As it did last year, “green space” is becoming increasingly important as we lose more of it to development. Today's policy makers are more aware of how urban forests cool cities, lower energy consumption, and reduce greenhouse gases.

Cities will require more time and input from arborists as they add green to their infrastructure. If your public works department doesn't have an arborist on staff, it will need one soon, cautions Robert Benjamin, an urban forestry consultant who serves on the board of directors for both the International Society of Arboriculture and the Society of Municipal Arborists.

Global warming heats up. It's increasingly difficult to ignore those calling for more trees and open-space parks to reduce global warming.

NASA reports that 2005 was the hottest year in more than a century, followed closely by 2004, 2003, 2002, and 1998 (an El Niño year). Meanwhile, the U.S. Department of Agriculture (USDA) Forest Service proclaims the loss of open space as one of four major threats to our forests and grasslands; 34 million acres were lost between 1982 and 2001, and an estimated 44 million more will be lost to housing by 2030.

In 2005 the U.S. Conference of Mayors asked mayors to commit to a Climate Protection Agreement by launching urban forest restoration projects, anti-sprawl land-use policies, and campaigns urging the public as well as state and federal governments to meet or beat the greenhouse gas reduction target suggested for the United States in the Kyoto Protocol—7% reduction from 1990 levels by 2012. By late last year, 333 mayors had signed the agreement.

Return of the trees. Natural tree cover in cities has declined by as much as 30% over the last several decades, according to American Forests, a nonprofit environmental group. Many cities are working to reverse that trend.

Last year Los Angeles announced a plan to add 1 million shade trees. Other cities that have launched sizable tree-planting programs include Baltimore, Chicago, Denver, Minneapolis, and Washington, D.C. More work is ahead, though: American Forests recommends an average 40% tree canopy for regions east of the Mississippi and in the Pacific Northwest; 25% in the Southwest and West. Many cities currently do not meet these recommendations.

The right mix. State University of New York's College of Environmental Science researchers in Syracuse found that cities can improve air quality and reduce greenhouse gases in the atmosphere by planting the right mix of trees. They suggest that urban forests include no more than 10% of any one species, 20% of any one genus, and 30% of any one family.

Federal funding declines. Last September, the Washington Post reported that although Bush administration officials say trees are a priority, spending on the federal Urban and Community Forest Program has declined by 25% in the past four years, from a high of $36 million annually to a proposed $27 million in 2007.

Attack of the killer insects. Invasive disease and insects remain problematic, particularly wood-boring insects such as the emerald ash borer and Sirex wood-wasp, which cause high mortality rates in ash and pine trees, respectively.