The processes that determine the patterns of community composition have long been central to the study of ecology. Naturally occurring environmental gradients have proved a useful tool in exploring these patterns. Our objective was to examine the pattern of species and trait associations along a productivity gradient and to relate these results to the different processes suggested to affect species coexistence within plant communities. We analyzed whether putative key traits (specific leaf area, height, stomatal conductance and leaf succulence) were over- or under-dispersed in areas of the alpine tundra that differed in aboveground primary production, which may relate to the role of resource competition and abiotic filters in these communities. We hypothesized that in the more productive community species composition would be less similar among plots (greater divergence) and the species would have less similar traits (their traits would be overdispersed) within plots, suggesting an increased role of priority effects and an increased role of limiting similarity via competitive interactions. We sampled species composition in 45 1m² plots along a productivity gradient, and measured the aforementioned traits on 3-5 individuals for each species. We harvested aboveground biomass from two 10cm² subplots within each plot to assess productivity.

Results/Conclusions

We found that species composition differed among communities. Contrary to our hypothesis, plots were more similar to each other in composition in the productive communities as compared to unproductive communities, indicating greater compositional convergence at high productivity. This convergence indicates that increased priority effects at higher productivity may not occur. Trait composition also varied along the gradient, with species having greater leaf area and height and lower stomatal conductance and leaf succulence at higher productivity. At all levels of productivity, species traits within a plot were more dissimilar than expected by chance, indicating that limiting similarity may play a role across the entire range of productivities we sampled. However, coexisting species differed more in their traits in more productive communities, indicating that the strength of limiting similarity may increase with productivity. Thus, the pattern of species and trait associations suggest that the role of competition in affecting species coexistence may increase across productivity gradients.