Characteristics
Rocky Mountain Douglas-fir is a large tree, typically reaching 35–45 m in height and 1 m in diameter, with exceptional specimens known to 67 m tall, and 2 m diameter. It commonly lives more than 500 years and occasionally more than 1,200 years. The bark on young trees is thin, smooth, gray, and covered with resin blisters. On mature trees, it is moderately thick (3–6 cm), furrowed and corky though much less so than coast Douglas-fir.
The shoots are brown to gray-brown, smooth, though not as smooth as fir shoots, and finely pubescent with scattered short hairs. The buds are a distinctive narrow conic shape, 3–6 mm long, with red-brown bud scales. The leaves are spirally arranged but slightly twisted at the base to be upswept above the shoot, needle-like, 2–3 cm long, gray-green to blue-green above with a single broad stomatal patch, and with two whitish stomatal bands below.
The male (pollen) cones are 2–3 cm long, and are typically restricted to or more abundant on lower branches. Pollen cones develop over 1 year and wind-dispersed pollen is released for several weeks in the spring.
The mature female seed cones are pendent, 4–7 cm long, 2 cm broad when closed, opening to 3–4 cm broad. They are produced in spring, purple (sometimes green) at first, maturing orange-brown in the autumn 5–7 months later. The seeds are 5–6 mm long and 3–4 mm broad, with a 12–15 mm wing. Both coast Douglas-fir and Rocky Mountain Douglas-fir produce abundant crops of seed approximately every 2–11 years. Seed is produced annually except for about 1 year in any 4-to-5-year period.
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