Initial Performance of Trees in an Urban Stormwater Bioretention System

Stormwater runoff is an issue in urban areas as impervious surfaces increase. Various bioretention systems that incorporate trees have been developed but are expensive, technically complex, and require large amounts of space, which limits their widespread use as a stormwater solution. This study investigated a novel design for a bioretention system that is less complex and may hold promise as an inexpensive approach to capturing runoff and growing trees near impervious surfaces. The system comprises a large bed of gravel surrounding a trench of topsoil planted with trees. The gravel bed captures and retains stormwater from adjacent hardscapes. Roots extending outward from the topsoil into the gravel bed absorb and recycle captured water through transpiration. A full-scale field prototype of this system was constructed adjacent to a parking lot on the campus of Virginia Tech in Blacksburg, VA in spring 2020. The system was planted with three London planetree (Platanus × acerifolia ‘Morton Circle’), which were compared to three control trees of the same species planted concurrently in nearby native control soil. The purpose of this study was to monitor stormwater capture by the bioretention system and evaluate tree growth and physiological function during the second growing season following planting. Because the topsoil in the gravel bed was of better quality than the native control soil, trees in the gravel bed exhibited greater water use efficiency and had larger and denser crowns during the study period. Under prevailing precipitation conditions, the gravel bed rarely captured runoff exceeding its volume capacity, and the residence time of water in the system rarely lasted more than 48 hours before draining via deep infiltration into the underlying subsoil.