Study finds hemlock trees dying rapidly, affecting forest carbon cycle

New research by U.S. Forest Service Southern Research Station (SRS) scientists and partners suggests the hemlock woolly adelgid is killing hemlock trees faster than expected in the southern Appalachians and rapidly altering the carbon cycle of these forests. SRS researchers and cooperators from the University of Georgia published the findings in the most recent issue of the journal Ecosystems. "The study marks the first time that scientists have tracked the short-term effects hemlock woolly adelgid infestations are having on the forest carbon cycle," said Chelcy Ford, SRS ecologist and co-author of the paper.

Eastern hemlock, a keystone species in the streamside forests of the southern Appalachian region, is already experiencing widespread decline and mortality because of hemlock woolly adelgid (a tiny nonnative insect) infestation. The pest has the potential to kill most of the region's hemlock trees within the next decade. As a native evergreen capable of maintaining year-round transpiration rates, hemlock plays an important role in the ecology and hydrology of mountain ecosystems. Hemlock forests provide critical habitat for birds and other animals; their shade helps maintain the cool water temperatures required by trout and other aquatic organisms in mountain streams.

Scientists conducted the study in mixed hardwood forests along the edges of two streams at the SRS Coweeta Hydrologic Laboratory, a 5,600-acre research facility and experimental forest in the Nantahala Mountain Range of western North Carolina.

Researchers compared rates of decline of adelgid-infested hemlock trees to a small number of girdled (severely wounded the bark of a tree to initiate tree mortality) trees that were not infested. Researchers tracked changes in the carbon cycle of these hemlock stands over a 3-year period. Scientists measured components of the forest carbon cycle – including tree growth, leaf litter and fine root biomass, and soil respiration – over the 3-year period.

"While we expected that girdled trees would decline quickly, we were surprised to find that hemlock declines just as quickly from adelgid infestation," said Ford. "This research shows that hemlock woolly adelgid infestation is rapidly impacting the carbon cycle in these tree stands. The study also supports the widely held belief that adelgid-infested hemlock trees in the South are declining much faster than the reported 9-year decline of some infested hemlock trees in the Northeast."

The study showed, among other things, that very fine roots in the girdled and hemlock woolly adelgid-infested plots declined by 38 percent and 22 percent, respectively, during the 3-year period. In addition, in the first year after girdling and infestation, researchers found soil respiration was approximately 20 percent lower than they expected.

The authors suggest that infrequent frigid winter temperatures in the southern Appalachians may not be enough to suppress adelgid populations. The authors believe this could be one explanation of why infested hemlocks appear to be declining faster in the South than in the Northeast. The authors also point out that other tree species are quick to occupy the space given up by their dying hemlock neighbors.

"Perhaps because of increased light in the canopy and reduced competition for soil nutrients and water, other species are already increasing their growth," said Ford. "We'll continue to monitor this, but, it's still too early to predict just how different these forests will look 50 or 100 years from now."

Source: Southern Research Station - USDA Forest Service