Forest community changes following hemlock woolly adelgid infestation in southern New England
Melanie J. Small
ABSTRACT
The hemlock wooly adelgid (Adelges tsugae), a small aphid-like insect introduced from Japan, has caused widespread Tsuga canadensis mortality throughout the Mid-Atlantic and southern New England region over the last twenty years. We examined long-term changes (1952-2002) in hemlock-dominated stands following the introduction of the woolly adelgid in 1987. Major findings included a shift in the dominance to hardwood canopy species, increasingly rich herbaceous layers, the appearance and expansion of several invasive shrub and vine species, and increased sapling stem density, particularly in clonal species. Hemlock basal area declined by 70% from 1982 (pre-adelgid) to 2002. Black oaks (Quercus velutina, Q. coccinea, Q. rubra) increased from 28% of canopy basal area in 1982 to 41% in 2002. Saplings of Sassafras albidum increased from 0 to 1900 stems/ha and Acer rubrum from 4 to 1100 stems/ha over the past 20 years. Formerly hemlock-dominated communities on nearby ledge and ravine sites became more compositionally distinct. Ledge site canopies became dominated by black oaks. The density of Sassafras albidum saplings in the understory increased and few herbaceous species were present. The canopies of ravine sites became, dominated by black oaks and Fagus grandifolia. Ravine sites had a greater diversity of herbaceous and sapling species, particularly with S. albidum, Prunus serotina and Acer rubrum. Hemlock stands studied here are expected to experience complete hemlock mortality due to woolly adelgid infestation by 2006.With the loss of a dominant forest component and depending upon site type, hemlock stands infested with hemlock woolly adelgid may convert to oak and mixed hardwood forests with multi-species dominance.