Is there hope for hemlocks?

by | Feb 26, 2024

“While we cannot save every hemlock in our forests, we can prevent the species from going extinct.” – Jason Frye,  NCFS B.R.I.D.G.E Program project leader.

In the years following the introduction of the hemlock woolly adelgid (HWA) (Adelges tsugae), a sap-sucking insect native to Asia, hemlocks have steadily declined across their native range, leaving behind the skeletal remnants of what was once a prominent overstory tree dominating the forests of the Appalachian Mountains. The question on many minds is whether hope exists for our ‘redwoods of the east’ or if they’ve reached a tipping point, leaving them unable to rebound.

The native hemlock range extends from Canada into the southern Appalachian Mountains, spreading westward into parts of Minnesota and descending eastward into the foothills. Two species of hemlock occur in North Carolina: the eastern hemlock (Tsuga canadensis) and the Carolina hemlock (Tsuga caroliniana). Eastern hemlocks are long-lived, overstory trees towering heights up to 100 feet, with the oldest known trees exceeding 800 years old. In the Piedmont, more than 200 miles east of the edge of their natural range, a relic population of eastern hemlocks exists in Cary, North Carolina, on a series of north-facing bluffs along Swift Creek. Sustained by a cooler, moist microclimate than the surrounding area, these trees have been able to persist long after the last ice age, or glacial maximum.

Carolina hemlock is a southern Appalachian endemic, restricted to elevations between 2,100 and 4,000 feet along the rocky slopes and ridges of the Blue Ridge Mountains. North Carolina is home to around 80% of the naturally occurring Carolina hemlock, with smaller populations in Virginia and Georgia. They are less common than the eastern hemlock. With the onset of the HWA invasion, combined with other environmental stressors, their dwindling numbers are a cause for concern, prompting the species to be under review for listing under the Endangered Species Act.

Hemlocks in DuPont State Recreational Forest (DSRF) that have succumbed to HWA infestation. Photo by NCFS.

If you are wondering what all the fuss is about, dead hemlocks scattered across the Western North Carolina landscape are not only an eyesore, but they play an important ecological role in the health of our forests in the Appalachian Mountains.

Dead hemlocks at Linville Falls. Photo by NCFS.

If you have ever hiked in the mountains during summer, you have probably noticed the cool reprieve once under the canopy of a hemlock forest. In the southern Appalachians, hemlocks primarily grow near streams. The shaded cove environment they create maintains the cool temperatures conducive to the survival of our native brook trout and several species of salamanders, including the eastern hellbender. Hemlock forests provide critical stopover sites during migration for several neotropical bird species such as the blackpole warbler, and they provide habitat for over 90 species of bird, including the Blackburnian warbler, blue-headed vireo, black-throated green warbler and northern saw-whet owl. Young hemlocks in the understory provide thermal cover and a vital food source for various game species including ruffed grouse, wild turkey, white-tailed deer and the eastern cottontail rabbit during our cold winter months. American black bears commonly use hemlock tree cavities for denning. The microclimate created by hemlocks also supports a myriad of plants, some of which are listed as threatened or endangered at the state or federal level like the pirate bush (Buckleya distichophylla), a hemiparasitic shrub found nowhere else in the world but a small region of the southern Appalachians. Hemlocks are an essential component of the forests of the Appalachian Mountains.

So, you may be asking yourself, how did HWA get here and why is such a tiny insect so destructive? 

HWA is native to Asia and was introduced to the Pacific Northwest in the 1920s, where it feeds on western hemlock (Tsuga heterophylla). An adapted suite of natural predators has prevented the species from becoming a pest in both regions. Unfortunately, hemlocks in the east were not as lucky. Since its introduction in 1951 near Richmond, VA through infected hemlocks imported from Japan, HWA has continued to spread across much of the native hemlock range from wind or hitchhiking on birds and other wildlife. HWA was first discovered in North Carolina in 1995 in Stokes and Surry counties.

During most times of the year, HWA can be recognized because of the woolly covering it creates to protect itself. Photo by NCFS.

Adelgids are aphid-like insects that feed exclusively on conifers. HWA settles at the base of hemlock needles on branch tips and uses their piercing-sucking mouthparts to deplete their host of stored nutrients. In their native range, HWA is subjected to heavy predation, potentially leading to their complex life cycle having two overlapping generations a year. The spring generation, or progredien, is active from early spring through midsummer. The second, known as the sistens generation, is the overwintering generation, present from summer to early spring of the following year. To further complicate things, only females exist in Eastern North America where and are parthenogenic, meaning they reproduce asexually. In Asia, male HWA feed on tiger-tail spruce (Picea torano) and fortunately, North Carolina’s native red spruce (Picea rubens) does not seem to support their development, leading to an abbreviated life cycle.

With no natural predators to keep their populations in check and the ability to reproduce without a mate, HWA can rapidly multiply and overwhelm trees, starving them of vital nutrients. Heavy infestations cause premature needle drop and branch dieback, beginning at the lower branches and progressing to the crown. If left untreated, trees can die in as little as four years, but typically die within 10 years following an initial infestation. Environmental stress, such as heat and drought, can further exacerbate their decline.

Is there hope for North Carolina’s redwoods of the East?

In the mid-1900s, Fraser fir (Abies fraseri) was seemingly met with a similar fate by the invasive balsam woolly adelgid (Adelges piceae). However, it still prevails today in our red spruce—fraser fir Forests. Fortunately, with continued research leading to more advanced integrated pest management programs (IPM), there is hope and many researchers and forest managers remain optimistic about the future of hemlocks and their recovery to our forests.

One of the first eastern hemlock treated at Dupont State Recreational Forest. Photo by NCFS.

IPM is an approach that uses the knowledge of a perceived pest’s biology with a combination of techniques including chemical and biological control, habitat manipulation, and the use of resistant plant varieties in a way that makes it more difficult for a pest population to thrive while also minimizing risk to human health, wildlife and the environment. The N.C. Forest Service (NCFS) implements an IPM strategy incorporating chemical and biological control while tracking the program’s progress through annual surveys and monitoring. Hemlocks are treated with a systemic insecticide, providing protection for up to five years. Biological control agents are employed in infested areas that are too close to streams for treatment and along steep cliffsides.

Laricobius beetles or Lari beetles as they are sometimes referred to, are predator beetles native to the Pacific Northwest. Adults and larvae have a voracious appetite for the overwintering generation of HWA that feed on western hemlock. The beetles are released in select areas under the guidance of the USDA Forest Service (USFS) Forest Health Protection Program and APHIS, and in cooperation with the NCDA&CS Beneficial Insects Program. The goal is to keep hemlocks alive through temporary insecticide protection while simultaneously establishing predator populations in nearby untreated areas. This approach allows Laricobius beetle populations to increase as chemical treatments wear off, reaching a predator-prey balance our native hemlocks can tolerate, ultimately reducing the need to treat hemlocks over time chemically.

Treating hemlocks and the biocontrol release program began at DuPont State Recreational Forest in 2007 and South Mountains State Park the following year. In 2009, the NCFS assisted the N.C Wildlife Resources Commission (NCWRC) with treatments on several state game lands. These combined efforts resulted in approximately 5,000 hemlocks treated in the first few years.

In 2014, restoring hemlocks in our state became a prime objective for Agriculture Commissioner Steve Troxler, prompting the formation of the Hemlock Restoration Initiative (HRI). HRI is a partnership program between the NCFS, N.C. Department of Agriculture and Consumer Services (NCDA&CS), USFS, WNC communities, local government agencies, university researchers and conservation organizations working under the common goal of minimizing the damage caused by HWA and restoring hemlocks to our southern Appalachian Mountain forests. Together, they identify and establish conservation areas, provide education and guidance to landowners and oversee the HWA integrated pest management program.

Eastern hemlock seedling with the remnants of a large hemlock killed by HWA in the background. Photo by NCFS.

In 2018, the NCFS B.R.I.D.G.E Program was recruited to help treat hemlocks and has since become one of the most powerful assets for treating hemlocks in the Appalachian Mountains. To date, NCFS B.R.I.D.G.E crews have treated more than 60,000 hemlocks and helped preserve our highest priority hemlock conservation areas across Western North Carolina. As of 2023, more than 120,000 hemlocks across our forests have been treated or are within areas with biocontrol releases.

During annual surveys, Laricobius beetles are consistently found near release sites and sometimes miles from the nearest release area, suggesting they are surviving and spreading. Since they only feed on the overwintering generation, it is unlikely the beetles alone will be enough to save our hemlocks. In the Pacific Northwest, two species of silver fly (genus Leucotaraxis) are known predators of HWA and are commonly associated with Laricobius beetles in western hemlock forests. Both are promising candidates and are currently being evaluated as potential biocontrol agents to tackle the spring generation of HWA (Dietschler, 2021).

New hope is on the horizon for hemlocks, and predatory insects and insecticides are not the only tools at our disposal. Silviculture prescriptions have long been used to alter forest stands to provide timber and related forest products for human benefit (Gottschalk, 1995). It has also proved to be an innovative approach for transforming forest structure and composition to mitigate the impacts of insects and diseases and improve forest health. A new study published in Forest Ecology and Management by Albert E. Mayfield III, research entomologist with the USFS, and Robert M. Jetton, associate professor of Forest Health at N.C. State University, concluded that creating gaps in the forest canopy around eastern hemlocks may give them the upper hand against HWA infestations. The study found that the reduced competition and more available sunlight resulted in increased new shoot growth and annual basal area with significantly less branch dieback despite being infested with HWA. Increased shoot growth is especially encouraging because one of the first responses to an HWA infestation is that the tree stops producing new shoots.

Although they did not see a decline in HWA populations with elevated sunlight levels as seen in a previous study (Brantley, 2017), trees were healthier, leading researchers to believe hemlocks not limited by competition and resources can better tolerate HWA infestations. Jetton sees silviculture as another tool in the IPM toolbox. He is optimistic that it will benefit biological control efforts by allowing trees to hold on long enough while predator populations are established, thereby reducing the need for chemical treatments. Mayfield and Jetton’s end goal is to develop a stand-level prescription that forester managers can utilize as part of their IMP program to minimize the impacts of HWA.

Sources:

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

Mayfield III, 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, 121374. https://doi.org/10.1016/j.foreco.2023.121374