Rockfall-Relevant Terrain Roughness in Disturbed Mountain Forests: Assessing the Effects of Lying Deadwood Using UAV Remote Sensing

Natural disturbances such as the 2018 windthrow Vaia and subsequent bark beetle outbreaks can strongly affect the protective function of mountain forests against rockfall. This study evaluated the relationship between the volume of horizontal deadwood, placed as natural barriers and their resulting terrain roughness relevant for rockfall mitigation, to improve the representation of biological legacies in rockfall modelling. The relation was assessed by including both manually placed logs and fresh windthrown conditions. We also examined how roughness changes over time in a windthrown site, analysing the impact of decay and settling on these structures. To achieve this, we combined UAV-based remote sensing with field surveys to quantify deadwood volume and monitor temporal changes. The RGB-based vegetation index proved to be an effective and robust tool for improving the accuracy of roughness quantification derived from deadwood structures. A logarithmic model based on the biomass volume best performs in representing the mean obstacle height of the 10 and 20% of the highest structures produced by the lying logs, further showing a saturation effect beyond approximately 200 m³ /ha of deadwood. Six years after the Vaia event, surface roughness remained high in unmanaged sites. Overall, these results highlight the relevance of lying deadwood as a quantifiable and persistent roughness component that can be incorporated into rockfall modelling to better represent the residual protective function of disturbed mountain forests.