This series of posts will highlight the plants that help you celebrate the Yuletide season.
“Chestnuts roasting on an open fire
Jack Frost nipping at your nose
Yuletide carols being sung by a choir
And folks dressed up like Eskimos”
The lyrics to The Christmas Song were written during the summer of 1944 and recorded by Nat King Cole in 1946. They were meant to be the acme of the spirit of the winter season. How much do you know about the tree behind the tradition? The American Chestnut’s story of loss and the brink of hope seems almost too fitting for the Christmas season.
By the time those lyrics were written, the American Chestnut (Castanea dentata) was nearing extinction. The forests of the Eastern United States were once filled with this fast-growing species with numerous specimens reaching nearly 100 feet tall and 10 feet in diameter. It provided sustenance for deer, turkeys, bears and passenger pigeons. Its wood was used for a variety of purposes since it was both lightweight and strong. Logging was not the cause of the demise of the American Chestnut.
In 1904, American Chestnuts in the Bronx Zoological Park were reportedly dying, infected with a Chetsnut blight fungus (Cryphonectria parasitica). It was determined that this pathogen had been imported to the United States along with chestnut trees from Asia. Because those varieties had long coexisted with the fungus, they had immunity to the disease, but our American Chestnuts were woefully susceptible. By the 1940s, the fungus had spread throughout the entire range of the American Chestnut. Once infected, the trees died within a decade as the fungus cuts off the lifeline connecting the photosynthetic leaves to the rest of the tree. Because the fungus didn’t completely kill all the roots, the trees vainly sprouted new shoots from the stumps, but eventually these too would fall victim to Chestnut blight. By 1950, nearly 4 billion trees were dead. The loss of the American Chestnut not only changed a Christmas tradition, but also drastically altered the composition of our Eastern forests as slower-growing species like oaks began to fill the empty ecological niche.
Very few specimens of American Chestnuts survive today, and their discovery is always newsworthy. Adult trees have been identified in Ohio, Tennessee, Alabama, Kentucky, Pennsylvania and Georgia. These specimens have escaped infection for a variety of reasons including micro-niche conditions and possibly superior genetics. The largest stand of American Chestnuts can be found in West Salem, Wisconsin. These were planted by an early settler in the area, and because they are outside the natural range of the American Chestnut, they largely escaped the blight. However, in 1987 scientists discovered that even these trees had become infected, and they have been the focus of intense research efforts to save them.
A number of different strategies are being used to ensure that the American Chestnut does not permanently disappear. The American Chestnut Foundation is coordinating restoration efforts. One strategy involves back-crossing American Chestnuts with the blight-resistant Chinese Chestnut. By crossing these two varieties, disease-resistance is conferred. These offspring are then back-crossed to the American Chestnut six more times, selecting for blight-resistance along the way. By 2005 the program had yielded the first blight-resistant chestnuts that were 15/16ths American Chestnut. Because these offspring are 94% American Chestnut, they retain many of the properties of this species. These are being re-introduced in test plots by the U.S. Forest Service. As of this year, nearly 80% of the back-crossed saplings are surviving. Other breeding studies are being conducted with the remaining wild specimens of American Chestnut trees.
More modern technologies are even being applied for the salvation of the American Chestnut. In an NSF-funded project, the genomes of various tree species including the American and Chinese Chestnuts have been sequenced in order to identify the specific genes involved in blight-resistance. This knowledge will provide better genetic markers for screening in traditional breeding programs as well as target genes for engineering. To facilitate these efforts, researchers have developed new techniques for propagating and transforming American Chestnut plant material. As scientists learn more about plant disease resistance in other plant species (like crop plants), those genes also become candidates for engineering into the American Chestnut. For example, transgenic American Chestnuts containing a gene from wheat that helps confer resistance have been planted for field tests. Hopefully the genome of the Chinese Chestnut will reveal additional resistance genes that can be introduced transgenically.
Scientists are also using other methods to tame the Chestnut blight fungus. One way to combat the microbe that felled the giant trees is by using another microbe- a naturally-occurring virus that attacks the Chestnut blight fungus. This is a method that may save mature trees already infected with the fungus like those in West Salem, Wisconsin. It works to heal infected trees in a way similar to modern human vaccinations. Scientists isolate the virus from other infected American Chestnuts and inoculate blight-infected trees. The virus weakens the fungus enough so that the trees can recover. The initial results have proven promising, but this is not the magic bullet solution that they were hoping for.
With all of these efforts, roasting chestnuts over an open fire may once again be a common American Christmas tradition, but science still has a long way to go to bring this species back from functional extinction. Even then, it remains to be seen whether crossbred or transgenic (aka GMO) American Chestnuts will be accepted by the general public. However, there does seem to be some poetic justice in a restoration solution that involves a small piece of foreign DNA to protect these giant trees from a foreign deadly microbe.
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