- The Washington Times - Wednesday, June 7, 2006

James L. Sherald of the National Park Service dislikes the toothy look that one American elm gives the line of trees along the Mall.

The Augustine ascending elm to which he points extends above the otherwise uniform canopy and instead of branching out, grows upright. The tree, he says with a thankful sigh, is susceptible to Dutch elm disease (DED), so it will be replaced with another type of American elm once it dies.

The American elm, or Ulmus americana, is admired for its beauty, height, expanding canopy and ability to withstand urban stresses, along with the fact that it is an American icon, says Mr. Sherald, chief of the Natural Resources and Science division of the Center for Urban Ecology in Northwest, part of the National Park Service.

The National Capital Region parks, which include portions of Maryland, Virginia and West Virginia and all of the District, have about 2,500 to 2,700 American elms of a total of 16,000 trees.

“It’s really the perfect tree for the nation’s capital despite the Achilles’ heel of being susceptible to Dutch elm disease,” says Mr. Sherald, who holds a doctorate in plant pathology. “It’s the latticework that holds the park landscape together.”

Like the District, other cities are cautiously bringing back disease-tolerant varieties of the American elm, Mr. Sherald says.

DED, which began infecting American elms in the 1930s, reached the District in 1947 and claimed thousands of trees in the city and its parks. Elm bark beetles transmitted the fungus that causes DED from diseased trees to healthy ones. The healthy trees responded by restricting water movement in infected portions, causing the trees to wilt and eventually die.

Before 1970, the District lost about 1 percent of American elms each year, a rate that increased to 5 percent to 7 percent in the early 1980s, according to a 1993 report by the Save-the-Elms Task Force.

The 14-member task force, which worked under the auspices of the National Capital Planning Commission, called for a strategy that combined several methods of disease control, including the cornerstone method of sanitation, to reduce tree loss to 3 percent or less annually. Sanitation, still used on parks grounds today, involves detecting and destroying diseased trees or sections of trees. Fungicides are used to arrest DED in some infected trees, and a method called root graft control is used to sever roots between diseased and healthy trees.

Trees affected by DED can be saved if less than 5 percent of the crown is infected, but if symptoms are widespread, the tree must be removed, says Robert DeFeo, regional horticulturalist for the National Park Service, which is a part of the U.S. Department of the Interior.

“You want to get the infected trees out because the beetle goes from tree to tree,” Mr. DeFeo says.

The park service plants half of new American elms from seedlings, which may or may not be susceptible to DED, and the rest from clones of the elm that are DED-tolerant. The clones, technically called cultivars, are created by planting cuttings of a tree, which then take root.

“Breeding has been tried. That’s not what has given us resistance,” Mr. Sherald says.

In the 1970s, the U.S. Department of Agriculture inoculated thousands of trees in an effort to identify DED-tolerant American elms, says Alden “Denny” Townsend, a retired research geneticist for the U.S. National Arboretum in Northeast.

The USDA selected the best trees with disease tolerance and propagated cuttings from them to develop trees with the same genetic makeup of the originals, Mr. Townsend says. The trees were grown for four years, then inoculated with spores of the fungus that causes DED, he says. The propagation and inoculation process was repeated several times to achieve the most resistant trees, he says.

“We inoculated all of these trees with huge numbers of spores,” says Mr. Townsend, who holds a doctorate in plant genetics. “We overdid it, but we wanted to get the best disease-tolerant trees.”

The USDA moved the American elm program to the arboretum in 1984, at which point the agency had identified 10 DED-tolerant trees, Mr. Townsend says. In a final test, the 10 trees were evaluated intensely for DED tolerance. The best two were named and released in 1995 to the nursery industry. They were named Valley Forge and New Harmony.

Another cultivar Mr. Townsend and Mr. Sherald propagated was one they had found to be growing on Jefferson Drive near the Mall, hence the tree’s name, which is written as Ulnus americana “Jefferson.”

“Unlike some of the trees, this one roots very easily,” Mr. Sherald says.

The Jefferson elm, a variety of American elm, was inoculated with the DED fungus and showed wilting symptoms two to three feet along a branch or infected area before it began to recover. In comparison, an American elm that is not resistant to DED will continue to wilt through the entire tree.

In the 1990s, the park service began using its nursery at Daingerfield Island in Alexandria to grow American elms from cultivars such as the Jefferson and from seedlings. The nursery has a thousand elms in various stages of growth, including about 100 to 200 elms that are ready for planting.

“Part of the reason for the existence of the nursery is to grow trees that are not commercially available, either because they’re unique trees, or we want them of a size to match up with the size that is out there,” says Barry Stahl, horticulturalist and nursery manager for the National Park Service.

The elms are grown for three years in the nursery and another three years in the field before being planted in the parks, Mr. Stahl says. By that point, their trunks have a 6-inch circumference and are about 15 feet tall, he says.

“It might be a little bit smaller, [but] it’s not like putting in a tiny sapling next to a large tree. It’s a mature tree at that point,” Mr. Stahl says.

Most of the 75 to 100 elms the park service replaces each year come from the nursery, Mr. DeFeo says.

“We’re trying to get some diversity in a monoculture of trees,” he says about the American elm.


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