Cities are designed and managed ecosystems, largely under the control of human preferences and capacities. And yet, because of human influences on urban climate, spatial heterogeneity, and introduced species, cities are also hotspots of plant species richness. These ecosystem modifications can drive changes in biodiversity, including variation in the richness and dominance of species, and also the filters on functional traits. Accordingly, community diversity may respond differently to human and biophysical influences depending on whether evaluating functional, phylogenetic, or taxonomic diversity. To begin with, we tested the simple hypothesis that functional, phylogenetic, and taxonomic diversity in urban plant communities would respond similarly to drivers, whether climatological or anthropogenic. For functional diversity, we considered both the changes in community-mean trait values as well as changes in multivariate trait dispersion amongst 20 cities along a climatological gradient in the US and Canada. For phylogenetic diversity, we calculated the phylogenetic relatedness of each city. For taxonomic diversity, we considered species richness as well as measures of species diversity. Dimensions of biodiversity were quantified using the direction and magnitude of diversity response for each city along a continent-wide climate gradient.
Using a continental-scale study of 20 cities in the US and Canada, we compared functional, phylogenetic, and taxonomic diversity of urban forests. Contrary to expectations, we found that urban tree functional richness decreased as a function of minimum winter temperature, while species richness increased as minimum temperature increased. Similarly, functional diversity (as measured by Rao’s quadratic entropy) decreased as a function of winter temperature, despite an equivalent increase in Shannon diversity. Functional dispersion across cities was negatively correlated with minimum winter temperatures, with higher dispersion values associated with wet cities than arid cities. Thus, mean trait values responded to climate contexts in the same direction as trait dispersion values, but functional response of urban plant communities had an inverse relationship with taxonomic diversity. Taxonomic and functional diversity are sometimes used as step towards or proxy for phylogenetic diversity, but their inverse relationship here suggests this may be a flawed approach. More likely, functional and taxonomic diversity are responding differently to spatial heterogeneity and to climatological or anthropogenic filters.