Habitat specialization and the role of trait lability in structuring diverse willow (genus Salix) communities

The co-occurrence of closely related species is challenging to explain because biotic filters are expected to limit the ecological similarity of species within communities. To investigate the mechanisms important in facilitating species' co-occurrence in diverse willow and poplar communities, we examined functional diversity and community phylogenetic structure along a hydrologic gradient. We focused on traits related to drought tolerance, leaf hydraulics, and recruitment, and examined species' phylogenetic relatedness and trait lability using a molecular phylogeny. Within habitats, species exhibited phenotypic clustering, and across the landscape, species distributions were correlated with their functional traits in a manner consistent with environmental filtering. With increasing water availability, communities changed from being phylogenetically even to being phylogenetically clustered. We suggest that this shift results from environmental filtering acting on conserved traits in wet habitats and labile traits in dry habitats. Taken together, these results suggest that environmental filtering is important to community assembly along the entire hydrologic gradient within this system. Although many of the traits important to habitat specialization in upland habitats are phylogenetically labile, species' habitat affinity is phylogenetically conserved overall, indicating that niche conservatism can occur as an emergent property despite trait lability. This study demonstrates the complementary nature of trait and community phylogenetic analyses and how these methods can be used to better understand the processes involved in community assembly along environmental gradients.