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| A Hemlock Woolly Adelgid was attached to a hemlock tree in the Rock Creek Park area of the Cherokee National Forest in Erwin,
Tenn. Ron Campbell/Johnson City Press/AP |
Hemlocks threatened by an unwelcome guest
Scientists are working to stop the hemlock woolly adelgid from killing trees in the Eastern US and spreading northward.
from the August 2, 2007 edition
Page 2 of 3
In the meantime, working with partners, the US Forest Service is establishing hemlock nurseries in South America and Arkansas's adelgid-free Ozark Mountains to ensure that if and when a solution is found, restoration is possible.
In the Eastern US, hemlocks come in two varieties, the Eastern hemlock in the north and the Carolina hemlock in the south. In southern Appalachia, where hemlocks tend to cluster around water, their shade helps keep streams cool.
"We're at the tipping point for where temperatures can support trout populations," says ecologist James Vose, project leader at the Coweeta Hydrologic Laboratory in Otto, N.C. "Hemlock is really important for providing the shade. It keeps the environment cool enough for trout to survive."
During the summer, air temperatures beneath hemlock trees are 5 degrees C. lower than in the open air, the equivalent of being 300 miles to the north, says Julian Hadley, a plant eco-physiologist at Harvard Forest in Petersham, Mass. "It has a strong effect on the microclimate," he says.
In the Northeast, where the hemlock grows more evenly throughout the forest, they provide an important overwintering site for deer. They also slow the spring runoff by shielding snow from the sun. In both North and South, fallen hemlock needles create a mat of organic matter on the forest floor. Slow to decompose because of high tannin content, the mat provides habitat and nutrients for many other species.
But maybe the greatest worry is the hardest to quantify. During the first half of the 20th century, a blight eliminated the American chestnut from Eastern forests, another keystone species. How much more can a forest ecosystem take before it collapses and shifts radically?
"As ecologists, we worry about 'What is the tipping point?' " says Dr. Vose. "It seems that when you lose these keystone species, you have the possibility of reaching that tipping point sooner."
In Tennessee's Great Smoky Mountains National Park hemlocks tower 150 feet and can reach 500 years old. There, scientists are protecting hemlock stands with insecticidal soap or horticultural oil sprayed from trucks. (The Arnold Arboretum also sprays trees near roads with the oil, which suffocates the bugs.) Where the hemlock grows near campsites and spraying is impossible, scientists directly apply imidacloprid, a pesticide, by injection or by soaking the soil around the tree's base.
But none of these treatments can be applied on a large scale. This fact has scientists looking for the perfect natural predator, one that can breed in its new environment and survive the Northeastern winters, but not become a pest in its own right. After testing many beetles with mixed results, scientists think they have a good candidate. Laricobius nigrinus, a beetle native to the Pacific Northwest, can survive New England's cold snaps, and can only complete its life cycle on the hemlock woolly adelgid.
"I'm quite hopeful that this enemy and overwinter mortality could actually save the hemlocks in Massachusetts," says Joe Elkinton, a professor of entomology at the University of Massachusetts, Amherst, who has wrapped trees, adelgid, and beetle together in netting with promising results. "I'm confident we can stem the tide."
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