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

Abstract

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.

How drought-induced forest die-off alters microclimate and increases fuel loadings and fire potentials

“…Die-off events are expected to continue as changes in the frequency, intensity and duration of extreme episodes related to global climate change continue to modify growing conditions for forests globally (Intergovernmental Panel on Climate Change 2014). Consequently, concern regarding tree mortality under climate change is growing Martinez-Vilalta et al 2012;Allen et al 2015). Whereas some regions may become hotter and wetter, Mediterranean-climate-type regions, among others, are predicted to become hotter and drier (e.g.…”

Forest fires and climate change: causes, consequences and management options

“…Their results indicate that the measured increases in dead wood mass and increased near-ground solar radiation associated with the die-off could lead to increases in fire spread of up to 30%. This is a striking finding given the greater number of die-off events from increasing drought in many regions of the world (Allen et al 2015) Impacts on ecosystems' species composition and functioning are expected as new fire regimes approximate to the extremes of the historical range of variability that current ecosystems have evolved within. In their review, Fairman et al (2016) analyse the impacts of increasing wildfire frequency on temperate forests, using the fire tolerant (resprouter) and fire sensitive (obligate seeder) eucalypt forests of south-eastern Australia as a case study.…”

Plants and Drought in a Changing Climate

“…While higher CO 2 concentrations may boost plant growth and simultaneously help plants conserve water, the co-occurrence of hot conditions during drought could exacerbate plant stress, and potentially lead to increased damage to tissues and higher rates of mortality [65][66][67]. This increase in plant stress could occur even if drought frequency remains constant and have major impacts on forest structure and functioning in a hotter world [68]. The water saving benefits to plants of high CO 2 have the potential to counteract high temperature and VPD which occur during drought events [69].…”

“…Concurrent elevations in temperature during drought are likely to increase VPD further, as well as increase metabolic needs of plants [66]. Although experimental evidence is still limited, the existing studies find that hot droughts will kill trees faster [76], suggesting that in the future, even droughts of short duration may prove lethal, leading to more tree killing events even when drought frequency stays constant [68]. However, our mechanistic understanding of the process of mortality is still limited, and this constrains our ability to predict mortality responses to future climate.…”