November 30, 2006

A Short History of the Eastern Hemlock, Part I

About 16,000 years ago, glaciation from the last ice age finally began to retreat after millennia of occupation. As the glaciers melted and filled scrapes in the landscape with fresh water, the animals and plants followed, once only able to live in the temperate climes of southern North America.

The eastern hemlock (Tsuga canadenis) was one of these pioneers, albeit a slow, steady one. Spreading north at about 100 - 400 meters per year (incidentally about the same rate of large ungulates like elk), the hemlocks wouldn't reach the extent of their expansion, around the glacier-crafted Great Lakes, until about 2,000 years ago.

Over the Thanksgiving break, I finally got a chance to do a little exploring. I took the fam up to Laurel Hill State Park in Western PA to hike through the remnants of an old growth eastern hemlock forest set aside for environmental education.

It's tough to find old growth forests anywhere in the eastern US nowadays, especially old growth areas dominated by hemlock. Laurel Hill has a small but beautiful remnant of the tree's unique habitat.

Hemlocks typically grow on cool, moist, north-facing slopes along streams. The density of these trees tends to diminish as you progress up slope.

DOWNSLOPE Laurel Hill Creek runs through the hemlock
stand in the valley bottom.

UPSLOPE Notice the hardwood forest beyond the hemlock
stand. Hardwoods like red maple prefer the drier conditions
of higher altitudes.

Hemlocks are conifers, but produce small cones relative to the height/breadth of a mature tree. They are a long-lived species, maturing in 300 years, able to reproduce for 450; the oldest recorded hemlock was 988 years old.

In Laurel Hill, most of the hemlocks were young, especially around the periphery, but once you reached the center of the forest, there stood the remnants of an old stand, trees reaching 100+ ft., alive since William Penn roamed what would become Pennsylvania.

ANTIQUITIES These trees in the center of the stand are
over 300 years old.

Hemlocks are a part and sometimes a progenitor of a very special understory habitat and support a variety of other organisms. Next post I'll talk about some more of the ecology of the eastern hemlocks, including what are called "ecotones," the animal and plant species hemlocks support and why it is essential to protect and preserve these ancient conifers.


Molles, M.C. (2005). Ecology: Concepts and applications. New York, NY: McGraw-Hill.


  1. Thank you for sharing this! I have a tough time picking a favorite tree in this world, but hemlocks (especially Western hemlocks) are right up there at the top of my list!

  2. Jeremy, if you ever have the time and inclination to drive a little farther north in PA, I'd be happy to show you around some other remnant old-growth hemlock forests, especially those in the Bald Eagle and Rothrock State Forests. But you wouldn't want to delay too much longer, since the woolly adelgid has reached our area now. All the old trees have been killed off in some old-growth hemlock stands to the south of us, such as Sweetroot Natural Area and The Helocks Natural Area.

    Good post. I'm looking foprward to Part 2.

  3. (Oops, that's "The Hemlocks Natural Area.")

  4. "...alive since William Penn roamed what would become Pennsylvania."

    Realizations like that always fill me with's like a living link back into history.

    I'm visiting from the Festival of the Trees. Thanks for an excellent and informative post.

  5. I live on the western slope of the Sutton mountains in Québec 10 miles north of the Vermont border and we have the exact same pattern as observed at Laurel Hill, the Hemlock stand is situated on and at the bottom of the slope where small streams start, while the red maple dominates up on the plateau dryer and more exposed to the south . Our Hemlocks are pretty large, had unfortunetaly to cut out a dozen to clear our building sight (put them back in the house as planks), but managed to keep most of the stand (80%) intact, which is where the larger ones are. I've noticed that a very tasty wild mushroom "golden chanterelle" grows in symbiosis with the more mature Hemlock.