Plant species loss due to forest succession in Alpine pastures depends on site conditions and observation scale

Cessation of agricultural management and subsequent natural forest succession has been the primary land use change in the Southern Alps over the past 50 years. It is generally assumed that early stages of succession host more plant species than grazed pastures, but that this richness is partly lost as the density of woody species increases. Based on vegetation surveys on eight sites in the Italian Alps, we found the effects of forest succession on plant species richness to depend strongly on environmental conditions. The relationship between plant species richness and wood cover at the sites ranged from non-detectable over hump-shaped, to monotonically decreasing. Linear mixed-effects models indicate that high mean annual temperature is associated with a strong decrease in plant species richness and in the number of red-list species along the pasture-to-forest gradient. Sampling plant species composition at a range of scales allowed us to rule out artefacts caused by modified species–area relationships as a consequence of changes in wood cover. Multi-scale sampling also indicated that the primary loss of plant species richness by forest succession is in plant species with low abundance. Our data further allow assessment of the risk of species loss in mountain grasslands in the Southern Alps, which is highest on sites with higher mean annual temperature. These areas should receive concentrated attention and support for biodiversity conservation.