Climate change-driven increases in drought frequency and severity could compromise forest ecosystems and the terrestrial carbon sink. While the impacts of single droughts on forests have been widely studied, 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 1900-2018. We measured the impact of multiple droughts on tree growth in ~1200 tree-ring chronologies from the International Tree Ring Data Bank, on tree mortality from the U.S. Forest Inventory and Analysis dataset, and on ecosystem water content from satellite-measured vegetation optical depth.
Results/Conclusions
We find that subsequent droughts generally have a more deleterious impact than initial droughts, but this effect differs enormously by clade and ecosystem, with conifer-dominated ecosystems exhibiting increased vulnerability to multiple droughts. Temperate conifer and tropical broadleaf ecosystems showed the largest increase in response to multiple droughts in the satellite dataset. 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.