Functional traits measured under field conditions in native populations: Investigating patterns of habitat specialization in fifteen co-occurring willow and poplar species.

Thirteen willow (Salix) species occur in southeastern Minnesota and often co-occur within the same wetlands. This high local diversity is challenging to explain since closely related species are often functionally similar and density-dependent interactions such as competition and susceptibility to pests and pathogens should limit their co-occurrence. However, if willow species are partitioning resources, or if they are phylogenetically structured so that closely related species rarely co-occur, then the impact of these density-dependent processes could be reduced. In this study, I examined the role of niche partitioning in maintaining local willow diversity by documenting species distributions in plots across a water availability gradient and comparing species physiology in the field and greenhouse. By taking a phylogenetic approach, I also investigated whether willow communities exhibit phylogenetic community structure and whether there is evidence for environmental filtering.