Ten species of hemlock tree exist worldwide, with four of those species native to North America (NA). The eastern United States is home to two of the native NA species, Canadian or eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana).
Eastern hemlock’s range expands from the northern Georgia mountains and Alabama northward to Canada. Eastern hemlock tends to thrive in moist, protected cove forests and riparian (associated with rivers and streams) zone environments. It grows on northern or eastern elevated slopes ranging from 2,000 to 5,000 feet as well. Additionally, Carolina hemlocks grow as dispersed populations of fewer trees along the Appalachian Mountains of northeast Georgia, western North Carolina, northwest South Carolina, eastern Tennessee, and southwest Virginia. In South Carolina, eastern hemlock occurs in northern Oconee, Pickens, and Greenville counties along with isolated stands of Carolina hemlock. Unlike eastern hemlock, Carolina hemlock thrives on drier montane (mountainous) sites.
Though harvested as a timber species during the early settlements of colonial America, hemlock’s popularity eventually gave way to the use of the bark as a tanning agent for leather. The mountains of South Carolina still hold the randomly preserved hemlock log, stripped of its bark and left to decay.
But if hemlock is not a commercially important timber species today, and the tanning industry no longer relies on the bark, then why are both eastern and Carolina hemlock still of interest or value?
Eastern hemlock trees are often considered keystone species in a plant community. This means that other species within the given ecosystem rely heavily on these species. Eastern hemlocks can live upwards of 800 years, reach heights of 175 feet, and are often a component of remaining old-growth forests along with American Beech. Hemlocks are evergreen trees, providing shelter, protection, and nesting or bedding sites year around to wildlife. As conifers, these trees produce cones with seed supplies for small mammals, which in turn are often fed upon by raptors and other animals. Young hemlock seedlings provide white-tailed deer with food to browse, when other plant resources are low. Shallow rooted hemlocks growing along rivers and streams moderate water quality and provide shade effects at the water’s edge for aquatic insects to thrive. These insects are in turn fed on by local cold-water fish. Brook trout for example, is a nice catch for avid anglers.
Healthy hemlock trees have the potential to remain in the forest landscape for many years, providing ecosystem services to other species. Forests with a hemlock component are known to support black bear, migratory birds and owls, toads, turtles, salamanders, water shrew, grouse, turkey, snakes, bats, butterflies, and moths to name just a few.
Hemlocks are slow growing and shade tolerant trees. Both characteristics promote hemlock survival beneath the canopy of other dense overstory trees in mixed forests, but also help hemlock establish as a climax species in predominately hemlock forests. Not much grows directly beneath hemlock trees in the thick duff layer that forms below. However, the surrounding forest floor created by hemlocks is home to many treasured woodland plants and unlimited microorganisms. These are just a few attributes that make hemlock a foundation species, but hemlocks are not only ecologically important. These trees provide unique recreational and aesthetically pleasing niches within private and public forests for those who enjoy the outdoors.
With so many positive attributes associated with these stately trees and the forests where they live, what could go wrong?
Well, an invasive species like hemlock woolly adelgid (HWA) is capable of turning the tide for hemlock growth and survival. This tiny, yet powerfully damaging insect is currently changing forest health because it has caused the death of many hemlock trees in the eastern US. HWA is providing a model for a sequence of effects resulting from the spread of an introduced, non-native species and for the battle against the loss of natural resources, especially the rare and limited Carolina hemlock.
For more information on hemlock woolly adelgid, see LGP 1020, Hemlock Woolly Adelgid: Biology and Management.