Trees in the City: Battling Stormwater Pollution
By April Reese
In Washington, D.C., one of the most anticipated signs of spring is the blossoming of the cherry trees around the Tidal Basin, a man-made inlet along the Potomac River, not far from the National Mall. Every year, people gather beneath their boughs to take in the fragrance of their pink-and-white blossoms or picnic next to their trunks.
But those trees — a gift from Japan in 1912 — serve a practical purpose as well as an aesthetic one. The cherry grove and thousands of other trees that make up the city’s urban forest also help protect the Chesapeake Bay watershed, sponging up rainfall that otherwise would fall onto pavement and rush into storm drains, carrying pollutants picked up along the way into the Potomac River and, eventually, the Bay.
“If you have impervious surfaces, where the water can’t permeate in, after it fills up, it just runs off,” explains David Nowak, an urban forest management expert with the USDA Forest Service’s Northern Research Station and member of American Forests’ Science Advisory Board. “If you have trees, they can intercept the water.”
Trees intercept this water in two ways. First, rain falls on their canopies, where the leaves and bark hold onto the water until it evaporates into the air or is gradually released to the soil below, which slows the rate of water accumulation on the ground. Second, the rain falls directly to the soil, and the trees’ roots absorb the water and send it through the tree in a process called evapotranspiration, which leads to the water exiting the tree as water vapor through the leaves.
And that’s good news for the Bay, says Karen Cappiella, research program director with the Center for Watershed Protection in Ellicott City, Maryland. “If you have less runoff, you have less pollutants,” she says. When trees absorb water and its accompanying pollutants, they often transform the pollutants into less harmful substances.
But trees can only do their job if they remain a part of the urban landscape. Unfortunately, cities across the country — including many in the Chesapeake Bay watershed — have lost trees over the past few decades.
In a recent study published in Urban Forestry & Urban Greening, Forest Service researchers Nowak and Eric Greenfield found that urban tree cover nationwide is decreasing at a rate of about four million trees per year.
Within the Chesapeake Bay watershed, a separate analysis by American Forests found that Leesburg in northern Virginia lost 71 percent of its urban tree canopy between 1992 and 2001. Combined, those trees provided almost $9 million worth of stormwater management and air-pollution-reduction services — about the same amount of money the city later ended up spending on man-made stormwater projects to control runoff.
“We often say that what happens on the land is mirrored in the water,” says Erik Fisher of the Chesapeake Bay Foundation. “We know forest is the best land cover for water quality.”
And as tree cover has declined, the area covered by parking lots, streets and other hard surfaces has expanded, worsening the stormwater runoff problem. The Chesapeake Bay has become so polluted that the federal government put it on what Fisher calls a “pollution diet” in 2010, which requires the five states in the watershed — Delaware, Maryland, New York, Virginia and West Virginia, plus Washington, D.C. — to reduce the amount of nitrogen, phosphorous and other pollutants that flow into the Bay.
While some of that pollution can be traced to agricultural and industrial sources, urban runoff also contributes to the Bay’s poor health, especially with fewer trees around to absorb it.
“I think that from the very beginning we can sort of trace the decline of the Bay to the decline of forest cover in the watershed, which was either converted to agricultural use or urban development,” Fisher says.
The good news is that communities within the Chesapeake Bay watershed, as well as those in other watersheds across the country, have begun to understand the connection between urban trees and watershed health, and new initiatives have sprung up to encourage cities and residents to reverse the decline.
Perhaps the most ambitious initiative is a 2003 directive under the Chesapeake Bay Program, the federal-state partnership aimed at cleaning up the Bay, which encourages cities in the watershed to come up with goals for expanding their urban tree canopy. Maryland is leading the pack, with 36 communities committed to participate, most of them in Baltimore County. Baltimore itself has pledged to double its canopy, from 20 percent to 40 percent by 2036.
“That’s a good number to strive for,” says Steve Koehn, Maryland’s state forester. “That can be achieved in any number of ways, through parks, street trees, what have you. But 40 percent of tree cover has a lot of value, and it’s an achievable goal.”
Even cities that are fortunate to still have a lot of trees are doing their part. Annapolis, for instance, whose tree canopy already exceeds 40 percent, wants to reach 50 percent by 2036.
Those goals are being helped along by various incentives, such as the Chesapeake Bay Trust’s tree-planting grants for communities and Baltimore County’s coupon program, which gives homeowners discounts on the purchase of trees to encourage planting on private lands.
“Trees are a very cost effective way to get those reductions in pollutants when you compare them to more structural practices,” Cappiella says. “So it’s becoming more popular.”
At the federal level, the U.S. Environmental Protection Agency (EPA) is offering grants for “green infrastructure” projects designed to reduce stormwater runoff and improve water quality. In February, the agency began accepting applications for a new round of grants; it plans to distribute a total of $950,000, divvied up among 10 to 20 different projects. EPA is accepting applications through April 6th.
To help communities and individuals understand the benefits of urban forests and figure out how many more trees they need to plant, the Forest Service has created an important urban-forest assessment tool called iTree that communities — or even individuals — can use to estimate and map how much forest cover is in a particular area and what the potential is for expanding the canopy and the ecosystem benefits received from the urban forest, such as reducing pollution and improving water quality.
The Forest Service’s Nowak, who co-authored the report on tree cover decline and helped develop the canopy assessment tool, says he hopes more communities will follow the lead of Baltimore, Washington, D.C., Minneapolis and other cities that have made restoring the urban forest a priority.
“Tree-planting campaigns are helping to increase, or at least reduce the loss of, urban tree cover, but reversing the trend may demand more widespread, comprehensive and integrated programs that focus on sustaining overall tree canopy,” he says.
“Our urban forests are under stress, and it will take all of us working together to improve the health of these crucial green spaces,” adds Forest Service Chief Tom Tidwell. “It’s not too late to restore our urban forests — the time is now to turn this around.”
Discover more ways that urban forests impact your life and find out how American Forests is working to preserve urban forests across the country on our Forests & Cities page.