Predicting long-term forest development following hemlock mortality

The hemlock woolly adelgid (Adelges tsugae Annand.), an introduced pest specializing on eastern hemlock (Tsuga canadensis (L.) Carr.), threatens to cause widespread hemlock mortality in New England forests. In this study, we used a stem-based model of forest dynamics (SORTIE) to predict forest development in a northeastern forest both with and without eastern hemlock. In all simulations, forest development was explained by species-specific life-history characteristics such as growth, mortality, and recruitment as they relate to light availability. Forest composition after 500 years depended on the relative abundances of late-successional species: eastern hemlock was long-lived but did not easily gain or yield space; American beech (Fagus grandifolia Ehrh.) gained dominance quickly and soon comprised nearly all basal area in a stand unless hemlock was present; and yellow birch (Betula alleghaniensis Britton) persisted if beech and hemlock were absent. Early-successional species thrived in the forest if late-successional species were not present. We conclude that the long-term impact of the hemlock woolly adelgid on forest composition in northeastern forests will depend both on initial species composition and on the extent of hemlock death. If 50% of the overstory basal area in hemlock is removed by the adelgid, and if the adelgid does not persist at densities high enough to cause extended damage, hemlock stands will continue to be dominated by hemlock. Mixed hemlock-hardwood stands will be dominated by the late-successional species remaining. If hemlock death is near 90% of overstory basal area, hemlock is likely to continue to persist in low densities while the other late-successional species gain dominance. If hemlock death is complete, all stands will be dominated by beech and yellow birch where they are present, no matter what their initial abundances. Hardwood stands will not be affected by the adelgid.