Hemlock Holding Its Own For Now at the Landscape Level

Hemlock woolly adelgid infestation. Photo by Chris Evans, courtesy of Bugwood.org.

Despite ongoing destruction by the non-native invasive hemlock woolly adelgid, hemlock forests in the eastern United States appear to be holding their own for now, according to findings by U.S. Forest Service researchers recently published in the journal Biological Invasions.

The key word is “appear,” said Talbot Trotter, the study’s lead author and a research ecologist with the Forest Service Northern Research Station (NRS). Sonja Oswalt, forester with the Southern Research Station Forest Inventory and Analysis (FIA) unit co-authored the study along with NRS research forester Randall Morin and NRS research entomologist Andrew Liebhold.

In many regions such as the southern Appalachians, the loss of hemlock is devastatingly obvious to land managers and the public, so when the Forest Service scientists analyzed regional FIA data to get a big picture view of the status of hemlock in the eastern U.S., the results surprised them. “In analyzing FIA data from the 1950s through 2007, we expected to see a more pronounced impact on hemlock stands,” according to Trotter.

Instead the data suggest that increasing tree density associated with the past century of reforestation and succession in the eastern U.S. may have offset the negative impacts of the adelgid at the regional scale. “Other studies show massive declines of eastern hemlock in specific areas,” said Oswalt. “But we’re looking at a much larger scale with a much larger sample, trying to indicate what is happening on the landscape from Georgia to Maine.”

A native of Japan, the hemlock woolly adelgid arrived in Virginia in the 1950s and for decades remained a primarily urban pest. That changed by 1980, when the effects of infestation began to be evident in forests within the tree’s native range. Hemlock’s native range forms a triangle from northern Georgia and South Carolina through the Appalachian Mountains into Pennsylvania, Canada and Minnesota.

Hemlock trees in the United States do not have natural defenses against hemlock woolly adelgid, which coupled with a lack of natural predators for the adelgid has resulted in high levels of tree mortality in the 18 states where it is known to have spread, particularly in southern states.

This study, which is based on forest data through 2007, may have caught hemlock at a tipping point in the balance between losses from hemlock woolly adelgid and increases due to forest regrowth.

“Again, we’re looking at the broader scale, and the data shows that hemlock is still present on the large scale,” said Oswalt. “This means that there’s still hope for this species on the landscape, still time to try to find solutions.”

The study included data from 432 counties in 21 states: Alabama, Connecticut, Delaware, Georgia, Kentucky, Maine, Maryland, Massachusetts, Michigan, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Vermont, Virginia, West Virginia, and Wisconsin.

Adapted from news release by Forest Service Northern Research Station.

Read the full text of the article.

For more information, contact Talbot Trotter at www.nrs.fs.fed.us/people/contact/rttrotter .

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