Co-occurrence patterns among related species can reveal both the evolution of ecological specialization and the role of relatedness in species interactions within communities.  Relating these co-occurrence patterns to relevant environmental gradients can further distinguish between the possible effects of ecological similarity and competition among related species.  In this study, we examined the relationships between evolutionary and ecological determinants of species distributions in the exceptionally diverse sedge genus Carex.  Using a robust six-gene phylogeny for 60 Carex species, we assessed the phylogenetic structure of Carex assemblages across a range of forest and wetland plant communities on Mont Saint-Hilaire in southern Quebec, Canada.  First, we compared the degree to which related species have diverged in their ecological tolerances along dominant environmental gradients such as soil moisture, soil pH, and canopy openness.  We tested whether more closely related species are more likely to occur in similar environments, whether they are more likely to occur together in the same communities, or both.  We also examined how the degree of relatedness within communities varies along environmental gradients.  

Results/Conclusions

Since some ecological traits, such as flood tolerance, have been conserved in the evolution of Carex, while others have been more labile, closely related species have more similar distributions along the soil moisture gradient than along other gradients such as soil pH.  Across the wide range of habitats on the mountain, Carex species within communities were more closely related than expected, and this pattern was stronger in forests than in wetlands.  These results suggest that environmental sorting outweighs competition in structuring the forest and wetland plant communities.  They also provide insight into the role of specific environmental factors in the evolutionary history of an ecologically diverse plant genus.