• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

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 [125]. Growth typically slows dramatically between 90 and 140 years of age [184].

Rocky Mountain Douglas-fir grows much more slowly than coast Douglas-fir [115] and is also more cold tolerant [53]. 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 [53,155,156,212]. Differences in cold-hardiness have been observed between northern Idaho populations and northwestern Montana populations of Rocky Mountain Douglas-fir [211].

Roots: 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 1^st or 2^nd 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 [53]. Rocky Mountain Douglas-fir in Colorado that were 22 to 24 feet (6.7 to 7.3 m) tall and 60 to 80 years old had root systems that extended 2.7 to 5 feet (0.82 to 1.52 m) vertically and 10 to 21 feet (3 to 6.4 m) laterally [39]. In a 27 to 53 inch- (69 to 135 cm) deep soil taproots were 50% of final depth in 3 to 5 years and 90% in 6 to 8 years [53]. Richardson [217] reported root growth rates averaging 2.9 inches (7.4 cm), ranging from 1.1 to 6 inches (2.8- 15.3 cm), per year in Rocky Mountain Douglas-fir near Merritt, British Columbia; rates were lower than others had reported because soil water and texture were highly variable and coarse fragments limited growing space.

Rocky Mountain Douglas-fir is ectomycorrhizal [110] and sometimes ectendomycorrhizal; 2,000 fungal associates have been reported. Colonization is not inoculum-limited except in nursery conditions [53].

RAUNKIAER [209] LIFE FORM:

REGENERATION PROCESSES:

Pollination: | Rocky Mountain Douglas-Fir 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 [11].

Seed production: Douglas-fir (both 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" [196]. Age at first reproduction is 12 to 15 years (both varieties). The magnitude of the cone crop is affected by the number of primordia that develop rather than by the number formed. Accordingly, the current year's crop is in large part influenced by the abortion rate of the previous year's primordia. However, even with low rates of primordia abortion, frost and insect infestation can reduce cone production [53]. Finley [87] reported estimates of the size of Rocky Mountain Douglas-fir's cone crop: in Washington and Oregon the number of cones per tree averaged 1,126 and ranged from 151 to 6,000; in British Columbia the average was 1,300 with a range of 1,000 to 4,000. Each cone contains 20 to 30 seeds [196].

Seed dispersal: | Rocky Mountain Douglas-Fir Douglas-fir has winged seeds that are dispersed primarily by wind and gravity [196]. 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 [236]. 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 in close proximity [117]. According to Burns and Honkala [53] 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 [53]. Small amounts of seeds are dispersed by mice, chipmunks, and squirrels [115,238]. Rocky Mountain Douglas-fir seeds are disseminated about twice as far as seeds of ponderosa pine [227].

Seed banking: | Rocky Mountain Douglas-Fir Consumption of seed by birds, mammals, and insects reduces natural regeneration of Rocky-Mountain Douglas-fir [53]. Caching of cones by red squirrels, and consumption by chipmunks, mice, voles, and birds reduces seed quantity considerably [115,238]. Between seedfall and germination, much seed is consumed by white-footed deer mice, creeping voles, chipmunks, shrews, birds, juncos, varied thrush, blue and ruffed grouse, and song sparrows [53]. Insect consumption of seed is discussed in the "Management Considerations" section of this species summary.

Germination: Most germination occurs within 150 days of seedfall, but seeds remain viable for 1 or occasionally 2 years [53]. Light exposure and, even more importantly, stratification affect the germination rate of Rocky Mountain Douglas-fir seeds [155]. In northwestern Montana, seed "soundness" averaged 39% to 43% during good seed crop years but was only 11% during poor years [236]. Average germinative capacity of Rocky Mountain Douglas-fir seed (collected in north-central Colorado) ranged from 68 to 94% under various controlled stratification periods and germination temperatures [196]. Poor seed crop years are characterized by low seed viability, possibly because of high frequency of self-fertilization [53].

Seedling establishment/growth: Coast Douglas-fir exhibits a strong preference for moist mineral soil, while Rocky Mountain Douglas-fir establishes in mineral soil and organic seedbeds less than 2 inches (5 cm) thick [53,226,227]. In western larch-Rocky Mountain Douglas-fir forests in Montana, however, natural stocking of Rocky Mountain Douglas-fir in clearcuts following site preparation was higher on undisturbed litter than on exposed surfaces [112,229]. See the 'Fire Effects' section of this summary for information on interactions of fire and seedling establishment. Bai and others [26], using constructed seedbeds with 6 treatments on seedling emergence and growth, showed that Rocky Mountain Douglas-fir germinated best in manure and fescue litter and worst in Rocky Mountain Douglas-fir litter; means and standard errors are given below:

Seedling growth during the 1^st year of establishment is slow. Seedlings are dormant from the onset of drought in summer until the following spring [53]. Seedling survival is best under partial shade in relatively dry habitats [151,226]. Establishment of Rocky Mountain Douglas-fir requires shade on southern aspects, particularly in the southern portion of its range; existing vegetation ameliorates drought and temperature extremes [53,134,228]. Coffman [62] observed survival of planted Rocky Mountain Douglas-fir in varying amounts of brush in south-central New Mexico; with brush at least 5 feet (1.5 m) away survival was 38.5%, with brush 2 to 5 (0.6 to 1.5 m) feet away survival was 47.7%, with brush 0 to 2 feet away (0 to 0.6 m) survival was 56.0%, and 66.5% of those planted under brush canopy survived (percentages are average 3-month survival for all aspects). On mesic or wet sites existing vegetation may be more inhibitive than facultative. Coast Douglas-fir grows best with weed control as competitive shading limits growth [53]. Kidd [148] observed strong negative effects of grass competition on the height, diameter and lateral leader length of Rocky Mountain Douglas-fir in a clearcut in a moist western redcedar habitat type in Idaho. Where competitive effects are predominant it is usually where grasses or sedges are rhizomatous and extract water from the same soil that Rocky Mountain Douglas-fir does [205,242].

In a clearcut in eastern Arizona at 9,100 feet (2,774 m), 6-year-old Rocky Mountain Douglas-fir averaged 21.3 inches (54 cm) tall, with a standard deviation of 9.7 inches (24.6 cm). On the same site Rocky Mountain Douglas-fir roots averaged 3.1 inches (7.9 cm) in height after the 1^st growing season and averaged 7 inches (17.6 cm) tall after 5 growing seasons [133]. A study of survival and growth of seedlings planted after various types of site preparation showed that growth at post-treatment year 3 was significantly (p<0.1) less on dozer-scarified plots than on untreated control, broadcast burned, or burned pile plots [177]. On dry lower quality sites, growth of Rocky Mountain Douglas-fir is often best when seedlings have root systems and ectomycorrhizal root tips located in decaying wood or humic layers [101].

Growth is extremely slow past age 200 years, but growth rates favorably respond to thinning (mechanically or by fire) at any age [53,115]. Younger trees' growth rates are more responsive to release from competition; in central Idaho, Rocky Mountain Douglas-fir shorter than 20 feet (6 m) at time of release had higher growth rates than those taller than 20 feet  (6 m) at release [167].

Asexual regeneration: Rocky Mountain Douglas-fir does not reproduce asexually under natural conditions [53]. Cuttings for regeneration purposes have had only limited success; only trees less than 10 years old have produced cuttings that could establish. Also, cuttings that do establish generally exhibit a trailing growth habit before growing upward [117].

SITE CHARACTERISTICS:

Soils: Rocky Mountain Douglas-fir grows on a wide variety of soils and parent materials. Substrates may be of igneous, sedimentary, or metamorphic origin. In some areas, particularly near the Great Plains, Rocky Mountain Douglas-fir is more common on basic parent materials such as limestone, andesite, and basalt [80,246]. In southern and central Utah, Rocky Mountain Douglas-fir shows a strong affinity for soils derived from sandstone and limestone, and often occurs on sites with shallow, rocky soils and a large amount of bare ground [274]. In the Sangre de Cristo Range, Colorado, acidic soils on north-facing slopes are dominated by Rocky Mountain lodgepole pine and/or Rocky Mountain Douglas-fir; more basic soils on southern aspects are dominated by quaking aspen or white fir [12]. In the Gros Ventre Mountains and other areas in northwestern Wyoming and Montana, pure stands of Rocky Mountain Douglas-fir develop on calcareous substrates [158].

Elevation: Rocky Mountain Douglas-fir grows "mostly" at elevations between 1,800 and 8,000 feet (550-2,440 m) in the northern part of its range [53]. In British Columbia, Douglas-fir (both varieties) grows from sea level to 2,500 feet (762 m); in Washington and Oregon it grows generally between sea level and 5,000 feet (1,520 m) (locally higher). Mesic ponderosa pine vegetation types with Rocky Mountain Douglas-fir invading generally are between 4,000 and 5,000 feet (1,219 to 1,524 m) [31]. In the central Rocky Mountains, Rocky Mountain Douglas-fir grows best between 6,000 and 8,000 feet (1,830 to 2,590 m), and between 8,000 and 9,500 feet (2,440 to 2,900 m) in the southern Rocky Mountains. Rocky Mountain Douglas-fir may grow as low as 5,100 feet (1,550 m) in canyons in central Arizona, or as high as 10,700 feet (3,262 m) on Mount Graham in southeastern Arizona [53]. In Utah Rocky Mountain Douglas-fir grows between 5,000 and 10,000 feet (1,525 to 3,050 m) [265].

Water table: Under laboratory control Rocky Mountain Douglas-fir seedlings survived non-aerated submersion for 3 days (75% survival) and 7 days (23% survival); none survived 10 days [173].

SUCCESSIONAL STATUS:

Rocky Mountain Douglas-fir is successional to ponderosa pine and Rocky Mountain lodgepole pine but, in the absence of major disturbance, the longer-lived ponderosa pine is codominant longer than Rocky Mountain lodgepole pine [3]. Rocky Mountain Douglas-fir has invaded and increased on big sagebrush-grasslands because of decreased fire frequency, climate change, and grazing pressure [19,54]. The species has also expanded into grasslands and meadows of the southwestern United States; approximately 55% of high elevation meadows in the Jemez Mountains of northern New Mexico have been invaded by conifers since the early part of the century [68]. Rocky Mountain Douglas-fir is successional to quaking aspen and has, with other shade tolerant conifers, invaded quaking aspen stands where fire exclusion has reduced clonal reproduction [141]. Rocky Mountain Douglas-fir has also increased in open long-needled pine forests (Arizona pine, piño blanco, Durango pine, Apache pine, Chihuahua pine, and piño triste) in Durango and Chihuahua, Mexico [93].

The historic "ponderosa pine savannah" was, particularly in the central and northern Rocky Mountains, part of a mosaic of differing densities and species proportions resulting from temporal and spatial variability in fire regimes and climatic patterns [68,140]. Fire suppression favors increased Rocky Mountain Douglas-fir because it is less fire resistant and slower growing than ponderosa pine when juvenile [227]. In the northern part of Rocky Mountain Douglas-fir's range, in eastern Washington and Oregon, prior to settlement the mosaic of stand types was not generally observable within a 15,000 acre area. There still existed a variety of stand densities in the northern Rockies and eastern Cascade Range, but disturbance was generally larger scale [131]. Ponderosa pine forests were also heavily logged, usually by high-grading, creating higher-density and more even-aged stands, and thus making them more susceptible to increased insect and disease frequency [23].

Rocky Mountain Douglas-fir invasion and increased density on sagebrush grasslands and ponderosa pine stands have resulted from changes in fire regimes, climatic variation, selective logging, and interactions thereof over the last 100 to 150 years [26,140,227,249,251]. Today, some sagebrush communities have sapling-sized (0.8 to 5.1 inches (2 to 13 cm) in diameter) Rocky Mountain Douglas-fir mixed with the former community. On other sites, slightly older stands (7.2 to 11.8 inches (18 to 30 cm) diameter) exist, grass cover is much reduced, and remains of big sagebrush are the only evidence of the former community. In the Galena Study Area, near Butte, Montana, forested area has increased from 48% in 1878 to 75% in 1984 (of 40 reference sites) [20].

In Glacier National Park, as in other areas at the wet/cool extreme of its former range, ponderosa pine is "not reproducing," and mesic shade-tolerant conifers are replacing it. There has been a concomitant increase in fuel loading and likelihood of stand-replacing fire [163]. See the "Fire Ecology" and "Fire Effects" sections of this species summary for more information on fire's influence on succession.

Hartwell and others [109] observed forest compositional changes in 3 elevation ranges in the Bitterroot Mountains (Bass and Blodgett creeks of western Montana), showing a large increase in Rocky Mountain Douglas-fir's relative basal area in the "ponderosa pine zone;" in other zones it either decreased or was relatively constant. Their results are presented below:

SEASONAL DEVELOPMENT:

Vegetative bud and lateral bud growth initiate in late March or April following increased enzymatic activity in March [53]. Vegetative bud burst is in late May, shoot elongation occurs in April through mid-July. Male and female cone bud primordia are present at vegetative bud break but are non-differentiable from primordia of vegetative buds until April or May [11,53]. Timing of Rocky Mountain Douglas-fir phenological events can vary greatly depending on weather, elevation, and latitude. Yearly weather variations can greatly alter timing of seedfall at a single location because cones open and release seeds as they dry [196].

Phenology of Rocky Mountain Douglas-fir in northern Idaho, western Montana, and Yellowstone Park was described as follows [231]:

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Related categories for SPECIES: Pseudotsuga menziesii var. glauca | Rocky Mountain Douglas-Fir