Tide slowly turning in war against against decimating spores
Grand stands of native chestnut trees, now absent from our forest landscapes, are the stuff of legends.
Hailed in its heyday as the “Redwood of the East,” the once mighty and magnificent North American chestnut was a towering, commonplace presence across woodland countrysides, from the middle of Mississippi throughout Appalachia and New England up into Canada.
American chestnuts thrived in nearly all of Tennessee, save some of the western lowlands.
Throughout its 200 million acre range in the eastern United States, chestnuts provided a source of forage for the entire food chain.
Prior to the tree’s calamitous demise in the early and mid-20th Century, chestnuts were “the single most important food source for a wide variety of wildlife from bears to birds,” according to the Tennessee chapter of the American Chestnut Foundation’s website. It was an “essential component of the entire eastern US ecosystem.”
The wood and nuts served as a staple of survival and prosperity for humans as well. Prized not just as a food source for people and livestock, Chestnut trees were a nearly boundless source of sturdy lumber.
Chestnut is estimated to have at one point been the highest timber-volume tree in Tennessee.
Insidious Interloper Introduced
But nature’s native chestnut bounty is no more in this country.
“Oh mighty, magic chestnut tree, how did you slip away from me?” asked Dolly Parton in a musical ode to the arboreal archetype.
The answer was a foreign invader.
A fungus blight imported from across the Pacific Ocean on Oriental varieties of chestnut trees in the late 1800s and early 1900s took hold in New York. And over the course of just a few decades, the relentless pestilence — which was relatively innocuous to the Asian chestnut varieties — devastated the non-resistant American chestnut.
“There is no example of a forest disease that so quickly and completely eliminated its host,” Dr. Hill Craddock, a biology professor and chestnut-breeding expert at the University of Tennessee-Chattanooga, told Center Hill Sun. “It spread very quickly in concentric circles, killing literally billions of trees.”
Craddock said the distinction between “epidemic” and “pandemic” is tragically vital in the tale of the American chestnut tree’s rapid descent toward oblivion.
“This was a true pandemic. There were no unaffected individuals,” he said. “The chestnut blight pandemic may be the worst ecological disaster in the history of North America since the Ice Ages.”
The blight pathogen, which kills back infected trees by scoring and splitting the bark with necrotic cankers, reached Tennessee in the 1930s. By the 1950s, mature American chestnut had been all but obliterated from forests throughout the United States.
However, if there was anything resembling a bright side to the blight plague, it was this: The fungus cankers terminate the tree’s trunk and canopy growth, but the root systems lived on.
And the chestnut is famous for its ability to send up fast-growing new shoots.
“If you walk in the woods today, you can still find chestnut trees sprouting from the base,” said Craddock. “So, therein lies the hope that we can bring them back.”
Craddock, 56, is a celebrity and savant among chestnut restoration enthusiasts. He’s committed his professional life to battling the blight through means of plant breeding and public education about how to engineer a rebound. In a 2004 Smithsonian magazine profile headlined “Chestnutty,” he was described as a “chestnut evangelist.”
This spring, Craddock led a group of regional foresters on a tour of a university chestnut-breeding plot he is managing near Tennessee Tech’s Hyder-Burks Agricultural Pavilion.
“The strategy we’re using is to create a hybrid between the blight-resistant Asian species and the American chestnut,” said Craddock.
Backcrossing to the Future
The breeding process he is using to develop blight-resistant trees is known as “backcrossing.” Parent trees of both Chinese and American are crossed, then back-bred to successive generations to eliminate most of the Chinese tree’s physical characteristics — except for blight resistance.
“If we start with a tree that’s fifty-fifty Chinese-American and we backcross that to an American, we get a tree that is three-quarters American and one-quarter Chinese. When we backcross that generation, then we get a tree that’s seven-eighths American. Backcross that, and then we get a tree that is fifteen-sixteenths American.”
Craddock selects for trees that have the look or “form” of the American chestnut, and also display blight resistance.
“When the trees get up to four or five years old and two or three inches in diameter, we deliberately inoculate the tree with the chestnut blight fungus,” he said.
Trees that show acceptable resistance are used in future crosses, thus bringing forth evermore resistant varieties that are, genetically speaking, very close to native chestnuts.
Seeds from the most resistant strains are then planted in a seed orchard. “In those trees we expect to recover full resistance,” he said.
So, in a nutshell, things are looking up for the iconic giant that once shaded the eastern United States and showered sustenance onto forest floors.
“Ultimate success,” however, is measured not in decades, or even human lifespans, but more like centuries, said Craddock.
“We are talking about ecosystem restoration” he said. “We need a tree that can survive and reproduce on its own under natural conditions. We are hoping to be able to release these trees into the woods in a way that allows natural selection to take over.”
For that to occur, trees must display a level of blight resistance enabling them to survive and reproduce in the wild. “In my lifetime, I think we will have initiated those plantings,” Craddock said.
“A hundred or two hundred years from now, I think we will be able to measure success in another way: if we have naturally reproducing populations of chestnut trees in the forest,” he said.
Interested in learning more about chestnut trees, or obtaining young trees that have been bred for blight-resistance? Contact Dr. Craddock through the American Chestnut Foundation at firstname.lastname@example.org, or visit the Tennessee chapter of the American Chestnut Foundation website at tnacf.org.
(Photo at top of page: Krystal Kate Place of Chattanooga is pollinating chestnut flowers. The bags over the blossoms are to prevent pollen pollution.)