Silviculture is the art and science of manipulating the architecture of a forest. Silvicultural methods to control HWA are intended to be used in conjunction with, and may increase the efficacy of, other control methods. Adopting these “forest architecture” methods is an ongoing research topic with much still unknown. The USDA Forest Service Southern Research Station and NC State University are two of the many institutions participating in this research. The information below highlights what has been observed so far.
There are currently two main strategies for silvicultural control of HWA: the creation of canopy gaps and replanting.
Canopy gaps
The manual creation of gaps in the canopy above an individual or group of hemlocks, usually through felling or girdling surrounding trees, increases the amount of sunlight that reaches hemlocks growing in the densely shaded understory of the typical hemlock forest. There is research indicating that this increased amount of sunlight improves the health of hemlocks that are infested with HWA. For example, a recent study cut small gaps in the canopy and monitored the hemlocks below. They found that canopy removal increased the health of the hemlock trees, and the growth of those that were infested with HWA (Mayfield et al., 2023).
Secondarily, this increased sunlight may also protect trees from freeze damage (Brantley et al., 2017). A hemlock with less physiological stress, such that freeze damage can induce, may be more prepared to withstand HWA infestations.
Increased levels of sunlight may lead to increased levels of photosynthesis for the hemlock (Mayfield & Jetton, 2020). Hemlocks in large gaps put on abundant new growth compared to hemlocks in denser shade. This increase in growth and carbon production could help the hemlock balance out the loss of photosynthetic capacity from defoliation by HWA. The canopy gaps may also increase the amount of water available to the hemlocks by the removal of competing neighbor trees. The water that these trees had been using is now available for the hemlocks. This increase in photosynthesis and water availability may allow the tree to withstand an HWA infestation by regrowing shoots that were fed upon by the previous season’s adelgids. (Mayfield et al., 2023).
Although the results are variable, some studies show that HWA may be negatively phototactic, which means they do not like sunlight. Increased sunlight may lead to decreased HWA settlement, taking away its chance to carry out its life stages. In one study, the amount of attached HWA on hemlocks was THREE times greater in seedlings that were shaded than on those that were in sunny conditions (Mayfield & Jetton, 2020). However, this phenomenon has not been observed in every canopy gap study (e.g., Mayfield et al., 2023).
There is much that still remains unclear, including the ideal gap size and the long-term effects on the hemlocks. Due to this, we do not recommend implementing this method on your hemlock trees at this time. You can hear more about the research being done by listening to this podcast episode featuring North Carolina’s own, hemlock silviculture researchers Bud Mayfield and Robert Jetton. If you are interested in helping with this research, keep an eye out for occasional HRI-facilitated volunteer workdays with these researchers.
Replanting
Hemlock reforestation in areas that have had high hemlock mortality is critical for the restoration of habitats built on the foundation of hemlock trees and to support their unique ecological benefits (see our page: The Importance of Hemlocks). It is a long and arduous process due to both hemlocks’ slow growth as well as sometimes difficult access into backcountry sites. The success can also depend on many factors. This includes, but is not limited to, manipulating light levels, spacing, weed management, and herbivory. Planting hemlock seedlings without planning for all of the above might lead to low survivorship.
Early research suggests that increased sunlight can help the hemlocks grow quickly, but the area must be monitored and controlled for shade-intolerant species that will otherwise outcompete hemlock in that environment (Gonzalez et al., unpublished).
There are three methods in consideration for replanting hemlock stands: reforesting with off-site stock, reforesting with species already resistant to HWA, such as western hemlock or Chinese hemlock, and reforesting with hybrid or native hemlocks bred for resistance.
Stocks:
For this method, foresters grow native hemlock seedlings en masse in greenhouse or nursery conditions, which will then be planted in a forest. Stock seedlings must meet intense quality assessment requirements to stand a chance at growth after replanting. (Grossnickle & Folk, 1993). Even after a seedling becomes established, it is still completely vulnerable to HWA, and other control methods, such as chemical or biological, must still be implemented. Organizations implementing this method include the North Carolina Forest Service’s (NCFS) Linville River Nursery. If you are interested in obtaining seedlings from the NCFS Linville River Nursery, follow this link.
Chinese and western hemlock planting:
These species of hemlock exist in areas where HWA is native. They have coevolved with the insect and developed a natural resistance. Though these species will still be attacked by HWA, there will not be fatal damage induced. These species could offer similar ecological benefits as our native hemlocks, but may have different relationships with our native animal, plant, and insect species that depend on them or have different growth characteristics that impact their effect on the ecosystem.
Hybrid and native species bred for resistance:
Work has been underway to breed a resistant strain of hemlock that incorporates either hybridizing natives with resistant, exotic hemlock species or finding potentially-resistant individuals of our native hemlocks and amplifying that resistance through selective breeding. Once developed, a resistant, or even partially resistant, hybrid or native strain, could be included in replanting and combined with other control strategies as part of integrated pest management of HWA. For more information on this strategy, please see our page titled Genetic Research.
For more information on the ongoing research about silvicultural solutions for HWA control, check out these publications:
Bolstad, P. V., Elliott, K. J., & Miniat, C. F. 2018. Forests, shrubs, and terrain: top-down and bottom-up controls on forest structure. Ecosphere. 9(4). https://doi.org/10.1002/ecs2.2185
Brantley, S. T., Mayfield, A. E., Jetton, R. M., Miniat, C. F., Zietlow, D. R., Brown, C. L., & Rhea, J. R. 2017. Elevated light levels reduce hemlock woolly adelgid infestation and improve carbon balance of infested eastern hemlock seedlings. Forest Ecology and Management. 385: 150–160. https://doi.org/10.1016/j.foreco.2016.11.028
Evans, A. 2006. The Distribution of Hemlock Woolly Adelgid in Trees, Forest Stands, and Regions. Chapter 9: Silviculture. PhD Dissertation. Yale University.
Grossnickle S.C., Folk R.S. 1993. Stock quality assessment: Forecasting survival or performance on a reforestation site. Tree Planters’ Notes. 44(3): 113-21
Jonas S.Z., Xi W., Waldron J.D., Coulson R.N. 2012. Impacts of hemlock decline and ecological considerations for hemlock stand restoration following hemlock woolly adelgid outbreaks. Tree For. Sci. Biotechnol. 6(1): 22-6.
Lapham, M., Miniat, C. F., Mayfield, A. E., Jetton, R. M., Brantley, S. T., Zietlow, D. R., Brown, C., & Rhea, J. R. 2018. Shade and hemlock woolly adelgid infestation increase eastern hemlock foliar nutrient concentration. Forest Science. 64(6), 577–582. https://doi.org/10.1093/forsci/fxy025
McAvoy, T. J., Mays, R., Johnson, N. G., & Salom, S. M. 2017. The effects of shade, fertilizer, and pruning on eastern hemlock trees and hemlock woolly adelgid. Forests. 8(5). https://doi.org/10.3390/f8050156
Marshall R. 1927. The growth of hemlock before and after release from suppression. Harvard Forest Bulletin. Petersham, Massachusetts.
Mayfield, A. E., & Jetton R.M. 2020. Differential sunlight exposure affects settling behaviour of hemlock woolly adelgid crawlers. Agricultural and Forest Entomology. 22(4): 309–318. https://doi.org/10.1111/afe.12382
Mayfield, A. E., Jetton, R. M., Mudder, B. T., Whittier, W. A., Keyser, T. L., & Rhea, J. R. 2023. Silvicultural canopy gaps improve health and growth of eastern hemlocks infested with Adelges tsugae in the southern Appalachian Mountains. Forest Ecology and Management. 546. https://doi.org/10.1016/j.foreco.2023.121374
Editors: Mcmanus, K. A., Shields, K. S., & Souto, D. R. 1999. Proceedings: Symposium on Sustainable Management of Hemlock Ecosystems in Eastern North America. Durham, New Hampshire. www.fs.fed.us/ne