Vegetation changes with climate change in the Grandes Rousses mountain range

Questions: We assessed interactions between climate change, bedrock types
and snow cover duration on the trajectories of taxonomic and functional
composition of subalpine plant communities. We predict (i) an increase in
species richness on siliceous bedrock due to a reduced competition and a
decrease in richness on calcareous bedrock due to increasing drought
stress, (ii) decreasing snow cover duration should induce a higher shrub
encroachment in hollows as compared to ridges (iii) increasing growing
season temperature should induce taller sizes and more conservative growth
traits, in particular in hollows. Location: Subalpine belt of the Grandes
Rousses mountain range, south western Alps (France). Methods: 189
vegetation plots were sampled in 1997 and 2017-2018. The duration of snow
cover was assessed during two years in 1995-1997 and five functional
traits were measured on 108 species in 2021. We performed multivariate
analyses, quantified community weighted-means (CWM) of traits and used
ANOVAs to detect responses to local-scale factors and changes in snow
cover, temperature and precipitation since 1997 according to a nearby
meteorological station. Results: Overall, taxonomic composition weakly
changed and changes were more dependent on the position of communities
along the snow cover duration gradient than on their bedrock type. The
abundance of drought-tolerant species increased at the border of hollows
and there was, over all communities, a slight increase in the abundance of
dwarf shrubs and tall herbaceous species, a strong decrease in short
herbaceous species and, thus, an overall decrease in species richness.
There were important overall changes in CWM of size traits, in particular
leaf area which increased the most in hollows irrespective of bedrock
types. Conclusion: In this subalpine site the effects of decreasing snow
cover duration overwhelmed the effects of bedrocks, which may explain the
overall increase in competitive species and decrease in species richness.