| Abstract | The eastern hemlock (Tsuga canadensis) is a shade-tolerant, late-successional conifer that provides unique riparian and upland habitat in the deciduous forest landscape (DeGraaf et al. 1992). Hemlocks influence microclimate, soil chemistry, and forest floor environment, and contribute to regional diversity (Benzinger 1994). The eastern hemlock may be particularly important in the mid-Atlantic region, where it is one of few native conifers found within the eastern deciduous forest and a major component of many remaining old-growth forest stands.
Because the eastern hemlock is a long-lived "climax" species, some plants and animals may have evolved in association with hemlock stands (Benzinger 1994). For example, several bird species, such as solitary vireo (Vireo solitarius), black-throated green warbler (Dendroica virens), and blackburnian warbler (Dendroica fusca) depend on, or strongly prefer, hemlock habitats (Benzinger 1994). Water shrews (Sorex palustris), a species of special concern in the northeast, also have been found to be closely associated with hemlock stands (Sciascia and Pehak 1995). At Shenandoah National Park (Shenandoah NP), the federally endangered Shenandoah salamander (Plethodon shenandoah) occurs in dense hemlock stands (Mitchell 1991, Watson et al. 1994). In addition, plant species, such as painted trillium (Trillium undulatum), grow primarily under the canopy of hemlock trees (Radford et al. 1968). Because riparian hemlocks shade adjacent waters and help to maintain cool stream temperatures, many streams that support naturally reproducing populations of brook trout (Salvelinus fontinalis) are associated with hemlock forests (Ross and Bennett 1995). Aside from their ecological qualities, eastern hemlock stands are highly valued for their aesthetic and recreational appeal.
At Shenandoah NP, the aesthetic, recreational, and ecological values of hemlock stands are threatened by the hemlock woolly adelgid (Adelges tsuga Annand; HWA). The HWA was first detected in Shenandoah NP in the winter of 1988 and now infests all eastern hemlock stands at Shenandoah NP, causing significant mortality of hemlock trees (Watson et al. 1994). Biodiversity associated with hemlock stands could be at risk if the current trend of HWA infestation and resulting mortality continues. The National Park Service endeavors to protect and maintain the natural heritage of its lands, particularly under the threat of an invasive exotic species such as the HWA.
Baseline information on the biotic components of hemlock ecosystems is fundamental to the protection and restoration of biodiversity and to the maintenance of ecosystem dynamics in hemlock stands at Shenandoah NP. The objectives of this study were to collect information on the biotic components of hemlock ecosystems at Shenandoah NP and to test the use of the ecosystem profile method (Mahan et al. 1998) in estimating biodiversity. Investigating entire natural communities is a formidable task because one must take a high diversity of species into account. To focus on a more manageable unit, ecologists usually restrict their attention to some component of the larger system such as guilds (Root, 2001). The guild concept defined by Root (1967) provides a manageable, functional unit for studying organization of natural communities. |
