What Comes After Hemlock Woolly Adelgid?

Eastern hemlock stand along North Carolina stream. Photo by Paul Bolstad, courtesy of Bugwood.org.

The hemlock woolly adelgid, an exotic invasive insect that feeds on eastern and Carolina hemlocks, now occupies about half the range of native hemlock forests in the eastern United States. Once infested, hemlocks lose vigor and die within a 4 to 10 years. Most managers and scientists accept that as time goes on native hemlocks will mostly disappear from eastern forests. Many also wonder and speculate about what type of forest landscape will emerge as hemlocks die out.

Doug Streett, project leader of the Southern Research Station Insects, Diseases, and Invasive Plants unit, works with Texas A&M University colleagues on simulation models designed to predict and provide visualizations of the long-term effects of hemlock dieback. One project focuses specifically on the Southern Appalachians, an area where hemlock functions as the keystone species, providing habitat for various terrestrial organisms and birds as well as the shade that keeps mountain streams cool and suitable for coldwater aquatic species.

For a recent study, the scientists used LANDIS-II, a forest landscape model capable of simulating real species in real landscapes. LANDIS-II uses maps of current vegetation, ecological zones, and disturbances (e.g. wind, fire, insect and harvest effects) and simulates the dispersal, growth, and death of interacting tree and shrub species to determine forest succession.

Using the Grandfather Ranger District on the Pisgah National Forest in North Carolina as the area of study, the researchers used LANDIS-II to examine 11 distinct ecological zones, focusing on 36 tree species. LANDIS-II simulated the results for two possible future scenarios: one with hemlock trees and the other without them. The simulation showed that in a landscape without hemlocks more shade-intolerant trees such as pines and oaks grew, leading to a change in forest composition. There were conflicting findings for rhododendron and mountain laurel, the two main understory shrubs found in the study areas. The study showed very little change in abundance of the rhododendron, while mountain laurel showed a significant increase in abundance. 

Forest managers can use the information obtained by using LANDIS-II and real world observation studies on hemlock decline to plan effective forest restoration strategies. These might include the introduction of hemlock species resistant to the hemlock woolly adelgid and the control of encroachments of mountain laurel and other vegetation resulting from the decimation of native hemlocks.

Read more about the project.  

For more information: Doug Streett at dastreett@fs.fed.us

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