View a PowerPoint presentation that describes how trees and forests help maintain healthy watersheds by reducing stormwater and non-point source pollution that impacts many of urban and suburban streams and waterways.

Download Trees and Forests Maintain Healthy Watersheds

As our landscapes change, it has an impact on the health of our streams and rivers. What to do on the land, or what we cover it with, affects the quantity (volume) and quality (pollutant levels) of the rainfall that enters our waterways – what many call stormwater.

When it rains in urban and suburban areas, rainwater washes pollutants such as nutrients, chemicals and heavy metals off paved surfaces, lawns or bare soils into storm drains that lead to streams and rivers. Because of the increased amount of paved or impervious surfaces, larger quantities of rainwater reach the streams quickly causing flash flooding, streambank scouring and sedimentation of streambeds. Stormwater from urban and suburban areas has become a major pollutant in Pennsylvania impairing 4,170 miles of streams and accounting for one third of the problem facing our waterways. For municipalities that are designated MS4 communities by the EPA and DEP, they are being tasked with finding ways to reduce the negative impacts of stormwater runoff and for some, combined sewer overflows that cause untreated sewage to flow into waterways because the pipes can not handle the increased volumes during rain events.

So how to do these municipalities begin to reduce stormwater runoff and the pollution associated with it. Well, they can increase the size of underground pipes and the capacity of their sewage treatment plants which might cost billions of dollars. Or they can begin to look for some greener and cheaper solutions.

One of those green and cheap solutions involves the planting of large canopy trees. A great deal of research by the USDA Forest Service and others has shown that trees (and forests more importantly) reduce stormwater runoff and pollution in several ways.

First, trees work like large umbrellas, intercepting rainfall in their canopies, composted of leaves, twigs, branches and trunk. Average interception by deciduous trees (those that loose their leaves in the fall) can range from 700 to 1,000 gallons per year, while an evergreen can intercept more than 4,000 gallons per year. Large canopy trees trees planted over impervious surface will provide more benefit with time, although small trees can help.

Next, trees and especially forests, allow for infiltration of rain into the soil, where it should be going to recharge groundwater, become filtered and slowly work its way to streams as subsurface flows. In one study of a North Carolina watershed (Kays, 1980), the mean soil infiltration rate went from 12.4 in/hr to 4.4 in/hr when the site was converted from forest to suburban lawn.

Trees absorb, and use tremendous amounts of water for growth, moving it from the roots back into the atmosphere through evapotranspiration, thereby consuming stormwater. A single mature oak tree can transpire over 40,000 gallons of water per year.

Trees are very good at removing nutrients (nitrates and phosphates) and contaminates (such as heavy metals, pesticides, solvents, oils, and hydrocarbons) from the soil and water. This process is called phytoremediation. In one study, a single sugar maple growing roadside, removed 60 mg of cadmium, 140 mg of chromium, 820 mg of nickel, and 5200 mg of lead in a single growing season, storing those pollutants in their wood (Coder, 1996).

Lastly, trees and other woody vegetation growing along our streams and rivers (riparian areas) provide a wealth of benefits, and research at the Stroud Water Center and elsewhere have shown that stream health is dependent on the presence of woody vegetation along its banks. Those riparian forest buffers filter sediments from streams during storm events; remove nitrogen and phosphorus leaching from adjacent lands; provide stability to the stream banks; shade and modify stream temperatures, critical for habitat and pollution reduction; provide aquatic and wildlife habitat for many species; reduce stream velocity; and reduce down stream flooding.

Urban Tree Planting Strategies to Reduce Stormwater

Recently, Pennsylvania Community Forests, a non-profit guiding the statewide urban & community forestry program, was awarded a $300,000 PennVest Grant to plant 1157 trees in 17 different municipalities (MS4) in Northeastern Pennsylvania in an effort to reduce over 700,000 gallons of stormwater and non-point source pollution that is entering local streams and rivers when it rains. This was one of four tree planting projects that were funded statewide, by a state agency that typically provides loans to municipalities for water and sewer infrastructure projects such as new pipes in the ground or sewage treatment plants. PennVest received federal stimulus funding from the American Recovery and Reinvestment Act of 2009 which required that a portion of the water pollution prevention projects be green infrastructure. The planting of trees with water infrastructure dollars is an innovative and cost effective way to begin to tackle the issue of stormwater and improve water quality.

Because stimulus funding from ARRA was being used to fund the stormwater tree plantings, Lynn Conrad, NEPA Urban Forestry Program Administrator, Bernard Dincher, NEPA Urban Forestry Program Technical Specialist, and Vincent Cotrone, Penn State Extension Urban Forester quickly prepared five tree planting contracts, coordinated with seventeen MS4 communities to select and mark planting sites, and bid out this “shovel ready work” to landscape contractors that were in need of work for the fall of 2009. Due to the economic downtown, the prices for contract planted trees have not been this low since 1989, even with paying prevailing wages to workers and a 1 year guarantee on the plantings. While the plantings are designed to help the local watershed, they are also helping the local economy by purchasing locally grown trees and by keeping workers employed late into the fall and next spring planting trees in communities.

Similar work is being completed in Philadelphia, where the Pennsylvania Horticultural Society is retrofitting street tree planting pits to accept stormwater running down paved street. As the stormwater works through the tree pits, the water is filtered biologically of some of its pollutants and the trees are watered – a win-win situation.

One common issue facing our urban forests is the fact that trees are dying prematurely. Many are planted improperly, setting them up for failure. Many do not receive regular maintenance. And few are adequately protected during construction projects. To help remedy this issue, the Forest Service has created this Tree Owner’s Manual.

Just like the owner’s manual that comes with automobiles and appliances, the Tree Owner’s Manual includes a parts list, instructions for installation, tips for troubleshooting common issues, recommended service, and more.

To download a copy, visit USDA Forest Service Tree Owners Manual

View a short video about Penn State University’s population of American Elm trees that survived Dutch Elm Disease but are now facing a new problem.