Patterns of urban forest composition in Utah’s growing mountain communities

Abstract

Urban forests provide critical ecosystem services in cities of the Western United States, including regulating thermal extremes, supporting biological diversity, and providing cultural and recreational services. However, these services may come with trade-offs such as heavy water demand in arid environments. Thus, afforestation and its effect on the water budget – as well as other ecosystem services – can be contingent on the species composition of urban forests. Choice of tree species, in turn, is influenced by historical contingencies and development context. The objective of this study was to identify differences in tree species composition between four broad classes of urban development in Heber Valley, Utah, with classes defined by establishment age, lot size, and location within the urban-suburban environment. Publicly available information was used to categorize residential and commercial areas, and standard forestry techniques were used to collect data on trees in a stratified random survey of lots in each category. Older, established housing had the highest tree basal area and species richness per hectare, and exurban (rural, dispersed housing) developments had significantly higher species diversity than new tract housing. Because it appears that exurban communities are being replaced by tract housing, there is evidence that tree diversity may be lost. Another important aspect of community structure in urban forests is the ratio of conifers to broadleaf trees because of fundamental differences in water use patterns. Conifers were twenty-five percent of the average lot basal area in exurban and thirty-five percent in established neighborhoods, as opposed to five percent in tract housing. If functional groups are used as predictors of water use in irrigated urban systems, water demand is likely to increase with the expansion of low-diversity, angiosperm dominated tract forests in the Western US in the coming decades.