Tsuga canadensis, also known as eastern hemlock,听eastern hemlock-spruce,听or听Canadian hemlock, and in the French-speaking regions of Canada as听pruche du Canada, is a听听听听to eastern North America. It is the听听of Pennsylvania.听Eastern hemlocks are widespread throughout much of the听, the听, the听, and听. They have been introduced in the听听and mainland Europe, where they are used as ornamental trees.
Eastern hemlock populations in North America are threatened in much of their range by the spread of the invasive听, which infests and eventually kills trees. Declines in population from hemlock wooly adelgid infestation have led to听Tsuga canadensis听being listed as Near Threatened on the听.
Eastern hemlocks are long lived trees, with many examples living for more than 500 years. They can grow to heights of more than 30 metres (100听ft), and are tolerant of shade, moist soil, and slopes. Hemlock wood is used in construction, and for听. Historically its bark was an important source of听听for the leather听听industry.听Eastern hemlocks are popular as ornamental trees, thanks to their tolerance of a wide variety of soil and light conditions, as well as their characteristic drooping branches.
Description[
The eastern hemlock grows well in shade and is very long lived, with the oldest recorded specimen, found in听, being at least 554 years old.听The tree generally reaches heights of about 31听m (102听ft),听but exceptional trees have been recorded up to 53听m (174听ft).听The diameter of the trunk at breast height is often 1.5听m (4听ft 11听in), but again, outstanding trees have been recorded up to 1.75听m (5听ft 9听in).听The trunk is usually straight and听, but very rarely is forked.听The crown is broadly conic, while the brownish听听is scaly and deeply fissured, especially with age.听The twigs are a yellow-brown in color with darker red-brown听, and are densely pubescent. The听听are ovoid in shape and are very small, measuring only听1.5 to 2.5听mm (0 to听1鈦�8听in) in length. These are usually not resinous, but may be slightly so.
The leaves are typically 15 to 20听mm (0.59 to 0.79听in) in length, but may be as short as 5听mm (0.20听in) or as long as 25听mm (1听in). They are flattened and are typically distichous, or two-ranked. The bottom of the leaf is glaucous with two broad and clearly visible听听bands, while the top is a shiny green to yellow-green in color. The leaf margins are very slightly toothed, especially near the apex. The seed听听are ovoid in shape and typically measure听1.5 to 2.5听cm (5鈦�8听to 1听in) in length and听1.0 to 1.5听cm (3鈦�8听to听5鈦�8听in) in width. The scales are ovate to cuneate in shape and measure听8 to 12听mm (3鈦�8听to听1鈦�2听in) in length by听7.0 to 10听mm (1鈦�4听to听3鈦�8听in) in width. The apex is more or less rounded and is often projected outward. Twenty-four diploid听听are present within the trees'听.
Wood[]
The wood is soft, coarse-grained, and light buff in color. Air-dried, a cubic foot (0.028听m3) weighs 28 pounds (13听kg). The lumber is used for general construction and crates. Because of its unusual power of holding spikes, it is also used for railroad ties. Untreated, the wood is not durable if exposed to the elements. As a fuel, it is low in value. The wood is also a source of pulp for paper manufacturing.
Distribution and habitat[]
T. canadensis听occurs at sea level in the north of its distribution,听but is found primarily at elevations of 600鈥�1,800听m (2,000鈥�5,900听ft). It ranges from northeastern听听eastward through southern听听and into听, and south in the听听to northern听听and Alabama.听Disjunct populations occur in the听, northern Alabama, western听听and into听, as well as western Wisconsin and eastern Minnesota.听In Canada, it is present in Ontario and all provinces to the east except Newfoundland and Labrador.听Its range completely overlaps that of the closely related听.
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 highland climates, with cool and humid conditions. Precipitation in the areas where it grows is typically 740听mm (29听in) to more than 1,270听mm (50听in) per year. The lower number is more typical of northern forests that receive heavy snowfall; 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 1,520听mm (60听in). In the north of its range, the temperatures in January average 鈭�12听掳C (10听掳F), while in July they average only 16听掳C (61听掳F). In these areas, the frost-free season can last fewer than 80 days. In contrast, the southern end of the range experiences up to 200 days without frost and January temperatures as high as 6听掳C (43听掳F).
Hemlock woolly adelgid[]
The species is currently threatened by the 听(Adelges tsugae), a sap-sucking听听introduced from听听to the United States in 1924, and first found in the native range of eastern hemlock in the late 1960s.听The adelgid has spread very rapidly in southern parts of the range once becoming established, while its expansion northward is much slower.[]听Virtually all 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.[]听Attempts to save representative examples on both public and private lands are on-going. A project named "Tsuga Search", funded by the听, is being conducted to save the largest and tallest remaining eastern hemlocks in the Park. Through Tsuga Search, hemlocks have been found with trunk volumes up to 44.8 m鲁 within the park,听making it the largest eastern evergreen conifer, eclipsing in volume both eastern white pine (Pinus strobus) and听听(Pinus taeda). The tree is currently listed as a听听species in the听, but this is based largely on its wide distribution and because the adelgid populations have not reached the northern areas of its range.
A 2009 study conducted by scientists with the听听听suggests the hemlock woolly adelgid is killing hemlock trees faster than expected in the southern Appalachians, and rapidly altering the听听of these forests. According to听, the pest could kill most of the region's hemlock trees within the next decade.[]听According to the study, researchers found "hemlock woolly adelgid infestation is rapidly impacting the carbon cycle in [hemlock] tree stands," and "adelgid-infested hemlock trees in the South are declining much faster than the reported 9-year decline of some infested hemlock trees in the Northeast."
In a 2009 case study, 听from the U.S. Forest Service,听, and the听听released 900听听beetles into a stand of adelgid-infested hemlocks near听.听L. nigrinus, which is native to the Pacific Northwest, naturally preys on the hemlock wooly adelgid. The particular site near Lansing was chosen because its hemlocks are only lightly infested with the woolly adelgid, and enough trees are found to sustain a long-term study. The site will be left untreated with pesticides for 10 years to study how well the听L. nigrinus听beetles become established; if the experiment proves successful, researchers expect the population will take two to three years to build to levels where they can be readily detected.
Ecology[]
The tree can be found living in association with many forest mushrooms, such as听.
Paleoecology[]
The mid-听decline of hemlock populations is a much-studied phenomenon.听From its foundation in the early Holocene (around 16,000 BP) in what is now the southeastern US,听T. canadensis听expanded rapidly and successfully into its potential range.听However,听听analyses show the hemlock population experienced a pronounced decline approximately 5,500 BP that lasted for about 1,000 years. Continued research points to other, though less dramatic, dips in Holocene hemlock populations.听Pathogens, insects, and climatic change, and a combination of these, have all been proposed to explain these anomalies. The eastern hemlock increased again after the major decline, but did not recover its former place as a dominant species.
Exceptional trees[]
Due to it being a long-lived tree, several very large or otherwise impressive trees exist along the east coast of North America. One organization, the Eastern Native Tree Society (ENTS), has been particularly active in discovering and measuring these trees. In the southern Appalachians, many individuals reach 45 metres (148听ft) tall, and one tree has been measured in the Great Smoky Mountains National Park to 52.8听m (173听ft 3听in) tall, though this tree is now dead from hemlock woolly adelgids; the tallest now surviving, the "Noland Mountain tree", is 51.8听m (169听ft 11听in) tall.听Altogether, ENTS has confirmed four trees to heights of 51听m (167听ft) or more by climb and tape drop. In the Northeast, the tallest accurately measured tree is 44听m (144听ft). This tree, named the Seneca hemlock, grows in Cook Forest State Park, PA. Above 43掳N latitude, the maximum height of the species is less, under 39听m (128听ft). In New England, ENTS has measured hemlocks to 42听m (138听ft), although trees above 39听m (128听ft) are extremely rare in New England. By 44掳N, the maximum height is probably not more than 35听m (115听ft). Diameters of mature hemlocks range from 0.75鈥�1.8听m (2听ft 6听in听鈥� 5听ft 11听in), with trees over 1.6听m (5听ft 3听in) in diameter being very rare. In New England, the maximum diameter is 1.4听m (4听ft 7听in).
Trunk volume is the third dimension to receive attention by ENTS. Many eastern hemlocks have been modeled to over 30听m3听(1,100听cu听ft) trunk volume, and the largest has been calculated to be 44.8听m3听(1,580听cu听ft),听making it the largest natural evergreen conifer in the eastern United States. The center of maximum size development for the species is the southern Appalachians, especially the Great Smoky Mountains.
Cultivation[]
Tsuga canadensis听has long been a popular tree in cultivation. The tree's preference for partial shade and tolerance of full shade allows it to be planted in areas where other conifers would not easily grow. In addition, its very fine-textured foliage that droops to the ground, its pyramidal growth habit, and its ability to withstand hard pruning make it a desirable听. In cultivation, it prefers sites that are slightly acidic to neutral with nutrient-rich and moist but well-drained soil. 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 susceptibility to attack by the hemlock woolly adelgid, though this is treatable.听Its tendency to shed needles rapidly after being cut down renders it unsuitable as a听.
It was introduced to British gardens in 1736.听In the听, 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听, but is considered inferior for this usage compared to听听(western hemlock); it is not well adapted to the UK climate and as a consequence often has a poorly developed, forked and sinuous trunk there.听In听, it is the most frequently seen hemlock in cultivation, and is also used in听.
Cultivars[]
Over 300 听have been selected for use, many of them being dwarf forms and shrubs. A partial list of popular cultivars includes:
- 'Beehive' 鈥� a very small dwarf shrub typically growing to 1听m (3听ft 3听in) high and 1.5 m wide, resembling a spreading beehive in form
- 'Bennett' 鈥� a dwarf shrub reaching 1听m (3听ft 3听in) high and 1.5听m (4听ft 11听in) wide, with upper branchlets that first ascend and then arch upper, this selection prefers partial shade.
- 'Cole's Prostrate' 鈥� a听听form that can also be used in听听as an alternative to the prostrate听, it slowly grows to only 30听cm (12听in) tall with a 1.3-metre (4.3听ft) spread, with the central stems eventually becoming visible. It also prefers partial shade.
- 'Gentsch White' 鈥� a dwarf shrub growing to 1.3听m (4听ft 3听in) 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 to keep it compact and create more tip growth.
- 'Jeddeloh' 鈥� a dwarf shrub reaching to 1听m (3听ft 3听in) high and 1.5听m (4听ft 11听in) wide, showing a small concavity in the centre, it is an alternative to the bird's-nest spruce (听'Nidiformis'). This cultivar has gained the听's听.
- 'Pendula' 鈥� an upright weeping form whose height is dependent on how long it is staked, but is typically seen 0.6鈥�1.5听m (2听ft 0听in听鈥� 4听ft 11听in) tall with a 1.5-metre (4.9听ft) spread. It has also won the AGM.
- 'Sargentii' 鈥� a popular large weeping shrub that grows to 3听m (9.8听ft) tall with a wide spread up to 6听m (20听ft), it features numerous pendulous branches and is most effectively employed near water, in rock gardens, or on embankments.
Uses[]
American pioneers made tea from the tree's leafy twigs and used its branches as brooms.听Tea can be made from the needles. The inner bark, which is best in winter and coming into spring, can be eaten raw or boiled; it can also be used to make flour.