Effects of Urbanization on Tree Species Functional Diversity in Eastern North America

Urban forests provide ecosystem services for millions of people. Numerous introductions have elevated tree species richness in cities, which may enhance functional diversity. However, few studies have examined changes in tree community composition or functional diversity with urbanization, even though functional diversity, and not species number per se, is directly linked with ecosystem function and associated services. We combined tree abundance data from both urban and extra-urban forest inventory plots for seven metropolitan areas in eastern North America to analyze changes in species composition, Shannon's diversity, and functional diversity with urbanization. As expected, urban tree diversity was reduced at local scales, and the effect varied with land use. Rarefaction analysis indicated that at large scales, urban tree species pools were equal with respect to species or functional diversity compared to extra-urban forests, but in urban areas at small scales this diversity is not realized because of low tree density. Ordination revealed that with urbanization, introduced species increased in importance, and regional variation in species composition became more homogenous. Increasing tree density and/or tree cover through changes in management practices and urban design could facilitate local scale urban tree diversity using existing species pools, which are functionally diverse. Monitoring of forests at large spatial scales that include urban areas, and the use of methods that account for abundance and functional trait variation can provide insights into the effects of urbanization on tree diversity at multiple scales.