On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene
Rapidly expanding literature highlights patterns, mechanisms, projections and consequences of tree mortality and broad-scale forest die-off events due to drought accompanied by warmer temperatures—“hotter drought”, a projected characteristic of the Anthropocene. Despite recent observational, experimental, and modeling studies suggesting greater vulnerability of trees to hotter drought and associated effects of pests and pathogens, substantial questions remain within the research, management and policy-making communities about risk of future tree mortality. We summarize key mortality-relevant findings differentiating between those implying lesser versusgreater levels of vulnerability.
Evidence suggesting lesser vulnerability relate to CO2 fertilization and increased water use efficiency; observed and modeled estimates of forest growth and canopy greening; widespread woody plant densification and encroachment; compensatory physiological, morphological, and genetic mechanisms; dampening ecological feedbacks; and potential mitigation by forest management. In contrast, numerous recent studies indicate negative tree physiological responses related to carbon metabolism, hydraulics, pest and pathogen impacts, and increased vulnerability to mortality under hotter drought. Other research indicating greater vulnerability relates to rising background mortality rates, projected increases in drought frequency and duration, potential warming feedbacks from die-off, and synergies with wildfire. We use these findings to identify ten contrasting perspectives shaping the vulnerability debate that generally are not considered explicitly in concert. We contend global tree vulnerability is being underestimated by confounding focus (on climate trends rather than drought events, and on drought stress rather than mortality) or by mortality probabilities being discounted due to prediction difficulties. Most importantly, ongoing debates about specific aspects of mortality obscure the profound collective implications of a critical set of global vulnerability drivers with low uncertainty: 1) spatial prevalence of drought occurrence; 2) widespread projected warming; 3) non-linear increase in atmospheric moisture demand with temperature; 4) shorter survival periods during hotter drought, consistent with fundamental physiology and resulting in a non-linear increase in lethal drought frequency with warming; and 5) mortality events being more rapid than subsequent recovery. We recommend research, management, and policy-making communities respond specifically and urgently. More generally, we caution against current underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene.