New mechanisms for measuring ecosystem values make it possible for cities to be both environmentally green and financially savvy.

— By Alexis Harte

A sea of rooftops has replaced forest in Leesburg, Virginia.

John Wimberly lives in the tree-covered neighborhood of Chevy Chase in Washington, DC. Trees dominate the landscape there, reaching high into the sky to form a green canopy and protective umbrella over the community. Without the trees, the neighborhood would change completely.

That possibility became very real when a senior care facility in the neighborhood proposed removing 30 percent of the tree cover on its 16-acre site as part of an expansion. The proposed building would be placed on a wooded portion of the site rather than on a grassy area.

The impact, while felt most heavily in Chevy Chase, would actually be felt citywide. With an existing tree canopy of 28 percent, the District of Columbia is already trying to address its tree deficit. A previous study by American Forests recommended the city enact policies that lead to a net gain in trees. Setting a canopy cover goal is a first step.

"The District could realistically increase canopy cover 7 percent to maximize benefits from this natural capital," says Gary Moll, vice president of urban forestry for American Forests.

The problem is not unique to Washington, DC; in fact, it's a story often repeated in communities across the country. Incremental, project-by-project changes add up to citywide tree deficits. The momentum of growth and development is plowing down urban forests-and eroding natural capital. Urban land cover is expanding by about 20 percent every 10 years while the urban tree deficit increases by about 30 percent during the same time period. Even more frustrating, citizens-as individuals-are unsure how to change this trend.

That's where American Forests hopes people will take advantage of its CITYgreen software, which allows individuals or communities to calculate the magnitude of a community's tree deficit and the corresponding impact of air, water, and energy. Different versions are geared toward different skill levels. Novices can get a fast, free tree analysis of their city, town or watershed from American Forests' website (www.americanforests.org). GIS professionals can use CITYgreen for ArcGIS to perform an advanced analysis.

Satellite images and GIS tools make it easy for officials to begin to consider trees as part of a community's tangible financial assets. Think of an urban forest as the principal of an investment. When it's large enough, that principal provides many environmental benefits, among them cleaning air, slowing stormwater and recharging groundwater, filtering pollutants from city streets before they enter waterways, and cooling communities.

These "ecosystem services" are like the interest that flows from the principal. Without a sufficient urban forest to work for cities, ecosystem services will dwindle. The goal is to protect the principal and use the interest. Yet cities too often install costly built infrastructure rather than allow nature to help manage air and water systems.

What constitutes a "sufficient" urban forest? While ideal canopy amounts vary by region, American Forests recommends that every community set tree canopy goals and offers general guidelines for different regions of the country on its website. Communities that don't meet federal clean air and water regulations need to first determine their tree canopy cover, then set canopy goals with an eye toward using the ecosystem services trees provide to help them reach compliance. The real trick is enacting policies and programs that allow a community to achieve its canopy goal.

In Leesburg, Virginia, for example, urban forester Jay Banks has watched tree canopy dwindle since 1997. The city didn't balance its "green" and "gray" infrastructure and so had to engineer a stormwater system to handle all the flow, he recalls. Residents learned just how big those stormwater culverts were when Hurricane Isabel flushed so much stormwater into the drains that a van got caught in the flooding and washed through a culvert under a four-lane road.

But engineering elaborate infrastructure to cart water downsteam to prevent flooding solves one problem while creating another-too little water in drought years.

"My area of the country gets nearly 4 feet of rainfall a year," says Robert Zimmerman, director of the Charles River Watershed Association in Waltham, Massachusetts, yet sections of his part of the state are literally running out of water.

Exacerbating the problem are combined sewer and stormwater systems that tend to overload in heavy rains, releasing untreated sewage into waterways. Those systems are in some of the nation's largest and oldest cities: Atlanta, Portland, Toledo, and Washington, DC. The U.S. Environmental Protection Agency (EPA) is cracking down on enforcing regulations for these systems.

Lessons From Nature

Taking a lesson from nature, engineers throughout the country are allowing rainwater to slowly infiltrate back into the soil, recharge water tables, and filter water pollutants before it enters waterways.

In Philadelphia, the water department uses state grants to promote landscaping that allows water to seep into the ground without entering the sewer system. "One gallon saved by keeping water on the land in the first place is one gallon less that you need to put in a tank, so that's some value to the city," says watershed director Howard Neukrug.

In Lodi, California, trees have made the city's list of best management practices-one strategy in its Stormwater Management Plan, designed to meet federal clean water requirements.

Many cities that suffer from poor air quality are close to or exceed EPA air quality standards for ozone pollution levels. Exceeding these standards is not only dangerous to public health, it can jeopardize millions or even billions in federal funding for transportation projects.

New research released this year shows that air pollution is responsible for up to 7 percent of cardiopulmonary deaths in the United States. In cities like Los Angeles, which has the highest air pollution rates in the country, the percentage of deaths could be nearly twice that number.

More trees can help in the effort to reduce emissions. Research by the U.S. Forest Service, Georgia Tech, and other scientific institutions shows trees to be effective at improving air quality and reducing pollution. EPA's Early Action Compact allows communities to use trees as part of the solution in their local plans, given that it is possible to measure trees' absorption of ozone, carbon dioxide, sulfur dioxide, particulate matter, and nitrogen dioxide.

San Antonio, Texas, is analyzing and quantifying its ecosystem services via a "green data layer"-a digital map of its urban ecosystem prepared by American Forests. "San Antonio's green data layer will assist in Early Action Compact Programs because tree benefits can be quantified," explains Dorothy Birch of the Alamo Area County Council of Governments.

People have begun to check out just how hard the trees in their community are working. More than 250 people looking for answers to local tree issues have clicked onto American Forests' website for a fast, free urban ecosystem analysis that will check the amount and general value of tree cover in their city, town, or watershed.

What happens next varies by community. Some take the findings directly to their mayor or city council. Others draw public attention by sharing the information with a local newspaper. In Texas' Lower Rio Grande Valley, the analysis was used as the basis for a public service announcement. And in Chesterfield County, Virginia, the information appeared as part of a slide show for the community's green infrastructure task force.

Back in Washington, DC's Chevy Chase neighborhood, where the senior care facility has proposed an expansion, John Wimberly and his neighbors used a more detailed CITYgreen analysis to propose an alternative development scenario. The analysis they presented showed the impact on stormwater and water quality if the expansion was built by clearing trees from the wooded land as opposed to using the nonwooded land. The planning board, the developer, and the neighborhood are now discussing alternatives.

Charts show how tree canopy and agricultural land was lost between 1992 (above) and 2001 (below) as the town of Leesburg, Virginia, began to develop. That trend means a loss of the ecosystem services trees provide to clean air and water.

In Leesburg, Virginia, urban forester Jay Banks plans to share his CITYgreen findings with the city's Tree Commission. Quantifying the ecosystem services provided by tree canopy will help Loudoun County as community leaders consider setting an urban growth boundary.

Elected officials and city managers working on air and water issues can integrate their urban forests into planning decisions using more detailed data and tools available from American Forests. An Urban Ecosystem Analysis conducted by American Forests in 1998 for Roanoke, Virginia, revealed the city's tree cover shrank from 40 percent in 1973 to 35 percent in 1997.

It took the city council less than a year to pass an Urban Forestry Plan. Among its top priorities: setting a 10-year 40 percent tree canopy goal citywide which city officials expect to have a ripple effect on all planning and management decisions.

The rapidly degrading natural landscape in our urban areas looks to be one of this country's largest conservation issues. American Forests believes the best conservation solution is to build a better understanding of trees' vital role in the city. Understanding the magnitude of loss, calculating the value of natural services provided by the ecosystem, and setting tree canopy goals will help communities make wise land-use decisions.

These decisions can protect the natural functions of the land, before they are lost forever. A city's investment in and commitment to its tree canopy will pay big clean air and water dividends for years to come. AF

Cheryl Kollin has been director of urban forestry at American Forests for 11 years. [TOP]