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The Future of Hemlock Trees in the Eastern U.S. Remains Dicey

Photo of Hemlock trees in the eastern U.S. have not yet been severely impacted by the hemlock woolly adelgid but the landscape may be at a tipping point. David Lee, USDA Forest ServiceHemlock trees in the eastern U.S. have not yet been severely impacted by the hemlock woolly adelgid but the landscape may be at a tipping point. David Lee, USDA Forest ServiceSnapshot : The arrival of the hemlock woolly adelgid, an invasive insect from Asia, threatens the stability and sustainability of hemlock in the eastern U.S. An analysis of landscape level data provided by the Forest Inventory and Analysis (FIA) Program shows that while some local impacts have been severe, at a landscape scale, hemlock is not yet severely impacted. But the researchers predict, if the adelgid continues to go unchecked, the fate of hemlocks remains unknown.

Principal Investigators(s) :
Trotter, III, R. Talbot 
Research Location : Eastern United States
Research Station : Northern Research Station (NRS)
Year : 2013
Highlight ID : 464

Summary

Since its introduction, the non-native hemlock woolly adelgid has spread to infest hemlock trees in at least 18 states in the eastern U.S. Previous studies have documented highly variable rates of hemlock mortality among infested stands making it difficult to estimate regional impacts. Recent data from the Forest Service's Forest Inventory and Analysis (FIA) Program collected from 432 eastern U.S. counties reveals several surprising and conflicting regional patterns. First, the median live and dead hemlock basal area has generally increased over the last two decades across the eastern U.S. This is the case in both infested and uninfested counties. Second, the median percentage of hemlock which is alive has decreased over the past 20 years, again in both infested and uninfested counties. Third, the ages of infestations are negatively correlated with the percentage of live hemlock, as might be expected given the known impact adelgids can have on a stand through time; however, this relationship depends on the exclusion of uninfested counties, as counties infested more than 12 years and uninfested counties have similar percentages of live hemlock. Combined, these data suggest increasing tree density associated with the past century of reforestation and succession in the eastern U.S. may be currently overwhelming the negative impacts of the adelgid at the regional scale, but the long-term stability of this situation is not known. Data from long-infested counties suggest the landscape may be at a tipping point, leaving hemlock trees vulnerable at the landscape scale.