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'''Tsuga''' (from , 栂; the name for ''Tsuga sieboldii'' ) is a genus of conifers in the family Pinaceae. They are commonly referred to as 'Hemlocks', though they are unrelated to other plants with the same name. All of the species are evergreen trees that range in height from 20 to 60 metres tall and are found exclusively in relatively moist areas with little or no water stress. There are between 8 and 10 species within the genus depending on the authority, with 4 occurring in North America and 4 to 6 in East Asia. Many species are utilised in horticulture and numerous cultivars have been developed for use in the garden.
The wood obtained from hemlocks is important in the timber industry, especially for use as wood pulp. The populations of two eastern North American species, namely ''T. canadensis'' and ''T. caroliniana'', are currently being rapidly reduced by a sap-sucking insect accidentally introduced from Asia called the Hemlock Woolly Adelgid, or ''Adelges tsugae''. While this bug does not seriously affect the hemlocks in its native range, nor the two western North American species, it is unchecked by both resistance and predators in the eastern United States. 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.

Contents

Description

Taxonomy

Ecology

References

Description

''Tsuga''s are medium-size to large evergreen trees, ranging from 20 to 65 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. Spur shoots, which are present in many gymnosperms, are absent to moderately developed.^[1] The young twigs as well as the distal portions of stem are flexuous and pendent. The stems are rough due to peglike projections that persist after the leaves fall.
The leaves are flattened to slightly angular and range in size from 8 to 40 mm in length and 1.5 to 3 mm in width. They are borne singly and are arranged either pectinately on the stem or rarely radially. Towards the base the leaves narrow abruptly to a petiole that is set on a forward-angled, twig-like projection. The petiole is twisted at the base so that it is almost parallel with the stem. Sheaths are absent. The apex is either notched, acute, or rounded. 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 for in the subgenus ''Hesperopeuce''. They have one resin canal that is present beneath the single vascular bundle. When sprouting from seed, 4 to 6 cotyledons are present. The buds persist into winter and are ovoid or globose, usually rounded at the apex and not resinous.^[2]
The cones are borne on year-old twigs and are small, ranging in length from 15 to 35 mm long. In the subgenus ''Hesperopeuce'' they are larger at 35 to 70 mm in length. The pollen cones grow solitary from lateral buds, though sometimes they occur in clustered umbels from a single bud. They are ovoid, globose, or ellipsoid in shape and brown in colour. 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 terminal or rarely lateral, occurring on second year branchlets. They occur solitary and are pendulous, though they are also erect in some species. They are ovoid-globose, oblong, or oblong-cylindric in shape and are sessile or at least very nearly so. Maturation occurs within the first year and the seeds are shed shortly thereafter, or they may persist for several years. 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, and they rarely have a somewhat protruding apical cusp. 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. They germinate epigeally.^[3]

Taxonomy

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''.
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, 35-70 mm long and cylindrical rather than ovoid. Some botanists treat it in a distinct genus as ''Hesperopeuce mertensiana'', though it is more generally only considered distinct at the rank of subgenus.

Ecology

Mountain Hemlock foliage and cones

Western Hemlock ''Tsuga heterophylla'' is the largest species, reaching heights of 70 m. It is a particularly common timber tree in the Pacific Northwest of North America and is the state tree of Washington. The tree is also planted for timber in northwest Europe and other temperate areas with high rainfall and cool summers.
Two eastern North American species''T. canadensis'' and ''T. caroliniana'' are under serious threat by a sap-sucking insect called the Hemlock Woolly Adelgid (''Adelges tsugae''). This aphid was introduced accidentally from eastern Asia. Extensive mortality has occurred, particularly east of the Appalachian Mountains. The Asian species, and also the two west American hemlocks, are relatively resistant to this pest. ''Tsuga'' species are used as food plants by the larvae of some Lepidoptera species including Autumnal Moth and The Engrailed, and older caterpillars of the Gypsy Moth.

References

1. ''Tsuga''
2. ''Tsuga''
3. Sections on Picea and Tsuga


★ Gymnosperm Database: ''Tsuga'' description

★ Arboretum de Villardebelle: photos of cones of all the species

★ US Forest Service Hemlock Woolly Adelgid website