Tsuga

Jump to navigation Jump to search
style="background:#Template:Taxobox colour;"|Tsuga
Tsuga heterophylla
Tsuga heterophylla
style="background:#Template:Taxobox colour;" | Scientific classification
Kingdom: Plantae
Division: Pinophyta
Class: Pinopsida
Order: Pinales
Family: Pinaceae
Genus: Tsuga
Carrière
Species
  • Subgenus Tsuga

Tsuga canadensis  Eastern Hemlock
Tsuga caroliniana  Carolina Hemlock
Tsuga chinensis  Taiwan Hemlock
Tsuga diversifolia  Northern Japanese Hemlock
Tsuga dumosa  Himalayan Hemlock
Tsuga forrestii  Forrest's Hemlock
Tsuga heterophylla  Western Hemlock
Tsuga sieboldii  Southern Japanese Hemlock

  • Subgenus Hesperopeuce (Engelm.) Ueno

Tsuga mertensiana  Mountain Hemlock

Tsuga (from Template:Lang-ja, ; the name for Tsuga sieboldii ) is a genus of conifers in the family Pinaceae. The common name hemlock is derived from a perceived similarity in the smell of the crushed foliage to that of the unrelated herb poison hemlock; see hemlock for other senses of the word. Unlike the herb, the species of Tsuga are not poisonous. There are between eight and ten species within the genus depending on the authority, with four occurring in North America and four to six in eastern Asia.[1][2][3][4][5]

Description

File:Tsuga-diversifolia.JPG
Tsuga diversifolia foliage and cones in snow

They are medium-sized to large evergreen trees, ranging from 20–60(–79) m tall, with a conical to irregular crown, with 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 that 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.[1][2][3][4][5]

File:MountainHemlock 6739.jpg
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 winge 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.[1][2][3][5][4]

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.,[6] though it is more generally only considered distinct at the rank of subgenus.[1]

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.[1][3]

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.[1][3]

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).[7] 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. Tsuga species are also used as food plants by the larvae of some Lepidoptera species including Autumnal Moth and The Engrailed, and older caterpillars of the Gypsy Moth. 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 which activate growth of the Rhizina spores.[8]

Uses

The wood obtained from hemlocks is important in the timber industry, especially for use as wood pulp. Many species are utilised in horticulture, and numerous cultivars have been selected for use in gardens.

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 Farjon, A. (1990). Pinaceae. Drawings and Descriptions of the Genera. Koeltz Scientific Books ISBN 3-87429-298-3.
  2. 2.0 2.1 2.2 Rushforth, K. (1987). Conifers. Helm ISBN 0-7470-2801-X.
  3. 3.0 3.1 3.2 3.3 3.4 Earle, C. J. (2006). "Tsuga". Gymnosperm Database. Retrieved 2007-05-16.
  4. 4.0 4.1 4.2 Wu, Z.-Y., & Raven, P. H. (1999). "Tsuga". Flora of China. Beijing: Science Press. Retrieved 2007-05-16.
  5. 5.0 5.1 5.2 Taylor, R. J. (1993). "Tsuga". Flora of North America North of Mexico, Vol. 2. Oxford University Press. Retrieved 2007-05-16.
  6. Page, C. N. (1990). Pinaceae. Pp. 319-331 in: Kubitzki, K., ed. The Families and Genera of Vascular Plants. Springer-Verlag, Berlin.
  7. United States Forest Service: Hemlock Woolly Adelgid website
  8. Phillips, D. H., & Burdekin, D. A. (1992). Diseases of Forest and Ornamental Trees. Macmillan ISBN 0-333-49493-8.

Template:Pinaceae

cs:Jedlovec da:Hemlock de:Hemlocktannen eo:Cugo it:Tsuga lt:Cūga nl:Hemlockspar no:Hemlokk fi:Hemlokit