Hanging On

Reports of the Eastern hemlock’s demise may be premature. Despite the invasion of a harmful insect, stands of the elegant conifer appear to be holding their own in the eastern United States — for the moment.

“When we started this project we really expected to see large-scale losses of hemlock at the landscape scale,” says Sonja Oswalt of U.S. Forest Service’s Southern Research Station, one of four co-authors of a new hemlock survey. “We were surprised to find that, at the broad scale, hemlock loss is nowhere near as dire as expected.”

Two native species of hemlock — eastern and Carolina — grow in the eastern United States. The trees typically don’t make up a big part of eastern forests, but high densities of eastern hemlock are found in New England and the mountains of the Southeast and Mid-Atlantic. The Carolina hemlock is found only on rocky mountain slopes in the Southern Appalachian region.

Stands of both species, however, have been badly damaged by infestations of an insect called the hemlock woolly adelgid. Introduced into the United States from East Asia, the adelgid feeds at the base of hemlock needles, defoliating and eventually killing trees. Since the insect was first noticed in the 1950s, it has expanded its range at between 4.7 and 12.7 miles a year. Currently, it infests about 45 percent of the range of hemlocks in the United States and 41 percent of all hemlock trees.

To gauge the adelgid’s impact, the Forest Service researchers analyzed 20 years of data collected across 433 counties that stretch from southern Maine into northern Georgia. Overall, they found an overall increase in live-tree hemlock basal area in both counties infested with hemlock woolly adelgid and those without infestations. The study concludes that infestations have not yet reduced the overall abundance of hemlock, even in states where hemlock woolly adelgid has been active for decades.

The authors caution, however, that the trend of increasing hemlock volume may not last. Net growth rates appear to be slowing in areas where the insect has been active for 10 or 20 years, and as time goes on tree deaths may begin to outstrip gains.

In addition, “even though this is unexpectedly good news about hemlock survival on the larger landscape, we don’t want to downplay the localized effects,” says Oswalt. “In eastern forests where hemlocks are often the keystone species they can support over 1,000 species birds, animals, and insects. The loss of hemlock stands in many of these areas is nothing less than devastating.” David Malakoff | September 27, 2011

Source: Morin, Randall S.; Oswalt, Sonja N.; Trotter, III, Robert T.; Liebhold, Andrew M. Status of Hemlock in the Eastern United States. e-Science Update SRS–038. See: http://www.srs.fs.usda.gov/pubs/38492

Image USDA Forest Service