The first of multiple unsuccessful efforts to restore the American chestnut tree began in the 1920s, barely two decades after the tree was decimated by an invasive fungus. In the late 1980s, after more than 20 years of inactivity, another wave of restoration efforts began, and a recently published paper offers positive signs on the progress.

“It’s a cautious optimism,” said co-author Sara Fitzsimmons, the director of restoration at The American Chestnut Foundation and a researcher at Penn State.

The paper, published in New Forests, analyzed current restoration efforts and found researchers are on the right track to successfully breeding a blight-resistant American chestnut tree. And while it concludes that day is still decades away, the findings mark a significant milestone in the quest to revive the functionally extinct tree.

“This is a conservation rescue mission that is much more complicated than anything else I’m aware of,” said lead author Kim Steiner, who is also a professor of forest biology at Penn State. 

The degree of difficulty in restoration efforts is in part due to the pervasive nature of the pathogen. It lives on other hosts, including some oaks, and the vulnerability of American chestnuts to the blight is “nearly universal,” the paper states. To overcome those challenges, rescue efforts can only be successful by developing a blight-resistant American chestnut.

Two main processes are being researched. Both involve moving a small number of blight-resistant genes into the genome of the American chestnut while still preserving its other qualities and characteristics.

In one process, the partially dominant resistance gene comes from the Chinese chestnut. Researchers cross the two species to create a progeny — the term used for offspring — that is half American, half Chinese.

Researchers then “backcross” the progenies carrying the resistance gene with an American chestnut, which helps preserve all characteristics of the American chestnut other than blight susceptibility.

The second process involves genetic transformation, an operation similar to what led to the creation of Bt corn. Researchers in the early 1990s identified a blight-resistance gene in wheat, which they have been able to transfer to embryonic material of the American chestnut through lab procedures. From there, scientists needed to create roots and shoots.

“That was the biggest challenge in taking it from the lab into the field — making it a tree,” Fitzsimmons said. “Getting the chemicals and everything right took probably 10 years to really get it to a point where you say, ‘I’ve got a tree that once I plant it out will survive.’”

Both restoration approaches have strengths, Steiner said, but also “question marks.”

“I think going forward both are going to be useful,” he said.

One of the question marks surrounding the backcrossing approach is whether the hybrid trees will achieve an average resistance suitable to withstand the blight. Steiner estimated the most resistant seedlings from the test populations will be created around 2022. Even then, the average resistance of the progeny “would not be what we think we need,” he said.

But what those seedlings would presumably have, Steiner said, is genetic variation, which means some might be resistant enough to survive. And if so, “we could contemplate doing restoration plantings,” Steiner said.

The key question mark surrounding the transformation process is genetic variability. After nearly 30 years of research, the transgenic trees have suitable resistance, Steiner said. What they do not have is genetic variability, and long-term survival would not be feasible without it.

“You’re not restoring anything if you just put a clone back into nature,” he said. “That’s not a restoration. That’s a band aid.”

Fitzsimmons said researchers this summer will cross transgenic trees with backcrossed trees. Because there are two different resistance mechanisms — the gene from the Chinese chestnut and the wheat gene — at play, researchers want to see what the effect is.

“Our hope of course is that you would have more resistance, but we have no idea,” Fitzsimmons said. “Plants and biology do really strange things.”

Steiner said there will be an impact when large numbers of blight-resistant American chestnuts are reintroduced into nature, but he emphasized that will be “a lot smaller than the effect of the disappearance.”

“You have to cover such a huge amount of area for it to be felt on an ecological scale,” Fitzsimmons said. “You won’t see the ecological benefits for decades, maybe even for centuries.”

Immediate impact could be felt economically. Fitzsimmons said the trees’ nuts could once again be sold as a crop. And while it would still be decades down the road, she also said “part of the hope” is that American chestnut could fill a traditional forestry role.

With scientific research, outsiders can allow their expectations and desires to outgrow reality. Steiner understands the paper’s findings might disappoint those who believed full-scale restoration was imminent, but where they see decades more of waiting, Steiner sees decades of possibility.

“I think there’s some satisfaction in being involved in a project that’s so difficult that it might not be completed until after you’re gone but to know that you’re helping them make it happen,” Steiner said. 

“If it weren’t so difficult, it wouldn’t be worth doing.”

Kevin Stankiewicz: kstankiewicz@post-gazette.com and Twitter @kevin_stank.

First Published: June 11, 2017, 4:00 a.m.