Impacts of Vegetation on Drought Trends

Vegetation conditions interact with and feed back to atmospheric anomalies such as droughts through water, energy and other exchanges on the land surface. Various mechanisms have been proposed to understand the related physical processes based on observational and numerical analyses of historical droughts. This study aims to understand the complex impacts and mechanisms of vegetation on future droughts under changing climate. Future trends in droughts were projected by comparing a drought index for present and future periods using seven dynamically downscaled regional climate change scenarios for the continental United States. The impacts of vegetation were examined by comparing changes in drought intensity and frequency between two types of land cover in each of six climate regimes. The ensemble results indicate that future droughts would increase in both intensity and frequency, mainly in the central Great Plains during all seasons, and in the Southeast and Southwest U.S. during warm seasons. The roles of vegetation depend on climate regime. Future droughts would increase more significantly on forest lands (grasslands) than the corresponding farmlands (drylands) in warm and moist/dry climate regimes. Vegetation, therefore, can amplify future droughts. In contrast, they would be less significant in cold and moist regimes, suggesting that the capacity of vegetation in amplifying future droughts would exist only in certain climate circumstances. The complex impacts and regional dependence raise new challenges for climate models to provide reliable drought simulation and projection, and for forest management to develop adaptation and mitigation strategies for climate change.