Forest vulnerability to drought controlled by bedrock composition

Spatial variability in forest dieback during the severe drought in California between 2011 and 2017 can be explained by variations in bedrock composition and thus weatherability, according to analyses of the drought responses a series of geologically distinct sites. Forests are increasingly threatened by climate-change-fuelled cycles of drought, dieback and wildfires. However, for reasons that remain incompletely understood, some forest stands are more vulnerable than others, leaving a patchwork of varying dieback and wildfire risk after drought. Here, we show that spatial variability in forest drought response can be explained by differences in underlying bedrock. Our analysis links geochemical measurements of bedrock composition, geophysical measurements of subsurface weathering and remotely sensed changes in evapotranspiration during the 2011-2017 drought in California. We find that evapotranspiration plummeted in dense forest stands rooted in weathered, nutrient-rich bedrock. By contrast, relatively unweathered, nutrient-poor bedrock supported thin forest stands that emerged unscathed from the drought. By influencing both subsurface weathering and nutrient supply, bedrock composition regulates the balance of water storage and demand in mountain ecosystems. However, rather than enhancing forest resilience to drought by providing more water-storage capacity, bedrock with more weatherable and nutrient-rich minerals induced greater vulnerability by enabling a boom-bust cycle in which higher ecosystem productivity during wet years drives excess plant water demand during droughts.