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 to 77 percent removal of nitrate, 52 to 58 percent of phosphorus, 75 to 80 percent of zinc, and 78 to 92 percent of chromium (see Managing Stormwater for Urban Sustainability Using Trees and Structural Soils). 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 the following.
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) 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.
The Minnesota Stormwater Manual contains recommendations for trees suitable for Minnesota conditions such as salt tolerance. Proprietary devices are available that consist of the structural framework into which the soil medium and tree is installed and above which the sidewalk is constructed. See the section on trees.
Incentives for implementing trees for stormwater management can include providing stormwater management credit in development or redevelopment rules. Some metro watershed districts are considering allowing credit for the interception of rainfall by trees. For example, the Capitol Region Watershed District is considering a volume reduction credit equal to 0.15 inches over the area of the tree canopy. Other credits have been established in municipalities across the country. Pine Lake, Georgia site runoff requirements are lessened by 10 gallons/inch for trees less than 12 inches diameter at breast height (DBH) and those greater than 12 inches DBH are credited 20 gallons/inch. The following table presents this and other stormwater credit approaches.
Examples of stormwater credits for urban trees
Link to this table
|Pine Lake, GA
|Saved trees contribute to site runoff requirements: < 12 inches DBH = 10 gal/in; > 12 inches DBH = 20 gal/in
|A portion of impervious cover underneath the canopy of new or existing interceptor trees from approved species list may be subtracted from the site’s overall impervious cover:
New deciduous = 100 ft2; New evergreen = 200 ft2Existing = ½ existing canopy
|2007 Stormwater Quality Design Manual
|A portion of impervious cover underneath the canopy of new or existing trees from approved species list and within 25 feet of impervious surfaces may be subtracted from the site’s overall impervious cover.
|Stormwater Management Manual
|100 ft2 impervious area reduction for each new tree within 20 feet of impervious surfaces and from approved species list.
|Indianapolis Urban Forestry
In addition to regulatory tools and design detail modifications for development and redevelopment, financial incentives can encourage private landowners to plant trees on their property. These incentives can take many forms, ranging from free or low cost seedlings or other native tree stock to financial rebates or reduced fees offered by utilities or local governments. Tree seedling giveaways may be coupled with educational programs and may also coincide with nationally recognized days such as Arbor Day. Various utilities across the country offer incentives to preserve or plant trees in certain areas of the yard to maximize their cooling benefits.
Every urban tree planting site requires regular inspection and maintenance such as watering, weed control, pruning, and pest management. Fertilization is usually not needed for newly planted trees, but may be beneficial later, depending on soil and growing conditions. The Tree Care Industry Association (2004) provides guidance on tree fertilization. Inspection, replacement, and removal of tree shelters and stakes should also be part of a maintenance plan.
The cost of preventing tree loss during development and redevelopment and the incorporation of trees into stormwater management BMPs will largely be at the expense of developers, except for staff time for ordinance and detail development. The cost for incentivizing planting of trees in existing development will vary depending on the intensity of the effort and the set maximum cost-share or rebate. Cost savings as a result of increasing tree cover in urban areas was studied in Fayetteville, Arkansas where increasing tree canopy from 27 percent to 40 percent was estimated to reduce stormwater runoff by 31 percent. This runoff reduction was estimated to result in a savings of $43 million in capital improvement based on a $2/cubic ft. cost for stormwater management (American Forests, UEA of Benton and Washington Counties, Arkansas, 2002). A similar study on Portland’s declining tree canopy found that tree replacement would cost at least $5 billion, but the volume reduction and pollutant removal benefits from the trees were estimated to save the city $11 million per year in stormwater management costs. Volume and pollutant removal benefits increase with the age of the trees. Trees planted for stormwater management are planted in uncompacted soils to maximize the stormwater management benefits; this practice increases the lifetime of the tree as compared to the general practice of planting trees in compacted soil.