Urban environments are characterized by change. Plants surviving in the urban setting must contend with unique environmental conditions and disturbance regimes imposed by humans. Soil is integral to plant growth and development and is often the most disturbed natural feature within the urban landscape. Yet, even with such severe pressures and frequent disturbance regimes, a large number of species are able to persist and even thrive in city environments. The global success of these urban plant species is often attributed to 1) preadaptation to urban environmental conditions or 2) intraspecific trait variability (ITV). Although trait-based techniques have become a standard method for providing mechanistic insights into species’ responses to variable environmental conditions, few studies have empirically considered how different plant species respond to and/or tolerate unique urban conditions. The aim of this study, therefore, was to identify the influence of soil on the establishment and persistence of urban plant species and whether these plants exhibit trait variability in response to different soils types.
To test these ideas, I conducted a greenhouse study on 69 urban plant species collected as seeds from natural populations found in Baltimore City, MD. Seeds were grown in either topsoil or “urban” soil collected from abandoned vacant lots in Baltimore City. Trait values for relative germination success, survival, height, and number of flowering buds were measured weekly throughout the study period. At 15 weeks, specific leaf area (SLA) measurements were taken and plants were harvested for above- and belowground biomass.
Overall, I found taxon-specific differences among plant species in response to soil treatments. Eleven (16%) of the species either failed to germinate or did not survive the duration of the study period in either soil type. Of the remaining species, results were highly variable with some species performing better (i.e. greater germination success and survival and higher values for most traits) in topsoil and others in urban vacant lot soil. Furthermore, many species exhibited significant trait differentiation in SLA, height, flower production, and above- and belowground biomass between soil treatments. These results suggest that preadaptation and ITV both play important roles in influencing plant species establishment and persistence in urban environments and that the relative importance of these strategies may be species-specific. Ultimately, this information is critical for predicting the effects of different urban environmental conditions on plant community structure and function.