Restoration
American Chestnut Conservation and Restoration
The American Chestnut Foundation (TACF) is leading an unprecedented mission to restore the American chestnut tree to its native range. The iconic American chestnut was devastated by chestnut blight in the early 1900’s. Employing complementary scientific strategies of traditional breeding, biotechnology, and biocontrol, TACF is working to restore a disease-resistant and genetically diverse population of American chestnuts in the eastern forests of the United States.
Breeding a Tree for Restoration
What qualities must a chestnut tree possess in order to be successfully restored and to fulfill its ecological role?
- Sufficient disease-resistance and competitive ability to achieve canopy status and produce abundant crops of chestnut mast under natural forest conditions
- The ability to pass on disease-resistance and competitive ability when naturally bred with wild-type chestnuts and backcross materials; and
- Sufficient genetic diversity to adapt to the wide range of environments in the chestnut’s native range and in changing climates.
The History of American Chestnut Restoration
Since the blight was first identified in the US, many methods of restoration have been implemented. With a challenge of this geographic and temporal scale, there is no silver bullet or singular method that will save and return the American chestnut to its native range.
In the early 1900s, the USDA sent legendary plant explorer Frank Meyer to China to find a suitable replacement for the American chestnut.
Throughout the 1920s – 1940s, when those imported Chinese chestnuts failed to entirely meet the ecological needs once met by the American chestnut, the USDA and other government entities began breeding all available chestnut materials together. Those products were another step toward restoration, but still yet failed to meet all the necessary elements for species restoration.
In the 1950s, at the height of the nuclear era, mutational breeding was all the rage. Scientists Ralph Singleton and Albert Dietz applied this technique to American chestnuts, exposing thousands of seeds to doses of radiation in hopes that disease-resistance would be one of the results of multiple mutations. The results of that work is still on display at Sugarloaf Mountain in Maryland.
The 1970s saw the discovery of hypovirulence, a phenomenon wherein the virulence of the chestnut blight is reduced through the introduction of a virus.
In the 1980s, Charles Burnham hypothesized that a traditional plant breeding method, specifically backcross breeding, could result in a fully blight-resistant American chestnut. Building on the work began by Meyer and his USDA colleagues, these materials are now an essential element in the ongoing efforts for species restoration.
In the 1990s, Drs. Bill Powell and Chuck Maynard created a method for genetically modifying American chestnuts, and thus developed a pipeline for transgenic tree testing and deployment.
In the early 2000s, Darling 58 (D58) became another milestone in a long-ranging restoration process which will take additional decades to complete. In order to utilize D58 or other genetically modified organisms as part of the restoration process, federal deregulation must be secured. Visit the Darling 58 page for more information.
Silviculture and Reforestation
Returning the American chestnut to the forest is TACF’s restoration goal. While we continue our work to develop trees with enough blight resistance to make that goal a reality, we prepare by experimenting with silvicultural practices and reforestation methods to better understand the best ways to approach species reintroduction.
These experimental plantings can use wild-type American chestnuts, various levels of hybrids, and, in very limited and highly regulated cases, transgenic American chestnuts. The transgenic American chestnut trees in Figure 1 were planted as part of a silvicultural trial looking at various establishment methods including tree shelters, herbicide application, and direct-seeding vs. seedling planting.
Stephen Hoy stands among 15-year old wild-type American chestnuts in Penn State University’s Stone Valley Forest.
Silviculture and Reintroduction Trials
Bruce Wakeland stands beside a wild-type American chestnut at the Whipperman planting in northern Indiana. A mixture of American chestnut, sugar maple, black cherry, and tulip poplar bareroot seedlings were machine-planted in an abandoned crop field to investigate the varying growth rates and compatibility of these four species.
Silvicultural and reintroduction trials provide an opportunity to experiment with planting chestnuts on field and forested sites (Figure 2).
Silvicultural trials allow us to learn how chestnut grows under different forest management scenarios. Existing trials have examined planting in gaps of various sizes, clearcuts, closed canopy, shelterwoods, and multi-step management prescriptions. For example, a Green Mountain National Forest planting, managed in partnership with the US Forest Service and University of Vermont, looks at chestnut growth, cold tolerance, and winter injury under three canopy treatments – open, partial, and closed canopy.
Reintroduction trials differ slightly in that these plantings look at techniques for establishment such as planting stock type (seed vs. bare root vs. containerized seedlings), planting density (spacing between trees) and time of planting (fall vs. spring). They also include site preparation, methods of protection from wildlife, and fertilization.
Often reintroduction and silvicultural trials go hand-in-hand, for instance a North Carolina State University study examined growth and survival differences between chestnuts planted in an open canopy vs. a shelterwood, with and without fertilization at the time of establishment.
Silviculture and Reforestation
A primary practice of TACF which involves silviculture and reintroduction is assisted migration. Assisted migration is a forest management practice which provides human-driven movement of tree species, often to non-indigenous locales, in response to pest, pathogen, and climate stresses. As the climate changes, some have hypothesized that some tree species will be unable to adapt quickly enough to these deviations, potentially resulting in local and regional extirpation, or even species extinction. Assisted migration cannot be completed without sound and effective artificial regeneration practices, which can often be more expensive, laborious, and unsuccessful than natural regeneration.
In the case of American chestnut, a majority of diversity of the species can be found in the extreme southern extent of the range (Figure 3). Unfortunately, the trees at the southern extent are also the most at risk for extirpation due to climate change and pests and pathogens, such as Phytophthora root rot, which are either limited to or worse in the southern Appalachians. TACF is working to identify and rescue as many individuals as possible. While harvesting chestnuts from these trees is the easiest means to do this, most of these trees are either not fruiting or do not produce viable nuts. In those cases, grafting is the only means of conservation. These nuts and/or grafts are then placed into Germplasm Conservation Orchards (GCOs) for long-term conservation and breeding. Conversely, at and beyond the northern extent of the range, experimental chestnut introductions may assist with the northern migration of the species predicted by many climate change models.
One of the best ways you can assist TACF in its conservation and restoration mission is to locate wild-type American chestnut trees. We hope you will participate in this activity and let us know the details of your find! We ask that you collect a leaf and twig sample, press it between stiff paperboard, and mail it in with a completed Tree Locator Form. You can find the form, mailing addresses, and more information about American chestnut identification.
A map of haplotype diversity of American chestnut throughout the historical native range of the species. While only D-type cytoplasm is found within populations to the north (blue dots), multiple haplotypes (orange dots, inclusive O, M, and P haplotypes) are found throughout the southern extent of the range, suggesting a top conservation priority should focus on the southern and southwestern locations. Map courtesy of Taylor Perkins.
Mineland Reforestation
Mined land reforestation plantings represent another avenue to help TACF achieve its research and restoration goals. By working with the Appalachian Regional Reforestation Initiative (ARRI), Green Forests Work (GFW), and other partners, TACF has assisted in the planting of more than 1.8 million seedlings of various high-value hardwood species, including American chestnut, and the reforestation of nearly 3,000 acres on both publicly- and privately-owned mined lands in eight states since 2009 (Figure 4). By restoring degraded lands to native forest types, mined land plantings meet multiple objectives. These plantings not only improve wildlife habitat and decrease forest fragmentation, but also result in improved air and water quality, increased carbon sequestration, invasive exotic species suppression, and have economic benefits.
