Bedrock lithology determines forest demographic responses to drought

Forest vulnerability to drought depends not only on climate and traits, but also on bedrock controls on water dynamics, which remain poorly resolved. Here we integrate satellite-derived regolith water loss rate, a proxy for near-surface water retention capacity, with tree-ring records from 849 trees across 40 sites, carbon isotope measurements and mortality observations to examine forest demographic drought responses across karst and non-karst forests in southwest China. Bedrock lithology emerged as a key structural constraint on spatial variation in drought vulnerability, alongside climate, soil and trait variables. Notably, we identify a lithology-mediated inversion of demographic vulnerability: in karst forests, younger and smaller trees showed lower resistance and recovery than older and larger trees, especially under prolonged drought, whereas non-karst forests showed contrasting patterns. Site-level isotope and mortality evidence indicated physiological stress and growth decline consistent with this contrast, highlighting bedrock lithology as a geophysical constraint on forest demographic drought vulnerability. Drought threatens forests worldwide, but bedrock effects are overlooked. This study suggests that bedrock lithology reverses demographic drought responses in karst forests, leaving younger and smaller trees especially vulnerable to prolonged drought.