Functional traits of urban forest patch vegetation across multiple gradients of urbanization intensity

Myla F.J. Aronson, Department of Ecology, Evolution and Natural Resources, Rutgers University, New Brunswick, NJ, Lea R. Johnson, Division of Research and Conservation, Longwood Gardens, Kennett Square, PA, Michelle L. Johnson, Northern Research Station, NYC Urban Field Station, USDA Forest Service, Bayside, NY, Matthew E. Baker, Geography & Environmental Systems, University of Maryland Baltimore County, Baltimore, MD, Lindsay Darling, The Morton Arboretum, Robert Fahey, Natural Resources and the Environment, University of Connecticut, Storrs, CT, Dexter H. Locke, National Socio-Environmental Synthesis Center (SESYNC), Baltimore, MD, John Paul Schmit, National Capital Region Inventory and Monitoring Network, Office of Natural Resources and Science, US National Park Service, Washington, DC and Nancy F. Sonti, Northern Research Station, Baltimore Urban Field Station, USDA Forest Service, Baltimore, MD

Background/Question/Methods
Urban forest patches – natural habitats that support spontaneously regenerating forest vegetation co-produced by human actions, biotic interactions, and environmental factors – are an important type of green space in cities. Urban forest patches are refugia for biodiversity, harboring native plants and animals, while also providing recreational, educational, and cultural opportunities for urban residents. However, many forest patches are often small and fragmented, have an aging canopy, high potential for invasion, and experience regeneration failure from a variety of pressures including human use, soil and atmospheric pollution, urban heat island effects, and overabundant herbivores. In this study, we sought to understand if community functional traits shift with increasing urbanization and whether factors related to environmental filtering or biotic interactions were of greater influence in understanding urban floral assemblages. Traits dictate the relative ability of species to establish, persist and reproduce in particular environments, thus examining trait shifts in response to urbanization can help identify factors that drive species persistence or extinction. Traits dictate the ability of a species to establish, persist and reproduce in particular environments and studies of trait shifts in response to urbanization inform us on the factors that drive species persistence or extinction. We assessed and synthesized vegetation data from 19 datasets and >4800 plots encompassing two counties beyond the Combined Statistical Areas for New York, Philadelphia, Baltimore/Washington, and Chicago to capture a rural to urban gradients. We examined nativity, growth form, clonality, and spinescence for 1226 unique species found across metropolitan regions and their adjacent rural surroundings.
Results/Conclusions
Across the entire urban woodland flora, 25% were non-native. The majority of species were herbaceous (62.1%), shrubs (13.5%), or trees (15.9%) ; 43.1% were clonal, and 7.1% had spines. Significantly more non-native species exhibited herbaceous growth forms than expected. Significantly more native species had spines, but less were clonal than expected. Individual tree species response to increasing impervious surface cover was assessed using Threshold Taxa Analysis. Across all regions, the relative density of species that decreased with increasing urbanization were entirely native and most were non-clonal and without spines. Species that increased with increasing landcover had similar traits, but only 61.1% of increasing species were native; 38.9% were non-native trees. Further analysis of all strata will enable trait-based predictions of species’ response to urbanization as well as the relative importance of local and landscape-scale factors on community assembly of these urban forest patches.