A tree may grow in brooklyn, but Gary Moll of American Forests, a conservation group, would like to see more of them there - and in Dallas, Des Moines, and Detroit as well.
But the reality is that just the opposite is happening. Over the past 15 years, the number of trees in many US cities has declined by about 30 percent, while the space covered by concrete and other solid surfaces has risen by 20 percent, says Mr. Moll.
He estimates that to redress the imbalance, metropolitan areas need 634 million more trees.
That's a lot of new trees, especially in a time of belt-tightening municipal budgets. But in some cities, such as Atlanta and Detroit, volunteer groups are stepping in to help - planting and caring for trees on public land, and educating the public about why they matter.
Trees are natural pollution-control devices. They absorb carbon dioxide (a byproduct of burning fossil fuels) and return oxygen to the air. Their leaves, branches, and and trunks help slow the runoff of storm water.
They also provide sound buffering, shade, and measurably cooler temperatures on hot summer days, and breeding and roosting places for local and migratory birds, whose habitat has been disappearing.
The savings that result from these environmental benefits can really add up. Research has shown that substantial increases in the number of city trees can reduce storm-water and pollution-control expenditures by millions of dollars.
Unfortunately, there's more involved than simply planting more trees. Cities also have to keep them alive - and that can sometimes be a problem.
In some cases, as with elms, disease has been a factor. Generally, though, much of the problem stems from lax maintenance, outright neglect, and stressful growing conditions.
One solution, practiced by The Greening of Detroit, is to avoid tiny saplings and instead plant good-sized trees. Most of the 35,000 trees planted by the nonprofit group over the past 13 years have been about 12 feet tall.
A tree this size, says Rebecca Salminen Witt , executive director of the organization, makes a more immediate impact and stands a better chance of survival.
Landscape architect Henry Arnold of Princeton, N.J., likes to think of trees as beautiful, economical public utilities. Often, however, they are shortchanged in planning underground infrastructure, and as a result, wind up crowded for space. "Trees are not looked upon with the same seriousness as other utilities," he says.
This is beginning to change as word spreads that tree size and longevity count more than quantity. "The benefit of an urban tree is proportional to its crown size or volume," Mr. Arnold explains. "One tree that lasts 50 years is worth more than 20 trees that last only 10 years."
Consequently, attention is shifting toward doing a better job of improving growing conditions. To thrive, city trees need better soil and more of it than they commonly receive, plus good drainage and aeration.
When trees are planted too hastily, without proper attention to soil and selection, they may never mature or produce the desired benefits. In fact, many live only seven to 10 years.
"A lot of community groups like to push tree planting because it's fun and easy to get the public involved," says Steve Cothrel, president of the Society of Municipal Arborists. "But if you don't maintain the trees, you can end up with a lot of headaches and potential hazards."
Education, therefore, is a priority for organizations like The Greening of Detroit and Trees Atlanta, which teach neighborhood volunteers proper aftercare, from pruning to watering.
In both cities, the goal is to maintain each tree three or four years after being planted, while it's getting established. Ideally, community groups that plant trees shoulder this responsibility.
"We ask our planting partners to sign maintenance agreements," says Ms. Witt, "but practically speaking, it doesn't always get done."
The effort is backstopped, therefore, by a small full-time staff and by the Green Corps, trained high school summer workers.
In order to have trees planted, neighborhood groups must have a water supply (even if it's just permission to use someone's garden hose), and generally a willingness to do some modest fundraising. "We find that encourages them to take better care of the trees," Witt says.
In the case of Chicago, the mayor has taken the lead in crusading for trees. Disappointed by the city's defoliated landscape and urban heat islands (summer hot zones), Richard M. Daley vowed to plant half a million trees upon taking office in 1989.
His recollection of growing up when the city had a magnificent canopy of elm trees led to dusting off Chicago's long-ignored motto, urbs in horto ("City in a Garden"), which dates to 1837.
The city is so committed to this identity that nearly 400,000 trees have been planted and 53 miles of tree-studded median strips built. Along the lakefront, trees are strategically planted to benefit wildlife migration and create an "O'Hare for birds."
"Tourists are frankly shocked because they don't picture the city of broad shoulders being a garden," says Barry Burton, an assistant to the mayor.
Few would argue about the aesthetic appeal of trees. To get broad-based support from local governments and civic organizations, however, means going beyond that.
Bill Sullivan and Frances Kuo, University of Illinois researchers, have studied the impact of trees on Chicago's public-housing residents, and believe the social benefits to city dwellers are every bit, if not more, compelling than the environmental ones.
To gauge this, they studied a Chicago public-housing complex of 28 architecturally identical high-rise apartment buildings. They wanted to see how the site's limited trees impacted the lives of residents.
Compared to people in places without trees, people in buildings with trees enjoyed better relations with neighbors and reduced violence. The message, Mr. Sullivan says, is simple: Nature should be at every doorstep, or very nearby.
The city is attempting to heed this advice by designing ample green space into new public-housing projects.
Like any community, though, Chicago is challenged by trees-versus-development issues.
To rebuild a stretch of Lake Shore Drive, 400 mature trees had be removed. A city policy, however, requires replacing them inch-for-inch, based on trunk size. As a result, 1,000 new trees are being planted.
Atlanta, a city under intense development pressure, is especially mindful of its trees. Orange warning signs are posted where trees are to be removed. People have 15 days to appeal the plan to the city tree commission.
Marcia Bansley, executive director of Trees Atlanta, a nonprofit citizens' group, knows how tricky it can be to balance saving trees with development.
Her organization has done a lot of tree rescuing and planting, and people often look to the group for assistance. But it's not possible to save every tree. "It's a fine line between saving trees and working toward the overall good of the community," she says.
In Atlanta's case there is a particular sense of urgency because the city has grown so rapidly in recent decades. Suburban sprawl has eaten up surrounding woodlands and made Atlanta's commutes the longest in the country.
To encourage more urban dwelling, especially among former suburbanites, cities need to be as green as possible. Trees Atlanta has helped tremendously in that regard by planting more than 14,000 shade trees downtown since 1985. Trees have been planted alongside business-district sidewalks, in parking lots, around downtown churches and subway stations, and in pocket parks.
For businesses, trees can have drawbacks. They make messes, buckle sidewalks, interfere with utility lines, and obscure storefronts. But there's an upside, too, says Kathleen Wolf of the University of Washington's Center for Urban Horticulture.
Her research shows that people are willing to spend more on products in business districts with trees than without them. Trees send a message of care, quality, and welcome, and can give a district a distinct character that customers like.
Communities still have much to do in reaching low- income neighborhoods with the message about planting and caring for trees and in enlisting residents as "citizen foresters," but Jim Lyons, executive director of GCA Casey Trees, an endowment fund of the Garden Club of America, is convinced the rewards are worth it.
"Trees represent the fabric that helps pull communities together and gives them something to care about and commit to in terms of their own love of neighborhoods," he says.