Current Research in EEPS: Dr. Cliff Riebe, University of Wyoming
Forests are increasingly threatened by climate-change-fueled 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, I show that spatial variability in drought-related forest dieback can be explained by differences in bedrock composition through its effects on subsurface weathering and nutrient supply, which together regulate the balance of water storage and demand in mountain ecosystems. During the historic 2011–2017 drought in the Sierra Nevada, California, evapotranspiration plummeted in dense forest stands that are rooted in highly weathered, nutrient-rich bedrock. In contrast, relatively unweathered, nutrient-poor bedrock supported thin forest stands that emerged unscathed from the drought. Hence, rather than enhancing forest resilience to drought by providing more water storage capacity, bedrock with more weatherable minerals induces greater vulnerability by enabling a boom-bust cycle in which higher ecosystem productivity during wet years drives greater plant water demand that cannot be satisfied during droughts. This boom-bust cycle is amplified by the correlation between nutrient and weatherable mineral concentrations in regional bedrock and by a positive feedback between previously underappreciated, abiotic, bottom-up controls on subsurface water storage capacity and more widely recognized, biotic, top-down controls on root distributions and subsurface weathering.