THE AMERICAN CHESTNUT TREE. ITS DECLINE AND RESTORATION
By Gregory R. Weaver
Published in The Tennessee Conservationist
magazine July/August 2009.
We were walking through one
of Tennessee's rarest forests. Although the Great Smoky Mountains National Park
is famous for its massive old growth forests and record sized trees of many
species, the smallish trees we were admiring were not much to look at. I was
guiding a hike for the Tennessee Chapter of the American Chestnut Foundation
along the crest of Sugarlands Mountain, one of the few places in Tennessee where
American chestnut is still a common tree and where natural nut production and
new seedling establishment occurs. We were seeing the remnant of an ancient,
once common forest type, now known in only a few isolated groves in Tennessee.
Unlike the American chestnuts of Tennessee's past forests, the trees now growing
in the Smokies rarely exceed thirty feet in height before Cryphonectria
parasitica, better known as chestnut blight, kills them back to ground
It wasn't always like this. From shortly after the end of the last ice age until the early twentieth century, American chestnut (Castanea dentata) was King of the Forest. Trees fourteen feet in diameter are recorded in old books about the Smokies. To this day, a stump more than eleven feet in diameter is visible in the Greenbrier section of the park. The trees grew over one hundred twenty feet tall. American chestnut was the most common tree in Tennessee's Appalachian Mountains, constituting twenty five percent or more of the forest, and occurring in pure stands of several hundred acres on the most favorable sites. Nor was chestnut confined to the mountains. Although it became progressively less important toward Western Tennessee, chestnut still composed ten percent of the forest on the Highland Rim around Middle Tennessee and was known in scattered groves as far west as the Memphis area . Nationally, the tree's range extended from Mississippi to Maine and west to Illinois.
Unlike the oaks and walnuts, chestnuts produced large crops of nuts every year. These plentiful and tasty nuts fed wildlife and humans. The high quality, rot resistant wood served people from "cradle to grave", supplying furniture lumber, rail fences, coffins and wood for buildings, many of which still stand today. There was no need for copper based preservatives. Chestnut endured naturally.
Despite its success in dominating the Eastern forests, American chestnut proved tragically susceptible to the Asian fungus cryphonectria parasitica. This exotic pathogen was accidentally introduced into the United States in the late nineteenth century on nursery stock imported from Asia. The Asian chestnut trees had co-evolved with the fungus and had natural resistance to it. However for American chestnut, it was an entirely different matter. In 1904, Herman Merkel, a forester in the Bronx Zoo, noticed that the chestnut trees in the zoo were dying, and thus was documented the first observation of chestnut blight. Interestingly, the fungus was unknown at the time in Asia where it had originated, since the Asian chestnut trees were resistant to it and most infections produced little change in the appearance of the Asian chestnut trees.
Over the next half century the blight spread outward from New York City. Despite heroic efforts to stop the blight, including clear cutting huge swaths of forest in Pennsylvania, the pandemic marched relentlessly down the Appalachians at about fifty miles per year, reaching Tennessee in the 1920's. By the end of the 1930's, American chestnut was in serious decline across the state. What had been among the most common and important trees had been reduced to insignificance.
That might have been the end of the story. Another tragic extinction of a key species. But why are there American chestnut trees still living on Sugarlands Mountain eighty years later? As it turns out, chestnut blight cannot effectively penetrate the soil because of antagonistic microorganisms. The old chestnut root systems that were established before blight arrived still live in undisturbed forests like those found in the Smokies. They continue to send up sprouts which grow for a few years, become infected, die back to the ground, and the process repeats over and over. These "stump sprouts" are common in Tennessee. For plant scientists, that means that even though the big trees are gone, there is still a lot of natural diversity in the chestnut gene pool available for plant breeding.
Occasionally some of these trees reach respectable size before being killed by blight infection. Trees eighteen to twenty three inches in diameter at breast height (dbh) and up to eighty feet tall are known in Tennessee. Trees of this size may flower, but chestnuts are not self fertile, and require another nearby tree for pollination. Except in a few places like Sugarlands Mountain where American chestnuts are plentiful, the larger flowering trees are isolated and natural nut production does not occur. This makes it unlikely that American chestnut could ever evolve resistance to blight on its own. With almost no natural nut production, and almost no natural blight resistance, the tree needs human help to reproduce and to have any chance of overcoming chestnut blight.
The American Chestnut Foundation was established in 1983 with the goal of returning the American chestnut tree to the forests of the Eastern United States. Building on early research at the Connecticut Agricultural Experiment Station and the United States Forest Service, the American Chestnut Foundation employs a scientific breeding program to develop a bight resistant, phenotypically American chestnut tree that uses crosses between Chinese and American chestnut trees to capture the blight resistance trait of the Chinese tree. The offspring of this first cross are then backcrossed to American chestnut and the process is repeated for multiple generations to dilute the Chinese chestnut traits of short, bushy habitus and recover the American upright, big timber traits. At each generation, the trees are grown for eight to ten years and then they are deliberately infected with blight fungus. Fewer than ten percent of the trees in each genetic line will possess suitable levels of b light resistance. These few trees are then crossed with other trees, that were also deemed acceptably resistant, to produce the next generation. After six generations, a tree will emerge that is indistinguishable from American chestnut, except that it will have the blight resistance of the Chinese chestnut, and it will breed true for blight resistance in succesive generations.
Tree breeding programs have been established in each of the fifteen state chapters of the American Chestnut Foundation, with the mission of each state chapter to find living, flowering American chestnut trees within its state that are adapted to the local climate and ecosystem, and perform crosses to incorporate those trees into the breeding program. To ensure genetic diversity, at least twenty parent trees are desirable in each state. As the rare flowering native American chestnut trees are discovered, volunteers use ladders and bucket trucks to access the trees' flowers and apply pollen to perform crosses between the desired trees in June. In the fall, the volunteers return to collect the nuts. These seeds are assigned to members to plant and grow in orchards.
In Tennessee, we are well on our way to incorporating the recommended twenty lines of trees into our program. Every year observant citizens tell us about flowering American chestnut trees they have found in the woods and these trees are used as parents to produce seeds for the next generation. Our member volunteers are now growing third and fourth generation trees in orchards around the state. As each generation matures and becomes ready for testing, we move closer to the goal of reestablishing the American chestnut as King of the Forest, so that forests dominated by American chestnut will no longer be as rare as the forest atop Sugarlands Mountain.
For more information, visit the American Chestnut Foundation, Tennessee Chapter's website at http://www2.volstate.edu/tnchestnut/
Gregory R. Weaver, M.D. is President of the Tennessee Chapter of the American Chestnut Foundation and is Chief of Medical Imaging at Baptist Hospital in Nashville, Tennessee.
TN-TACF Chapter website