Deriving and assessing forest gap thresholds to prevent shallow landslides in Swiss mountain forests

Protective forests are crucial for maintaining slope stability and play a vital role in mitigating natural hazards in mountain environments. Forest gaps influence shallow landslide occurrences and significantly impact forests, shaping their development, resistance, resilience, overall ecology, and biodiversity. Understanding the extent of forest gaps is thus essential for effective forest management, balancing protective effects with ecological functions and the promotion of regeneration. We determine practice-oriented gap dimension thresholds (length, width, area), assess how slope gradient and vegetation influence these dimensions, and discuss the trade-offs between protective capacity and adaptive management. We compiled a comprehensive dataset from landslide inventories and remote sensing data across six landslide-prone regions in Switzerland and statistically analysed 632 forest gaps. The probability of landslide occurrence increases with gap length, width, and area. This is consistent across different slope gradients. For gaps with more than three trees and a vegetation cover percentage exceeding 25 %, the dimensions of landslide-associated gaps are significantly larger than those without landslides. We suggest applying logistic regression to identify thresholds that indicate landslide occurrence probabilities, thus improving awareness of the related risk levels. Based on 25 % and 33 % probability levels, we recommend a maximum gap length and width of 22 m, with a gap area of 500 m²; where natural regeneration is ensured, these parameters could be increased to 30 m by 32 m and 1000 m², respectively. The suggested threshold values should be regarded as references and require on-site evaluation. Our outlined approach relies on dialogue and close cooperation with forest management practitioners.