The Health of Trees in Bioretention: A Survey and Analysis of Influential Variables

Bioretention is a commonly used stormwater control measure that, through biogeochemical processes, can improve water quality and reduce runoff volume generated from impervious surfaces. Vegetation has been shown to improve bioretention treatment performance and lifespan, yet guidance for plant selection in bioretention systems remains relatively general, particularly for trees. While numerous benefits of urban trees are understood, including heat island mitigation, air quality improvement, and the like, knowledge of their potential contributions to stormwater management as a component of bioretention is minimal. Critical to tree function in these systems is the trees' ability to maintain health in the unique substrate and hydrologic regime found in the bioretention environment. This study investigated tree health in bioretention systems in the southeastern United States using three-dimensional composite indicators of crown volume and surface area. Five tree species were found to be in a less-healthy state when planted in bioretention practices compared with similar urban trees, whereas only bald cypress (Taxodium distichum) exhibited greater health in bioretention. Differences in tree health were attributed to a lack of alignment between typical bioretention conditions and species-specific growing preferences. Regression models were created using random forest methods to identify bioretention parameters that impact tree health. Parameters relating to bioretention media composition, media chemistry, and tree species selection and planting location (upslope, midslope, or bottom of the bioretention system) were found to have the most influence on tree health. Results from this study suggest that tree health in bioretention may be improved if species selection is based on bioretention media analysis and consideration of species compatibility with the growing conditions found in bioretention. (C) 2018 American Society of Civil Engineers.