Continental-scale homogenization of US urban lawn vegetation
Similar management practices within urban residential areas across large spatial scales are thought to lead to the homogenization of ecosystem structure and function compared with the diverse natural ecosystems that these systems replaced. At the same time, urban vegetation potentially is also strongly influenced by the gradients in temperature and precipitation that shape natural vegetation. Residential lawns are generally highly managed and are very common in the urban landscape, and therefore are a good system in which to study the potential homogenizing effects of urban land use and management. We measured plant species in residential lawns in seven climatically distinct cities across the country (Baltimore, MD, Boston, MA, Los Angeles, CA, Miami, FL, Minneapolis-St. Paul, MN, Phoenix, AZ, and Salt Lake City, UT) to assess how the vegetation structure and composition in residential lawns compared across cities and with the region’s native ecosystems. We recorded species identities and abundance in residential lawn quadrats and in similar quadrats in native ecosystems, and then compared the species composition, Shannon-Wiener diversity, and richness among cities. We also examined how gradients of precipitation and temperature across cities influenced the structure and composition of lawn vegetation.
Lawn species composition was more similar to the composition of other lawns than to native vegetation in all seven cities. Lawns generally shared more species with lawns in other cities (5-48% of species) than with their own native vegetation (2-21%), the exception being Miami, which shared more species with its native vegetation than with any other lawn (17% with native, <9% with other lawn). Lawn functional group composition was different from and more homogenous than native structure, with a higher proportion of graminoid cover (72-93% of lawn plots, 2-51% of native plots) and a smaller range of forb cover (7-27% of lawn plots, 4-77% of native plots). Lawns tended to have lower species richness and diversity than native vegetation. Lawn species richness and diversity were well explained by the precipitation gradient. Lawns in cities with higher annual precipitation were more diverse and had higher species richness than lawns in drier cities (r2 = 0.49 for diversity, r2 = 0.85 for richness). Similarity of lawn vegetation decreased with increasing difference in temperature and precipitation among sites. Our results showed that lawns across the US were far more homogenous than native habitats, but also that climatic gradients strongly influenced lawn vegetation composition.