Plant community assembly in Mediterranean grasslands: understanding the interplay between grazing and spatio‐temporal water availability

Abstract

AbstractQuestionsHow does grazing affect taxonomic diversity and functional structure of Mediterranean grassland communities? How do spatial and inter‐annual variations in water availability, as a proxy for productivity, modulate grazing effects? Are shifts in taxonomic diversity systematically mirrored by analogous changes in functional diversity along these gradients?LocationMediterranean grasslands in central Spain.MethodsWe surveyed grassland plant communities in 3 yrs with contrasting mean annual rainfall (total n = 441 plots). Grazing gradients were quantified by periodic visual observation. DEM and annual rainfall data were used to quantify water availability. We examined the effects of grazing and spatio‐temporal water availability on taxonomic diversity (TD; species richness), functional diversity (FD; Rao's Q) and community mean trait values (CMT) for three key plant traits (specific leaf area –SLA–, height and seed mass). Functional redundancy was discussed through the relationship between TD and FD trends.ResultsThe results for TD, FD and CMT showed that environmental filtering determined the differences between plots with different grazing and water availability conditions. In contrast to seed mass FD, FD of vegetative traits (height and SLA) was highly decoupled from TD as a result of both spatial and inter‐annual variations in water availability. Grazing reduced TD and functional redundancy only in the wettest year (i.e. in the absence of drought filtering), but selected for species with grazing tolerance and grazing avoidance strategies (reflected by high SLA CMT and low height CMT, respectively) in all water availability conditions.ConclusionsOur results highlight the importance of grazing and both spatial and temporal variation in water availability as drivers of the assembly of Mediterranean grassland communities. The complex and decoupled responses that we found confirm that future studies should combine the use of different analytical methods to elucidate the multiple facets of community change, along with an optimal characterization of livestock pressure and its potential interactions with habitat productivity.