Droughts preceding tree mortality events have increased in duration and intensity, especially in dry biomes

The thresholds of drought duration and intensity required to provoke pulses of tree mortality across Earth's biomes remain unclear. Using globally-extensive updated databases of drought-associated tree mortality, we report substantial diversity in the types of drought events that cause tree death in different forest types. Tree-killing droughts are longer, more intense and have higher completeness (proportion of extreme drought within long-lasting droughts) in dry versus wet biomes. Mortality-inducing droughts are more intense and show higher completeness in angiosperm-dominated forests. We find a marked tendency towards long-lasting and more severe and complete droughts in recent years, particularly in more arid sites. Warming-amplified aridity is a main factor underpinning these variations. Differences in "sampling effort" across regions make it challenging to characterize the high variability in drought-induced tree mortality events. In this work we demonstrate the need to create, continuously update, and refine more extensive field-based tree mortality monitoring programs globally.