In the face of rapid global change, a key challenge in ecology today is to understand the functional impacts of shifts in species diversity and composition. Ongoing urbanization is known to be a major driver of biodiversity loss. A combination of species invasions, environmental filtering and human management actions lead to patterns of biotic homogenization both within and between cities. The quality of many vital urban ecosystem services such as storm water infiltration, pollination and soil remediation, however, is dependent on the ability of city habitats to maintain functionally diverse urban plant communities. We present results from a three-year field experiment which manipulated both functional and phylogenetic diversity of seed additions into vacant lots in Baltimore, MD to determine how increasing the diversity of the urban species pool impacted community assembly and establishment in vacant lots, a common urban habitat type that is often discussed as a potential location for urban ecosystem restoration projects. Twenty-five vacant lots were cleared in the spring of 2014, and 21 received seed additions while 4 were left to recolonize spontaneously from the soil seed bank or ambient regional species pool. All treated lots were mowed biannually, but no additional management (e.g. weeding) was performed. Plant community diversity was measured in each lot annually from 2014-2016, as well as in 5 additional unmanipulated vacant lots.
We find that simply clearing lots and allowing them to recolonize spontaneously initially increased species diversity compared to unmanipulated sites. Over time though, vacant lots receiving more phylogenetically diverse seed mix additions led to more diverse established plant communities, compared to unmanaged lots and unseeded but cleared lots. Functional diversity of seed mix additions, however, based on a suite of traits commonly used to describe grassland community diversity, did not have an effect on plant community establishment success. Our findings suggest that overcoming dispersal filters via seed additions can enhance the diversity of urban plant communities, even in disturbed urban habitats. Additionally, we discuss preliminary findings linking manipulated diversity of the plant communities to urban ecosystem services, including availability of floral resources for pollinators and soil characteristics. Establishing biodiversity experiments in cities which can directly inform urban environmental management policy requires extensive collaboration with non-academic partners. Thus, we also share practical lessons learned from the establishment of this large urban community engagement project and discuss how findings from this experiment are informing follow-up studies.