Increasing impacts of soil dryness on forests across the globe

Climate change leads to soil drying in many regions via reduced precipitation and/or increased atmospheric water demand. This threatens the functioning of global vegetation, particularly forests, which currently absorb a substantial fraction of human carbon emissions. Analyses of forest responses to droughts typically focus on single events and span spatial scales ranging from individual sites to continents. A global analysis of drought impacts on forests and their evolution over time under ongoing climate change is lacking. 
In this study, we quantify and analyse the evolution of the effects of soil moisture drought events on forests across the globe during 2001–2023. We identify dry events from a reanalysis soil-moisture dataset using percentile-based thresholds per grid pixel. Further, we evaluate forest responses using satellite-based vegetation indices, including NDVI and NIRv, and normalize anomalies by the pixel-based standard deviation to ensure comparability across regions. Using this approach, we find a steady increase in the global area exhibiting negative forest responses to soil dryness between 2001 and 2023. Additionally, regions with more negative forest responses tend to show faster increases over time than regions with mildly negative responses. We further hypothesize that this increased magnitude of severe vegetation responses to dryness may be related to three factors: increasing soil dryness and/or compound occurrence with atmospheric dryness, increased forest sensitivity to dryness, and changing spatial patterns of soil dryness occurrence. Understanding how these factors contribute to aggravated forest responses to dryness is essential for predicting the land carbon sink and implications for local water and energy cycling.