Forest Ecosystem Response to Hemlock Woolly Adelgid ( Adelges Tsugae) in the Southern Appalachian Mountains

The forests of eastern North America are faced with the loss of Tsuga canadensis (eastern hemlock) as the introduction of Adelges tsugae (hemlock woolly adelgid, HWA) an invasive insect native to Japan, has resulted in widespread T. canadensis mortality. Tsuga canadensis ranges from the southern Appalachian Mountains to the Great Lakes and into Canada. The dense canopies of T. canadensis create a cool, damp microclimate, and these conditions, combined with the tannin chemistry and nutrient poor litter of the species, slows decomposition, creating acidic nutrient-poor soils. As a result, understories under T. canadensis trees are typically species poor. Due to its control over the microclimate and edaphic conditions, T. canadensis is considered a foundation species. HWA was introduced to the United States in the 1950s and feeds on the ray parenchyma cells of T. canadensis, causing defoliation and death. Since its introduction, HWA has spread throughout much of the range of T. canadensis, reaching Great Smoky Mountains National Park in 2002. HWA has caused widespread mortality across its introduced range. In order to document changes in southern Appalachian forests that have resulted from this mortality, long-term vegetation monitoring plots were installed in 2003 prior to widespread infestation. These plots were resampled in 2008/2009 and 2017. Chapter 2 of this thesis focuses on forest community shifts across ecological gradients and changes in species diversity that have occurred in response to the loss of T. canadensis. Non-metric multi-dimensional scaling revealed that herbaceous-layer species composition in northern hardwood, acid hardwood, montane cove, and hemlock forests tended to converge through time with changes occurring along gradients of decreasing elevation, sapling density, and R. maximum basal area, and increasing total potential incident radiation and pre-HWA T. canadensis importance value (IV). The seedling and sapling strata of plots with greater pre-HWA IV of T. canadensis and lower basal area of R. maximum generally exhibited greater compositional change between 2003 and 2017. Changes in the overstory stratum were relatively unidirectional. Species richness, evenness, and Shannon-Wiener diversity changed in the herbaceous-layer and the seedling and sapling strata, with the degree and direction of change varying with strata. Chapter 3 examines changes in soil chemistry that have occurred following HWA based upon soil samples collected from vegetation monitoring plots. Between 2003 and 2017, pH and the concentration of Mg increased while CEC and concentration of P decreased. Percent saturation of H decreased while percent saturation of Ca, Mg, and K increased between 2003 and 2017. The soil chemistry of plots across a range of pre-adelgid T. canadensis importance is become more similar to that of soils associated with hardwood species.

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