Maintaining rockfall protection in mountain forests under climate change: optimizing management for sustainable stem size distributions

Climate change threatens the long-term effectiveness of mountain forests, which provide crucial protection against rockfall. Maintaining this protection function requires a sustainable stem size distribution, yet how to adapt forest management for this purpose remains unclear. This study uses a simulation-based optimization approach, integrating the dynamic forest model ForClim with the Simulated Annealing optimization algorithm, to identify adaptive management strategies for Swiss forests. We first established sustainable stem size distributions for managed protection forests in four elevation zones − lower montane to subalpine − under historical climate, leading to a so-called target profile. These represent a novel indicator enabling foresters to tailor silvicultural interventions towards improving rockfall protection. Subsequently, we assessed climate change impacts on these distributions. Our simulations show that climate change will alter stem size distributions, particularly at higher elevations where a reduction of soil water availability will hinder regeneration and growth. This leads to fewer trees, especially smaller ones. We developed optimized management regimes to counteract this effect, recommending specific adjustments depending on elevation zone and management type, such as less frequent and less intensive harvesting with larger minimum removal DBH in higher-elevation mountain forest plentering, and adjustments to target DBH and harvest intensity in lower-elevation plentering. This study demonstrates that adapting silvicultural interventions can preserve the desired forest structure under climate change, without fundamental regime shifts. These findings provide practical guidance for forest managers, enabling them to proactively respond to climate change impacts and ensure the long-term functionality of rockfall protection across elevation zones.