Study shows a shrinking urban forest

Report on Baltimore is 1st to track all of a city's trees

December 15, 2003|By Heather Dewar | Heather Dewar,SUN STAFF

Baltimore is losing its urban forest at the rate of 110,000 trees a year, and if current trends continue, the city's tree population will be cut in half by 2103.

That's the conclusion of a pioneering study, the first attempt to track a city's worth of trees over many years.

About six out of every 100 Baltimore trees died each year between 1999 and 2001, said David J. Nowak, the U.S. Forest Service scientist who led the study. With plantings factored in, the city lost about four of every 100 trees annually, but researchers don't know whether that's a high rate of loss, Nowak said.

"No one's ever followed the health of an urban forest over time, so we have nothing to compare it to," he said. "That's why we did it. We don't know what the rates of change are."

City arborist Marion J. Bedingfield and community activist Guy Hager said the study confirms the suspicion that despite a stepped-up tree-planting program, the city is gradually losing the struggle to protect its tree canopy.

Last year, the city's forestry department removed 2,000 trees on city property and planted 3,400 replacements -- an increase over tree-planting rates in the 1990s, according to Bedingfield. Meanwhile, community groups planted about 2,300 trees along streets and streams.

But not all trees growing on private property are replaced, and many of the newly planted saplings die. The efforts -- which receive financial help from the Forest Service, the state, private donors and volunteer laborers -- are not enough to replace all the city's dead or dying trees, said Hager, program director of the nonprofit Parks and People Foundation.

"I think it's like the bird in the mining shaft, telling us we have some problems in our local environment, and we are seeing tree death as a result," said Hager, whose organization coordinates community tree plantings.

Urban forests are rarely studied and poorly understood, but they are becoming more important as cities and suburbs consume more land. Besides the obvious benefits, such as providing summer shade and reducing cooling costs, trees also filter water and air pollution.

Nowak's research group estimates that Baltimore's 2.6 million trees removed 491 tons of air pollution last year -- a major benefit in a city where summer smog levels often violate federal standards and can cause breathing problems. It would cost $2.7 million for businesses to do the same cleanup work, the forest researchers found.

By 2103, the study projects, the city will have 1.2 million trees, although Nowak cautioned that long-term projections are always uncertain.

To make their estimates, the Forest Service researchers marked off 200 plots on a variety of public and private sites, including factory grounds and upscale residential neighborhoods.

Earlier studies have focused on street trees because those are planted by public agencies, which keep records. But trees on roadsides and in parks account for only 5 percent to 10 percent of a city's tree cover, Nowak found.

The researchers counted and assessed the health of every tree on those plots in 1999 and 2001, and compared the tallies. They plan to study the same 200 plots every summer, creating a long-term record that no other city has, Nowak said.

Because 1999 and 2001 were unusually dry, tree mortality might have been higher than in years with more typical rainfall, Hager said.

On the up side, city trees may not have the same competition for light, water and nutrients that wild trees do, Nowak said, because they are more likely to be planted, watered and fertilized. But they have enemies a wild tree never encounters.

"Dirty air, dirty water, [road] salting, dogs, soil compaction -- and there's always people," Bedingfield said. "Vandalism, weed whackers, lawn mowers, car wrecks" all threaten city trees.

On average, Nowak found, a Baltimore tree dies at age 15, but survival varies with location.

"The closer you are to downtown, the less the life expectancy is," Bedingfield added. "You can figure seven to 13 years downtown. Closer to the suburbs the years go up, to 70 or 80."

Highway embankments are the toughest sites for trees -- about 20 percent of trees growing along major highways, rail yards or port areas die each year. Many of these trees are aggressive, nonnative species that sprouted there on their own, like ailanthus, or tree of heaven.

Ailanthus and white mulberry have the highest death rates, the study found. Foresters consider these trees invaders, but they provide environmental benefits, Nowak said.

Flowering dogwoods are also in trouble, with about one in six dying every year from a fungus. City arborists try to plant 40 varieties of trees along city streets to slow the spread of disease, Bedingfield said.

Hager said residents should plant more of the trees that thrive in Baltimore. The trees with mortality rates of 2 percent or less, were red maple, white oak, mockernut hickory, white pine, pagoda dogwood, tupelo and hornbeam.

"These are trees that are acclimatized to our urban conditions -- which means poor air quality, poor water quality and, during the drought, no water at all and our not-very-great soil," Hager said.

Hager said the city should use the study as a jumping-off point for a long-range plan.

"We have a 20 to 25 percent tree canopy in the city now. What do we want? What can we achieve? Where should we be planting?" Hager said. "We don't have a plan, and we need one."

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