Forest mortality constitutes a major uncertainty in projections of climate impacts on terrestrial ecosystems and carbon‐cycle feedbacks. Recent drought‐induced, widespread forest die‐offs highlight ...that climate change could accelerate forest mortality with its diverse and potentially severe consequences for the global carbon cycle, ecosystem services, and biodiversity. How trees die during drought over multiple years remains largely unknown and precludes mechanistic modeling and prediction of forest die‐off with climate change. Here, we examine the physiological basis of a recent multiyear widespread die‐off of trembling aspen (Populus tremuloides) across much of western North America. Using observations from both native trees while they are dying and a rainfall exclusion experiment on mature trees, we measure hydraulic performance over multiple seasons and years and assess pathways of accumulated hydraulic damage. We test whether accumulated hydraulic damage can predict the probability of tree survival over 2 years. We find that hydraulic damage persisted and increased in dying trees over multiple years and exhibited few signs of repair. This accumulated hydraulic deterioration is largely mediated by increased vulnerability to cavitation, a process known as cavitation fatigue. Furthermore, this hydraulic damage predicts the probability of interyear stem mortality. Contrary to the expectation that surviving trees have weathered severe drought, the hydraulic deterioration demonstrated here reveals that surviving regions of these forests are actually more vulnerable to future droughts due to accumulated xylem damage. As the most widespread tree species in North America, increasing vulnerability to drought in these forests has important ramifications for ecosystem stability, biodiversity, and ecosystem carbon balance. Our results provide a foundation for incorporating accumulated drought impacts into climate–vegetation models. Finally, our findings highlight the critical role of drought stress accumulation and repair of stress‐induced damage for avoiding plant mortality, presenting a dynamic and contingent framework for drought impacts on forest ecosystems.
Forest ecosystems store approximately 45% of the carbon found in terrestrial ecosystems, but they are sensitive to climate-induced dieback. Forest die-off constitutes a large uncertainty in ...projections of climate impacts on terrestrial ecosystems, climate–ecosystem interactions, and carbon-cycle feedbacks. Current understanding of the physiological mechanisms mediating climate-induced forest mortality limits the ability to model or project these threshold events. We report here a direct and in situ study of the mechanisms underlying recent widespread and climate-induced trembling aspen (Populus tremuloides) forest mortality in western North America. We find substantial evidence of hydraulic failure of roots and branches linked to landscape patterns of canopy and root mortality in this species. On the contrary, we find no evidence that drought stress led to depletion of carbohydrate reserves. Our results illuminate proximate mechanisms underpinning recent aspen forest mortality and provide guidance for understanding and projecting forest die-offs under climate change.
Understanding the pathways through which drought stress kills woody vegetation can improve projections of the impacts of climate change on ecosystems and carbon-cycle feedbacks. Continuous in situ ...measurements of whole trees during drought and as trees die hold promise to illuminate physiological pathways but are relatively rare. We monitored leaf characteristics, water use efficiency, water potentials, branch hydraulic conductivity, soil moisture, meteorological variables, and sap flux on mature healthy and sudden aspen decline-affected (SAD) trembling aspen (Populus tremuloides) ramets over two growing seasons, including a severe summer drought. We calculated daily estimates of whole-ramet hydraulic conductance and modeled whole-ramet assimilation. Healthy ramets experienced rapid declines of whole-ramet conductance during the severe drought, providing an analog for what likely occurred during the previous drought that induced SAD. Even in wetter periods, SAD-affected ramets exhibited fivefold lower whole-ramet hydraulic conductance and sevenfold lower assimilation than counterpart healthy ramets, mediated by changes in leaf area, water use efficiency, and embolism. Extant differences between healthy and SAD ramets reveal that ongoing multi-year forest die-off is primarily driven by loss of whole-ramet hydraulic capability, which in turn limits assimilation capacity. Branch-level measurements largely captured whole-plant hydraulic limitations during drought and mortality, but whole-plant measurements revealed a potential role of other losses in the hydraulic continuum. Our results highlight the importance of a whole-tree perspective in assessing physiological pathways to tree mortality and indicate that the effects of mortality on these forests’ assimilation and productivity are larger than expected based on canopy leaf area differences.
Plant hydraulics mediate terrestrial woody plant productivity, influencing global water, carbon, and biogeochemical cycles, as well as ecosystem vulnerability to drought and climate change. While ...inter‐specific differences in hydraulic traits are widely documented, intra‐specific hydraulic variability is less well known and is important for predicting climate change impacts. Here, I present a conceptual framework for this intra‐specific hydraulic trait variability, reviewing the mechanisms that drive variability and the consequences for vegetation response to climate change. I performed a meta‐analysis on published studies (n = 33) of intra‐specific variation in a prominent hydraulic trait – water potential at which 50% stem conductivity is lost (P50) – and compared this variation to inter‐specific variability within genera and plant functional types used by a dynamic global vegetation model. I found that intra‐specific variability is of ecologically relevant magnitudes, equivalent to c. 33% of the inter‐specific variability within a genus, and is larger in angiosperms than gymnosperms, although the limited number of studies highlights that more research is greatly needed. Furthermore, plant functional types were poorly situated to capture key differences in hydraulic traits across species, indicating a need to approach prediction of drought impacts from a trait‐based, rather than functional type‐based perspective.
Why is Tree Drought Mortality so Hard to Predict? Trugman, Anna T.; Anderegg, Leander D.L.; Anderegg, William R.L. ...
Trends in ecology & evolution,
June 2021, 2021-Jun, 2021-06-00, Volume:
36, Issue:
6
Journal Article
Peer reviewed
Open access
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.
Drought mortality has wide-ranging ramifications from environmental conservation to climate change mitigation efforts. Thus far, mortality prediction efforts using physiology alone have found limited success.Physiological interactions, such as within-species trait variation, trait covariation, and trait–environment covariation, can reverse or confound mortality predictions.Ecological complexity, particularly the degree to which biotic mortality agents are linked to stress physiology, is highly variable. Thus, the presence of biotic agents has strong potential to reverse or confound mortality predictions.We present a framework to integrate our understanding of complex drought physiology and biotic mortality agents.Future work is needed to understand where and when biotic mortality agents might amplify patterns of physiological stress and where and when the effects of biotic agents might be largely decoupled from physiological stress.
Plants influence the atmosphere through fluxes of carbon, water and energy
, and can intensify drought through land-atmosphere feedback effects
. The diversity of plant functional traits in forests, ...especially physiological traits related to water (hydraulic) transport, may have a critical role in land-atmosphere feedback, particularly during drought. Here we combine 352 site-years of eddy covariance measurements from 40 forest sites, remote-sensing observations of plant water content and plant functional-trait data to test whether the diversity in plant traits affects the response of the ecosystem to drought. We find evidence that higher hydraulic diversity buffers variation in ecosystem flux during dry periods across temperate and boreal forests. Hydraulic traits were the predominant significant predictors of cross-site patterns in drought response. By contrast, standard leaf and wood traits, such as specific leaf area and wood density, had little explanatory power. Our results demonstrate that diversity in the hydraulic traits of trees mediates ecosystem resilience to drought and is likely to have an important role in future ecosystem-atmosphere feedback effects in a changing climate.
Airborne pollen has major respiratory health impacts and anthropogenic climate change may increase pollen concentrations and extend pollen seasons. While greenhouse and field studies indicate that ...pollen concentrations are correlated with temperature, a formal detection and attribution of the role of anthropogenic climate change in continental pollen seasons is urgently needed. Here, we use long-term pollen data from 60 North American stations from 1990 to 2018, spanning 821 site-years of data, and Earth system model simulations to quantify the role of human-caused climate change in continental patterns in pollen concentrations. We find widespread advances and lengthening of pollen seasons (+20 d) and increases in pollen concentrations (+21%) across North America, which are strongly coupled to observed warming. Human forcing of the climate system contributed ∼50% (interquartile range: 19-84%) of the trend in pollen seasons and ∼8% (4-14%) of the trend in pollen concentrations. Our results reveal that anthropogenic climate change has already exacerbated pollen seasons in the past three decades with attendant deleterious effects on respiratory health.
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.
Drought‐induced tree mortality is projected to increase due to climate change, yet predictions of tree mortality remain limited, in part because within‐species variations in ecophysiology 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. We find that climate stress has exceeded physiological and ecosystem‐level tolerance or compensating mechanisms by triggering extensive mortality at dry range edges, providing a foundation for future mortality projections in empirical distribution and mechanistic vegetation models.
Globally documented widespread drought‐induced forest mortality has important ramifications for plant community structure, ecosystem function, and the ecosystem services provided by forests. Yet the ...characteristics of drought seasonality, severity, and duration that trigger mortality events have received little attention despite evidence of changing precipitation regimes, shifting snow melt timing, and increasing temperature stress. This study draws upon stand level ecohydrology and statewide climate and spatial analysis to examine the drought characteristics implicated in the recent widespread mortality of trembling aspen (Populus tremuloides Michx.). We used isotopic observations of aspen xylem sap to determine water source use during natural and experimental drought in a region that experienced high tree mortality. We then drew upon multiple sources of climate data to characterize the drought that triggered aspen mortality. Finally, regression analysis was used to examine the drought characteristics most associated with the spatial patterns of aspen mortality across Colorado. Isotopic analysis indicated that aspens generally utilize shallow soil moisture with little plasticity during drought stress. Climate analysis showed that the mortality‐inciting drought was unprecedented in the observational record, especially in 2002 growing season temperature and evaporative deficit, resulting in record low shallow soil moisture reserves. High 2002 summer temperature and low shallow soil moisture were most associated with the spatial patterns of aspen mortality. These results suggest that the 2002 drought subjected Colorado aspens to the most extreme growing season water stress of the past century by creating high atmospheric moisture demand and depleting the shallow soil moisture upon which aspens rely. Our findings highlight the important role of drought characteristics in mediating widespread aspen forest mortality, link this aspen die‐off to regional climate change trends, and provide insight into future climate vulnerability of these forests.
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.
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