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Trees are long-lived organisms that integrate climate conditions across years or decades to produce secondary growth. This integration process is sometimes referred to as ‘climatic memory.’ While widely perceived, the physiological processes underlying this temporal integration, such as the storage and remobilization of non-structural carbohydrates (NSC), are rarely explicitly studied. This is perhaps most apparent when considering drought legacies (perturbed post-drought growth responses to climate), and the physiological mechanisms underlying these lagged responses to climatic extremes. Yet, drought legacies are likely to become more common if warming climate brings more frequent drought. To quantify the linkages between drought legacies, climate memory and NSC, we measured tree growth (via tree ring widths) and NSC concentrations in three dominant species across the southwestern USA. We analyzed these data with a hierarchical mixed effects model to evaluate the time-scales of influence of past climate (memory) on tree growth. We then evaluated the role of climate memory and the degree to which variation in NSC concentrations were related to forward-predicted growth during the hot 2011–2012 drought and subsequent 4-year recovery period. Populus tremuloides exhibited longer climatic memory compared to either Pinus edulis or Juniperus osteosperma, but following the 2011–2012 drought, P. tremuloides trees with relatively longer memory of temperature conditions showed larger (more negative) drought legacies. Conversely, Pinus edulis trees with longer temperature memory had smaller (less negative) drought legacies. For both species, higher NSC concentrations followed more negative (larger) drought legacies, though the relevant NSC fraction differed between P. tremuloides and P. edulis. Our results suggest that differences in tree NSC are also imprinted upon tree growth responses to climate across long time scales, which also underlie tree resilience to increasingly frequent drought events under climate change.

Widespread tree mortality following droughts has emerged as an environmentally and economically devastating 'ecological surprise'. It is well established that tree physiology is important in understanding drought-driven mortality; however, the accuracy of predictions based on physiology alone has been limited. We propose that complicating factors at two levels stymie predictions of drought-driven mortality: (i) organismal-level physiological and site factors that obscure understanding of drought exposure and vulnerability and (ii) community-level ecological interactions, particularly with biotic agents whose effects on tree mortality may reverse expectations based on stress physiology. We conclude with a path forward that emphasizes the need for an integrative approach to stress physiology and biotic agent dynamics when assessing forest risk to drought-driven morality in a changing climate.

In trees, large uncertainties remain in how nonstructural carbohydrates (NSCs) respond to variation in water availability in natural, intact ecosystems. Variation in NSC pools reflects temporal fluctuations in supply and demand, as well as physiological coordination across tree organs in ways that differ across species and NSC fractions (e.g., soluble sugars vs starch). Using landscape-scale crown (leaves and twigs) NSC concentration measurements in three foundation tree species (Populus tremuloides, Pinus edulis, Juniperus osteosperma), we evaluated in situ, seasonal variation in NSC responses to moisture stress on three timescales: short-term (via predawn water potential), seasonal (via leaf δ13C) and annual (via current year’s ring width index). Crown NSC responses to moisture stress appeared to depend on hydraulic strategy, where J. osteosperma appears to regulate osmotic potentials (via higher sugar concentrations), P. edulis NSC responses suggest respiratory depletion and P. tremuloides responses were consistent with direct sink limitations. We also show that overly simplistic models can mask seasonal and tissue variation in NSC responses, as well as strong interactions among moisture stress at different timescales. In general, our results suggest large seasonal variation in crown NSC concentrations reflecting the multiple cofunctions of NSCs in plant tissues, including storage, growth and osmotic regulation of hydraulically vulnerable leaves. We emphasize that crown NSC pool size cannot be viewed as a simple physiological metric of stress; in situ NSC dynamics are complex, varying temporally, across species, among NSC fractions and among tissue types.

Drought-induced tree mortality is projected to increase due to climate change, which will have manifold ecological and societal impacts including the potential to weaken or reverse the terrestrial carbon sink. Predictions of tree mortality remain limited, in large part because within-species variations in ecophysiology due to plasticity or adaptation and ecosystem adjustments could buffer mortality in dry locations. Here, we conduct a meta-analysis of 50 studies spanning >100 woody plant species globally to quantify how populations within species vary in vulnerability to drought mortality and whether functional traits or climate mediate mortality patterns. We find that mortality predominantly occurs in drier populations and this pattern is more pronounced in species with xylem that can tolerate highly negative water potentials, typically considered to be an adaptive trait for dry regions, and species that experience higher variability in water stress. Our results indicate that climate stress has exceeded physiological and ecosystem-level tolerance or compensating mechanisms by triggering extensive mortality at dry range edges and provides a foundation for future mortality projections in empirical distribution and mechanistic vegetation models.

Widespread drought-induced forest mortality (DIM) is expected to increase with climate change and drought, and is expected to have major impacts on carbon and water cycles. For large-scale assessment and management, it is critical to identify variables that integrate the physiological mechanisms of DIM and signal risk of DIM. We tested whether plant water content, a variable that can be remotely sensed at large scales, is a useful indicator of DIM risk at the population level. We subjected Pinus ponderosa Douglas ex C. Lawson seedlings to experimental drought using a point of no return experimental design. Periodically during the drought, independent sets of seedlings were sampled to measure physiological state (volumetric water content (VWC), percent loss of conductivity (PLC) and non-structural carbohydrates) and to estimate population-level probability of mortality through re-watering. We show that plant VWC is a good predictor of population-level DIM risk and exhibits a threshold-type response that distinguishes plants at no risk from those at increasing risk of mortality. We also show that plant VWC integrates the mechanisms involved in individual tree death: hydraulic failure (PLC), carbon depletion across organs and their interaction. Our results are promising for landscape-level monitoring of DIM risk.

Summary Determining physiological mechanisms and thresholds for climate‐driven tree die‐off could help improve global predictions of future terrestrial carbon sinks. We directly tested for the lethal threshold in hydraulic failure – an inability to move water due to drought‐induced xylem embolism – in a pine sapling experiment.In a glasshouse experiment, we exposed loblolly pine (Pinus taeda) saplings (n = 83) to drought‐induced water stress ranging from mild to lethal. Before rewatering to relieve drought stress, we measured native hydraulic conductivity and foliar color change. We monitored all measured individuals for survival or mortality.We found a lethal threshold at 80% loss of hydraulic conductivity – a point of hydraulic failure beyond which it is more likely trees will die, than survive, and describe mortality risk across all levels of water stress. Foliar color changes lagged behind hydraulic failure – best predicting when trees had been dead for some time, rather than when they were dying.Our direct measurement of native conductivity, while monitoring the same individuals for survival or mortality, quantifies a continuous probability of mortality risk from hydraulic failure. Predicting tree die‐off events and understanding the mechanism involved requires knowledge not only of when trees are dead, but when they begin dying – having passed the point of no return.

© 2019 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd. Drought-induced tree mortality is expected to increase in future climates with the potential for significant consequences to global carbon, water, and energy cycles. Xylem embolism can accumulate to lethal levels during drought, but species that can refill embolized xylem and recover hydraulic function may be able to avoid mortality. Yet the potential controls of embolism recovery, including cross-biome patterns and plant traits such as nonstructural carbohydrates (NSCs), hydraulic traits, and nocturnal stomatal conductance, are unknown. We exposed eight plant species, originating from mesic (tropical and temperate) and semi-arid environments, to drought under ambient and elevated CO 2 levels, and assessed recovery from embolism following rewatering. We found a positive association between xylem recovery and NSCs, and, surprisingly, a positive relationship between xylem recovery and nocturnal stomatal conductance. Arid-zone species exhibited greater embolism recovery than mesic zone species. Our results indicate that nighttime stomatal conductance often assumed to be a wasteful use of water, may in fact be a key part of plant drought responses, and contribute to drought survival. Findings suggested distinct biome-specific responses that partially depended on species climate-of-origin precipitation or aridity index, which allowed some species to recover from xylem embolism. These findings provide improved understanding required to predict the response of diverse plant communities to drought. Our results provide a framework for predicting future vegetation shifts in response to climate change.

The circadian clock is a molecular timer of metabolism that affects the diurnal pattern of stomatal conductance (gs), amongst other processes, in a broad array of plant species. The function of circadian gs regulation remains unknown and here, we test whether circadian regulation helps to optimize diurnal variations in stomatal conductance. We subjected bean (Phaseolus vulgaris) and cotton (Gossypium hirsutum) canopies to fixed, continuous environmental conditions of photosynthetically active radiation, temperature, and vapour pressure deficit (free-running conditions) over 48 h. We modelled gs variations in free-running conditions to test for two possible optimizations of stomatal behaviour under circadian regulation: (i) that stomata operate to maintain constant marginal water use efficiency, or (ii) that stomata maximize C net gain minus the costs or risks of hydraulic damage. We observed that both optimization models predicted gs poorly under free-running conditions, indicating that circadian regulation does not directly lead to stomatal optimization. We also demonstrate that failure to account for circadian variation in gs could potentially lead to biased parameter estimates during calibrations of stomatal models. More broadly, our results add to the emerging field of plant circadian ecology, where circadian controls may partially explain leaf-level patterns observed in the field.

The Craig-Gordon type (C-G) leaf water isotope enrichment models assume a homogeneous distribution of enriched water across the leaf surface, despite observations that Δ18 O can become increasingly enriched from leaf base to tip. Datasets of this 'progressive isotope enrichment' are limited, precluding a comprehensive understanding of (a) the magnitude and variability of progressive isotope enrichment, and (b) how progressive enrichment impacts the accuracy of C-G leaf water model predictions. Here, we present observations of progressive enrichment in two conifer species that capture seasonal and diurnal variability in environmental conditions. We further examine which leaf water isotope models best capture the influence of progressive enrichment on bulk needle water Δ18 O. Observed progressive enrichment was large and equal in magnitude across both species. The magnitude of this effect fluctuated seasonally in concert with vapour pressure deficit, but was static in the face of diurnal cycles in meteorological conditions. Despite large progressive enrichment, three variants of the C-G model reasonably successfully predicted bulk needle Δ18 O. Our results thus suggest that the presence of progressive enrichment does not impact the predictive success of C-G models, and instead yields new insight regarding the physiological and anatomical mechanisms that cause progressive isotope enrichment.

A better understanding of plant stomatal strategies holds strong promise for improving predictions of vegetation responses to drought because stomata are the primary mechanism through which plants mitigate water stress. It has been assumed that plants regulate stomata to maintain a constant marginal water use efficiency and forego carbon gain when water is scarce. However, recent hypotheses pose that plants maximize carbon assimilation while also accounting for the risk of hydraulic damage via cavitation and hydraulic failure. This "gain-risk" framework incorporates competition in stomatal regulation because it takes into account that neighboring plants can "steal" unused water. This study utilizes stomatal models representing both the water use efficiency and carbon-maximization frameworks, and empirical data from three species in a potted growth chamber experiment, to investigate the effects of drought and competition on seedling stomatal strategy. We found that drought and competition responses in the empirical data were best explained by the carbon-maximization hypothesis and that both drought and competition affected stomatal strategy. Interestingly, stomatal responses differed substantially by species, with seedlings employing a riskier strategy when planted with a high water use competitor, and seedlings employing a more conservative strategy when planted with a low water use competitor. Lower water users in general had less stomatal sensitivity to decreasing Ψ L compared to moderate to high water users. Repeated water stress also resulted in legacy effects on plant stomatal behavior, increasing stomatal sensitivity (i.e., conservative behavior) even when the seedling was returned to well-watered conditions. These results indicate that stomatal strategies are dynamic and change with climate and competition stressors. Therefore, incorporating mechanisms that allow for stomatal behavioral changes in response to water limitation may be an important step to improving carbon cycle projections in coupled climate-Earth system models.

Adaptation across climate gradients can provide the raw material needed for evolutionary response to climate change. In migratory species, studies of local climate adaptation are made challenging by seasonal movement, where it is unclear to what extent individuals track their local climate niches across the annual cycle. In the migratory songbird yellow warbler ( Setophaga petechia ), we test the hypothesis that individuals track similar climates between their breeding and wintering ranges. Further, we examine whether adaptation to local climate might lead to morphological differences among populations and different demographic responses to temporal climate variability. We find a correlation between wintering and breeding precipitation but not temperature regimes at the level of the individual bird. Specifically, birds from the driest wintering regions migrate to the driest breeding regions. Additionally, we find an association between bill size and breeding season precipitation which, given documented climate-associated genomic variation, suggests adaptation to local precipitation gradients might exist on the breeding grounds. Finally, we show geographic variation in the effect of precipitation on demography, with higher precipitation associated with population increases in some regions and declines in others. Taken together, our results suggest that variation in climate optima exists across the breeding range of yellow warblers and provide a potential mechanism for parallel selection across the annual cycle.

Species interactions mediate tree responses to water limitation because competition and/or facilitation alter plant physiology and growth. However, because it is difficult to isolate the effects of plant–plant interactions and water limitation from other environmental factors, the mechanisms underlying tree physiology and growth in coexisting plants under drought are poorly understood. We investigated how species interactions and water limitation impact the physiology and growth of trembling aspen (Populus tremuloides), narrowleaf cottonwood (Populus angustifolia) and ponderosa pine (Pinus ponderosa) seedlings in a controlled environment growth chamber, using aspen as a focal species. Seedlings were grown in pots alone or with a con- or hetero-specific seedling, and were subjected to a water limitation treatment. Growth, water status and physiological traits were measured before, during and after the treatment. Under well-watered conditions, the presence of another seedling affected growth or biomass allocation in all species, but did not impact the physiological traits we measured. Under water limitation, the presence of a competing seedling had a marginal impact on seedling growth and physiological traits in all species. Throughout the study, the magnitude and direction of seedling responses were complex and often species-specific. Our study serves as an important step toward testing how species’ interactions modify physiological responses and growth in well-watered and water-limited periods.

The mechanisms governing tree drought mortality and recovery remain a subject of inquiry and active debate given their role in the terrestrial carbon cycle and their concomitant impact on climate change. Counter-intuitively, many trees do not die during the drought itself. Indeed, observations globally have documented that trees often grow for several years after drought before mortality. A combination of meta-analysis and tree physiological models demonstrate that optimal carbon allocation after drought explains observed patterns of delayed tree mortality and provides a predictive recovery framework. Specifically, post-drought, trees attempt to repair water transport tissue and achieve positive carbon balance through regrowing drought-damaged xylem. Furthermore, the number of years of xylem regrowth required to recover function increases with tree size, explaining why drought mortality increases with size. These results indicate that tree resilience to drought-kill may increase in the future, provided that CO2 fertilisation facilitates more rapid xylem regrowth.

Future droughts are expected to become more severe and frequent under future climate change scenarios, likely causing widespread tree mortality in the western USA. Coping with an uncertain future requires an understanding of long-term ecosystem responses in areas where prolonged drought is projected to increase. Tree-ring records are ideally suited for this task. We developed 24 tree-ring chronologies from 20 U.S. Forest Service Forest Inventory and Analysis (FIA) plots in the southwestern USA. Climate variables were derived from the PRISM climate dataset (800-m grid cells) to capture the bimodal precipitation regime of winter snow and summer monsoonal rainfall, as well as warm-season vapor-pressure deficit (VPD) and winter minimum temperature. Based on mixed linear models, radial growth from 1948 to 2013 for four conifer species (Pinus edulis, Juniperus osteosperma, Pinus ponderosa, and Picea engelmannii) responded negatively to warm-season VPD and positively to cold-season precipitation. Pinus spp. benefited from warm-season precipitation linked to the North American monsoon, and Pinus spp. and J. osteosperma radial growth increased with warmer cold-season minimum temperature. However, warmer cold-season minimum temperatures countered the beneficial influence of cold-season precipitation for radial growth in Pinus spp. and J. osteosperma, while P. engelmannii was unaffected. Also, enhanced drying effects of warm-season VPD associated with decreased cold-season precipitation negatively affected radial growth of Pinus spp. and P. engelmannii. Of the four conifer species studied, Pinus spp. are most affected by droughts since 1948, while P. engelmannii and J. osteosperma appear to be more resilient. Investigating seasonal climate responses and interaction effects on radial growth in areas impacted by severe drought helps identify species that may be particularly at risk from climate change impacts in the Anthropocene.

We investigated the response of a model facultative CAM plant (Mesembryanthemum crystallinum) and its competition with a C3 grass (Bromus mollis) to changes in rainfall regime. Seedlings of M. crystallinum and B. mollis in both monoculture and mixtures growing in shallow and deep pots were subjected to three levels of intra-seasonal rainfall variability and rainfall seasonality in both high water and low water conditions. Response of plants were evaluated by nocturnal carboxylation and biomass. A high rate of water drainage beneath root zones in coarse soil led to a negative response of M. crystallinum and B. mollis in monoculture under increased intra-seasonal rainfall variability. Seasonal rainfall shifts to later dates during the growing season generally favored the growth of M. crystallinum and B. mollis in monoculture, with the exception of high water stress conditions whereby drought-intolerant species B. mollis was disfavored. Rainfall seasonality but not intra-seasonal rainfall variability affected nocturnal carboxylation by M. crystallinum in monoculture. We suggest that soil texture, root depth, and rainfall gradient are important mediators of plant growth under increased intra-seasonal rainfall variability. Drought severity and the ability of a plant to tolerate drought and can greatly affect its response to the seasonal timing of rainfall. Nocturnal carboxylation by M. crystallinum in response to rainfall variability depends on the timescale.

Author(s): Yu, Kailiang; D'Odorico, Paolo; Collins, Scott L; Carr, David; Porporato, Amilcare; Anderegg, William R. L; Gilhooly, William P; Wang, Lixin; Bhattachan, Abinash; Bartlett, Mark; Hartzell, Samantha; Yin, Jun; He, Yongli; Li, Wei; Tatlhego, Mokganedi; Fuentes, Jose D | Abstract: Plants with crassulacean acid metabolism (CAM) are increasing in distribution and abundance in drylands worldwide, but the underlying drivers remain unknown. We investigate the impacts of extreme drought and CO2 enrichment on the competitive relationships between seedlings of Cylindropuntia imbricata (CAM species) and Bouteloua eriopoda (C4 grass), which coexist in semiarid ecosystems across the Southwestern United States. Our experiments under altered water and CO2 water conditions show that C. imbricata positively responded to CO2 enrichment under extreme drought conditions, while B. eriopoda declined from drought stress and did not recover after the drought ended. Conversely, in well‐watered conditions B. eriopoda had a strong competitive advantage on C. imbricata such that the photosynthetic rate and biomass (per individual) of C. imbricata grown with B. eriopoda were lower relative to when growing alone. A meta‐analysis examining multiple plant families across global drylands shows a positive response of CAM photosynthesis and productivity to CO2 enrichment. Collectively, our results suggest that under drought and elevated CO2 concentrations, projected with climate change, the competitive advantage of plant functional groups may shift and the dominance of CAM plants may increase in semiarid ecosystems.

Climate change-driven drought stress has triggered numerous large-scale tree mortality events in recent decades. Advances in mechanistic understanding and prediction are greatly limited by an inability to detect in situ where trees are likely to die in order to take timely measurements and actions. Thus, algorithms of early warning and detection of drought-induced tree stress and mortality could have major scientific and societal benefits. Here, we leverage two consecutive droughts in the southwestern United States to develop and test a set of early warning metrics. Using Landsat satellite data, we constructed early warning metrics from the first drought event. We then tested these metrics' ability to predict spatial patterns in tree physiological stress and mortality from the second drought. To test the broader applicability of these metrics, we also examined a separate drought in the Amazon rainforest. The early warning metrics successfully explained subsequent tree mortality in the second drought in the southwestern US, as well as mortality in the independent drought in tropical forests. The metrics also strongly correlated with spatial patterns in tree hydraulic stress underlying mortality, which provides a strong link between tree physiological stress and remote sensing during the severe drought and indicates that the loss of hydraulic function during drought likely mediated subsequent mortality. Thus, early warning metrics provide a critical foundation for elucidating the physiological mechanisms underpinning tree mortality in mature forests and guiding management responses to these climate-induced disturbances.

The widely documented phenomenon of nighttime stomatal conductance gsn could lead to substantial water loss with no carbon gain, and thus it remains unclear whether nighttime stomatal conductance confers a functional advantage. Given that studies of gsn have focused on controlled environments or small numbers of species in natural environments, a broad phylogenetic and biogeographic context could provide insights into potential adaptive benefits of gsn . We measured gsn on a diverse suite of species (n = 73) across various functional groups and climates-of-origin in a common garden to study the phylogenetic and biogeographic/climatic controls on gsn and further assessed the degree to which gsn co-varied with leaf functional traits and daytime gas-exchange rates. Closely related species were more similar in gsn than expected by chance. Herbaceous species had higher gsn than woody species. Species that typically grow in climates with lower mean annual precipitation - where the fitness cost of water loss should be the highest - generally had higher gsn . Our results reveal the highest gsn rates in species from environments where neighboring plants compete most strongly for water, suggesting a possible role for the competitive advantage of gsn .

For nearly two decades, the American Southwest has been in the grips of a long-term ‘megadrought’, punctuated by a number of short and severe ‘global change type droughts’ (i.e., negative precipitation anomalies co-occurring with high temperature). These events have caused widespread mortality of the drought-sensitive pinon pine (Pinus edulis) while co-dominant Juniperus spp. have historically been much more drought tolerant and thus have rarely died. However, a severe drought occurred in 2018 that rapidly ( 60% canopy dieback) at hot, dry, low elevation sites, and was associated with drought-induced hydraulic damage. There was no evidence that biotic agents could be the primary drivers of this dieback, implicating the acute effects of drought as the main causal agent. The speed and scale of this drought-induced juniper dieback seems to be historically unprecedented in the region and foreshadows an uncertain future for pinon-juniper woodlands as the region continues to get warmer and drier.

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.

Aim The differential responses of plant species to climate change are of great interest and grave concern for scientists and conservationists. One underexploited resource for better understanding these changes are the records held by herbaria. Using these records to assess the responses of different groups of species across the entire flora of California, we sought to quantify the magnitude of species elevational shifts, to measure differences in shifts among functional groups and between native and introduced species, and to evaluate whether these shifts were related to the conservation of thermal niches. Location California. Methods To characterize these shifts in California, we used 681,609 georeferenced herbarium records to estimate mean shifts in elevational and climatic space of 4426 plant taxa. We developed and employed a statistical method to robustly analyse the data represented in these records. Results We found that 15% of all taxa in California have ranges that have shifted upward over the past century. There are significant differences between range shifts of taxa with different naturalization statuses: 12% of endemic taxa show significant upward range shifts, while a greater proportion (27%) of introduced taxa have shifted upward. We found significant differences between the proportion of significant range shifts across taxa with different seed sizes, but did not find evidence for differences in shift based on life-form (annual versus perennial, herbaceous versus woody). Main conclusions Our analyses suggest that introduced species have disproportionately expanded their ranges upward in elevation over the past century when compared with native species. While these shifts in introduced species may not be exclusively driven by climate, they highlight the importance of considering the interacting factors of climate-driven range shifts and invasion to understand how floras are responding in the face of anthropogenic change.

© 2018, The Ecological Society of Japan. 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.

Extreme drought is increasing in frequency and intensity in many regions globally, with uncertain consequences for the resistance and resilience of ecosystem functions, including primary production. Primary production resistance, the capacity to withstand change during extreme drought, and resilience, the degree to which production recovers, vary among and within ecosystem types, obscuring generalized patterns of ecological stability. Theory and many observations suggest forest production is more resistant but less resilient than grassland production to extreme drought; however, studies of production sensitivity to precipitation variability indicate that the processes controlling resistance and resilience may be influenced more by mean annual precipitation (MAP) than ecosystem type. Here, we conducted a global meta-analysis to investigate primary production resistance and resilience to extreme drought in 64 forests and grasslands across a broad MAP gradient. We found resistance to extreme drought was predicted by MAP; however, grasslands (positive) and forests (negative) exhibited opposing resilience relationships with MAP. Our findings indicate that common plant physiological mechanisms may determine grassland and forest resistance to extreme drought, whereas differences among plant residents in turnover time, plant architecture, and drought adaptive strategies likely underlie divergent resilience patterns. The low resistance and resilience of dry grasslands suggests that these ecosystems are the most vulnerable to extreme drought - a vulnerability that is expected to compound as extreme drought frequency increases in the future. (C) 2018 Elsevier B.V. All rights reserved.

International audience; nitori Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land-atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global mong network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.

Climate change is expected to drive increased tree mortality through drought, heat stress, and insect attacks, with manifold impacts on forest ecosystems. Yet, climate-induced tree mortality and biotic disturbance agents are largely absent from process-based ecosystem models. Using data sets from the western USA and associated studies, we present a framework for determining the relative contribution of drought stress, insect attack, and their interactions, which is critical for modeling mortality in future climates. We outline a simple approach that identifies the mechanisms associated with two guilds of insects - bark beetles and defoliators - which are responsible for substantial tree mortality. We then discuss cross-biome patterns of insect-driven tree mortality and draw upon available evidence contrasting the prevalence of insect outbreaks in temperate and tropical regions. We conclude with an overview of tools and promising avenues to address major challenges. Ultimately, a multitrophic approach that captures tree physiology, insect populations, and tree-insect interactions will better inform projections of forest ecosystem responses to climate change.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; International audience; Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere–atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function

Widespread drought-induced mortality of woody plants has recently occurred worldwide, is likely to be exacerbated by future climate change and holds large ecological consequences. Yet despite decades of research on plant-water relations, the pathways through which drought causes plant mortality are poorly understood. Recent work on the physiology of tree mortality has begun to reveal how physiological dysfunction induced by water stress leads to plant death; however, we are still far from being able to predict tree mortality using easily observed or modeled meteorological variables. In this review, we contend that, in order to fully understand when and where plants will exceed mortality thresholds when drought occurs, we must understand the entire path by which precipitation deficit is translated into physiological dysfunction and lasting physiological damage. In temperate ecosystems with seasonal climate patterns, precipitation characteristics such as seasonality, timing, form (snow versus rain) and intensity interact with edaphic characteristics to determine when and how much water is actually available to plants as soil moisture. Plant and community characteristics then mediate how quickly water is used and seasonally varying plant physiology determines whether the resulting soil moisture deficit is physiologically damaging. Recent research suggests that drought seasonality and timing matter for how an ecosystem experiences drought. But, mortality studies that bridge the gaps between climatology, hydrology, plant ecology and plant physiology are rare. Drawing upon a broad hydrological and ecological perspective, we highlight key and underappreciated processes that may mediate drought-induced tree mortality and propose steps to better include these components in current research.

Global patterns of drought-induced forest die-off indicate that many forests may be sensitive to climate-driven mortality, but the lack of understanding of how trees and saplings die during drought hinders the projections of die-off, demographic bottlenecks and ecosystem trajectories. In this study, we performed a severe controlled drought experiment on saplings of Pinus edulis Engelm. and Juniperus osteosperma (Torr.) Little, two species that both experienced die-off in a recent 'climate change-type' drought. We examined the roles of carbohydrate and hydraulic changes in multiple tissues as the saplings died. We found that saplings of both species exhibited large degrees of loss of hydraulic conductivity prior to death. Neither species exhibited significant changes in carbohydrate concentrations in any tissue during the relatively short and severe imposed drought. Native hydraulic conductivity successfully predicted the degree of canopy mortality in both species, highlighting the importance of drought characteristics and tree attributes in influencing physiological pathways to mortality. The relationships elucidated here, as well as the differences between our results and previous findings in adult trees, can help inform mortality mechanisms in climate-vegetation models, especially for young trees, and to understand species response to severe drought across ontogeny.

Forest die-off around the world is expected to increase in coming decades as temperature increases due to climate change. Forest die-off will likely affect understory plant communities, which have substantial influence on regional biological diversity, ecosystem function, and land-atmosphere interactions, but how die-off alters these plant communities is largely unknown. We examined changes in understory plant communities following a widespread, drought-induced die-off of trembling aspen (Populus tremuloides) in the western United States. We assessed shrub and herbaceous cover and volume in quadrats in 55 plots located across a wide range of levels of aspen mortality. We measured species richness and composition of herbaceous plant communities by recording species presence and absence in 12 sets of paired (1 healthy, 1 dying) aspen plots. Although understory composition in healthy and dying stands was heterogeneous across the landscape, shrub abundance, cover, and volume were higher and abundance of herbaceous species, cover, and volume were lower in dying aspen stands. Shrub cover and volume increased from 2009 to 2011 in dying stands, which suggests that shrub growth and expansion is ongoing. Species richness of herbs declined by 23% in dying stands. Composition of herbs differed significantly between dying and healthy stands. Richness of non-native species did not differ between stand types. The understory community in dying aspen stands was not similar to other shrub-dominated plant communities in the region and may constitute a novel community. Our results suggest that changes in understory plant communities as forests die off could be a significant indirect effect of climate change on biological diversity and forest communities.

Drought impacts on forests, including widespread die-off, are likely to increase with future climate change, although the physiological responses of trees to lethal drought are poorly understood. In particular, in situ examinations of carbon starvation and its interactions with and effects on infestation and hydraulic vulnerability are largely lacking. In this study, we conducted a controlled, in situ, repeated defoliation experiment to induce carbon stress in isolated trembling aspen (Populus tremuloides) ramets. We monitored leaf morphology, leaves per branch, and multitissue carbohydrate concentrations during canopy defoliation. We examined the subsequent effects of defoliation and defoliation-induced carbon stress on vulnerability to insect/fungus infestation and hydraulic vulnerability the following year. Defoliated ramets flushed multiple canopies, which coincided with moderate drawdown of nonstructural carbohydrate reserves. Infestation frequency greatly increased and hydraulic conductivity decreased 1 year after defoliation. Despite incomplete carbohydrate drawdown from defoliation and relatively rapid carbohydrate recovery, suggesting considerable carbohydrate reserves in aspen, defoliation-induced carbon stress held significant consequences for vulnerability to mortality agents and hydraulic performance. Our results indicate that multiyear consequences of drought via feedbacks are likely important for understanding forests’ responses to drought and climate change over the coming decades.

Drought-induced vegetation mortality has been documented on every vegetated continent in recent decades and constitutes a major uncertainty in climate change impacts on terrestrial ecosystems and carbon cycle feedbacks. While recent research has focused on specific failure mechanisms during drought-induced forest die-off, a broader understanding of the physiology of trees under drought, especially changes in growth and carbon allocation, is needed for determining the sensitivity of forests to drought and interacting mechanisms during forest mortality. I present here multi-tissue and high-resolution temporal dynamics of tree carbon resources during moderate experimental and natural drought in trembling aspen (Populus tremuloides) forests, a major forest type in western North America that recently experienced widespread drought-induced die-off. Drought led to substantial declines in inferred carbon uptake. Tree carbohydrate concentrations, however, largely increased in concert with substantial decreases in growth and severe declines in root biomass. These findings highlight that growth declines, especially in fine roots which are important to water uptake, and increased carbon allocation to root non-structural carbohydrates are key responses to drought in aspen and could play an important role in widespread die-off. They suggest multi-year consequences of drought and carbon-hydraulic interconnections. They underscore the need for a more integrated multi-tissue, multi-process, and multi-year perspective of climate-induced forest mortality.

Sudden and widespread forest die-back and die-off (e.g., Huang & Anderegg, 2012) and increased mortality rates (e.g., Peng et al., 2011) in many forest ecosystems across the globe have been linked to drought and elevated temperatures (Allen et al., 2010, Fig. 1). Furthermore, these observations have caused a focus on the physiological mechanisms of drought-induced tree mortality (e.g. McDowell et al., 2008) and many studies, both observational and manipulative, have been carried out to explain tree death during drought from a physiological perspective.

[EN] Non-structural carbohydrates (NSC) in plant tissue are frequently quantified to make inferences about plant responses to environmental conditions. Laboratories publishing estimates of NSC of woody plants use many different methods to evaluate NSC. We asked whether NSC estimates in the recent literature could be quantitatively compared among studies. We also asked whether any differences among laboratories were related to the extraction and quantification methods used to determine starch and sugar concentrations. These questions were addressed by sending sub-samples collected from five woody plant tissues, which varied in NSC content and chemical composition, to 29 laboratories. Each laboratory analyzed the samples with their laboratory-specific protocols, based on recent publications, to determine concentrations of soluble sugars, starch and their sum, total NSC. Laboratory estimates differed substantially for all samples. For example, estimates for Eucalyptus globulus leaves (EGL) varied from 23 to 116 (mean = 56) mg g(-1) for soluble sugars, 6-533 (mean = 94) mg g-1 for starch and 53-649 (mean = 153) mg g-1 for total NSC. Mixed model analysis of variance showed that much of the variability among laboratories was unrelated to the categories we used for extraction and quantification methods (method category R-2 = 0.05-0.12 for soluble sugars, 0.10-0.33 for starch and 0.01-0.09 for total NSC). For EGL, the difference between the highest and lowest least squares means for categories in the mixed model analysis was 33 mg g-1 for total NSC, compared with the range of laboratory estimates of 596 mg g-1. Laboratories were reasonably consistent in their ranks of estimates among tissues for starch (r = 0.41-0.91), but less so for total NSC (r = 0.45-0.84) and soluble sugars (r = 0.11-0.83). Our results show that NSC estimates for woody plant tissues cannot be compared among laboratories. The relative changes in NSC between treatments measured within a laboratory may be comparable within and between laboratories, especially for starch. To obtain comparable NSC estimates, we suggest that users can either adopt the reference method given in this publication, or report estimates for a portion of samples using the reference method, and report estimates for a standard reference material. Researchers interested in NSC estimates should work to identify and adopt standard methods., M.G.R. was funded by McMaster fellowship (1158.C). S.P. was funded by Juan de la Cierva contract (MCI project) and project ARBALMONT/786-2012 (OPAN, MAAMA, Spain). F.P. was funded by Fondecyt 11121175. U.N. and M.T. were funded by the Estonian Ministry of Education and Science, grant IUT-8-3. N.G.M. and L.T.D. were funded by DOE-BER. H.D.A. was funded by LANL-LDRD. J.M.-V. was funded by the Spanish Government (CGL 2010-16376). S.H. was funded by the Montana Institute on Ecosystems' Graduate Enhancement Award from NSF EPSCoR Track-1 NSF-IIA-1443108. Valuable comments from Dr Mauricio Mencuccini (University of Edinburgh), Dan Binkley (Colorado State University) and two anonymous reviewers were also greatly appreciated.