Divergent forest sensitivity to repeated extreme droughts

Climate change-driven increases in drought frequency and severity could compromise forest ecosystems and the terrestrial carbon sink1–3. While the impacts of single droughts on forests have been widely studied4–6, understanding whether forests acclimate to or become more vulnerable to sequential droughts remains largely unknown and is crucial for predicting future forest health. We combine cross-biome datasets of tree growth, tree mortality and ecosystem water content to quantify the effects of multiple droughts at a range of scales from individual trees to the globe from 1900 to 2018. We find that subsequent droughts generally have a more deleterious impact than initial droughts, but this effect differs enormously by clade and ecosystem, with gymnosperms and conifer-dominated ecosystems more often exhibiting increased vulnerability to multiple droughts. The differential impacts of multiple droughts across clades and biomes indicate that drought frequency changes may have fundamentally different ecological and carbon-cycle consequences across ecosystems. Drought frequency will probably increase under climate change, posing a potential risk to forests. Forest response is variable, but subsequent droughts generally have a negative impact at the tree and ecosystem scales, with systems dominated by conifers particularly vulnerable.