Introduction

Eastern Hemlock (Tsuga canadensis) is one of the most gorgeous, and ecologically fascinating of Ontario’s native coniferous trees. It is one of 9 species of hemlock (Tsuga sp.) in the world. Only one other hemlock grows in eastern North America, Carolina Hemlock (Tsuga carolina), distributed in the montane forests of southern Appalachian Mountains of North Carolina and Virginia. Western North America is home to two species, the Mountain Hemlock (Tsuga mertensiana), a narrow, conical tree subalpine forests in the Coast and Rocky Mountains, and the Western Hemlock (Tsuga heterophylla), the largest hemlock species, growing in the mild, rain-drenched Pacific Coastal forests, especially on Haida Gwaii and Vancouver Island, where it attains it’s most massive form. The other five species are found in Asia. Three of these (Tsuga chinensis, Tsuga forrestii, and Tsuga dumosa) are found in the mountains of southern China. The other two (Tsuga sieboldii, and Tsuga diversifolia) are found on Japan. Only the T. diversifolia, the Northern Japanese Hemlock, is found in similar ecological situations as Eastern Hemlock. Northern Japanese Hemlocks are found in high-snowfall zones and beech forests (with Japanese Beech, Fagus crenata), very similar to Eastern Hemlock, which often occurs in American Beech (Fagus grandifolia) forests. Hemlocks as a whole are closely related to true firs (genus Abies), which includes the Balsam Fir (Abies balsamea).

Description 

Hemlock is a large coniferous tree in the pine family (Pinaceae), growing up to 50 m. It is evergreen, retaining its needle-like leaves throughout the winter. Needles are ~1.5cm long, arranged opposite to each other, giving young twigs a flattened appearance. Seed cones are 2.5cm long, and similar in appearance to spruces (Picea sp.). Ecologically, Hemlock is considered a late-successional tree species as it dominates older-growth forests, thriving in shaded areas, and contrastingly, not tolerant of major disturbances, such as wildfires. It has a very distinctive growth form – a pyramidal crown with weeping branches. This is an ecological adaptation to the heavy shade and high snowfall in the areas where Eastern Hemlock is most abundant.

Hemlock obtained its name similarly to the crushed foliage of poison hemlock (Conium maculatum), a poisonous, European forb. There are also several closely-related species to the European poison-hemlock native to Norfolk County, the most common being water-hemlock (Cicuta maculatum), which is a common understory plant in deciduous swamps (typically dominated by Silver Maple, Acer saccharinum). All poison-hemlocks are members of the carrot family (Apiaceae).

Hemlock is distributed throughout the Great Lakes Region and Appalachian Mountains. It is most common in northern Ontario and Michigan’s upper peninsula, where some of the largest concentrations of older-growth Eastern Hemlock trees occur in North America. Algonquin Provincial Park is home to over 40% of the remaining old-growth Great Lakes forests in Ontario, and much of these strands are Eastern Hemlock. In those areas, Eastern Hemlock is very common, especially in moist forest ecosystems, especially with Sugar Maple (Acer saccharum), Yellow Birch (Betula lutea), American Beech, and White Pine (Pinus strobus).

In much of the Lake Erie region (“Carolinian Life Zone”), Eastern Hemlock is a regionally uncommon species: attributable to the prevalence of hardwood species, warm microclimates, and a history of forest disturbance / deforestation, which suppresses Hemlock growth. However, it is more common in southern Norfolk County, where varied terrain creates favourable habitats for Hemlock, especially moist, cool microclimates. Hemlock is most abundant on north-facing slopes, and moist bottomlands. It is also common in moist hardwood forests, especially in Sugar Maple – Beech strands as scattered specimens.

An older strand of Eastern Hemlock can be found within the eastern portion of Backus Woods. LPBLT also protects substantial populations of Hemlock on a substantial proportion of its properties, including the Hammond Hemlock Slough, Jackson-Gunn Old-Growth Forest, Fisher’s Creek Nature Reserve, Stackhouse Forest Sanctuary and Arthur Langford Nature Preserve.

Hemlock Wooly Adelgid 

In the southern Great Lakes region, Hemlock is threatened by the introduced Hemlock Woolly Adelgid (Adelges tsugae), a sap-sucking insect that feeds on Hemlock foliage, originally introduced from Japan. In the southern Appalachians, Hemlock Woolly Adelgid has caused extensive Hemlock mortality. Currently, large populations are restricted to Haldimand and Niagara Regional Municipalities in Canada. Hemlock Woolly Adelgid is currently not present in Norfolk County, but future spread is likely.

Biocontrols for Hemlock Woolly Adelgid have been approved in both Canada and the United States. Two of the most widely-utilized biocontrol insects for Hemlock Woolly Adelgids are Laricobius beetles and Leucotaraxis silver flies. When utilizing these biocontrol insects, the hope of conservationists is to reduce Woolly Adelgid populations below the mortality threshold for Eastern Hemlock.

Fun facts

• Eastern Hemlock is one of the most shade-tolerant trees in North America, it can grow under a dense forest canopy for years before reaching full height.
• Hemlock forests help keep stream water cool, benefiting cold-water fish species like brook trout.
• In old-growth forests, Eastern Hemlocks can live over 500 years, making them one of the oldest tree species in Canada.
• Hemlock is an important tree species for many northern wildlife species – such as the American Red Squirrel (Tamiasciurus hudsonicus), and Northern Flying Squirrel (Glaucomys sabrinus)!

 

Written by Matthew Palarchio, HBSc in Environmental Sciences, Western University

Photos (L-R): Hemlock foliage (Canva stock photo), all other photos are provided by Matthew Palarchio

 

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