The aim of the study is to investigate the relative importance of plant-plant interactions with regard to flooding and drought effect on perennial plant performances in wetlands. Flooding is expected to be the major driver and, accordingly, the importance of drought is hardly if ever taken into account. Focusing on five widespread species, the growth, the survival and the competitive ability of plants were monitored on permanent plots spread along two elevation gradients. Flooding duration and drought intensity were found to vary substantially along the ~ 0.5 meter range elevation gradient. Flooding and drought alternate over the hydrological year and the pin-point surveys were thus conducted over the course of one year. The data were modeled taking into account survival, recruitment and competitive growth throughout flooding and drying out periods. Flooding and drought both directly impacted the plant performances and their competitive effect, with the effect of drought being much more general among species and of higher magnitude than flooding. The importance of competition was found to be high for all species, particularly during the drying out period. It varied more along the flooding gradient than along the drought gradient. The higher flooding tolerance shown by the studied species compared to drought may be related to species specific growth timing together with efficient response traits. These results offer new insights into the filters operating over the species pools. This suggests that the drying out period and drought conditions may be even more important for species’ relative success and the importance of competition than the flooding pattern. The general applicability of this result, obtained in mild Atlantic climate and fertile wetlands, remains to be studied.

Summary: 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 whether and how local relationships between diversity, disturbance and productivity are affected by habitat filtering and regional climate. 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. 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 whether the variation in taxonomic and functional metrics corresponded to the DEH predictions as well as to determine the relationship between those metrics. 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. 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 multifaceted studies of biodiversity and the need for a more systematic quantification of regional controls in community assembly studies.