Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought (original) (raw)
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Multiple strategies for drought survival among woody plant species
Functional Ecology, 2016
1. Drought-induced mortality and regional dieback of woody vegetation are reported from numerous locations around the world. Yet within any one site, predicting which species are most likely to survive global change-type drought is a challenge. 2. We studied the diversity of drought survival traits of a community of 15 woody plant species in a desert-chaparral ecotone. The vegetation was a mix of chaparral and desert shrubs, as well as endemic species that only occur along this margin. This vegetation boundary has large potential for drought-induced mortality because nearly all species are at the edge of their range. 3. Drought survival traits studied were vulnerability to drought-induced xylem cavitation, sapwood capacitance, deciduousness, photosynthetic stems, deep roots, photosynthetic responses to leaf water potential and hydraulic architecture. Drought survival strategies were evaluated as combinations of traits that could be effective in dealing with drought. 4. The large variation in seasonal predawn water potential of leaves and stem xylem ranged from À6Á82 to À0Á29 MPa and À6Á92 to À0Á27 MPa, respectively. The water potential at which photosynthesis ceases ranged from À9Á42 to À3Á44 MPa. Architecture was a determinant of hydraulic traits, with species supporting large leaf area per sapwood area exhibiting high rates of water transport, but also xylem that is vulnerable to drought-induced cavitation. Species with more negative midday leaf water potential during the growing season also showed access to deeper water sources based on hydrogen isotope analysis. 5. Drought survival mechanisms comprised of drought deciduousness, photosynthetic stems, tolerance of low minimum seasonal tissue water potential and vulnerability to drought-induced xylem cavitation thus varied orthogonally among species, and promote a diverse array of drought survival strategies in an arid ecosystem of considerable floristic complexity.
Isohydric species are not necessarily more carbon limited than anisohydric species during drought
Tree Physiology, 2016
Isohydry (i.e., strong regulation of leaf water potential, Ψ l) is commonly associated with strict stomatal regulation of transpiration under drought, which in turn is believed to minimize hydraulic risk at the expense of reduced carbon assimilation. Hence, the iso/ anisohydric classification has been widely used to assess drought resistance and mortality mechanisms across species, with isohydric species being hypothetically more prone to carbon starvation and anisohydric species more vulnerable to hydraulic failure. These hypotheses and their underlying assumptions, however, have rarely been tested under controlled, experimental conditions. Our objective is to assess the physiological mechanisms underlying drought resistance differences between two co-occurring Mediterranean forest species with contrasting drought responses: Phillyrea latifolia L. (anisohydric and more resistant to drought) and Quercus ilex L. (isohydric and less drought resistant). A total of 100 large saplings (50 per species) were subjected to repeated drought treatments for a period of 3 years, after which Q. ilex showed 18% mortality whereas no mortality was detected in P. latifolia. Relatively isohydric behavior was confirmed for Q. ilex, but higher vulnerability to cavitation in this species implied that estimated embolism levels were similar across species (12-52% in Q. ilex vs~30% in P. latifolia). We also found similar seasonal patterns of stomatal conductance and assimilation between species. If anything, the anisohydric P. latifolia tended to show lower assimilation rates than Q. ilex under extreme drought. Similar growth rates and carbon reserves dynamics in both species also suggests that P. latifolia was as carbon-constrained as Q. ilex. Increasing carbon reserves under extreme drought stress in both species, concurrent with Q. ilex mortality, suggests that mortality in our study was not triggered by carbon starvation. Our results warn against making direct connections between Ψ l regulation, stomatal behavior and the mechanisms of droughtinduced mortality in plants.
The ecohydrological context of drought and classification of plant responses
Ecology Letters, 2018
Many recent studies on drought-induced vegetation mortality have explored how plant functional traits, and classifications of such traits along axes of, for example, isohydry-anisohydry, might contribute to predicting drought survival and recovery. As these studies proliferate, the consistency and predictive value of such classifications need to be carefully examined. Here, we outline the basis for a systematic classification of plant drought responses that accounts for both environmental conditions and functional traits. We use non-dimensional analysis to integrate plant traits and metrics of environmental variation into groups that can be associated with alternative drought stress pathways (hydraulic failure and carbon limitation), and demonstrate that these groupings predict physiological drought outcomes using both synthetic and measured data. In doing so, we aim to untangle some confounding effects of environment and trait variations that undermine current classification schemes, advocate for more careful treatment of the environmental context within which plants experience and respond to drought, and outline a pathway towards a general classification of drought vulnerability.
Title Multiple strategies for drought survival among woody plant species Permalink
2015
1. Drought-induced mortality and regional dieback of woody vegetation are reported from numerous locations around the world. Yet within any one site, predicting which species are most likely to survive global change-type drought is a challenge. 2. We studied the diversity of drought survival traits of a community of 15 woody plant species in a desert-chaparral ecotone. The vegetation was a mix of chaparral and desert shrubs, as well as endemic species that only occur along this margin. This vegetation boundary has large potential for drought-induced mortality because nearly all species are at the edge of their range. 3. Drought survival traits studied were vulnerability to drought-induced xylem cavitation, sapwood capacitance, deciduousness, photosynthetic stems, deep roots, photosynthetic responses to leaf water potential and hydraulic architecture. Drought survival strategies were evaluated as combinations of traits that could be effective in dealing with drought. 4. The large variation in seasonal predawn water potential of leaves and stem xylem ranged from À6Á82 to À0Á29 MPa and À6Á92 to À0Á27 MPa, respectively. The water potential at which photosynthesis ceases ranged from À9Á42 to À3Á44 MPa. Architecture was a determinant of hydraulic traits, with species supporting large leaf area per sapwood area exhibiting high rates of water transport, but also xylem that is vulnerable to drought-induced cavitation. Species with more negative midday leaf water potential during the growing season also showed access to deeper water sources based on hydrogen isotope analysis. 5. Drought survival mechanisms comprised of drought deciduousness, photosynthetic stems, tolerance of low minimum seasonal tissue water potential and vulnerability to drought-induced xylem cavitation thus varied orthogonally among species, and promote a diverse array of drought survival strategies in an arid ecosystem of considerable floristic complexity.
Annals of Forest Science, 2006
The extreme drought event that occurred in Western Europe during 2003 highlighted the need to understand the key processes that may allow trees and stands to overcome such severe water shortages. We therefore reviewed the current knowledge available about such processes. First, impact of drought on exchanges at soil-root and canopy-atmosphere interfaces are presented and illustrated with examples from water and CO 2 flux measurements. The decline in transpiration and water uptake and in net carbon assimilation due to stomatal closure has been quantified and modelled. The resulting models were used to compute water balance at stand level basing on the 2003 climate in nine European forest sites from the CARBOEUROPE network. Estimates of soil water deficit were produced and provided a quantitative index of soil water shortage and therefore of the intensity of drought stress experienced by trees during summer 2003. In a second section, we review the irreversible damage that could be imposed on water transfer within trees and particularly within xylem. A special attention was paid to the inter-specific variability of these properties among a wide range of tree species. The inter-specific diversity of hydraulic and stomatal responses to soil water deficit is also discussed as it might reflect a large diversity in traits potentially related to drought tolerance. Finally, tree decline and mortality due to recurrent or extreme drought events are discussed on the basis of a literature review and recent decline studies. The potential involvement of hydraulic dysfunctions or of deficits in carbon storage as causes for the observed long term (several years) decline of tree growth and development and for the onset of tree dieback is discussed. As an example, the starch content in stem tissues recorded at the end of the 2003's summer was used to predict crown conditions of oak trees during the following spring: low starch contents were correlated with large twig and branch decline in the crown of trees.
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.