Underestimation of Global Vulnerability to Tree Mortality and Forest Die-Off from Hotter Drought in the Anthropocene — A Synthesis of Contrasting Evidence and Perspectives

Abstract:

Drought accompanied by warmer temperatures - “hotter drought” - is an emerging characteristic of the Anthropocene. Tree mortality and forest die-off due to hotter drought are the focus of rapidly expanding scientific literature. Despite recent observational, experimental, & modeling studies suggesting increased tree vulnerability to hotter drought and associated pests and pathogens, no consensus exists regarding future mortality risks. We summarize key findings, differentiating between those implying lesser versus greater levels of vulnerability. Evidence of lesser vulnerability includes forest benefits of elevated [CO2] and increased water-use efficiency; increases in forest growth, canopy greening, & woody-plant biomass, density, and extent; compensatory physiological, morphological, & genetic mechanisms; dampening ecological feedbacks; & potential mitigation by forest management. In contrast, recent studies document more rapid mortality under hotter drought due to negative tree physiological responses & accelerated biotic attacks. Additional evidence suggesting greater vulnerability includes rising background mortality rates and forest die-off events; projected increases in drought occurrence; limitations of vegetation models such as inadequately represented mortality processes; & 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 critical set of global vulnerability drivers 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 & can become lethal under warming, increasing the frequency of lethal drought; & 6) mortality happens rapidly relative to growth intervals needed for forest recovery. These high-confidence drivers, along with greater vulnerability evidence, support an overall viewpoint of greater forest vulnerability globally. We provide a synthesis including tree mortality thresholds, & highlight urgent research challenges.