During summer, the lacy leaves of an eastern hemlock tree provide a soft-lit, cool and quiet reprieve in many New York area parks. However, this luxury may soon become only a memory if a deadly insect continues to kill these trees along the east coast, particularly in Westchester County and New York City.
The hemlock wooly adelgid, an insect originally from Asia, was transmitted to Virginia in the 1950s where the hemlocks had no natural defenses against it. By the early to mid-1980s, the adelgid had reached New York and Connecticut, killing thousands of hemlocks in the process. James D. Lewis, Ph.D., assistant professor of plant ecology at the Louis Calder Center Biological Field Station in Armonk, N.Y., has been researching the adelgid since 1998 at the Calder Center; Ward Pound Ridge, the largest park in Westchester County; Black Rock Forest, a private preserve north of West Point; and the Delaware Water Gap, a national recreational area.
Enlisting the help of doctoral students J.T. Mates-Muchin and Greg Turner as well as Rob Sproule (FCO ’02), Lewis is trying to determine the direct effects of the adelgid on the hemlocks and, as a result, the potential ecological and economic effects on the forest and its habitants. Besides being aesthetically pleasing, hemlocks traditionally help reduce run-off into streams, preventing erosion, and filter nitrogen out of acid rain. According to Lewis, there is a possibility that oak and maple trees in the New York area will naturally replace the hemlock.
“Other hardwood species potentially would take over the roles of the hemlock, but there is a time gap,” Lewis said. “When the number of trees is knocked off by 80 percent, there is a reduced capacity of the forest to protect itself from erosion and acid rain.” In addition to erosion and acid rain, Lewis said the water, fish and stream organisms could also be negatively affected by a lack of hemlock protection. “For the other trees, this may possibly be a good thing, but for the animals, it is negative because it is removing food, bedding and nesting sites,” Lewis said. As the project entered its fourth field season this summer, Lewis began working with William Giuliano, Ph.D., assistant professor of vertebrate population biology at the Calder Center, to determine precisely how the animals are being affected. The areas of hemlocks Lewis is studying known as stands are showing as much as a 90 percent mortality rate within the first five years after the adelgid is introduced.
While it is not clear whether the adelgids are blocking the sap as they feed on branches and twigs, the end result is that nutrients are not able to reach the hemlock needles. Most of the first-year needles die and fall off. Over time, the older needles also die, killing the hemlock. “Right now we have no effective ways to counteract the adelgid. This is going to be a persistent problem,” Lewis said. “If you want to protect these hemlocks, there is going to be constant management. You can’t spray once and have [the problem]go away.”
Lewis has received funding from the United States Department of Agriculture to continue research through this year and has applied for two other grants. One would extend research specifically on the nitrogen processing to determine what is happening in the tree itself and why. The other, which was submitted by Amy Tuininga, Ph.D., and Jacqui Johnson, Ph.D., of the department of biological sciences, would continue the assessment of the adelgid’s impact on the hemlocks.