Rocky Mountain Douglas-fir

The Rocky Mountain Douglas-fir (Pseudotsuga menziesii subsp. glauca) is an evergreen conifer native to the interior mountainous regions of western North America, from central British Columbia and southwest Alberta, 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 Wyoming, and western Montana, 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 subsp. menziesii), of which Rocky Mountain Douglas-fir is variously treated as a variety (Pseudotsuga menziesii var. glauca), subspecies, 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.

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. Unlike the Coast Douglas-fir, Rocky Mountain Douglas-fir foliage has no noticeable scent.

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. The male (pollen) cones are 2-3 cm long, dispersing yellow pollen in spring.

There are two varieties, which have proved distinct on genetic testing (Li & Adams, 1989):

• 'Blue Douglas-fir' or 'Colorado Douglas-fir' (var. glauca) in the southern Rocky Mountains. Leaves strongly blue-green, 2-3 cm long. Cone scale bracts broader, often reflexed.
• 'Gray Douglas-fir' or 'Fraser River Douglas-fir' (var. caesia) in the northern Rocky Mountains. Leaves gray-green to green above, 2-2.5 cm long. Cone scale bracts slender, not reflexed.

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.

Ecology

Rocky Mountain Douglas-fir grows more slowly than Coast Douglas-fir and is also much more cold tolerant. Its presence in variable habitats is due to genetic differentiation rather than ecological amplitude. Variation 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, across the divide between the 'Gray' and 'Blue' varieities.

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.

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. 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.

The age at first reproduction is 12 to 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 spring and winter (in British Columbia, Idaho, and Montana) elk browse on south- and southwest-facing Rocky Mountain Douglas-fir and 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 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 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.

Uses

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.

References

• Peng Li & W. T. Adams. Rangewide patterns of allozyme variation in Douglas-fir. Canad. J. Forest Res. 19: 149-161 (1989).

External links

• Arboretum de Villardebelle - cone photos (http://www.pinetum.org/cones/PScones.htm)
• Gymnosperm Database: Pseudotsuga menziesii subsp. glauca (http://www.botanik.uni-bonn.de/conifers/pi/ps/glauca.htm)
• US Forest Service Fire Effects Database (http://www.fs.fed.us/database/feis/plants/tree/psemeng/index.html)