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Trees for Stormwater Management

It is often said that one of the best things you can do to manage stormwater is to plant a tree. When it rains on natural areas, such as forests or other vegetated landscapes, some rainfall runs off into nearby waterways, while a larger percentage soaks into the ground. Tree roots create pathways for rainfall to infiltrate into the soil, which helps to recharge groundwater. Through the process of evapotranspiration, water can evaporate from the soil, or can be taken up by plants and returned to the atmosphere via transpiration.

In suburban and urban areas where land use is increasingly dominated by hard surfaces such as rooftops, driveways, sidewalks, and roads, less rainfall is able to infiltrate into the soil, which creates a high percentage of stormwater runoff. In South Carolina, stormwater that is collected in pipes and ditches flows directly to nearby waterways, untreated. Because stormwater runoff is untreated, it becomes a conduit for transferring pollution on land to waterways, which can degrade water quality and harm aquatic life. Managing stormwater to reduce runoff and protect water quality requires a multifaceted approach; however, incorporating trees into stormwater management plans is a great way to include natural solutions that promote infiltration.

Figure 1. Pre- and Post- Development Impacts on Stormwater Infiltration (US EPA, 2003).

Figure 1. Pre- and Post- Development Impacts on Stormwater Infiltration (US EPA, 2003).

Urban Trees Reduce Runoff Volumes

Trees play many important roles in urban and suburban environments. They add beauty to the landscape, provide shade and a cooling effect for city streets, and help to clean the air. Trees can also be a great stormwater management tool (Figure 2). When rain falls on the canopy of a tree, the leaves and bark slow the water down. Water can evaporate directly off the leaves, flow down the trunk, or drip slowly onto the ground, allowing for infiltration. Rainwater that soaks into the ground can be absorbed by the tree’s roots and eventually is released as water vapor through the leaves in a process called evapotranspiration. The more trees in an area, whether it’s an urban or rural setting, the less runoff generated that is capable of transporting pollutants. And since there are generally more pollution sources in urban areas, trees are especially important there. Trees can filter pollutants both out of the water that moves through their cells, and out of the air.

Figure 2. Urban tree impacts on hydrology and water quality (Cappiella et al., 2016).

Figure 2. Urban tree impacts on hydrology and water quality (Cappiella et al., 2016)

Urban trees can significantly reduce runoff volume and save communities money by minimizing the use of engineered devices and best management practices to minimize flooding and stormwater pollution. The USDA Forest Service has developed a software suite called i-Tree (see http://www.itreetools.org/), which helps communities plan their urban forests and quantifies the effects of planting trees, including how they affect air quality, home energy use, streamflow, and water quality.

Table 1. Rainfall intercepted and the estimated annual cost-savings in three selected southern cities (USDA FS iTree).

Year Completed i-Tree Reference City Number of Trees Studied Annual Stormwater Benefits (dollars) Rainfall Intercepted Annually by Trees (million gallons)
2006 Charleston, SC 15,244 $171,406 28.3
2005 Charlotte, NC 85,146 $2,077,393 209.5
2009 Orlando, FL 68,211 $539,151 283.7

Considerations for the Selection and Success of Urban Trees

Urban settings can limit the lifespan of trees if not selected and sited appropriately. Soil compaction, lack of space, and poor drainage are just a few factors that create challenging conditions for many trees. In order to be a cost-effective stormwater management tool, tree longevity is important, and there are several steps that an engineer, planner, or horticulturist can take to help extend the life of urban trees.

  • For general planting recommendations, refer to the Clemson Extension Home & Garden Information Center fact sheet, Planting Trees Correctly. http://www.clemson.edu/extension/hgic/plants/landscape/trees/hgic1001.html
  • Follow your city or county’s ordinances on tree protection, landscaping, or street trees. A 2003 study found 103 South Carolina jurisdictions with tree ordinances of some kind, and that number has likely increased (London & Duffy, 2003).
  • Choose plants that are adapted to thrive under the site’s conditions. The South Carolina Urban Tree Species Guide, published by the SC Forestry Commission, provides detailed information about how to choose the best species of tree for the site based on sunlight conditions, soil type, water availability, amount of space, and temperature. EPA also published an urban forestry guide called Stormwater to Street Trees.
  • Utilize a tree pit, suspended pavement, or other engineered tree planting structure to reduce soil compaction and increase tree success.

Tree Planting Organizations

The Arbor Day Foundation, American Forests, and the Earth Day Network are only a few of the national organizations that promote planting trees for their multitude of benefits.

Trees SC is a local non-profit that focuses specifically on trees planted along city streets, sidewalks, or on commercial property in South Carolina. The goal of Trees SC is to foster stewardship of urban and community forests through education, advocacy, and networking. Local organizations that promote urban tree planting and gardening include Trees Greenville, Columbia Green, and the Charleston Parks Conservancy. Through these organizations and others, grants may be available for planting trees and other green infrastructure practices.

South Carolina currently has 39 communities designated as a Tree City USA through the Arbor Day Foundation! To reach this status, communities must have a tree board or department and a community tree ordinance, dedicate funds to urban forestry, and celebrate Arbor Day.

Citations:

  1. Cappiella K, Claggett S, Cline K, Day S, Galvin M, MacDonagh P, Sanders J, Whitlow T, Xiao Q (2016) Recommendations of the Expert Panel to Define BMP Effectiveness for Urban Tree Canopy Expansion. Center for Watershed Protection and Virginia Tech.
  2. London D & Duffy E (2003) Status of Tree Ordinances in South Carolina. South Carolina Forestry Commission and The Strom Thurmond Institute of Government and Public Affairs, Clemson University. 2003. Status of Tree Ordinances in South Carolina. Available online: http://www.sti.clemson.edu
  3. South Carolina Forestry Commission’s Urban Tree Species Guide. Available online: https://www.state.sc.us/forest/urbsg04.htm
  4. U.S. Department of Agriculture Forest Service’s i-Tree: Tools for Assessing and Managing Forests and Community Trees. Available online: http://www.itreetools.org
  5. U.S. Environmental Protection Agency (2003) Protecting Water Quality from Urban Runoff. Document No. EPA 841-F-03-003
  6. U.S. Environmental Protection Agency (2013) Stormwater to Street Trees. Document No. EPA 841-B-13-001

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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