Vegetation trajectories in proglacial primary successions within Gran Paradiso National Park: A comparison between siliceous and basic substrates

Global warming has a major impact on alpine environments and glacier retreat is the most noticeable consequence of rising temperatures. Ice-free screes and moraines are subjected to the colonization of biota thus providing a unique opportunity for the study of primary successions. Plant primary succession trajectories depend on several environmental factors, but bedrock type is one of the most important drivers of change. We studied the primary plant successions along two proglacial chronosequences, i.e. on siliceous and calcschist substrates, within the Gran Paradiso National Park (North-Western Italy). Both the chronosequences encompassed about 200 years and were located at high elevation (2,450–3,000 m a.s.l.). We focused on three functional species pools, namely early-successional pioneer, late-successional grassland and companion species. Plant species and phytosociological communities were recognized as indicators of several environmental features, such as sediment mobility and particle size. Despite the basic substrate generally hosts more plant species, we found a greater species richness and diversity in the gneissic chronosequence. The higher heterogeneity of the siliceous debris likely promoted plant establishment and vegetation development towards climax community, even if it was not reached. Conversely, the chronosequence on the calcschist substrate showed higher plant cover in the more developed stages, but still dominated by several pioneer species.