Resurrection of the American Chestnut
Resurrecting something from the dead is a classic scary story plotline, but there is nothing spooky about efforts to bring back our American chestnuts (Castanea dentata). The grand, incredibly valuable tree has been functionally extinct for the past century, but efforts are underway to restore it as a component of our eastern forests.
(In case you missed it, be sure to read our previous article, American Chestnut: Ghosts of Our Eastern Forests, to learn about this wonderful species and its tragic demise.)
Some American chestnut trees were able to survive infections of the chestnut blight that ravaged the species in the early 20th century, and several individuals are still persisting today. It is thought that natural resistance to the fungus was uncommon but present in our pre-blight chestnut populations, but the wood was so valuable that most trees were cut down when the blight began sweeping across the Appalachians. This likely set the long path to recovery back, but enough American chestnuts survived blight infections to give hope for the species. The few survivors were either isolated from the historical chestnut range or were graced by a degree of natural resistance and hypovirulence, meaning a virus infected and weakened the fungus that was attacking the trees. Natural Castanea dentata resistance, hypovirulence biocontrols, and blight-resistant genes of Castanea mollissima and C. crenata (Chinese and Japanese chestnuts) are the key components of efforts to resurrect American chestnut in our forests.
Humans have been breeding traits into plants and animals for millenia. When two individuals have a desirable trait (for example, large fruit, fast growth, or resistance to disease), their reproduction is controlled so that their offspring are only a result of the two selected individuals. The offspring are tested for these desired traits, and the ones that have them are retained as breeding stock and subsequently bred with each other, their parents, or other individuals with the selected traits. This process of “artificial selection” is a time-honored practice that has given humans most domesticated plants and animals, from barley to beagles.
This technique has been followed to increase chestnut blight resistance for decades. Many individuals and organizations have attempted to breed blight resistance into American chestnuts, but the most successful and notable has been the American Chestnut Foundation. In 1989, ACF began a breeding operation of their own in the Shenandoah Valley of Virginia. They first gathered pollen from surviving American chestnuts that had some blight resistance, and used it to hand-pollinate Chinese chestnuts. When not being hand-pollinated, the female flowers of the Chinese chestnuts were covered in plastic bags to ensure that the offspring were only the result of the American-Chinese cross. Then the scientists waited years, until the seeds that were produced from these initial crosses germinated and grew to reproductive age. This second generation was inoculated with chestnut blight, and the individuals that survived best were selected to receive pollen from another surviving American chestnut tree. Over the years, the process has continued, until now, where the line is 94% American chestnut and 6% Chinese chestnut. These trees, dubbed the “Restoration Chestnut”, are being planted experimentally in state and federal forests, on private lands, and on reclaimed surface mines across the chestnut’s former range.
The backcrossing technique has proven to be useful, but modern genetic modification allows a more rapid and effective defense of the species. In 1990 the American Chestnut Foundation began a partnership with the State University of New York’s College of Environmental Science and Forestry to create transgenic American chestnuts, where genes that provided blight resistance to other species, including Chinese chestnut, wheat, and other crops, are inserted into the American chestnut genome in a laboratory. The resulting chestnuts are so far proving to be highly resistant to the blight and may soon be approved for use in the restoration effort, to either cross with the “Restoration Chestnut” and its descendants, or to be planted as is. This is a monumental moment in ecology and ultimately human history. If approved by the U.S. government, this will be the first wild plant that is genetically modified and introduced into natural ecosystems. If it works, this treatment may hold hope for the other species that are currently succumbing to invasive pests, including elms, hemlocks, ashes, and likely many more as globalization rages on.
While transgenic American chestnuts are being researched more fully, work on hypovirulence is a promising tool to add to the fight against chestnut blight. Even fungi get viruses, and some of the pathogens infecting chestnut blight are potent enough to allow the tree to survive its fungal infection. The chestnut blight that devastated the American chestnut made its way to Europe in 1938 and caused severe damage to the population of European chestnut (Castanea sativa) there, but a virus attacking the fungus prevented full-on devastation akin to the fate of the American chestnut. Hypovirulence has been found in many of the surviving American chestnut trees and will likely play an important part in the effort to restore C. dentata, but doesn’t seem to be as effective as viral strains in Europe. This is likely due to higher genetic diversity of the blight in North America, but this topic is currently the subject of intensive research.
American chestnut is well on its way to being resurrected. Promising new work, in conjunction with impressively patient traditional breeding techniques, will likely result in American chestnuts that are nearly identical to their forebears, with the notable exception of a tolerance to the chestnut blight. It is possible that future residents of North America will find themselves once again under a vast canopy of towering American chestnuts, providing food for humans and our forest-dwelling fauna. But, as noted in American Chestnut: Ghosts of Our Eastern Forests, our landscape, and planet, are not what they were in the year 1903. Today, invasive plants smother our highly fragmented forests, overabundant deer populations eat every seedling that is native, and there are little to no disturbance events like fire that promote diversity. Furthermore, it is projected that many now-common eastern species will see dramatic range shifts and possibly extinction due to climate change. Can the American chestnut survive yet another cataclysm? Can we?