Nationwide deadwood mapping reveals rising mountain forests vulnerability

Forest mortality is increasing globally under climate change, making detailed, large-scale monitoring essential for understanding ecosystem responses and guiding adaptive forest management. In this study, we present the first nationwide spatio-temporal assessment of standing deadwood in Switzerland, covering the period from 2018 to 2023, based on a semantic segmentation model applied to centimeter-scale high-resolution aerial imagery. We reveal a consistent upslope concentration of standing deadwood, with highest shares occurring around mid to high elevations (~1,500 m), despite declining forest cover. Relative increases of up to 43% were observed in overlapping survey areas, following the 2018 drought. Random forest models, interpreted using SHAP analysis, identified maximum temperature anomalies and conifer dominance as the key predictors of standing deadwood. The consistent accumulation of standing deadwood at higher elevations suggests increasing vulnerability of mountain forests, with implications for carbon storage, biodiversity, and disturbance susceptibility under ongoing climate change. Our results highlight the value of high-resolution remote sensing for large-scale monitoring of forest mortality. They advance understanding of the climatic and forest compositional drivers on forest mortality and offer a reproducible and transferable framework to support assessments of spatial patterns relevant for climate-adaptive forest management.