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US Forest Service study finds hemlock still abundant despite adelgid infestation

NEWTOWN SQUARE, Pennsylvania, September 27, 2011 An analysis of two decades of data collected by the U.S. Forest Service's Forest Inventory and Analysis (FIA) program shows that the live volume of hemlocks in the eastern United States is increasing despite infestations of hemlock woolly adelgid that have decimated local populations.

The information comes from an e-Science Update co-authored by scientists from two U.S. Forest Service research stations, the Northern Research Station (NRS) and the Southern Research Station (SRS), and published by SRS.

"The update provides an overview of the status and extent of hemlocks in the eastern United States based on FIA inventories conducted by SRS and NRS," according to Randall Morin, a research forester with the NRS' FIA program and the primary author. "It also incorporates research by NRS scientists and the results of pest surveys conducted by the Forest Service's State and Private Forestry Program."

Two native species of hemlock eastern and Carolina grow in eastern North America. Although a minor component in most of the forests of the eastern United States, high densities of eastern hemlock are found in New England and the mountains of the Southeast and Mid-Atlantic. The Carolina hemlock, similar in appearance to the eastern hemlock, is found only on rocky mountain slopes of the Southern Appalachian region.

Researchers conducted the analysis on 20 years of data collected across 433 counties stretching from southern Maine into northern Georgia. They found an overall increase in live-tree hemlock basal area in counties infested with hemlock woolly adelgid as well as those without infestations.

A tiny insect introduced into the United States from East Asia, the hemlock woolly 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," according to Andrew Liebhold, an NRS research entomologist. "Hemlock woolly adelgid currently infests about 45 percent of the range of hemlocks in the United States and 41 percent of the total hemlock basal area." These percentages are up 26 percent and 25 percent respectively since 2004.

"The analysis also showed that the general regional trend in the East over the past 50 years has been one of increasing hemlock volume, even with infestation by the hemlock woolly adelgid," according to Sonja Oswalt, an SRS forester and co-author. "Even though the insect has caused substantial negative impacts on hemlock at stand-level scales, analysis of FIA data suggests 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 that the trend of increasing hemlock volume may not last much longer. "Despite increasing hemlock volume over the last four decades across most of the eastern United States, the regions with long-established populations of hemlock woolly adelgid are also the regions where hemlock is accumulating slowest," Morin said. "Net growth rates decrease as years of infestation increase, and mortality rates increase, with mortality starting to equal net growth in areas where hemlock woolly adelgid has been present for 10 to 20 years." As time goes on the trend of increasing abundance may begin to reverse.


Contact: Jane Hodgins
USDA Forest Service - Northern Research Station

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