Résumé
1. According to the dynamic equilibrium hypothesis (DEH), plant species
richness is locally controlled by productivity and disturbance. Given that
regional conditions widely affect local environmental variables such as
soil nutrient availability, the DEH predictions could be improved by
considering how climate influences local controls of species richness.
Further, a trait-based approach to community assembly has the potential to
reveal a deeper, mechanistic understanding of species richness variation
across environments. Here we bring together DEH and trait-based community
assembly expectations to examine if and how local relationships between
diversity, disturbance and productivity are affected by habitat filtering
and regional climate. 2. We specifically tested how gradients of local
nutrient availability and disturbance intensity interact with climatic
conditions to drive the species richness of grassland communities.
Further, we recast the DEH through a functional lens by exploring how
disturbance-diversity and nutrient availability-diversity relationships
are shaped by the functional space occupied by species in a community and
species packing within this functional space. 3. The functional space
occupied by co-occurring species and the way they are functionally packed
are quantified using multi-trait indices calculated with five core plant
functional traits. Working with grassland communities spread across
differing regional climatic conditions, we used mixed models to test if
the variation in taxonomic and functional metrics corresponded to the
dynamic equilibrium model's predictions as well as to determine the
relationship between those metrics. 4. Contrary to the expectations based
on the relation between species richness and the functional components
considered, taxonomic and functional metrics did not vary in accordance
along environmental gradients. Climate strongly interacted with the local
environment to modulate local diversity patterns, sometimes even inversing
a given trend and falsifying the DEH predictions. 5. Synthesis. Our
findings quantitatively highlight the interplay between regional and local
environmental gradients in driving community assembly. We demonstrate
that, depending on climatic conditions, observed patterns of both
taxonomic and functional community composition can be opposite to expected
productivity-diversity and disturbance-diversity relationships. This
emphasizes the relevance of multi-faceted studies of biodiversity and the
need for a more systematic quantification of regional controls in
community assembly studies.