Terrain and Landform Influence on Tsuga canadensis (L.) Carrière (Eastern Hemlock) Distribution in the Southern Appalachian Mountains

We examined the relationships between hemlock distribution and abundance and terrain attributes for the Coweeta Basin in the southern Appalachian Mountains. Field measurements were combined with GIS mapping methods to develop predictive models of abundance and distribution of Tsuga canadensis (L.) Carrière (eastern hemlock) and evaluate the co-occurrence of Rhododendron maximum L. (rosebay) and Kalmia latifolia L. (mountain laurel). Terrain variables were derived from USGS DEM 30-meter digital maps. Elevation, slope, aspect, terrain shape index, landform, and distance from stream were calculated from field measurements and the digital data. Terrain attributes such as elevation (r 5 0.97, p , 0.0001), distance to stream (r 5 0.94, p , 0.0001), and terrain shape index (r 5 0.61, p 5 0.0015) were good predictors of T. canadensis abundance. Terrain shape index explained 56% of the variation in R. maximum percent aerial cover (r 5 0.56, p 5 0.005). In the Coweeta Basin, T. canadensis was distributed as few, large trees mostly concentrated in near-stream locations, and it was closely associated with R. maximum. Tsuga canadensis mortality due to Adelges tsugae Annand (hemlock wooly adelgid) will result in a minor decrease in basin-wide basal area, but will substantially reduce near-stream basal area, and will also remove the largest trees in nearstream environments. In similar landscapes across the southern Appalachians, where T. canadensis co-occurs with R. maximum, riparian shading will likely remain unchanged. INTRODUCTION Forest vegetation patterns in the southern Appalachians are related to complex environmental gradients associated with climate, soils, and topography, as well as disturbance history, from natural events and direct and indirect human actions (Harmon et al. 1983). Tsuga canadensis (L.) Carrière (eastern hemlock) responds strongly to this environmental gradient (Elliott et al. 1999). Tsuga canadensis, a long-lived, shade tolerant conifer, can persist for decades in the forest understory, live 250 to 300 yr, with the oldest examples surviving up to 400 yr (Godman and Lancaster 1990, Orwig and Foster 1998, Bonneau et al. 1999, Young et al. 2002). Tsuga canadensis grows in a wide range of topograhic positions including steep slopes, deep gorges, and riparian borders. Tsuga canadensis ranges from western Nova Scotia to southern Ontario and northeastern Minnesota, extending south throughout New England and along the Appalachian Mountains into northern Georgia and Alabama (Elias 1987). In general, T. canadensis is more abundant and evenly distributed in the northeast than in the southern and western portions of its range (Evans 1995, Williams and Schmidt 2000), where it reaches greatest abundance in cool, moist microclimates (Goodman and Lancaster 1990). In the southern Appalachians, T. canadensis distribution varies from isolated individual trees to scattered clusters, pure small stands (defined as at least five trees with adjacent canopies), or mixed stands. *email address: [email protected] Received October 15, 2008; Accepted June 3, 2009. CASTANEA 75(1): 1–18. MARCH 2010