Natural selection, along with biochemical and architectural constraints can limit the trait combinations expressed by plants, creating functional trade-offs across species. These trade-offs often play a critical role in limiting species distributions by preventing them from performing well under all environmental conditions. In this study, we examined the role of functional trade-offs in limiting species distributions at two geographic scales, focusing on species in the genus Salix (the willows). First, we examined whether species exhibited niche differentiation across a local water availability gradient, and examined the extent that plant function and functional similarity influence species distributions and patterns of co-occurrence. For this analysis, we examined species distributions in relation to physiological and functional data collected in the field and in a greenhouse common garden. We also estimated a phylogeny of the species to examine patterns of species phylogenetic community structure and trait evolution. Second, we examined whether there was evidence for a trade-off between freezing tolerance and growth that could explain species range limits. This analysis involved comparing species freezing tolerances and growth rates under different environmental conditions and determining the extent that these traits related to species distributions.
We found evidence that functional trade-offs are important in determining species distributions both within local plant communities and across broader geographic distributions. On a local level, species exhibited niche differentiation across a water availability gradient. This differentiation is driven by a functional trade-off between drought tolerance and relative growth rate. Traits related to species drought tolerance and recruitment strategies also demonstrated correlated evolution with species water availability niches. At a broader geographic scale, species demonstrated a trade-off between freezing tolerance and growth. This trade-off is primarily the result of species dependence on photoperiod cues for growth up-regulation. The strong correlation between species growth rates, their freezing tolerances, and their modeled geographic ranges suggests that this trade-off could be important in limiting their geographic distributions. This research demonstrates the importance of functional trade-offs in determining species distributions at two geographic scales in an ecological and economically important genus. It also demonstrates the value of integrative research that draws on physiological, evolutionary and ecological methodology.