A continuous upward shift of the upper treeline is observed in many mountain regions worldwide. Such forest expansion is causing significant impacts on ecosystem services, especially as it may reduce avalanche hazards. Forests mitigate the release of snow avalanches mainly through snowpack stabilization. Trees prevent the creation of continuous weak layers thus decreasing the probability of slab release. Forest density, size of forest gaps, tree height and tree species are key factors to estimate the forest protective effect against snow avalanches. Field observations indicate that a certain tree height is crucial for the prevention of avalanche release in young forest stands and that tree height should be at least one-and-a-half to two times taller than the snow cover, with required tree height increasing with slope inclination. In this study, we analysed the relationship between avalanche occurrence, tree height and snow height within the long-term afforestation research area Stillberg near Davos, Switzerland. A decreasing number of avalanche releases was observed in relation to changes in tree height in the afforestation during the last 49 years. Furthermore, the spatial distribution of avalanche release areas shifted from covering most of the studied area of Stillberg to three local avalanche release areas within gullies. These gullies were characterized by lower tree densities but larger tree heights and larger snow accumulation. The analysis of all avalanches at Stillberg indicated that the ratio of tree height to snow height had contrasting effects on the release of loose snow avalanches and slab avalanches; a high ratio hindered the formation of slab avalanches, but under certain conditions some loose snow avalanches released. Areas prone to loose snow avalanches typically have few or small Larix and lack evergreen trees. Overall, the Stillberg afforestation after almost 50 years, together with local temporary protection measures for the gullies could be effective in mitigating avalanche activity.