Forest hazard protection against snow avalanches after windstorms in mountain areas

Windstorms are natural disturbances that are predicted to increase in frequency in the next future due to climate change, with consequent increasing risk of damages to forests. In the case of protection forest, such damages change the forest structure and therefore the protection capacity. Previous studies analysed post-event conditions and the recovery time of abated forest within small study areas (<10 ha), not accounting for larger spatial scales. In this study, we propose a new methodology with the aim to provide tools for spatial assessment and monitoring of forests affected by windstorm over large areas and previously voted to protection against snow avalanches. Four indices have been used (vegetation height model, surface roughness, stored volume height and adapted tree parameters) of which the latter two have been specifically developed for this goal. We selected and periodically recorded using photogrammetric surveys (deriving dense point clouds) two windthrown areas, in order to assess the performance of the proposed indices and to investigate the long term changes in protection effects (Disentis, CH) and the influence of snow cover (Franza, IT). Results showed that stored volume height and the adapted tree parameters resulted to be the best indicators to capture the forest conditions and the standing trees, respectively. Analysing the Disentis (CH) area, we concluded that the minimum level of protection capacity occurs 10 years after the storm event. However, even after 29 years the forest has not yet completely recovered in terms of mitigation efficiency against natural hazards. This study provided new insights on the long-term protection efficiency of windthrown forests, proposing two new indices to spatially assess and monitor their evolution.