Tsuga canadensis (eastern hemlock) in the southern Appalachian mountains
Kincaid, Joshua Andrew
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The goal of this research is to provide baseline data on the compositional characteristics, regeneration dynamics, and disturbance regimes of Tsuga canadensis forests in the southern Appalachian Mountains prior to large-scale forest alterations induced by climatic changes and hemlock woolly adelgid (Adelges tsugae Annand.) infestation. To achieve this goal, I established 50 plots within mature Tsuga canadensis forests of Great Smoky Mountains National Park (GSMNP), a landscape of biogeographical importance because it is situated near the southern range terminus of Tsuga canadensis. Cluster analysis identified four Tsuga canadensis forest types, which differed compositionally and environmentally in terms of slope aspect, litter depth, tree species richness and diversity. Detrended correspondence analysis (DCA) identified landscape-scale gradients in Tsuga canadensis forest composition and regeneration, reflecting differences in soil nutrient availability and sand content, and geographical location. Models relating Tsuga canadensis regeneration to site factors emphasize understory Rhododendron maximum cover, litter depth, soil nutrient availability and sand content as important controls on seedling and sapling abundance. Local indices of dispersion indicate significant within-plot spatial aggregation of regeneration. Models relating the local indices of dispersion to site factors show that the spatial aggregation of regeneration is controlled by soil nutrient availability, canopy disturbance, and geographical location. These results elucidate the contrasting suites of forest-environmental factors controlling the abundance and spatial pattern of regeneration, and that these controls shift in importance as seedlings and saplings grow larger. Forest-environmental characteristics from a subset of four forest stands in the Cataloochee Watershed of GSMNP show that sites with successful Tsuga canadensis regeneration are located above 1000 m in elevation on well-drained, moderately steep slopes and have the greatest canopy openness. Structural characteristics and disturbance regimes at these four forest stands elucidate a history of more or less continuous regeneration at sites where Tsuga canadensis seedling and sapling establishment is currently successful. Moreover, disturbance chronologies show that forest stands with successful Tsuga canadensis regeneration are disturbed more frequently by canopy disturbance events, especially those of moderate intensity.