Research on decline of eastern hemlock (Tsuga canadensis) at the Aquatic Ecology Laboratory
Eastern hemlock (Tsuga canadensis) occurs in cool, moist, hillside and ravine environments throughout the eastern United States. Hemlock stands and forests are valued as riparian and forest habitat and as a commercial timber and horticultural species. Hemlock stands are also frequently targeted as desirable recreational areas on public lands because of their distinctiveaesthetic, recreational, and ecological qualities. In the last two decades, substantial declines in eastern hemlock have been observed throughout its range, resulting in extensive Federal and state concern . Widespread hemlock defoliation and mortality has largely been attributed to the hemlock woolly adelgid (aphid-like Adelges tsugae, HWA), an exotic insect that is native to Japan. Resource managers expect the adelgid to continue to spread and consequently the entire hemlock ecosystem may be forest threatened. The impact of the removal of this important climax forest species on the ecology of Appalachian forests is poorly understood, but has the potential for significant disturbance to biotic communities by changing the energy inputs, micro-climatic environments, and physical habitat structure available to other vegetation, bird, mammal, and aquatic communities. Consequently, there is an urgent need to characterize the contribution of hemlock forests to biological diversity and functional stability in large, forested landscapes , and to identify contributing or ameliorating environmental conditions (both abiotic and biotic) that influence hemlock decline. Such information could provide the basis for future restoration strategies and serve as indicators of potential risk to hemlock forests not yet infested. At the request of the National Park Service, the Leetown Science Center (LSC) intiated several studies that are designed to determine the potential long-term consequences to aquatic invertebrates, fish, and bird communities due to hemlock forest decline. The overall goal of this research is to discern the influence of hemlock forests on biotic communities, and to provide a landscape perspective on what we stand to lose in terms of biological diversity should hemlock forests die. In Delaware Water Gap National Recreation Area (DEWA), we conducted a comparative study designed to determine the potential long-term consequences to aquatic invertebrate and fish communities due to hemlock forest decline. We began by conducting a landscape analysis of the Park using Geographic Information Systems (GIS), and used the results to select 14 hemlock and hardwood site-pairs that were similar in topography (i.e., slope, terrain shape, aspect, light levels) and stream size but differed in forest composition (hemlock vs mixedhardwood). This paired watershed approach provided a powerful means to discern the influence of hemlock forests on stream communities. We found aquatic invertebrate diversity to be strongly influenced by forest composition. Specifically, streams draining hemlock forests supported on average 37% more taxa than streams draining hardwood forests, though the significance and magnitude of the forest effect depended on stream type (as determined by terrain characteristics and stream size). In addition, 10% of invertebrate taxa encountered in DEWA occurred significantly more often in streams draining hemlock. In contrast, total invertebrate densities and the probability of occurrence of rare taxa were higher in streams draining hardwood forests. Trophic composition also differed between forest types with hemlock-dominated watersheds supporting more predators and fewer scrapers (algivores). This suggests that stream ecosystem function (e.g., rates of nutrient and carbon processing) might also differ between forest types (follow the links at left to down load the full report). In a follow-up study, we are conducting landscape analyses of Shenandoah National Park using GIS and remote sensing to assess possible landscape factors influencing the rate of decline of eastern hemlock. In Shenandoah National Park (SHEN), hemlock defoliation and tree mortality have advanced at such a rapid pace that near complete mortality of some hemlock stands occurred within 3-4 years, while other stands have been less severely impacted. In other areas, such as Delaware Water Gap National Recreation Area, the adelgid is present in large numbers but no significant mortality of hemlocks has been observed. The patchy nature of hemlock decline suggests that landscape-level processes may affect hemlock mortality either by regulating the dispersal potential of hemlock wooly adelgid, or by affecting the sensitivity of the trees themselves. Consequently, prevailing winds, number and distance to roads, proximity to migrating bird habitats, and stand composition, edge, and topography may contribute to hemlock wooly adelgid dispersal and ultimately to the extent of mortality. Likewise, sub-optimal habitat characteristics such as low soil moisture may be related to landscape variables such as aspect and elevation, which may act synergistically with hemlock wooly adelgid infestation to induce mortality. Through this study, we are attempting to assess the relationship between hemlock stand mortality and landscape features. This information will be used to develop empirical models designed to predict stand vulnerability from landscape attributes. Our goal is to provide tools to National Park managers that can be used to design pro-active strategies in areas where hemlock wooly adelgid has not spread such as Great Smoky Mountains National Park (GSNP), or where infestation is recent and significant mortality has yet to be evidenced, such as Delaware Water Gap National Recreation Area. More details on our research can be found by following the links at left. For further infomation, contact: Craig Snyder or John Young USGS-BRD, Aquatic Ecology Laboratory 1700 Leetown Road Kearneysville, WV 25430 (304) 724-4460		Hemlock Decline Research at AEL: Studies in Delaware Water Gap National Recreation Area Studies in Shenandoah National Park Quick Links: Stream ecology linked to threatened hemlocks in Delaware Water Gap National Recreation Area A Landscape-based sampling design to assess biodiversity losses from eastern hemlock decline Landscape correlates of hemlock decline in Shenandoah National Park (Adobe .PDF) Satellite image mapping of hemlock decline in Shenandoah National Park In partnership with: