Urbanization induces dramatic land-use changes, which are often reflected in unique biodiversity patterns. Urban habitats, for example, have been found to support high proportions of introduced ‘urban exploiters’ in exchange for native species. Although this trend has been observed in many cities, leading to the characterization of urban communities as globally homogenized, there still lacks a general understanding of the processes responsible for shaping these patterns. I conducted an experimental common garden study designed to identify the relative influence of different assembly processes (i.e. environmental filtering, competition, and dispersal) on native plant communities assembling in urban habitats. The main purpose was to identify which factors limit the representation of native plant species in urban vacant lot habitats, with the broader aim of using this information to guide future city planning efforts that are focused on enhancing biodiversity in abandoned urban green spaces. Native communities consisting of 10 species, representing a variety of functional strategies, were seeded in 2m x 2m plots filled with either topsoil or subsoil fill material used in vacant lot demolition projects. Additionally, a weeding treatment (weeded vs. unweeded) was used to isolate interspecific competition from urban plant species that either recruited into the plots from the surrounding region or were present in the soil seed bank. Species and functional trait data were collected in October 2016, during peak biomass of the first growing season. Native plant community responses to treatments were assessed via mixed effect models and permutational multivariate ANOVA (PERMANOVA) tests on native species richness, Shannon-Weaver diversity, and species and functional composition data.
Preliminary results showed that native Shannon-Weaver diversity was influenced by soil type. Yet, contrary to predictions regarding urban soil environmental filtering effects, native diversity was significantly higher in urban fill soil treatments. This suggests that soil conditions in vacant lot habitats are conducive to native plant species germination and growth, at least in early years. This effect, however, may not persist over time as nutrients and soil moisture become more limiting. I found no effect of soil or competition on native community richness, species or functional composition, which indicates that dispersal limitation may play a key role in observed patterns of low native species diversity in urban habitats. Future study efforts will include additional measures of species and functional diversity taken biannually, in order to assess long-term effects of community assembly processes on native species in urban vacant lot habitats.