Hemlocks Could Disappear From New England Forests

Eastern hemlock trees might be gone from New England forests by the end of the century, thanks to an introduced insect and warming climate.

Already the hemlock woolly adelgid has nearly eliminated hemlocks from southern Connecticut and Virginia's Shenandoah National Park.

Scientists from the University of Massachusetts predict that as winter temperatures become milder, the adelgid will also kill off vast tracts of hemlock in northern states.

Since arriving in Virginia from Japan over 50 years ago, adelgids have spread from northern Georgia to southern Maine. Like an aphid, the tiny insect sucks on sap at the base of tree needles. This weakens hemlocks; they drop needles, turn gray and often die within six years.

At the northern extent of the adelgid's range, infested eastern hemlocks can survive a decade or longer, and the insect's expansion has slowed in Massachusetts. Cold winters so far hold the destructive insect at bay along the southern borders of Vermont, New Hampshire and Maine, and keep it out of upper-state New York.

Researchers have now determined the extent to which adelgids can survive freezing temperatures, and with this new information mapped how far the invasion will spread as the climate warms. Adelgid populations remain too small to expand when from December to March the temperature averages 23 °F (-5 °C), or reaches a minimum of 14 °F (-10 °C) for 79 days, or drops to -31 °F (-35 °C).

As winters become milder, this insect will hop northward and up hillsides to higher elevations. By 2100, winters could be warm enough for hemlock woolly adelgid to thrive throughout hemlock's range in the northeastern United States.

Not only will hemlocks then disappear from the landscape, but their current dominance of New England forests means that whole ecosystems will be transformed. Creatures that only live alongside eastern hemlocks, like the black-throated green warbler and several aquatic invertebrates, will also vanish.