Researchers hope to give the American chestnut an edge on climate change

As the earth warms and rainfall changes, trees are expected to migrate north in search of the climate they have adapted to. Scientists predict that trees will move faster than their natural abilities through the spread of seeds.

This led scientists from the University of Vermont to try to kickstart this process for an already besieged tree: the American chestnut.

“We are simultaneously looking to restore the chestnut in our experiment, as well as test how well it will perform in a future environment if moved a little further north,” said Peter Clark, the lead researcher of the study.

After a bacterial 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 the American chestnut with the Chinese chestnut.

John Emery sprinkles deer repellent on chestnut trees. (Jesse Costa / WBUR)

They are those hybrids that researchers at the University of Vermont are using in an “assisted migration” experiment – the process of planting seedlings outside their traditional habitat in an effort to give them an edge on climate change.

Four years ago, Peter Clark and his team planted over 900 two- and three-year-old seedlings in New Hampshire, north of the historic chestnut area.

“We were really surprised that the American chestnut showed this incredible ability to grow and persist, even in these extremely cold environments,” said Clark.

In his study, out of nine transplanted tree species, chestnut ranked second for growth and survival. That doesn’t mean it was easy for the chestnut hybrids – they grew fast in the spring, but the cold snaps damaged their roots in the winter. Yet, over time, over 400 chestnuts continue to grow.

“They will potentially serve as a really important source for the future chestnut plant in that region,” Clark said.

It’s too early to know for sure whether assisted migration works, but scientists expect to know more in about two decades.

The orange rash of downy mildew on an American chestnut tree.  (Jesse Costa / WBUR)
The orange rash of downy mildew on an American chestnut tree. (Jesse Costa / WBUR)

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 over a dozen sites in this program that are testing what types of human interventions can help trees be more resilient in the future.

He believes that assisted tree migration studies such as Clark’s will be instrumental in learning how to manage forests with climate change.

“Knowledge and experience in the field will be crucial as we make these management decisions moving forward,” said Peterson.

Scientists collaborate with forest managers to design these studies to be as close as possible to replicating a natural environment so that managers can apply lessons learned, said Tony D’Amato, professor of silviculture and forest ecology at the University of Vermont and one of the authors of the study on assisted chestnut migration.

He also stressed the importance of human-assisted migration in the past, when indigenous peoples helped spread the chestnut seeds that formed the original American chestnut. “The biggest movement historically would have been on indigenous people, given how important it was to their culture for staple food.”

Face an uphill battle

Chestnut hybrid that resists bacterial infections through masonry.  (Jesse Costa / WBUR)
A chestnut hybrid that resists bacterial infections through masonry. (Jesse Costa / WBUR)

While researchers hope that chestnut hybrids take root in northern climates, the reality is that these trees are considered to be functionally extinct. Chestnuts can reproduce in nature when trees are close to each other. But wild American chestnuts very rarely reach maturity; the disease prevents them from growing to their full potential.

On a recent fall day in Weston, John Emery, the local chapter director for the American Chestnut Foundation, points to a skinny, skinny chestnut about 2 feet tall with a scraped section of bark.

“When a young male deer rubs its horns on the bark in the fall, it creates an opening for the bacterial fungus to enter,” he says.

In addition to downy mildew, deer are one of the biggest threats to the tree. In addition to scraping the bark, the deer eat the green tops of the seedlings. To address this, use deer sprays and mesh nets.

Emery and other volunteers planted around 6,000 chestnut groves in the orchard in an effort to breed mildew-resistant chestnut hybrids. However, usually less than 5% of hybrids inherit enough stamina to be good candidates for further breeding. To identify trees resistant to late blight, they deliberately apply the fungus. Then they raise the hybrids from the surviving trees by hand pollinating.

A healthy young chestnut hybrid in Lincoln.  (Jesse Costa / WBUR)
A healthy young chestnut hybrid in Lincoln. (Jesse Costa / WBUR)

Emery supports all efforts to restore chestnuts to the environment, including human-assisted climate migration.

“Squirrels and blue jays only move chestnuts for a mile a year. And it’s not very fast compared to climate change ”.

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