Patterns in Residential Urban Forest Structure Along a Synthetic Urbanization Gradient

The environmental sciences increasingly need to understand the ecological effects of urbanization. This is especially true for the urban forest, a major component of the urban environment that is relied on to provide ecosystem services such as air pollution removal and storm water interception. The urbanization gradient is a popular organizing concept for assessing ecological response to varying urbanization intensity, and recent methodological improvements have moved beyond simple distance-based gradients to more sophisticated synthetic gradients based on urbanization indicators such as population density and impervious surface intensity. Although these synthetic gradients provide a more complete picture of urbanization than any one indicator alone can provide, it is unclear how synthetic gradients relate to ecological structure. In this study, we collected field data on urban forest structure from 150 residential properties over a 40-km transect in Minnesota's Twin Cities metropolitan area. We then applied factor analysis to a set of nineteen urbanization indicators and extracted two primary urbanization trends strongly related to distance from the urban core and residential neighborhood density, respectively. We related the synthetic gradient to urban forest structure using polynomial regression models. The synthetic gradient explained nine of the fourteen urban forest structural variables assessed and improved on a simple distance-based gradient by explaining patterns in tree canopy cover. Our findings demonstrate the need to consider the effects of secondary urbanization trends on ecological structure. These results support the continued application of synthetic gradient approaches to understanding the relationships between urbanization and ecological structure.