By the 1950s, two non-native pathogens had killed almost all American chestnut trees. “There’s a lot of interest in breeding a chestnut that looks like American chestnut with the disease resistance of Chinese chestnut,” says U.S. Forest Service research forester Stacy Clark. “However, there hasn’t been much research on reintroducing disease-resistant trees to the forest.”
Clark and SRS research forester Callie Schweitzer recently contributed to a new study on chestnut reintroduction. Cornelia Pinchot, currently a research ecologist at the Forest Service Northern Research Station, led the study while she was a PhD student in the University of Tennessee’s Tree Improvement Program. Results from the study were recently published in the journal New Forests.
Clark and her colleagues received BC2F3 hybrid chestnuts from The American Chestnut Foundation’s Meadowview Research Farm in Virginia, and grew them for one year in a nursery before planting them into the study in 2009. The American Chestnut Foundation and the Connecticut Agricultural Experiment Station have been breeding disease-resistant chestnut trees for decades, and the BC2F3 hybrids have higher levels of blight resistance than the American chestnut.
In cooperation with the University of Tennessee, the scientists planted American chestnut, Chinese chestnut, and the hybrid chestnut in the Daniel Boone National Forest, Kentucky. The study was part of a larger Forest Service study designed to improve forest health and promote natural oak regeneration before non-native gypsy moths invade.
The scientists used three different silvicultural methods – shelterwood harvest, thinning, and midstory tree removal. The treatments created a gradient of light levels, from open canopy, to intermediate, to closed canopy.
Seedling survival and growth were tracked until 2012. “We also compared chestnut seedlings to other trees and shrubs,” says Schweitzer. “Our goal was to see how well chestnut competes for growing space and sunlight.”
Natural American chestnut sprouts can survive on the shady forest floor for decades, waiting for a disturbance to send some light their way, but it is not known how planted seedlings will perform. Their unique combination of shade tolerance and quick growth means managers might be able to use a range of silvicultural strategies to restore the species.
The results showed that most chestnut seedlings grew fastest in open canopy plots of the shelterwood harvests. However, the other understory plants in these sunny plots also grew quickly, overwhelming chestnut’s ability to compete for space and sunlight.
Pinchot and the other scientists found that the intermediate light in the thinning treatment gave chestnut the biggest advantage over other understory plants. “It’s critical to know how chestnut will interact with other species – especially since these ecosystems have changed dramatically since chestnut was lost,” says Clark.
One ecosystem threat has remained constant – root rot disease. Root rot is caused by Phytophthora cinnamomi, a water mold that lives in the soil and attacks trees’ roots.
The water mold was accidentally introduced to the U.S. in the late 1700s or early 1800s, and by the late 1800s, it had killed many American chestnuts. Trees that grew in moist and poorly drained sites throughout the southeast succumbed to root rot, and the chestnut blight fungus finished off most of the survivors.
The scientists found the water mold in every soil sample they collected. It was suspected to have killed about 75 percent of the American chestnut trees as well as a large number of hybrid seedlings.
In one hybrid family, only 21 percent of the seedling survived. The other hybrid family did better – 57 percent of seedlings survived, and 90 percent of the Chinese chestnut trees survived; Chinese chestnut has natural resistance to root rot disease.
Before planting American chestnut, the scientists recommend testing the sites for the presence of P. cinnamomi, and making sure sites are well-drained.
For more information, email Stacy Clark at email@example.com.