Faster than expected: 5-year re-surveys reveal accelerating plant colonization in two proglacial forelands of the Gran Paradiso National Park (NW Italian Alps)

Proglacial vegetation communities are facing major challenges today due to fast glacier retreat and global warming, resulting in possible changes in plant colonization patterns. By re-surveying permanent plots along two alpine chronosequences (encompassing 5 to 165 years from deglaciation) with a 5-year time span, we aimed to (i) assess short-term vegetation changes and (ii) compare current vegetation trajectories with those predicted by the chronosequence approach. We analysed the number and cover of total, pioneer, and alpine grassland species and calculated the slopes of the trajectories of the chronosequence and observed on permanent plots. As expected, the number and cover of total, pioneer, and grassland species increased during the 5 years of the study. Moreover, we observed a significant acceleration in plant colonization (21 and 45 times faster increase in species richness and cover, respectively) compared to the chronosequence predictions. These dramatic changes in vegetation dynamics of proglacial plant communities were probably induced by the increased temperatures and longer growing seasons occurring in the Alps.