Drought‐related tree mortality: addressing the gaps in understanding and prediction

Meir, Patrick; Mencuccini, Maurizio; Dewar, Roderick C.

doi:10.1111/nph.13382

Cited by 119 publications

(119 citation statements)

References 59 publications

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“…Research related to many aspects of tree mortality has increased rapidly over the past decade, with multiple reviews related to particular portions of the proliferating science on drought and heat-related mortality (McDowell et al 2008, Raffa et al 2008, Choat et al 2012, Hicke et al 2012a, Martínez-Vilalta et al 2012a, Anderegg et al 2013b, Oliva et al 2014, Teskey et al 2014, Zeppel et al 2014, Hartmann et al 2015, Meir et al 2015, Sperry and Love 2015. We point readers to these reviews for more details on particular aspects of tree mortality.…”

Section: Key Findings From Recent Research On Tree Mortality and Forementioning

confidence: 99%

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

2015

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Abstract. Patterns, mechanisms, projections, and consequences of tree mortality and associated broadscale forest die-off due to drought accompanied by warmer temperatures-''hotter drought'', an emerging characteristic of the Anthropocene-are the focus of rapidly expanding literature. Despite recent observational, experimental, and modeling studies suggesting increased vulnerability of trees to hotter drought and associated pests and pathogens, substantial debate remains among research, management and policy-making communities regarding future tree mortality risks. We summarize key mortalityrelevant findings, differentiating between those implying lesser versus greater levels of vulnerability. Evidence suggesting lesser vulnerability includes forest benefits of elevated [CO 2 ] and increased water-use efficiency; observed and modeled increases in forest growth and canopy greening; widespread increases in woody-plant biomass, density, and extent; compensatory physiological, morphological, and genetic mechanisms; dampening ecological feedbacks; and potential mitigation by forest management. In contrast, recent studies document more rapid mortality under hotter drought due to negative tree physiological responses and accelerated biotic attacks. Additional evidence suggesting greater vulnerability includes rising background mortality rates; projected increases in drought frequency, intensity, and duration; limitations of vegetation models such as inadequately represented mortality processes; warming feedbacks from die-off; and wildfire synergies. Grouping these findings we identify ten contrasting perspectives that shape the vulnerability debate but have not been discussed collectively. We also present a set of global vulnerability drivers that are known with high confidence: (1) droughts eventually occur everywhere; (2) warming produces hotter droughts; (3) atmospheric moisture demand increases nonlinearly with temperature during drought; (4) mortality can occur faster in hotter drought, consistent with fundamental physiology; (5) shorter droughts occur more frequently than longer droughts and can become lethal under warming, increasing the frequency of lethal drought nonlinearly; and (6) mortality happens rapidly relative to growth intervals needed for forest recovery. These high-confidence drivers, in concert with research supporting greater vulnerability perspectives, support an overall viewpoint of greater forest vulnerability globally. We surmise that mortality vulnerability is being discounted in part due to difficulties in predicting threshold responses to extreme climate events. Given the profound ecological and societal implications of underestimating global vulnerability to hotter drought, we highlight urgent challenges for research, management, and policy-making communities.

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Section: Key Findings From Recent Research On Tree Mortality and Forementioning

confidence: 99%

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

2015

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“…16) show smaller effects than are seen in field observations and experiments. Field-based studies suggest that severe hydrological droughts tend to reduce both forest growth rates (via reduced photosynthesis) and carbon stocks (via increased tree mortality) (18). At least two factors account for the relatively small effects of hydrological droughts seen in simulations.…”

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confidence: 99%

Projections of future meteorological drought and wet periods in the Amazon

Duffy

Brando

Asner

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

306

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Future intensification of Amazon drought resulting from climate change may cause increased fire activity, tree mortality, and emissions of carbon to the atmosphere across large areas of Amazonia. To provide a basis for addressing these issues, we examine properties of recent and future meteorological droughts in the Amazon in 35 climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We find that the CMIP5 climate models, as a group, simulate important properties of historical meteorological droughts in the Amazon. In addition, this group of models reproduces observed relationships between Amazon precipitation and regional sea surface temperature anomalies in the tropical Pacific and the North Atlantic oceans. Assuming the Representative Concentration Pathway 8.5 scenario for future drivers of climate change, the models project increases in the frequency and geographic extent of meteorological drought in the eastern Amazon, and the opposite in the West. For the region as a whole, the CMIP5 models suggest that the area affected by mild and severe meteorological drought will nearly double and triple, respectively, by 2100. Extremes of wetness are also projected to increase after 2040. Specifically, the frequency of periods of unusual wetness and the area affected by unusual wetness are projected to increase after 2040 in the Amazon as a whole, including in locations where annual mean precipitation is projected to decrease. Our analyses suggest that continued emissions of greenhouse gases will increase the likelihood of extreme events that have been shown to alter and degrade Amazonian forests.he responses of tropical forests to severe droughts exert strong influences on the global carbon cycle (1, 2). During periods of extended soil water stress, changes in forest physiology and structure reduce the capacity of forests to cycle and store carbon (3-5). In the 2000s, for example, more than half of the Amazon experienced droughts that were severe enough to cause increased tree mortality and reduced tree growth (6). In response to these droughts, 1-2% of the carbon stocks of Amazonian forests were committed to the atmosphere (6, 7), while large amounts were combusted due to widespread fires. Although similar droughts have occurred for millennia (8), and forests have apparently recovered from such events (9), climate change could intensify hydrological extremes over the basin (10). Together with warmer climatic conditions, an increase in the frequency and severity of droughts could push the Amazon region into a new climate envelope (11), potentially releasing to the atmosphere a large part of the 120 Pg of carbon stored in these forests (12). These effects on the carbon stocks of the Amazon could be even more pronounced if forest fires become more common in the region.There is disagreement about the potential responses of Amazon forests to climate change (13). Earlier versions of climate−vegetation models projected that as climate changed, many forests growing in southeast Amazonia wo...

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“…None of these definitions provide testable mechanisms that can precisely explain the physiology of plant stress responses that lead to mortality. Also, no existing process model can accurately predict when a plant will die from severe water stress (Zeppel et al, 2011;Anderegg et al, 2012aAnderegg et al, , 2012bBrodribb and Cochard, 2009;McDowell et al, 2013;Adams et al, 2013;Xu et al, 2013;Meir et al, 2015;Mencuccini et al, 2015). A quantifiable definition of drought mortality is needed that resolves death on the daily time scale so that we can predict if the timing of drought alleviation (through rain or irrigation) will allow plant recovery.…”

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confidence: 99%

Dead or Alive? Using Membrane Failure and Chlorophyll a Fluorescence to Predict Plant Mortality from Drought

et al. 2017

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Climate models predict widespread increases in both drought intensity and duration in the next decades. Although water deficiency is a significant determinant of plant survival, limited understanding of plant responses to extreme drought impedes forecasts of both forest and crop productivity under increasing aridity. Drought induces a suite of physiological responses; however, we lack an accurate mechanistic description of plant response to lethal drought that would improve predictive understanding of mortality under altered climate conditions. Here, proxies for leaf cellular damage, chlorophyll a fluorescence, and electrolyte leakage were directly associated with failure to recover from drought upon rewatering in Brassica rapa (genotype R500) and thus define the exact timing of drought-induced death. We validated our results using a second genotype (imb211) that differs substantially in life history traits. Our study demonstrates that whereas changes in carbon dynamics and water transport are critical indicators of drought stress, they can be unrelated to visible metrics of mortality, i.e. lack of meristematic activity and regrowth. In contrast, membrane failure at the cellular scale is the most proximate cause of death. This hypothesis was corroborated in two gymnosperms (Picea engelmannii and Pinus contorta) that experienced lethal water stress in the field and in laboratory conditions. We suggest that measurement of chlorophyll a fluorescence can be used to operationally define plant death arising from drought, and improved plant characterization can enhance surface model predictions of drought mortality and its consequences to ecosystem services at a global scale.

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Drought‐related tree mortality: addressing the gaps in understanding and prediction

Cited by 119 publications

References 59 publications

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

Projections of future meteorological drought and wet periods in the Amazon

Dead or Alive? Using Membrane Failure and Chlorophyll a Fluorescence to Predict Plant Mortality from Drought

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