A well-documented story of near extinction is worth re-telling here. The American chestnut (Castanea dentata) used to be one of the most prolific and important trees in North America. It was prized for the beauty of its hardwood and the bounty of nuts it annually produced.
It’s estimated that at its peak, there were more than 3 million American chestnut trees in North America. The average mature chestnut was five feet thick and 100 feet tall. In the Appalachian mountains, where it was most common, it is believed that the American chestnut accounted for 25 percent of the total trees. Many of the poverty-stricken residents of the Appalachians depended on chestnuts for their income.
That all changed in 1904.
That’s when an airborne Asian fungus (Cryphonectria parasitica), was introduced to New York on chestnut trees imported from Asia. (Asian chestnut trees are resistant to the fungus, because the two evolved in tandem.) Known as chestnut blight, the disease spread 50 miles a year and within a couple of decades had killed billions of trees.
The American chestnut isn’t completely extinct because shoots, or suckers, commonly sprout from the stumps of dead trees. Here’s a photo of shoots growing from the a chestnut stump, taken on a hike at Wolf Hill Forest Preserve in Smithfield, RI, this weekend. But these sprouts usually grow no more than 12-15 feet before chestnut blight returns and kills them.
In the U.S., the American Chestnut Foundation is trying to develop a species of American chestnut that is resistant to chestnut blight using a technique called backcrossing, in which a single trait can be transferred from one species to another. The plan is to transfer the resistance to chestnut blight of the Chinese chestnut (Castanea mollissima) to the American chestnut by breeding the two trees.
In backcrossing, an American chestnut and Chinese chestnut are bred to create a tree that’s one half each. It is bred with another American chestnut, creating a tree that’s 3/4 American and 1/4 Chinese. Every time the new tree is bred with another American chestnut, the amount of Chinese chestnut is reduced by a half. Breeders can select which traits to leave in and out. The American Chestnut Foundation says that trees that are 15/16 American and 1/16 Chinese have the disease resistance to chestnut blight but are otherwise indistinguishable from a “true” American chestnut.
You may have read that in May, a stand of American chestnut trees that have so far escaped chestnut blight was discovered in Georgia. Scientists hope to study these trees and see if hardier American chestnut varieties can be bred from them.
More about backcrossing here. (I find this fascinating, though I do not pretend to understand how they do this, please don’t ask me!). If you’re interested in learning more, consider joining or donating to the American Chestnut Foundation. And if you’re north of the border, learn about the activities of the Canadian Chestnut Council.
The American Chestnut Foundation hopes to begin test-planting blight-resistant American chestnut trees in eastern forests sometime in the next year. Once they’ve proved to be resistant, they will be made available for wider distribution. Perhaps if this happens, future generations will be able to enjoy the American chestnut tree as much as our ancestors did.
Top photo courtesy of the American Chestnut Foundation.