Interactions between invasive herbivores and their long-term impact on New England hemlock forests.
The introduction of the hemlock woolly adelgid (Adelges tsugae) in the eastern United States has caused extensive damage to eastern hemlock (Tsuga canadensis) stands. The presence of other herbivores can affect adelgid density and hemlock health. Here, we report the results of long-term work monitoring hemlock forests in a 7,500 km2 latitudinal transect of southern New England. In biannual surveys between 1997 and 2011, we assessed change in hemlock basal area, stand-level hemlock defoliation, the density of four exotic herbivores (A. tsugae, Fiorinia externa, Nuculaspis tsugae and an unidentified Nuculaspis sp.) and explored relationships between biotic and abiotic variables. Hemlock basal area decreased by approximately 60% over the 14 years of the study. Hemlock stand defoliation increased locally in some stands but on average stand defoliation did not change throughout the study period/area. Local increases in defoliation were probably driven by the northward expansion by A. tsugae, and to a certain extent by the co-occurring presence of F. externa and Nuculaspis scales. Average F. externa density increased during the study and also expanded dramatically northward, although its density did not impact hemlock stand defoliation and only reduced hemlock basal area in some stands. Stand-level variation in F. externa densities was most strongly affected by the presence of Nuculaspis scales, which were present in about 80% of the stands. While A. tsugae continues to be the primary threat to eastern hemlock, its impact needs to be studied in the context of a broader herbivore community.