Tree cover in American cities is shrinking, and it could be costing taxpayers billions of dollars.

A new U.S. Forest Service analysis of 20 U.S. cities, including Atlanta Ga. (above), pegs urban tree loss at about 4 million trees a year. That decrease translates into an astronomical annual loss in environmental services, such as reduced heating and cooling costs, when you consider that each tree represents as much as $2,500 in such services during a its lifetime (a return rate three times greater than tree care costs), the forest service says.

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Urban tree-planting campaigns have made a difference, but not nearly enough to offset development. “Tree cover loss would be higher if not for the tree planting efforts cities have undertaken in the past several years,” David Nowak of the U.S. Forest Service’s Northern Research Station said in a press release. “Reversing the trend may demand more widespread, comprehensive and integrated programs that focus on sustaining overall tree canopy.”

The good news is that the forest service is providing a free tool to help urban planners assess the problem in their own jurisdictions. To help in quantifying the cover types within an area, a free tool, i-Tree Canopy, allows users to photo-interpret a city using paired Google images, such as these views of Atlanta, Ga.:

Indeed, that’s how the Nowak and his colleague Eric Greenfield made their assessment of the 20 cities included in the initial study, published recently in Urban Forestry & Urban Greening. They evaluated differences between the most recent digital aerial photographs possible and imagery dating as close as possible to five years prior to that date. Here are some highlights from the study:

• Tree cover in 17 of the 20 cities analyzed in the study declined; 16 cities saw increases in impervious cover, including pavement and rooftops
• Of the 20 cities analyzed, the greatest percentage of annual loss in tree cover occurred in New Orleans, Houston and Albuquerque.
• Researchers expected to find a dramatic loss of trees in New Orleans and said that it is most likely due to the devastation of Hurricane Katrina in 2005.
• Tree cover ranged from a high of 53.9 percent in Atlanta to a low of 9.6 percent in Denver; total impervious cover varied from 61.1 percent in New York City to 17.7 percent in Nashville.
• Cities with the greatest annual increase in impervious cover were Los Angeles, Houston and Albuquerque.
• Only one city, Syracuse, N.Y., exhibited an overall increase in tree cover.

U.S. Forest Service Chief Tom Tidwell urges the public to realize that it is not too late to act. “Our urban forests are under stress, and it will take all of us working together to improve the health of these crucial green spaces,” Tidwell said in a press release. “Community organizations and municipal planners can use i-Tree to analyze their own tree cover, and determine the best species and planting spots in their neighborhoods.”

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Cities included in the U.S. Forest Service study were: Albuquerque, N.M., Atlanta, Ga., Baltimore, Md., Boston, Mass., Chicago, Ill., Denver, Colo., Detroit, Mich., Houston, Texas, Kansas City, Mo., Los Angeles, Calif., Miami, Fla., Minneapolis, Minn., Nashville, Tenn., New Orleans, La., New York City, Pittsburgh, Pa., Portland, Ore., Spokane, Wash., Syracuse, N.Y., and Tacoma, Wash.

IMAGES:

Aerial views of Atlanta, Ga., in 2010. (Courtesy Google Earth)

Paired aerial views of the same several blocks of Atlanta, Ga., in 2005 (left) and 2010 (right). (Courtesy Google Earth)