HRI partners with scientists, many with ties to North Carolina State University (NCSU), who are taking a long-term view on the restoration of hemlocks. These scientists work to conserve the diverse genetic lineages of North Carolina’s native eastern and Carolina hemlocks using a variety of different approaches, including collecting seeds for long-term storage and via traditional breeding programs. These projects are aimed at providing the tools land managers will need to restore hemlock forests that have been devastated by HWA.
Researchers are attempting to integrate resistance into our hemlock forests in multiple ways. Some are studying naturally resistant eastern and Carolina hemlocks, searching for resistant genes. Others are hybridizing the hemlock species from HWA’s native ranges (ie. Chinese and western hemlock) with our more susceptible species. Both require developing an understanding of the mechanisms for resistance, which is a challenging project in and of itself.
Identifying natural resistance
There are several approaches to developing HWA resistance within the tree itself and researchers are looking into each. The most straightforward method is to find hemlocks in our forests that seem to already have resistance to or tolerance of HWA and then take samples of these trees for testing and breeding purposes.
Tree Species in Peril- the Lingering Hemlock Search:
While our native hemlocks don’t show widespread inherent resistance to HWA, some individual trees may possess some resistance or tolerance that can be combined and increased through breeding. Tree Species in Peril (TSIP) is a project of The Nature Conservancy investigating genetic resistance to pests in vulnerable North American tree species. Professionals from federal government agencies, state programs, non-profits, and universities are involved in this far-reaching project. The eastern hemlock is one of the trees that TSIP is actively researching. Researchers are especially interested in healthy hemlocks that are found in areas where most other hemlocks show high levels of decline. These hemlocks that appear to be dying more slowly than the ones around it are called Lingering Hemlocks. The qualifications that make a hemlock a potential ‘Lingering Hemlock’ can be found here. However, identifying potentially resistant hemlocks can be quite complicated since there are many environmental reasons why an infested hemlock that has not been chemically treated may remain relatively healthy. If those other factors can be eliminated through testing, then hemlocks that possess some genetic resistance to HWA can be identified. More information about the Lingering Hemlock Project can be found here.
Hybridization
Another method of creating resistance to HWA is to breed our mostly resistance-lacking eastern and Carolina hemlocks with other species of hemlocks that do possess innate resistance to HWA. So far, researchers have been unable to cross the more widespread eastern hemlock with other hemlock species, but the Carolina hemlock has been successfully crossed with the Chinese hemlock to produce hybrid offspring that exhibit intermediate levels of HWA-resistance, including some individuals that long-term testing has shown are very resistant to HWA. These hybrids, although resistant, have limitations. They must be reproduced from cuttings and lack the genetic diversity that would make them candidates for helping repopulate our forests with hemlocks. However, this first step is promising that similar efforts in the future could produce more hemlocks that possess genetic resistance to HWA and could at the very least be used as ornamentals and for landscaping. Underscoring this point, the US National Arboretum has released ornamental Carolina-Chinese hemlock hybrid cultivars named “Traveler” and “Crossroads” that is currently undergoing commercial production and is expected to be available for public purchase in the near future.
Diversity and Gene Conservation
Because the hemlocks are in decline, it is important to conserve the genetic diversity of the species. There are many organizations who are pursuing this effort. One, named Camcore, actively collects seeds from hemlocks to conserve this genetic diversity. Camcore is a non-profit, international tree breeding organization based at North Carolina State University. Its work focuses on maintaining a broad genetic base of tree species, many of which are especially suited to plantation forestry as well as several threatened conifers native to the southern US, including the eastern and Carolina hemlock.
A major emphasis for the group is the establishment of ex situ (off site) genetic conservation through seed storage and the planting of seed orchards. Camcore has collected seeds from over 700 eastern in 73 populations and 130 Carolina hemlock in 19 populations since 2003. Seed storage through both cold storage and cryogenic deep-freeze helps maintain a large number of seeds from across the range of eastern and Carolina hemlock. Seed orchards provide a way for conserved lineages to continue producing new seeds. These efforts will go far towards ensuring that the genetic resources of North Carolina’s two native hemlock species will be preserved. In March, 2020 HRI volunteers helped plant a new Carolina hemlock seed orchard just outside of Linville Gorge.You can also learn more about Camcore’s work with hemlocks and other tree species on their website. The collected seeds are preserved for long-term storage or planted to serve as a conservation orchard for future seed collection.
Camcore has also been working closely with the USDA Forest Service Southern Research Station to develop a long-term plan to help reintroduce hemlocks to southern Appalachian forests so that these majestic trees can make a successful return to our forest when the time is right. The plan involves a variety of management strategies including silvicultural techniques along with traditional chemical and biological control strategies.
How you can help:
TreeSnap App (available on iOS and Android)
You can help us find Lingering Hemlocks! The TreeSnap app lets you report hemlock trees observed to have apparent resistance to HWA. This data will help scientists locate trees for research projects like tree breeding and genetic conservation. Check out this flyer for more information.
Forest Restoration Alliance Workdays:
The Forest Restoration Alliance (FRA) is a non-profit group of researchers at NCSU focused on developing genetic resistance to adelgid in native hemlocks and firs. To achieve its goals of long-term forest stability the group researches mechanisms for tree resistance and, based on the successful program established by The American Chestnut Foundation, works to breed hemlocks, firs and other native trees that are resistant to invasive forest pests.
One way that the HRI supports the work of the FRA is through co-hosting volunteer workdays. These days are an opportunity for volunteers to get an inside look at the detailed, exciting work of the search for HWA-resistant hemlocks, and volunteers provide hands on help to scientists in maintaining valuable seedlings and test plots at the FRA selective breeding facility in Waynesville, North Carolina. If you would like to volunteer at an upcoming FRA work day, please fill out our Volunteer Sign Up Form. Check our events page for upcoming work days. We typically host one or two work days with the FRA each season.
For more information on the ongoing research about genetic solutions for HWA control, check out these publications:
Bentz, S., Gouker, F., Olsen, R., & Pooler, M. 2023. Tsuga ‘Traveler’ and ‘Crossroad’—The first adelgid-resistant interspecific hemlock hybrids. HortScience. 58(3): 289–290. https://doi.org/10.21273/HORTSCI16918-22
Feng, Y. Y., Shen, T. T., Shao, C. C., Du, H., Ran, J. H., & Wang, X. Q. 2021. Phylotranscriptomics reveals the complex evolutionary and biogeographic history of the genus Tsuga with an East Asian-North American disjunct distribution. Molecular Phylogenetics and Evolution. https://doi.org/10.1016/j.ympev.2020.107066
Havill N.P., Montgomery M.E. 2008 The role of arboreta in studying the evolution of host resistance to the hemlock woolly adelgid. Arnoldia. 65(3):1-9.
Holman, G., Tredici, P. del, Havill, N., Lee, N. S., Cronn, R., Cushman, K., Mathews, S., Raubeson, L., & Campbell, C. S. 2017. A New Species and Introgression in Eastern Asian Hemlocks (Pinaceae: Tsuga). Systematic Botany. 42(4), 733–746. https://doi.org/10.1600/036364417X696474
Jetton, R. M., Dvorak, W. S., & Whittier, W. A. 2008. Ecological and genetic factors that define the natural distribution of Carolina hemlock in the southeastern United States and their role in ex situ conservation. Forest Ecology and Management. 255(8–9): 3212–3221. https://doi.org/10.1016/j.foreco.2008.01.032
Potter, K. M., Campbell, A. R., Josserand, S. A., Nelson, C. D., & Jetton, R. M. (2017). Population isolation results in unexpectedly high differentiation in Carolina hemlock (Tsuga caroliniana), an imperiled southern Appalachian endemic conifer. Tree Genetics and Genomes. 13(5). https://doi.org/10.1007/s11295-017-1189-x