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Tsuga

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This article is about the genus. For other uses, see Tsuga (disambiguation).

Tsuga
Tsuga heterophylla
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Gymnospermae
Division: Pinophyta
Class: Pinopsida
Order: Pinales
Family: Pinaceae
Subfamily: Abietoideae
Genus: Tsuga
Carrière
Synonyms[1][2]
  • Micropeuce Gordon, not validly published
  • Hesperopeuce Lemmon
  • × Tsugo-piceo-tsuga Van Campo & Gaussen
  • × Hesperotsuga C.N.Page
  • Nothotsuga Hu ex C.N.Page.
  • Pinus Linnaeus sect. Tsuga Endlicher

Tsuga (/ˈsuːɡə/,[3] from Japanese 栂 (ツガ), the name of Tsuga sieboldii) is a genus of conifers in the subfamily Abietoideae. The common name hemlock is derived from a perceived similarity in the smell of its crushed foliage to that of the unrelated plant poison hemlock. Unlike the latter, Tsuga species are not poisonous.[4]

Eight to ten species are within the genus (depending on the authority), with four species occurring in North America and four to six in eastern Asia.[5][6][7][2][8]

Description

Tsuga diversifolia foliage and cones in snow

They are medium-sized to large evergreen trees, ranging from 10–60 m (33–197 ft) tall, with a conical to irregular crown, the latter occurring especially in some of the Asian species. The leading shoots generally droop. The bark is scaly and commonly deeply furrowed, with the colour ranging from grey to brown. The branches stem horizontally from the trunk and are usually arranged in flattened sprays that bend downward towards their tips. Short spur shoots, which are present in many gymnosperms, are weakly to moderately developed. The young twigs, as well as the distal portions of stem, are flexible and often pendent. The stems are rough due to pulvini that persist after the leaves fall. The winter buds are ovoid or globose, usually rounded at the apex and not resinous. The leaves are flattened to slightly angular and range from 5–35 mm long and 1–3 mm broad. They are borne singly and are arranged spirally on the stem; the leaf bases are twisted so the leaves lie flat either side of the stem or more rarely radially. Towards the base, the leaves narrow abruptly to a petiole set on a forward-angled pulvinus. The petiole is twisted at the base so it is almost parallel with the stem. The leaf apex is either notched, rounded, or acute. The undersides have two white stomatal bands (in T. mertensiana they are inconspicuous) separated by an elevated midvein. The upper surface of the leaves lack stomata, except in T. mertensiana. They have one resin canal that is present beneath the single vascular bundle.[5][6][7][2][8]

Tsuga mertensiana foliage and cones

The pollen cones grow solitary from lateral buds. They are 3–5(–10) mm long, ovoid, globose, or ellipsoid, and yellowish-white to pale purple, and borne on a short peduncle. The pollen itself has a saccate, ring-like structure at its distal pole, and rarely this structure can be more or less doubly saccate. The seed cones are borne on year-old twigs and are small ovoid-globose or oblong-cylindric, ranging from 15–40 mm long, except in T. mertensiana, where they are cylindrical and longer, 35–80 mm in length; they are solitary, terminal or rarely lateral, pendulous, and are sessile or on a short peduncle up to 4 mm long. Maturation occurs in 5–8 months, and the seeds are shed shortly thereafter; the cones are shed soon after seed release or up to a year or two later. The seed scales are thin, leathery, and persistent. They vary in shape and lack an apophysis and an umbo. The bracts are included and small. The seeds are small, from 2 to 4 mm long, and winged, with the wing being 8 to 12 mm in length. They also contain small adaxial resin vesicles. Seed germination is epigeal; the seedlings have four to six cotyledons.[5][6][7][2][8]

Taxonomy

Mountain hemlock, T. mertensiana, is unusual in the genus in several respects. The leaves are less flattened and arranged all round the shoot, and have stomata above as well as below, giving the foliage a glaucous colour; and the cones are the longest in the genus, 35–80 mm long and cylindrical rather than ovoid. Some botanists treat it in a distinct genus as Hesperopeuce mertensiana (Bong.) Rydb.,[9] though it is more generally only considered distinct at the rank of subgenus.[5]

Tsuga canadensis boughs shedding older foliage in autumn

Another species, bristlecone hemlock, first described as Tsuga longibracteata, is now treated in a distinct genus Nothotsuga; it differs from Tsuga in the erect (not pendulous) cones with exserted bracts, and male cones clustered in umbels, in these features more closely allied to the genus Keteleeria.[5][7]

Ecology

The species are all adapted to (and are confined to) relatively moist, cool temperate areas with high rainfall, cool summers, and little or no water stress; they are also adapted to cope with heavy to very heavy winter snowfall and tolerate ice storms better than most other trees.[5][7] Hemlock trees are more tolerant of heavy shade than other conifers; they are, however, more susceptible to drought.[10]

Threats

The two eastern North American species, T. canadensis and T. caroliniana, are under serious threat by the sap-sucking insect Adelges tsugae (hemlock woolly adelgid).[11] This adelgid, related to the aphids, was introduced accidentally from eastern Asia, where it is only a minor pest. Extensive mortality has occurred, particularly east of the Appalachian Mountains. The Asian species are resistant to this pest, and the two western American hemlocks are moderately resistant. In North America, hemlocks are also attacked by hemlock looper.[12] Larger infected hemlocks have large, relatively high root systems that can bring other trees down if one falls. The foliage of young trees is often browsed by deer, and the seeds are eaten by finches and small rodents.

Old trees are commonly attacked by various fungal disease and decay species, notably Heterobasidion annosum and Armillaria species, which rot the heartwood and eventually leave the tree liable to windthrow, and Rhizina undulata, which may kill groups of trees following minor grass fires that activate growth of the Rhizina spores.[13]

Uses

The wood obtained from hemlocks is important in the timber industry, especially for use as wood pulp. Many species are used in horticulture, and numerous cultivars have been selected for use in gardens. The bark of the hemlock is also used in tanning leather. The needles of the hemlock tree are sometimes used to make a tea and perfume.

Species

Accepted species[1][14][2]
Formerly included[1]

Moved to other genera: Cathaya Keteleeria Nothotsuga Picea Pseudotsuga Taxus

References

  1. ^ a b c "Tsuga". World Checklist of Selected Plant Families (WCSP). Royal Botanic Gardens, Kew.
  2. ^ a b c d e Fu, Liguo; Li, Nan; Elias, Thomas S.; Mill, Robert R. "Tsuga". Flora of China. Vol. 4. Retrieved 2007-05-16 – via eFloras.org, Missouri Botanical Garden, St. Louis, MO & Harvard University Herbaria, Cambridge, MA.
  3. ^ Sunset Western Garden Book, 1995:606–607
  4. ^ Farjon, A. (2010). A handbook of the world's Conifers. Vol. 2. Brill Publishers. pp. 533–1111.
  5. ^ a b c d e f Farjon, A. (1990). Pinaceae. Drawings and Descriptions of the Genera. Koeltz Scientific Books. ISBN 3-87429-298-3.
  6. ^ a b c Rushforth, K. (1987). Conifers. Helm. ISBN 0-7470-2801-X.
  7. ^ a b c d e Earle, Christopher J., ed. (2018). "Tsuga". The Gymnosperm Database. Retrieved 2007-05-16.
  8. ^ a b c Taylor, Ronald J. (1993). "Tsuga". In Flora of North America Editorial Committee (ed.). Flora of North America North of Mexico (FNA). Vol. 2. New York and Oxford: Oxford University Press. Retrieved 2007-05-16 – via eFloras.org, Missouri Botanical Garden, St. Louis, MO & Harvard University Herbaria, Cambridge, MA.
  9. ^ Page, C. N. (1990). "Pinaceae". In Kubitzki, K. (ed.). The Families and Genera of Vascular Plants. Berlin: Springer-Verlag. pp. 319–331.
  10. ^ "Implementation and Status of Biological Control of the Hemlock Woody Adelgid" (PDF). US Forest Service. December 2011. Retrieved 28 July 2013.
  11. ^ United States Forest Service: Hemlock Woolly Adelgid website
  12. ^ USDA Forest Service NA-PR-05-92
  13. ^ Phillips, D. H., & Burdekin, D. A. (1992). Diseases of Forest and Ornamental Trees. Macmillan ISBN 0-333-49493-8.
  14. ^ "Tsuga". County-level distribution maps from the North American Plant Atlas (NAPA). Biota of North America Program (BONAP). 2013.
  15. ^ Lepage, B. A. (2003). "A new species of Tsuga (Pinaceae) from the middle Eocene of Axel Heiberg Island, Canada, and an assessment of the evolution and biogeographical history of the genus". Botanical Journal of the Linnean Society. 141 (3): 257–296.
  16. ^ Popkin, Gabe (30 January 2018). "First New Species of Temperate Conifer Tree Discovered in More Than a Decade". National Geographic News. Retrieved 31 January 2018.
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