COS 70-5
Comparative urban biogeography: tree community and trait assembly patterns for eleven major U.S. metropolitan areas
Cities contain a unique composition of managed and unmanaged plants, many of which are non-native to the region. This proliferation of exotic species has been documented to have a homogenizing effect on the biodiversity of urban systems. Mechanisms behind similar tree compositions across cities are still under debate. Trait selection and climate suitability may influence planting choices by land managers. We ask the questions, How does tree biodiversity and community assembly vary between geographic regions? and How do geographic regions vary in tree traits and climate suitability? We hypothesize that urban tree composition will be regionally specific and dependent on climate suitability and valued species traits. To test these hypotheses, we compiled an extensive comparative dataset of urban tree biodiversity. This includes I-Tree and other metropolitan survey results from 11 U.S. cities in three geographic regions: Southwest, Northeast, and Midwest. Each survey includes 50-350 plots in a metropolitan region. Similarity of biodiversity and abundance was tested using rarefaction curves and principal components analyses. We noted traits for each species likely to be chosen by managers, including shade, edible fruit, showy flowers, and seasonal color change. Biome and geographic origin of each species was identified to account for climate suitability.
Results/Conclusions
In total, our dataset contains records of over 10,000 individual trees of 440 different species in 2300 sample points. Principal components analyses of tree species composition shows that western coastal cities, like Los Angeles and Orange County, were similar in species composition, while desert cities like Riverside and Phoenix clustered together. Northeast cities, like Baltimore and Syracuse, were also clustered. In addition, rarefaction curves indicate that large, dense cities, like D.C. (121 sp. in 5 ha) have the highest relative tree biodiversity as compared to smaller cities like Riverside (55 sp. in 5 ha). Northeast cities contain more trees, but individual rarefaction indicates a lot of species repetition. An average of 58 species are found per 100 trees in Los Angeles, but only 37 species per 100 trees in Baltimore. While trait and biome analyses are still ongoing, we expect to see trait assemblies based on regional cultural preferences, and more biome representation in regions with less climate extremes. Our ongoing results indicate that climate and local preferences influence regional biodiversity patterns, consistent across cities. Our research will help create a broader and more robust set of generalizations about the formation of urban tree assemblies in major metropolitan areas.