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.

森林越来越受到气候变化引发的干旱、枯死和野火循环的威胁。然而，由于仍未完全了解的原因，一些林分比其他林分更脆弱，在干旱后留下了不同的枯死和野火风险。在这里，我们表明森林干旱响应的空间变异性可以通过底层基岩的差异来解释。我们的分析将基岩成分的地球化学测量、地下风化的地球物理测量和 2011-2017 年加利福尼亚干旱期间蒸发蒸腾的遥感变化联系起来。我们发现，茂密的森林中蒸发量急剧下降，这些森林植根于风化、营养丰富的基岩中。相比之下，相对未风化、营养贫乏的基岩支撑着薄薄的林分，而这些林分却没有受到干旱的影响。通过影响地下风化和养分供应，基岩成分调节山区生态系统中水储存和需求的平衡。然而，不是通过提供更多的蓄水能力来增强森林对干旱的抵御能力，而是具有更多耐候性和营养丰富的矿物质的基岩通过实现繁荣-萧条循环导致更大的脆弱性，在这种循环中，潮湿年份更高的生态系统生产力会导致植物对水分的过度需求干旱。