Hemlocks Declining Fast
Eastern hemlock is a keystone species in the streamside forests of the Southern Appalachian region. Its shade is key in maintaining the cool water temperatures required by trout and other aquatic organisms. Hemlock branches shelter nesting songbirds, many of which are neotropical migratory species already in decline.
Around 2004, eastern hemlocks in the Southern Appalachians began showing signs of infestation by the hemlock woolly adelgid, a tiny nonnative insect which has spread steadily south since it was introduced into the Northeast area in the 1950s. Once a hemlock is infested, it generally dies within 10 years, but hemlocks in the Southern Appalachians are dying much faster, with untreated trees dying within 3 to 5 years. Warming temperatures don’t help.
“Really cold winters, even cold snaps, will slow down the hemlock woolly adelgid,” says Jim Vose, project leader of the SRS Forest Watershed Science unit. “Unfortunately, we have not had those conditions in the Southern Appalachians for the past several winters. The warm weather has been ideal for the adelgids to reproduce and spread.”
Water Flow and Temperature
In July 2007, Vose and ecologist Chelcy Ford published a study that provided the first estimates on the impact the loss of eastern hemlock will have on the water dynamics of the Southern Appalachian mountains. The data came from experiments the researchers are conducting along the streams at the SRS Coweeta Hydrologic Laboratory in Otto, NC, where hemlocks are dying rapidly.
To estimate the impact the loss of hemlocks will have on the water balance, the researchers measured transpiration rates—the rates at which trees return water to the atmosphere—for hemlocks of different sizes over a period of 2 years. “We found substantial rates of transpiration for individual hemlocks,” says Ford. “Some of the larger trees transpired as much as 49 gallons of water a day.”
The researchers predict that as hemlocks die, forest transpiration could decrease as much as 30 percent in the winter and spring. The loss of the trees’ transpiration capacity could mean greater levels of moisture in the soil. Stream flow could increase, saturating streamside riparian zones. The loss of canopy could also change the temperature of streams; it’s been estimated that hemlock shade cools some streams by as much as 9 degrees Fahrenheit.
“We’re at the point where rising stream temperature could impact trout populations,” says Vose. “The shade provided by hemlock helps keep the environment cool enough for trout to survive.”
At Coweeta, researchers have already started studies on how to mitigate the loss of hemlock by planting other species—among these, the American chestnut and eastern white pine, but the loss of eastern hemlock will profoundly change southern forests and the groups of species they shelter.
“It’s unlikely that any other evergreen native to the Southern Appalachians will fill the role of eastern hemlock,” says Ford.