Mighty Elms Sought in Evolutionary Quest

In a daisy-filled meadow that looks onto houses and a fast-food chili restaurant, a stand of gangly 5- to 7-foot saplings could represent the genetic future of the American elm.

The species that once formed stately arches over city streets might one day improve its ability to fight off Dutch elm disease in the same state that allowed the deadly fungus into the country about 70 years ago.

Researchers are planting clusters of cuttings in Ohio from the rare and scattered trees that recovered after infection so they can cross-pollinate in the wild.

The hope is to jump-start elm evolution, said Jim Slavicek, a geneticist at the U.S. Forest Service research station. When the fungus evolves to overcome today’s tolerant trees decades from now, new survivors might be found in the Ohio woods.

“The American elm is part of our heritage and through this project, we hope to make it part of our legacy,” he said.

Never a dominant tree in U.S. forests, fast-growing elms since Colonial times were urban favorites for their leafy canopies. They tolerated air pollution, road salt and weather extremes.

No one knows how many millions of elms died since Dutch elm disease arrived with a shipment of logs at a Cleveland harbor in 1930, but estimates range as high as 90%.

As with other diseases from overseas, such as the blight that all but wiped out the chestnut, the trees didn’t have thousands of years to develop resistance.

A few cities, such as Minneapolis and Washington, D.C., are protecting their remaining trees by quickly pruning or chopping down diseased specimens and injecting fungicide in trunks.

About 3 billion elms are in U.S. forests, but 80% are young trees with trunks less than 5 inches thick. The disease typically attacks mature trees.

The few giants still standing are probably not immune, just lucky, Slavicek said.

The nation’s largest elm, in a field in Buckley, Mich., died from Dutch elm last spring. It was 112 feet tall and had a 115-foot-wide crown -- creating the false hope that size made it invincible, said Deborah Gangloff, executive director of American Forests. The Washington-based environmental group identifies “champions” of many tree species. Surviving trees should be protected, she said, but genetic research is needed to preserve the species.

Since the 1960s, the U.S. Department of Agriculture screened about 100,000 trees that survived the epidemic, but identified few that naturally tolerate the disease. The rest were simply never exposed, said Denny Townsend of the department’s Agricultural Research Service laboratory in Glenn Dale, Md.

Slavicek is planting five tolerant types -- four identified by Townsend’s team and one from Princeton University

“This sounds like quite a large step in the preservation of the American elm,” Gangloff said.

Leaves on tolerant trees die on 13% to 55% of the branches the first year, but the trees bounce back in following years, Townsend said.

Since the 1990s, the USDA has made cuttings available of the trees that seemed to fare the best. The varieties are “Valley Forge,” from a tree grown from a seed mailed to researchers, and “New Harmony,” from a Springfield, Ohio, elm that survived the fungus but died in floods.

But those trees are clones, Slavicek said. Urban trees produce seeds that land on lawns and pavement, with few if any growing to maturity. There’s no chance to mix genes -- the engine of evolution.

“We actually need to go back into the woods and plant these tolerant strains in areas where the trees can naturally produce seeds and generate new trees,” he said. As the fungus evolves, he said, the trees will too, while maintaining some genetic diversity in the species.

Back at the Forest Service laboratory in this city about 25 miles north of Columbus, researchers are getting a sneak preview of the outcome. They’re analyzing results from trees that were crossbred in 1992 and infected with the fungus last year.

“It looks like we’re getting some great results from these crosses,” Slavicek said.

The lab will grow clones from those trees and test them in a few years to confirm that the results are due to genes and not the environment.