Canadian Hemlock Slowly Vanishing - NYT

"When I drive down the rural roads near my house in northwestern Connecticut, I cannot help but keep my eyes open to what is happening in the surrounding woods (and in other people's gardens). And with all of the talk lately about the demise of Canadian hemlocks, I am constantly assessing the health of these beloved trees. It is deceptive to think that they are fine just because one still sees them along the roadside.

Their enemy is the wooly adelgid (AH-dell-jid), a tiny insect that poisons hemlocks while feeding on their needles. The infestation is long and protracted - a thinning of the needles, a period of regrowth and then, most often, a slow decline, taking more than 10 years to reach its sad conclusion.

Michael Montgomery, a research entomologist for the United States Forest Service at the Center for Forest Health Research in Hamden, Conn., explains that, unlike most pests and diseases, the wooly adelgid thrives on healthy trees, not weak ones. Botanists have given this unusual syndrome a fittingly unwieldy name: negative density dependent feedback.

The Canadian hemlock, Tsuga canadensis, is an important element of the Northeastern landscape - not only aesthetically because of its drooping, densely needled branches - but also for its role in providing shelter and sustenance for a variety of wildlife. After emigrating from Asia to the West Coast in the 1920's, the adelgid made its way east, decimating trees from Maine to North Carolina. Right now the only solutions are to spray pesticides or horticultural oil, hardly an option for an area as large as the Adirondacks, let alone the East Coast.

One really can't help but question what will be the end result of this loss to our local landscape. And last fall, at a meeting of horticulturists in New York, I was given some surprising answers by Peter del Tredici, a senior research scientist at the Arnold Arboretum (Harvard's botanical garden). While expounding on nature's complicated relationship with the garden, Mr. del Tredici examined how nature confounds us with its unpredictable moves forward. Forecasts typically envision the hemlock's demise and replacement by similar trees, a conifer for a conifer, so to speak. So I assumed he would describe the spread of native white pine. But Mr. del Tredici pointed to a different turn of events. As hemlocks recede, the black birch and a variety of other deciduous trees are beginning to make their appearance, often accounting for more than 90 percent of the woody stock replacing the Canadian hemlock in the landscape."

EDIT