Résumé
Global change is affecting the distribution and population dynamics of plant species across the planet, leading to trends such as shifts in distribution toward the poles and to higher elevations. Yet, we poorly understand why individual species respond differently to warming and other environmental changes, or how the trait composition of communities responds. Here we ask two questions regarding plant species and community changes over 42 years of global change in a temperate montane forest in Québec, Canada: (1) How did the trait composition, alpha diversity, and beta diversity of understory vascular plant communities change between 1970 and 2010, a period over which the region experienced 1.5 C of warming and changes in nitrogen deposition? (2) Can traits predict shifts in species elevation and abundance over this time period? For 46 understory vascular species, we locally measured six aboveground traits, and for 36 of those (not including shrubs), we also measured five belowground traits. Collectively, they capture leading dimensions of phenotypic variation that are associated with climatic and resource niches. At the community level, the trait composition of high-elevation plots shifted, primarily for two root traits: specific root length decreased and rooting depth increased. The mean trait values of high-elevation plots shifted over time toward values initially associated with low-elevation plots. These changes led to trait homogenization across elevations.