The Weizmann Tree Lab

Despite gaining significant attention in recent years, it remains unclear whether mycorrhizal fungi distribute meaningful amounts of resources among trees in ways that increase the fitness of the receiving trees. To investigate this, we used pairs of shaded and unshaded Pinus halepensis or Quercus calliprinos saplings, growing in both inter- and intra-specific combinations outdoors in forest soil. We examined the combined effects of indirect and direct below-ground connections on tree performance and Non-Structural Carbohydrate (NSC) pools. Although we did not observe any growth benefits, shaded recipient oaks exhibited higher levels of root and branch NSC compared to their control counterparts, which were not connected below-ground. This finding suggests a potential benefit of establishing below-ground connections. However, no such benefits were observed among the unshaded pairs or in the other inter- and intra-specific pairs of pines and oaks. We monitored the carbon (C) flow from a ¹³CO₂-labelled donor pine tree to a below-ground connected oak tree and demonstrated C transfer from pines to shaded oaks. We also identified the main fungal symbionts interacting with pines and oaks. Our findings indicate that the effects of below-ground C transfer are context-dependent, manifesting in nuanced alterations in plant NSC that are not readily apparent through conventional growth metrics. Read the free Plain Language Summary for this article on the Journal blog.

Tree species differ in their carbon (C) allocation strategies during environmental change. Disentangling species-specific strategies and contribution to the C balance of mixed forests requires observations at the individual tree level. We measured a complete set of C pools and fluxes at the tree level in five tree species, conifers and broadleaves, co-existing in a mature evergreen mixed Mediterranean forest. Our study period included a drought year followed by an above-average wet year, offering an opportunity to test the effect of water availability on tree C allocation. We found that in comparison to the wet year, C uptake was lower in the dry year, C use was the same, and allocation to belowground sinks was higher. Among the five major C sinks, respiration was the largest (ca. 60%), while root exudation (ca. 10%) and reproduction (ca. 2%) were those that increased the most in the dry year. Most trees relied on stored starch for maintaining a stable soluble sugars balance, but no significant differences were detected in aboveground storage between dry and wet years. The detailed tree-level analysis of nonstructural carbohydrates and δ¹³C dynamics suggest interspecific differences in C allocation among fluxes and tissues, specifically in response to the varying water availability. Overall, our findings shed light on mixed forest physiological responses to drought, an increasing phenomenon under the ongoing climate change.

In trees, nonstructural carbohydrates (NSCs) serve as long-term carbon storage and long-distance carbon transport from source to sink. NSC management in response to drought stress is key to our understanding of drought acclimation. However, the molecular mechanisms underlying these processes remain unclear. By combining a transcriptomic approach with NSC quantification in the leaves, stems, and roots of Populus alba under drought stress, we analyzed genes from 29 gene families related to NSC signaling, translocation, and metabolism. We found starch depletion across organs and accumulation of soluble sugars (SS) in the leaves. Activation of the trehalose-6-phosphate/SNF1-related protein kinase (SnRK1) signaling pathway across organs via the suppression of class I TREHALOSE-PHOSPHATE SYNTHASE (TPS) and the expression of class II TPS genes suggested an active response to drought. The expression of SnRK1α and β subunits, and SUCROSE SYNTHASE6 supported SS accumulation in leaves. The upregulation of active transporters and the downregulation of most passive transporters implied a shift toward active sugar transport and enhanced regulation over partitioning. SS accumulation in vacuoles supports osmoregulation in leaves. The increased expression of sucrose synthesis genes and reduced expression of sucrose degradation genes in the roots did not coincide with sucrose levels, implying local sucrose production for energy. Moreover, the downregulation of invertases in the roots suggests limited sucrose allocation from the aboveground organs. This study provides an expression atlas of NSC-related genes that respond to drought in poplar trees, and can be tested in tree improvement programs for adaptation to drought conditions.

Recent findings suggest that trees can survive high levels of drought-induced xylem embolism. In many cases, the embolism is irreversible and, therefore, can potentially affect post-drought recovery and tree function under recurring droughts. We examined the development of embolism in potted Aleppo pines, a common species in hot, dry Mediterranean habitats. We asked (1) how post-drought recovery is affected by different levels of embolism and (2) what consequences this drought-induced damage has under a recurring drought scenario. Young trees were dehydrated to target water potential (Ψ_x) values of −3.5, −5.2 and −9.5 MPa (which corresponded to ~6%, ~41% and ~76% embolism), and recovery of the surviving trees was measured over an 8-months period (i.e., embolism, leaf gas-exchange, Ψ_x). An additional group of trees was exposed to Ψ_x of −6.0 MPa, either with or without preceding drought (Ψ_x of −5.2 MPa) to test the effect of hydraulic damage during repeated drought. Trees that reached −9.5 MPa died, but none from the other groups. Embolism levels in dying trees were on average 76% of conductive xylem and no tree was dying below 62% embolism. Stomatal recovery was negatively proportional to the level of hydraulic damage sustained during drought, for at least a month after drought relief. Trees that experienced drought for the second time took longer to reach fatal Ψ_x levels than first-time dehydrating trees. Decreased stomatal conductance following drought can be seen as \u201cdrought legacy,\u201d impeding recovery of tree functioning, but also as a safety mechanism during a consecutive drought.