High intensity land use patterns and increasing pressure on water resources demands creative stormwater management. Trees dissipate the energy of falling raindrops to help prevent erosion and buffer intense rainfalls. Urban tree roots have the potential to penetrate compacted soils and increase infiltration rates in open space areas, stormwater basins and subsurface stormwater storage (structured soil). Uptake of water from trees limits the volume of runoff discharged downstream, and their canopies offer interception of rainfall and shading (cooling) in an urban environment. Trees also absorb nutrients that could otherwise run off to local receiving waters.
This fact sheet provides an overview of the benefits of protecting existing trees and planting new trees in stormwater treatment practices of new development or redevelopment sites and includes activities that can be implemented by an MS4.
Urban forestry strategies can help satisfy many of the MS4 stormwater management requirements in a cost effective manner. Trees, forests, and other natural areas effectively manage water through interception, evapotranspiration, and infiltration. Together, these processes can significantly reduce peak stormwater rates and volumes, naturally filter runoff, enhance ground water recharge, stabilize base flows and reduce erosion in streams.
Trees also take up nutrients and various pollutants through their root systems. A study of the City of Fayetteville, Arkansas, estimated that increasing tree canopy from 27 percent to 40 percent would reduce stormwater runoff by 31 percent (American Forests, UEA of Benton and Washington Counties, Arkansas, 2002).
A study at University of California at Davis evaluated pollutant removals for structural soils, soils designed to meet requirements for pavement support while still allowing sufficient pore space to support tree roots. Three soil types averaged 73-77 percent removal of nitrate, 52-58 percent of phosphorus, 75-80 percent of zinc and 78-92 percent of chromium (see Managing Stormwater for Urban Sustainability Using Trees and Structural Soils in Additional Resources). The term “phytoremediation” has been used to describe the ability of certain trees to take up and alter contaminants that occur in soil and shallow ground water. This has become an effective and low cost remediation approach for brownfield restoration.
Regulatory tools can be adopted, perhaps as part of a tree ordinance, to reduce forest clearing during development, as well as to prevent inadvertent injury to trees. Some of these techniques include:
Urban development and redevelopment sites provide many opportunities (e.g. during the installation of stormwater treatment practices) to plant new trees that provide water quality treatment and storage of stormwater runoff from impervious surfaces. Many stormwater treatment practices such as roadside rain gardens have not traditionally been considered appropriate locations for planting trees. Research on the benefits of trees, however, shows they have enormous potential to improve the efficiency of these practices through nutrient uptake and runoff reduction.
To encourage tree planting in stormwater treatment practices, detailed guidance has been developed by the Center for Watershed Protection (CWP) (see Additional Resources) for the selection of appropriate species, identification of areas suitable for planting, and modification of the design or planting environment. The CWP resource also provides conceptual designs for the following stormwater management features: