Human legacies differentially organize functional and phylogenetic diversity of urban herbaceous plant communities at multiple spatial scales

Background/Question/Methods

Based on a robust body of literature investigating patterns in urban plant diversity along urban-rural gradients, we expect to see reduced functional and phylogenetic diversity, relative to species number, within cities. Few studies, however, have tied common patterns of urban plant diversity—particularly biotic homogenization—to specific structuring processes, or have explored patterns across multiple scales. In this study, we consider all three fundamental components of biodiversity to develop and test a conceptual model for explaining patch-to-patch variation in local diversity in urban environments. We apply our model to a case study of surveyed patterns of herbaceous plant community diversity in a system of vacant lots scattered across an urban residential neighborhood in Baltimore, MD. Herbaceous plant surveys were conducted in Baltimore, MD, during the summer of 2012 in habitat patches that fall along a gradient of human management from “low” (the building footprint section of vacant lots) to “medium” (remnant back gardens in vacant lots). Using community phylogenetic approaches, we focus first on patterns of functional and phylogenetic diversity within and between vacant lots—and within and between human legacy groups within these lots—and secondly, on the relationship between functional and phylogenetic diversity that emerges.

Results/Conclusions

We found significant local taxonomic, functional and phylogenetic compositional divergence between remnant gardens and building footprints. However, the majority of variation between these human legacy groups was at the scale of turnover, from location to location. Building footprint plant communities tended to be more phylogenetically clustered than communities in remnant gardens, and there were significant differences between groups in community weighted means of functional traits that commonly vary along urban-rural gradients, including plant height, seed mass, life cycle duration and native vs. non-native origin of species. We found no evidence of phylogenetic signal for any functional traits, suggesting trait-based filtering at the scale of the urban species pool. We contend priority effects, as a result of differential human legacies, maintain compositional divergence between local communities, regardless of similar contemporary structuring processes, similar abiotic environmental conditions and lack of dispersal barriers between legacy groups. We discuss implications for urban restoration plans, as well as potentially profitable future research directions to further explore the complex interactive processes that structure plant communities across the urban landscape.