Eastern hemlock (Tsuga canadensis) stands across the eastern United States experience widespread decline and mortality due to the introduced hemlock woolly adelgid (Adelges tsugae; HWA). We investigated the overstory and understory vegetation dynamics associated with chronic HWA infestation over a 13-year period. From 1995-1998, forest structure, composition, health, and physical site characteristics were examined in permanent plots established at ten sites in central Connecticut. Plots were resampled at 4 to 5 year intervals through 2008 for overstory dbh, foliar loss, crown classification, T. canadensis mortality levels, and understory vegetation dynamics.  
Results/Conclusions

Overstory mortality was variable across sites, originally ranging from 5-98%; by 2008 seven sites have mortality levels > 80%. Interestingly, three sites at the northern extent of the study area have lower mortality levels of 17 to 38%. Hemlock mortality has resulted in a shift in forest structure, to forests dominated by Betula lenta (black birch), Acer rubrum (red maple), and Fagus grandifolia (American beech). Black birch sapling density increased from 125-250 ha^-1 to >4,000 ha^-1 at some sites. In heavily damaged stands, black birch has recently recruited into the tree size category (>8cm dbh). Seedling layer vegetation mirrors saplings in composition, and the relative density of red maple seedlings has increased from 10% to > 50% at several sites. The recent migration of the co-occuring invasive elongate hemlock scale (Fiorinia externa), adds an additional layer of complexity, potentially speeding the decline of hemlock forest health in HWA infested stands. Despite 15 years of adelgid infestation, healthy hemlocks remain at some sites, which contrasts with initial predictions of immediate decline post-HWA invasion.