| Tsuga | |
|---|---|
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Gymnospermae |
| Division: | Pinophyta |
| Class: | Pinopsida |
| Order: | Pinales |
| Family: | Pinaceae |
| Subfamily: | Abietoideae |
| Genus: | Tsuga (Endl.)Carrière |
| Synonyms[1][2] | |
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Tsuga (/ˈsuːɡə/,[3] fromJapanese栂 (ツガ), the name ofTsuga sieboldii) is agenus ofconifers in the subfamilyAbietoideae ofPinaceae, the pine family. TheEnglish-languagecommon name "hemlock" arose from a perceived similarity in the smell of its crushed foliage to that of the unrelated planthemlock.[4] Unlike the latter,Tsuga species are not poisonous.[5]
The genus comprises eight to ten species (depending on theauthority), with four species occurring inNorth America and four to six in easternAsia.[6][7][8][2][9]
They are medium-sized to largeevergreentrees, ranging from 10–60 metres (33–197 feet) tall, with a conical to irregular crown, the latter occurring especially in some of the Asian species. The leading shoots generally droop. Thebark 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 spurshoots, which are present in manygymnosperms, 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 withpulvini that persist after theleaves fall. The winterbuds are ovoid or globose, usually rounded at the apex and not resinous.
The leaves are flattened to slightly angular and range from 5–35 millimetres (3⁄16–1+3⁄8 inches) long and1–3 mm (1⁄32–1⁄8 in) 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 apetiole 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 whitestomatal bands (which are inconspicuous onT. mertensiana) separated by an elevated midvein. The upper surface of the leaves lack stomata, except those ofT. mertensiana. They have one resin canal that is present beneath the single vascular bundle.[6][7][8][2][9]
Thepollen cones grow solitary from lateral buds. They are3–5 mm (1⁄8–3⁄16 in) – usually up to5 mm or3⁄16 in – in length, 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. Theseed cones are borne on year-old twigs and are small ovoid-globose or oblong-cylindric, ranging from15–40 mm (5⁄8–1+5⁄8 in) long, except inT. mertensiana, where they are cylindrical and longer,35–80 mm (1+1⁄2–3+1⁄4 in) in length; they are solitary, terminal or rarely lateral, pendulous, and aresessile or on a short peduncle up to4 mm (3⁄16 in) long. Maturation occurs in 5–8 months, and theseeds 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. Thebracts are included and small. The seeds are small, from2 to 4 mm (3⁄32 to5⁄32 in) long, and winged, with the wing being8 to 12 mm (5⁄16 to1⁄2 in) in length. They also contain small adaxial resinvesicles. Seed germination isepigeal; the seedlings have 4–6cotyledons.[6][7][8][2][9]
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 (1+3⁄8–3+1⁄8 in) long and cylindrical rather than ovoid. Some botanists treat it in a distinct genus asHesperopeuce mertensiana (Bong.) Rydb.,[10] though it is more generally only considered distinct at the rank of subgenus.[6]
The oldest fossils attributed to the genus are twigs, known from the Early Cretaceous of Inner Mongolia, China, though their relationship to modernTsuga is not unambiguous. The earliest pollen attributed to the genus is known from the Upper Cretaceous of Poland, dating to around 90 million years ago. Abundant remains are only known fromEocene onwards, when the modernTsugacrown group is thought to have begun to diversify.[11][12] While formerly present in the regionTsuga became extinct in Europe during theMiddle Pleistocene epoch around 780-440,000 years ago, due to unfavourable climate change caused by the ongoingQuaternary glaciation.[13]
Another species, bristlecone hemlock, first described asT. longibracteata, is now treated in a distinct genusNothotsuga; it differs fromTsuga in the erect (not pendulous) cones with exserted bracts, and male cones clustered in umbels, in these features more closely allied to the genusKeteleeria.[6][8]
| Phylogeny ofTsuga[14][15] | |||||||||||||||||||||||||||||||||||||||||||||||||||
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The above phylogeny is in marked conflict with earlier studies, which foundT. mertensiana as basal to the rest of the genus.[6]
Moved to other genera:
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 wintersnowfall and tolerateice storms better than most other trees.[6][8] Hemlock trees are more tolerant of heavy shade than other conifers; they are, however, more susceptible to drought.[24]
The two eastern North American species,T. canadensis andT. caroliniana, are under serious threat by the sap-suckinginsectAdelges tsugae (hemlock woolly adelgid).[25] This adelgid, related to theaphids, was introduced accidentally from eastern Asia, where it is only a minor pest. Extensive mortality has occurred, particularly east of theAppalachian 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 byhemlock looper.[26] 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 bydeer, and the seeds are eaten byfinches and smallrodents.
Old trees are commonly attacked by variousfungal disease and decay species, notablyHeterobasidion annosum andArmillaria species, which rot the heartwood and eventually leave the tree liable towindthrow, andRhizina undulata, which may kill groups of trees following minor grass fires that activate growth of theRhizina spores.[27]
The wood obtained from hemlocks is important in the timber industry, especially for use aswood pulp. Many species are used inhorticulture, and numerouscultivars have been selected for use in gardens.The bark is also used intanning leather.[28] The needles are sometimes used to make a tea and perfume.[citation needed]
Tsuga is the Japanese name for Hemlock (the conifer, not the poisonous herbConium maculatum inApiaceae).
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