Publications

The effect of water stress on mutualistic interactions can play a key role in forest response to drought. We tested the effects of water availability on the mutualism between Aleppo pine seedlings and ectomycorrhizal fungi (EMF) originating from soil spore banks of pine forests along a steep climatic gradient. The composition of the EMF communities in the soil varied along the climatic gradient, whereas the communities on pine seedlings' roots which were inoculated with these soil spore banks remained largely consistent. Water stress reduced seedling growth, which was influenced by EMF presence and not by inoculum origin. These results suggest that even under water shortage, pine seedlings maintain similar partnerships despite variable EMF availability. This raises important questions regarding the role of EMF richness at different stages of tree growth and at different ecological scales and improves our understanding of forest regeneration dynamics under extreme climatic conditions.

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.

As major terrestrial carbon sinks, forests play an important role in mitigating climate change. The relationship between the seasonal uptake of carbon and its allocation to woody biomass remains poorly understood, leaving a significant gap in our capacity to predict carbon sequestration by forests. Here, we compare the intra-annual dynamics of carbon fluxes and wood formation across the Northern hemisphere, from carbon assimilation and the formation of non-structural carbon compounds to their incorporation in woody tissues. We show temporally coupled seasonal peaks of carbon assimilation (GPP) and wood cell differentiation, while the two processes are substantially decoupled during off-peak periods. Peaks of cambial activity occur substantially earlier compared to GPP, suggesting the buffer role of non-structural carbohydrates between the processes of carbon assimilation and allocation to wood. Our findings suggest that high-resolution seasonal data of ecosystem carbon fluxes, wood formation and the associated physiological processes may reduce uncertainties in carbon source-sink relationships at different spatial scales, from stand to ecosystem levels.

The mutualistic interaction between trees and ectomycorrhizal fungi (EMF) can have a major effect on forest dynamics and specifically on seedling establishment. Here, we compared the EMF community composition associated with the roots of young saplings and mature trees of two co-habiting Pinaceae: Pinus halepensis and Cedrus deodara growing together in a post-fire forest plot, using fungal ITS metabarcoding. We found that the differences in the EMF community between the two sapling groups were mostly attributed to changes in the relative abundance of specific fungal species, with little species turnover. Specifically, Tomentella showed high abundance on pine roots, while Tuber, Russula and Sebacina were more common on the roots of cedars. The physical proximity to a specific host species was correlated with the EMF community composition of young saplings. Specifically, regardless of the sapling's own identity, the roots of saplings growing next to mature cedars had higher abundance of Tuber species, while Tomentella coerulea (Höhn. & Litsch), Russula densifolia (Secr. ex Gillet) and Tuber nitidum (Vittadini) dominated saplings next to mature pines. Cedar saplings' shoot structure was correlated with a specific EMF species. Overall, these results suggest that when germinating next to mature trees, the EMF community of saplings could be determined by extrinsic factors such as the small-scale distribution of mature trees in the forest.

High abundance of legumes in drylands suggests that symbiotic nitrogen fixation provides an advantage in water-limited environments. However, the interactive effect of nitrogen availability and water scarcity on the nitrogen fixation strategies of dryland legumes remain largely unexplained. We conducted two experiments to test the effects of nitrogen availability and drought on symbiotic nitrogen fixation in two drought-adapted Acacia tree species. Seedlings were grown under deficient and sufficient levels of nitrogen and with and without an imposed drought to test the effect of resource availability on nitrogen fixation and plant growth. We found that seedlings that grew in extreme deficiency of nitrogen reached a similar biomass as seedling that grew with a sufficient supply of nitrogen, showing a high nitrogen-use efficiency. Nitrogen fixation was strongly downregulated (reduction of 90% in biomass allocation to nodules) when plants received sufficient nitrogen supply. Under nitrogen deficiency, drought had a slight negative effect on nodule biomass and total biomass. Under sufficient nitrogen, drought reduced nitrogen availability enough to induce an increase in symbiotic nitrogen fixation in the risk-averse A. raddiana but not in the risk-taking A. tortilis. We conclude that strong regulation of nitrogen fixation together with low nitrogen demand, and flexible strategies of carbon and nitrogen allocation, increase the chance of legume tree survival and establishment in dry and unpredictable environments.

Root exudation, the export of low-molecular weight organic carbon (C) from living plant roots to soil, influences microbial activity, nutrient availability, and ecosystem feedbacks to climate change, but the magnitude of this C flux at ecosystem and global scales is largely unknown. Here, we synthesize in situ measurements of root exudation rates and couple those to estimates of fine root biomass to estimate global and biome-level root exudate C fluxes. We estimate a global root exudate flux of 13.4 (10.120.2) Pg C y⁻¹, or about 9% (714%) of global annual gross primary productivity. We did not find differences in root mass-specific exudation rates among biomes, though total exudate fluxes are estimated to be greatest in grasslands owing to their high density of absorptive root biomass. Our synthesis highlights the global importance of root exudates in the terrestrial C cycle and identifies regions where more in situ measurements are needed to improve future estimates of root exudate C fluxes.

Drought stress causes multiple feedback responses in plants. These responses span from stomata closure and enzymatic downregulation of photosynthetic activity to structural adjustments of xylem biomass and leaf area. Some of these processes are not easily reversible and may persist long after the stress has ended. Despite a multitude of hydraulic model approaches, simulation models still widely lack an integrative mechanistic description of how this sequence of physiological to structural tree responses may be realized that is also simple enough to be generally applicable. Here, we suggest an integrative, sequential approach to simulate drought stress responses. First, decreasing plant water potential triggers stomatal closure alongside a downregulation of photosynthetic performance, thereby effectively slowing down further desiccation. A second protective mechanism is introduced by increasing the soil-root resistance, represented by a disconnection of fine roots after a threshold soil water potential has been reached. Further decreases in plant water potential due to residual transpiration and loss of internal stem water storage consistently lead to a loss of hydraulic functioning, which is reflected in sapwood loss and foliage senescence. This new model functionality has been used to investigate the responses of tree hydraulics, carbon uptake, and transpiration to soil and atmospheric drought in an extremely dry Aleppo pine (Pinus halepensis Mill.) plantation. Using the hypothesis of a sequential triggering of stress-mitigating responses, the model was able to reflect carbon uptake and transpiration patterns under varying soil water supply and atmospheric demand conditions - especially during summer - and respond realistically regarding medium-term responses, such as leaf and sapwood senescence. We could show that the observed avoidance strategy was only achieved when the model accounted for very early photosynthesis downregulation, and the relatively high measured plant water potentials were well reproduced with a root-soil disconnection strategy that started before major xylem conductance losses occurred. Residual canopy conductance was found to be pivotal in explaining dehydration and transpiration patterns during summer, but it also disclosed the fact that explaining the water balance in the driest periods requires water supply from stem water and deep soil layers. In agreement with the high drought resistance observed at the site, our model indicated little loss of hydraulic functioning in Aleppo pine, despite the intensive seasonal summer drought.

Seedling establishment under natural conditions is limited by numerous interacting factors. Here, we tested the combined effects of drought, herbaceous competition, and ectomycorrhizal inoculation on the performance of Aleppo pine seedlings grown in a net-house. The roots of all pine seedlings were strongly dominated by Geopora, a fungal genus known to colonize seedlings in dry habitats. Ectomycorrhizal fungi (EMF) inoculum significantly increased seedling height, biomass, and the number of side branches. However, under either competition or drought, the positive effect of EMF on seedling biomass and height was greatly reduced, while the effect on shoot branching was maintained. Further, under a combination of drought and competition, EMF had no influence on either plant growth or shape. The discrepancy in pine performance across treatments highlights the complexity of benefits provided to seedlings by EMF under ecologically relevant settings.

Summary Plant water uptake from the soil is a crucial element of the global hydrological cycle and essential for vegetation drought resilience. Yet, knowledge of how the distribution of water uptake depth (WUD) varies across species, climates, and seasons is scarce relative to our knowledge of aboveground plant functions. With a global literature review, we found that average WUD varied more among biomes than plant functional types (i.e. deciduous/evergreen broadleaves and conifers), illustrating the importance of the hydroclimate, especially precipitation seasonality, on WUD. By combining records of rooting depth with WUD, we observed a consistently deeper maximum rooting depth than WUD with the largest differences in arid regions???indicating that deep taproots act as lifelines while not contributing to the majority of water uptake. The most ubiquitous observation across the literature was that woody plants switch water sources to soil layers with the highest water availability within short timescales. Hence, seasonal shifts to deep soil layers occur across the globe when shallow soils are drying out, allowing continued transpiration and hydraulic safety. While there are still significant gaps in our understanding of WUD, the consistency across global ecosystems allows integration of existing knowledge into the next generation of vegetation process models.

We used five mature Picea abies continuously labeled with ¹³C-depleted CO₂ in a broadleaf-dominated Swiss forest to assess the spatial extent and lag time of carbon fluxes to ectomycorrhizal fungi differing in hyphal development and host association. We traced labeled carbon into ectomycorrhizal sporocarps collected for two seasons at different distances from labeled Picea. Picea-derived photosynthate reached conifer-specific sporocarps up to 612 m away and reached other sporocarps only 06 m away. At 06 m, genera of lesser hyphal development acquired more Picea-derived photosynthate than those of greater hyphal development, presumably from preferential fungal colonization of inner root zones by the former genera. Correlations of sporocarp δ¹³C with daily solar radiation integrated for different periods indicated that carbon fluxes from Picea to sporocarps peaked 1721 days after photosynthesis. Thus, these results provided rough estimates of the spatial extent and temporal lags of carbon transfer from Picea to ectomycorrhizal fungi.

Ectomycorrhizal fungi (EMF) are common belowground tree symbionts, supplying trees with water and nutrients. In return, large amounts of C assimilated by trees can be allocated into EMF. However, the chemical forms in which the C is transferred from trees to fungi under field conditions are mostly unknown. In this study, we aimed to unravel the fate of tree-derived C in EMF. We conducted ¹³CO₂ pulse labelling of Pinus halepensis trees in two forest sites with adjacent EMF sporocarps, combined with a non-targeted metabolomics profiling of root and sporocarp tissues. ¹³C was measured in sporocarps of Tricholoma terreum and Suillus collinitus up to 3 m from pine stems. C was assimilated in the labelled trees' needles and transferred to their roots. Starting from Day 2 after labelling, the C was transferred to adjacent sporocarps, peaking on Day 5. We identified more than 100 different labelled metabolites of different chemical groups present in roots and sporocarps. Of them, 17 were common to pine roots and both EMF species, and additional eight common to roots and one of the two EMF. The major labelled metabolites in the root tips were amino acids and tricarboxylic acid intermediates. The major labelled metabolites in sporocarps were amino acids, nucleotides, and fatty acids. We also identified labelled carbohydrates in all tissues. Labelling patterns diverged across different tissues, which can hint at how the C was transferred. Considering the young tree as a sole C source for these sporocarps, and with a diurnal assimilation of 5.4 g C, the total monthly C source is ~165 g C. On average, there were 10 sporocarps around each tree, each requiring ~1 g C. Therefore, a 10 g C investment would make 6% of total tree C allocation, and about 12% of net primary productivity. Overall, we found that this significant and ubiquitous transfer of metabolites from tree roots to EMF sporocarps is more rapid and chemically diverse than once thought. 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.

Root systems form a significant part of tree biomass and function. Yet, roots are hidden from our eyes, making it difficult to track the belowground processes. By contrast, our capacity to detect aboveground changes in trees has been continuously improving using optical methods. Here, we tested two fundamental questions: (1) To what extent can we detect aboveground responses to mechanical damage of the root system? (2) To what extent are roots redundant? We applied three different non-destructive remote sensing means: (1) optical means to derive leaf greenness, (2) infrared means to detect the changes in leaf surface temperature and (3) spectral means to derive five vegetation indices (i.e. the photochemical reflectance index (PRI), the chlorophyll photosynthesis index (CI_Red-edge), the anthocyanin reflectance index 1, the structure insensitive pigment index and the normalized difference water index (NDWI)). We recorded the above metrics for hours and days and up to a month following induced root damage in three key Mediterranean tree species: Aleppo pine (Pinus halepensis Mill.), Palestine oak (Quercus calliprinos Webb.) and Carob (Ceratonia siliqua L.). To induce root damage, we removed 25, 50 and 75 percent of the root system in each species and compared it with control saplings. Tree aboveground (canopy) responses to root damage increased over time and with damage level. Leaf warming (up to 3^◦C) and decreased PRI were the most significant and rapid responses, with temperature differences being visible as early as 2 days following root damage. NDWI and greenness were the least sensitive, with responses detectable only at 75 percent root damage and as late as 14 or 30 days following root damage. Responses varied vastly among species, with carob being the most sensitive and pine being the least. Changes in leaf temperature and PRI indicated that leaf transpiration and photosynthesis were impaired by root damage. Although trees build roots in excess, mechanical damage will eventually decrease transpiration and photosynthesis across tree species.

Deforestation of tropical forests has been a critical issue affecting climate change mitigation and biodiversity conservation. Reforestation strives to remedy this situation, yet it is futile as long as deforestation of primary forests continues. Since deforestation is partly motivated by the demand for valuable tropical wood, reforestation should focus not only on planting native tree species, but specifically on a high diversity of native tree species with high-quality wood. However, the eco-physiological information required for growing such species is limited, and their resilience to drought events is unknown. Here, we focused on four native tropical wood tree species identified as suitable for Brazils Atlantic Forest reforestation. Growth, carbon assimilation, water-use and xylem hydraulics were studied in seedlings of the two legume species Dalbergia nigra and Plathymenia foliolosa and the two non-legume species Cariniana legalis and Zeyheria tuberculosa. Seedlings were monitored weekly for 9 consecutive weeks, three to five weeks of which under induced drought. Growth and carbon assimilation were 2565 per cent higher in the legume vs. non-legume species. In turn, non-legume species mostly avoided the drought by stomatal closure, producing a 50 per cent higher water-use efficiency (WUE) compared with the legume species. The average water potential at 50 per cent stomatal conductivity (Ψ _gs50) for legume species was −2.6 MPa, whereas for non-legume species it was −0.85 MPa. Still, each species showed a unique set of responses, indicating different growth strategies under mesic and xeric conditions. Our results indicate a divergence among legume and non-legume species, driven by a trade-off between plant productivity (carbon assimilation and growth) and plant safety (stomatal regulation and WUE). All in all, the four species of juvenile potted plants demonstrated a high capacity for recovery from drought, which supports their potential role in future reforestation under climate change.

Drought stress is imposing multiple feedback responses in plants. These responses span from stomata closure and enzymatic downregulation of photosynthetic activity to structural adjustments in leaf area. Some of these processes are not easily reversible and may persist long after the stress ended. Unfortunately, simulation models widely lack an integrative mechanistic description on how this sequence of tree physiological to structural responses occur.Here, we suggest an integrative approach to simulate drought stress responses. Firstly, a decreasing plant water potential triggers stomatal closure alongside a downregulation of photosynthetic performance. This is followed by a disconnection of roots and soil and the reliance on internal stem water storage or water uptake from deep soil layers. Consistently, loss in hydraulic functioning is reflected in sapwood loss of functionality and foliage senescence. This new model functionality has been used to investigate responses of tree hydraulics, carbon uptake and transpiration to soil- and atmospheric drought in an extremely dry Aleppo pine (Pinus halepensis L.) plantation.Using the hypothesis of a sequential triggering of stress-mitigating responses, the model was able to reflect the carbon uptake and transpiration patterns under varying soil water supply and atmospheric demand especially during summer and responded realistically regarding medium-term responses such as leaf and sapwood senescence. In agreement with the high drought resistance observed at the site our model indicated little loss of hydraulic functioning in Aleppo pine, despite the intensive seasonal summer drought.

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.

Severe droughts limit tree growth and forest productivity worldwide, a phenomenon which is expected to aggravate over the next decades. However, how drought intensity and climatic conditions before and after drought events modulate tree growth resilience remains unclear, especially when considering the range-wide phenotypic variability of a tree species. We gathered 4632 Aleppo pine (Pinus halepensis Mill.) tree-ring width series from 281 sites located in 11 countries across the Mediterranean basin, representing the entire geographic and bioclimatic range of the species. For each site and year of the period 19502020, we quantified tree-growth resilience and its two components, resistance and recovery, to account for the impact of drought and the capacity to recover from it. Relative drought intensity of each year was assessed using SPEI (Standardized Precipitation Evapotranspiration Index), a climatic water deficit index. Generalized additive mixed models were used to explore the non-linear relationships between resilience and its two components and drought intensity, preceding and following years climatic conditions. We found that P. halepensis radial growth was highly dependent on the SPEI from September of the previous year to June of the current year. Trees growing under more arid bioclimates showed higher inter-annual growth variability and were more sensitive to drought, resulting in an increased response magnitude to pre-, during and post-drought conditions. In contrast to our expectations, drought intensity only slightly affected resilience, which was rather negatively affected by favorable preceding conditions and improved by favorable following conditions. Resilience and its components are highly dependent on preceding and following years climatic conditions, which should always be taken into account when studying growth response to drought. With the observed and predicted increase in drought frequency, duration and intensity, favorable conditions following drought episodes may become rare, thus threatening the future acclimation capacity of P. halepensis in its current distribution.

Introduction: Soil water availability is a key factor in the growth of trees. In arid deserts, tree growth is limited by very dry soil and atmosphere conditions. Acacia tree species are distributed in the most arid deserts of the globe, therefore they are well adapted to heat and long droughts. Understanding why some plants do better than others in some environments is a key question in plant science. Methods: Here we conducted a greenhouse experiment to continuously and simultaneously track the whole-plant water-balance of two desert Acacia species, in order to unravel their physiological responses to low water availability. Results: We found that even under volumetric water content (VWC) of 5-9% in the soil, both species maintained 25% of the control plants, with a peak of canopy activity at noon. Moreover, plants exposed to the low water availability treatment continued growing in this period. A. tortilis applied a more opportunistic strategy than A. raddiana, and showed stomatal responses at a lower VWC (9.8% vs. 13.1%, t₄= -4.23, p = 0.006), 2.2-fold higher growth, and faster recovery from drought stress. Discussion: Although the experiment was done in milder VPD (~3 kPa) compared to the natural conditions in the field (~5 kPa), the different physiological responses to drought between the two species might explain their different topographic distributions. A. tortilis is more abundant in elevated locations with larger fluctuations in water availability while A. raddiana is more abundant in the main channels with higher and less fluctuating water availability. This work shows a unique and non-trivial water-spending strategy in two Acacia species adapted to hyper-arid conditions.

Phosphorus (P) availability to forest trees is often limited by local soil conditions that increase its fixation to soil minerals. In certain regions, atmospheric-P inputs can compensate for low soil-P availability. Among atmospheric-P sources, desert dust is the most dominant. However, the effects of desert dust on P nutrition and its uptake mechanisms by forest trees are currently unknown. We hypothesized that forest trees that naturally grow on P-poor soils or soils with high soil-P fixation capacity can acquire P from desert dust deposited on their leaves via direct foliar uptake, bypassing the soil, thus promoting tree growth and productivity. We performed a controlled greenhouse experiment with three forest tree species: Palestine Oak (Quercus calliprinos) and Carob (Ceratonia siliqua), native to the NE edge of the Saharan desert, and Brazilian peppertree (Schinus terebinthifolius), native to the Atlantic Forest in Brazil, which is located on the western part of the trans-Atlantic Saharan dust route. To simulate natural dust deposition events, the trees had desert dust applied directly upon their foliage and were monitored for growth and final biomass, P levels, leaf surface pH and the rate of photosynthesis. The dust treatment increased the P concentration significantly by 33-37% in Ceratonia and Schinus trees. On the other hand, trees that received the dust displayed a 17-58% reduction in biomass, probably related to particle coverage of the leaf surface that inhibited photosynthesis by 17-30%. Overall, our findings show that direct P uptake from desert dust can be an alternative P uptake pathway for multiple tree species under P-deficient conditions, with implications for forest trees' P economy.

Survival and growth of woody species in the Mediterranean are mainly restricted by water availability. We tested the hypothesis that Mediterranean species acclimate their xylem vulnerability and osmotic potential along a precipitation gradient. We studied five predominant co-occurring Mediterranean species; Quercus calliprinos, Pistacia palaestina, Pistacia lentiscus, Rhamnus lycioides and Phillyrea latifolia, over two summers at three sites. The driest of the sites is the distribution edge for all the five species. We measured key hydraulic and osmotic traits related to drought resistance, including resistance to embolism (Ψ₅₀) and the seasonal dynamics of water and osmotic potentials. The leaf water potentials (Ψ_l) of all species declined significantly along the summer, reaching significantly lower Ψ_l at the end of summer in the drier sites. Surprisingly, we did not find plasticity along the drought gradient in Ψ₅₀ or osmotic potentials. This resulted in much narrower hydraulic safety margins (HSMs) in the drier sites, where some species experienced significant embolism. Our analysis indicates that reduction in HSM to null values put Mediterranean species in embolism risk as they approach their hydraulic limit near the geographical dry edge of their distribution. Read the free Plain Language Summary for this article on the Journal blog.

Understanding the causes of the arrest of species distributions has been a fundamental question in ecology and evolution. These questions are of particular interest for trees owing to their long lifespan and sessile nature. A surge in data availability evokes a macro-ecological analysis to determine the underlying forces limiting distributions. Here we analyse the spatial distribution of >3,600 major tree species to determine geographical areas of range-edge hotspots and find drivers for their arrest. We confirmed biome edges to be strong delineators of distributions. Importantly, we identified a stronger contribution of temperate than tropical biomes to range edges, adding strength to the notion that tropical areas are centres of radiation. We subsequently identified a strong association of range-edge hotspots with steep spatial climatic gradients. We linked spatial and temporal homogeneity and high potential evapotranspiration in the tropics as the strongest predictors of this phenomenon. We propose that the poleward migration of species in light of climate change might be hindered because of steep climatic gradients.

Key message: Desert Acacia trees photosynthesize during the hot dry summer, and use stored carbon for summer growth. Trees that grow in hyper-arid environments can provide important insight into the role of carbon use and carbon storage for tree survival and growth in extreme conditions. Acacia trees, in particular, experience some of the most arid conditions in which trees can grow on the planet, enduring high temperatures, high radiation and drought. Here we measured for two years photosynthesis along the day, stem circumference growth, and seasonal changes in non-structural carbohydrates in adult Acacia trees in their natural hyper-arid habitat (Arava desert, southern Israel). The peak of net CO₂ assimilation was at midday in all seasons, even during summer when vapor pressure deficit was at maximum of 6 kPa and light intensities were at high levels (1800 µmol photons m⁻² s⁻¹). Tree growth started in the spring and increased in the hot summer season and during the dry fall season (autumn). Starch concentrations in the branches were highest in the winter and spring (17% in dry matter) decreasing in the summer and fall (7% in dry matter). Our observations indicated that carbon assimilated during the winter was stored in the branches as starch reserves, which were later used for tree growth in summer. Still, most of the growth was subsidized by concurrent assimilation during the dry season. These findings show that Acacia trees are able to photosynthesize in conditions that other trees cannot, indicating a strong potential to contribute to ecosystem carbon sequestration in warming and drying climates.

Cavitation resistance has often been viewed as a relatively static trait, especially for stems of forest trees. Meanwhile, other hydraulic traits, such as turgor loss point (psi(tlp)) and xylem anatomy, change during the season. In this study, we hypothesized that cavitation resistance is also dynamic, changing in coordination with psi(tlp). We began with a comparison of optical vulnerability (OV), microcomputed tomography (mu CT) and cavitron methods. All three methods significantly differed in the slope of the curve,psi(12) and psi(88), but not in psi(50) (xylem pressures that cause 12%, 88%, 50% cavitation, respectively). Thus, we followed the seasonal dynamics (across 2 years) of psi(50) in Pinus halepensis under Mediterranean climate using the OV method. We found that psi(50) is a plastic trait with a reduction of approximately 1 MPa from the end of the wet season to the end of the dry season, in coordination with the dynamics of the midday xylem water potential (psi(midday)) and the psi(tlp). The observed plasticity enabled the trees to maintain a stable positive hydraulic safety margin and avoid cavitation during the long dry season. Seasonal plasticity is vital for understanding the actual risk of cavitation to plants and for modeling species' ability to tolerate harsh environments.

Premise Tree growth is a fundamental biological process that is essential to ecosystem functioning and water and element cycling. Climate exerts a major impact on tree growth, with tree species often requiring a unique set of conditions to initiate and maintain growth throughout the growing season. Still, little is known about the specific climatic factors that enable tree growth in savannah and desert tree species. Among the global tree species, Acacia tortilis occupies one of the largest distribution ranges (crossing 6500km and 54 latitudes), spanning large parts of Africa and into the Middle East and Asia. Methods Here we collected climate data and monitored Acacia tortilis tree growth (continuous measurements of stem circumference) in its southern and northern range edges in South Africa (SA) and Israel (IL), respectively, to elucidate whether the growthclimate interactions were similar in both edges. Results Growth occurred during the summer (between December and March) in SA and in IL during early summer and autumn (AprilJune and OctoberNovember, respectively). Surprisingly, annual growth was 40% higher in IL than in SA. Within the wide distribution range of Acacia tortilis, our statistical model showed that climatic drivers of tree growth differed between the two sites. Conclusions High temperatures facilitated growth at the hot and arid IL site, while high humidity permitted growth at the more humid SA site. Our results confer an additional understanding of tree growth adaptation to extreme conditions in Acacia's world range edges, a major point of interest with ongoing climate change.

Afforestation in drylands is an active ecosystem restoration strategy shown to increase ecosystem services in these regions, which are highly prone to land degradation. Nevertheless, seedling recruitment is difficult in such projects due to various biotic and abiotic stresses. This paper proposes a methodology for providing tree seedlings in drylands with 3D-printed soil-based bioclimatic envelopes. The workflow suggests key aspects in transforming locally sourced soils to 3D printable, bioclimatically performable materials. Essential robotic tooling aspects and processing parameters are proposed. In addition, ways to embed site-specific data to algorithmically conceive a customized envelope according to the tree species are suggested.

The mycorrhizal symbiosis between fungi and plants is among the oldest, ubiquitous and most important interactions in terrestrial life on Earth. Carbon (C) transfer across a common mycorrhizal network (CMN) was demonstrated over half a century ago in the lab (Reid & Woods, 1969), and later in the field (Simard et al., 1997a). Recent years have seen ample progress in this research direction, including evidence for ecological significance of carbon transfer (Klein et al., 2016). Furthermore, specific cases where the architecture of mycorrhizal networks have been mapped (Beiler et al., 2015) and CMN-C transfer from mature trees to seedlings has been demonstrated (Orrego, 2018) have suggested that trees in forests are more connected than once thought (Simard, 2021). In a recent Perspective, Karst et al. (2023) offered a valuable critical review warning of over-interpretation and positive citation bias in CMN research. It concluded that while there is evidence for C movement among plants, the importance of CMNs remains unclear, as noted by others too (Henriksson et al., 2023). Here we argue that while some of these claims are justified, factual evidence about belowground C transfer across CMNs is solid and accumulating.

The current projections of climate change might exceed the ability of European forest trees to adapt to upcoming environmental conditions. However, stomatal and leaf morphological traits could greatly influence the acclimation potential of forest tree species subjected to global warming, including the single most important forestry species in Europe, European beech. We analysed stomatal (guard cell length, stomatal density and potential conductance index) and leaf (leaf area, leaf dry weight and leaf mass per area) morphological traits of ten provenances from two provenance trials with contrasting climates between 2016 and 2020. The impact of meteorological conditions of the current and preceding year on stomatal and leaf traits was tested by linear and quadratic regressions. Ecodistance was used to capture the impact of adaptation after the transfer of provenances to new environments. Interactions of trial-provenance and trial-year factors were significant for all measured traits. Guard cell length was lowest and stomatal density was highest across beech provenances in the driest year, 2018. Adaptation was also reflected in a significant relationship between aridity ecodistance and measured traits. Moreover, the meteorological conditions of the preceding year affected the interannual variability of stomatal and leaf traits more than the meteorological conditions of the spring of the current year, suggesting the existence of plant stress memory. High intraspecific variability of stomatal and leaf traits controlled by the interaction of adaptation, acclimation and plant memory suggests a high acclimation potential of European beech provenances under future conditions of global climate change.

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 23 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.

Xylem embolism impairs hydraulic conductivity in trees and drives drought-induced mortality. While embolism has been monitored in vivo in potted plants, and research has revealed evidence of embolism in field-grown trees, continuous in situ monitoring of cavitation in forests is lacking. Seasonal patterns of embolism were monitored in branchlets of Aleppo pine (Pinus halepensis) trees growing in a dry Mediterranean forest. Optical visualization (OV) sensors were installed on terminal branches, in addition to monthly sampling for micro computed tomography scans. We detected 208 cavitation events among four trees, which represented an embolism increase from zero to c. 12% along the dry season. Virtually all the cavitation events occurred during daytime hours, with 77% occurring between 10:00 and 17:00 h. The probability for cavitation in a given hour increased as vapor pressure deficit (VPD) increased, up to a probability of 42% for cavitation when VPD > 5 kPa. The findings uniquely reveal the instantaneous environmental conditions that lead to cavitation. The increased likelihood of cavitation events under high VPD in water-stressed pines is the first empirical support for this long hypothesized relationship. Our observations suggest that low levels of embolism are common in Aleppo pine trees at the dry edge of their distribution.

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.

Safeguarding Earths tree diversity is a conservation priority due to the importance of trees for biodiversity and ecosystem functions and services such as carbon sequestration. Here, we improve the foundation for effective conservation of global tree diversity by analyzing a recently developed database of tree species covering 46,752 species. We quantify range protection and anthropogenic pressures for each species and develop conservation priorities across taxonomic, phylogenetic, and functional diversity dimensions. We also assess the effectiveness of several influential proposed conservation prioritization frameworks to protect the top 17% and top 50% of tree priority areas. We find that an average of 50.2% of a tree species range occurs in 110-km grid cells without any protected areas (PAs), with 6,377 small-range tree species fully unprotected, and that 83% of tree species experience nonnegligible human pressure across their range on average. Protecting high-priority areas for the top 17% and 50% priority thresholds would increase the average protected proportion of each tree species range to 65.5% and 82.6%, respectively, leaving many fewer species (2,151 and 2,010) completely unprotected. The priority areas identified for trees match well to the Global 200 Ecoregions framework, revealing that priority areas for trees would in large part also optimize protection for terrestrial biodiversity overall. Based on range estimates for >46,000 tree species, our findings show that a large proportion of tree species receive limited protection by current PAs and are under substantial human pressure. Improved protection of biodiversity overall would also strongly benefit global tree diversity.

Mycorrhizal fungi can colonize multiple trees of a single or multiple taxa, facilitating bidirectional exchange of carbon between trees. Mycorrhiza-induced carbon transfer was shown in the forest, but it is unknown whether carbon is shared symmetrically among tree species, and if not, which tree species are better donors and which are better recipients. Here, we test this question by investigating carbon transfer dynamics among five Mediterranean tree species in a microcosm system, including both ectomycorrhizal (EM) and arbuscular (AM) plants. Trees were planted together in \u201ccommunity boxes\u201d using natural soil from a mixed forest plot that serves as a habitat for all five tree species and their native mycorrhizal fungi. In each box, only the trees of a single species were pulse-labelled with ¹³CO₂. We found that carbon transfer was asymmetric, with oak being a better donor, and pistacia and cypress better recipients. Shared mycorrhizal species may have facilitated carbon transfer, but their diversity did not affect the amount, nor timing, of the transfer. Overall, our findings in a microcosm system expose rich, but hidden, belowground interactions in a diverse population of trees and mycorrhizal fungi. The asymmetric carbon exchange among cohabiting tree species could potentially contribute to forest resilience in an uncertain future.

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.

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.

The unprecedented heatwave which hit the Pacific northwest of North America in late June-early July 2021 impacted ecosystems and communities, yet evidence for and analysis of this impact are still missing. Here we bring a unique dataset quantifying the impact on conifer trees, which are keystone species of many northwest ecosystems. Moreover, we take advantage of this exceptional event as a broad, extreme, 'field experiment' to test a fundamental theory in plant physiology and prepare our forests for a harsher future. Overall, the data collected confirm the role of hydraulic vulnerability in drought-induced injury to trees.

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.

Water shortage is a severe environmental factor causing growth disruption and yield-\loss in many agricultural plant species. As fruit trees are likely to suffer from the effects of severe drought in the future, wild relatives of cultivated crops can provide plant breeders a unique material to improve the drought resistance of modern crop varieties. We conducted a drought and rewatering greenhouse experiment along 51 days with young trees of almond (the desert wild species Prunus ramonensis vs. the commonly used rootstock hybrid Prunus dulcis×Prunus persica) and plum (the montane wild species Prunus ursina vs. the rootstock Prunus cerasifera × Prunus persica). To decipher the drought resistance mechanisms in these trees we monitored physiological responses. Expression dynamics of cellular water channels from the plasma intrinsic protein (PIP) aquaporin family were measured in the almond species. Our results indicate a higher drought resistance in wild almond compared to the rootstock, but not in the wild plum species. Under drought, P. ramonensis had~ninefold higher photosynthesis activity,~50-fold higher water-use efficiency and lower vulnerability to embolism than the rootstock. In the almond species, PIP downregulation was linked with maintenance of hydraulic conductivity, and vice versa for upregulation. This study implies that there is a link between drought resistance in wild tree species and their native habitat conditions, with an advantage for the desert, but not the montane, species. Finally, our study highlights the need to protect and conserve wild relatives of fruit tree species, partly as potential plant materials to be used by breeders to improve the resilience of orchard tree species to drought.

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.

Drought will increasingly threaten forest ecosystems worldwide. Understanding how competition influences tree growth response to drought is essential for forest management aiming at climate change adaptation. However, published results from individual case studies are heterogeneous and sometimes contradictory. We reviewed 166 cases from the peer-reviewed literature to assess the influence of stand-level competition on tree growth response to drought. We monitored five indicators of tree growth response: mean sensitivity (inter-annual tree ring width variability); association between inter-annual growth variability and water availability; resistance; recovery; and resilience to drought. Vote counting did not indicate a consistent effect of competition on mean sensitivity. Conversely, higher competition for resources strengthened the association between water availability and inter-annual growth rates. Meta-analysis showed that higher competition reduced resistance (p < 0.001) and improved recovery (p < 0.05), but did not consistently affect resilience. Species, site and stand characteristics, and drought intensity were insignificant or poor predictors for the large variability among the investigated cases. Our review and meta-analysis show that competition does not affect the response of tree growth to drought in a unidirectional and universal way. Although density reduction (thinning) can alleviate growth declines during drought, the effects on growth after stress are uncertain. The large variability among investigated cases suggests that local-scale processes play a crucial role in determining such responses and should be explicitly evaluated and integrated into specific strategies for adaptation of forests to climate change.

Carbon (C) allocation in trees involves three different aspects, namely, the fate of C in terms of tree compartment, C compound, and physiological process (often termed C flux). A review and synthesis of the recent literature on C allocation dynamics in Mediterranean pines, with an emphasis on stress conditions, reveals some important patterns. First, C allocation to stem growth is highly plastic in phenology and magnitude, whereas C allocation to needle growth is sensitive to growing conditions (e.g. drought, competition), but genetically programmed with respect to timing and magnitude. Second, starch is a major C reserve in Mediterranean pines, providing C under limited photosynthesis for weeks and months, replenished during recovery before growth is resumed, and stored at high concentrations in the root system. Third, C respiration is dominated by the roots, while growth and litter production are dominated by the foliage. Relocation fluxes occur under summer drought, i.e. from stem to foliage, and from roots to stem. Although minor in magnitude, these C fluxes might represent an important drought acclimation mechanism for pines in the Mediterranean.

Declining performance of avocado orchards growing in a clayey soil irrigated with treated wastewater (TWW) for more than 5 years has been observed in Israel. Measures studied to mitigate this were freshwater (FW), blended TWW:FW in a 1:1 ratio (MIX), low-frequency TWW-irrigation (LFI), TWW irrigated tuff trenches (TUF) and TWW as the control treatment. This study reports on the response of avocado leaf gas exchange, intrinsic water use efficiency, leaf water potential and leaf hydraulic conductance (Kleaf) to the mitigation measures applied to a 'Hass' avocado orchard (Persea americana Mill.) previously irrigated with TWW for 7 years. Stomatal conductance (gs) of FW, MIX, TUF and LFI was greater than that for TWW on most measurement dates, which increased net leaf CO2 assimilation rate (Aleaf) and intercellular CO2 concentration (Ci). Increased gs reduced intrinsic water use efficiency [WUE, determined from either 13C discrimination (iWUE) or gas exchange (WUEi)] as compared to TWW. Although the WUE in TWW was higher than that in mitigation treatments on most measurement dates, the lower assimilation rates at leaf (Aleaf) and whole tree (Atree) levels observed in TWW indicate that TWW irrigation had negative effects on tree physiological performance. A strong negative relationship between WUEi (Aleaf/gs) and gs was found in all treatments, with highest WUEi and lowest gs in TWW. Leaf hydraulic conductance was also lowest for TWW, which may indicate that lower gas exchange was linked to lower water availability to the leaves. Overall, the increased gs, Aleaf, Atree and Kleaf in FW, MIX and TUF relative to TWW indicate that these mitigation measures are good candidates for improving orchard performance negatively affected by long-term irrigation with TWW in clayey soils. Comparing all treatments, FW was the most effective.

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 ~1001000-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.

Lemon (Citrus limon) is a fruit tree with major agricultural importance around the Mediterranean basin and is considered to be highly drought resistant. In this study, we tested the effect of two months summer-desiccation on physiological and yield parameters of mature lemon trees growing under Mediterranean climate during three consecutive years. We also examined the efficiency of current irrigation regime, which is based on reference evapo-transpiration. We measured leaf gas exchange and water potential (Ѱl), monitored sap flow and soil moisture and followed flowering, fruit set and fruit size. Lemon trees showed an isohydric stomatal regulation, as stomata maintained leaf water potential >-2 MPa. Summer desiccation caused a gradual decrease in diurnal tree water use, starting immediately after cessation of irrigation, with leaf gas exchange practically halted at the end of the drought period. Tree function recovered following re-irrigation, and fruit yields were not reduced, but even mildly increased during the first year. In contrast, summer desiccation during two consecutive years caused long-term effects of tree activity decrease, significantly lower yield, main branch collapse and even tree mortality. Irrigation amounts matched closely tree water-use amounts; soil moisture was maintained around 26% (v/v); and irrigation responded dynamically to meteorological changes, indicating that current irrigation regime represents highly efficient water management. The lemon desiccation protocol relied on the physiological capacity of this species to avoid short-term drought effects through stomatal closure. Still, this protocol must be managed carefully, to reduce risk to trees and save yields.

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 2070 g C m−1 yr−1 and 4080 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.

Fixed carbon (C) is a central resource, whose dynamic allocation by the tree determines its ability to establish, grow and compete with other species. Traditionally, tree C allocation has been estimated by isotopic or mass balance approaches. Here we developed a setup of 13CO2 pulse-labeling, with a flux-coupled, three-phase detection, allowing us to follow C quantitatively. We examined C allocation patterns in 2-year-old potted saplings of five evergreen forest tree species that co-occur in a typical mixed forest in the Mediterranean region: two early succession conifers (Pinus, Cupressus) and three late succession broadleaf species (Quercus, Ceratonia, Pistacia). Across the five species, C moved from leaves to stem and fine roots following an exponential decay, with parameters that are consistent with the ecological role of each species. Eight days post-labeling, conifers allocated ~30% of the C belowground, and broadleaves

Green walls can improve indoor air-quality by reducing concentrations of carbon dioxide (CO2) and other air pollutants. Our study focused on the spider plant, Chlorophytum comosum, and devils ivy, Epipremnum aureum, both common green-wall plants that have been found to be efficient CO2 absorbers. Both species have multiple variants with varying degrees of leaf green-white segmentation. Since photosynthesis depends on the concentration of leaf chlorophylls, we hypothesized that green variants are more efficient carbon absorbers than green-white variants. In addition, we tested the hypothesis that the photosynthetic rate of plants is affected by pot volume, as suggested by previous studies. We used a portable gas exchange system to determine the rate of photosynthesis of the study plants. No evidence was found for better photosynthetic performance in the green vs. green-white variants of each species. In fact, our results suggest the opposite. It was observed that a spider plants assimilated carbon more efficiently when grown in a larger pot volume. In conclusion, our study shows that in terms of carbon assimilation, green-white variants of spider plants are the better choice for indoor green walls. Their efficiency can be improved dramatically by increasing pot volume.

Adaptations of fruit trees to future climate are a current research priority due to the rapid increase in air temperature and changes in precipitation patterns. This is aimed at securing sustainable food production for our growing populations. Key physiological traits in trees conferring drought tolerance are resistance to embolism and stomatal control over water loss. Recently, we have shown in the field that a native wild pear species performs better under drought than two cultivated pear species. A comparative greenhouse study was conducted to investigate traits associated with drought tolerance in four ecotypes of a wild pear species (Pyrus syriaca Boiss), compared with a wild pear species (Pyrus betulifolia Bunge) commonly used as a pear rootstock. Seed sources were collected from semi-arid, sub-humid and humid sites across northern Israel. Measurements of water relations, leaf physiology, hydraulic conductivity and percent loss of hydraulic conductivity (PLC) were conducted under standard irrigation, drought and recovery from drought. The four P. syriaca ecotypes maintained significantly higher leaf gas exchange values and water-use efficiency and had lower PLC than the rootstock species under prolonged drought as well as during recovery. Across the four ecotypes, stomatal closure occurred at stem water potential (ψ) around -3.5 MPa; however, ψ at 50% PLC ranged from -4.1 MPa in the humid ecotype to -5.2 MPa in one of the semi-arid ecotypes, rendering the latter with a higher hydraulic safety margin (the ψ difference between stomatal closure and 50% PLC). Divergence of the ecotypes in xylem vulnerability to embolism closely matched the mean annual precipitation at their seed sources. Thus, selection of pear ecotypes from populations in semi-arid sites may be better than the currently used plant material for preparing our cultivated species for hotter and drier future climate.

Ancient burial caves represent some of the most important sources of information on human history. In a world heritage site in Israel, such caves are under threat due to tree root growth penetrating from the ceilings and causing a risk of cave collapse. To facilitate historical conservation, while avoiding cutting of the mixed forest above the caves, we identified the tree species responsible for the damage, facilitating their specific cutting. Accurate identification of tree roots balances the needs to protect human structures on one side, while conserving the majority of vegetation on the other. Our approach is applicable to the management of ancient sites, as well as to urban management more broadly. Summary: 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.

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 (19181964); and (3) analyzed the carbon-13 composition in those samples, as an indicator for drouht stress. Over the course of five years (20142018) 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 2050% to 05 %, depending on the species. Comparing the carbon-13 composition in current and historical leaf samples showed an increase of 1.53.5 associated with a decrease in precipitation of 230 %, 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.

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.

Aims: In plant eco-physiology, less negative (enriched) carbon 13 (13C) in the leaves indicates conditions of reducing leaf gas exchange through stomata, e.g. under drought. In addition, 13C is expected to be less negative in non-photosynthetic tissues as compared with leaves. However, these relationships in δ 13C from leaves (photosynthetic organs) to branches, stems and roots (non-photosynthetic organs) are rarely tested across multiple closely related tree species, multiple compartments, or in trees growing under extreme heat and drought. Methods: We measured leaf-to-root 13C in three closely related desert acacia species (Acacia tortilis, A. raddiana and A. pachyceras). We measured δ 13C in leaf tissues from mature trees in southern Israel. In parallel, a 7-year irrigation experiment with 0.5, 1.0 or 4.0 L day−1 was conducted in an experimental orchard. At the end of the experiment, growth parameters and δ 13C were measured in leaves, branches, stems and roots. Important Findings: The δ 13C in leaf tissues sampled from mature trees was ca. −27, far more depleted than expected from a desert tree growing in one of the Earths driest and hottest environments. Across acacia species and compartments, δ 13C was not enriched at all irrigation levels (−28 to ca. −27), confirming our measurements in the mature trees. Among compartments, leaf δ 13C was unexpectedly similar to branch and root δ 13C, and surprisingly, even less negative than stem δ 13C. The highly depleted leaf δ 13C suggests that these trees have high stomatal gas exchange, despite growing in extremely dry habitats. The lack of δ 13C enrichment in non-photosynthetic tissues might be related to the seasonal coupling of growth of leaves and heterotrophic tissues.

NA

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.

Root exudates are part of the rhizodeposition process, which is the major source of soil organic carbon (C) released by plant roots. This flux of C is believed to have profound effects on C and nutrient cycling in ecosystems. The quantity of root exudates depends on the plant species, the period throughout the year, and external biotic and abiotic factors. Since root exudates of mature trees are difficult to collect in field conditions, very little is known about their flux, especially in water-limited ecosystems, such as the seasonally hot and dry Mediterranean maquis. Here, we collected exudates from DNA-identified roots in the forest from the gymnosperm Cupressus sempervirens L. and the evergreen angiosperm Pistacia lentiscus L. by 48-h incubations on a monthly temporal resolution throughout the year. We examined relationships of the root exudate C flux to abiotic parameters of the soil (water content, water potential, temperature) and atmosphere (vapor pressure deficit, temperature). We also studied relationships to C fluxes through the leaves as indicators of tree C balance. Root exudation rates varied significantly along the year, increasing from 6 μg C cm -2 root day-1 in both species in the wet season to 4- and 11-fold rates in Pistacia and Cupressus, respectively, in the dry season. A stepwise linear mixed-effects model showed that the three soil parameters were the most influential on exudation rates. Among biotic factors, there was a significant negative correlation of exudation rate with leaf assimilation in Cupressus and a significant negative correlation with leaf respiration in Pistacia. Our observation of enhanced exudation flux during the dry season indicates that exudation dynamics in the field are less sensitive to the low tree C availability in the dry season. The two key Mediterranean forest species seem to respond to seasonal changes in the rhizosphere such as drying and warming, and therefore invest C in the rhizosphere under seasonal drought.

Many ecologically important forest trees from dry areas have been insufficiently investigated for their ability to adapt to the challenges posed by climate change, which hampers the implementation of mitigation policies. We analyzed 14 common-garden experiments across the Mediterranean which studied the widespread thermophilic conifer Pinus halepensis and involved 157 populations categorized into five ecotypes.Ecotype-specific tree height responses to climate were applied to projected climate change (2071-2100 ad), to project potential growth patterns both locally and across the species' range.We found contrasting ecotypic sensitivities to annual precipitation but comparatively uniform responses to mean temperature, while evidence of local adaptation for tree height was limited to mesic ecotypes. We projected intriguing patterns of response range-wide, implying either height inhibition or stimulation of up to 75%, and deduced that the ecotype currently experiencing more favorable (wetter) conditions will show the largest inhibition. Extensive height reductions can be expected for coastal areas of France, Greece, Spain and northern Africa.Our findings underline the fact that intraspecific variations in sensitivity to precipitation must be considered when projecting tree height responses of dry forests to future climate. The ecotype-specific projected performances call for management activities to ensure forest resilience in the Mediterranean through, for example, tailored deployment strategies.

The mutualistic symbiosis between forest trees and ectomycorrhizal fungi (EMF) is among the most ubiquitous and successful interactions in terrestrial ecosystems. Specific species of EMF are known to colonize specific tree species, benefitting from their carbon source, and in turn, improving their access to soil water and nutrients. EMF also form extensive mycelial networks that can link multiple root-tips of different trees. Yet the number of tree species connected by such mycelial networks, and the traffic of material across them, are just now under study. Recently we reported substantial belowground carbon transfer between Picea, Pinus, Larix and Fagus trees in a mature forest. Here, we analyze the EMF community of these same individual trees and identify the most likely taxa responsible for the observed carbon transfer. Among the nearly 1,200 EMF root-tips examined, 50%-70% belong to operational taxonomic units (OTUs) that were associated with three or four tree host species, and 90% of all OTUs were associated with at least two tree species. Sporocarp C-13 signals indicated that carbon originating from labelled Picea trees was transferred among trees through EMF networks. Interestingly, phylogenetically more closely related tree species exhibited more similar EMF communities and exchanged more carbon. Our results show that belowground carbon transfer is well orchestrated by the evolution of EMFs and tree symbiosis.

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.

Trees of the genus Prunus produce some of the most widely consumed fruits globally. The combination of climate change-related warming and increased drought stress, scarcity of freshwater resources for irrigation, and increasing demands due to population growth creates a need for increased drought tolerance in these tree species. Recently, we have shown in the field that a native wild pear species performs better under drought than two cultivated pear species. Here, a comparative field study was conducted in Israel to investigate traits associated with drought tolerance in almond (cultivated Prunus dulcis (Mill.) D. A. Webb vs wild Prunus ramonensis Danin) and plum (cultivated Prunus domestica L. vs wild Prunus ursina Kotschy). Measurements of xylem embolism and shoot and root carbon reserves were done along a year, including seasonal drought in the wild and a 35-day drought experiment in the orchards. Synchronous measurements of native xylem embolism and shoot water potential showed that cultivated and wild almond trees lost similar to 50% of hydraulic conductivity at -2.3 and -3.2 MPa, respectively. Micro-CT images confirmed the higher embolism ratio in cultivated versus wild almond, whereas the two plum species were similar. Dynamics of tissue concentrations of nonstructural carbohydrates were mostly similar across species, with higher levels in cultivated versus wild plum. Our results indicate an advantage for the wild almond over its cultivated relative in terms of xylem resistance to embolism, a major risk factor for trees under drought stress. This result is in line with our previous experiment on pear species. However, the opposite trends observed among the studied plum species mean that these trends cannot be generalized. It is possible that the potential for superior drought tolerance in wild tree species, relative to their cultivated relatives, is limited to wild species from dry and hot habitats.

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.

In the published version of this editorial paper, the sentence in the first paragraph should be corrected as shown below: This special section of Plant and Soil contains 13 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.

Severe droughts have the potential to reduce forest productivity and trigger tree mortality. Most trees face several drought events during their life and therefore resilience to dry conditions may be crucial to long-term survival. We assessed how growth resilience to severe droughts, including its components resistance and recovery, is related to the ability to survive future droughts by using a tree-ring database of surviving and now-dead trees from 118 sites (22 species, >3,500 trees). We found that, across the variety of regions and species sampled, trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. In angiosperms, drought-related mortality risk is associated with lower resistance (low capacity to reduce impact of the initial drought), while it is related to reduced recovery (low capacity to attain pre-drought growth rates) in gymnosperms. The different resilience strategies in these two taxonomic groups open new avenues to improve our understanding and prediction of drought-induced mortality.

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: \u201cExposing the Hidden Half - Root Research at the Forefront of Science\u201d, 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.

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.

Plant traits-the morphological, anatomical, physiological, biochemical and phenological characteristics of plants-determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to biodiversity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait-based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits-almost complete coverage for 'plant growth form'. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait-environmental relationships. These traits have to be measured on individual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives.

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.

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 trees to increasing atmospheric CO2 concentrations is often mediated by the availability of nutrients. However, little is known about the influence of CO2 enrichment on nutrient availability in forests with mature trees. We studied processes in the soil under five 35-m-tall Norway spruce trees (Picea abies) in NW Switzerland that were exposed to a mean CO2 concentration of 550 ppm for 5 growing seasons using free air CO2 enrichment (FACE). We compared them with values from the soil under five control trees. Ceramic suction cups were installed in the soil under each tree to collect soil solution, and ion-exchange resin bags were buried in the soil to absorb ammonium and nitrate. Soil cores taken at the end of the experiment were used to measure the gross production of ammonium and nitrate by the 15N dilution technique. Although temporally and spatially variable, the nitrate concentration was higher in the soil solution under CO2-enriched trees. This effect was reflected in the resin bag extracts, which additionally indicated a trend of increased ammonium availability. Dissolved reduced N concentration (mainly dissolved organic N), however, was lower in the soil solution under CO2-enriched trees. K and Mg, and to a lesser extent Na and sulfate concentrations increased in the soil solution. P concentrations mostly remained below the detection limit. In spite of the higher concentrations of nitrate in soil extracts, gross N mineralization and nitrification rates were not affected by FACE. Needles from CO2-enriched trees contained slightly more N. No difference was observed for other nutrients. Overall, these results support the hypothesis of a priming effect, i.e., that FACE led to the production of more root exudates, which in turn stimulated soil biological activity, including mineralization, over a time-span of at least several years. However, these tall trees showed no growth response to elevated CO2; hence, they gained no advantage from increased nitrate in the soil solution, presumably owing to other growth constraints including P and Mg availability.

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.

Among forest types, the Mediterranean maquis is specifically exposed to fluctuations in water availability. Therefore, monitoring the water-use patterns of its major tree species is key in quantifying the local and regional water balance. However, the traditional measurement methods of tree water-use at high spatial scales are difficult and labor-intensive, thus indirect methods become useful. Evaporation of water from the stomatal pores on the leaf surface involves evaporative cooling, and hence the differences between the leaf temperature and its surrounding air temperature (ΔT _leaf-air ) can serve as a reliable estimator for tree water-use. Here, we used direct measurement of transpiration rate (E) with a gas exchange system, simultaneously with ground Thermal Infra-Red (TIR) imaging to study the relationship between ΔT _leaf-air and E in dominant tree species of the Mediterranean maquis. Controlled experiments were conducted in parallel with measurements in the forest, on five tree species of contrasting leaf shapes (Conifers: Pinus halepensis; Cupressus sempervirens; Broadleaf: simple: Quercus calliprinos; Ceratonia siliqua; compound: Pistacia lentiscus). Next, we used a quantitative approach, applying a leaf energy balance model to estimate E from the TIR images and compared it to the direct measurement of the gas exchange system. We report evaporative cooling across the five species, replicated in tree saplings and in mature trees in the forest. The conifers were significantly cooler than broadleaves by up to ∼4 °C and produced narrower ΔT _leaf-air ranges. Estimations of E from ΔT _leaf-air were relatively close to the observed E, with some overestimations. Our observations show that TIR imaging can detect transpiration-related differences in ΔT _leaf-air among species and can be used to estimate E in natural environments. Yet, the dependence of ΔT _leaf-air on E is species-specific and thus, empirical associations must be developed separately for each of the species.

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 1520 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 sites 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. 

Tree mortality is a key driver of forest dynamics and its occurrence is projected to increase in the future due to climate change. Despite recent advances in our understanding of the physiological mechanisms leading to death, we still lack robust indicators of mortality risk that could be applied at the individual tree scale. Here, we build on a previous contribution exploring the differences in growth level between trees that died and survived a given mortality event to assess whether changes in temporal autocorrelation, variance, and synchrony in time-series of annual radial growth data can be used as early warning signals of mortality risk. Taking advantage of a unique global ring-width database of 3065 dead trees and 4389 living trees growing together at 198 sites (belonging to 36 gymnosperm and angiosperm species), we analyzed temporal changes in autocorrelation, variance, and synchrony before tree death (diachronic analysis), and also compared these metrics between trees that died and trees that survived a given mortality event (synchronic analysis). Changes in autocorrelation were a poor indicator of mortality risk. However, we found a gradual increase in inter- annual growth variability and a decrease in growth synchrony in the last similar to 20 years before mortality of gymnosperms, irrespective of the cause of mortality. These changes could be associated with drought-induced alterations in carbon economy and allocation patterns. In angiosperms, we did not find any consistent changes in any metric. Such lack of any signal might be explained by the relatively high capacity of angiosperms to recover after a stress-induced growth decline. Our analysis provides a robust method for estimating early-warning signals of tree mortality based on annual growth data. In addition to the frequently reported decrease in growth rates, an increase in inter-annual growth variability and a decrease in growth synchrony may be powerful predictors of gymnosperm mortality risk, but not necessarily so for angiosperms.

Is tree mortality increasing? Are recent mortality events related to climate change? Which tree species are the most affected? Many case studies have been published in the last decade, but the necessary large-scale and long-term knowledge is still missing.Here we combined data from forest surveys and satellite imagery, to create the first spatial tree mortality history at the national scale. Israel is a small country with only 7% forest cover, but its large environmental diversity and mosaic of many, small, forest areas makes it a good 'miniature model' for the task.Tree mortality events have been increasing significantly since 1991 and correlated well with drought. Among mortality events, 24% of the loss was directly related to drought, and 58% to fire, with 69% of fires occurring over a drought background. Conifers were disproportionally more affected than native broadleaved trees.This is the first national-scale study of tree mortality dynamics, and it confirms the suspected increase in this phenomenon in recent decades, and the dominant role of drought. Our study opens a way to a better, multi-source monitoring future for forest management and ensuring forest sustainability under climate change.

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.

The combined effects of local adaptation and phenotypic plasticity influencing plant performance are relevant to understand the capacity for genetic responses to climate change. Pima halepensis is a native species of low- to mid-elevation Mediterranean forests with a high ecological value in drought-prone areas. Thus, it is of utmost importance to determine its adaptive structure for key traits such as growth or survival. Here, we analyse a highly unbalanced dataset collated from different common-garden networks that cover the distribution range of the species. A total of 82 range-wide populations were evaluated in nine Mediterranean trials located in Israel, Italy and Spain. A climate classification of populations allowed for the definition of six different groups, or ecotypes, which showed contrasting performances for tree height and survival at age 15. The effects of ecotypic differentiation and among-ecotypes genetic variation in plasticity were disentangled by fitting stability models accounting for interaction and heteroscedasticity in genotype-by-environment tables. For growth, a Finlay-Wilkinson model suggested high predictability of ecotypic plastic responses in P. halepensis, as described by different linear reaction norms. However, differences in mean height of ca. 15% among ecotypes dominated infra-specific patterns of tree growth across trials, pointing to preponderance of genotypic adaptation over differential ecotypic plasticity in this species. For survival, ecotypic differences were approximately constant across trials, suggesting lack of genotype-by-environment effects. Sub-humid cool climate populations from the eastern Mediterranean (e.g., Greek populations) showed general adaptation and high sensitivity to improved growing conditions, as opposed to populations from the driest ecological extreme of the species (e.g., south Spain and Maghreb populations), which exhibited specific adaptation to harsh environments. Altogether, our results indicate a general adaptive syndrome by which less reactive ecotypes to ameliorated conditions (e.g., non-water-limited) would be associated with high survival rates and low growth. The reported ecotypic differentiation constitutes the basis for tailoring infra-specific responses to climate and disentangling the relationship between adaptive variation and resilience towards climatic warming for this exemplary Mediterranean pine.

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.

Tree growth enhancement under elevated [CO2] is much smaller than originally anticipated; yet carbon overabundance can lead to increased wood carbon storage and to stomata] downregulation and hence reduced water use. Notably, all three outcomes increase tree drought resistance. Here we studied growth, water relations, and nonstructural carbohydrates of 60 lemon saplings growing in CO2-controlled rooms at the same greenhouse, under 400, 650, and 850 ppm (CO2]. At each [CO2] level, 10 saplings were exposed to 1-month dry-down after 2 months of standard irrigation, followed by re-watering for another month. The other 10 saplings served as controls. Under drought, tree growth was maintained at elevated, but not ambient, CO2, linked with mild vs. severe tree water stress (leaf water potential of 3.5 at elevated and 5.5 MPa at ambient [CO2]). Stomatal downregulation with increasing [CO2] meant that leaf transpiration and diurnal plant water-use were 13-46% lower at elevated vs. ambient [CO2] but photosynthesis was still 15-25% higher. CO2-induced increases in root and shoot starch were transient but significant. Our results suggest that when predicting tree growth in a warmer and drier future, concomitant atmospheric CO2 concentration must be considered. In young lemon trees, elevated CO2 partially compensated for drought effects on tree growth and water status, and might delay some of the effects of the anthropogenic climate change.

Broad-scale forest die-off associated with drought and heat has now been reported from every forested continent, posing a global-scale challenge to forest management. Climate-driven die-off is frequently compounded with other drivers of tree mortality, such as altered land use, wildfire, and invasive species, making forest management increasingly complex. Facing similar challenges, rangeland managers have widely adopted the approach of developing conceptual models that identify key ecosystem states and major types of transitions between those states, known as "state-and-transition models" (S&T models). Using expert opinion and available research, the development of such conceptual S&T models has proven useful in anticipating ecosystem changes and identifying management actions to undertake or to avoid. In cases where detailed data are available, S&T models can be developed into probabilistic predictions, but even where data are insufficient to predict transition probabilities, conceptual S&T models can provide valuable insights for managing a given ecosystem and for comparing and contrasting different ecosystem dynamics. We assembled a synthesis of 14 forest die-off case studies from around the globe, each with sufficient information to infer impacts on forest dynamics and to inform management options following a forest die-off event. For each, we developed a conceptual S&T model to identify alternative ecosystem states, pathways of ecosystem change, and points where management interventions have been, or may be, successful in arresting or reversing undesirable changes. We found that our diverse set of mortality case studies fit into three broad classes of ecosystem trajectories: (1) single-state transition shifts, (2) ecological cascading responses and feedbacks, and (3) complex dynamics where multiple interactions, mortality drivers, and impacts create a range of possible state transition responses. We integrate monitoring and management goals in a framework aimed to facilitate development of conceptual S&T models for other forest die-off events. Our results highlight that although forest die-off events across the globe encompass many different underlying drivers and pathways of ecosystem change, there are commonalities in opportunities for successful management intervention.

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-time-period 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.

This is a protocol to evaluate gross primary productivity (GPP) of a forest stand based on the measurements of trees 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.

Green roofs are expected to absorb and store carbon in plants and soils and thereby reduce the high CO₂ concentration levels in big cities. Sedum species, which are succulent perennials, are commonly used in extensive green roofs due to their shallow root system and ability to withstand long water deficiencies. Here we examined CO₂ fixation and emission rates for Mediterranean Sedum sediforme on green-roof experimental plots. During late winter to early spring, we monitored CO₂ concentrations inside transparent tents placed over 1 m² plots and followed gas exchange at the leaf level using a portable gas-exchange system. We found high rates of CO₂ emission at daytime, which is when CO₂ concentration in the city is the highest. Both plot- and leaf-scale measurements showed that these CO₂ emissions were not fully compensated by the nighttime uptake. We conclude that although carbon sequestration may only be a secondary benefit of green roofs, for improving this ecosystem service, other plant species than Sedum should also be considered for use in green roofs, especially in Mediterranean and other semi-arid climates.

Climate change is expected to exacerbate drought for many plants, making drought tolerance a key driver of species and ecosystem responses. Plant drought tolerance is determined by multiple traits, but the relationships among traits, either within individual plants or across species, have not been evaluated for general patterns across plant diversity. We synthesized the published data for stomatal closure, wilting, declines in hydraulic conductivity in the leaves, stems, and roots, and plant mortality for 262 woody angiosperm and 48 gymnosperm species. We evaluated the correlations among the drought tolerance traits across species, and the general sequence of water potential thresholds for these traits within individual plants. The trait correlations across species provide a framework for predicting plant responses to a wide range of water stress from one or two sampled traits, increasing the ability to rapidly characterize drought tolerance across diverse species. Analyzing these correlations also identified correlations among the leaf and stem hydraulic traits and the wilting point, or turgor loss point, beyond those expected from shared ancestry or independent associations with water stress alone. Further, on average, the angiosperm species generally exhibited a sequence of drought tolerance traits that is expected to limit severe tissue damage during drought, such as wilting and substantial stem embolism. This synthesis of the relationships among the drought tolerance traits provides crucial, empirically supported insight into representing variation in multiple traits in models of plant and ecosystem responses to drought.

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 ¹³C 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 onoff 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 enhanced N availability.

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 ~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 ~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.

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 suffcient 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, confrming 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 ∼25 days) but delayed cambial growth (by ∼15 days) than Fagus and Quercus, the result of a competitive strategy to flush early, while having lower NSC levels.

Drought-induced tree mortality has been observed globally and is expected to increase under climate change scenarios, with large potential consequences for the terrestrial carbon sink. Predicting mortality across species is crucial for assessing the effects of climate extremes on forest community biodiversity, composition, and carbon sequestration. However, the physiological traits associated with elevated risk of mortality in diverse ecosystems remain unknown, although these traits could greatly improve understanding and prediction of tree mortality in forests. We performed a meta-analysis on species' mortality rates across 475 species from 33 studies around the globe to assess which traits determine a species' mortality risk. We found that species-specific mortality anomalies from community mortality rate in a given drought were associated with plant hydraulic traits. Across all species, mortality was best predicted by a low hydraulic safety margin-the difference between typical minimum xylem water potential and that causing xylem dysfunction-and xylem vulnerability to embolism. Angiosperms and gymnosperms experienced roughly equal mortality risks. Our results provide broad support for the hypothesis that hydraulic traits capture key mechanisms determining tree death and highlight that physiological traits can improve vegetation model prediction of tree mortality during climate extremes.

Forest trees compete for light and soil resources, but photoassimilates, once produced in the foliage, are not considered to be exchanged between individuals. Applying stable carbon isotope labeling at the canopy scale, we show that carbon assimilated by 40-meter-tall spruce is traded over to neighboring beech, larch, and pine via overlapping root spheres. Isotope mixing signals indicate that the interspecific, bidirectional transfer, assisted by common ectomycorrhiza networks, accounted for 40% of the fine root carbon (about 280 kilograms per hectare per year tree-to-tree transfer). Although competition for resources is commonly considered as the dominant tree-to-tree interaction in forests, trees may interact in more complex ways, including substantial carbon exchange.

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.

The carbon sink intensity of the biosphere depends on the balance between gross primary productivity (GPP) of forest canopies and ecosystem respiration. GPP, however, cannot be directly measured and estimates are not well constrained. A new approach relying on canopy transpiration flux measured as sap flow, and water-use efficiency inferred from carbon isotope analysis (GPP_SF) has been proposed, but not tested against eddy covariance-based estimates (GPP_EC). Here we take advantage of parallel measurements using the two approaches at a semi-arid pine forest site to compare the GPP_SF and GPP_EC estimates on diurnal to annual timescales. GPP_SF captured the seasonal dynamics of GPP_EC (GPP_SF = 0.99 × GPP_EC, r² = 0.78, RMSE = 0.82, n = 457 d) with good agreement at the annual timescale (653 vs 670 g C m^-2 yr^-1). Both methods showed that GPP ranged between 1 and 8 g C m^-2 d^-1, and the GPP_SF/GPP_EC ratio was between 0.5 and 2.0 during 82% of the days. Carbon uptake dynamics at the individual tree scale conformed with leaf scale rates of net assimilation. GPP_SF can produce robust estimations of tree- and canopy-scale rates of CO₂ uptake, providing constraints and greatly extending current GPP_EC estimations.

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

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 thaliana PDI 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.

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 (Ψ_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.

This study aims to test the hypothesis that as leaf water potential decreases, stomatal 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: Pinus halepensis (isohydric) and Quercus calliprinos (anisohydric) growing together in two different sites during seven field campaigns over 14months. Intrinsic water-use efficiency (WUE_i) was calculated from gas exchange ratios, and independently from carbon isotopic composition, δ¹³C, of annual tree-ring sub-sections in four representative growth years.As expected, g_s was greatly restricted already at VPD5kPa. Consequently, mean transpiration rates were 0.2-2.2 and 0.5-3.9mmolm²s^-1 in coexisting pines and oaks, respectively. Mean δ¹³C values were 1.5 higher in tree-rings of the pine compared to the oak trees, consistent with the differences in WUE_i between 75 and 64μmol CO₂mol^-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 ~30% higher water-use (or ~70% lower water yield) in oak stand compared to pine stand, related to its tendency toward anisohydric behavior. This sets the limit for typical 300treesha^-1 oak and pine stands at the 460 and 360mm 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.

We review observational, experimental, and model results on how plants respond to extreme climatic conditions induced by changing climatic variability. Distinguishing between impacts of changing mean climatic conditions and changing climatic variability on terrestrial ecosystems is generally underrated in current studies. The goals of our review are thus (1) to identify plant processes that are vulnerable to changes in the variability of climatic variables rather than to changes in their mean, and (2) to depict/evaluate available study designs to quantify responses of plants to changing climatic variability. We find that phenology is largely affected by changing mean climate but also that impacts of climatic variability are much less studied, although potentially damaging. We note that plant water relations seem to be very vulnerable to extremes driven by changes in temperature and precipitation and that heatwaves and flooding have stronger impacts on physiological processes than changing mean climate. Moreover, interacting phenological and physiological processes are likely to further complicate plant responses to changing climatic variability. Phenological and physiological processes and their interactions culminate in even more sophisticated responses to changing mean climate and climatic variability at the species and community level. Generally, observational studies are well suited to study plant responses to changing mean climate, but less suitable to gain a mechanistic understanding of plant responses to climatic variability. Experiments seem best suited to simulate extreme events. In models, temporal resolution and model structure are crucial to capture plant responses to changing climatic variability. We highlight that a combination of experimental, observational, and/or modeling studies have the potential to overcome important caveats of the respective individual approaches.

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 μmol CO ₂ 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.

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 (Ψ _l) of -2.8 MPa, suggesting isohydric stomatal regulation. In trees under extreme drought treatments, stomatal closure reduced CO ₂ uptake to -1μmol 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.