Predicting trunk flare diameter to prevent tree damage to infrastructure

Tree roots often come in conflict with elements of the built environment, particularly when planted in limited soil locations. For street trees located between roadways and sidewalks, minimum planting width requirements can be calculated to prevent large supporting roots from lifting or growing over paved surfaces. In this study, we used diameter at breast height (DBH) to predict trunk flare diameter for common shade tree species from four different communities in the United States (Milwaukee, Minneapolis, Saint Paul, and Tampa). These predictive models were then used to calculate minimum width requirements to reduce infrastructure damage given the maximum expected DBH measurements for each species based on existing urban forest inventory data in the communities studied. For all ten taxa tested, DBH was a significant predictor of trunk flare diameter (minimum R² = 0.81), indicating that this commonly used urban forestry measurement can be used to design minimum growing space based on selected species to potentially prevent root and infrastructure conflicts. The methods employed in this paper can be easily replicated by other researchers in order to create guidelines for species and environments not captured in our data set. Alternatively, broader functions for estimating trunk flare based on DBH are provided for species based on natural habitat type (i.e., upland, wetland, variable).