Human settlements in arid and semi-arid regions often feature urban forests consisting mainly of planted trees and shrubs. Tree species composition in such locations is a function of property owner preferences and plant availability. In more affluent regions such as the western United States where many species are available from nurseries and water is relatively cheap, forests in urbanized areas can create an “oasis effect” where daytime temperatures are several degrees below those of surrounding natural landscapes. Cooling, in turn, can reduce irrigation demand for lawns and gardens. However, if species composition is determined largely by social factors, trends in residential preference or nursery availability can influence which species are planted. To measure trends in tree species choices over time, and to assess the influence of those choices on urban water demand, we sampled street and residential trees in Cache County, Utah, choosing properties of different home ages. We examined the alternative hypothesis that economic factors would drive species choices by measuring relationships between tree species composition and assessed property valuation. Finally we surveyed residents of a subset of sampled properties about their tree management behaviors and species choice factors.
Results/Conclusions
Richness and diversity of tree species was significantly greater for properties where the home was built between 1951 and 1990 than for older or younger age cohorts. More trees were found around homes built after 1950 than before. No correlation was found between assessed home valuation and either species diversity or composition. However, we did find temporal trends in species choices. Frequency of larger, shade-producing species such as Norway maple (Acer platanoides) and northern catalpa (Catalpa speciosa) was lower around newer homes, while smaller species such as flowering pear (Pyrus sp.) and green ash (Fraxinus pennsylvanica) were more common around newer homes. While the latter are relatively drought-tolerant they produce less shade, potentially increasing demand for water to irrigate lawns and gardens. Property owner surveys showed that most residents water their lawns more than once per week but rarely water their trees. Slightly over half of residents had planted at least some of the trees on their properties, with appearance and ease of care the most important factors guiding species choice. Trends in urban forest composition could exacerbate future water scarcity caused by climate change and rapid urban growth.