Abstract: Invasive insects can cause substantial short-term as well as long-term impacts on forest structure, composition, and function. Invasive insects have led to the decline and removal of several dominant tree species across broad regions in eastern forests affecting wildlife habitat, understory dynamics and aboveground biomass. The invasive hemlock woolly adelgid (HWA; Adelges tsugae) has impacted many eastern hemlock (Tsuga canadensis) forests with widespread foliar loss, mortality and subsequent compositional changes, although the extent and rate of decline varies tremendously across sites. Vegetation dynamics associated with hemlock mortality have been described in several eastern forests, although data that includes dynamics exceeding 1 or 2 decades is rare. Data spanning twenty-five years from permanent plots located across eight hemlock-dominated locations in south-central Connecticut was used to 1) document long-term hemlock mortality, 2) quantify the changes in aboveground density and basal area over time, and 3) characterize the understory vegetation dynamics that have unfolded with the decline and loss of hemlock.
After 25 years of adelgid infestation, dramatic shifts in forest structure have occurred, with very few healthy hemlocks remaining at most sites. Overstory hemlock mortality was variable across sites, ranging from 5-98% initially in the mid-1990s, to exceeding 80% in all infested sites examined by 2020. Out of 854 hemlock stems originally sampled in infested stands in the 1990s, less than 50 remained alive by 2020. Total overstory basal area and density represent only 40-60% of pre-adelgid levels. The original control site was infested with HWA by 1998 but only experienced 20% mortality by 2020. Black birch (Betula lenta) sapling density increased from < 125 ha-1 to >4,000 ha-1 at some sites. In heavily damaged stands, black birch has already recruited into the tree size category (8 – 14 cm dbh) and now represents up to 40 % of the basal area. Seedling layer vegetation mirrors saplings in overall composition, and the number of species present on the sites have fluctuated but increased in most plots. The invasive Japanese stilt grass (Microstegium vimineum), increased from 5% to over 95% total understory cover at one location. Few hemlock saplings or seedlings remain at most sites, indicating that this species may soon be effectively eliminated from these sites.
