Biological legacies as nature-based solutions to maintain protective effects in alpine mountain forests

Protective forests play a fundamental role in mountain areas. Natural disturbances, such as windthrow, fire, and bark-beetle outbreaks are affecting their stability and the provision of protection against rockfall. These events generate “biological legacies” (standing and lying deadwood) that may partially mitigate the post-disturbance protection gap, but the service life of these legacies and their remaining protective contribution are rarely quantified or parameterized in operational rockfall modelling. This thesis examines how disturbance legacies evolve spatially and temporally and how they can be translated into model-ready inputs to support scenario-based risk assessment and post-disturbance management in Norway spruce - dominated protective forests. Two complementary pathways are developed to represent biological legacies in rockfall simulations. First, UAV LiDAR and RGB surveys are combined with field inventories to quantify deadwood-driven surface roughness and to link lying deadwood volume (m³/ha) to terrain-roughness classes applicable as rockfall model inputs. Optimized RGB-derived vegetation indices improve downed-log detection under variable conditions, strengthening reproducibility. Six years after disturbance, uncleared windthrow stands still exhibit substantial roughness where lying deadwood exceeds approximately 200 m³ ha⁻¹, indicating persistent mitigation potential during settling and early decay. Second, standing deadwood (snags) is represented through decay-dependent reductions in maximum impact energy dissipation. Rapid decay classification is combined with class-wise dry densities to derive a density-based decay index relative to fresh wood. Using a density-fracture-energy relationship, the index is translated into decay-specific tree factors for rockfall modelling, enabling systematic reductions in dissipated energy with advancing decay and facilitating stand-level scenario assessments. Finally, long-term moisture monitoring of windthrown Norway spruce logs using TDR sensors indicates that deadwood moisture dynamics are strongly influenced by decay state and microsite conditions, with differences in event-driven recharge and seasonal dry-down patterns. Stand-level upscaling suggests that deadwood can provide substantial near-surface water storage (up to approximately 92 m³/ha), supporting regeneration-favourable microsites and linking biological-legacy retention to stand recovery processes. Overall, the thesis delivers a practical framework to integrate biological legacies into rockfall modelling, reducing uncertainty in post-disturbance assessments and supporting adaptive, restoration-oriented management to shorten the protection gap in alpine protective forests.