Community assembly along climatic gradient: Contrasting pattern between- and within- species

Grime’s CSR classification of functional strategies in terms of competitors, stress-tolerators and ruderals provides a helpful framework for understanding and predicting vegetation responses to environmental changes. To evaluate the importance of alternative processes that structure plant communities, it is useful to disentangle the community functional variation into interspecific and intraspecific components and assess their degree of co-variation. Few efforts have been made to investigate the habitat filtering theory in all the interspecific and intraspecific components of a plant community. We hypothesized that under intense climatic conditions, such as drought and cold, functional diversity would decrease towards the dominance of stress tolerant strategies, and that such trends would be reinforced by similar variation at the intraspecific level. We investigated the effect of climatic variation on functional diversity and on community-weighted mean along an elevation gradient in central Italy that ranges from dry and warm climatic conditions at lower elevation to cold and moist ones at higher elevation. We ran regression models to disentangle the total community components for both functional diversity and community-weighted mean into interspecific effect, intraspecific effect and their covariation along the climatic gradient. Our observations were in line with the theory of habitat filtering: we found lower diversity of the Grime strategy for species at both warmer and colder climatic conditions, with dominance of the stress-tolerant strategy. Similarly, the intraspecific effect was lower in cold conditions but higher under drier conditions, which seems to indicate that different processes act at the level of individuals. Given the important intraspecific variability observed in this study, it can be proposed that investigations of vegetation communities should take the role of intraspecific variability into greater consideration.