Novel indices for analysis of protection against snow avalanches in wind-disturbed forest

Windthrows are natural disturbances affecting the structure of forests. Such events are predicted to increase in magnitude and frequency due to climate change. The altered forest structure influences forests’ protection capacity against the release of snow avalanches. Previous studies investigated windthrown forest characteristics and their recovery time in study areas smaller than 10 ha, not accounting for larger spatial scales. In this study, we developed a new method for the spatial assessment and monitoring of forests affected by large-scale windstorms to evaluate the protection against the release of snow avalanches. We propose the use of two indices: (1) stored volume height, assessing the uniformity of the biomass on the ground, and (2) adapted tree parameters, evaluating the characteristics of the standing trees. We implemented the algorithm in the R environment and published the scripts on https://github.com/TommBagg/Wind-disturbed_forest_analysis.

The developed indices were applied in two unmanaged windthrow areas periodically surveyed with photogrammetric techniques (deriving dense point clouds) to investigate the long-term changes in protective effects (Disentis, Switzerland) and the short-term influence of snow cover (Franza, Italy). The first area was affected by the storm Vivian in 1990 and four surveys have been performed (1991, 2001, 2009 and 2019), while the storm Vaia hit the Franza area in 2018 and three surveyed were performed (October 2019, October 2020 and December 2020). Analyzing the Disentis area, over 29 years, the minimum level of protective capacity was observed ten years after the storm event. As forest recovery proceeded, forest protective capacity increased again and natural regeneration has progressively provided an anchoring effect against the release of snow avalanches, 29 years after the windthrow event. However, a careful evaluation of the gaps between growing trees (outcome of the adopted tree parameters index) should be performed, as deadwood in these gaps decays, facilitating the potential avalanche formation. The stored volume height was further used to evaluate the forest protection in relation to the snow cover height in the Franza (IT) area, evaluating the necessary snow amount to smooth the rough surface created by the biomass on the ground. This study provided new insights into the long-term protective efficiency of windthrow forests, introducing two new indices to spatially assess and monitor their development over time.