Tsuga canadensis

Tsuga canadensis

Tsuga canadensis, also known as Eastern or Canadian Hemlock, and in the French-speaking regions of Canada as Pruche du Canada, is a coniferous tree native to eastern North America. It ranges from northeastern Minnesota eastward through southern Quebec to Nova Scotia, and south in the Appalachian Mountains to northern Georgia and Alabama. Scattered outlier populations occur in several areas east and west of the Appalachians. It is the state tree of Pennsylvania.

Description

The eastern hemlock generally reaches heights of about 30 metres (100 feet), but exceptional trees have been recorded up to 53 metres (173 feet). The diameter of the trunk at breast height is often 1.5 metres (5 feet), but again, outstanding trees have been recorded up to 1.75 meters (6 feet). The trunk is usually straight and monopodial, but very rarely is forked. The crown is broadly conic, while the brownish bark is scaly and deeply fissured, especially with age. The twigs are a yellow-brown in colour with darker red-brown pulvini, and are densely pubescent. The buds are ovoid in shape and are very small, measuring only 1.5 to 2.5 mm (0.05 to 0.1 inches) in length. These are usually not resinous, but may be slightly so.

The leaves are typically 15 to 20 mm (0.6 to 0.9 inches) in length, but may be a short as 5 mm (0.2 inches) or as long as 25 mm (1 inch). They are flattened and are typically distichous, or two-ranked. The abaxial surface of the leaf, or underside, is glaucous with 2 broad and clearly visible stomatal bands, while the adaxial surface, i.e. the upper-side, is a shiny green to yellow-green in colour. The leaf margins are very slightly dentate, or toothed, especially near the apex. The seed cones are ovoid in shape and typically measure 1.5 to 2.5 cm (0.6 to 1 inch) in length and 1 to 1.5 cm (0.4 to 0.6 inches) in width. The scales are ovate to cuneate in shape and measure 8 to 12 mm (0.3 to 0.5 inches) in length by 7 to 10 mm (0.3 to 0.4 inches) in width. The apex is more or less rounded and is often projected outward. 24 diploid chromosomes are present within the tree's DNA.

Distribution and habitat

T. canadensis occurs at sea level in the north of its distribution, but is found primarily at elevations of 600 to 1800 metres. It ranges from from northeastern Minnesota eastward through southern Quebec and into Nova Scotia, and south in the Appalachian Mountains to northern Georgia and Alabama. Disjunct populations occur in the southeastern Piedmont, western Ohio and into Illinois, as well as eastern Minnesota. In Canada it is present in all provinces east from and including Ontario, but discludng Newfoundland. In the USA it is found in all states east of and including Minnesota, Wisconsin, Indiana, Kentucky, Tennessee and Alabama, but discluding Florida. Its range completely overlaps that of the closely related Tsuga caroliniana.

It is found primarily on rocky ridges, ravines and hillsides with relatively high levels of moisture.

Climate

Eastern hemlock is generally confined to areas with cool and humid climates. Precipitation in the areas where it grows is typically between 740 mm (29 inches) to more than 1270 mm (50 inches) per year. The lower number is more typical of northern forests that receive heavy snowfall, and the higher number is common in southerly areas with high summer rainfall. Near the Atlantic coast and in the southern Appalachians where the trees often reach their greatest heights, annual rainfall often exceeds 1520 mm (60 inches). In the north of its range the temperatures in January average -12°C, while in July they average only 16°C. In these areas less than 80 are without frost. In contrast to this, the southern end of the range experiences up to 200 days without frost and January temperatures as high as 6°C.

Ecology

The eastern hemlock grows well in shade and is very long lived with the oldest recorded specimen being at least 554 years old. The future of the species is currently under threat due to the hemlock woolly adelgid (Adelges tsugae), a sap-sucking bug accidentally introduced from East Asia to the United States in 1924. The Adelgid has spread very rapidly in southern parts of the range once becoming established, while its expansion northward is much slower. Virtually all of the hemlocks in the southern Appalachian Mountains have seen infestations of the insect within the last five to seven years, with thousands of hectares of stands dying within the last two to three years. Attempts to save representative examples on both public and private lands are on-going. At exceptionally important project named "Tsuga Search", funded by the Great Smoky Mountains National Park, is being conducted to save the largest and tallest remaining Eastern Hemlocks in the Park. It is through Tsuga Search that Hemlocks have been found with trunk volumes of up to 44.8 m³ within the Park, making the it the largest eastern evergreen conifer, eclipsing in volume both Pinus strobus (Eastern White Pine) and Pinus taeda (Loblolly Pine). The tree is currently listed as a least concern species in the IUCN Red List, but this is based largely on its wide distribution and the fact that the adelgid populations have not reached the northern areas of its range.

Paleoecology

Hemlock was very abundant in the northern-eastern USA and eastern Canada during the early Holocene, when it underwent a sudden decline in abundance, almost disappearing from the pollen record around 5,000 years ago. It later returned, but never became as abundant as it had been previously. The causes of this 'hemlock decline' are unknown, but it is likely to have been due to some form of pest or disease.

Exceptional trees

Due to it being a particularly long lived tree, several very large or otherwise impressive trees exist up and down the east coast of North America. One organisation, the Eastern Native Tree Society (ENTS), has been particularly active in discovering and measuring these trees. In the southern Appalachians many individuals reach to 45 m tall, and one tree has been measured in the Great Smoky Mountains National Park to 52.8 m tall, though this tree is now dead from hemlock woolly adelgid; the tallest now surviving, the "Noland Mountain tree", is 51.8 m tall. Altogether, ENTS has confirmed four trees to heights of 51 m or more by climb and tape drop. In the Northeast, the tallest accurately measured tree is 44 m. This tree, named the Seneca Hemlock, grows in Cook Forest State Park, PA. Above 43°N latitude, the maximum height of the species is less, under 39 m. In New England, ENTS has measured Hemlocks to 42 m, although trees above 39 m are extremely rare in New England. By 44°N, the maximum height is probably not more than 35 m. Diameters of mature Hemlocks range from 0.75–1.8 m, with trees over 1.6 m diameter being very rare. In New England, the maximum diameter is 1.4 m.

Trunk volume is the third dimension to receive attention by ENTS. The largest Eastern Hemlock has been calculated to be 44.8 m³. This makes the hemlock the largest natural evergreen conifer in the eastern United States. But recent research by ENTS places the hemlock in the first position. Many eastern hemlocks have been modeled to over 30 m³ trunk volume. The center of maximum size development for the species is the southern Appalachians and within that region, the Great Smoky Mountains.

Cultivation

Tsuga canadensis has long been a popular tree in cultivation. The tree is equally tolerant of full shade and full sun, making it a versatile tree. In addition, its very fine textured foliage that droops to the ground, its pyrimidal growth habit and its ability to withstand hard pruning make it a desirable ornamental tree. In cultivation it prefers sites that are slightly acidic to neutral with nutrient-rich soil that is moist but well-drained. It is most often used as a specimen, for a screen, or in small group plantings, though it can also be trained as a dense formal hedge. It should not be used on roadsides where salt is used in winter, as its foliage is sensitive to salt spray. It is also poorly adapted as a windbreak tree, as wind exposure causes dieback in winter. It has several drawbacks, such as a fairly low tolerance of urban stress, intolerance for very wet or very dry soils, and it is susceptible to attack by the hemlock woolly adelgid, though this is treatable. Its tendency to shed needles rapidly after being cut down renders it unsuitable as a Christmas tree.

It was introduced to British gardens in 1736. In the UK it is encountered frequently in gardens both large and small, as well as some parks and is most common in the eastern areas of the country. It is sometimes employed as a hedge, but is considered inferior for this usage compared to Tsuga heterophylla (Western Hemlock); it is not well adapted to the UK climate and as a consequence there often has a poorly developed, forked and sinuous trunk. In Germany it is the most frequently seen hemlock in cultivation and is also used in forestry.

Cultivars

Over 300 cultivars have been selected for use, many of them being dwarf forms and shrubs. A partial list of popular cultivars includes:

  • 'Beehive' - a very small dwarf shrub typically growing to 1 m high and 1.5 m wide, resembling a spreading beehive in form.
  • 'Bennett' - a dwarf shrub reaching 1 m high and 1.5 m wide, with upper branchlets that first ascend and then arch upper. This selection prefers partial shade.
  • 'Cole's Prostrate' - a groundcover form that can also be used in bonsai as an alternative to the prostrate junipers. It slowly grows to only 30 cm tall with a 1.3 m spread, with the central stems eventually becoming visible. It also prefers partial shade.
  • 'Gentsch White' - a dwarf shrub growing to 1.3 m tall with an equal spread and new spring growth that turns creamy-white in autumn through winter, creating a dramatic contrast with the dark green old growth. It is easily scorched by the sun and requires partial shade. It is recommend to feather shear annually in order to keep it compact and create more tip growth.
  • 'Jeddeloh' - a dwarf shrub reaching to 1 m high and 1.5 m wide, showing a small concavity in the centre. It is an alternative to the bird's nest spruce (Picea abies 'Nidiformis')
  • 'Pendula' - an upright weeping form whose height is dependent on how long it is staked, but is typically seen 0.6–1.5 m tall with a 1.5 m spread.
  • 'Sargentii' - a popular large weeping shrub that grows to 3 m tall with a wide spread up to 6 m. It features numerous pendulous branches and is most effectively employed near water, in rock gardens or on embankments.

References

External links

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