Effects of tree spacing and thinning on root reinforcement in mountain forests of the European Southern Alps

Root reinforcement is a mechanism through which forests contribute to the prevention and mitigation of shallow instabilities in soils, one of the main hazards in mountain areas. This study aims to assess how spatial tree distribution and whether thinning operations affect root reinforcement in the most common forest types of the European Southern Alps. We measured size and position of the trees of 119 stands, belonging to sweet chestnut, European beech, Norway spruce, and spruce-beech-silver fir forest types. We developed, calibrated and validated a model for estimating root reinforcement at the stand level, using the spatial distribution of tree diameter as the input variable. Finally, we simulated a thinning of 18% stand basal area, selecting removal trees from smaller diameter classes (low thinning) or from random clusters (random cluster thinning), and assessed its effect on root reinforcement. Root reinforcement statistically differed among forest types and the lowest values were found in the sweet chestnut forest type. Irrespective of the forest type, low thinning did not significantly modify root reinforcement, while random cluster thinning reduced it five-fold.