Quantifying patch size distributions of forest disturbances in protected areas across the European Alps

Aim: Natural disturbances are key drivers of forest ecosystem dynamics and are highly sensitive to global change. Despite their importance, central disturbance characteristics remain unknown for many forests worldwide. Here, we quantified an important component of the forest disturbance regime—the distribution of patch sizes—in strictly protected areas by asking: (i) How are patch sizes of naturally occurring disturbances distributed across the Alps and how can they best be quantified? (ii) Are patch size distributions stochastic or can they be explained by environmental drivers? (iii) What are the return periods of extreme disturbance events?. Location: European Alps. Methods: We analysed satellite-based disturbance maps for the period 1986–2020 across a network of 12 strictly protected areas, modelling patch sizes of all observed disturbance patches as well as of annual extreme events. We tested the influence of temperature, precipitation, topographic complexity and forest type on patch size distributions. Results: Disturbance patch sizes across the Alps (median 0.36 ha, 5th percentile 0.18 ha and 95th percentile 1.71 ha) as well as their annual extremes (0.72 ha, 0.18–7.11 ha) are best described by a Fréchet distribution. The size of annual extreme events significantly increased with intra-annual temperature amplitude (+0.98 ha with a one standard deviation increase) and the share of evergreen trees (+0.63 ha). On average, disturbance patches of 5.5 ha (95% credible interval 2.6–17.5 ha) occur once every 30 years, whereas patches of 2.6 ha (1.2–7.0 ha) occur once every 10 years. Main Conclusions: Disturbances caused by natural agents are generally small and stochastic across the Alps. Extreme events are driven by climate, suggesting sensitivity of disturbance patch sizes to climate change. Our results provide a baseline for monitoring climate-induced changes in forest disturbance regimes, and provide important information for the management and conservation of forest ecosystems.