Naturalness of forest composition affects vulnerability to climate change and disturbances in Alpine mountain landscapes

Abstract

European mountain forests have been strongly shaped by past human activities, which have influenced their structure and composition. Assessing the natural tree-species composition of current forest landscapes is essential for evaluating their biodiversity potential and for informing management prioritization. High levels of compositional naturalness are often associated with greater ecosystem functioning, but it remains unclear whether forest landscapes that are closer to their potential forest composition are also less vulnerable to future climate change and natural disturbances. Using a process-based forest landscape model, we quantified the naturalness and the vulnerability to disturbances across a large forested area in the Italian Alps. We developed a spatially-explicit index to evaluate how closely current tree species composition matches potential forest composition. We then simulated future forest dynamics under multiple climate change and disturbance scenarios, using two different initial vegetation conditions on the same landscape - potential vs. current forest - and compared their vulnerability based on changes in species dominance, vegetation structure, and height heterogeneity. Results indicate that current forests exhibit generally low naturalness compared with their potential forest composition, reflecting historical management and agro-silvopastoral practices. The naturalness score changed depending on elevation across the landscape: forests at low (<1500 m a.s.l.) and high (>2100 m a.s.l.) elevations had low naturalness, while those in the mid-elevation range (1500-2100 m) exhibited medium to high levels of naturalness. Vulnerability to disturbances under climate change differed markedly between the two initial vegetation conditions. Current forest was more susceptible to bark beetle outbreaks, driven by past promotion of Norway spruce and further amplified by warming. In contrast, the potential forest was more vulnerable to wind disturbance, likely due to old-growth characteristics, such as greater height heterogeneity and canopy roughness, that increase blowdown susceptibility. This study provides the first assessment of forest naturalness using spatially explicit dynamic landscape modelling. Given the projected intensification of natural disturbances under future climates, our findings suggest that promoting more natural forest conditions alone may not guarantee higher resilience to climate-induced disturbances. Instead, management approaches should aim at increasing landscape-level structural and compositional heterogeneity in a balanced manner to minimizing future disturbance vulnerability.