Stand Dynamics Associated with Chronic Hemlock Woolly Adelgid Infestations in Southern New England

Vegetation dynamics were monitored for 6 years in eight hemlock (Tsuga canadensis (L.) Carriere) stands with varying levels of hemlock woolly adelgid (HWA) (Adelges tsugae Annand) damage in southcentral Connecticut to examine the patterns of overstory hemlock mortality and subsequent community reorganization associated with chronic HWA infestations. Since 1995, overstory and understory mortality has risen 5 to 15% per year to overall values of 50 to 99%. There has been no sign of tree recovery and the health and vigor of remaining trees deteriorated in all infested stands. Results suggest that trees on some sites can remain alive for more than 10 years following initial infestation and remain standing for 6 to 8 years following mortality. Rapid recolonization of black birch (Betula lenta L.) occurred at most sites along with low densities of red maple (Acer rubrum L.), and oak (Quercus L.) species. Seedling densities increased in moderately damaged stands and thinned but increased rapidly in height, reaching 3 to 7 m tall in heavily damaged stands. The few hemlock seedlings found contained HWA infestation. Shrub cover remained low following infestation while herbaceous cover, consisting primarily of hay-scented fern (Dennstaedtia punctilobula (Michx.) Moore), has gradually increased over time. Cover of the invasive Japanese stilt grass (Microstegium vimineum (Trin.) A. Camus.) increased in one of the stands. Tree-ring analysis of hardwood and declining hemlock trees, coupled with age-structure analysis of newly established birch saplings, was effective in determining the timing of initial HWA impact in stands of unknown infestation date. Over the last 10 years, radial growth of hemlock declined precipitously in most stands, while oak and maple growth exhibited concomitant, large annual increases. Results suggest dramatic stand structure and composition changes accompany heavy HWA infestations that continue long after hemlock mortality.