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Pseudotsuga includes five to seven species, three in western North America and four in eastern Asia. The habit of Douglas-firs is rather variable, as P. menziesii is a massive tree with a columnar trunk, the tallest in the Pinaceae (up to 100 m), while the Asian species rarely reach half this height, with trunks usually dividing to form a broad crown. The Mexican P. lindleyana is probably the smallest in the genus, developing a slender conical crown, often only 25–30 m tall; the Californian P. macrocarpa is only slightly taller but is commonly much stouter. The bark is generally scaly and grey-brown, but later thickens, forming deep longitudinal cracks. Most species retain their lower branches into maturity, though abscission of lower branches is common in mature specimens of P. menziesii, which often have massive, clear boles. The branchlets are pubescent or glabrous, prominently grooved, and with raised, circular leaf scars. The buds are not resinous and are covered in red-brown, triangular scales. The needle-like leaves are dark green or glaucous-blue, rigid and spirally arranged. They form two untidy ranks, and are twisted at the base. They are grooved on the non-stomatal side, linear, with an acute or obtuse apex (American species) or emarginate apex (Asian species). Stomata are restricted to the lower surface, in two bands separated by the midrib. The male strobili are solitary, produced on branchlets in all parts of the crown, and buds are 1–2 cm long with distinctive red-brown perular scales, rather similar to those of Abies and Picea. The female cones are erect at pollination but become pendulous, and develop near the end of second-year shoots, mainly in the upper crown but some also on low branches where exposed to good light. They mature in five to seven months, during which time they change from green or purple to brown. The cone scales are spirally arranged around a central rachis, and the cone falls whole after opening at maturity. The cone scales are broad, variously shaped, and pedicellate. They are persistent, and open to release the seeds. The bract scales are large, straight or reflexed, trilobate and exserted. There are two seeds on each cone scale, partially enclosed in a membranous cup, which extends to form a persistent wing (Farjon 1990).
Pseudotsuga is long overdue a modern taxonomic review. In particular, many problems stem from the description by Flous (1934a, 1934b) of an excessively large number of ill-founded taxa throughout the American range of the genus, based on cone characters that proved poorly formulated and unreliable, and a subsequent highly conservative reaction against this interpretation by Little (1952), resulting in reduction to the generally familiar two species P. macrocarpa and P. menziesii – which reduction, however, overlooks apparently genuinely distinct taxa in Mexico. Within P. menziesii, only var. glauca (Beissn.) Franco (Blue or Rocky Mountains Douglas-fir) and the nominate var. menziesii (Green or Coast Douglas-fir) are currently accepted (Farjon 2001). Var. glauca shows very marked ecological differences from var. menziesii, which are matched by strong genetic differentiation (Li & Adams 1989), and it should probably be recognised at subspecific rank. Genetically it is subdivided into two (Li & Adams 1989) – a southern subgroup (subsp. glauca var. glauca) and a northern subgroup (subsp. glauca var. caesia) – and the divide is sharp, situated at about 44° N, running west-southwest along the Bighorn and Shoshone Rivers, across Yellowstone and along the Snake River.
The mainland Asian taxa are nearly as complex and require further study. Flora of China (Fu et al. 1999c) recognises P. brevifolia, P. forrestii and P. sinensis (with the Taiwanese var. wilsoniana), principally distinguished by leaf length and seed scale characters, and this treatment is followed here. Pseudotsuga brevifolia W.C. Cheng & L.K. Fu (syn. P. sinensis var. brevifolia (W.C. Cheng & L.K. Fu) Farjon & Silba) is characterised by short, broad leaves (0.7–1.5 0.2–0.3 cm), the underside of which is flat, as in P. sinensis. It was introduced from northern Vietnam in 2003 and has so far proved hardy in western England, but is very slow-growing (K. Rushforth, pers. comm. 2008). It occurs in southwestern Guanxi and in Guizhou, but is more widespread across northeast Vietnam where it occurs on karst limestone ridges, with at least seven other conifer genera (Amentotaxus, Cephalotaxus, Nageia, Pinus, Podocarpus, Taxus, Tsuga, and possibly Calocedrus and Xanthocyparis); it may also occur in southeast Yunnan (K. Rushforth, pers. comm. 2008).
A genus of five or six species of large evergreen trees in western N. America, Mexico, China, Formosa, and Japan. Buds slender, acute, not resinous. Leaves set spirally, but spreading and crowded into two opposite rows, linear, grooved above, with two bands of stomata beneath. Cones pendulous, with persistent scales (as in Picea); bracts much longer than the scales and always a conspicuous feature of the cone (in Picea the bracts are very small and never exposed; the cones of Abies often have exserted bracts, but they are erect, and the scales are deciduous). The generic name Pseudotsuga implies a resemblance to Tsuga, but in that genus the cones resemble those of Picea (except in being smaller) and, as in Picea, the branchlets are roughened by the persistent leaf-bases, whereas in Pseudotsuga they are more or less smooth (as in Abies).