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Rocky Mountain Douglas-fir

Pseudotsuga menziesii var. glauca is the botanical name for the Rocky Mountain variety of the Douglas-fir.

Rocky Mountain Douglas-fir is native to the inland mountains of the Pacific Northwest and the Rocky Mountains from central British Columbia south to northern and central Mexico. Its range is fairly continuous from central British Columbia south through eastern Washington and eastern Oregon to central Idaho, western Wyoming, and western Montana; it is restricted to mountain topography in Utah, Nevada, Colorado, New Mexico, Arizona, and northern and central Mexico. Populations of Rocky Mountain Douglas-fir are very isolated in Texas, Coahuila, Nuevo León, Zacatecas, Durango, Chihuahua, and Sonora.

Rocky Mountain Douglas-fir is a coniferous, evergreen tree. Open-growing trees often have branches over the length of the bole, while trees in dense stands lack lower limbs. The bark on saplings is photosynthetically active, smooth, and covered with resin blisters; mature individuals have thick, deeply-furrowed, corky bark. At about age 40 (in the northern Rockies), bark becomes thick and corky. Bark thickness in the northern Rockies is about 1 inch (2.5 cm) on 12-inch (30 cm) diameter trees, and 2.5 inches (6 cm) on 24-inch (60 cm) diameter trees.

Rocky Mountain Douglas-fir mature cones are 1.6 to 2.8 inches (4 to 7 cm) long . Male strobili are approximately 0.8 inch (2 cm) long and females 1.2 inches (3 cm). Needles are 0.6 to 1.4 inch (15-35 mm) long. Rocky Mountain Douglas-fir grows 100 to 130 feet (30 to 42 m) high [120] (occasionally up to 160 feet (48 m) [53]). Diameter seldom exceeds 5 feet (152 cm).

The oldest accurately-dated Rocky Mountain Douglas-fir, 930 years old, is on the El Malpais National Monument 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.

Rocky Mountain Douglas-fir grows much more slowly than coast Douglas-fir and is also more cold tolerant. Its presence in variable habitats is due to genetic differentiation rather than ecological amplitude. Races with respect to tolerance of different environmental conditions are easily detected. Differences in cold-hardiness have been observed between northern Idaho populations and northwestern Montana populations of Rocky Mountain Douglas-fir.

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; average lifespan of fine roots is usually between several days and several weeks.

Pollen cones 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. Douglas-fir (both Coast and Rocky Mountain varieties) produces abundant crops of seed approximately every 2 to 11 years. Seed is produced annually except for about 1 year in any 4- to 5-year period.

Age at first reproduction is 12 to 15 years. Douglas-fir has winged seeds that are dispersed primarily by wind and gravity. In western Montana clearcuts, seeds were dispersed up to 800 feet (244 m) uphill from their source, but seedfall between 600 and 800 feet (183-244 m) was only 7% of that found in uncut stands. Other studies determined that seedfall in clearcuts beyond 265 feet (80 m) 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 0.6 to 1.2 miles (1 to 2 km) distant, but most Douglas-fir seeds fall within 330 feet (100 m) of their source. Small amounts of seeds 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.

Rocky Mountain Douglas-fir is a valuable timber tree. The wood is exceptionally strong and is used for structural timber as well as poles, plywood, pulp, dimensional lumber, plywood, railroad ties, mine timbers, log cabins, posts and poles, fencing, and firewood. Other uses listed include "machine-stress-rated lumber," finger-jointed studs, glued-laminated beams, pallets, furniture, cabinets, doors, and window frames.

In spring and winter (in British Columbia, Idaho, and Montana) elk browse on south- and southwest-facing Rocky Mountain Douglas-fir and Pacific ponderosa pine stands, particularly when shrubs and/or grasses are productive. In summer, elk generally are found at higher elevations (outside the Rocky Mountain Douglas-fir and Pacific ponderosa pine zones). During fall elk use stands of Rocky Mountain lodgepole pine, subalpine fir, western larch, or grand fir with high canopy cover.

In parts of Yellowstone National Park, elk browsing is so pervasive that young Rocky Mountain Douglas-fir are stunted at 3 to 4.5 feet (1-1.5 m) in height, with live branches trailing very close to the ground, and branches on the upper 2/3rds of the tree dead. Low-elevation and south-facing open-structure Rocky Mountain Douglas-fir types are often important winter range for white-tailed 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 decimate 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.

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