[restoration chestnuts] over the top.”
These restoration chestnuts at Meadowview Research Farm show resistance to the blight. (Credit: American Chestnut Foundation)
At the University of Maryland’s Biotechnology Center in Shadyside, virologist Donald Nuss has been dissecting the American strains of hypovirulence, trying to understand why they don’t spread as easily in the wild here as they do in Europe.
His funding comes from the National Institutes of Health, which is interested in how viruses work; the chestnut hypovirulence is one of the easiest ways to study this, Nuss says.
Nuss has cloned the hypovirulence and inserted it into a transgenic chestnut blight whose effects on trees are far less severe. So far, neither the hypovirulence or his transgenic blight seem able to spread efficiently on their own in the wild, which would be essential for becoming effective across the landscape.
Scientists think the problems lie partly in the large number of strains in which both blight and hypovirulence occur. There is a lot of incompatibility, which retards spreading; also, European chestnuts probably have a little more natural resistance than American chestnuts, which allows the hypoviruses to work more easily there.
The hypovirus here may make the blight too weak, so that it can’t spread in a less destructive form; in effect, vaccinating the chestnuts it encounters against the full-strength blight. Meanwhile, the original blight is able to remain dormant in dozens of non-chestnut tree species, from which it respreads by wind and by birds.
Another hope lies with engineering a transgenic chestnut. A chestnut with a disease-resistant wheat gene has already been produced experimentally by researchers William Powell and Charles Maynard at the State University of New York’s Environmental Science and Forestry school in Syracuse.
Powell says a $5.6-million project that includes sequencing all the genes in the chestnut is two years from completion. He expects that this will allow researchers to produce a chestnut that is pure American except for the addition of a few genes from the Chinese chestnut that confer disease-resistance.
One of the funders of that project is Duke Energy, which is interested in the chestnut’s potential to reclaim coal-mining land, but also in its promise for sequestering carbon dioxide. A Purdue University study shows that the growth rate, size and longevity of chestnuts let them store more carbon, and at a faster rate, than any other hardwood.
There’s also an ancient chestnut tree that Fred Hebard directs you to on your route home from Meadowview. The “Amherst tree” is so large, so gnarled with age, and so rare that, like a few dozen other long-surviving chestnuts, it has been named.
To develop resistance to the blight, young trees are inoculated with samples of the chestnut blight fungus. (Credit: American Chestnut Foundation)
It sits alone in the middle of a pasture near Amherst, Virginia, full of healed-over cankers, its crown wracked by storms, but enduring. Gary Griffin, Hebard’s PhD mentor at Virginia Tech, says these most ancient survivor trees almost all share a few characteristics. “They have some natural resistance, they are infected by the hypovirulence, and they have very good growing environments.”
Griffin, an emeritus professor of plant pathology, has been working since 1973 grafting tissue from old survivors (and younger ones that have made it to about 15 inches in diameter) onto American chestnut rootstock, crossing these to one another. There is plenty of evidence that genetic resistance to disease can be recovered by crossing even trees with relatively low resistance; but it is taking awhile — “We’re about halfway there,” he ventures.
Griffin has one tree, grafted in the early 1980s, that is now 24 inches in diameter and close to 70 feet tall. “I have no problem with what Fred is doing trying to produce a hybrid,” he says, “but a lot of people also just want to bring back the pure American tree.”
Just as the chestnut blight appears here to stay, so does the movement to restore the chestnut to its place in the forest. In Carroll County, Maryland, in partnership with the American Chestnut Foundation and American Forests, more than 18,000 school children each year participate in a science curriculum built around experimental chestnut orchards.
Nor has the chestnut itself ever really gone away, notes Essie Burnworth, head of the ACF’s Maryland chapter: “There are millions of them around, sprouting from old stumps, sitting as seedlings in the forest understory, just waiting for light to grow.”
Burnworth explains that American chestnuts have an extraordinary ability to “release,” or spurt toward the light when surrounding canopy trees die. Fred Hebard says he’s seen understory chestnuts only an inch in diameter that show 60 years of growth rings, followed by growth that approaches an inch a year after they get access to light.
Chestnut hybrids, grown at the Hashawa Environmental Center in Carroll County, MD. (Credit: Melissa Boyle)
A project to spot chestnuts sprouting within sight of the Appalachian Trail has so far turned up more than 40,000, Burnworth says. Many clear-cuts literally explode with long-suppressed chestnuts racing for the light.
All evidence is that if the blight can be overcome, the chestnut can outcompete most any other hardwood to become part of the forest canopy. “Maybe only yellow poplar, on excellent yellow poplar sites, might outgrow it,” says Kim Steiner.
Fred Paillet, a University of Arkansas geoscientist who often writes on chestnuts, has taken the long view. He cites pollen profiles from North American lakes that show virtually all hemlocks simply vanished from the forests some 5,000 years ago — probably of a disease still unknown — and then reappeared throughout their range a few centuries later.
“The American chestnut, considering it’s been around millions of years, can in the long term probably take care of itself as long as wild woodlands and rodents and jays exist to forage and spread the nuts.” Paillet wonders whether it’s possible for the chestnut to someday be seen as virtually “invasive;” a problem, he writes, “I would gladly live with.”
— Tom Horton writes from Maryland’s Eastern Shore
This article was published in the Winter 2010 issue of American Forests magazine.