This genus has been sponsored and new text is being prepared, with publication expected in 2023.
A small genus of coniferous trees, here considered to include 4 species and 3–4 varieties, distributed in western North America and in eastern Asia. The American P. menziesii var. menziesii is one of the world’s tallest trees and can reach over 100 m; the other North American taxa and the Asian species rarely exceed 50 m and typically develop broad crowns. The bark is distinctive; generally scaly and grey-brown with prominent resin blisters when young, it later thickens considerably taking on a corky appearance and develops deep fissures in the boles of mature trees. Most species retain their lower branches into maturity, though these are commonly shed in mature specimens of P. menziesii, which often have massive, clear boles, especially in the wild. Branchlets pubescent or glabrous, prominently grooved, with raised, circular leaf scars. Vegetative buds fusiform-conical, relatively short, covered in red-brown, triangular scales. Leaves needle like, green or glaucous-blue, spirally arranged forming two irregular ranks, with two stomatal bands beneath; upper surface with a central groovel; base twisted; apex obtuse or emarginate. Pollen (male) cones borne individually but often clustered on branchlets, pendulous, 1–2 cm long in bud with distinctive red-brown perular scales, similar to those of Abies and Picea. Seed (female) cones erect at pollination, soon pendulous, borne near the ends of second-year shoots, mainly in the upper crown but some also on low branches in open grown trees; maturing from green or greenish-purple to brown over 5–7 months, falling intact after opening at maturity; cone scales broad, variously shaped, pedicellate, spirally arranged around a central rachis. The bract scales are diagnostic: large, straight or reflexed, trilobate and exserted. Seeds two per scale, partially enclosed in a membranous cup, which extends to form a persistent wing. (Farjon 1990; Grimshaw & Bayton 2009; Debreczy & Rácz 2011; Farjon 2017).
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).