Douglas-Fir (Pseudotsuga)

Description

Pseudotsuga is a genus of evergreen coniferous trees in the family Pinaceae (subfamily Laricoideae). Common names include Douglas fir, Douglas-fir, Douglas tree, and Oregon pine. Pseudotsuga menziesii is widespread in western North America and is an important source of timber. The number of species has long been debated, but two in western North America and two to four in eastern Asia are commonly acknowledged. Nineteenth-century botanists had problems in classifying Douglas-firs, due to the species' similarity to various other conifers better known at the time; they have at times been classified in Pinus, Picea, Abies, Tsuga, and even Sequoia. Because of their distinctive cones, Douglas-firs were finally placed in the new genus Pseudotsuga (meaning "false hemlock") by the French botanist Carrière in 1867. The genus name has also been hyphenated as Pseudo-tsuga. They are medium-sized to large evergreen trees, ranging from 10-60 m (33-197 ft) tall, with a conical to irregular crown, 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 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. 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 wing 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.