Intra- and interspecific trait variation and abundance of dry grassland plants

One of the few laws in ecology is that communities consist of few common and many rare taxa. Functional traits may help to identify the underlying mechanisms of this community pattern, since they correlate with different niche dimensions. However, comprehensive studies are missing that investigate the effects of species mean traits (niche position) and intraspecific trait variability (ITV, niche width) on species abundance. In this study, we tested three predictions: species abundance a) increases (or decreases) with species mean traits, b) is highest for intermediate species mean traits, and c) increases with ITV. We measured three plant functional traits (specific leaf area, leaf dry matter content, plant height) at 21 local dry grassland communities (10m x 10m) and analyzed the effect of these traits and their variation on species abundance at the local and regional scale. In our analyses we compared phylogenetic-corrected with standard models, in order to reveal whether the relationships between traits and species abundance are influenced by the species’ phylogeny. At the local scale, we found that species abundance increased with species mean leaf dry matter content and the intraspecific variations of leaf dry matter content and plant height. In contrast, at the regional scale, plants with a higher ITV of plant height were less abundant. We found no evidence that the consideration of phylogenetic-relationships influenced significantly our findings nor that species with intermediate traits were more abundant. Overall, our results indicate that the tolerance towards environmental conditions rather than competitive ability drives species abundance. Hereby, ITV may be particularly beneficial in heterogeneous environments at the local scale. However, the contrasting effect of ITV at the regional scale shows that trait-abundance relationships seem to be highly scale-dependent.