- Lessons LearnedManaging Biological Invasion on Hemlock Hill (Massachusetts)
Hemlock woolly adelgid (Adelges tsugae; HWA), an introduced organism, has decimated the native eastern hemlock (Tsuga canadensis) across much of its range in North America. In 1997, after decades of slow northward spread, HWA was detected at the Arnold Arboretum in Boston, Massachusetts.
The 106 ha Arnold Arboretum, founded in 1872, contains more than 15,000 scientific plant specimens and attracts over 250,000 annual visitors. While this National Historic Landmark is internationally renowned for its extensive collections of rare trees, in Boston it also well known for Hemlock Hill, a magnificent natural site that is home to the city's largest stand of eastern hemlock. More than a natural area, the 8.8 ha Hemlock Hill is an historic site whose early public use included frequent visits in the 1840s from Margaret Fuller and other members of the Transcendentalist circle. While most of the hill is believed to be second growth forest dating to the early to mid-19th century, younger hemlock on the north-facing slope were planted shortly after the 1938 hurricane. With few trails and minimal management, Hemlock Hill has long been appreciated as a place of seemingly wild nature in the midst of the city.
Today, the Arboretum's Hemlock Hill is a landscape transformed by HWA. Large gaps mark the loss of trees, while some survivors, greatly diminished by infestation, stand as relics among growing swaths of successional vegetation. A great many other hemlock trees, beneficiaries of recent protection treatment, offer strong signs of recovery.
Our ten years of managing HWA is a story of decision-making in a rapidly changing informational environment. We began with many uncertainties and traveled a path of pivots and about-faces led by growing knowledge of our own site, analysis of outcomes elsewhere, and key findings from the research community.
The Arboretum was hardly among the first sites to deal with HWA. First detected in Richmond, Virginia, in the [End Page 129] early 1950s, HWA spread rapidly, impacting hemlock populations in the Mid-Atlantic before reaching southern New England. Across much of the range of infestation, the ultimate consequence of HWA was near to complete hemlock mortality within 4 to 12 years. There were few exceptions. With the prospect of losing one of Boston's most significant natural sites and an integral part of local history, Arboretum managers addressed challenges of a scope not seen since the 1938 hurricane.
The process began with questions. What would be the rate of decline for our hemlocks? How many trees could we protect and at what costs to the larger ecosystem? Could a biocontrol under development save our trees? Although these and other questions would remain unanswered for years, management goals drawn from our organizational mission provided a strong compass for initial decision-making. Protecting visitor and staff safety, protecting the larger environment, and preserving a still undetermined number of hemlocks were our key priorities. But where to start?
We determined that obtaining reliable, site-specific information about the spread of the infestation and rates of hemlock decline would be essential to planning an effective management response. Monitoring the health of our hemlocks required mapping locations and assigning an accession number for each tree, a process accomplished over the course of several months with GPS survey equipment and BG-Base, a database designed for management of living botanical collections (BG-Base, Topsham ME). This significant investment of staff time was abundantly repaid in data that detailed the progression and severity of the infestation, as measured by crown health. We found that from 1998 to 2002, the number of trees in poor health increased from 30% to 70%. By 2003, Hemlock Hill was a sickly grey-green color. Data from other sites indicated that we could expect large numbers of hazardous and dead trees within two to three years.
That winter, we visited forests in Connecticut that had been closed to the public because of hundreds of disintegrating dead hemlocks. Further, we learned that the hazardous snags had not only precluded salvage operations but also efforts to contain rapidly growing populations of invasive plants. Foreseeing similarly grim prospects for Hemlock...