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