Abiotic and biotic factors influencing eastern hemlock (Tsuga canadensis) health and hemlock woolly adelgid (Adelges tsugae) success in the southern Appalachian Mountains
Mech, Angela Marie
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For over 30 years, the exotic hemlock woolly adelgid (Adelges tsugae Annand) (Hemiptera: Adelgidae) has caused widespread deterioration of eastern hemlock [Tsuga canadensis (L.) Carrière] in the eastern U.S. In the southeastern portion of the range, this infestation is expected to cause the local extinction of T. canadensis, a foundation species in the Southern Appalachian Mountains. Tsuga canadensis and Adelges tsugae were examined at the southernmost part of T. canadensis’s contiguous range in the Chattahoochee National Forest of Georgia to better understand the effects of abiotic and biotic factors in this region. Adelges tsugae were exposed to high temperatures (20 – 40°C) to assess if summer temperatures observed at the edge of the Southern Appalachians influence A. tsugae survivorship. Results from laboratory bioassays found a positive correlation between increasing temperatures and duration of exposure (2 – 8 days), and A. tsugae mortality. The effects of high temperatures were cumulative and the ability of A. tsugae to survive markedly decreased above 30°C. Field observations supported these results as A. tsugae mortality (9 – 63%) increased at warmer sites. A predictive model was created that can be used to estimate percent A. tsugae mortality based on temperature recordings from T. canadensis sites. We also documented the rate of T. canadensis dieback and A. tsugae densities over a four-year period, and factors that may influence these rates/densities. The average dieback rate of T. canadensis in Georgia (6.3% year-1) was found to be lower than rates reported in other locations in the Southeast (20 – 33% year-1). None of the sampled stand or tree characteristics (e.g. diameter, elevation, years infested, and A. tsugae density) were found to significantly affect T. canadensis dieback rates. Adelges tsugae densities were positively influenced by the length of T. canadensis new growth. When comparing release and non-release sites, biological control beetles were not found to significantly affect T. canadensis dieback rates or A. tsugae densities.