Hemlock Hill Management and Research
Hemlock Hill Research Tree Removals: Winter 2004/2005
The hemlock woolly adelgid, an insect native to eastern Asia, has been feeding on hemlock trees in Massachusetts since the mid 1990s. First discovered on the Arboretum’s Hemlock Hill in 1997, this insect has forced many trees into severe decline and eventual mortality. The Arnold Arboretum is conducting research on Hemlock Hill to investigate the effects of hemlock woolly adelgid on forest ecosystems and management.
As part of its research program, the Arnold Arboretum removed approximately 120 hemlock trees during the 2004/2005 winter. Since the spring of 2004, the areas in which these trees were located had been designated as research plots and enclosed with six-foot-high vinyl deer fencing to prevent disturbance of the soil and vegetation. Since the trees’ removal, we have begun to track patterns of revegetation and changes in soil chemistry in order to better understand the impacts of hemlock mortality on the Arboretum environment. We anticipate that the study will conclude in the fall of 2007.
Hemlock Hill Management Plan
In 1997, the hemlock woolly adelgid (HWA) was discovered on the Arboretum's Hemlock Hill. This tiny insect, a relative of the aphid, feeds with lethal effect on the hemlock trees of eastern North America. Native to eastern Asia, HWA was first detected in Virginia in the early 1950s and has since spread throughout the mid-Atlantic and southern New England. Sadly, researchers studying the insect have observed very high mortality rates within infested forests.
Arboretum staff has monitored the condition and rates of decline of roughly 1,900 eastern hemlocks (Tsuga canadensis) on Hemlock Hill since 1997. It is believed that almost 100 percent of the population is infested with HWA. Over the past six years, nearly 350 trees have died and been removed. Thankfully, a number of hemlocks growing along the base of the Hill can be reached with a spray truck. These trees have been treated annually with horticultural oil and remain in relatively good condition; we hope to maintain their health far into the future. Unfortunately, extreme topography and lack of road access prevent the treatment of the remainder of the Hemlock Hill population. Of these trees, over 70 percent are in a state of severe decline and will probably die within the next two to ten years.
After several years of preparation, the Arboretum’s management plan for Hemlock Hill was completed in early 2004. As outlined below, the plan integrates ecological stewardship with our goals of public access and safety. It also includes research programs that will contribute to larger efforts to effectively manage HWA-infested forests.
- Removal of hazardous trees . We will continue to remove hemlocks that are in severe decline and present a safety hazard.
- Preservation of hemlocks along the base of Hemlock Hill . We will continue our work to preserve these trees.
- Native regeneration . We will encourage the growth of native tree species by removing competing invasive plants. We envision for the future a mixed deciduous and evergreen forest in areas where hemlocks have been lost to HWA.
- Research Programs on Hemlock Hill . The HWA infestation offers an unusual opportunity to study the environmental impacts of hemlock death and hemlock removal in an urban environment, including changes in soil nutrient cycling and microclimate, as well as the spread of invasive species during the period of vegetative succession. To support research on these changes, the Arboretum established three large study plots on Hemlock Hill during the spring of 2004. Comprising a total of roughly 2,000 square meters, these plots (each containing two subplots) were fenced in while their hemlocks were still standing to prevent soil and vegetation disturbance.
During the remainder of 2004, we collected baseline data on soil characteristics and vegetation composition on the six subplots. Approximately 120 hemlock trees, between four and 30 inches in diameter, were removed from four of the six subplots during the winter of 2005, and two of the subplots remained as uncut controls. On two of the subplots where hemlocks were removed, slash was chipped and left onsite to determine its impact on soil characteristics and subsequent vegetation development, while the other two were left bare. - Simultaneously, we began investigating the resistance of Chinese hemlock (Tsuga chinensis) to HWA. This species, which was planted on Hemlock Hill in 1999, appears to be extremely resistant to HWA.
This research is a collaboration between the Arboretum and the Harvard Forest , a research institute for the study of forest ecology. For more information, contact Richard Schulhof at 617.384-5241 or hemlocks@arnarb.harvard.edu.
The Arnold Arboretum is operated through a public/private partnership between the City of Boston and Harvard University. For additional information about hemlock woolly adelgid, visit the Harvard Forest website.