Enhanced Drought Exposure Increasingly Threatens More Forests Than Observed

Forest protection and afforestation have been identified as a means to partially offset anthropogenic CO2 emissions. Yet, increasingly frequent observations of drought-induced tree mortality are reported. Here, we applied a risk analysis framework for global drought-induced forest mortality by examining extreme reductions in greenness and water content of forest canopies during past mortality events as well as growth recovery of surviving individual trees following stand-scale mortality events. We defined a drought-induced mortality risk index (DMR) that explains 80% of documented tree mortality. Rising CO2 alleviated the increase of DMR with short-term drought, however, the observed DMR increases with long-term drought no matter whether considering plant responses to CO2. DMR in sites where tree mortality has been observed significantly increased since the 1980s. More than that, drought exposure threatened 0.28 billion hectares of forested areas. Our framework highlights how climate change-induced drought, especially hotter-droughts, threatens the sustainability of global forests.