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Increased temperature and prolonged soil moisture reduction have distinct impacts on tree photosynthetic properties. Yet, our knowledge of their combined effect is limited. Moreover, how species interactions alter photosynthetic responses to warming and drought remains unclear. Using mesocosms, we studied how photosynthetic properties of European beech and downy oak were impacted by multi-year warming and soil moisture reduction alone or combined, and how species interactions (intra- vs inter-specific interactions) modulated these effects. Warming of +5°C enhanced photosynthetic properties in oak but not beech, while moisture reduction decreased them in both species. Combined warming and moisture reduction reduced photosynthetic properties for both species, but no exacerbated effects were observed. Oak was less impacted by combined warming and limited moisture when interacting with beech than in intra-specific stands. For beech, species interactions had no impact on the photosynthetic responses to warming and moisture reduction, alone or combined. Warming had either no or beneficial effects on the photosynthetic properties, while moisture reduction and their combined effects strongly reduced photosynthetic responses. However, inter-specific interactions mitigated the adverse impacts of combined warming and drought in oak, thereby highlighting the need to deepen our understanding of the role of species interactions under climate change., New Phytologist, 236 (2), ISSN:0028-646X, ISSN:1469-8137

Trees remain sufficiently hydrated during drought by closing stomata and reducing canopy conductance (Gc) in response to variations in atmospheric water demand and soil water availability. Thresholds that control the reduction of Gc are proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the link between Gc and the ability of stem tissues to rehydrate at night remains unclear. We investigated whether species-specific Gc responses aim to prevent branch embolisms, or enable night-time stem rehydration, which is critical for turgor-dependent growth. For this, we used a unique combination of concurrent dendrometer, sap flow and leaf water potential measurements and collected branch-vulnerability curves of six common European tree species. Species-specific Gc reduction was weakly related to the water potentials at which 50% of branch xylem conductivity is lost (P50). Instead, we found a stronger relationship with stem rehydration. Species with a stronger Gc control were less effective at refilling stem-water storage as the soil dries, which appeared related to their xylem architecture. Our findings highlight the importance of stem rehydration for water-use regulation in mature trees, which likely relates to the maintenance of adequate stem turgor. We thus conclude that stem rehydration must complement the widely accepted safety–efficiency stomatal control paradigm. ISSN:0028-646X ISSN:1469-8137

Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD). Heat and VPD have been identified as increasingly important drivers of plant functioning in terrestrial biomes and are significant contributors to recent drought-induced tree mortality. Despite this, few studies have isolated the physiological response of plants to high VPD, heat, and soil drought, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. I will present diverse experimental approaches to disentangle atmospheric and soil drivers of plant functions across scales. I will further discuss recent findings suggesting that high temperature and VPD can lead to a cascade of impacts, including reduced photosynthesis, foliar overheating, and higher risks of hydraulic failure, independently of soil moisture changes.

Stable isotope compositions of carbon (δ13C) and oxygen (δ18O) in plant carbohydrates such as photosynthetic assimilates or cellulose are widely applied tools to reconstruct climate and plant physiological responses. In contrast, applications of hydrogen isotope composition (δ2H) in plant carbohydrates are limited because of previous methodological constrains and limited knowledge on processes causing hydrogen isotope fractionations. To better understand the individual climatic drivers of isotopic variations in tree rings, particularly for δ2H, we performed a controlled experiment over one growing season in climate chambers with saplings of broadleaf and conifer tree species. The growing conditions resembled conditions that can be typically found in the field: vapor pressure deficit (VPD; 1.0, 1.6, and 2.2 kPa), air temperature (T; 25 and 30 °C), and soil drought (D, well-watered and extreme dry). After 5 months of treatment, δ13C, δ18O, and δ2H of water, sugars and starch in stems and leaves, as well as in cellulose of the recent year tree rings were measured. For δ2H analyses of plant carbohydrates, we applied a newly developed hot water vapour equilibration method (Schuler et al., 2022, doi.org/10.1111/pce.14193). Across all species, first results show that the three elements in tree-ring cellulose respond differently to the climatic drivers: both δ2H and δ13C values increased with increases in D and VPD, but the VPD responses were more pronounced under high than under low T conditions. In contrast, δ18O values were affected by T and VPD, while the VPD response was more pronounced under wet than under dry soil D conditions. Thus, the combination of δ2H and δ13C values could be used to identify D occurrences independent of VPD conditions, while δ18O value is a better indicator for T and VPD responses. In the following steps, the isotopic variations in tree-ring cellulose will be linked to those in water, sugars, and starch and their concentrations in leaf and stem material, as well as to various other structural and functional traits which have been measured throughout the experiment (Schönbeck et al., 2022, doi.org/10.1111/pce.14425). With this unique experimental design, we aim to provide new knowledge facilitating the interpretation of stable isotope patterns in tree rings under field conditions.

Despite a warm and dry climate, the city of Nouakchott has been facing constant flooding for almost a decade, making part of the city inhabitable and posing long-term health threats. Groundwater levels are relatively constant over the year, except for October, when the groundwater table rises at the end of the rainy season, resulting in an almost doubled flooded area in the city compared to drier periods. Saltwater intrusion maintains a constant level in the water table beneath the city. However, the infiltration of most of Nouakchott’s used water acts as systematic artificial aquifer recharge, thus increasing the risk of groundwater saturation excess and flooding. Hence, in comparison to the driest decade (1971-1980), flooding in the city today cannot only be attributed to the slight increase in precipitation over the last decade. This project hypothesizes that increasing the resilience to urban flooding in the city of Nouakchott can be achieved by using salt-tolerant plants to lower the water table level. This work presents a joined interdisciplinary ecohydrology and plant physiology approach for monitoring and modeling the transpiration and dewatering capacity of different local tree species. The project aims to provide scenarios for an integrated and sustainable afforestation strategy for Nouakchott. In addition to increasing the city’s resilience to flooding, the role that afforestation could play to enhance the provision of sustainable services for the people and the economy (e.g., shade in the streets, potential fruit harvesting and wood market, etc.) will also be discussed. The first field campaign of the project allowed to monitor five observation wells with automatic water depth measurements and 12 sap flow sensors on three tree species. Eventually, to reinforce the relatively scarce groundwater data, a spatiotemporal time series of the city's flooded areas was also reconstructed using remote sensing data, and its reliability to calibrate an eco-hydrogeological model will be discussed.

This is a pre-copyedited, author-produced PDF of an article accepted for publication in Tree Physiology following peer review. The version of record [Faster drought recovery in anisohydric beech compared with isohydric spruce. Tree Physiology (2023)] is available online at: https://doi.org/10.1093/treephys/tpad009. Deposited by shareyourpaper.org and openaccessbutton.org. We've taken reasonable steps to ensure this content doesn't violate copyright. However, if you think it does you can request a takedown by emailing help@openaccessbutton.org.

Temperature (T) and vapour pressure deficit (VPD) are important drivers of plant hydraulic conductivity, growth, mortality, and ecosystem productivity, independently of soil water availability. Our goal was to disentangle the effects of T and VPD on plant hydraulic responses. Young trees of Fagus sylvatica L., Quercus pubescens Willd. and Quercus ilex L. were exposed to a cross-combination of a T and VPD manipulation under unlimited soil water availability. Stem hydraulic conductivity and leaf-level hydraulic traits (e.g., gas exchange and osmotic adjustment) were tracked over a full growing season. Significant loss of xylem conductive area (PLA) was found in F. sylvatica and Q. pubescens due to rising VPD and T, but not in Q. ilex. Increasing T aggravated the effects of high VPD in F. sylvatica only. PLA was driven by maximum hydraulic conductivity and minimum leaf conductance, suggesting that high transpiration and water loss after stomatal closure contributed to plant hydraulic stress. This study shows for the first time that rising VPD and T lead to losses of stem conductivity even when soil water is not limiting, highlighting their rising importance in plant mortality mechanisms in the future., Plant, Cell & Environment, 45 (11), ISSN:0140-7791, ISSN:1365-3040

Heat and drought affect plant chemical defenses and thereby plant susceptibility to pests and pathogens. Monoterpenes are of particular importance for conifers as they play critical roles in defense against bark beetles. To date, work seeking to understand the impacts of heat and drought on monoterpenes has primarily focused on young potted seedlings, leaving it unclear how older age classes that are more vulnerable to bark beetles might respond to stress. Furthermore, we lack a clear picture of what carbon resources might be prioritized to support monoterpene synthesis under drought stress. To address this, we measured needle and woody tissue monoterpene concentrations and physiological variables simultaneously from mature piñon pines (Pinus edulis) from a unique temperature and drought manipulation field experiment. While heat had no effect on total monoterpene concentrations, trees under combined heat and drought stress exhibited ~ 85% and 35% increases in needle and woody tissue, respectively, over multiple years. Plant physiological variables like maximum photosynthesis each explained less than 10% of the variation in total monoterpenes for both tissue types while starch and glucose + fructose measured 1-month prior explained ~ 45% and 60% of the variation in woody tissue total monoterpene concentrations. Although total monoterpenes increased under combined stress, some key monoterpenes with known roles in bark beetle ecology decreased. These shifts may make trees more favorable for bark beetle attack rather than well defended, which one might conclude if only considering total monoterpene concentrations. Our results point to cumulative and synergistic effects of heat and drought that may reprioritize carbon allocation of specific non-structural carbohydrates toward defense. Supplementary Information The online version contains supplementary material available at 10.1007/s00442-021-05058-8.

Increasing severity and frequency of drought is predicted for large portions of the terrestrial biosphere, with major impacts already documented in wet tropical forests. Using a 4-year rainfall exclusion experiment in the Daintree Rainforest in northeast Australia, we examined canopy tree responses to reduced precipitation and soil water availability by quantifying seasonal changes in plant hydraulic and carbon traits for 11 tree species between control and drought treatments. Even with reduced soil volumetric water content in the upper 1m of soil in the drought treatment, we found no significant difference between treatments for predawn and midday leaf water potential, photosynthesis, stomatal conductance, foliar stable carbon isotope composition, leaf mass per area, turgor loss point, xylem vessel anatomy, or leaf and stem nonstructural carbohydrates. While empirical measurements of aboveground traits revealed homeostatic maintenance of plant water status and traits in response to reduced soil moisture, modeled belowground dynamics revealed that trees in the drought treatment shifted the depth from which water was acquired to deeper soil layers. These findings reveal that belowground acclimation of tree water uptake depth may buffer tropical rainforests from more severe droughts that may arise in future with climate change.

Biological diversity and competition among species in ecosystems are sensitive to changes in macronutrient supply and nutrient availability. Human activity is intensively and extensively altering macronutrient cycles from a regional to a global scale with rates that can far exceed natural ones. Moreover, anthropogenic pollution exposes ecosystems to additional nutrients and stressors. These processes, although not well studied, can have a strong impact on ecosystem composition and productivity. In this study, we characterize the atmospheric deposition of bioavailable macronutrients from air pollution and study their impact on plant (oat) productivity and soil quality at a site in the Bois-Chamblard forest outside of Lausanne, Switzerland by Lake Geneva.To evaluate the importance of atmospheric deposition as a nutrient path for soil and plants, we set up a mesocosm experiment where plants and bare soil were exposed to atmospheric deposition for four months (during Spring and Summer, 2021) and compared against replicates not exposed to atmospheric deposition. Carbon (C), N, P in plant and soil, as well as soil enzymatic activity, fungi and bacterial communities are quantified for each member of the mesocosm experiment. Quantification of the total nitrogen (N) and phosphorous (P), gas- and aerosol-species (inorganic/organic species and metals) in rain water, dry deposition and airborne particles and soil is carried out.We find that plants exposed to atmospheric deposition display higher photosynthetic activity, larger N content and higher capacity to compete for nutrients in the soil. The soil community in the atmospheric deposition treatment shown higher nitrification rate and enzymatic activity towards lignin decomposition compared to the control. These results indicates that atmospheric pollutants act as plant fertilizers fostering their control on soil microbial community and accelerating soil nutrient stocks consumption.

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., Enhanced drought frequency and magnitude have impacted tree mortality, leading to multiple examples of regional-scale dieback. This Review outlines the mechanisms leading to mortality, including carbon starvation and hydraulic failure.

1. Many experiments have shown that biodiversity promotes ecosystem functioning and stability and that this relationship varies with resource availability. However, we still have a poor understanding of the underlying physiological and ecological mechanisms driving diversity effects and how they may interact with soil nutrient availability. 2. We collected data in a grassland experiment factorially manipulating fertilization, species richness (SR), functional composition (slow-growing vs. fast-growing species) and functional diversity in resource economic traits. We measured above-ground productivity (AP), nitrogen (N) uptake, photosynthesis and water use efficiency by combining a 15N labelling approach with productivity, gas exchange and stable isotope measurements in 3 years differing in rainfall. 3. We found that sown SR increased AP, N uptake and photosynthesis, suggesting that SR is the most important driver of ecosystem productivity and nutrient cycling. Similarly, photosynthesis was affected by functional composition but not by functional diversity. Water use efficiency was reduced by sown SR for communities dominated by slow-growing species but not for communities dominated by fast-growing species. Fertilization increased productivity, N uptake and water use efficiency. The positive effects of high SR on ecosystem functions were independent of fertility levels. 4. Synthesis. Our results provide evidence that high species richness in temperate grasslands could enhance productivity and reduce the negative impacts of drought events. Multiple factors and community characteristics are important in driving enhanced ecosystem functioning in biodiverse grasslands and seem to affect functioning and stability through different mechanisms. ISSN:0022-0477

Future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees' sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil moisture is largely unknown. Leaf gas exchange measurements were performed within a long-term (17 years) irrigation experiment in a drought-prone Scots pine-dominated forest in one of Switzerland's driest areas on trees in naturally dry (control), irrigated, and 'irrigation-stop' (after 11 years of irrigation) conditions. Seventeen years of irrigation increased photosynthesis (A) and stomatal conductance (g(s)) and reduced g(s) sensitivity to increasing VPD and soil drying. Following irrigation-stop, gas exchange decreased only after 3 years. After 5 years, maximum carboxylation (V-cmax) and electron transport (J(max)) rates in irrigation-stop recovered to similar levels as to before the irrigation-stop. These results suggest that long-term release from soil drought reduces the sensitivity to VPD and that atmospheric constraints may play an increasingly important role in combination with soil drought. Moreover, our study indicates that structural adjustments lead to an attenuation of initially strong leaf-level acclimation to strong multiple-year drought. Acclimation to irrigation increased gas exchange in Pinus sylvestris, but reduced the sensitivity to short-term changes. In addition, structural adjustments led to an attenuation of initially strong leaf-level acclimation.

1. Despite increasing reports of severe drought and heat impacts on forest ecosystems, community-level processes, which could potentially modulate tree responses to climatic stress, are rarely accounted for. While numerous studies indicate a positive effect of species diversity on a wide range of ecosystem functions and services, little is known about how species interactions influence tree responses to climatic variability. 2. We quantified the intraspecific variation in 16 leaf and wood physiological, morphological and anatomical traits in mature beech trees (Fagus sylvatica L.) at six sites located along a climatic gradient in the French Alps. At each site, we studied pure beech and mixed stands with silver fir (Abies alba Mill.) or downy oak (Quercus pubescens Willd.). We tested how functional traits differed between the two species mixtures (pure vs. mixed stands) within each site and along the climatic gradient. 3. We found significant changes in many traits along the climatic gradient as conditions progressively got drier and warmer. Independent of the mixture, reduced leaf-level CO2 assimilation, stomatal size and thicker leaf cuticles, consistent with a more conservative resource use strategy, were found. At the drier sites, higher foliar stable carbon isotopic composition (δ13C), thicker mesophyll tissues and lower specific leaf area (SLA) in pure stands suggest that beech had more acquisitive traits there compared to mixed stands. At the wetter sites, trees in beech-silver fir mixtures had higher chlorophyll concentration, lower δ13C, larger xylem vessels and higher SLA, suggesting a more acquisitive resource use strategy in mixed stands than in pure stands. 4. Our work revealed that species interactions are significant modulators of functional traits, and that they can be just as important drivers of intraspecific trait variation as climatic conditions. We show that downy oak mixtures lead to an adaptive drought response by common beech in dry environments. In contrast, in milder climates, interactions with silver fir seem to increase beech resource acquisition and productivity. These findings highlight a strong context dependency and imply that incorporating local interspecific interactions in research on climate impacts could improve our understanding and predictions of forest dynamics., Functional Ecology, 35 (11), ISSN:0269-8463, ISSN:1365-2435

Short-term plant respiration (R) increases exponentially with rising temperature, but drought could reduce respiration by reducing growth and metabolism. Acclimation may alter these responses. We examined if species with different drought responses would differ in foliar R response to +4.8°C temperature and -45% precipitation in a field experiment with mature pinon and juniper trees, and if any differences between species were related to differences in photosynthesis rates, shoot growth and non-structural carbohydrates (NSCs). Short-term foliar R had a Q10 of 1.6 for pinon and 2.6 for juniper. Pinon foliar R did not respond to the +4.8°C temperatures, but R increased 1.4x for juniper. Across treatments, pinon foliage had higher growth, lower NSC content, 29% lower photosynthesis rates, and 44% lower R than juniper. Removing 45% precipitation had little impact on R for either species. Species differences in the response of R under elevated temperature were related to substrate availability and stomatal response to leaf water potential. Despite not acclimating to the higher temperature and having higher R than pinon, greater substrate availability in juniper suggests it could supply respiratory demand for much longer than pinon. Species responses will be critical in ecosystem response to a warmer climate. This article is protected by copyright. All rights reserved.

Drylands play a dominant role in global carbon cycling and are particularly vulnerable to increasing temperatures, but our understanding of how dryland ecosystems will respond to climatic change remains notably poor. Considering that the area of drylands is projected to increase by 11%–23% by 2,100, understanding the impacts of warming on the functions and services furnished by these arid and semi‐arid ecosystems has numerous implications. In a unique 13‐year ecosystem warming experiment in a south‐western U.S. dryland, we investigated the consequences of rising temperature on Achnatherum hymenoides, a widespread, keystone grass species on the Colorado Plateau. We tracked individual‐ and population‐level responses to identify optimal strategies that may have been masked if considering only one level of plant response. We found several factors combined to affect the timing and magnitude of plant responses during the 13th year of warming. These included large warming‐induced biomass increases for individual plants, an 8.5‐day advancement in the growing season and strong reductions in photosynthetic rates and population cover. Importantly, we observed a lack of photosynthetic acclimation and, thus, a warming‐induced downregulation of photosynthetic rates. However, these physiological responses were concurrent with warmed‐plant increases in growing season length and investment in photosynthetic surfaces, demonstrating the species’ ability to balance carbon fixation limitations with warming. These results, which bring together ecophysiological, phenological, reproductive and morphological assessments of plant responses to warming, suggest that the extent of change in A. hymenoides populations will be based upon numerous adaptive responses that vary in their direction and magnitude. Plant population responses to climatic warming remain poorly resolved, particularly for Earth's drylands, and our in situ experiment assessing multiple strategies offers a novel look into a warmer world. A free Plain Language Summary can be found within the Supporting Information of this article.

Droughts are a rising concern for terrestrial ecosystems, particularly for forests where drought-induced reductions in tree growth and survival are reported. Biodiversity has long been acknowledged as an important component modulating ecosystem functions, including mitigating their vulnerability to climate-related stresses. Yet the impact of tree diversity on forest vulnerability to drought is unclear. In this review, consistent mechanisms are identified by which tree diversity could reduce vulnerability to drought and emerging evidence is revealed that tree diversity is not systematically positively related to drought resistance in forests. A path is suggested to further increase our knowledge on this subject in the face of climate change, proposing standardization of methods to quantitatively establish diversity impacts on the drought resistance of forests.

SummaryThe future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees’ sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil moisture is largely unknown.Leaf gas exchange measurements were performed within a long-term (17 years) irrigation experiment in a Scots pine-dominated forest in one of Switzerland’s driest areas on trees in naturally dry (control), irrigated, and‘irrigation-stop’ (after 11 years of irrigation) conditions.Seventeen years of irrigation increased photosynthesis (A) and stomatal conductance (gs) and reduced the gs sensitivity to increasing VPD but not to soil drying. Following irrigation-stop, gas exchange did not decrease immediately, but after three years, had decreased significantly in irrigation-stop trees. Vcmax and Jmax recovered after five years.These results suggest that long-term release of soil drought reduces the sensitivity to atmospheric evaporative demand and that atmospheric constraints may play an increasingly important role in combination with soil drought. In addition, they suggest that structural adjustments lead to an attenuation of initially strong leaf-level acclimation to strong multiple-year drought.

C.G. was supported by the Swiss National Science Foundation SNF (5231.00639.001.01). A.V. was supported by WSL visiting fellowship and IMAGINA project (Prometeo program 2019/110-Generalitat Valenciana). The CEAM Foundation is supported by Generalitat Valenciana.

Continuous and long-term monitoring of water use are required to reduce uncertainties in modelling forest transpiration. Since drought threatens the vitality and survival of forests worldwide, understanding and modelling responses to drought are of particular interest. Tree species undergo strong selective pressure to develop specialized mechanisms for regulating water-use dynamics during unfavourable climatic conditions. To cope with drought a tree can adjust its “water-use strategy”, by 1) altering the regulation of water release through the leaves to the atmosphere, 2) adjusting the water storage capacitances, or 3) changing the hydraulic conductivity of the xylem, impacting the water flux. There is thus a pressing need to understand the variability of such hydraulic mechanisms, between and within tree species, and quantify how they impact forest transpiration.We strive to elucidate hydraulic mechanisms in European tree species by combining, for the first time, three hydraulic components (stomatal conductance regulation, storage water capacity and wood anatomical traits) to identify water-use strategies and mechanistically model their effect on water use under increasing drought and warming. We constructed a European monitoring network, integrating ongoing meteorological measurements (e.g., temperature, relative humidity, global radiation and soil moisture) with sap flow (SF) and dendrometer (DM) measurements, as well as wood anatomical properties collected from the same tree individuals. Currently, the network includes 22 sites stretching from Spain till Finland (latitudinal range: 40° - 62° N), with a total of 281 individuals (14 tree species) and hourly-resolution monitoring of SF and DM from ~2011-2018. This large temporal coverage ensures a broad range of dry and wet conditions at each site, while the extensive climatological range of sites promotes the detection of intra-specific variability in hydraulic mechanisms.Focussing on four common European tree species (Fagus sylvatica, Quercus petraea, Pinus sylvestris and Picea abies), we present initial results from a Swiss temperate forest, where combining SF, DM and wood anatomy allowed us to disentangle species-specific differences in water-use strategies. Building upon these empirical observations, we were able to quantify the impact of these inter-specific differences on water use. Moreover, a mechanistic water transport model was used to assess stem water content, stem water potential (i.e., an indicator for hydraulic vulnerability), and subsequently turgidity within the cambium (i.e., crucial for wood formation) during the summer drought of 2015. Our efforts will advance process-based understanding of drought impacts on water use and could constrain predictions of forest transpiration under changing climatic conditions.

Shifting forest dynamics Forest dynamics are the processes of recruitment, growth, death, and turnover of the constituent tree species of the forest community. These processes are driven by disturbances both natural and anthropogenic. McDowell et al. review recent progress in understanding the drivers of forest dynamics and how these are interacting and changing in the context of global climate change. The authors show that shifts in forest dynamics are already occurring, and the emerging pattern is that global forests are tending toward younger stands with faster turnover as old-growth forest with stable dynamics are dwindling. Science , this issue p. eaaz9463

The leaf area to sapwood area ratios of trees (Al:AS) can shift to maintain homeostatic gas exchange per unit leaf area in response to climate variability. We tested the hypothesis that trees alter their Al:AS ratios in response to long-term warming and reduced precipitation in order to maintain leaf-specific gas exchange rates under more stressful conditions. Whole-tree Al:AS was measured on mature piñon pine (Pinus edulis Engelm.) and one-seed juniper (Juniperus monosperma (Engelm.) Sarg.) trees after 5 years (2012-16) of chronic exposure to increased temperature (+4.8 °C), precipitation reduction (-45%), or both simultaneously. No difference was found in Al:As among treatments for either species. Associated with this lack of shift in Al:As were large changes in pre-dawn leaf water potential and stomatal conductance, consistent with theoretical expectations of interactions between leaf and whole-tree hydraulic supply. Our results suggest that a lack of whole-tree acclimation in Al:As results in the reductions in plant gas exchange and water status associated with long-term warming and reduced precipitation in semi-arid woodlands.

International audience; Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.e., vapor pressure deficit (VPD)] will be increasingly important for plant functioning, becoming the major control of plant water use in the twenty-first century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime [mean annual precipitation (MAP); seasonal drought intensity (PDRY)] and two functional traits related to foliar and wood economics spectra [leaf mass per area (LMA); wood specific gravity (WSG)]. We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates.

It has been known for a long time that drought intensity is a critical variable in determining water stress of Mediterranean tree species. However, not as much attention has been paid to other drought characteristics, for example the timing of the dry periods. We investigated the impact of the timing and intensity of extreme droughts on growing season length, growth and water-use efficiency of three tree species, Pinus nigra ssp. Salzmannii J.F. Arnold, Quercus ilex ssp. ballota (Desf.) Samp. and Quercus faginea Lam. coexisting in a continental Mediterranean ecosystem. Over the study period (2009-13), intense droughts were observed at annual and seasonal scales, particularly during 2011 and 2012. In 2012, an atypically dry winter and spring was followed by an intense summer drought. Quercus faginea growth was affected more by drought timing than by drought intensity, probably because of its winter-deciduous leaf habit. Pinus nigra showed a lower decrease in secondary growth than observed in the two Quercus species in extremely dry years. Resilience to extreme droughts was different among species, with Q. faginea showing poorer recovery of growth after very dry years. The highest intra- and inter-annual plasticity in water-use efficiency was observed in P. nigra, which maintained a more water-saving strategy. Our results revealed that the timing of extreme drought events can affect tree function to a larger extent than drought intensity, especially in deciduous species. Legacy effects of drought over months and years significantly strengthened the impact of drought timing and intensity on tree function., This work was supported by the Spanish Ministry of Economy and Competitiveness with the grants VERONICA (CGL2013-42271-P), SEDIFOR (AGL2014-57762-R), the project of the Autonomous Community of Madrid REMEDINAL3-CM (S2013/MAE-2719) and ECOMETAS (CGL2014-53840-REDT). D.B. and A.G. were supported by the Laboratory of Excellence ARBRE (ANR-12-LABXARBRE-01) supported by the French National Research Agency. The PTEF is supported by the French National Research Agency through the Laboratory of Excellence ARBRE (ANR-11-LABX-0002-01).

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (FD ), stomatal conductance (Gs ), hydraulic conductivity (KL ) and xylem anatomy in pinon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower Gs and FD were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of Gs and FD to ambient levels only for pinon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced FD , Gs and KL . Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in Gs , FD and KL took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi-arid regions.

Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD). VPD has been identified as an increasingly important driver of plant functioning in terrestrial biomes and has been established as a major contributor in recent drought-induced plant mortality independent of other drivers associated with climate change. Despite this, few studies have isolated the physiological response of plant functioning to high VPD, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. An abundance of evidence suggests that stomatal conductance declines under high VPD and transpiration increases in most species up until a given VPD threshold, leading to a cascade of subsequent impacts including reduced photosynthesis and growth, and higher risks of carbon starvation and hydraulic failure. Incorporation of photosynthetic and hydraulic traits in 'next-generation' land-surface models has the greatest potential for improved prediction of VPD responses at the plant- and global-scale, and will yield more mechanistic simulations of plant responses to a changing climate. By providing a fully integrated framework and evaluation of the impacts of high VPD on plant function, improvements in forecasting and long-term projections of climate impacts can be made.

Trees may survive prolonged droughts by shifting water uptake to reliable water sources, but it is unknown if the dominant mechanism involves activating existing roots or growing new roots during drought, or some combination of the two. To gain mechanistic insights on this unknown, a dynamic root-hydraulic modeling framework was developed that set up a feedback between hydraulic controls over carbon allocation and the role of root growth on soil-plant hydraulics. The new model was tested using a 5 yr drought/heat field experiment on an established pinon-juniper stand with root access to bedrock groundwater. Owing to the high carbon cost per unit root area, modeled trees initialized without adequate bedrock groundwater access experienced potentially lethal declines in water potential, while all of the experimental trees maintained nonlethal water potentials. Simulated trees were unable to grow roots rapidly enough to mediate the hydraulic stress, particularly during warm droughts. Alternatively, modeled trees initiated with root access to bedrock groundwater matched the hydraulics of the experimental trees by increasing their water uptake from bedrock groundwater when soil layers dried out. Therefore, the modeling framework identified a critical mechanism for drought response that required trees to shift water uptake among existing roots rather than growing new roots.

Understanding how climate alters plant-soil water dynamics, and its impact on physiological functions, is critical to improved predictions of vegetation responses to climate change. Here we analyzed how belowground interactions for water shift under warming and drought, and associated impacts on plant functions. In a semi-arid woodland, adult trees (pinon and juniper) and perennial grasses (blue grama) were exposed to warming and precipitation reduction. After 6 years of continuous treatment exposure, soil and plant water isotopic composition was measured to assess plant water uptake depths and community-level water source partitioning. Warming and drought modified plant water uptake depths. Under warming, contrasting changes in water sources between grasses and trees reduced belowground water source partitioning, resulting in higher interspecific competition for water. Under drought, shifts in trees and grass water sources to deeper soil layers resulted in the maintenance of the naturally occurring water source partitioning among species. Trees showed higher water stress, and reduced water use and photosynthesis in response to warming and drought. This case study demonstrates that neighboring plants shift their competitive interactions for water under prolonged warming and drought, but regardless of whether changes in moisture sources will result in increased competition among species or maintained partitioning of water resources, these competitive adaptations may easily be overridden by climate extremes.

Stomata regulate CO2 uptake for photosynthesis and water loss through transpiration. The approaches used to represent stomatal conductance (gs ) in models vary. In particular, current understanding of drivers of the variation in a key parameter in those models, the slope parameter (i.e. a measure of intrinsic plant water-use-efficiency), is still limited, particularly in the tropics. Here we collected diurnal measurements of leaf gas exchange and leaf water potential (Ψleaf ), and a suite of plant traits from the upper canopy of 15 tropical trees in two contrasting Panamanian forests throughout the dry season of the 2016 El Nino. The plant traits included wood density, leaf-mass-per-area (LMA), leaf carboxylation capacity (Vc,max ), leaf water content, the degree of isohydry, and predawn Ψleaf . We first investigated how the choice of four commonly used leaf-level gs models with and without the inclusion of Ψleaf as an additional predictor variable influence the ability to predict gs , and then explored the abiotic (i.e. month, site-month interaction) and biotic (i.e. tree-species-specific characteristics) drivers of slope parameter variation. Our results show that the inclusion of Ψleaf did not improve model performance and that the models that represent the response of gs to vapor pressure deficit performed better than corresponding models that respond to relative humidity. Within each gs model, we found large variation in the slope parameter, and this variation was attributable to the biotic driver, rather than abiotic drivers. We further investigated potential relationships between the slope parameter and the six available plant traits mentioned above, and found that only one trait, LMA, had a significant correlation with the slope parameter (R2 = 0.66, n = 15), highlighting a potential path towards improved model parameterization. This study advances understanding of gs dynamics over seasonal drought, and identifies a practical, trait-based approach to improve modeling of carbon and water exchange in tropical forests.

Plant waxes, including n-alkanes, are commonly used for a wide range of paleo-applications. Several common traits of n-alkanes that are used as paleo-proxies include chain length distribution and average chain length (ACL), as well as plant wax carbon and hydrogen isotopic compositions. The effect of climate on plant wax traits has been the subject of many studies, but a common challenge with modern calibrations is disentangling the effects of species (genetic), temperature, and precipitation from one another. Here, we explore the effect of temperature and drought, independently and combined, on plant wax composition of the species Juniper monosperma in a large ecosystem-scale field manipulation experiment. We find that n-alkane concentrations significantly increase with temperature, but other parameters (including ACL) are not affected. These results support physiological studies that identify n-alkanes as an important barrier to water loss within the plant cuticle. Combined with prior studies, it appears that changes in ACL within sediments are likely controlled by changes in species composition rather than directly by changes in climate. We find little variation in the carbon isotopic composition (δ13C) of n-alkanes across the treatments whereas bulk leaf δ13C values are higher in the heat and drought treatment. Because leaf δ13C values represent a weighted C assimilation signal, these values reflect differences in leaf gas exchange among treatments, whereas the n-alkanes are synthesized when water availability is higher and differences among treatments are not significant enough to influence their values. These results have important implications for using n-alkane traits, including ACL and δ13C values, for paleoenvironmental reconstructions.

Coastal shoreline forests are vulnerable to seawater exposure, the impacts of which will increase due to sea-level rise, but the long-term adaptation strategies and vulnerability of coastal forests are not well understood. We used whole-tree transpiration, leaf water potential, tree-ring width, and tree-ring δ13C (a proxy for intrinsic water use efficiency, iWUE) to examine the long-term adaptation strategies of red maple (Acer rubrum) trees at the coastal interface (i.e., shoreline) and nearby upland in Maryland, USA. Red maple trees that grew along the shoreline and were exposed to slightly saline water (up to two PSU) had higher transpiration rates than those growing in the nearby upland forest during a wet year, but these differences disappeared during a normal precipitation year. Shoreline trees grew more slowly than upland trees over the last four decades, but these growth differences have disappeared in the last six years. Shoreline and upland red maple trees had similar variation in iWUE, indicating that higher transpiration rates of the seawater-exposed trees did not translate into differences in water use efficiency. There were no differences in predawn and midday water potential between upland and shoreline trees, suggesting no additional water stress occurs in shoreline trees. These findings indicate that mature red maple in our coastal study site maintains gas exchange and growth at a consistent or homeostatic level under slight soil salinity.

Summary Model scenarios of climate change predict that warming and drought will occur simultaneously in the future in many regions. The capacity of woody species to modify their physiology and morphology in response to environmental conditions is widely recognized, but little is known about the responses of trees to reduced precipitation and increased temperature acting simultaneously. In a semi-arid woodland, we assessed the responses in physiological (needle emergence, maximum photosynthesis, stomatal conductance, water use efficiency (WUE) and shoot elongation) and morphological (needle length and thickness, and leaf mass per area (LMA)) foliar traits of pinon pine (Pinus edulis) in response to three years of a 45% reduction in precipitation, a 4.8 °C increase in air temperature and their simultaneous effects. A strong change in physiological and morphological traits in response to reduced precipitation was observed. Precipitation reduction delayed needle emergence, decreased photosynthesis and stomatal conductance, increased WUE, decreased shoot elongation and induced shorter needles with a higher LMA. Trees subjected to simultaneous reductions in precipitation and warming demonstrated a similar response. However, atmospheric warming did not induce a response in any of the measured traits. Physiological and morphological traits of trees in this semi-arid climate were more responsive to changes in soil moisture than air temperature. Long-term exposure to seasonal drought stress in arid sites may have resulted in strong plastic responses to this first stressor. However, atmospheric warming probably was not experienced as a stress for trees in this warm and dry climate. Overall, our results indicate that in semi-arid ecosystems where tree functioning is already highly limited by soil water availability, atmospheric warming as anticipated with climate change may have less impact on foliar trait responses than previously thought.

Climate warming should result in hotter droughts of unprecedented severity in this century. Such droughts have been linked with massive tree mortality, and data suggest that warming interacts with drought to aggravate plant performance. Yet how forests will respond to hotter droughts remains unclear, as does the suite of mechanisms trees use to deal with hot droughts. We used an ecosystem‐scale manipulation of precipitation and temperature on piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees to investigate nitrogen (N) cycling‐induced mitigation processes related to hotter droughts. We found that while negative impacts on plant carbon and water balance are manifest after prolonged drought, performance reductions were not amplified by warmer temperatures. Rather, increased temperatures for 5 years stimulated soil N cycling under piñon trees and modified tree N allocation for both species, resulting in mitigation of hotter drought impacts on tree water and carbon functions. These findings suggest that adjustments in N cycling are likely after multi‐year warming conditions and that such changes may buffer reductions in tree performance during hotter droughts. The results highlight our incomplete understanding of trees' ability to acclimate to climate change, raising fundamental questions about the resistance potential of forests to long‐term, compound climatic stresses. Pacific Northwest National Laboratories; Swiss Federal Research Institute; Swiss Forest Lab; Swiss National Science Foundation SNF, Grant/Award Number: 31003A_159866; U.S. Geological Survey; U.S. Department of Energy Office of Science, Grant/Award Number: DESC‐0008168; Spanish Government, Grant/Award Number: CGL2015‐69773‐C2‐2‐P; Generalitat Valenciana, Grant/Award Number: BEST/2016/289; Los Alamos National Laboratory.

No claim to original US government works New Phytologist © 2018 New Phytologist Trust Tree mortality rates appear to be increasing in moist tropical forests (MTFs) with significant carbon cycle consequences. Here, we review the state of knowledge regarding MTF tree mortality, create a conceptual framework with testable hypotheses regarding the drivers, mechanisms and interactions that may underlie increasing MTF mortality rates, and identify the next steps for improved understanding and reduced prediction. Increasing mortality rates are associated with rising temperature and vapor pressure deficit, liana abundance, drought, wind events, fire and, possibly, CO2 fertilization-induced increases in stand thinning or acceleration of trees reaching larger, more vulnerable heights. The majority of these mortality drivers may kill trees in part through carbon starvation and hydraulic failure. The relative importance of each driver is unknown. High species diversity may buffer MTFs against large-scale mortality events, but recent and expected trends in mortality drivers give reason for concern regarding increasing mortality within MTFs. Models of tropical tree mortality are advancing the representation of hydraulics, carbon and demography, but require more empirical knowledge regarding the most common drivers and their subsequent mechanisms. We outline critical datasets and model developments required to test hypotheses regarding the underlying causes of increasing MTF mortality rates, and improve prediction of future mortality under climate change.