As the earth warms and precipitation patterns change, trees are expected to migrate north in search of the weather to which they are suited. Scientists predict that trees will have to move faster than their natural abilities through seed propagation.
This has led some scientists at the University of Vermont to try to restart this process for an already beleaguered tree: the American chestnut.
“We are simultaneously trying to restore the chestnut in our experiment, as well as testing its performance in a future environment if it is moved a bit further north,” said study lead researcher Peter Clark.
After a blight fungus decimated American chestnut trees in the eastern United States in the mid-20th century, dedicated naturalists kept the species alive by breeding hybrids of American chestnut with Chinese chestnut.
It’s these hybrids that University of Vermont researchers are using in an experiment in ‘assisted migration’ – the process of planting seedlings outside of their traditional habitat in a bid to give them a head start on climate change. .
Four years ago, Peter Clark and his team planted more than 900 two- and three-year-old seedlings in New Hampshire, north of the chestnuts’ historic range.
“We were really surprised that the American chestnut showed this amazing ability to grow and persist, even in these extremely cold environments,” Clark said.
In his study, out of nine species of trees that were transplanted, chestnuts ranked second in terms of growth and survival. That’s not to say it was easy for the chestnut hybrids – they grew quickly in the spring, but cold snaps damaged their roots in the winter. Yet over time, more than 400 chestnuts continue to grow.
“They will potentially serve as a very important source for future chestnut establishment in this area,” Clark said.
It’s too early to know for sure if assisted migration works, but scientists expect to know more in about two decades.
The University of Vermont group is part of a large network of researchers studying tree migration called Adaptive Silviculture for Climate Change. Courtney Peterson, a researcher at Colorado State University, coordinates more than a dozen sites in this program that are testing the kinds of human interventions that can help trees be more resilient in the future.
She thinks that studies of assisted tree migration like Clark’s will be essential for learning how to manage forests in the face of climate change.
“Knowledge and expertise on the ground will be crucial as we make these management decisions going forward,” Peterson said.
Scientists work with forest managers to design these studies to mimic a natural setting as closely as possible so managers can apply lessons learned, said Tony D’Amato, a professor of silviculture and forest ecology at the University. of Vermont and one of the authors of the study on chestnut-assisted migration.
He also pointed to the importance of human-assisted migration in the past, when indigenous peoples helped spread the chestnut seeds that formed the original range of American chestnut trees. “The most significant movement historically would have been on Indigenous peoples, given the importance of their culture to the staple diet.”
Facing an uphill battle
While researchers hope chestnut hybrids take root in northern climates, the reality is that these trees are considered functionally extinct. Chestnut trees can reproduce in the wild when the trees are close together. But wild American chestnut trees very rarely reach maturity; the plague prevents them from growing to their full potential.
On a recent fall day in Weston, John Emery, the local chapter director of the American Chestnut Foundation, points to a slender, lanky chestnut tree about 2 feet tall with a scraped section of bark .
“When young male deer in the fall rub their antlers on the bark, it creates an opening for the blight fungus to enter,” he says.
Besides blight, deer are one of the biggest threats to the tree. In addition to scraping off the bark, deer eat the green tops of seedlings. To tackle this problem, he uses deer sprays and mesh netting.
Emery and other volunteers planted about 6,000 chestnut trees in the orchard with the goal of creating blight-resistant chestnut hybrids. Yet typically less than 5% of hybrids inherit sufficient resistance to be good candidates for further selection. To identify blight-resistant trees, they deliberately apply the fungus. Then they raise the hybrids from the surviving trees by hand pollination.
Emery supports all efforts to restore chestnuts to the environment, including human-assisted climate migration.
“Squirrels and blue jays only move chestnuts a mile a year. And that’s not really very fast compared to climate change.
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