Walking down into the floodplain at the Nature Conservancy’s preserve in Benson is like walking into a sea of American elm saplings.
The trees criss-cross about 30 acres on the valley floor in tight rows.
“American elm is a foundation tree species in floodplain forests,” says Leila Wilson, an ecologist with the U.S. Forest Service. “These are systems which are facing severe threats from non-native pests and pathogens, but we also know from other impacts, right? Land use change, conversion to agriculture, now climate change, and changing precipitation and temperature patterns. So these are systems that are in peril.”
Wilson knows these trees well. Each one comes from a seed she harvested from a tree whose flowers she isolated with little plastic bags, then hand-fertilized using pollen collected in the lab.
That pollen came from big old elms scattered across the Northeast.
After all that care, Wilson is in Benson to pump these baby trees full of a fungus that’s likely to kill about a third of them.
“We're going to be infecting them with the Dutch elm disease fungus. So drilling a little hole into each tree, and then injecting a solution that contains 100,000 spores of the fungus into each tree,” she says as she drills the first hole.
It’s bittersweet, she says.
“I feel like we should warn them and apologize to them,” she says wistfully as she works. “Because we’re torturing them. And it’s not nice. I don’t like that part.”
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Big, old elms are rare these days. But that wasn’t always the case.
If you took a walk along the banks of a river in Vermont a hundred years ago, you’d likely find yourself looking up mostly at towering American elm trees. About 50 years ago, Dutch elm disease wiped out most of these trees, but a few survived. Now, scientists like Wilson want to use these survivor trees to bring American elms back.
The disease is caused by a fungus that’s spread by beetles that feed on elm twigs. From there, the fungus infiltrates the tree’s vascular system, clogging up vessels called xylem that carry water up into the leaves, a little like arteries.
As the fungus spreads, the tree cauterizes the vessels in an effort to stop it. This makes the disease snowball, until the leaves and the tree’s canopy die.
It’s a little reminiscent of heart disease in humans.
Scientists aren’t entirely sure why, but some trees appear to be more resistant than others. It could be that they have more resilient vessels. It could also be that the beetles have a preference for certain kinds of trees.
Regardless, scientists hope that by identifying and then breeding these resistant individuals, they can bring more big American elms back to the landscape of the East Coast.
Wilson is hopeful that many, if not most, of the saplings at this site in Benson will survive this test. She and her colleagues will look for survivor trees by rating the lushness of their canopies.
All in all, it will take about two years to know for certain which trees made it and which ones succumbed.
The ones that survive will have their seeds harvested to be used to grow a nursery stock that can be planted as part of floodplain restoration projects around the region.
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American elms are of particular interest to scientists because they are big, long-lived trees with roots that can survive being underwater for weeks at a time. Mature elms slow down floodwaters and filter out sediment. They thrive along flood-prone rivers, in ecosystems scientists say are increasingly important to protect as human-caused climate change consolidates rain into more extreme deluges across the region.
When elms died, green ash moved in to replace the role they filled. But those trees are increasingly under threat from emerald ash borer, and are dying en masse.
A 2022 University of Vermont study found that planting trees like elms in floodplains could save Vermont $1 billion over the next century and cut flooding damage by almost 20%.
Gus Goodwin is a scientist with the Nature Conservancy who is caring for the elms in Benson. From the nursery, he points out an adult elm that was killed by Dutch elm disease, on the border of the neighboring hayfield.
The top of the tree is curled up and bare, like a witch’s broom.
Goodwin says losing an iconic tree like the American elm has a human cost as well as an ecological one.
“I think because it's such a beloved piece of our cultural history and planted throughout so many towns and so many of our civic spaces, there's that kind of like community loss too,” he said.
Just over the fence from the nursery, he points out a big, healthy American elm. This one could be hundreds of years old. Shaped like an overflowing vase, it towers over the saplings the scientists are inoculating with spores, and serves as a reminder of what used to be.
“There's just something special about walking into a big forest with big trees, that's messy, that has all this kind of complexity and diversity,” Goodwin said. “And you can't get that without big trees.”
Scientists hope with this work in Benson, they can bring a little of that carbon-rich mess back to New England’s river banks.
