Cities are starting to treat trees less as decoration and more like public utilities now that they can calculate how much money trees save by cutting air pollution, storm runoff and energy costs.

Charlotte, Cincinnati and Salem, Ore., are among a growing number of cities that have adopted or toughened ordinances that protect trees, require them as part of development or provide incentives for homeowners to plant them. The efforts reflect growing recognition that trees can be as crucial to urban and suburban living as sewers, roads and water-treatment plants.

"Tree cover not only produces beauty but services that the city has to go out and buy otherwise," says Gary Moll, vice president of the Urban Forest Center at American Forests, a non-profit conservation group in Washington, D.C. "It's a whole lot cheaper than building concrete infrastructure."

Tree cover has plummeted since the mid-1980s in all 25 metropolitan areas American Forests studied with satellite and aerial imagery.

The "green infrastructure" strategy springs from imaging software that allows cities to quantify the financial benefits trees bring because they:

*Clean air by filtering pollutants and producing oxygen. A 35% decline in the Charlotte metropolitan area's tree cover from 1984 to 2003 led to a similar reduction in the amount of carbon monoxide, ozone and other pollutants that trees removed from the air, American Forests research says.

*Reduce the need for huge storm-water systems that prevent rain from washing oil, auto coolant, pesticides and other chemicals into rivers and lakes. Cities in 10 counties in the Atlanta area had to spend $2 billion on storm-water facilities to handle runoff caused by the loss of trees over more than two decades, the analysis shows. Salem asks developers to plant trees in parking lots; the city hopes to increase its tree canopy from 18% to 25%.

*Lower energy costs by providing shade and cooling the air. Cincinnati encourages homeowners to protect trees on and near their properties.

"You can tell them these two trees each can save you $55 a year" in air-conditioning bills, says Dave Gamstetter, natural resource manager for the Cincinnati Park Board. "That's the key: the pocketbook."