EarthTalk: Do urban trees really help reduce pollution and clean the air?

Dear EarthTalk: Do urban trees really help reduce pollution and clean the air?

— John Alderman, Washington , D.C.

Back in 1872 Frederick Law Olmsted, the granddaddy of American landscape architecture and the designer of New York 's Central Park , proclaimed that trees were the "lungs of the city."

While Olmsted's statement may have been more philosophical than scientific, researchers have since found that city trees do indeed perform important environmental functions like soaking up ground-level pollutants and storing carbon dioxide, which helps offset global warming.

Each year in Chicago , for example, the windy city's urban tree canopy removes 15 metric tons of carbon monoxide, 84 metric tons of sulfur dioxide, 89 metric tons of nitrogen dioxide, 191 metric tons of ozone and 212 metric tons of particulates, according to David Nowak, project leader of the U.S. Forest Service's Urban Forest Ecosystem Research Unit.

Trees absorb these gaseous pollutants via the tiny pores in their leaves and break them down into less harmful molecules during photosynthesis.

In Sacramento , California , a public-private partnership called Sacramento Shade spearheaded the planting of more than 200,000 trees around the city in the mid-1990s. In a study assessing Sacramento 's bolstered tree cover, Greg McPherson of the Western Center for Urban Forest Research found that the region's urban forest removes more than 200,000 metric tons of carbon dioxide from the atmosphere each year, saving taxpayers as much as $3 million annually in pollution cleanup costs.

Meanwhile, the tree cover in New York City helps remove enough airborne toxins to save taxpayers as much as $10 million a year in pollution mitigation costs, according to Nowak. The Big Apple's five boroughs are home to more than 5 million trees, covering nearly 17 percent of its public and private land, he added.

Gary Moll, a vice president at the nonprofit group American Forests, asserts that trees are the "ultimate urban multi-taskers," absorbing carbon dioxide and acting as filters, sponges, humidifiers, heat shields, and wind blockers. Under Moll's supervision, American Forests is assessing the costs and benefits of city tree cover across the country. The group uses a combination of satellite data, field surveys, and computer modeling technology to measure regional tree canopy and calculate its dollar value.

All told, Olmsted was right in his assessment of the importance of city trees. Indeed, planting trees in urban environments may be one of the best medicines available to help restore our ailing cities.

Dear EarthTalk: What are ìwildlife corridors? And how do they help preserve wildlife and biodiversity?

— J. J. Harris, Hilo , Hawaii

Wildlife corridors are stretches of land that connect otherwise fragmented pieces of wildlife habitat. Since many mammals and birds require large ranges of undeveloped land in order to survive, linking smaller habitats together is key to maintaining strong populations. Ecologists consider wildlife corridors crucial because they increase the total amount of habitat available for species, while counteracting the fragmentation that has resulted from human activity.

First espoused by Harvard biologist Edward O. Wilson in the 1960s and later by environmentalists considered on the fringe, the wildlife corridor concept has since become an institutionalized technique for managing at-risk wildlife populations.

The benefits — including greater biodiversity, larger wildlife populations, wider ranges of food sources and shelter, and increased long-term genetic viability due to population interbreeding — are now well known and undisputed by wildlife professionals. Corridor projects have sprung up from coast to coast, in some cases implemented by the U.S. Fish and Wildlife Service itself in the name of protecting threatened and endangered species.

Environmental advocacy groups are also engaged in the creation and expansion of wildlife corridors throughout North America and beyond. The Bozeman, Montana”“based American Wildlands, for instance, runs the Corridors of Life project, which uses scientific modeling to locate the best potential public and private lands for conversion to wildlife corridors throughout the Northern Rocky Mountains . According to executive director Rob Ament, the group is working with the government and with private landowners to protect parcels of land it deems key to conserving viable populations of wild animals.

Meanwhile, the Richmond, Vermont”“based Wildlands Project is also committed to the establishment of a connected system of wild areas. Since its founding in 1991, the group has commissioned several scientific studies on the viability of creating wildlife corridors and restoring populations of wolves and other ailing species in different parts of North America .

The wildlife corridor concept is not limited to North America . Central American nations have come together with leading conservation organizations including the World Resources Institute and the Wildlife Conservation Society to create the Mesoamerican Biological Corridor — also known as El Paseo Pantera ("The Pantherís Path") — to link key wildlife habitat from Mexico to Panama.

Many conservationists feel that this project is an important experiment "because it is taking place in poor tropical countries where the greatest diversity of life exists but where biodiversity is also under the greatest threat," said preeminent Canadian environmentalist David Suzuki, who hopes that someday the concept can expand to South America, Asia, and Africa.

Got an environmental question? Mail it to: EarthTalk, c/o E/The Environmental Magazine , P.O. Box 5098 , Westport , CT 06881 . Or submit your question at www.emagazine.com or email us at This email address is being protected from spambots. You need JavaScript enabled to view it. .


Related Links

U.S. Forest Service Urban Forest Ecosystem Research Unit
Sacramento Shade
Western Center for Urban Forest Research
American Forests

American Wildlands
The Wildlands Project
Mesoamerican Biological Corridor Project (Web site in Spanish)

ENN would like to thank E/The Environmental Magazine for their permission to reprint this article.