The Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca) is an evergreen conifer native to the interior mountainous regions of western North America, from central British Columbia and southwest Alberta in Canada southward through the United States to the far north of Mexico. The range is continuous in the northern Rocky Mountains south to eastern Washington, eastern Oregon, Idaho, western and south-central Montana and western Wyoming, but becomes discontinuous further south, confined to "sky islands" on the higher mountains in Utah, Colorado, Arizona and New Mexico, with only very isolated small populations in eastern Nevada, westernmost Texas, and the north of Coahuila, Chihuahua, and Sonora in Mexico. It occurs from 600 m altitude in the north of the range, up to 3,000 m, rarely 3,200 m, in the south.
Further west towards the Pacific coast, it is replaced by the related Coast Douglas-fir (Pseudotsuga menziesii var. menziesii), of which Rocky Mountain Douglas-fir is variously treated as a variety, subspecies (Pseudotsuga menziesii subsp. glauca), or as a distinct species (Pseudotsuga glauca). To the south, it is replaced by Mexican Douglas-fir (Pseudotsuga lindleyana) in high mountains as far south as Oaxaca.
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. Unlike the Coast Douglas-fir, Rocky Mountain Douglas-fir foliage has no noticeable scent.
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 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.
There are two varieties, which have proved distinct on genetic testing (Li & Adams, 1989):
The oldest accurately-dated Rocky Mountain Douglas-fir, 1275 years old, is in New Mexico. This longevity is apparently an anomaly; growing on a relatively barren lava field has protected it from fire, animals, and humans. Growth typically slows dramatically between 90 and 140 years of age.
Root morphology is variable, but when unimpeded, a taproot forms within several years. "Platelike" root morphologies occur where growth is impeded. The most prominent lateral roots begin in the 1st or 2nd year of growth. Most roots in surface soil are "long ropelike laterals of secondary and tertiary origin". Fine-root production is episodic in response to changing environmental conditions; the average lifespan of fine roots is usually between several days and several weeks.
Rocky Mountain Douglas-fir reaches reproductive maturity at 12-15 years. It has winged seeds that are dispersed primarily by wind and gravity. In western Montana clearcuts, seeds were dispersed up to 250 m (800 feet) uphill from their source, but seedfall between 180-250 m (600-800 feet) was only 7% of that found in uncut stands. Other studies determined that seedfall in clearcuts beyond 80 m (265 feet) from seed trees was about 3% of seedfall in uncut stands where seed trees are close together. Well-stocked stands have resulted from seedfall from sources 1-2 km (0.6-1.2 miles) distant, but most Douglas-fir seeds fall within 100 m (330 feet) of their source. Small amounts of seed are dispersed by mice, chipmunks, and squirrels. Rocky Mountain Douglas-fir seeds are disseminated about twice as far as seeds of Ponderosa Pine.
Rocky Mountain Douglas-fir grows on a variety of sites across its wide geographic range. It grows at lower elevations adjacent to and within bunchgrass communities and is also found in upper-elevation subalpine forests. It tends to be most abundant in low- and middle-elevation forests, where it grows over a wide range of aspects, slopes, landforms, and soils.
In parts of Yellowstone National Park, Elk browsing is so intensive that young Rocky Mountain Douglas-fir are stunted at 1-1.5 m (3-4.5 feet) in height, with live branches trailing very close to the ground, and branches on the upper two thirds of the tree dead. Low-elevation and south-facing open-structure Rocky Mountain Douglas-fir types are often important winter range for White-tailed Deer and Mule Deer. Moose winter in low-elevation Rocky Mountain Douglas-fir types in areas where willow thickets, the preferred winter habitat, are lacking; in such areas Rocky Mountain Douglas-fir is an important moose food.
Chipmunks, mice, voles, and shrews eat large quantities of conifer seeds from the forest floor, and clipped cones are a staple and major part of storage of red squirrels. These animals store a large amount of Rocky Mountain Douglas-fir cones or seeds. American Marten commonly den in hollow logs.
Numerous species of songbirds extract seeds from Douglas-fir cones or forage for seeds on the ground. The most common are the Clark's Nutcracker, Black-capped Chickadee, Mountain Chickadee, Boreal Chickadee, Red-breasted Nuthatch, Pygmy Nuthatch, Red Crossbill, White-winged Crossbill, Dark-eyed Junco, and Pine Siskin. Migrating flocks of Dark-eyed Juncos may consume vast quantities of seeds and freshly germinated seedlings. Woodpeckers commonly feed in the bark of Rocky Mountain Douglas-fir. Blue Grouse forage on needles and buds in winter; they and other birds rely heavily on Rocky Mountain Douglas-fir communities for cover.