Assessment of protective effect of wind-disturbed forest against snow avalanches

Mountain forests provide various ecosystem services, including the protection against snow avalanches, which is essential for Alpine communities. However, storms, one of the primary drivers of large-scale forest disturbances in protective forests, can change the forest structure and thus the protective effect against avalanche formation and release. This can potentially lead to a protection gap, where the forest cannot fulfil its protective function. The assessment of the remaining protective effect of these areas against snow avalanches is crucial for decisions regarding the most effective post-disturbance forest and risk management. Previous studies have shown that unmanaged windthrow areas often exhibit a high protective effect against avalanches as the lying stems and root plates lead to a high roughness and prevent the formation of spatially continuous weak layers, but their protective effect changes over time due to decomposition. However, an objective, reliable, and easy-to-apply assessment for monitoring the protective effect is still lacking.

 

Building on a recent study that introduced protective effect indices in windthrow areas derived from drone-based photogrammetric data, we refined and integrated this approach into a semi-automated and comprehensive framework for supporting the assessment of said protective effect of windthrow areas against snow avalanche release. The framework includes 1) the processing of a dense point cloud representing the deadwood structure, 2) the detection of remaining standing trees and their crowns, 3) the determination of the critical snow depth required to cover the deadwood and reduce surface roughness below a defined threshold favourable for avalanche release, 4) adding a relevant snow slab thickness, and 5) assessing the return period of such snow depths. The output of this framework are spatial protective effect indices identifying critical zones for potential avalanche release.

 

We demonstrate the applicability of the framework in various windthrow case study sites, exhibiting different characteristics in severity, deadwood structure, number of standing trees and slope steepness. The results show that the return period for the snow depth required for potential avalanche release is mostly above 30 years but locally varies strongly depending on the existing deadwood structure. This study is a crucial step toward providing an objective decision-support tool for practitioners and decision-makers to effectively manage windthrow areas in protective forests against snow avalanches.