The eastern hemlock (Tsuga canadensis) is a long-lived coniferous tree native to eastern North America. It is shade tolerant and has a high moisture requirement for establishment. Hemlock trees are important parts of northeastern forest ecosystems. Recent declines in the vitality of hemlock populations due to invasive pests pose a serious threat to the future of the species (Rooney and Waller 1998).
Eastern hemlock are coniferous trees that have flat, slender needles ranging from 2mm-5mm in length that are attached to the twig with a slender stalk (Godman & Lancaster, n.d.).
The leaves are dark green above and light green below with two white parallel lines.
The tree has a conical shape with a wide trunk that tapers into a thin top. Young trees have bark that appears scaly and light red. On mature trees the bark is deeply furrowed and reddish. It has small oval shaped cones that are a reddish colour when mature and range from 12 to 20 millimeters long. They mature over one year and seeds are dropped from mid-October into December.
Range and Preferred Habitat
Eastern hemlocks (Tsuga canadensis)are shade tolerant and thrive in acidic soils in moist, cool environments (Rooney et al, 2000). They can be found in certain areas across the United States including northern Michigan, New England, New York, and Eastern Canada including most of the maritime provinces and Quebec (Godman & Lancaster, n.d.). Its presence has been noted as diminished in the Great lakes, St-Lawrence and Acadian regions (Fuller 1998). This species has an extremely slow growth rate and individual trees are long-lived. In Quebec, eastern hemlock stands can be found on Mont St-Hilaire, Boise Muir and the Morgan Arboretum.
Hemlocks are commonly seen growing together in stands or in association with several other trees, including the Yellow Birch, Eastern White Pine, Red Spruce, White Spruce, Sugar Maple and American Beech (Kavanaugh and Kelvin 1986;Villeneuve and Brisson 2003).
Seedling Recruitment and Regeneration
There are a narrow range of acceptable sites for hemlock seedling establishment (Coffman 1978). A vast majority of hemlock seedlings die during their first year, over 80 percent (Friesner and Potzger 1994). Many factors prevent seedling establishment and survival. Hindrances include unfavorable moisture content, sun exposure, soil conditions, and predation. (Goerlich and Nyland 1997). The roots of hemlock seedlings are weak, grow slowly and are shallower than that of other tree species. This means that moisture in the surface of the soil is a critical factor in their survival (Friesner and Potzger 1994). Hemlock seedlings have a low light requirement. Seedlings will grow at a faster rate in the presence of sun exposure, but when exposed to light, they are at a higher risk of desiccation. Hemlocks very rarely regenerate in open areas (Goerlich and Nyland 1997). Despite their shade tolerance, hemlocks grow slower in the shade and are more susceptible to predators and pathogens (Rooney et al 2000). Research shows that hemlock seedlings are found most commonly in shady areas growing on rotten wood (Coffman 1978). Hemlock seedlings show an affinity for rotting wood because of the constant moisture supply they provide (Rooney and Waller 1998).
Factors Adversely Affecting Eastern Hemlock Populations
There are several factors that adversely influence eastern hemlock populations. The most significant factor is the presence of the insect Hemlock woolly adelgid (Adelges tsugae). It is an invasive species originating from Southern Japan that currently inhabits about half of the native range of eastern hemlocks and has been causing severe decreases in its overall population. The woolly adelgid establish themselves at the needle base of healthy hemlock foliage, causing defoliation by feeding on cellular fluid, which eventually leads to the mortality of the tree (Krapfl, et al. 2011).
Understory Tsuga canadensis, predominantly intermediate size trees, are more susceptible to mortality than those in the overstory, though both are adversely affected by the pest. Although the overstory population may have a higher survival rate, future recruitment will be compromised due to the loss of understory Tsuga canadensis making it the greatest threat to the long-term survival of eastern hemlock populations (Krapfl, et al. 2011).
Cultural Significance of Eastern Hemlock
Eastern hemlock has served many functional uses in industry and has been subject to over-harvesting. It was logged for timber and material for crates, boxes and paper pulp. The wood tends to be weak which prevents the hemlock from being a dominant timber tree. Hemlock bark was stripped and milled to extract tannin, which used in the process of dying leather (Nesom, 2012).
First Nations people also harvested parts of eastern hemlock for traditional medicine. The inner bark was used to make poultice for wounds and the vitamin C rich leaves were used for tea (Nesom, 2012).
Ecological Significance of Eastern Hemlock
The Eastern Hemlock is an important component of the eastern deciduous and mixed forests. Individual trees are long-lived and have impacts on the structure, function and composition of a forest (Fuller 1998). It has effects on the hydrological process within a forest as it has year-round transpiration rates that are highest during the spring (Ford & Vose 2007). This affects the water flow through a forest ecosystem. In the winter months, the canopy of the tree shields the ground below from the snow, and provides a habitat for ruffed grouse, wild turkey and larger browsing fauna such as white-tailed deer. However, an abundance of deer can have a negative effect on the hemlock population and its ability to regenerate since the deer feed on hemlock seedlings (Rooney et al 2000; Blanchette et al 2007). . Hemlock trees are known to moderate the temperature extremes of their understory in both the summer and the winter which creates unique and important habitats for plants and animals. These microclimatic pockets add to the complexity of the ecosystem and contribute to biodiversity (Lishaws et al, 2007).
Coffman, M.S. (1978) Eastern hemlock germination influenced by light, germination, media, and moisture content. Michigan Botanist, 17, 99-103.
Blanchette, P., Bourgeois, J.C., St.-Onge, S. (2007) Ruffed grouse winter habitat use in mixed softwood‐hardwood forests, Québec, Canada. Journal of Wildlife Management,71, 1758-1764.
Ford R., C and Vose M., J. Tsuga canadensis (l.) carr. mortality will impact hydrologic processes in southern Appalachian forest ecosystems (2007). Ecological Applications, 17(4), pp. 1156–1167.
Friesner, R.C., Potzger, J.E. (1994) Survival of hemlock seedlings in a relict colony under forest conditions. Butler University Botanical Studies, 6, 102-115.
Fuller, L. (1998). Ecological Impact of the mid-holocene Hemlock Decline in Southern Ontario, Canada Ecology, 79(7), pp. 2337–2351.
Godman, R., and Lancaster, K. n.d.. Eastern Hemlock. Cited 14 Oct 2012. Available from http://www.na.fs.fed.us/pubs/silvics_manual/Volume_1/tsuga/canadensis.htm
Goerlich, D., Nyland, R. (1997) Natural regeneration of eastern hemlock: a review. Proceedings: Symposium on Sustainable Management of Hemlock Ecosystems in Eastern North America.
Kavanaugh, K. & Kellman, M. (1986) Performance of Tsuga canadensis (L.) Carr. At the centre and northern edge of its range: A comparison. Journal of Biogeography, 13, 145-157.
Krapfl, K.J., et al. (2011). Early impacts of hemlock woolly adelgid in Tsuga Canadensis forest communities of the southern Appalachian Mountains. Journal of the Torrey Botanical Society, 138, 93-106.
Lishaws, S.C., Bergdahl, D.R., Costa, S.D. 2007. Winter conditions in eastern hemlock and mixed-hardwood deer wintering areas of Vermont. Canadian Journal of Forest Research. 37:697-703.
Nesom, Guy. “Eastern Hemlock.” Plant Guide. USDA, 2012. Available from http://plants.usda.gov/plantguide/pdf/cs_tsca.pdf
Rooney, T., McCormick, R., Solheim, S., Waller, D. (2000) Regional variation in recruitment of hemlock seedlings and saplings in the upper great lakes, USA. Ecological Applications, 10, 1119-1132.
Rooney, T., Waller, D. (1998) Local and regional variation In hemlock seeding establishment in the forests of the upper Great Lakes region, USA. Forest Ecology and Management, 111:211-224.
Villeneuve, N. & Brisson, J. (2003) Old-growth forests in the temperate deciduous zone of Quebec: Identification and evaluation for conservation and research purposes. The Forestry Chronicle, 79, 559-569.