Stand and Community Response to Hemlock Woolly Adelgid in Southern New England since 1995

Hemlock woolly adelgid (HWA), Adelges tsugae, an introduced aphid-like insect from Asia, is expanding its range across the northeastern United States through the range of Tsuga canadensis (eastern hemlock) and can severely reduce or eliminate this important late-successional species. As part of a study investigating stand- and landscape-level forest dynamics resulting from HWA infestation, we examined initial community response of eight T. canadensis stands in south-central Connecticut and then continued to monitor these stands every other year. Our major objectives were to assess mortality patterns in T. canadensis, evaluate subsequent changes in stand microenvironment, and relate these and stand composition to initial patterns of regeneration, understory response, and community reorganization. Tsuga canadensis damage varied broadly across the study area ranging from near zero to greater than 95 % mortality. All size and age classes sampled were attacked by HWA, although smaller trees exhibited higher mortality rates than larger trees. All remaining T. canadensis sampled in seven of the eight stands were infested with HWA and over 90 % suffered at least 50 % foliar loss. 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 over 10 years following initial infestation and remain standing for 6 to 8 years following mortality. Substantial accumulations of downed woody debris have developed in stands with severe HWA damage. Canopy gaps created by HWA damage significantly increased the amount of light reaching the forest floor and resulted in rapid understory vegetation responses. Prolific Betula lenta (black birch) establishment occurred in stands with moderate to severe T. canadensis mortality. In addition, opportunistic herbaceous species (Erechtites hieracifolia, Phytolacca americana) and non-native species (Ailanthus altissima, Microstegium vimineum) have recently invaded these stands. Due to mortality from HWA, T. canadensis seedlings were scarce in sampled stands, suggesting that advance regeneration and seedbanks will not be important mechanisms for T. canadensis reestablishment. Tsuga canadensis cannot sprout following defoliation and has no apparent resistance to HWA. 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. Therefore, dramatic reductions in T. canadensis across broad geographical areas appear imminent if HWA dispersal continues unimpeded and no effective natural enemies of HWA are found.