On November 3, about forty people from the USDA Forest Service and The American Chestnut Foundation (TACF) gathered virtually. It was the second biennial plan of work meeting between TACF and SRS.
Since the 1990s, the two organizations have worked together on American chestnut (Castanea dentata) restoration. In 2017 and in 2019, they committed to a Biennial Plan of Work that strengthens the partnership.
At the meeting, foresters, researchers, and organization leaders shared the latest advances in genetics and breeding, restoration research, and disease screening.
SRS research forester Stacy Clark organized the meeting, which was introduced by Lisa Thomson, TACF president and CEO, and Rob Doudrick, SRS station director. Doudrick provided an historical viewpoint of the partnership and work conducted over the last 25 years.
The work involves TACF, several university partners, and all three branches of the Forest Service: the National Forest System, Research and Development, and State and Private Forestry.
Partners from the Forest Service Southern Region attended as part of their week-long silvicultural recertification workshop. Through the National Advanced Silvicultural Program, certified silviculturists also attended and participated in the discussion.
Since 1983, TACF has been working to restore the American chestnut. Their science strategy is called 3BUR – a reference to chestnut’s three nutritious nuts in a spiky burr. TACF uses traditional approaches (tree breeding), integrated management approaches (biocontrol), and powerful new molecular tools (biotechnology).
Two major diseases decimated American chestnut: chestnut blight (Cryphonectria parasitica) and Phytophthora root rot (Phytophthora cinnamomi).
TACF is working to combine resistance to both diseases, as Jared Westbrook, the TACF director of science, discussed. Although most TACF researchers are focusing on traditional breeding, some are outcrossing blight-tolerant transgenic founder trees with backcross trees that are resistant to Phytophthora root rot.
“We are using multiple approaches to develop a disease-resistant American chestnut population,” says Westbrook.
SRS researchers are advancing fundamental knowledge of chestnut genetics. A reference genome of Chinese chestnut was recently published, as SRS research geneticist Dana Nelson discussed at the meeting. Nelson also shared new information about the structure of the ribosomal DNA genes of American and Chinese chestnuts and genetic mapping of disease resistance.
Chinese chestnut is blight resistant. Chinese chestnut and American chestnut have been backcrossed – a breeding process that aims to recover the timber-type form of American chestnut while keeping Chinese chestnut genes for blight resistance. Hybrids with above average blight resistance have about 85 percent of their genome from American chestnut.
In 2009, Clark began testing advanced hybrid chestnuts in test plots on three national forests in the Southern Region.
“These test plantings represent the largest, most robust research of advanced hybrid chestnut material in the world,” says Clark. Clark helped design the test plantings, which span 13 test sites and include 4,500 trees.
Results from the first ten years of field trials are encouraging in terms of growth rates and competitive ability, but many trees are now succumbing to the blight. Ten years is a very short time to monitor a tree that can live for centuries, as Clark points out.
Other challenges remain. Deer nibble on young plants, as do many insects. And Phytophthora root rot remains a threat.
Screening for resistance to Phytophthora root rot is underway, as Katie McKeever, the director of the Forest Service Resistance Screening Center discussed at the meeting. The Center is located at Bent Creek Experimental Forest in North Carolina and has partnered with TACF for many years.
Phytophthora is a water mold, so flooding and soggy soil makes trees more susceptible to infection. McKeever and her colleagues mimic these conditions, inoculate the seedlings, and monitor them for 20 weeks.
Most seedlings die. “When we give tours, I tell people that the seedlings are supposed to look terrible,” says McKeever. Any survivors are outplanted so that the assessment can continue under field conditions.
Chestnut blight does not affect the plant’s roots. Although American chestnut is functionally extinct, an estimated 430 million chestnut stems still sprout from century-old roots before dying of blight within a few years, over and over.
Sara Fitzsimmons, the TACF director of restoration, discussed the value of these remnant, wild-type populations. TACF and researchers from partner universities are embarking on a landscape genomics project. The results will help sustain the genetic diversity of the species and show where collection efforts should be focused.
Chestnut restoration involves long-term collaboration. The work has involved the University of Tennessee, Clemson University, Mississippi State University, and the University of Georgia.
“Collaboration increases our capacity to complete this research, from breeding and genetic work in the lab and orchards to planting and testing trees in the real world,” says Clark. “Together we can move forward.”
For more information, email Stacy Clark at firstname.lastname@example.org.