A dendroecological study of eastern hemlock decline in the Shenandoah National Park (Virginia)

The Eastern hemlock (Tsuga canadensis) population in the Shenanodaoh National Park (SNP) began to decline in 1988 (Blount 1996). The decline coincided with the arrival to the region of the hemlock woolly adelgid (Adelges tsugae), an insect pest. Dendro-ecological techniques were used to verify the onset of the HWA infestation and to assess if growth prior to the infestation reflected the presence of predisposing conditions that may have placed the hemlock trees in a vulnerable state. Additionally, tree-ring data was used to generate growth metrics from the last years of tree life that may be useful in determining if treatment of newly infested sites is warranted.; Hemlock growth response functions were generated and optimized for the period prior to the HWA infestation using data from increment tree cores extracted from infested individuals at two SNP sites. The response function was used to project growth after the infestation, which was then compared with actual growth. Hemlock growth after the HWA infestation was not within the normal range of growth as indicated by t-test of before vs. after means.; Analysis of indices and basal area increment give no indication of predisposing factors. Stand growth for 10 years prior to the adelgid attack was as vigorous as observed over the previous 100 years. Airborne pollution and previous drought may have been responsible for reduced growth from 1950–1976. Minimum radial and basal area growth for three years prior to tree death was documented.; The JABOWA-II forest growth model was qualitatively tested for ability to replicate SNP hemlock stand growth, applying site conditions, climate, and indigenous tree species. JABOWA-II successfully simulated a SNP hemlock stand with the same species diversity and similar species basal area. JABOWA-II was modified with a growth reduction factor that simulates HWA feeding and then exercised to quantify growth loss due to the HWA and to predict community succession after the hemlock were eliminated. When hemlock was not allowed to re-enter the site, red maple (Acer rubrum), dominated the site for at least 375 years. When hemlock was allowed to re-enter, it regained dominance in approximately 250 years.