The Health of Trees in Bioretention: A Survey and Analysis of Influential Variables
Tirpak, A., J. M. Hathaway, J. A. Franklin, and A. Khojandi.  2018.  Journal of Sustainable Water in the Built Environment.  (in press)

Abstract:
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 the nutrient removal performance and lifespan of these systems, yet guidance for plant selection in bioretention practices is still relatively empirical, particularly for trees. While numerous benefits of urban trees are understood, including heat island mitigation, air quality improvement, noise attenuation, etc., knowledge of their potential contributions to stormwater management as an integral component of bioretention is minimal. Critical to tree function in these systems is their ability to maintain health in the bioretention environment. This study investigated the health of trees in bioretention practices 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 to similar urban trees, while only bald cypress (Taxodium distichum) exhibited greater health in bioretention practices. Differences in tree health were attributed to a lack of alignment between typical bioretention conditions and species-specific growing preferences. Random forest regression models were used to identify bioretention parameters that impact tree health. Eight parameters related to bioretention media composition, media chemistry, and tree species selection and planting location were found to have the most influence on tree health. Results from this study suggest that tree health and function in bioretention practices may be improved if species selection is based on bioretention media analysis and consideration of species compatibility with the growing conditions found in bioretention practices.