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Tsuga canadensis
Conservation status

Apparently Secure (TNC)[1]
Scientific classification
Kingdom: Plantae
Division: Pinophyta
Class: Pinopsida
Order: Pinales
Family: Pinaceae
Genus: Tsuga
Species: T. canadensis
Binomial name
Tsuga canadensis
(L.) Carrière

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.[2]

Contents

Description

A line drawing of the leaves and cones from Britton & Brown's 1913 Illustrated flora of the northern states and Canada

The eastern hemlock grows well in shade and is very long lived with the oldest recorded specimen being at least 554 years old.[3] The tree generally reaches heights of about 31 metres (100 feet),[2] but exceptional trees have been recorded up to 53 metres (173 feet).[4] 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).[5] The trunk is usually straight and monopodial, but very rarely is forked.[6] The crown is broadly conic, while the brownish bark is scaly and deeply fissured, especially with age.[2] 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.[2][6]

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.[2][6]

Distribution and habitat

T. canadensis occurs at sea level in the north of its distribution,[6] but is found primarily at elevations of 600 to 1800 metres. It ranges from northeastern Minnesota eastward through southern Quebec and into Nova Scotia, and south in the Appalachian Mountains to northern Georgia and Alabama.[2] Disjunct populations occur in the southeastern Piedmont, western Ohio and into Illinois, as well as eastern Minnesota.[7][6] In Canada it is present in Ontario and all provinces to the east except Newfoundland. In the USA it is found in all states east of and including Minnesota, Wisconsin, Indiana, Kentucky, Tennessee and Alabama, but excluding Florida.[2] Its range completely overlaps that of the closely related Tsuga caroliniana.[8]

It is found primarily on rocky ridges, ravines and hillsides with relatively high levels of moisture.[2]

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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 the frost free season can last less 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°C.[8]

Hemlock wooly adelgid

Shoot infested with hemlock woolly adelgid

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. A 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,[9] making 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.[10]

A 2009 study conducted by scientists with the U.S. Forest Service Southern Research Station suggests the hemlock woolly adelgid is killing hemlock trees faster than expected in the southern Appalachians and rapidly altering the carbon cycle of these forests. According to Science Daily, the pest could kill most of the region's hemlock trees within the next decade. According to the study, researchers found that "hemlock woolly adelgid infestation is rapidly impacting the carbon cycle in [hemlock] tree stands," and that "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."[11]

In a 2009 local case study, entomologists from the U.S. Forest Service, Cornell University and the University of Massachusetts-Amherst released 900 Laricobius nigrinus beetles into a stand of adelgid-infested hemlocks near Lansing, New York. 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 there are enough trees to sustain a long-term study. The site will be left untreated with pesticides for 10 years to study how well the L. nigrinus 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.[12]

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.[9] 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. Many eastern hemlocks have been modeled to over 30 m³ trunk volume, and the largest has been calculated to be 44.8 m³,[9] 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 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 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.[13] 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.[14] 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.[14][15] In Germany it is the most frequently seen hemlock in cultivation and is also used in forestry.[16]

Cultivars

The weeping shrub form T. canadensis 'Sargentii'

Over 300 cultivars have been selected for use, many of them being dwarf forms and shrubs. A partial list of popular cultivars includes:[13][17]

  • '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

  1. ^ "Tsuga canadensis". NatureServe Explorer. NatureServe. http://www.natureserve.org/explorer/servlet/NatureServe?searchName=Tsuga+canadensis+. Retrieved 2007-07-05. 
  2. ^ a b c d e f g h Taylor, Ronald J. (1993), "Tsuga canadensis", in Flora of North America Editorial Committee (eds.), Flora of North America, 2, Oxford University Press, http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=233501320 
  3. ^ Gove, J.H.; Fairweather, S.E. (1988), "Tree-ring analysis of a 500-year old hemlock in central Pennsylvania", U.S. Forest Service General Technical Report NC-120, 1, pp. 483–489 
  4. ^ Blozan, Will (February 16, 2007), The Usis Hemlock Climb, http://www.nativetreesociety.org/fieldtrips/gsmnp/usis/usis_hemlock_climb.htm, retrieved 2007-06-08 
  5. ^ Blozan, Will (December 18, 2006), The Laurel Branch Leviathan Climb, http://www.nativetreesociety.org/tsuga/laurel_branch/laurel_branch_leviathan_climb.htm, retrieved 2007-06-08 
  6. ^ a b c d e Farjon, A. (1990). Pinaceae. Drawings and Descriptions of the Genera. Koeltz Scientific Books ISBN 3-87429-298-3.
  7. ^ Thompson, Robert S.; Anderson, Katherine H.; Bartlein, Patrick J. (1999), "Tsuga canadensis", Atlas of Relations Between Climatic Parameters and Distributions of Important Trees and Shrubs in North America, U.S. Geological Survey, http://pubs.usgs.gov/pp/p1650-a/pages/tscatrim.pdf, retrieved 2007-07-05 
  8. ^ a b Godman and, R. M.; Lancaster, K. (1965, revised December 1990). "Tsuga canadensis, Eastern Hemlock". Silvics of Forest Trees of the United States, Volume 1. United States Department of Forestry. http://na.fs.fed.us/pubs/silvics_manual/Volume_1/tsuga/canadensis.htm. Retrieved 2007-07-05. 
  9. ^ a b c Gymnosperm Database: Tsuga canadensis
  10. ^ Conifer Specialist Group (1998). Tsuga canadensis. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 12 May 2006.
  11. ^ Hemlock Trees Dying Rapidly, Affecting Forest Carbon Cycle
  12. ^ Predator Beetle to Battle Hemlock Pest
  13. ^ a b Rhodus, T. Tsuga canadensis. In: OSU Pocket Gardener. Ohio State University.
  14. ^ a b Mitchell, A. F. (1974). A Field Guide to the Trees of Britain and Northern Europe. Collins ISBN 0-00-212035-6
  15. ^ Mitchell, A. F. (1972). Conifers in the British Isles. Forestry Commission Booklet 33.
  16. ^ (German) Schmeil, O., Fitschen, J., & Seybold, S. (2006). Flora von Deutschland 93. Auflage, p. 424. Quelle & Meyer Verlag, Wiebelsheim. ISBN 3-494-01413-2.
  17. ^ Welch, H., & Haddow, G. (1993). The World Checklist of Conifers. Landsman's. ISBN 0-900513-09-8.

External links


Wikispecies

Up to date as of January 23, 2010

From Wikispecies

Tsuga canadensis

Taxonavigation

Main Page
Cladus: Eukaryota
Regnum: Plantae
Division: Pinophyta
Classis: Pinopsida
Ordo: Pinales
Familia: Pinaceae
Genus: Tsuga
Species: Tsuga canadensis

Name

Tsuga canadensis (L.) Carriere

Vernacular names

Suomi: Kanadanhemlokki
Türkçe: Kanada sugası

References

Data compiled from various sources by Mark W. Skinner. National Plant Data Center, Baton Rouge, LA 70874-4490 USA.

  • Traité gén. conif. 189. 1855
  • USDA, ARS, National Genetic Resources Program. Germplasm Resources Information Network - (GRIN) [Online Database]. [1]
Wikimedia Commons For more multimedia, look at Tsuga canadensis on Wikimedia Commons.

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