Look What's Killing Our Oldest Trees
First published 12.31.13 in Quest: The Science of Sustainability
Something is happening in the Appalachian forest, something deadly. The mighty stands of hemlock, the old-growth guardians of these ancient mountains, are succumbing to death by a billion cuts. Go out into the Alleghenies, the Blue Ridge or the Smokies, and seek out the rushing waters of a mountain stream. Chances are you’ll see what I’ve seen: a skeletal scene with hundreds of giant hemlock trees standing dead on the riverbank, their leafless limbs reaching up the sky like penitents begging for absolution. Both Eastern (Tsuga canadensis) and Carolina (T. caroliniana) hemlocks are falling victim to an exotic insect called the hemlock woolly adelgid (Adelges tsugae), and the riverside areas of the mountainous eastern United States will likely never be the same again.
Accidently introduced to Virginia in the 1950s through the importation of ornamental Japanese weeping hemlocks, the hemlock woolly adelgid (HWA) is a purplish insect less than a millimeter long. It’s detectable mainly by its egg casings, ghostly pale fibrous masses clotting the underside of the hemlock’s shoots. Over the course of several years the HWA larvae feed on the phloem sap that carries life-giving sucrose to the tree’s elegant flattened-needle leaves, eventually causing them to wither and fall from lack of nutrients. And as the leaves fall, the trees starve.
Hemlocks are a keystone species, critical to the East’s mountain waters. They are also our primary old-growth trees, living for over 800 years and anchoring a complex riverine ecological community. Their great shaggy forms, 175-feet tall and more, line our highland waterways and provide year-round shade and soil stabilization to keep streams running cool and clear. Removing hemlocks has increased siltation and exposes the rivers and forest floor to greatly increased sunlight, which results in warmer waters and enormous changes in the vegetative composition of the forest floor, with attendant degradation of wildlife habitat. HWA has spread throughout much of the hemlocks’ range and killed countless trees, a massive loss to biodiversity and the integrity of our mountain headwaters.
The Alliance for Saving Threatened Forests, a coalition composed of multiple universities and agencies based at North Carolina State, has arrived at two potential (and unfortunately long-term) methods of combatting HWA.
The first involves the remarkable discovery that a tiny fraction of hemlocks appear for unknown reasons to be largely resistant to HWA infestation; that is the bugs permeate these trees like any other, but they are somehow able to carry on regardless (a grove of veteran survivors in New Jersey is referred to by foresters as the “Bulletproof Stand”). Perhaps over time the secret of their hidden strength can be understood and shared with the few isolated hemlocks that have as yet managed to avoid infestation. The other gambit involves interbreeding and backcrossing (crossing a hybrid with one of its parents to accentuate desired traits) natives with Asian hemlocks that coevolved with HWA and therefore have inborn means of defying it.
Both of these methods will take a great deal of time, if they ever do succeed. Scott Salom, a Professor of Forest Entomology at Virginia Tech and recognized expert on HWA, tells me that the more direct method is to bring the battle to the bug via the introduction of predatory insects from Asia that have proven to be successful when released in sufficient numbers. The problem there, says Salom, is that “We’re unable to produce and release enough predators” to effectively counter the HWA onslaught: “There are maybe 1,000 predators applied to a hemlock stand infested with millions of HWA.” Valiant but vastly outnumbered, the predators simply would not be able to kill enough HWA to turn the tide.
Salom says the key might be “integrated pest management” whereby direct applications of chemical pesticides, which are highly effective in yards and small parks but impracticable in a sprawling forest environment, are coupled with predator releases and, perhaps in time, with the hybridization methods mentioned above.
Of course along with HWA our Southern forests are also enjoying such nonnative killers as the emerald ash borer, whose larvae are boring into the inner bark of an ash tree near you; the apocalyptic-sounding Thousand Cankers Disease caused by an exotic fungus that is overwhelming the black walnut (an important food source for native wildlife); and the Redbay ambrosia beetle, which injects red bay trees, uncommon along the Southern coast, with a symbiotic fungus that the beetle then feeds upon while the fungus slowly kills the tree. There is even a “balsam” wooly adelgid that feeds on Frazer firs, an essential high-altitude tree of the Southern Appalachians.
All of these pests arrived on our shores via the globalized economy, which must be made to take into account not just profits but the unintended losses inherent in the careless movement of goods around the globe. Who would have imagined that a few decorative saplings brought over by some nursery half a century ago would lead, unwittingly, to the death throes of the oldest trees in the eastern US?
Accidently introduced to Virginia in the 1950s through the importation of ornamental Japanese weeping hemlocks, the hemlock woolly adelgid (HWA) is a purplish insect less than a millimeter long. It’s detectable mainly by its egg casings, ghostly pale fibrous masses clotting the underside of the hemlock’s shoots. Over the course of several years the HWA larvae feed on the phloem sap that carries life-giving sucrose to the tree’s elegant flattened-needle leaves, eventually causing them to wither and fall from lack of nutrients. And as the leaves fall, the trees starve.
Hemlocks are a keystone species, critical to the East’s mountain waters. They are also our primary old-growth trees, living for over 800 years and anchoring a complex riverine ecological community. Their great shaggy forms, 175-feet tall and more, line our highland waterways and provide year-round shade and soil stabilization to keep streams running cool and clear. Removing hemlocks has increased siltation and exposes the rivers and forest floor to greatly increased sunlight, which results in warmer waters and enormous changes in the vegetative composition of the forest floor, with attendant degradation of wildlife habitat. HWA has spread throughout much of the hemlocks’ range and killed countless trees, a massive loss to biodiversity and the integrity of our mountain headwaters.
The Alliance for Saving Threatened Forests, a coalition composed of multiple universities and agencies based at North Carolina State, has arrived at two potential (and unfortunately long-term) methods of combatting HWA.
The first involves the remarkable discovery that a tiny fraction of hemlocks appear for unknown reasons to be largely resistant to HWA infestation; that is the bugs permeate these trees like any other, but they are somehow able to carry on regardless (a grove of veteran survivors in New Jersey is referred to by foresters as the “Bulletproof Stand”). Perhaps over time the secret of their hidden strength can be understood and shared with the few isolated hemlocks that have as yet managed to avoid infestation. The other gambit involves interbreeding and backcrossing (crossing a hybrid with one of its parents to accentuate desired traits) natives with Asian hemlocks that coevolved with HWA and therefore have inborn means of defying it.
Both of these methods will take a great deal of time, if they ever do succeed. Scott Salom, a Professor of Forest Entomology at Virginia Tech and recognized expert on HWA, tells me that the more direct method is to bring the battle to the bug via the introduction of predatory insects from Asia that have proven to be successful when released in sufficient numbers. The problem there, says Salom, is that “We’re unable to produce and release enough predators” to effectively counter the HWA onslaught: “There are maybe 1,000 predators applied to a hemlock stand infested with millions of HWA.” Valiant but vastly outnumbered, the predators simply would not be able to kill enough HWA to turn the tide.
Salom says the key might be “integrated pest management” whereby direct applications of chemical pesticides, which are highly effective in yards and small parks but impracticable in a sprawling forest environment, are coupled with predator releases and, perhaps in time, with the hybridization methods mentioned above.
Of course along with HWA our Southern forests are also enjoying such nonnative killers as the emerald ash borer, whose larvae are boring into the inner bark of an ash tree near you; the apocalyptic-sounding Thousand Cankers Disease caused by an exotic fungus that is overwhelming the black walnut (an important food source for native wildlife); and the Redbay ambrosia beetle, which injects red bay trees, uncommon along the Southern coast, with a symbiotic fungus that the beetle then feeds upon while the fungus slowly kills the tree. There is even a “balsam” wooly adelgid that feeds on Frazer firs, an essential high-altitude tree of the Southern Appalachians.
All of these pests arrived on our shores via the globalized economy, which must be made to take into account not just profits but the unintended losses inherent in the careless movement of goods around the globe. Who would have imagined that a few decorative saplings brought over by some nursery half a century ago would lead, unwittingly, to the death throes of the oldest trees in the eastern US?
