Eastern Hemlock

Tsuga canadensis
Sunday, June 24, 2012

           Eastern Hemlocks are a fast fading species in our area, and across their range. However, they have the potential to be massive, ancient trees. Some of the largest specimens are reported to have been over 2 meters in diameter at breast height, and 500-600 years old. One account puts the age of a massive specimen at almost 1,000 years.

           T. canadensis is the most common hemlock found in our area. Hemlocks differ from spruces and other evergreens because of the relative flatness of the spray of needles relative to the twig. In Eastern Hemlocks, these needles are typically around 1.5-2 cm in length and glossy green on the upper surfaces. The lower surfaces are marked with two lines of white stomata (microscopic pores used for exchanges of gases, particularly carbon dioxide, water vapor, and oxygen). These flat needles are set on slender, grey-brown twigs. The bark of very young trees is also gray-brown and smooth; however, as they age, the bark becomes red-brown and scaly and finally deeply ridged and furrowed. These conically or irregularly shaped trees can reach 50 m (164 ft) in height, but healthy adults are usually around 30 m.

           When a tree is around 15 years old, it begins to reproduce. Hemlocks are monoecious (male and female flowers are distinct, but both present). Male flowers form conelets in April to early June, and release pollen for up to six weeks. At the same time, the female conelets have developed, with two ovules present on each cone scale, opening to receive pollen at about the same time that the male cones begin to produce it. After the pollination period, the female cones mature and develop until the autumn, and seed dispersal typically begins in mid-October and continues into the winter.

           Both the seeds and seedlings are extremely sensitive to moisture, and lack of adequate water within the first year often causes seedling failure. Although the older trees are less sensitive, healthy hemlock stands still tend to be established in cool, moist areas. Hemlocks are able to tolerate shade better than many other species; however, their growth is very slow both due to an often crowded overstory and natural tendencies. As a result, trees only 5-8 cm in diameter at breast height can be up to more than a century old.  Eastern Hemlocks can be found in cool, moist forests across the northeastern US, extending into Canada. However, their range is dramatically decreased from what it once was, largely as a result of the hemlock wooly adelgid.

           Adelges tsugae, a small aphid-like insect smaller than the period on the end of this sentence, was imported from Japan in 1924. Until the 1980s, its effects were mostly confined to small parts of Virginia, Pennsylvania, New York, New Jersey, and Connecticut. However, it began a rapid range extension in the 1980s-1990s, and can now be found from Georgia to Maine. The wooly adelgid is so named because of the easily recognizable dry, white “wool” with which it covers itself and its egg masses. This insect harms the hemlock by sucking its sap, meanwhile injecting toxic saliva. The saliva causes needles on the branch to dry up and fall off within a few months. It also kills buds on the branches, preventing any new growth. Centuries-old hemlocks infested with the wooly adelgid can die within 4 years, or persist in a severely weakened state, vulnerable to other threats. There is no effective measure against this invasive insect, and as a result, hemlocks are becoming scarce even in the heart of their native range. No trees remain that measure up to historical records of size or age.

           These branches were collected from a small stand behind Clayton. Although all the trees were infested with the wooly adelgid, none had yet succumbed. A different story can be seen in the dead, brown hemlocks surrounding the lake, near the War Spur Overlook, or throughout the woods around MLBS.

 Hazel Galloway