Recent intensification of forest disturbances in the Alps and implications for carbon dynamics

Changes in forest disturbances can have strong impacts on forest structure and carbon dynamics. Yet, we lack consistent long-term data on forest disturbance regime shifts and their implications for carbon stocks. Observational evidence is mandatory for developing adaptation strategies in vulnerable regions such as the European Alps. Here, we present an observation-based, alpine-scale characterization of forest disturbances and their temporal evolution, together with modelled impacts on above-ground biomass (AGB) under different disturbance regimes. We applied a Landsat-based time series change detection approach to classify stand-replacing disturbances across the Alps for the period 1984-2024. Disturbance regimes were characterized using metrics such as event frequency, severity, return intervals and temporal trends. Disturbance regime parameters (probability scale, clustering degree and intensity slope) were derived by decade and used in a model inversion framework to assess AGB responses under different disturbance regimes. Our results indicate a marked intensification of disturbances in recent years. While disturbance peak years were synchronized between the western and the eastern Alpine ranges, the western Alps did not exhibit an increasing disturbance trend. In contrast, disturbance severity in the eastern Alps has significantly changed in the last decade compared to previous decades with both a mean rise in disturbances and a higher frequency of years characterized by an extreme number of disturbance events. Spatially, this increase was widespread across the eastern Alps and not confined to distinct hotspot areas. Outputs from dynamic carbon simulations showed that under the current disturbance regime (2014-2024) AGB can be reduced by 25% relative to the past disturbance regime (1984-1994), with convergence times between regimes spanning between 10 to 100 years. Overall, our findings provide robust observational evidence of an ongoing forest disturbance regime shift in the eastern Alps and demonstrate its substantial impacts on forest carbon dynamics. This work provides spatial and temporal information for understanding changes in carbon dynamics in alpine forests as well as an empirical foundation for improving disturbance-aware carbon modelling. The outcomes can inform adaptation and management strategies.