The United States is under attack from alien invaders who are waging war against city trees.

Millions of urban trees have been felled by the Asian longhorn beetle, the emerald ash borer, Dutch elm disease, sudden oak death syndrome, and yes, by construction managers who compact the soil with heavy machinery or pave over natural landscapes with parking lots.

James Urban, a landscape architect in Annapolis, Md., says the decline of city trees is "a national issue," due mainly to the degradation of soils and a street tree monoculture that can be devastating when trees are hit by a certain pest or disease.

Washington, D.C., once celebrated as the "city of trees," is just one example. According to a survey by American Forests, the city lost 64 percent of its heavy tree cover from 1973 to 1997, due to neglect, disease, and construction.

Urban has been a passionate advocate of changing the way Americans think about city trees since 1982, when he ran into a group of tree researchers while designing a new landscape for the National Geographic in Washington, D.C. It was "a fatalistic meeting," he said, "and now, almost 100 percent of my effort is focused on this problem."

According to Urban, "the biggest issue in cities is a lack of soil," caused by construction, compaction, and the move to bury utilities underground in areas formerly occupied by tree roots. Urban estimates that planting a tree properly on a downtown street can cost up to $5,000 each. "You have to figure out how to get the tree roots under the pavement," he says. Urban says a standard city sidewalk tree box should measure at least 6 by 10 feet, ideally connected to an underground soil system that gives the tree roots a place to take hold. "Size matters, and bigger is better," he said.

Another concern is the lack of diversity in street trees planted across the country, he said. "The tendency in my profession, landscape architecture, is to use a set of less than 10 trees - two species of red maple, ash, red and willow oak, linden, honey locust, zelkova and plane trees - you see them all over the country." The problem is that when a devastating disease or insect does hit, it could destroy most trees of the species affected.

"If we remember the potato famine in Ireland, when the big one comes, it hits everybody," Urban said. "And using the same species all over the country does not make sense. Regional diversity is as important as diversity within the forest."

Urban recommends broadening the palette of street trees dramatically to include the hardy rubber tree, the black locust, sugar maples, single-stem river birches, and several oaks, including Spanish, sawtooth, chestnut, overcup, and bur oaks. "We need 20 new species to make our forests diverse," he said.

Practicing what he preaches, Urban installed Princeton elms, a disease-resistant variety, and sugar maple along one streetscape at the Dumbarton Oaks gardens in Washington, D.C.

Dumbarton is losing a lot of trees to old age, storm damage, disease, and new construction. Gail Griffin, superintendent of gardens at Dumbarton, says more than 20 trees are on the garden's replacement schedule, and she would like to replace them with the same or similar species in keeping with the original design by Beatrix Farrand.

Griffin says, however, that "we try not to repeat the same mistakes more than two or three times," and that when necessary, "we've been switching to another species to suit what we need." An old English elm lost to Dutch elm disease was replaced with a lacebark elm, and Griffin plans to find a substitute for a black cherry recently lost to construction.

She and Urban also are considering Japanese torreya as a replacement for hemlocks weakened by insect attack. "They're not nearly as feathery as the hemlock, but up until now they seem pest tolerant," she said.

Urban says there are some trees that just can't be replaced, but he says a number of cities, including Chicago, Milwaukee, and Savannah, Ga., are making "very good progress" by practicing progressive urban forestry. "Chicago has made a 180-degree turnaround with Mayor Richard M. Daley," he said. "Daley said 'We're going to make this work,' and he did."

Since 1989, Chicago has planted 400 thousand new trees, with a target goal of half a million. Barry Burton, assistant to the mayor for landscape issues, said Mayor Daley ordered all city departments to replace trees on a regular basis after he noticed a huge loss in the city's tree canopy. Burton said when budget figures showed that new trees added only a two or three percent increase, "the ball really started rolling."

Burton said the mayor is also committed to diversification of species. The city's tree list now includes hackberrys and hawthorns, Turkish filberts, Kentucky coffee trees, and swamp white oaks. Burton favors two new disease-resistant elms. "The American elm that came out of the Morton Arboretum called Accolade promises to be really great," he said. "The latest one is called Century, and it promises to have that perfect large vase shape that we are missing so much right now."

Burton says Chicago has "very aggressive greening goals for the city overall." That includes 63 miles of landscaped medians, flowers and plants that absorb storm water runoff, and more than 70 green roofs either in planning stages, under construction, or completed, including one on top of city hall.

"It helps cool down buildings and saves energy," he said, "and it also helps with air pollution."

According to Burton, studies conducted with federal agencies indicate that every 11 trees the city plants removes the equivalent of one car's emissions. "It's a great way to make your city look beautiful and also improve the environment and air quality," he said.