Invasive pests threaten an ancient tree species
By Sydney Cromwell
To walk into a stand of eastern hemlock trees is to be transported to an ecosystem unlike any other. It suddenly becomes cool, shady and moist, an environment the hemlocks have preserved beneath their canopies since the last ice age.
“Usually you can sense, you can feel it,” said Justin Hart, professor and director of the University of Alabama’s Environmental Sciences program and Forest Dynamics Lab.
Within the canyons of Bankhead National Forest, particularly in the Sipsey Wilderness, that micro-environment attracts unique birds, amphibians, plants and quite a few human visitors, as well.
“It’s hard to describe them unless you come touch them,” Wild Alabama Executive Director Maggie Johnston said. “… They’re a beautiful species. I think they’re one of my favorite living things.”
In states from Georgia to Maine, however, death is slowly spreading among forests of towering eastern hemlocks, on the back of an invasive insect called the woolly adelgid.
“This is a treasure that we still have these, because they’re dying in most other places,” Johnston said.
The woolly adelgid has slowly spread over the eastern hemlock’s range for decades, devastating entire populations of trees. Once a tree is infested, it usually dies within five to 10 years.
Alabama represents the southernmost tip of the eastern hemlock’s range, with stands of trees found in Bankhead National Forest and other scattered spots across the northern part of the state.
For a while, Allison Cochran, a U.S. Forest Service wildlife biologist at Bankhead, said she thought the separation between eastern hemlock populations in Alabama might save the trees from the woolly adelgid’s spread. However, in 2020 the tiny insect was spotted in Alabama for the first time — “too close for comfort,” she said.
IN THE SHADE OF GIANTS
Alabama’s eastern hemlocks are a remnant of the Pleistocene ice age, nearly 12,000 years ago. Hart said many of the “relict” stands of trees that survived that ice age are still in the same locations today.
“These guys and their ancestors are the ones who actually repopulated the hemlock population, all the way up into Canada and beyond, when the ice receded,” Johnston said.
An individual eastern hemlock lives for hundreds of years, growing over 100 feet tall with a trunk spanning 4 to 5 feet in diameter. Cochran estimates there are 3,500-4,000 acres of hemlock forests in Bankhead, the largest population in the state, but they also occur in Little River Canyon, Redstone Arsenal, Mentone and other sites at the tail end of the Appalachian Mountain range.
“They have really dense canopies. So they’re the most shade-tolerant trees in North America,” Hart said.
Hemlocks thrive in moist, shady areas, often near streams, like the canyons at Bankhead. They share that ecosystem with species like the tulip poplar, white oak and beech, but Hart said the eastern hemlock is the dominant species and fills a niche that other trees cannot fully replicate.
“They’re called a foundation species,” he said. “… They regulate a whole bunch of different ecosystem processes when they’re dominant.”
Beneath the hemlock’s evergreen needles, Cochran said the cooler temperatures and sandstone bluffs are home to species such as green salamanders and gorge filmy fern, both rare in Alabama. The black-throated green warbler also spends the summer in Bankhead’s hemlocks, she said, which draws bird watchers to the forest.
In addition to birds, plants and amphibians, these hemlocks also keep streams cool during the summer, which is important for several aquatic species.
“It’s one of the main things that makes Bankhead truly a special gem in Alabama,” Cochran said. “… People come here just for that reason.”
AN UNSTOPPABLE INVASION
The hemlock woolly adelgid is native to East Asia and the west coast of North America. It was accidentally introduced to the eastern hemlock and Carolina hemlock from Japan in the 1950s.
In its native environments, cold weather and predator species keep the woolly adelgid from becoming too numerous or causing too much harm to the trees on which they feed.
But in eastern hemlocks they found a ready supply of food without any of those environmental checks and balances, and the stage was set for an ecological disaster.
While the woolly adelgid doesn’t travel far on its own legs and will spend almost its entire life cycle on a single tree, the insect can be carried by wind, wildlife, and unwitting humans to reach new trees. It has now spread almost to the entirety of the eastern hemlock’s range.
The first sign of a woolly adelgid infestation is usually the clusters of eggs found on the underside of hemlock needles. These look like small tufts of white wool, for which the tiny insect got its name.
Once they’ve inhabited a tree, woolly adelgids suck the sap out of its trunk and branches, causing the hemlock to lose its needles and be unable to produce new growth. The tree either slowly starves to death due to the loss of its needles or is weakened to the point that other environmental stressors, such as drought, can kill it more easily.
In Shenandoah National Park and the Blue Ridge Parkway, the National Parks Service estimates that 80% of the hemlock population has been lost since the arrival of the woolly adelgid. Other national parks and forests are seeing similar losses
Johnston said she first learned about the species while hiking in the Great Smoky Mountains National Park, where she came across a stand of hemlocks that had already been infested.
“There were hemlocks that were dying — now they’re all dead,” she said.
Hart regularly takes some of his students to visit the Joyce Kilmer Memorial Forest in North Carolina, another widespread infestation site, to learn about the consequences of invasive species.
“Before woolly adelgids came through there, I would have said that’s my favorite forest in the world,” he said. “All the hemlocks there are dead … and it’s completely changed the look of that forest.”
If the woolly adelgid reaches farther into Alabama, Bankhead and other eastern hemlock sites can expect a similar fate.
“In the South we’ve already lost a lot. We’re going to lose more,” said Rusty Rhea, an entomologist at the U.S. Forest Service Southern Region Forest Health Protection.
FIGHTING AGAINST FUNCTIONAL EXTINCTION
There are ways to kill the woolly adelgid or slow its spread, but on a practical level those treatments may not work fast enough to save more than a small minority of America’s eastern hemlocks.
Rhea said chemicals can be injected at the base of an individual tree and will protect it from the woolly adelgid for about five to seven years. However, treating trees one at a time is not feasible throughout an entire forest or national park.
Hart described these chemical injections as a “short-term Band-Aid.”
So far, Rhea said, those treatments are the primary method used by the Forest Service to save eastern hemlocks in a “few hundred conservation areas” that are ecologically significant or are located near public recreation areas.
“We can maintain some stands in perpetuity,” he said.
Predatory beetles have also been imported from Japan and the Pacific Northwest to feed on the woolly adelgid and provide a natural limit to their population growth. However, Rhea said research has shown the beetles are not active enough throughout the year to keep up with the woolly adelgid’s reproduction.
Rhea said there is a lot of public interest and support for saving the eastern hemlock, and research on other biological and chemical controls is ongoing.
“We’ve come a long way, based on what we knew when this thing got here,” Rhea said.
However, it’s always a race against time
At Bankhead, U.S. Forest Service forester Jason Harris said staff go out to search for signs of woolly adelgid infestation about three times a year. If — or when — the insect arrives, Cochran said they plan to use all treatment methods available to protect high-priority hemlock stands.
Saving all of Bankhead’s hemlocks, Harris said, is likely beyond their capabilities or resources.
“Generally, there isn’t anything that we can do should an outbreak occur,” Harris said.
“So far, I haven’t seen anyone successfully going out there and treating and saving entire stands from being lost. … It doesn’t seem like a realistic goal, I’m afraid to say,” Cochran said.
The woolly adelgid was found in Mentone in spring 2020, though it has already been in Georgia for a few years. Right now, one of the greatest sources of protection for Alabama’s hemlocks is the physical distance between tree populations, making it less likely for the woolly adelgid to successfully travel to new hosts.
Cochran said some well-intentioned people try to plant the eastern hemlock in their own backyards to support the species. Ironically, this can actually make the hemlocks more susceptible to infestation, as those new trees can decrease the travel distance between hemlock stands and make the woolly adelgid’s job easier.
Cochran said the best thing hikers and eastern hemlock supporters can do is clean their clothes and gear after visiting a hemlock forest, and be careful about carrying anything into or out of forests. The Forest Service also recommends using only local firewood, as a variety of invasive pests can hitch rides in firewood brought from outside sources.
If hikers in Bankhead spot white fuzz underneath hemlock needles, Cochran said they should let staff know.
Rhea and Hart both said researchers are also collecting seeds and genetic material throughout the eastern hemlock’s range, for a possible reforestation effort if the woolly adelgid can be controlled in the future.
“We’re a long way from even thinking about restoration,” Hart said.
Depending on the extent of the hemlock’s destruction, Cochran said the moist, shaded environments where they thrive may be lost as well, limiting the effectiveness of any attempts to repopulate the trees.
“It typically grows in its own canopy,” she said. “… They’re hard to get to grow. You don’t see a lot of young hemlock trees.”
Hart said the eastern hemlock is “heading toward functional extinction” in most of its range. That makes it all the more important to prevent the woolly adelgid from ever reaching Bankhead, Hart said.
“There is some hope that we can retain the species here at these disjunct populations,” he said. “… That should help with some kind of a restoration effort.”
Bankhead could become one of the last major sites in the United States with living, healthy eastern hemlocks — if they can be protected.
“They have a pretty big responsibility for maintaining hemlocks in Alabama,” Hart said.