ST. LOUIS • The old ash trees under the Gateway Arch are about to die, casualties of the imminent arrival of a small green beetle.
But the sapling picked to take their place, the London plane tree, is also threatened.
A black fungus has infected and killed tens of thousands of plane trees across Europe. And the disease, commonly called canker stain, has roots in the Mississippi River valley.
The National Park Service says no species is perfect. The London plane tree, administrators noted, is unusually resilient to pests and disease.
But tree experts say the real issue isn’t the plane tree, which most agree is a decent choice. Rather, it’s the decision to plant just one species along the Arch’s walkways — a point on which arborists said they repeatedly warned the park service.
“Anybody in my field would say don’t do a monoculture, because you can lose them all at once,” said Thomas C. Harrington, a professor of plant pathology at Iowa State University who studies canker stain disease.
He’d recommend no more than 100 trees of one species — far short of the 800 expected to line walkways at the Jefferson National Expansion Memorial.
“I don’t think they know what they’re getting into,” he said.
The park service consulted with about a dozen tree experts in recent deliberations. All of them argued against the single-species planting, said Hank Stelzer, chair of University of Missouri forestry, whose department was involved in the process.
“History has a way of repeating itself,” he said. “We lost the chestnut trees to the chestnut blight. Then we lost the American elm to the Dutch elm disease. And when all those left the urban environment, we said we’d put in green ash. It’s resistant to drought, heat. It’s pretty tough. Until this pest showed up.”
In North America, the emerald ash borer was first identified in Michigan in 2002, likely imported as an accidental stowaway in wooden packing materials.
By 2010, the beetle had been detected in 15 states, including Wayne County, about 150 miles south of St. Louis. The beetle is now in Madison County, Mo., about 90 miles south of St. Louis.
The park service has been preparing for years to find a replacement for the ash trees on the Arch grounds. It met privately with tree experts and publicly with residents, slowly culling more than 500 kinds of trees down to 68, eight, and then three.
At the end of May, it announced its decision: the London plane tree, Platanus x acerifolia, chosen for its broad leaves, peeling bark, height, hardiness and resistance to disease.
Replacing the trees will cost about $1 million, park service administrators said, part of an estimated $14 million in landscape improvements to the walkways leading to the Arch. The money is projected to come from private donations to CityArchRiver, the nonprofit agency coordinating the concurrent $380 million tax-supported renovation of the Arch grounds.
Meanwhile, canker stain is ravaging European plane trees. News reports and scientific papers outline the damage: thousands of trees already cut down in Italy; 80,000 affected in southern France; 42,000 at risk lining the 300-year-old Canal du Midi, one of Europe’s oldest man-made waterways.
And now the stain threatens the beloved plane trees along Paris’ Champs-Elysées.
The fungus travels, in part, through a tree’s water-conducting veins. And plane trees planted in rows graft their roots together.
“By the time you see one tree dying, it’s already moved to a couple of the trees down the road,” Harrington said. “You just can’t get ahead of it.”
Decades ago, paper companies experimented with monoculture plantations in the southeastern U.S. They planted rows of American sycamore, a parent to the hybrid London plane. But disease tore through the plantations, eventually shutting them down, Harrington said.
Planting a single species, said Stelzer, the Mizzou professor, is like playing Russian roulette.
“Who knows what the next malady will be?” Stelzer said. “All of us — the (state) Department of Conservation’s urban foresters, private consulting foresters, us — were strongly encouraging them to pick four or five different trees and mix it up a little.”
The park service never considered it, said Bob Moore, a historian at the Arch grounds who worked on choosing the tree. The Arch, including its landscape, is a national historic landmark.
Other kinds of trees will be planted in other areas around the park. But the single-species planting along the Arch walkways is one of the landscape’s defining features.
Architect Dan Kiley, considered by many to have been one of the country’s leading contemporary landscape designers, used the trees to frame and highlight the Arch. The landscape, Moore said, is one of the premier examples of mid-century modern architecture. And the monoculture has become Kiley’s signature.
“The landscape was every bit as important as the Arch itself,” Moore said. “What we have is an entire package that needs to be preserved.”
It’s a fight between history and ecology, said Andrew Wyatt, vice president of horticulture at the Missouri Botanical Garden. “A diversified landscape on one side, and a historic presence on the other,” he said.
“It’s whether you’re willing to roll the dice and take the risk that a disease would wipe out the whole monoculture,” Wyatt said.
“But I do have to say,” he added, “the London plane tree is a reasonably good choice.”
EDITOR'S NOTE: An earlier version of this story had an incorrect age for the Canal du Midi.