Fading tree-crown color, from robust deep greens to grayish tones, led to the discovery of the infestation in Thornton and Frazier Hollows, and by 1993 the insect was found in hemlocks throughout the park. Since 1991, formerly full-crowned trees (77% of the population) have been reduced to thin, partial crowns with much associated mortality. Interestingly, this negative trend slowed down during 1996–98, which led the park and researchers from the USGS Biological Resources Division to surmise that the harsh winter of 1995–96 contributed to adelgid decline. However, two recent, mild winters and a drought in 1998 may cause further hemlock decline, given that moisture stress hinders the hemlock’s ability to resist effects of the insect.

The park and the Biological Resources Division are working together to ferret out the risk factors associated with the infestation and to create a model that predicts associated mortality. Using Geographic Information Systems and statistical analyses of the park’s annual hemlock crown assessment data, the team found areas of higher risk potential. Heavier impacts seem to occur at lower elevations, probably indicating the importance of winter cold in controlling the insect. They also found that slope, light conditions (such as site aspect and position), and distance to streams correlated with hemlock condition. The results of this preliminary analysis are helping to guide 1999 research, which will look at several years of imagery and weather and climate data to correlate and model hemlock stand vulnerability as a result of site, landscape, and regional factors.

While research is aimed at understanding factors contributing to the infestation and decline, resource management activities seek to protect the tree species. Shenandoah has adopted the following objectives: (1) preserving a seed source for future hemlock reestablishment, (2) preserving individuals in important cultural sites and recreation areas, and (3) reducing the risk of falling trees in highly visited areas. In FY 1998, recognizing the dire situation of the hemlock forest, resource managers increased the frequency and breadth of their efforts to suppress the hemlock woolly adelgid. Nevertheless, treatments are still limited to areas accessible by vehicles equipped with hydraulic spray equipment, which are used to apply insecticidal soap or horticultural oil. This approach is employed by other federal agencies too. High-value individual hemlocks located far from roads may be treated with systemic insecticides on a case-by-case basis.

With such limited treatments, the long-term hope for the eastern hemlock in Virginia may be the introduction of natural, albeit nonnative, biological controls. The U.S. Forest Service and the Connecticut Agricultural Experiment Station are carefully testing several Asian insects on the East Coast with such a purpose in mind.

james_akerson@nps.gov
Forest Ecologist, Shenandoah National Park, Virginia

ZEBRA MUSSELS AT ST. CROIX
Following another year of active monitoring, education, access management, and research, the St. Croix National Scenic Riverway (Wisconsin and Minnesota) is still considered free of
reproducing populations of the exotic freshwater zebra mussel, which threatens native mussels. During the year there was no duplication of the 1997 finding of nearly 50 juvenile zebra mussels on a plate sampler, although the species was again
discovered on boats and sporadically on native mussels and other hard substrates.

Back to Chapter 4: Resource Disturbances

Jean Lafitte learns from 3-D seismic oil exploration experience
by Sandee Dingman

Parks cultivate partnerships to tackle noxious weeds
by Jeff Connor and Greg Waters

At what cost? Deciding whether to control exotic plants
by Sue Rutman

Source of chemicals that feminize Lake Mead fish discovered
by Roy Irwin

Protecting the natural “soundscape” in parks
by William B. Schmidt

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