Mutually inclusive mechanisms of drought-induced tree mortality (original) (raw)
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Drought-related tree mortality: addressing the gaps in understanding and prediction
The New phytologist, 2015
I. II. III. IV. References SUMMARY: Increased tree mortality during and after drought has become a research focus in recent years. This focus has been driven by: the realisation that drought-related tree mortality is more widespread than previously thought; the predicted increase in the frequency of climate extremes this century; and the recognition that current vegetation models do not predict drought-related tree mortality and forest dieback well despite the large potential effects of these processes on species composition and biogeochemical cycling. To date, the emphasis has been on understanding the causal mechanisms of drought-related tree mortality, and on mechanistic models of plant function and vegetation dynamics, but a consensus on those mechanisms has yet to emerge. In order to generate new hypotheses and to help advance the modelling of vegetation dynamics in the face of incomplete mechanistic understanding, we suggest that general patterns should be distilled from the d...
Triggers of tree mortality under drought
Nature, 2018
Severe droughts have caused widespread tree mortality across many forest biomes with profound effects on the function of ecosystems and carbon balance. Climate change is expected to intensify regional-scale droughts, focusing attention on the physiological basis of drought-induced tree mortality. Recent work has shown that catastrophic failure of the plant hydraulic system is a principal mechanism involved in extensive crown death and tree mortality during drought, but the multi-dimensional response of trees to desiccation is complex. Here we focus on the current understanding of tree hydraulic performance under drought, the identification of physiological thresholds that precipitate mortality and the mechanisms of recovery after drought. Building on this, we discuss the potential application of hydraulic thresholds to process-based models that predict mortality.
New Phytologist, 2013
The intensity and severity of drought is increasing globally (Huntington, 2006) and will influence forest ecosystems alongside rising temperatures, heat waves, and changing interactions between pests/pathogens and hosts (Bonan, 2008; Allen et al., 2010). Understanding the mechanisms underlying drought-induced forest mortality is important for modelling water and carbon fluxes, and predicting the impacts of forest die-off on ecosystem function, ecosystem services, biogeochemical cycles, and the climate system (Adams et al ...
Research frontiers for improving our understanding of drought-induced tree and forest mortality
The New phytologist, 2018
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 o...
A multi-species synthesis of physiological mechanisms in drought-induced tree mortality
I ncreasing forest mortality from global change has been observed in all forested biomes 1,2 and will have profound implications for future energy and element fluxes 3–5. Predictions of vegetation responses to future climate are uncertain due to the lack of realistic mortality mechanisms in vegetation models 3,6–9. Recent research supports at least two tightly interrelated physiological mechanisms associated with tree mortality by drought: (1) hydraulic failure through partial or complete loss of xylem function from embolism that inhibits water transport through the vasculature, leading to tissue desiccation; and (2) carbon starvation via imbalance between carbohydrate demand and supply that may lead to an inability to meet osmotic, metabolic and defensive carbon requirements 3,6,7,10–15. Hydraulic failure is most typically assessed via per cent loss of xylem conductivity (PLC) and carbon starvation via changes in tissue non-structural carbohydrate (NSC) concentrations 12–16. There has been significant debate over these co-occurring mechanisms of mortality, particularly regarding the prevalence of carbon starvation and whether reduced carbohydrate reserves can be lethal during drought 11,17–22. Although a number of studies on the mechanism of drought-induced mortality in trees have been conducted for a variety of tree species over the past decade, the prevalence of these mechanisms on a global scale remains uncertain. Differences in approach, variables measured, and species and life stage studied have limited global assessment of drought-induced tree mortality mechanisms. Here, we provide the first cross-species synthesis of tree drought mortality mechanisms. We used a standardized physiological framework to analyse drought-induced tree mortality across species and assessed hydraulic function as PLC, and carbohydrate status as NSC normalized relative to controls. We examined data from 19 recent experimental and observational studies on 26 species from around the globe. Most tree species were assessed in only one study, but for several species data were available from more than one study, resulting in 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.
Drought-induced tree mortality: ecological consequences, causes, and modeling
Environmental Reviews, 2012
Drought-induced tree mortality, which rapidly alters forest ecosystem composition, structure, and function, as well as the feedbacks between the biosphere and climate, has occurred worldwide over the past few decades, and is expected to increase pervasively as climate change progresses. The objectives of this review are to (1) highlight the likely ecological consequences of drought-induced tree mortality, (2) synthesize the hypotheses related to drought-induced tree mortality, (3) discuss the implications of current knowledge for modeling tree mortality processes under climate change, and (4) highlight future research needs. First, we emphasize the likely ecological consequences of tree mortality from ecosystem to biome to continental scales. We then document and criticize multiple non-exclusive tree mortality hypotheses (e.g., carbon starvation carbon supply is less than carbon demand; and hydraulic failuredesiccation from failed water transport) from a more comprehensive ecological perspective. Next, we extend a forest decline concept model, Manion's framework, by considering new emerging environmental conditions, for a more thorough understanding of the effects of climate change on forest decline. We find that an increase in drought frequency and (or) climate-change-type droughts may trigger increased background tree mortality rates and severe forest dieback events, accelerating species turnover and ecological regime shifts. The contribution of CO 2 fertilization, rising temperature within the optimal growth range, and increased nitrogen deposition may defer or reduce this trend in tree mortality, but such contributions will vary between locations, species, and tree sizes. Multiple hypotheses proposed for drought-induced tree mortality are discussed, but coupling carbon and water cycles could help resolve the debate. The absence of a physiological understanding of tree mortality mechanisms limits the predictive ability of current models from stand-level process-based models to dynamic global vegetation models. We thus suggest that longterm observations, experiments, and models should be tightly interwoven during the research process to better forecast future climate changes and evaluate their impacts on forests.