Urbanization influences plant diversity through mechanisms such as habitat transformation, altered environmental conditions and species introductions. However, changes in tree diversity with urbanization are poorly documented, despite the importance of diverse urban tree communities to urban ecosystem function and human well-being. Increasingly ecologists are advocating for a functional approach to biodiversity-ecosystem function research¾arguing that it is not the number of species per se, but the diversity of functional traits, and the diversity of species that express them, that drive ecosystem function. In order to study the effects of urbanization on tree species (SD) and functional diversity (FD), we compiled a unique dataset that consisted of plot-based tree inventory data from seven cities and their surrounding forests in eastern North America. Plots spanned a land use gradient from managed forests to urban. To assess changes in FD we compiled information on coarse indicators of plant functional strategy (maximum plant height, leaf nitrogen, seed mass and wood density) and combined these with our inventory data to calculate abundance weighted trait values and FD for each plot. The diversity of species pools was estimated by rarefaction. We hypothesized that despite minor changes in species diversity, functional diversity declines with more intensive land use due to environmental filtering.
Results/Conclusions
We found that SD and FD generally changed in concert across the seven urban centers, decreasing substantially from forest to urban plots. As expected, the density of trees was much lower in urban compared to forest plots. Based on a strong negative correlation between the area of hard surfaces and 1) tree density, 2) SD and 3) FD; we suggest that plot level diversity decreased in part because of a lack of available growing space. Despite the strong decreases in plot level diversity in urban areas, the taxonomic and functional diversity of species pools—estimated by rarefaction—were unaffected by urbanization. Ordination of species abundance suggested a tendency towards homogenization of urban tree communities, which may have important effects on tree diversity at the landscape scale with future urban expansion. We conclude that urban tree species pools in eastern North America are approximately as functionally and species diverse as nearby forests, suggesting that the provision of ecosystem services from urban trees will primarily depend on increasing tree density.