If a tree falls in the city. (Urban Issues).

Trees in urban settings have often been thought of as little more than decorative objects ripe for sacrifice in the name of development. But in addition to making city life more pleasant, trees also offer significant health and environmental benefits, trapping and converting air pollutants as well as catching and controlling stormwater runoff. To provide city planners and officials with solid evidence of the value of city foliage, the Washington, D.C.-based nonprofit organization American Forests has quantified the economic worth of trees to cities and towns.

Using satellite and aerial imagery and other data, researchers at American Forests have determined the amount of tree canopy cover for almost 20 cities and towns so far. Using the organization's own geographic information system--based software, they then calculated the amount of air pollution removed by urban forests and the lost benefits urban forests would have provided had they been left standing. They also calculated the effect of trees on stormwater runoff.

Overall, urban tree canopy cover has declined by 30% over the past three decades, says Cheryl Kollin, director of the Urban Forest Center at American Forests, and canopy tree cover has decreased in most of the surveyed areas. In Washington, D.C., for example, densely forested areas--those with at least 50% tree cover--decreased from 37.4% of the city's land to 13.4% over the period 1973-1997, while sparsely forested areas--those with less than 20% tree cover--increased from 51% to 71.8%. The result, according to American Forests, is a 34% increase in stormwater runoff. The lost trees also would have removed an estimated 354,000 pounds of air pollutants, including sulfur dioxide, carbon monoxide, nitrogen dioxide, ozone, and particulate matter of 10 microns or less.

Pollutant gases diffuse into leaves through the stomata and react in a variety of ways, converting into other compounds that sometimes actually benefit the tree. The effectiveness of trees at removing air pollution varies with the type of pollutant and the type of tree, says David Nowak, project leader of the U.S. Forest Service's urban forest ecosystem research unit in Syracuse, New York.

Trees mitigate solids pollution by filtering particles from the air, Nowak explains. "Trees that have sticky and hairy leaves or smaller leaf surfaces or smaller leaves tend to be better filters," he says. But unlike gaseous pollutants, particulates aren't converted or absorbed. An average of 50% of particles blow off the tree and back into the atmosphere. Most of the rest either fall to the ground when the tree sheds its leaves or are washed off by rain. "The soil is the ultimate resting place for a lot of these particles," Nowak says. The Forest Service is currently investigating whether these particles actually do less harm in the soil than in the air.

The increased costs to taxpayers as a result of the shrinking tree canopy are significant. In Washington, D.C., alone, processing the additional stormwater runoff costs about $226 million per year, and the additional air pollution abatement expenses add another $1 million annually, according to American Forests. "The economic value placed on air pollutants is established by state public service commissions from costs to society not reflected in the marketplace, such as rising health care costs from respiratory illnesses," Kollin says. And trees provide still other concrete benefits to urban dwellers, such as providing shade that can reduce residential cooling costs.

Everybody likes trees, but many city dwellers and city managers don't realize just how valuable they are. But according to Kollin, the average urban area still has about a 30% tree canopy cover. She says, "American Forests believes that cities could increase their tree canopy cover by at least ten percent and reap increased environmental benefits."