Publications

Root exudates are thought to play an important role in plant-microbial interactions. In return for nutrition, soil bacteria can increase the bioavailability of soil nutrients. However, root exudates typically decrease in situations such as drought, calling into question the efficacy of solvation and bacteria-dependent mineral uptake in such stress. Here we tested the hypothesis of exudate-driven microbial priming on Cupressus saplings grown in forest soil in custom-made rhizotron boxes. A 1-month imposed drought and concomitant inoculations with a mix of Bacillus subtilis and Pseudomonas stutzeri, bacteria species isolated from the forest soil, were applied using factorial design. Direct bacteria counts and visualization by confocal microscopy showed that both bacteria associated with Cupressus Interestingly, root exudation rates increased 2.3-fold with bacteria under drought, as well as irrigation. Forty four metabolites in exudates were significantly different in concentration between irrigated and drought trees, including phenolic acid compounds and quinate. When adding these metabolites as carbon and nitrogen sources to bacterial cultures of both bacterial species, 8 of 9 metabolites stimulated bacterial growth. Importantly, soil phosphorous bioavailability was maintained only in inoculated trees, mitigating drought-induced decrease in leaf phosphorus and iron. Our observations of increased root exudation rate when drought and inoculation regimes were combined, support the idea of root recruitment of beneficial bacteria, especially under water stress.

Montane treelines are defined by a threshold low temperature. However, what are the dynamics when the snow-free summer growth season coincides with a 6-month seasonal drought?

We tested this fundamental question by measuring tree growth and leaf activity across elevations in Mt. Hermon (2,814 m; in Israel and Syria), where oak trees (Quercus look and Q. boissieri) form an observed treeline at 1900 m.

While in theory, individuals can be established at higher elevations (minimum daily temperature > 6.5°C for > 4 months even at the summit), soil drying and vapor pressure deficit (VPD) in summer enforces growth cessation in August, leaving only 2–3 months for tree growth. At lower elevations, Q. look is replaced by Q. cerris (1,300 m) and Q. calliprinos (1,000 m) in accompanying Q. boissieri, and growth season length (GSL) is higher due to an earlier start in April. Leaf gas exchange continues during autumn, but assimilates are no longer utilized in growth. Interestingly, the growth and activity of Q. boissieri were equivalent to that of each of the other three species across the ~ 1 km elevation gradient. A planting experiment at 2100 m showed that seedlings of the four oak species survived the cold winter and showed budding of leaves in summer, but wilted in August.

Our unique mountain site in the Eastern Mediterranean introduces a new factor to the formation of treelines, involving a drought limitation on GSL. This site presents the elevation edge for each species and the southern distribution edge for both the endemic Q. look and the broad-range Q. cerris. With ongoing warming, Q. look and Q. boissieri are slowly expanding to higher elevations, while Q. cerris is at risk of future extirpation.

Earth's forests face grave challenges in the Anthropocene, including hotter droughts increasingly associated with widespread forest die-off events. But despite the vital importance of forests to global ecosystem services, their fates in a warming world remain highly uncertain. Lacking is quantitative determination of commonality in climate anomalies associated with pulses of tree mortality-from published, field-documented mortality events-required for understanding the role of extreme climate events in overall global tree die-off patterns. Here we established a geo-referenced global database documenting climate-induced mortality events spanning all tree-supporting biomes and continents, from 154 peer-reviewed studies since 1970. Our analysis quantifies a global "hotter-drought fingerprint" from these tree-mortality sites-effectively a hotter and drier climate signal for tree mortality-across 675 locations encompassing 1,303 plots. Frequency of these observed mortality-year climate conditions strongly increases nonlinearly under projected warming. Our database also provides initial footing for further community-developed, quantitative, ground-based monitoring of global tree mortality.

Drought-associated woody-plant mortality has been increasing in most regions with multi-decadal records and is projected to increase in the future, impacting terrestrial climate forcing, biodiversity and resource availability. The mechanisms underlying such mortality, however, are debated, owing to complex interactions between the drivers and the processes. In this Review, we synthesize knowledge of drought-related tree mortality under a warming and drying atmosphere with rising atmospheric CO2. Drought-associated mortality results from water and carbon depletion and declines in their fluxes relative to demand by living tissues. These pools and fluxes are interdependent and underlay plant defences against biotic agents. Death via failure to maintain a positive water balance is particularly dependent on soil-to-root conductance, capacitance, vulnerability to hydraulic failure, cuticular water losses and dehydration tolerance, all of which could be exacerbated by reduced carbon supply rates to support cellular survival or the carbon starvation process. The depletion of plant water and carbon pools is accelerated under rising vapour pressure deficit, but increasing CO2 can mitigate these impacts. Advancing knowledge and reducing predictive uncertainties requires the integration of carbon, water and defensive processes, and the use of a range of experimental and modelling approaches.

Inter-kingdom belowground carbon (C) transfer is a significant, yet hidden, biological phenomenon, due to the complexity and highly dynamic nature of soil ecology. Among key biotic agents influencing C allocation belowground are ectomycorrhizal fungi (EMF). EMF symbiosis can extend beyond the single tree-fungus partnership to form common mycorrhizal networks (CMNs). Despite the high prevalence of CMNs in forests, little is known about the identity of the EMF transferring the C and how these in turn affect the dynamics of C transfer. Here, Pinus halepensis and Quercus calliprinos saplings growing in forest soil were labeled using a 13CO2 labeling system. Repeated samplings were applied during 36 days to trace how 13C was distributed along the tree-fungus-tree pathway. To identify the fungal species active in the transfer, mycorrhizal fine root tips were used for DNA-stable isotope probing (SIP) with 13CO2 followed by sequencing of labeled DNA. Assimilated 13CO2 reached tree roots within four days and was then transferred to various EMF species. C was transferred across all four tree species combinations. While Tomentella ellisii was the primary fungal mediator between pines and oaks, Terfezia pini, Pustularia spp., and Tuber oligospermum controlled C transfer among pines. We demonstrate at a high temporal, quantitative, and taxonomic resolution, that C from EMF host trees moved into EMF and that C was transferred further to neighboring trees of similar and distinct phylogenies.

Over the 21st century, human-caused climate change is projected to vastly increase the occurrence of severe heat, which has deleterious health, economic, and societal impacts. Over the same period, global human population is expected to increase from 7.8 to 10.9 billion, placing more people in harm's way. Here, we combine projections of sustained heat from climate models with spatially explicit population projection scenarios. We find that: (1) by 2090, high climate change and population growth scenarios show a ~5-, ~10-, and ~100–1000-fold increase in the population exposed to a mean hottest monthly temperature of 30 ºC, 35 ºC, and 40 ºC, respectively; (2) globally, population growth, warming, and their interaction, are the major drivers for the increase in exposure at milder, harsher, and extreme, temperatures, respectively; and (3) differences between population growth scenarios show that policy can potentially reduce the level of increase in exposure by up to 70%. Based on our analyses, the major driver for the increased heat exposure is the dangerous combination between global warming and population growth in already-warm cities in regions like Africa, India, and the Middle East.

Although atmospheric CO2 concentration ([CO2]) continues to rise, the question of how tree carbon (C) allocation is affected by this change remains. Studies show that C assimilation increases under elevated CO2 (eCO(2)). Yet, no detailed study has determined the fate of the surplus C, i.e., its compartment and physiological process allocation, nor in multiple species together. In this project, we grew 2-year-old saplings of four key Mediterranean tree species (the conifers Cupressus sempervirens L. and Pinus halepensis Mill., and the broadleaf Quercus calliprinos Webb. and Ceratonia siliqua L.) to [CO2] levels of 400 or 700 p.p.m. for 6 months. We measured the allocation of C to below and aboveground growth, respiration, root exudation, storage and leaf litter. In addition, we monitored intrinsic water-use efficiency (WUE), soil moisture, soil chemistry and nutrient uptake. Net assimilation, WUE and soil nitrogen uptake significantly increased at eCO(2) across the four species. Broadleaf species showed soil water savings, which were absent in conifers. All other effects were species-specific: Cupressus had higher leaf respiration, Pinus had lower starch in branches and transiently higher exudation rate and Quercus had higher root respiration. Elevated CO2 did not affect growth or litter production. Our results are pivotal to understanding the sensitivity of tree C allocation to the change in [CO2] when water is abundant. Species-specific responses should be regarded cautiously when predicting future changes in forest function in a higher CO2 world.

It has been assumed that mixing of species with high physiological diversity reduces competition over water and light resources, compared to single-species forests. Although several mechanisms to explain this observation have been proposed, quantification of these effects is lacking. Here we studied water-use dynamics for five tree species in a mature, mixed, evergreen, and Mediterranean forest. We use empirical measurements of key tree structural attributes including root distribution, through DNA barcoding and soil cores, tree height and biomass along with measurements of species-specific water use for two years. These measurements at the tree-scale were used to parameterize an ecosystem model of coupled water, carbon and energy fluxes (Regional Hydro Ecologic Simulation System, RHESSys). Site-scale empirical measurements showed contrasting diurnal and seasonal transpiration and sap flow curves across tree species, with year-round activity in angiosperms, and mostly wet season-activity in gymnosperms. Water-use patterns matched the rooting depth patterns, with the deep- and shallow-rooted Ceratonia and Cupressus, showing year-round and seasonal behaviors, respectively. RHESSys estimates of species-specific and stand-scale transpiration, biomass and productivity across 20 years of climate variation showed substantial differences between mixed and monoculture scenarios. Stand-scale annual net primary productivity and transpiration increased by 20–70 g C m−1 yr−1 and 40–80 mm yr−1, respectively, for mixed stands relative to average fluxes aggregated across monocultures. Model results, collaborated by field data provide evidence for niche partitioning of the soil water resource among co-habiting tree species, and demonstrate that this mechanism can facilitate higher productivity and an enhanced forest carbon sink especially in semi-arid regions.

•Tree C allocation is estimated by either isotopic or mass balance approaches.•We combine these approaches to study C allocation in five evergreen treespecies.•Within 8 days, conifers allocated 30% of the C belowground, and broadleaves 

The high-mountain ecosystems of Central Asia are a biodiversity hotspot with unique plant communities and many endemic species. Intense human pressure and global warming have caused habitat destruction in these areas and a parallel increase in the number of endangered species. Lagochilus species are key medicinal herbaceous plants native to Central Asia, many of which have been recently added to the endangered of species in Uzbekistan. To assess the climate sensitivity of Lagochilus species, we (1) located populations of six species in their native sites across Uzbekistan, and assessed their health by partitioning to ontogenetic stages along five consecutive years; (2) collected plant materials from these species, as well as from old herbarium samples (1918–1964); and (3) analyzed the carbon-13 composition in those samples, as an indicator for drouht stress. Over the course of five years (2014–2018) of continuous monitoring, fluctuations in annual precipitation in the region indicated a decrease by ∼20 %, and the fraction of young plants in each population decreased from 20–50% to 0–5 %, depending on the species. Comparing the carbon-13 composition in current and historical leaf samples showed an increase of 1.5–3.5‰ associated with a decrease in precipitation of 2–30 %, depending on the site and species. Our results show the high sensitivity of Lagochilus populations’ regeneration to drying, among six species and in sites across Uzbekistan. On a multi-decadal temporal scale, the dramatic changes in carbon-13 indicate that the response to precipitation reduction is related with drought stress. Considering the expectation for drier and hotter climate in Uzbekistan in the coming decades, conservation of Lagochilus populations should become a priority in Central Asia.

The intimate connection between roots and soil particles is a prerequisite for the continuous flow of water and nutrients from soil to trees, and carbon flow from roots to the rhizosphere. The soil-root interface has been studied in multiple laboratory and greenhouse experiments. Yet its multiple roles and their differential contributions to tree health have rarely been experimentally studied on mature trees in the field. We took advantage of mature olive tree transplanting to test the physiological effects of breakage of the soil-root interface in situ. Eight olive trees were monitored following transplantation into a site located 4 km from their native site, along two years. Additional eight trees were monitored simultaneously at the native site, as control. To decrease mortality risk, transplanted trees were heavily pruned before transplanting, and were irrigated and fertilized in their new site. Transplanted trees had ~50% lower rates of leaf photosynthesis and transpiration; ~80% lower root starch content; and ~30% higher loss of xylem conductivity, than native trees. Leaf water potential (LWP) was similar across trees, becoming more negative in the transplanted trees only in the second year following transplanting. While starch content and xylem conductivity recovered in the second year, leaf gas exchange and LWP remained significantly lower than in native trees. Soil P and K were higher under transplanted trees that remained stressed than under trees that recovered. Breakage of the soil-root interface caused a multi-system stress to trees. The lack of persistent LWP response might indicate that loss of hydraulic conductivity was driven by root, rather than aboveground, embolisms. Degradation of starch in the roots indicates an increase in belowground sinks. In the long run, recovery of starch content means no carbon limitation; yet the prevailing effects on leaf activities suggest a long-term stress unrelated with water or carbon supply.

Tree roots have penetrated the ceiling of burial caves in a ~1,800‐years‐old necropolis in the Galilee, Israel, damaging the antiquities and risking the catacombs with collapse. Root identification was needed to enable selective cutting (at the species level), facilitating the conservation of the world heritage site, while maintaining the majority of trees growing on top of the burial caves. Samples of roots penetrating the cave ceilings were collected and identified both by using DNA barcoding, which has become a standard method for the reliable identification of organisms in their natural environment, and also by traditional morpho‐anatomical methods. However, woody plant species of the Mediterranean region are under‐represented in DNA databases. Therefore, we added relevant species to the ITS2 database by sequencing the internal transcribed spacer (ITS)2 of the 18S‐26S rDNA from sampled leaves of 19 woody species of the Mediterranean maquis. The identification of these tree species facilitated their selective removal, balancing antiquities, and nature conservation needs.

Carbon reserve use is a major drought response in trees, enabling tree survival in conditions prohibiting photosynthesis. However, regulation of starch metabolism under drought at the whole-tree scale is still poorly understood. To this end, we combined measurements of nonstructural carbohydrates (NSCs), tree physiology and gene expression. The experiment was conducted outside on olive trees in pots under 90 d of seasonal spring to summer warming. Half of the trees were also subjected to limited water conditions for 28 d. Photosynthesis decreased in dehydrating trees from 19 to 0.5 µmol m−2 s−1 during the drought period. Starch degradation and mannitol production were a major drought response, with mannitol increasing to 71% and 41% out of total NSCs in shoots and roots, respectively. We identified the gene family members potentially relevant either to long-term or stress-induced carbon storage. Partitioning of expression patterns among β amylase and starch synthase family members was observed, with three β amylases possibly facilitating the rapid starch degradation under heat and drought. Our results suggest a group of stress-related, starch metabolism genes, correlated with NSC fluctuations during drought and recovery. The daily starch metabolism gene expression was different from the stress-mode starch metabolism pattern, where some genes are uniquely expressed during the stress-mode response.

The ongoing increase in human population and the subsequent freshwater demands raise conflicts regarding conservation of riparian ecosystems. Identifying anthropogenic effects on these vulnerable nature resources is crucial for preventing future damages. Here we measured tree-ring width and isotopic carbon composition (delta C-13) in stem wood of protected Platanus orientalis trees at the Kziv Nature Reserve, a Mediterranean riparian ecosystem in northern Israel. In this reserve, water was pumped for human needs during 1976-2006 from the major spring feeding the stream. We show a negative exponential correlation between stem growth and pumped water amounts. Spring drawdown had a significant negative effect on growth even in years when amounts were reduced to similar to 2.10(6)m(3) year(-1), leveling off at around 5.10(6) m(3) year(-1). Precipitation and spring volume effects on growth were exposed only after pumping stopped, further indicating its role in inhibiting tree growth. During the pumping years, stem wood delta C-13 was up to 1.8%0 higher than after pumping cessation, indicating the drought stress imposed on trees. Our results provide an unequivocal evidence for the adverse effect of water pumping on riparian Platanus orientalis tree growth. Such effects, and their related tree mortality risk, must be considered in sustainable water management planning.

This special section of Plant and Soil contains 12 contributions based on presentations at the 10th Symposium of the International Society of Root Research (ISRR10), which took place in Israel from 8 to 12 July, 2018 (https://www.ortra.com/events/isrr10/Home.aspx). The symposium promoted fundamental understanding of the diverse aspects of root biology. The symposium, titled: “Exposing the Hidden Half - Root Research at the Forefront of Science”, assembled multiple disciplines in order to facilitate exploration of novel approaches and investigation of complex processes and mechanisms. The intersections of root physiology, root development, root architecture and root interactions with the environment were tackled by researchers from around the globe through oral presentations, poster sessions and discussions. Basic research at multiple scales (proteins, cells, tissues and the root system as a whole) and cutting-edge methodologies were highlighted as important means to advance plant health and growth, thereby supporting sustainable and nutritious food production.

Land surface models (LSMs) typically use empirical functions to represent vegetation responses to soil drought. These functions largely neglect recent advances in plant ecophysiology that link xylem hydraulic functioning with stomatal responses to climate.We developed an analytical stomatal optimization model based on xylem hydraulics (SOX) to predict plant responses to drought. Coupling SOX to the Joint UK Land Environment Simulator (JULES) LSM, we conducted a global evaluation of SOX against leaf- and ecosystem-level observations.SOX simulates leaf stomatal conductance responses to climate for woody plants more accurately and parsimoniously than the existing JULES stomatal conductance model. An ecosystem-level evaluation at 70 eddy flux sites shows that SOX decreases the sensitivity of gross primary productivity (GPP) to soil moisture, which improves the model agreement with observations and increases the predicted annual GPP by 30% in relation to JULES. SOX decreases JULES root-mean-square error in GPP by up to 45% in evergreen tropical forests, and can simulate realistic patterns of canopy water potential and soil water dynamics at the studied sites.SOX provides a parsimonious way to incorporate recent advances in plant hydraulics and optimality theory into LSMs, and an alternative to empirical stress factors.

Forestry at the dry edge of forest existence has been a long-lasting interest to arid land residents. Current land-use and climate changes have broaden this interest to the global society, seeking for improved tree and forest drought resistance across biomes. Within this transition, Israel is a hub for scientific research and testing ground alike. Israel is a small country, spanning over arid, semi-arid, and thermo-Mediterranean environments, with relatively little forest. Nevertheless afforestation efforts have been unprecedented (by percent change in land cover), and many of these forests serve today as a 'living lab' for forest and tree drought resistance research. Considering the ongoing warming and drying, tree physiologists, forest ecologists, and forest managers in Israel are running a race to the unknown. To ensure local forest sustainability for future generations, the gap between the increased drought risk on the one hand, and the rather stationary forest drought resistance on the other hand, must be closed. The aim of this review is to highlight the findings and achievements, as well as challenges, of this contemporary research. Examples from Israel are given at the Middle East, and the wider, global arid land, contexts. In particular, important progress has been made in the following aspects: (1) the physiology of tree drought resistance, including water-use traits and phenology; (2) the genetic approach, advancing from provenance trials, to the application of molecular tools in the identification of tree drought resistance mechanisms; (3) the canopy flux measurement approach, e.g. monitoring the exchange of carbon, water, and energy in water-limited forests; (4) the application of remote sensing tools, such as satellite-based vegetation indices; and (5) the application of adaptive management practices in increasing forest resilience under drought. Taken together, these advances represent a way forward, while indicating important gaps, and open questions to future research. Lessons learnt in forests in Israel should be useful and transferable to other forest ecosystems in the Middle East, the Mediterranean, and additional water-limited regions.

Water availability is becoming a limiting factor with increasing world population that challenges global food security. Thus, we need to enhance cultivation in increasingly drier and hotter climate and prepare fruit trees for the ongoing climate change. Wild tree species might offer vital information and plant material in face of these challenges.A year-long comparative field study was conducted to investigate the mechanisms underlying drought tolerance in pear species (cultivated Pyrus communis and Pyrus pyrifolia vs. the wild Pyrus syriaca).We confirmed the hypothesis of higher drought tolerance in wild pear compared to its cultivated relative. P. syriaca xylem had fewer, narrower vessels, and lower vulnerability to embolism. It showed higher intrinsic water-sue efficiency and more robust seasonal patterns of photosynthesis, hydraulic conductivity, and PIP (plasma intrinsic protein) aquaporin expression. Across species, we identified a ubiquitous gene (PIP1:5/1:6), nine drought-inhibited genes, and two drought-induced genes (PIP1:4 and 2:6/2:7, confirming previous studies).Our study highlights the potential of using wild relatives of fruit tree species to prepare key crops to a drier and hotter future. The study of PIPS leads the way to a more focused research of the role of these cellular water channels in minimizing tree water loss under drought, while ensuring hydration of specific tissues.

The response of tree leaf gas exchange to elevated CO2 concentrations has been investigated in numerous experiments along the past 30 years. Stomatal regulation is a major plant control over leaf gas exchange, and the response to the increasing CO2 will shape the biological activity of forests in the future. Here, we collected 144 records from 57 species on stomatal conductance in CO2 manipulation experiments on trees (340-980 ppm CO2). CO2-induced stomatal downregulation was calculated as the slope of the linear regression between stomatal conductance and [CO2]. Among tree species, the slope (a) of change in stomatal conductance per 100 ppm CO(2 )increase ranged between 0 and -151, indicating stomatal downregulation, and only four species showed upregulation. There was a significant divergence between evergreen gymnosperms (a = -3.6 +/- 1.0), deciduous angiosperms (a = -16.3 +/- 3.1) and evergreen angiosperms (a = -32.8 +/- 7.1). Gymnosperms were less sensitive to CO2 changes than deciduous angiosperms even when considering only field experiments. The significant role of tree functional group in predicting CO2-induced stomatal downregulation was detected in multiple mixed-effect models, with p values ranging between 0.0002 and 0.0295. The significantly higher stomatal sensitivity to CO2 of angiosperms versus gymnosperms might be related to the overall higher stomatal conductance of angiosperms; their thinner leaves, in turn losing water faster; and the decreasing atmospheric [CO2] at the time of their taxa diversification. We conclude that species differences must be taken into account in forecasting future forest fluxes. A plain language summary is available for this article.

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%–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 (−2.8 MPa vs. −2.3 MPa in non-stressed trees), shorter needles (5.5 vs. 7.7 mm) and lower chlorophyll content (0.6 vs. 1 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 15–20 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 site’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. 

Drought-induced tree mortality has been recently increasing and is expected to increase further under warming climate. Conversely, tree species that survive under arid conditions might provide vital information on successful drought resistance strategies. Although Acacia (Vachellia) species dominate many of the globe's deserts, little is known about their growth dynamics and water-use in situ. Stem diameter dynamics, leaf phenology, and sap flow were monitored during 3 consecutive years in five Acacia raddiana trees and five Acacia tortilis trees in the Arid Arava Valley, southern Israel (annual precipitation 20-70mm, restricted to October-May). We hypothesized that stem growth and other tree activities are synchronized with, and limited to single rainfall or flashflood events. Unexpectedly, cambial growth of both Acacia species was arrested during the wet season, and occurred during most of the dry season, coinciding with maximum daily temperatures as high as 45 degrees C and vapor pressure deficit of up to 9kPa. Summer growth was correlated with peak sap flow in June, with almost year-round activity and foliage cover. To the best of our knowledge, these are the harshest drought conditions ever documented permitting cambial growth. These findings point to the possibility that summer cambial growth in Acacia under hyper-arid conditions relies on concurrent leaf gas exchange, which is in turn permitted by access to deep soil water. Soil water can support low-density tree populations despite heat and drought, as long as recharge is kept above a minimum threshold.

Understanding which species are able to recover from drought, under what conditions, and the mechanistic processes involved, will facilitate predictions of plant mortality in response to global change. In response to drought, some species die because of embolism-induced hydraulic failure, whilst others are able to avoid mortality and recover, following rehydration. Several tree species have evolved strategies to avoid embolism, whereas others tolerate high embolism rates but can recover their hydraulic functioning upon drought relief. Here, we focus on structures and processes that might allow some plants to recover from drought stress via embolism reversal. We provide insights into how embolism repair may have evolved, anatomical and physiological features that facilitate this process, and describe possible trade-offs and related costs. Recent controversies on methods used for estimating embolism formation/repair are also discussed, providing some methodological suggestions. Although controversial, embolism repair processes are apparently based on the activity of phloem and ray/axial parenchyma. The mechanism is energetically demanding, and the costs to plants include metabolism and transport of soluble sugars, water and inorganic ions. We propose that embolism repair should be considered as a possible component of a hydraulic efficiency-safety' spectrum. We also advance a framework for vegetation models, describing how vulnerability curves may change in hydrodynamic model formulations for plants that recover from embolism.

Green roofs provide many ecosystem services such as regulation of building temperatures, reducing urban heat-island effects and draining rainwater. In addition, they are expected to reduce the high levels of CO2 concentrations in big cities. Previous CO2 fixation studies on green roofs were done by taking long-timeperiod samples using expensive equipment and with limited replication. To plan green roofs for optimal CO2 reduction, a simple method to quantify CO2 fixation rate in relation to plant species-composition is required. The method we tested is direct measurement of CO2 concentrations with a portable air-quality meter, which allows a large number of samples. Here we focus on differences in the photosynthetic effect between plots containing the local Mediterranean succulent, Sedum sediforme and plots containing various annuals. In a factorial design (presence or absence of Sedum crossed with presence or absence of annuals), we tested the effect of sedum and annual treatments on CO2 concentrations. To compare our results with a commonly used method, and to evaluate the role of the different species, we examined the photosynthetic activity at the single plant level under these treatments by using a portable gas-exchange measuring system. We found that our method can detect the effect of different green roof plots and can be used as a simple and reliable tool for green-roof planers. We found that annuals reduced CO2 concentrations, but only in the absence of Sedum. Sedum alone had no effect on CO2 concentrations. This emphasizes the importance of integrating plots with annual plants in Sedum-based green roofs. (C) 2017 Elsevier GmbH. All rights reserved.

Are mature forests carbon limited? To explore this question, we exposed ca. 110-year-old, 40-m tall Picea abies trees to a 550-ppm CO2 concentration in a mixed lowland forest in NW Switzerland. The site receives substantial soluble nitrogen (N) via atmospheric deposition, and thus, trees are unlikely N-limited. We used a construction crane to operate the free-air CO2 release system and for canopy access. Here, we summarize the major results for growth and carbon (C) fluxes. Tissue C-13 signals confirmed the effectiveness of the CO2 enrichment system and permitted tracing the continuous flow of new C in trees. Tree responses were individually standardized by pre-treatment signals. Over the five experimental years, needles retained their photosynthetic capacity and absorbed up to 37% more CO2 under elevated (E) compared to ambient (A) conditions. However, we did not detect an effect on stem radial growth, branch apical growth and needle litter production. Neither stem nor soil CO2 efflux was stimulated under elevated CO2. The rate at which fine roots filled soil ingrowth cores did not significantly differ between A- and E-trees. Since trees showed no stomatal responses to elevated CO2, sap flow remained unresponsive, both in the long run as well as during short-term CO2 on-off experiments. As a consequence, soil moisture remained unaffected. We trapped significantly more nitrate in the root sphere of E-trees suggesting a CO2-stimulated breakdown of soil organic matter, presumably induced by extra carbohydrate exudation (priming').Synthesis. The lack of a single enhanced C sink to match the increased C uptake meant a missing C sink. Increased C transport to below-ground sinks was indicated by C transfer to ectomycorrhiza and on to neighbouring trees and by increased C export to soil. We conclude that these tall Picea abies trees are not C limited at current CO2 concentrations and further atmospheric CO2 enrichment will have at most subtle effects on growth, despite

In deciduous trees growing in temperate forests, bud break and growth in spring must rely on intrinsic carbon (C) reserves. Yet it is unclear whether growth and C storage occur simultaneously, and whether starch C in branches is sufficient for refoliation. To test in situ the relationships between growth, phenology and C utilization, we monitored stem growth, leaf phenology and stem and branch nonstructural carbohydrate (NSC) dynamics in three deciduous species: Carpinus betulus L., Fagus sylvatica L. and Quercus petraea (Matt.) Liebl. To quantify the role of NSC in C investment into growth, a C balance approach was applied. Across the three species, >95% of branchlet starch was consumed during bud break, confirming the importance of C reserves for refoliation in spring. The C balance calculation showed that 90% of the C investment in foliage (7.0-10.5 kg tree(-1) and 5-17 times the C needed for annual stem growth) was explained by simultaneous branchlet starch degradation. Carbon reserves were recovered sooner than expected, after leaf expansion, in parallel with stem growth. Carpinus had earlier leaf phenology (by similar to 25 days) but delayed cambial growth (by similar to 15 days) than Fagus and Quercus, the result of a competitive strategy to flush early, while having lower NSC levels.

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.

A chloroplast protein disulfide isomerase (PDI) was previously proposed to regulate translation of the unicellular green alga Chlamydomonas reinhardtii chloroplast psbA mRNA, encoding the D1 protein, in response to light. Here we show that AtPDI6, one of 13 Arabidopsis thalianaPDI genes, also plays a role in the chloroplast. We found that AtPDI6 is targeted and localized to the chloroplast. Interestingly, AtPDI6 knockdown plants displayed higher resistance to photoinhibition than wild-type plants when exposed to a tenfold increase in light intensity. The AtPDI6 knockdown plants also displayed a higher rate of D1 synthesis under a similar light intensity. The increased resistance to photoinhibition may not be rationalized by changes in antenna or non-photochemical quenching. Thus, the increased D1 synthesis rate, which may result in a larger proportion of active D1 under light stress, may led to the decrease in photoinhibition. These results suggest that, although the D1 synthesis rates observed in wild-type plants under high light intensities are elevated, repair can potentially occur faster. The findings implicate AtPDI6 as an attenuator of D1 synthesis, modulating photoinhibition in a light-regulated manner.

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.