For the past six years, the Bay Program has worked to plant trees along barren streambanks with the aim of curbing nutrient runoff and improving stream habitat.
Now, it’s looking at increasing the number of trees along barren city streets and sidewalks, as well as other developed areas, to improve the health of urban waterways.
The Forestry Workgroup has been gathering opinions about setting a new goal for the Bay Program’s riparian forest buffer restoration effort. It could include the first “urban forest” restoration goals which, among other things, might call for planting 1 million trees in metropolitan areas throughout the watershed by 2010.
In addition, goals may seek up to 10 communities in each state set their own targets to increase their amounts of tree canopy.
The urban goal seeks to increase the overall number of trees, rather than focusing just on streams, because a highly developed city landscape has different hydrology.
In rural areas, much of the water enters a stream through groundwater or runoff, which is filtered by tree roots that draw out much of the nutrients and other contaminants before the water enters the stream.
Much of the urban landscape is covered with rooftops and pavement, which divert runoff into storm systems that pipe it into streams, bypassing any forest buffer. With most pollutants being carried by stormwater runoff, urban riparian buffers are less effective at cleaning up streams.
“The stream network really begins in the individual driveways of an urban area,” said Al Todd, U.S. Forest Service watershed program leader for the Northeast United States. “In those areas, the function of a buffer starts with the tree canopy in the watershed, rather than just along the stream.”
Protecting urban stream health means catching as much rainwater as possible before it hits hard surfaces. And trees can be a big help.
The leafy canopy of a tree can capture and slow the rate at which rain hits the ground, allowing more of it to be evaporated into the air or absorbed into the soil rather than runningoff solid surfaces or being piped to the stream.
In addition, urban trees can be used as part of new “low impact development” designs aimed at promoting water infiltration into the ground. For example, special openings in curbs can help divert street water to areas planted with trees—where pollutants are filtered and water absorbed—rather than having it flow down storm drains.
Trees can also be incorporated into “rain gardens,” landscaped depressions in the ground where runoff is steered so it can soak into the soil.
The idea for an urban tree goal emerged during a series of stakeholder meetings conducted by the Forestry Workgroup last fall, as it gathered input on setting a new streamside forest goal. “It was something that everyone really got behind and could support,” said Sally Claggett, U.S. Forest Service liaison to the Bay Program. “The most frequent comments we received related to doing more to conserve existing and newly planted buffers and doing more with trees in developed areas.”
She said the million-tree goal was based on the professional judgment of foresters who viewed it as an aggressive, but realistic goal that could be met with extensive help from volunteer groups. “It is a goal that we hope will motivate more urban tree planting,” she said.
Fitting these urban goals into the process has raised some concerns. “Although we can clearly make the link between urban trees and the kinds of water quality and stream health functions that we expect from buffers, these recommendations do go beyond what some feel was called for in developing a new riparian buffer goal,” Todd said.
But adding urban tree canopy also supports other commitments of the Chesapeake 2000 agreement, as well as the Bay Program’s recent Stormwater Directive, which promotes innovative runoff practices to reduce runoff and protect streams.
Any water absorbed by the trees, or allowed to soak into the soil, helps mimic natural hydrologic conditions in which most water enters streams via groundwater. Water rapidly gushing off streets, by contrast, gouges out streambanks and increases erosion that smothers aquatic life. It can also carry a wide variety of pollutants, and have a warmer temperature, which affects stream life.
American Forests, a conservation group, in 1999 estimated that the tree cover lost in the Baltimore-Washington metropolitan area from 1973 to 1997 area resulted in an additional 540 million cubic feet of runoff a year, which would have taken more than $1 billion in stormwater control facilities to manage.
The Forest Service has launched an effort to measure urban trees’ impact on stormwater flow and stream chemistry. The Baltimore-based study will result in better estimates of both the ecological, and economic, benefits of trees in urban areas, said David Nowak, a researcher with the U.S. Forest Service. But, he noted, “with trees, you get a multiple host of benefits for one price.”
For example, trees can also reduce air pollution in urban areas. An analysis recently completed by Nowak found that the 2.8 million trees in Baltimore reduce ozone pollution—a key component of urban smog—by 244 metric tons, particulate material by 200 metric tons, nitrogen dioxide by 156 million tons and sulfur dioxide by 81 million tons a year. The total annual air pollution control benefits of Baltimore trees is worth more than $7.5 million annually, he estimated.
In addition, cooling and shade from trees reduced energy demand by another $3.3 million a year. That would also have resulted in air pollution reductions from power plants.
A key part of any metropolitan area forest goal would be educating people about the multiple functions that trees perform, said Mike Galvin, an urban forester with the Maryland Department of Natural Resources. “Trees are part of the ‘green’ infrastructure,” he said. “It is important that people make the connection between the tree in front of their house and how that can help the Bay.”
Besides reducing air pollution and runoff, trees help maintain streambank stability and provide habitat, even in urban areas.
“I don’t know how the Bay Program can achieve its goals without trying to encompass urban areas in what they are trying to do,” Galvin said. “They are often the most degraded, most impacted parts of the system. We really need to mitigate there. And let’s face it—that’s where the people are.”