The fight for a fallen giant: Bringing back the American chestnut

by | May 3, 2024

More than a century ago, the American chestnut (Castanea dentata) was a common overstory tree across portions of eastern North American forests. These giants thrived on moist, well drained slopes and ridges across the Appalachians, towering more than 100 feet tall with an average diameter at breast height (DBH) of five to eight feet. Their historical range extended into the southeastern deciduous forests of Canada and as far south as Florida.

Photo taken in Western North Carolina published by the American Lumberman in January 1910 to illustrate the grand size of the American chestnut trees. Photo by Forest History Society.

Historical range of the American chestnut (Castanea dentata). Photo by the USDA.

According to legend, American chestnut trees were once so abundant in eastern North America that a squirrel could travel the chestnut canopy from Georgia to Maine without ever touching the ground. However, fossil pollen records and early forest inventory records suggest the American chestnut may not have been as dominant a tree species across its entire range as depicted. Early forest inventories conducted by Emma Lucy Braun, a prominent forest ecologist and botanist, suggest the species was of surprisingly limited dominance in many parts across the Appalachians, except for the central and southern ranges.

With their ability to rapidly sprout from stumps and reach maturity in as little as eight years, the American chestnut likely benefited from intensive logging of the past. This rapid regeneration, coupled with possible allelopathic properties that suppress competing trees, would have allowed them to quickly reclaim their place in the forest canopy following disturbance. This advantage may have been particularly significant in the northern part of their range, likely contributing to the historical accounts describing the American chestnut’s remarkable abundance across the landscapeNonetheless, American chestnut reigned as a keystone species with immense ecological value. With its strong, rot resistant wood and abundant annual crop of nutrient dense chestnuts, the American chestnut was once an invaluable hardwood for humans and wildlife before the chestnut blight decimated its populations in the early 1900s, leaving a lasting scar on eastern North American forests.

 

 

The invader: American chestnut blight

Chestnut blight is a canker disease caused by a fungal pathogen (Cryphonectria parasitica) introduced as early as 1876 with imported Chinese chestnut (Castanea mollissima) and Japanese chestnut (Castanea crenata) nursery stock. By 1950, the blight had spread throughout the Appalachian Mountain chain from Maine to northern Georgia, killing roughly 80% of all mature American chestnut trees. Windborne spores spread the fungal pathogen, infecting mature trees through bark furrows and wounds. Cankers develop and kill the cambium layer, girdling and killing the tree within a few years. Though the roots survive and sprout, the cycle repeats as the new growth matures, preventing trees from reaching maturity and reducing a once prominent canopy tree to an understory shrub. Although this cycle of death and rebirth has prevented the species from numerical extinction, the American chestnut has been considered functionally extinct for decades.

 

New hurdles and hope for American chestnut restoration

American chestnut resprouting from the root collar. Photo by Richard Gardner, bugwood.org.

Bringing back these fallen giants across their historical range is an arduous journey. Eradicating the pathogen is impossible as it is harbored on many host species. The American chestnut is also vulnerable to Phytophthora root rot (Phytophthora cinnamomi), presenting yet another hurdle for their recovery. Recent research from the U.S. Forest Service Southern Research Station adds another layer of complexity. While crossbreeding programs have yielded viable hybrids between American and Chinese chestnuts, their genetic compatibility may not align as well as initially thought. This means even the most advanced hybrids may never have enough resistance for successful restoration efforts despite inheriting blight resistance from the Chinese chestnut. Regardless of the challenges, researchers, government agencies and organizations such as The American Chestnut Foundation (TACF) remain fiercely optimistic about preventing the complete loss of the species. The fight to restore these fallen giants continues to adapt and forges on.

While TACF recently halted the development of the Darling 58, a genetically modified American chestnut with a wheat derived enzyme (OxO), due to concerns regarding variability in blight resistance and overall growth, their efforts continue. TACF is actively breeding beyond the B3F3 generation, which is 94% American chestnut with minimal physical characteristics of Chinese chestnut beyond blight resistance. At their Meadowview Research Farms in Virginia, TACF cultivates tens of thousands of trees at various breeding stages. The most promising, those exhibiting the strongest blight resistance, are planted on restoration trials across the Eastern U.S. to test their blight resistance and competitive ability.

The N.C. The Forest Service Nursery and Tree Improvement (N&TI) Program continues to research ways to improve, and in some cases, restore native tree species. As part of a collaborative effort with TACF, American chestnut plantings were established at Linville River Nursery and Morganton Forestry Center. Successful seedlings are then used for ongoing research projects such as phytophthora resistance.

Occasionally, mature American chestnut trees are discovered in remote areas, providing yet another glimmer of hope. DNA analysis can confirm their pure American lineage. If so, these survivors likely possess some natural resistance to the blight and may eventually help reestablish American chestnut on the landscape. TACF has an ongoing, large-scale search for surviving American chestnuts in the forest. If you believe you have found an American chestnut tree on your property, the N.C. Forest Service encourages you to report your observations to TACF by visiting the following link: https://tacf.org/identification/.

 

Sources:

Anagnostakis, S.L. 2000. Revitalization of the Majestic Chestnut: Chestnut Blight Disease. APSnet Features. Online. doi: 10.1094/APSnetFeature-2000-1200

Carlson, E., Stewart, K., Baier, K., McGuigan, L., and Powell W. 2021. Pathogen-induced expression of a blight tolerance transgene in American chestnut. Mol Plant Pathol. 2022 Mar;23(3):370-382. doi: 10.1111/mpp.13165. Epub 2021 Nov 28. PMID: 34841616; PMCID: PMC8828690.

Faison, Edward. 2014. Did American chestnut really dominate the eastern forest?. Arnoldia.

Gustafson, E. J., Miranda, B. R., Dreaden, T. J., Pinchot, C. C., and Jacobs, D. F. 2022. Beyond blight: Phytophthora root rot under climate change limits populations of reintroduced American chestnut. Ecosphere (Washington, D.C), 13(2).

Henderson, A. F., Santoro, J. A., & Kremer, P. 2023. Impacts of spatial scale and resolution on species distribution models of American chestnut (Castanea dentata) in Pennsylvania, USA. Forest Ecology and Management, 529, 120741.

Islam-Faridi, N., Hodnett, G.L., Zhebentyayeva, T. et al. 2024. Cyto-molecular characterization of rDNA and chromatin composition in the NOR-associated satellite in Chestnut (Castanea spp.). Sci Rep 14, 980. https://doi.org/10.1038/s41598-023-45879-6

Moonier, H., Oten, K. 2023. Chestnut Blight. Invasive Forest Pests. NC State Extension Publications.