March 15, 2012

The area covered by trees in many United States cities is declining, reports the U.S. Forest Service, and that loss of trees–as many as 4 million annually–translates into an astronomical loss in energy efficiency. According to the Forest Service, urban trees greatly decrease heating and cooling costs, by as much as $2,500 over the lifespan of a mature tree; improve air and water quality; and help control water drainage and erosion. The Forest Service calculates that the financial loss of an urban tree is, thus, three times greater that the cost of maintaining it.

Forest Service researchers David Nowak and Eric Greenfield arrived at their conclusion by comparing aerial photographs of 20 urban areas from the years 2002 to 2010 and evaluating the differences in the canopy coverage. They found that the tree cover in 17 out of the 20 cities had declined. New Orleans, Louisiana, had the largest decrease. (The research suggests that much of the tree loss there was a result of Hurricane Katrina in 2005.) Only one of the 20 cities–Syracuse, New York–showed an increase in the amount of tree cover.

Many urban trees are lost to the artifically made objects in a designed landscape. This is known as “impervious cover,” which includes rooftops and pavement–streets, sidewalks, and parking lots.

Trees across North America are under attack by a number of different diseases and pests. Urban trees are especially threatened by the emerald ash borer, the Japanese beetle, and a number of fungal diseases, including the Dutch elm disease.

Ginkgo trees are popular for city plantings because they are largely resistant to air pollution as well as various tree diseases. (Atoz)

Tree-planting programs in many cities have helped to slow tree loss but have not been able to reverse the larger trend. In an effort to make city planning more tree-conscious, the Forest Service is providing cities with a free software program entitled i-Tree Canopy. The program lets users analyze changes in an area’s tree coverage by pairing aerial photographs from different time periods. U.S. Forest Service Chief Tom Tidwell hopes the tool will help communities plant and maintain trees more effectively.

Additional World Book articles:

• Conservation
• Green building
• Ecology
• Trees Under Threat (a special report)