Quantifying the benefits of urban forest systems as a component of the green infrastructure stormwater treatment network

The use of green infrastructure for reducing stormwater runoff is increasingly common. One under-studied component of the green infrastructure network is the urban forest system. Trees can play an important role as the first line of defense for restoring more natural hydrologic regimes in urban watersheds by intercepting rainfall, delaying runoff, infiltrating, and transpiring captured stormwater. However, inadequate research quantifying the urban tree contribution to rainfall/runoff processes limits their promotion by stormwater managers. The purpose of this literature review is to highlight the limited research performed, document areas of need for quantifying the benefits of urban trees for stormwater management, and provide a basis for providing credits for trees in stormwater designs. Recent research has shown that urban trees can retain a sizable volume of annual rainfall in their crowns, delay the flow of stormwater runoff, substantially increase the infiltration capacity of urban soils, and provide transpiration of sequestered runoff for additional stormwater storage. Tree canopy effectiveness is highest during short, low-intensity storms and lower as rainfall volume and intensity increases. While soils are the best medium to store and filter stormwater, trees may be integrated with other runoff reduction strategies to bring more natural hydrologic processes to urban watersheds by taking advantage of multiple points of retention. Gaps remain in the body of research, but there is a basis for considering trees an integral part of the watershed-scale green infrastructure network that helps reduce the volume and intensity of urban stormwater runoff.