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

Craig D. Allen, David D Breshears, Nate G. McDowell

Research output: Contribution to journalArticle

572 Citations (Scopus)

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 [CO2] 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.

Original languageEnglish (US)
Article number129
JournalEcosphere
Volume6
Issue number8
DOIs
StatePublished - Aug 1 2015

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die-off
tree mortality
vulnerability
drought
mortality
research policy
research management
warming
policy making
Anthropocene
mortality risk
atmospheric moisture
forest growth
forest canopy
physiological response
woody plant
wildfires
water use efficiency
woody plants
wildfire

Keywords

  • Carbon starvation
  • Climate change
  • CO fertilization
  • Drought
  • ESA centennial paper
  • Extreme events
  • Forest die-off
  • Forests
  • Hydraulic failure
  • Insect pests
  • Pathogens
  • Tree mortality
  • Woodlands

ASJC Scopus subject areas

  • Ecology, Evolution, Behavior and Systematics
  • Ecology

Cite this

On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene. / Allen, Craig D.; Breshears, David D; McDowell, Nate G.

In: Ecosphere, Vol. 6, No. 8, 129, 01.08.2015.

Research output: Contribution to journalArticle

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KW - Tree mortality

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