There's a funny thing about people and trees," said Mayor Margaret Mallino, of University Park, which is noted for its streets lined with leafy oaks and Bradford pears, many planted in the 1930s and '50s when housing projects sprouted around the Capital Beltway.

Trees give people a sense of continuity as few things in life do, Mallino suggested. "In a community," she said, "the value of street trees means a lot."

University Park, like many other urban neighborhoods, is working hard these days to save its great old trees, gradual victims of disease, environmental damage and old age. In a 1987 survey of 20 cities, the American Forestry Association found that urban trees were dying at a faster rate than they were being planted; about half of the cities replaced only a quarter of those losses.

This fall, University Park is planting 130 young trees in front of many of its 900 homes to replace ones that died or had to be removed. Similar programs are underway in neighboring Mount Rainier and Brentwood.

In Northern Virginia, 28 new red bud and ornamental cherry trees are taking root along the Maple Avenue segment of the Washington & Old Dominion recreational trail, a gift of Vienna as part of 100 trees the town planted for its centennial. Last year, a coalition of Montgomery County residents, business people and government officials banded together as "Bethesda Ever Green" and landscaped the city's drab downtown with more than 75 trees.

Unlike in the past, these efforts are far more than beautification. Environmental education and concerns about global warming have made citizens more aware of the ecological importance of trees, which help prevent soil erosion, reduce air and noise pollution, cool urban "hot spots" and lower carbon dioxide buildup in the air.

Today, local officials and civic activists talk about the need to preserve "urban forests," and they stress that when it comes to preserving a heritage of green, every day is Arbor Day.

"It's politically sexy to talk about planting new trees, but what happens next year when they need pruning, mulching and maintenance?" said Holly Wagner, chairwoman of the Mount Rainier City Tree Commission, chartered as part of a tree-preservation ordinance the city enacted last year.

Last weekend, Wagner led a group of Mount Rainier residents and city officials through Queenstown Neighborhood Park to check the progress of 175 fledgling green ash, red maple and oak trees that they planted there last year along the northwest tributary of the Anacostia River. Otis Hayward, a former Mount Rainier council member, said the trees were donated by Maryland under its Green Shores Program, designed to reduce trash and toxic runoff into waterways that feed into the Chesapeake Bay.

Urban conditions are tough on trees, Wagner said, surveying the bank of saplings. Some had succumbed to the effects of drought, though Mount Rainier's volunteer fire department had watered them during the driest days. Upper leaves were pocked by pollution from passing cars. Some trees damaged by insects had become mere skeletons. Several trees near an apartment complex had been vandalized, their trunks broken off at the base. "Fortunately, trees are pretty resilient," Wagner said, eyeing bushy green shoots poking from the ground.

The health of a city tree, Wagner said, "is affected by a lot of things we don't think about -- salt from the street, oil runoff from parking lots, poor soil and drainage, underground gas leaks."

Maryland and Virginia have created positions for urban foresters -- and some local counties are hiring their own -- to advise municipalities on tree planting and maintenance, such as which varieties are best suited to withstand the rigors of city life. But "everyone's still learning," said Wagner, who teaches ornithology and plant ecology at George Washington University. For example, "how do you put in shade trees when space is limited and there are wires overhead?"

While stately to behold, Mount Rainier's prized American elms, the largest cluster in the state, are causing headaches during major street renovation. "To replace the sidewalks while preserving overlapping elms is very difficult," said Mayor Linda Nalls. "The contractor is working very closely with us to preserve those trees, but I fear we won't be able to get them to come back here to work for us again."

Last year, Mount Rainier was named an official Tree City U.S.A. The title is given by the National Arbor Day Foundation to towns and cities that adopt a comprehensive tree-management program and demonstrate broad-based citizen involvement in tree conservation. Mount Rainier's efforts include a computerized inventory of the city's trees, a program to monitor for Dutch elm disease and training public works crews on proper tree care and repair.

Several local jurisdictions have adopted programs modeled on the American Forestry Association's Global Releaf campaign, a three-year project with a goal of planting 100 million trees from coast to coast. Prince George's County ReLeaf, for example, has pledged to plant 100,000 new trees in the county by 1993.

"The effectiveness of the whole process of urban forestry depends on having everybody committed to the program and realizing the value of it," Mallino said.

"There's a big push on now to realize how important our green spaces are, especially to those of us in communities inside the Beltway that have large trees -- mature urban forests -- that are dying," she said.