An alien insect is causing decline in eastern hemlock forests, leading to the loss of native biodiversity, and opening the way for invasions of alien plants.
Hemlock woolly adelgid (Adelges tsugae) is an aphidlike insect native to Asia that feeds exclusively on hemlock (Tsuga spp.) trees. First documented in Richmond, Virginia, in 1951, hemlock woolly adelgid (HWA) now occurs in 13 states, from Georgia to New Hampshire. During the past decade, HWA has been associated with widespread, severe decline and mortality of eastern hemlock (T. Canadensis) trees. The insect also debilitates Carolina hemlock (T. caroliniana), the other hemlock species native to the eastern United States. The geographic range of Carolina hemlock is limited to the southernmost Appalachian Mountains, which has just recently been infested by HWA. Examples of National Park System areas affected by HWA include Great Smoky Mountains and Shenandoah National Parks, New River Gorge National River, Catoctin Mountain Park, and Delaware Water Gap National Recreation Area.
Eastern hemlock is an ecologically important and influential conifer that for thousands of years was a major component of forests over much of the eastern United States. It is an extremely shade-tolerant species, and with appropriate climatic and site conditions forms nearly pure stands that can persist for hundreds of years. Hemlock-dominated forests create characteristically dark, acidic soil conditions that control and limit fundamental ecosystem characteristics such as plant and animal species composition, productivity, nutrient cycling, decomposition, and succession dynamics. During the past 400 years, the distribution and abundance of eastern hemlock was dramatically reduced by land clearing and logging, especially for the tanning industry, which utilized the tannic acid contained in the bark.
The decline and loss of our remaining eastern hemlock stands could be more ecologically significant in some respects than the loss of American chestnut (Castanea dentate) in the early 1900s because of chestnut blight. Following the demise of American chestnut, an array of native oak and hickory species naturally expanded, and have functioned as “ecological surrogates” for chestnut, providing habitat and mast (fruits and nuts) critical to many species of wildlife. In contrast, the species most likely to expand in declining hemlock stands include deciduous trees, white pine (Pinus strobes), and invasive alien plants like “tree-of-heaven” (Ailanthus altissima), Japanese barberry (Berberis thunbergii), and Japanese stiltgrass (Microstegium vimineum) (Orwig and Foster 1996, Battles et al. 1999). These species will not provide habitat or ecological functions resembling those of eastern hemlock (fig. 1).
Evans, R. A. 2004. Hemlock woolly adelgid and the disintegration of eastern hemlock ecosystems. Park Science 22(2):53–56.
Available at http://www.nature.nps.gov/ParkScience/archive/PDF/Article_PDFs/ParkScience22(2)Fall2004_53-56_Evans_2438.pdf.