Disentangling Atmospheric and Water Shortage Stresses Is Key to Anticipate Forest Dynamics With Climate Change

ABSTRACT Aim Studies assessing changes in vegetation with climate change have not separated atmospheric and water shortage stresses, although they drive different vegetation patterns in relation to continentality, water balance and bedrock types. We aim to assess how these two stresses will likely impact forest communities. Location European Alps. Taxon Angiosperms and Conifers. Methods We selected from the literature 773 forest plots that were sampled for vegetation composition during the 20th century. We collected at each plot climate variables from databases and calculated several climate indices (including winter rainfall continentality and summer water balance) at two periods (1965–1994 and 1995–2024). We conducted a canonical correspondence analysis (CCA) on the vegetation (understory species) and environmental tables (including eight 1965–1994 climatic variables and bedrock type, exposure, and canopy cover). The significance of differences in climate conditions between the two periods was tested separately for each climate variable and each of the 19 community types identified by a cluster analysis conducted after the CCA. Results Elevation, continentality and bedrock were the three primary complex environmental variables affecting temperature, atmospheric and water shortage stresses, factors directly driving vegetation composition. Climate change induced an overall decrease in summer water balance and an increase in temperature and continentality. However, a stronger increase in water shortage stress was observed for high elevation sites from northern latitudes with a high summer water balance, while a slight decrease in atmospheric stress was observed for summer‐dry continental communities from the southwestern French and Italian Alps. Main Conclusions Atmospheric and water shortage stresses have specific effects on forest community composition and therefore need to be accounted for when predicting changes in vegetation composition with climate change.