Data collected via regional Forest Inventory and Analysis (FIA), by the United States Forest Service from the 1950s through 2007 reveal that in spite of the accumulation of the invasive, non-native pest of hemlock trees, the woolly adelgid, hemlock forests in the eastern U.S. appear to have sustained the impact well. The big picture they are assembling with this data suggests that while some regions, such as the southern Appalachians, have seen severe damage a more pronounced impact was expected in the northeast. The past century has seen significant reforestation and succession in eastern hemlock forests, perhaps the increase in tree density has offset mortality and damage incurred by woolly adelgid.
Hemlock woolly adelgid is native to Japan, it was first detected in the 1950s in Virginia but was primarily found in urban settings. By the 1980s the infestation had spread to hemlock’s native range which forms a triangle from eastern Canada west to Minnesota, south through the Appalachian mountains to northern Georgia and South Carolina and back northeast to Pennsylvania. In the United States the woolly adelgid lacks natural predators and native hemlocks also lack natural defenses, this unfortunate pairing has resulted in the high mortality in the 18 states where this pest has made its home. Impacts to southern hemlock forests have been the most severe.The findings of this study, based on data collected up to 2007, appears to have captured the hemlock population at its tipping point between losses from the woolly adelgid and increases from regrowth.
Repeating or continuing this study with data more recently collected via FIA may reveal if hemlock losses have begun again. Should increases in density of hemlock forests have continued in northern climes due to colder temperatures and longer winters impeding pest growth and spread, it would still not balance out massive losses to southern forests. The lack of southern hemlocks deprives the species of genetic variation.
Pests like the woolly adelgid can be devastating to the region’s culture, tourism, ecology and economy. Michael Rains, director of the U.S. Forest Service’s Northern Research Station and Forest Products Lab maintains that the Forest Service’s research is working aggressively to gain control over non-native insects, pests and diseases and make forests healthier, sustainable and increase natural resistance.
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. The full text of this study, entitled “Changes in the regional abundance of hemlock associated with the invasion of hemlock woolly adelgid,” as published in the journal Biological Invasions, is available at:http://www.nrs.fs.fed.us/pubs/45316