A Battle for Survival

The southern Appalachian region is home to two species of hemlock: the Eastern hemlock (Tsuga canadensis) and the Carolina hemlock (Tsuga caroliniana). The Carolina hemlock grows up to 60 feet and tends to grow at higher elevations on dry ridges near rock outcrops. The eastern hemlock grows up to 170 feet and can be found in moist, riparian environments at lower elevations. Southern Appalachian hemlocks are coniferous, evergreen trees and can be identified by their short, flat needles (1).

Due to their tall heights and dense pine needle coverage, hemlocks provide vital shade for riparian ecosystems. By helping to maintain a cool, moist habitat, many endemic species can be found near these forest giants. For example, endemic species such as the semi-parasitic pirate bush and the black-throated green warbler can often be found near eastern hemlocks. The shadiness hemlocks provide also help to maintain cool water temperatures for nearby rivers and streams. Vulnerable species such as brook trout and salamanders rely on cool water temperatures for survival. Because of their vast ecosystem benefits and impressive beauty, they have been coined the "Redwood of the East" by the National Forest Service.

HWA was first introduced in NC in 1995. A survey was conducted in 2001, and by the end of 2002, adelgid infestation was documented in 19 mountain counties of NC (7). Milder temperature and drought contribute to the exponential growth of the HWA (5). The eastern hemlocks in the Southern Appalachians experienced a release event, which occurs when a species has a boom in growth productivity and thrives in the wake of another species’ disturbance, following the chestnut blight of the early 20th century. Now, we’re beginning to see rhododendron experience a release event as eastern hemlocks suffer widespread population destruction due to the HWA. Hemlocks are a shade-tolerant species, and rhododendron is as well, allowing it to quickly propagate and occupy hemlocks’ ecological niche (19).

Although it is possible that hemlocks stands’ ecological niche could be filled by hardwood forests (19), the presence of rhododendron threatens this possibility for two reasons: rhododendron thrives in disturbed areas, and inhibits the seed germination of hardwood trees by preventing the recruitment and colonization of hardwood canopy-tree seedlings (19). Unlike hemlocks and hardwood trees whose roots efficiently prevent soil from eroding, rhododendron have weak and shallow roots which can negatively impact stream health in riparian ecosystems. Hemlocks’ deep roots also helped to filter out sediments and agricultural runoff before they reached the stream. When fish and other low trophic-level species are negatively impacted by loss of hemlocks, the entire food web experiences the effects. Rhododendron also germinates and spreads quickly and can dominate the understory, rapidly creating a monoculture (19). 

One way to help hemlocks survive is through hybridization. The U.S. National Arboretum has been creating hybrids between the eastern and Carolina hemlocks with two hemlock species from Asia, the Chinese hemlock (T. chinensis) and the northern Japanese hemlock (T. diversifolia) (9). Through these trials, crosses with the eastern hemlock were not viable, and only crosses between the Carolina and Chinese hemlock produced successful hybrids (10, 9). 

This cross, known as the Traveler, still has the symmetrical evergreen growth that the Carolina hemlock is known for, and produces large cones (10). Travelers were tested with woody adelgids and researchers found that fewer of the insects settled on the hybrids compared to the native trees (9). Of those insects that did settle fewer of them survived and the surviving ones grew slower than the insects that live on the native trees (9). 

After these trials Traveler seedlings were taken and planted in field trials in both Connecticut and North Carolina (10). Researchers are looking at survival, growth, and appearance of the adelgids in the field trial trees in Connecticut (9). These trials have been conducted since 2015 and in the North Carolina trial it was found that the Travelers have all survived, and have an average height of 3.5 meters (10). 

Using drone imagery, we can get a better sense of the complete picture of the Hemlocks at the Station. With the help of Dr. Susan Cohen, Troy Walton, Fleming Talton, and Peggy Mullin, we flew an EBee Fixed-Wing Mapping and Surveying Drone. This map shows the orthomosaic created from drone imagery overlaid with GPS coordinates of Hemlock locations:

Fortunately, there are two options for treating the HWAs. Soil application of imidacloprid and bark application of dinotefuran have proven effective in suppressing Hemlock Wooly Adelgids. Imidacloprid treatment is slow acting, and requires 3 months to observe adelgid mortality and at least 2 years post application to see full effects. However, it protects the hemlocks for about 5 years following just one soil treatment. This tactic will provide satisfactory milti-year suppression of adelgids in places where an immediate reduction in adelgids is not needed (20). Dinotefuran is a newer insecticide active ingredient, and because it is much more water soluble, it has greater mobility in trees than imidacloprid. This could be useful in a situation where a more immediate reduction of adelgids is necessary in order to preserve tree health. Dinotefuran is applied to and absorbed through the bark of hemlocks, which allows for rapid treatment and minimizes the amount of insecticide contacting the soil (21).

Biological predators are another solution being researched. Hemlock trees in western US coevolved with a native species of the HWA (13). That is why the HWA in western US have natural predators like the beetle (Laricobius nigrinus) and silver flies (Leucopis argenticollis and L. piniperda) (17 & 18). The HWA present in eastern US was imported from Asia so the Hemlocks here did not coevolve with them and there are no natural predators. However, there has been some success introducing these western predators to the east. The beetle feeds on the HWA during the winter but not the spring. So introducing the beetle by itself allows the HWA population to rebound in the spring (18). But the silver flies feed on the HWA during spring and summer. If the beetle and silver flies are introduced to an area together there is hope that there will be enough consistent predation of the HWA to diminish their population (17). These western species have not been in the east long enough for researchers to know if their populations can be naturally sustained. 

Breeding the HWA resistant Hemlocks and introducing biological predators aren’t the only methods being looked into. Silviculture is the the practice of growing and cultivating forests and researchers at NC State are trying to develop a silviculture tool that includes various management strategies like predation, resistance breeding, insecticides, and increased sunlight (12). Studies have shown that Hemlocks exposed to greater amounts of sunlight survive infestation longer. The HWA’s dislike of bright light is not a permanent solution, but it buys the trees some time (12). And there is concern that opening up the forest canopy to increase light exposure will allow other invasives to establish themselves and harm native shade-tolerant species that rely on the Hemlocks. The development of this silviculture tool is still in its early stages, but the hope is that this multifaceted approach will regulate the HWA enough to allow Hemlocks to “outgrow the insects’ impact and better tolerate the infestation” (12). If successful, this new management tool will provide foresters with the HWA prescriptions specific to their region - Hemlocks grow in various parts of eastern US and therefore different treatment combinations will be needed.

This story map was created by students at the Highlands UNC IE Field Site. We are a group of environmental science/studies majors who came to Southern Appalachia to learn about its unique biodiversity and landscape and the environmental issues it currently faces. We chose this project because we felt it was important to tell the Hemlock's story and inform people about the risks of biodiversity loss. Thank you to Brian Naess, Troy Walton, Dr. Susan Cohen, and Dr. Rada Petric for all their help and guidance with this project.

Works Cited

1. The Importance of Hemlocks – Hemlock Restoration Initiative. n.d.. Savehemlocksnc.org. Retrieved November 11, 2021, from  https://savehemlocksnc.org/hemlocks-hwa/the-importance-of-hemlocks/ 
2. Brantley, S. T., Miniat, C. F., Elliott, K. J., Laseter, S. H., & Vose, J. M. 2015. Changes to southern Appalachian water yield and stormflow after loss of a foundation species: LOSS OF FOUNDATION SPECIES AFFECTS WATER YIELD. Ecohydrology, 8:518–528.  https://doi.org/10.1002/eco.1521 
3. Omeka@ctl | uvm tree profiles: Eastern hemlock: What do humans use it for? n.d.. Retrieved November 11, 2021, from  http://libraryexhibits.uvm.edu/omeka/exhibits/show/uvmtrees/eastern-hemlock/what-do-humans-use-it-for- 
4. DeFosset, S. 2017. The Plight of the Hemlock. Forest History Today.  https://foresthistory.org/wp-content/uploads/2018/07/deFossett_Hemlock.pdf 
5.  C:\MYDOCU~2\PROCEE~1\hemlockA.PDF (harvard.edu) 
6.  https://savehemlocksnc.org/hemlocks-hwa/hemlock-woolly-adelgid/ 
7.  https://www.dec.ny.gov/animals/7250.html 
8.  https://www.ncforestservice.gov/forest_health/pdf/HWAReport.pdf 
9. Montgomery, M. E., S. E. Bentz, & R. T. Olsen. 2015. Development of Resistant Hybrid Hemlocks: . Development of resistant hybrid hemlocks - hemlock woolly adelgid - invasive species - forest disturbance processes - northern research station - USDA forest service. Retrieved November 11, 2021, from  https://www.nrs.fs.fed.us/disturbance/invasive_species/hwa/control_management/hybrid_hemlocks/ . 
10. Kaplan, K. 2020. New Hemlock Hybrid Withstands Killing Insect. USDA Agricultural Research Service. Retrieved November 11, 2021, from  https://www.ars.usda.gov/news-events/news/research-news/2020/new-hemlock-hybrid-withstands-killing-insect/ . 
11.  http://nyis.info/invasive_species/hemlock-woolly-adelgid/ 
12.  https://cnr.ncsu.edu/news/2020/09/a-new-hope-for-the-eastern-hemlock/  
13.  https://reeis.usda.gov/web/crisprojectpages/0206985-predators-of-the-hemlock-woolly-adlegid-in-the-pacific-northwest.html  
14.  https://www.nrs.fs.fed.us/disturbance/invasive_species/hwa/risk_detection_spread/  
15.  https://ag.umass.edu/landscape/fact-sheets/hemlock-woolly-adelgid-frequently-asked-questions  
16.  https://www.nrs.fs.fed.us/disturbance/invasive_species/hwa/control_management/chamaemyiid/  
17.  https://blogs.cornell.edu/nyshemlockinitiative/2018/09/13/breaking-new-ground-in-biocontrol-research/  
18.  https://www.srs.fs.usda.gov/compass/2020/09/22/hemlock-woolly-adelgids-their-predator-beetle-laricobius-nigrinus/  
19. Roberts, S. W., Tankersley, R., & Orvis, K. H. (2009). Assessing the potential impacts to Riparian ecosystems resulting from hemlock mortality in Great Smoky Mountains National Park. Environmental Management, 44(2), 335–345.  https://doi.org/10.1007/s00267-009-9317-5 
20.  https://www.ncforestservice.gov/forest_health/pdf/HWAReport.pdf  
21.  https://blogs.cornell.edu/foresthealth/insecticide-treatment-for-hwa/ 
22.  https://www.canr.msu.edu/resources/how_to_treat_hemlock_trees_for_hemlock_woolly_adelgid 
23.  https://www.nrs.fs.fed.us/disturbance/invasive_species/hwa/control_management/chemical_control/