Assessing forest resilience after windthrow: linking deadwood, regeneration, and landscape-scale susceptibility - A case study in a Pre-Alpine forest in Italy

Severe windstorms account for nearly half of all damage to European forests. South of the Alps, windstorms such as Storm Vaia in 2018 and Storm Alex in October 2020 have left forest managers uncertain on how to assist habitat recovery and how to prevent further damage in the intact portions of the forest. Since storms did not occur regularly in this part of Europe, questions remain about whether to remove windthrown deadwood and which stand, topographic, and microclimatic traits most strongly predispose forests to future windthrow. This study investigates the characteristics affecting the likelihood of windthrow and the effects of windthrow severity on forest regeneration and habitat recovery across three study sites in Alto Verbano, Lombardy, Italy. The objectives of the study were twofold: (i) to examine the relationship between deadwood accumulation and forest regeneration, and (ii) to assess the influence of forest structure, topography, and microclimate on windthrow susceptibility. We combined field sampling with drone-based optical imagery to collect detailed data on deadwood biomass and cover, as well as forest regeneration. We investigated the relationship between deadwood and regeneration indices using Generalized Linear Models and Structural Equation Modeling to test for effects both direct and indirect, i.e., through changes in topographical and environmental factors. To assess windthrow susceptibility, we used Sentinel-2 satellite data, integrated with microtopographic information from satellite products, to assess landscape-level vulnerability to windthrow using non-parametric machine learning methods. The results reveal a negative influence of deadwood on regeneration, irrespective of topographical features and threats to regeneration levels at the sites (i.e., ungulate browsing). On the other hand, forest structure, topography, and microclimate had a critical role in determining forest vulnerability to windthrow. These findings emphasize the importance of carefully considering deadwood management and future forest structural features when carrying out forest restoration actions. Assessing the consequences of deadwood accumulation and understanding forest vulnerability to windthrow are essential for optimizing forest regeneration and resilience.