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.

As the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization.

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The frequency of severe droughts is increasing in many regions around the world as a result of climate change [1][2][3] . Droughts alter the structure and function of forests 4,5 . Site-and region-specific studies suggest that large trees, which play keystone roles in forests 6 and can be disproportionately important to ecosystem carbon storage 7 and hydrology 8 , exhibit greater sensitivity to drought than small trees 4,5,9,10 . Here, we synthesize data on tree growth and mortality collected during 40 drought events in forests worldwide to see whether this size-dependent sensitivity to drought holds more widely. We find that droughts consistently had a more detrimental impact on the growth and mortality rates of larger trees. Moreover, drought-related mortality increased with tree size in 65% of the droughts examined, especially when community-wide mortality was high or when bark beetles were present. The more pronounced drought sensitivity of larger trees could be underpinned by greater inherent vulnerability to hydraulic stress 11-14 , the higher radiation and evaporative demand experienced by exposed crowns 4,15 , and the tendency for bark beetles to preferentially attack larger trees 16 . We suggest that future droughts will have a more detrimental impact on the growth and mortality of larger trees, potentially exacerbating feedbacks to climate change.Climate change has been linked to water deficits in many parts of the world, and future climate projections suggest that droughts are likely to increase in severity because of changes in the timing and magnitude of precipitation and rising temperature 1,2,14,17 . Across a wide range of biomes, drought leads to changes in forest composition, structure, productivity and climate interactions 5,18,19 . Drought has many important consequences for forest communities, as species composition and dominance are shaped by water availability and can change rapidly in response to drought 19,20 . Drought-induced forest decline results in climate feedbacks including reduced CO 2 uptake, reduced carbon stocks, increased albedo and decreased evapotranspiration 21 . The impact of drought on forest structure and function depends on which trees are most adversely affected; greater mortality of small trees may modify future forest succession whereas mortality of large trees causes disproportionate losses of carbon and ecosystem function 5-7 . It has not been clear whether large or small trees would suffer more under drought stress. Several studies have documented a greater impact of drought on large trees at a single site 4,9,10 , and a synthesis of data from the humid lowland tropics revealed a tendency for greater drought-related mortality increases in large trees 5 , but these patterns have never been systematically reviewed for forests worldwide.Here, we perform a meta-analysis of data from 40 drought events at 38 forest locations worldwide, ranging from semi-arid woodlands to tropical rainforests, to address whether trees of different size (seedlings excluded) respond differe...

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