Plant community assembly and the role of species identity, abundance, and intraspecific functional trait variability in alpine meadows

Trait-based ecology has provided important insights into the mechanisms of how plant communities are assembled and structured. Many key functional traits have been found to vary between communities, but also within species at relatively small scales. However, intraspecific variability of functional traits is usually taken into account only partially, if at all, in community analyses. In order to determine whether this deficiency is important, I quantified the full distribution of functional traits by sampling every individual in small scale, local meadow communities along a 900 m elevational gradient in Gunnison Co., Colorado, USA. I quantified the community-wide individual trait distributions (ITDs) of leaf mass per area (LMA), leaf area (LA) and plant height (PH) at five sites, including 2,253 plants belonging to 54 species. Variance partitioning analyses revealed that 14.4 ‰ÛÒ 43.8% of the variation in trait values occurs within species, suggesting that intraspecific trait variation may play an important role in community assembly. Moreover, the shape of ITDs changed substantially along the gradient, which shows that superficially similar communities can differ markedly in the relative abundance of functional traits. Null modeling detected shifts towards low LMA at the upper end of the elevation gradient, while the distributions of LA and PH do not differ from random expectations based on the traits of the regional species pool. This provides evidence for the influence of environmental filtering, and suggests that short growing seasons at high elevations may select for species with high rates of photosynthesis and production. However, this directional shift in LMA was evident even in the absence of intraspecific trait variation. Thus, despite the large trait differences found within species and differences in the ITDs among sites, intraspecific trait variability appears to be overshadowed by species turnover along the elevational gradient. An additional null model analysis based on species presence/absence resulted in a number of false positive results, indicating that species abundance is crucial for correctly detecting community assembly processes. This study reinforces the vital role of functional traits in community ecology, and suggests that variation associated with species identity and abundance structure is essential to community trait structure, but the importance of intraspecific variability might be context dependent