Ecological communities and their response to environmental gradients are increasingly being described by various measures of trait composition. Aggregated trait averages (i.e. averages of trait values of constituent species, weighted by species proportions) are popular indices reflecting the functional characteristics of locally dominant species. Because the variation of these indices along environmental gradients can be caused by both species turnover and intraspecific trait variability, it is necessary to disentangle the role of both components to community variability. For quantitative traits, trait averages can be calculated from ‘fixed’ trait values (i.e. a single mean trait value for individual species used for all habitats where the species is found) or trait values for individual species specific to each plot, or habitat, where the species is found. Changes in fixed averages across environments reflect species turnover, while changes in specific traits reflect both species turnover and within‐species variability in traits. Here we suggest a practical method (accompanied by a set of R functions) that, by combining ‘fixed’ and ‘specific averages’, disentangles the effect of species turnover, intraspecific trait variability, and their covariation. These effects can be further decomposed into parts ascribed to individual explanatory variables (i.e. treatments or environmental gradients considered). The method is illustrated with a case study from a factorial mowing and fertilization experiment in a meadow in South Bohemia. Results show that the variability decomposition differs markedly among traits studied (height, Specific Leaf Area, Leaf N, P, C concentrations, leaf and stem dry matter content), both according to the relative importance of species turnover and intraspecific variability, and also according to their response to experimental factors. Both the effect of intraspecific trait variability and species turnover must be taken into account when assessing the functional role of community trait structure. Neglecting intraspecific trait variability across habitats often results in underestimating the response of communities to environmental changes.

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This work shows the applicability of a set of protocols that can be widely applied to assess the impacts of global change drivers on species, communities and ecosystems.

Summary 1.Understanding the processes by which species sort themselves into communities remains a central puzzle for attempts to maintain biodiversity. It remains unclear whether any single assembly process is generally dominant or whether the influence of contrasting processes varies in a predictable way relative to biotic and abiotic gradients. Abundance-weighted niche overlap between species provides a powerful means of contrasting two major assembly processes -niche complementarity and environmental filtering. 2. We examined mean overlap for four vegetative functional traits, relative to that expected when abundances were randomly allocated to species co-occurring in experimental plots in a wet meadow. This provided a test of whether any single assembly process prevailed for the meadow as a whole and across all traits. The effects of mowing, fertilization and dominant species removal, and associated gradients of Simpson's dominance and biomass on the niche overlap of plots, were also examined. 3. Niche overlap was higher than expected at random for three of the four traits studied (height, leaf and stem dry matter content, leaf C:N ratio). However, niche overlap was lower than expected for specific leaf area. 4. Mowing was the treatment with the greatest effect on both niche overlap and biomass, with overlap significantly lower in the absence of mowing for three of the traits, while biomass was lower in mown plots. For three of the traits there was evidence of a significant decrease in overlap with increasing biomass, but not increasing dominance. None of the significant mowing effects on overlap remained when the effect of biomass had been removed. 5. Synthesis: These results suggest that the importance of niche differences between species in structuring grassland communities should increase with increasing biomass and decrease with disturbance in grassland communities. They also emphasize that contrasting community assembly processes may occur for different niche axes, even within a single community.