Despite a rich body of literature detailing variation in species diversity along environmental gradients, the underlying mechanisms controlling these patterns remain poorly understood. Janzen (1967) suggested that the latitudinal gradient in species range size is a result of the greater climatic variability occurring at higher elevations and latitudes which selects for species with broader physiological tolerance. Alternatively, as the environment becomes more stressful, stabilizing selection could limit ecological and evolutionary variation resulting in smaller range sizes. A unified approach that invokes both physiological and evolutionary considerations is needed to link species range sizes to environmental conditions. Functional traits, e.g. photosynthetic activity and leaf size, are a useful tool for linking physiological attributes like thermal tolerance to local environmental conditions. Quantifying the evolutionary and geographic variation in functional traits within and across elevational gradients will thus provide a measure of the effect of climate and climatic variability on species range sizes. Here, I quantify inter- and intra-specific variability in leaf function along elevational gradients in tropical and temperate ecosystems. This study tests the hypothesis that the range and co-existence of species can be predicted on the basis of functional trait variation in temperate and tropical plant communities.

Results/Conclusions

The results of this study show that quantifying both inter- and intraspecific variation in plant function in relation to elevational and latitudinal gradients may provide further understanding of patterns of species coexistence and distributions along environmental gradients. Specifically, this study shows that the mean and range of plant function is non-randomly distributed within and between species along elevational gradients in both temperate and tropical plant communities. Furthermore, while similar directional trends are found between mountain gradients, the mean and range of trait values differs for tropical and temperate plant communities. This study emphasizes the role of functional trait variation in structuring plant communities and provides insight into the mechanisms underlying species diversity along environmental gradients.