Changes of rock glacier vegetation in 25 years of climate warming in the Italian Alps

Rock glaciers are periglacial landforms sensitive to climate change, and a harsh environment for vegetation colonization due to the potential occurrence of surface instability. Changes of rock glacier vegetation would provide evidence of the importance of climate change impacts, as the limitation provided by physical disturbance could be overcome by plant responses to warming. We assess, through the resurvey of ten rock glaciers (six active, four inactive) in the Italian Central Alps, the vegetation changes in response to 25 years of climate warming. Vegetation was analyzed through the phytosociological method in 1995 and 2020 adopting the same field protocol for both surveys with 1208 relevés. The relation with climate was assessed by multivariate analysis. Air warming occurred in the study area and vegetation changed both in active and inactive rock glaciers, with increases of species richness, cover, changes in floristic composition. After 25 years the differences between active and inactive rock glaciers persisted but with an appreciable decrease mainly due to the larger vegetation changes showed by active rock glaciers. The multivariate analysis confirmed the relation between vegetation changes and air warming. The observed vegetation changes were compatible also with the impact of the decrease of the physical disturbance limitation likely due rock glacier stabilization, in particular for the active rock glaciers located at lower elevations, while the opposite trend could be hypothesized for those at higher elevations. The changes of floristic composition allowed assess that a gap-filling process, instead of species upward migration, produced the vegetation changes, with species persistence coupled to the recruitment of new species from the neighbor communities. Persistence of climate warming in future could lead to vegetation homogenization and biodiversity loss due to the extinction debt of several alpine species.