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Wild Side: Tiny insects and changing climate threaten majestic hemlocks

A tall eastern hemlock in the North Chagrin Reservation east of Cleveland, Ohio. (River Falls Journal photo by Dan Wilcox)

Hiking through the forests of my boyhood in northeastern Ohio again last week, I was reminded of how much I enjoy standing in a grove of stately hemlock trees. The fine needles and dense canopy of a stand of large hemlocks form a sheltered and shaded cool zone. In addition to being cathedral-like places for contemplation, groves of large hemlock trees provide important winter habitat for white-tailed deer, black bears and ruffed grouse.

Eastern hemlock is a native evergreen tree with short flattened needles on thin twigs. Each needle has two pale stripes on the underside. The narrowed leaf base distinguishes it from the similar leaves of balsam fir. 

Eastern hemlocks have the potential to grow very large and often occur in dense stands where the ground layer is covered with needles nearly bare of other vegetation. Hemlocks are slow-growing long-lived trees that grow well in shade. Hemlocks may take 250 to 300 years to reach maturity and may live for 800 years or more. A tree measuring 76 inches in diameter at breast height and 175 feet tall is among the largest recorded.

Eastern hemlocks occur throughout the northeastern two thirds of Wisconsin, but they don’t occur in our immediate area in the western part of the state. They range into northeastern Minnesota and east to Michigan, Newfoundland and south in the Appalachian Mountains into northern Alabama and Georgia. 

Eastern hemlocks are an important part of northern mixed forests, occurring with yellow birch, white pine, white spruce, balsam fir, sugar maple and Canada yew. Near the Great Lakes, in eastern Wisconsin along Lake Michigan, in the Upper Peninsula of Michigan near Lake Superior, and in northeastern Ohio in valleys near Lake Erie, eastern hemlock occur in tall stands mixed with beech, red oak, and sugar maple.

Hemlocks were logged heavily up to about 1910 for lumber and for the bark that was used for tanning leather. Regeneration has been slow following logging. Although they regularly produce many cones, their seeds and seedlings are small and delicate, easily killed by dry conditions and browsing by deer, mice, voles and squirrels. Seedling hemlocks are very shade tolerant, however, and can grow very slowly — patiently waiting for an opening in the canopy to let in some light.

Today eastern hemlocks are under attack by an exotic sap-sucking insect from Asia, the hemlock wooly adelgid. First observed in 1988, this insect has caused widespread mortality to hemlocks in the Appalachians and threatens to eliminate all the hemlocks from forest ecosystems in the eastern U.S. Efforts are underway to extend the lives of remaining healthy hemlock trees in some areas through applications of a systemic pesticide and introduction of adelgid predator insects.

It’s a real tragedy to see the huge trunks of dead hemlocks killed by wooly adelgids criss-crossing the trout streams in the mountains of North Carolina. Those streams have lost a good bit of the shade they need to keep the water cool and habitat conditions suitable for native brook trout.

Another threat to eastern hemlocks is climate change. The increasing frequency of drought restricts hemlock regeneration, killing the delicate seeds and seedlings. The increasing frequency of severe wind storms damages groves of hemlock by snapping them off and by wind throw of the shallow-rooted trees. One of my favorite hemlock groves near the Flambeau River in our deer hunting area in Wisconsin got clobbered by two wind storms in the last decade.

If the big hemlock trees could speak to us, what would they say?

Please send any comments and suggestions for this column to me at

--Dan Wilcox, outdoor columnist