Abstract Extreme drought and heat are driving forest die‐off globally, yet regional‐scale mapping of subsurface water storage capacity, governing forest vulnerability, remains a major challenge. Here, we demonstrate that airborne gamma‐ray spectrometry (GRS) of potassium (40K) decay provides a powerful proxy for subsurface constraints on water availability. In southwestern Australia, we found that spatial patterns of forest die‐off during an extreme 2023–2024 drought‐heat event were strongly predicted by surficial 40K concentrations. While GRS measures the upper ∼30 cm of soil, plot‐scale electrical resistivity tomography suggests that elevated 40K signals shallow granite or gneiss bedrock, to depths reaching several tens of meters due to leaching of mobile elements, remnant bedrock minerals, erosional thinning of regolith and biological nutrient recycling from K‐rich saprolite. Airborne GRS can map forest vulnerability across highly leached landscapes, which cover one‐third of Earth’s ice‐free land surface. This enables targeted adaptive management of forests for future drought‐heat intensification.