. Despite this increase, simultaneous measurements of malate concentration and respiratory flux confirm that only a small proportion of the total malate decarboxylation occurs in the mitochondria.
The natural abundance of carbon and hydrogen isotopic composition, expressed as a delta-C-13 value of plant dry matter and cellulose in the hypsophylls (husk leaves) of maize (Zea mays L.) was measured and compared with that of leaves and cobs. The delta-C-13 values of outer hypsophylls were usually 2 to 3 parts per thousand more negative than leaves or other tissues, and became more negative with increasing chlorophyll content, indicating significant local C3 pathway fixation of CO2 in the outer hypsophylls. The delta-D values indicated a significant part of hypsophyll cellulose was derived from heterotrophic sources (sucrose from C4 photosynthesis in other tissues). Isotopic mass balance calculations allowed quantitative estimation of these carbon sources and, in the samples examined, about 16% of hypsophyll cellulose was derived from local C3 photosynthesis, about 62% from local C4 photosynthesis, and about 22% from sucrose imported from other leaves.