Changes in a hemlock-dominated forest following woolly adelgid infestation in southern New England1

Abstract

Small, M. J., C. J. Small (Department of Botany, Connecticut College, New London, Connecticut 06320), and G. D. Dreyer (Goodwin-Niering Center for Conservation Biology and Environmental Studies, Connecticut College, New London, Connecticut 06320). Changes in a hemlock-dominated forest following woolly adelgid infestation in southern New England. J. Torrey Bot. 132: 458–470. 2005.—The hemlock wooly adelgid (HWA; Adelges tsugae), a small aphid-like insect introduced from Japan, has caused widespread hemlock (Tsuga canadensis) mortality throughout the mid-Atlantic and southern New England region over the last twenty years. We examined long-term (1952–2002) changes in hemlock-dominated stands before and after the appearance of the HWA in 1987 in the Connecticut College Arboretum, southeastern Connecticut. With HWA infestation, basal area of T. canadensis declined dramatically, dropping 70% from 1982 to 2002. Forest communities responded to the elimination of the dominant species by quickly filling various sized gaps. Black oaks (Quercus velutina, Q. coccinea, Q. rubra) increased from 28% of canopy basal area in 1982 to 41% in 2002. Sapling density increased markedly following HWA infestation, from 80 stems/ha in 1982 to nearly 5600 in 2002, with greatest increase in Sassafras albidum (0 to 1900 stems/ha) and Acer rubrum (4 to 1100 stems/ha). Ledge and ravine communities, formerly dominated by T. canadensis, became more compositionally distinct, with greater importance of black oaks on more xeric ledge sites and mixed canopy dominance in mesic ravine sites. Major trends associated with the decline of T. canadensis included a shift in canopy dominance to oak and mixed hardwoods, considerable understory development, including greater herb richness and abundance and increased density of clonal saplings, and expansion of several invasive shrubs and woody vines.