Phylogenetic and functional characteristics of household yard floras and their changes along an urbanization gradient.

Urban areas are among the most heavily managed landscapes in the world, yet they harbor a remarkable richness of species. Private yards are common habitats in urban areas and are places where cultivated species manage to escape cultivation and become part of the spontaneous species pool. Yards are novel ecosystems where community assembly is driven by both natural and anthropogenic processes. Phylogenetic diversity and functional traits are increasingly recognized as critical to understanding processes of community assembly. Recent evidence indicates that urban areas may select more closely related plant species from the pool of regionally occurring species than do nonurban areas, and that exotic species are phylogenetically clustered within communities. We tested whether phylogenetic diversity and functional trait composition in privately managed yards change along a gradient of housing density in the Minneapolis-Saint Paul metropolis, Minnesota, USA, in accordance with these predictions. We also identified characteristics of the spontaneous yard flora by comparing its phylogenetic diversity and functional composition with the "natural-areas" species pool represented by the flora of nearby Cedar Creek Ecosystem Science Reserve. Along the urbanization gradient, yards had more species per hectare in densely built regions than in lower-density regions, but phylogenetic diversity and functional composition did not change with housing density. In contrast, in comparison to species in natural areas, yard species were more closely related to each other and functionally distinct: They were more often short-lived, self-compatible, and had higher specific leaf area than species of Cedar Creek. The high number of exotic yard species increased the yard flora's phylogenetic relatedness in comparison to species of Cedar Creek, causing a degree of phylogenetic homogenization within yards. The urban environment and homeowners' preferences select for trait attributes and phylogenetic lineages that can colonize and persist in yards. As yard species disperse beyond household boundaries, their functional attributes will affect ecosystem processes in urban environments and beyond, such as accelerating decomposition rates. Limited phylogenetic diversity may reduce the potential of ecosystems to respond to environmental changes. As cities continue to expand globally, understanding the impacts of yard management for biodiversity and ecosystem services becomes increasingly important.