When Forests Fail: The Paradox of Tree-Induced Rockfalls in Protection Forests

Protective forests in mountainous regions play a crucial role in shielding human settlements from natural hazards, acting as effective barriers against rockfalls, avalanches, flowing water, and erosion, while providing partial protection against surface landslides and debris flows. However, under certain conditions, the same trees that constitute these forests may not only fail to fulfill their protective function, but can also become a source of hazard themselves, contributing, either directly or indirectly, to the initiation of rockfall events. This thesis investigates the relatively unexplored phenomenon of tree-induced rockfalls and provides the first quantitative data on its occurrence. A case study in a former chestnut grove region in the Canton of Ticino evaluated the effectiveness of orthophotos as a method to identify uprooting events, one of the main causes of tree-related rockfalls. Verification through GPS surveys, in addition to assessing the feasibility of this approach, taking into account different terrain morphologies and vegetation structure, allowed the characterization of uprooting under various factors, including slope, curvature, tree diameter, number of shoots and direction. It has been observed that this phenomenon is expanding across all types of terrain curvature and even on slopes below 30°, but slopes exceeding 30° and tree diameters over 30 cm were identified as the most influential factors. Subsequently, the analysis of vegetation-induced rockfall events, selected from the StorMe natural event database for the Ticino region, allowed a quantitative assessment of the phenomenon’s extent and a preliminary characterization of this natural hazard. The most notable finding is that its occurrence has increased markedly over recent decades. The areas most susceptible to this natural event are those with steep slopes (>40°) and complex terrain morphologies, including both concave and convex zones.