How multiple and interacting disturbances shape tree diversity in European mountain landscapes

Context: The relationship between disturbances and diversity remains uncertain, especially in forest landscapes where large spatial extents need to be considered, dynamics are slow, and disturbance interactions are common. Objectives: To analyse the individual and combined effects of ungulate browsing and wind disturbance on tree species diversity at the landscape scale. Methods: We used a dynamic forest landscape model to simulate the effects of browsing and wind disturbances (i.e., frequency and windthrow size) on tree species diversity in four mountain landscapes in Central Europe. Using boosted regression trees, we analysed the relative importance of each disturbance type for diversity at different layers (i.e., regeneration versus overstorey, and all tree sizes), the shapes of the diversity–disturbance relationships and the combined effect of wind and browsing disturbances on diversity. Results: Across all landscapes, browsing and windthrow were equally important for tree species diversity when considering all forest layers, but no consistent patterns could be observed for the regeneration and overstorey layer. The shape of the disturbance–diversity relationships differed between disturbance types. More frequent and severe windthrow events typically increased diversity almost linearly, while browsing showed a non-linear response with the highest diversity at intermediate browsing pressure. However, these relationships were not consistent across the four landscapes. Conclusions: Tree species diversity can be influenced by both browsing and windthrow disturbances. Forested landscapes are likely to experience multiple disturbances, and their relative influence on diversity needs to consider their different spatial and temporal scales.