In the Blue Mountains of northeastern Oregon, thinning or burning modified stand structure
and tree species composition slightly in closed-canopy Rocky Mountain Douglas-fir
and Pacific ponderosa pine/Rocky Mountain Douglas-fir stands in the stem exclusion
stage. Stands were dominated by large (≥21 inches (53 cm) in diameter), 70- to 100-year-old, even-aged
Rocky Mountain Douglas-firs and Pacific ponderosa pines, with smaller Rocky Mountain Douglas-firs
that had regenerated after extensive partial cutting. Thinning removed 4- to 10-inch (10-25 cm) diameter
Douglas-firs and ponderosa pines, while burning reduced the number of small-diameter Douglas-firs and killed Douglas-fir
seedlings. Thin-and-burn treatments resulted in greatest reduction in basal area,
reaching the targeted goal of 16.0 mò/ha. Thin-and-burn treatments were also most
successful in reducing conifer seedling density. All treatments caused minor changes
in understory vegetation. The authors suggested that repeated treatments were needed
in 10 to 15 years to bring stand structure and composition more in line with historical
conditions. They comment that one set of treatments is not likely to mitigate nearly
80 years of fire exclusion and fuel accumulation in low-elevation, dry forests. For
further information on the effects of thinning and burning treatments on Rocky
Mountain Douglas-fir and 48 other species, see the Research Project Summary
of Youngblood and others' [274] study.
For further information on Rocky Mountain Douglas-fir response to fire, see the following Fire Studies:
Nongame birds: In montane forests of Colorado, Rocky Mountain Douglas-fir snags are commonly used by cavity-nesting birds [217]. Numerous species of song birds nest in Douglas-fir foliage. In central Idaho, the Rocky Mountain Douglas-fir/pinegrass and Rocky Mountain Douglas-fir/white spirea habitat types are important to nesting Steller's jays, pine siskins, western tanagers, red-breasted nuthatches, and Cooper's hawks [249]. In Montana, pileated woodpeckers prefer western larch, Pacific ponderosa pine, black cottonwood (Populus trichocarpa), and quaking aspen over Rocky Mountain Douglas-fir for nesting [175].
Game birds: Blue grouse use open, dense, and intermediate density Rocky Mountain Douglas-fir stands (pure stands or with subalpine fir, Engelmann spruce, lodgepole pine, limber pine, Rocky Mountain juniper, or quaking aspen) [56]. However, males heavily use thickets of younger Rocky Mountain Douglas-fir (average about 5 inches (12.5 cm) in diameter) that have high density (up to 1,200 trees per acre) [171]. In northeastern Utah, blue grouse were observed to prefer Rocky Mountain Douglas-fir trees for roosts during the day and subalpine fir at night [202]. Pekins and others [202] suggest that management strategies "perpetuate large trees within Douglas-fir-subalpine fir habitat in areas occupied by blue grouse." Merriam's turkeys use tall, high-canopy coverage Rocky Mountain Douglas-fir for roosts [262]. Wakeling and Rodgers [262] suggest that logging should avoid known roosts and retain 20.2 m/ha of basal area.
Raptors: Flammulated owls most commonly use old growth Rocky Mountain Douglas-fir with or without ponderosa pine because these forests have a higher insect diversity and availability, open structure, and cavities for nesting [126]. Old growth that occurs in a landscape of closed canopy forest with little habitat variety are not used as much as old growth when it occurs in complex landscape with openings and dense thickets. Where old growth occurs in forests that are predominantly closed-canopy, prescribed surface fire may augment habitat, but surface fire applied over a large area without leaving thickets probably decreases habitat quality [166]. In the River of No Return Wilderness in central Idaho, boreal owls use mixed conifer stands (39%), spruce-fir stands (25%), Rocky Mountain Douglas-fir (18%), and quaking aspen stands (18%) for breeding territories [114]. Mexican spotted owls in the San Mateo Mountains of New Mexico showed a preference for greater density of large Rocky Mountain Douglas-fir, southwestern white pine, and Gambel oak. Canopy disturbance, through stand-replacing fire or logging, may have a negative impact on Mexican spotted owl habitat [121]. Sharp-shinned hawks, great-horned owls, and Cooper's hawks also nest and/or roost in Rocky Mountain Douglas-fir [186,207].
Rocky Mountain Douglas-fir habitat types provide excellent hiding and thermal cover for deer, elk, and bighorn sheep [64]. Dittberner and Olsen [78] rate Rocky Mountain Douglas-fir cover value as "good" for mule deer, white-tailed deer, small mammals, small nongame birds, and upland game birds in Colorado, Montana, Utah, and Wyoming.
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 [44,88,107,130,267]. At about age 40 (in the northern Rockies), bark becomes thick and corky [44,107]. 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 [1]. Monserud [184] found that bark thickness (both layers) is equal to 13% of diameter in northern Idaho and northwestern Montana, and in the eastern Cascade Range, bark thickness is equal to 0.0704+(0.1176*diameter). Rocky Mountain Douglas-fir mature cones are 1.6 to 2.8 inches (4 to 7 cm) long [120]. Male strobili are approximately 0.8 inch (2 cm) long and females 1.2 inches (3 cm) [53]. Needles are 0.6 to 1.4 inch (15-35 mm) long [267]. 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) [53,115]. The Flora of North America [90] provides morphological descriptions and identification keys for Rocky Mountain and coast Douglas-firs.
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 [185].
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 [53,156,157,213]. Differences in cold-hardiness have been observed between northern Idaho populations and northwestern Montana populations of Rocky Mountain Douglas-fir [212].
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 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 [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 [218] 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].
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 [119,138,267]. 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 [115]. Populations of Rocky Mountain Douglas-fir are very isolated in Texas, Coahuila, Nuevo Leon, Zacatecas, Durango, Chihuahua, and Sonora [117]. The Flora of North America provides a distributional map of Rocky Mountain Douglas-fir [90].
Coast Douglas-fir occurs naturally in British Columbia (generally west of the Continental Divide [53]), western Washington, western Oregon, California, and western Nevada [138].
Fire adaptations: In the pole and sapling stages Rocky Mountain Douglas-fir is susceptible to fire damage as bark is thin, photosynthetic, and resin-filled [67]. Trees develop fire-resistant bark in about 40 years on moist sites in the northern Rockies [88]. The thickness of the bark layers is about 12% to 13% of bole diameter in the northern Rockies [184]. Mature trees can survive moderately severe surface fires because the lower bole is covered by thick, corky bark that insulates the cambium from heat damage [1,3,88]. Fire scars are characterized by resin deposits that may increase the size of the scar in subsequent fires [44]. Rocky Mountain Douglas-fir usually forms obvious fire scars and can survive several centuries after injury, making the history of understory fire easily studied [19]. Rocky Mountain Douglas-fir is killed by crown damage; fine twigs and buds are particularly susceptible [137]. Fire resistance offered by thick bark is often offset by low-growing branches which may be retained even when shaded out and no longer green [67,88,161]. Trees that host Douglas-fir dwarf mistletoe (Arceuthobium douglasii) often accumulate dense brooms that increase likelihood of charring of the bole or torching [268].
Mature Rocky Mountain Douglas-fir is generally more fire resistant than spruces, true firs, lodgepole pine, western hemlock, western redcedar, and western white pine and slightly less fire resistant than ponderosa pine and western larch [107,265]. Rocky Mountain Douglas-fir is, however, slower growing and much less fire resistant than ponderosa pine or western larch in sapling and pole stages [137,228,264]. High fire frequency reduces the dominance of Rocky Mountain Douglas-fir relative to western larch and ponderosa pine because of the species' differential rates of growth and susceptibility to fire [16,88,159,228]. During pre-settlement times frequent fire often maintained ponderosa pine rather than Rocky Mountain Douglas-fir on drier sites, as Rocky Mountain Douglas-fir did not reach fire resistant size before the next fire [19]. On more mesic sites western larch was dominant as its bark is more fire resistant than ponderosa pine's and its deciduous habit allows it to recover from crown scorch more easily [228]. On moist sites Rocky Mountain Douglas-fir growth is rapid enough that some reach fire-resistant size before the next fire, allowing open stands to develop. In some grasslands and savannas, fire restricted Rocky Mountain Douglas-fir to rocky microsites with sparse herbaceous fuels. Fire suppression has allowed Rocky Mountain Douglas-fir to spread from these fire-safe sites and form extensive pole-sized stands in mountain grasslands [19].
Rocky Mountain Douglas-fir relies on wind-dispersed seeds to colonize burned areas where trees have been killed. Mineral soil exposed by burning provides a good seedbed. Seedling establishment begins a few years after fire and is restricted to within a few hundred yards of seed trees adjacent to the fire or relatively undamaged by the fire [237]. On xeric sites, Rocky Mountain Douglas-fir establishment is more successful in shade. On wet sites with thick litter layers, fire can aid establishment by reducing litter layer thickness. Oswald and others [195] observed that prescribed fire (in October) favored Rocky Mountain Douglas-fir establishment on a western redcedar/queencup beadlily habitat type by reducing the thickness of litter layers. Means are presented below. Different letters indicate means significantly different at p<0.05:
Treatment and litter depth Germination (%) Mean survival (%, 1 year) Mean height (3 yr, cm) Mean diameter (3 yr, cm) Burned 0-1 cm 59.3a 35.5a 7.6a 0.38b Unburned 0-1 cm 44.9b 32.8a 4.8b 0.32c Burned 2-4 cm 41.9b 18.9b 6.9a 0.40b Unburned 2-4 cm 6.4d 3.4c 4.9b 0.49a Burned >4 cm 23.1c 8.1c 5.5b 0.41b Unburned >4 cm 9.5d 3.8c 3.6b 0.31cFIRE REGIMES: FIRE REGIMES in moist Rocky Mountain Douglas-fir habitat types are mixed, ranging from low to moderate severity surface fires at relatively frequent intervals (7 to 20 years) to severe crown fires at long intervals (50 to 400 years) [149]. In some areas, large fires burn at several intensities, changing with shifts in stand structure, fuel loads, topography, and weather [16]. The result is a mosaic of burn patterns. Intense crown fires or repeat fires generally favor seral associates such as quaking aspen or Rocky Mountain lodgepole pine. In the Bob Marshall Wilderness in Montana, Rocky Mountain Douglas-fir-dominated sites were converted to Rocky Mountain lodgepole pine by 3 fires at 30- to 40-year intervals. Another site in the same area was converted from a Rocky Mountain Douglas-fir-western larch forest to a forest dominated by Rocky Mountain lodgepole pine as a result of a single severe fire [94].
Northwest: Where Pacific ponderosa pine and western larch were present in open stands, mean fire return interval was 5 to 30 years. Old-growth western larch stands where Pacific ponderosa pine was not present commonly have had either mixed severity fires at 30- to 75-year intervals or stand replacing fires at mean intervals of 120 to 350 years [24]. In 1900, the ponderosa pine savanna covered about 40 million acres (16 million ha) in the United States. The stand structure persists in dry areas; in more mesic areas, Rocky Mountain Douglas-fir has or is replacing it [23]. Prior to 1900, dry Douglas-fir habitat types in the northern Rocky Mountains experienced low- to moderate-severity surface fires at less than 30-year intervals [16,206]. Where Pacific ponderosa pine is a major associate, fires at 10-year intervals were common [161]. These frequent surface fires maintained relatively open stands of Rocky Mountain Douglas-fir or, more frequently, seral stands of Pacific ponderosa pine since ponderosa pine saplings are more fire-resistant than Rocky Mountain Douglas-fir saplings [16,88,159]. Fire suppression has resulted in long fire-free periods that have allowed Rocky Mountain Douglas-fir to establish. In some areas, dense Rocky Mountain Douglas-fir thickets have formed, providing continuous ladder fuels to the crown of overstory trees. Thus, fire exclusion has increased the potential for severe, stand-replacing fires. Fire maintains ponderosa pine on drier sites; on more mesic middle-elevation sites western larch may dominate because it can overcome crown scorch by growing a new crop of needles [228]. Stein [250] speculated that climate change over the last century has limited ponderosa pine regeneration at northern latitudes and upper elevations and in the Southwest (see below). Fires prevented Rocky Mountain Douglas-fir or grand fir from replacing ponderosa pine [23].
At the upper end of the Rocky Mountain Douglas-fir/pinegrass habitat type (ponderosa pine phase) in Montana, fires were mixed, sometimes spreading to crowns and occurring at an interval of 26 to 50 years, creating a many-aged structure. Conversely, moist sites on the Flathead National Forest allowed the development of even-aged classes; disturbances such as bark beetle epidemics and fire likely interacted to create the stand type. Many stands are intermediates between these 2 types. There were long fire-free periods before 1900: 41 to 97 years were the maximum fire-free intervals studied by Arno [23] in relatively moist Rocky Mountain Douglas-fir habitats of western Montana. This probably allowed some Rocky Mountain Douglas-fir development [23]. On dry Rocky Mountain Douglas-fir habitat types where limber pine is seral, limber pine (most commonly) establishes in the shade of Rocky Mountain Douglas-fir; fire when stands are young favors either grassland or an open stand of Rocky Mountain Douglas-fir [67].
Central Rocky Mountains: Fire was not as frequent as in the southern Rockies and because of this it sometimes resulted in patchy stand replacement fire in and a mixed fire regime (with mean intervals of 50 years of longer). Variable forest structure was also created by postfire regeneration in the crown fire areas. Postfire regeneration was episodic and controlled primarily by climatic factors (i. e. several age groups were in a single crown fire area). Logging has reduced the variability in ages. In central Colorado there is no evidence of high frequency surface fires as is seen in the interior ponderosa pine/bunchgrass types of the southern part of the range [140]. Fire suppression has allowed the development of dense, Rocky Mountain Douglas-fir sapling thickets and increased risk of stand replacement fire [183]. Frequent fire prevented Rocky Mountain Douglas-fir and white fir from replacing ponderosa pine. Surface fires have been excluded for about 60 to 90 years in these stands, increasing the likelihood of stand-replacing fire [23].
Southwest: A warmer, drier climate interacting with fire suppression has led to the decline of ponderosa pine/Arizona pine regeneration in the Southwest. Swetnam and Baisan [252] state that in the southwestern United States fires in the ponderosa pine zone, in which Rocky Mountain Douglas-fir is a major component, generally occurred after several wetter years that allowed fuel accumulation, while fires in mixed conifer stands were generally not preceded by wet years but rather by extreme drought. In the Sacramento Mountains of New Mexico, mean fire interval in the ponderosa pine zone range from 3 to 11 years and 4 to 14 years in mixed conifer types; fire occurrence on these sites (since the 1500s) was correlated with Palmer drought severity indices and influenced by El-Nino southern oscillation climate patterns [47]. In the southern Rockies, frequent 8-10 year surface fires in dry Douglas-fir habitat types maintained seral stands of ponderosa pine and/or southwestern white pine [140,183].
Alberta: In the lower subalpine zone (below 6,890 feet (2,100 m) on north aspects and 5900 feet (1,800 m) on others) of Kananaskis Provincial Park near Calgary, Rocky Mountain Douglas-fir and limber pine are present on drier areas that had mean fire intervals of 90 years between 1712 and 1920. Fires were generally large, greater than 2,500 acres (>1000 ha), and were "medium to high intensity" [113].
British Columbia: Mean fire interval in the Rocky Mountain Douglas-fir biogeoclimatic zone was 92.5 years over the last "200 to 600+" years. For the same general zone the estimated average fire size was 312 acres (125 ha), with a maximum of 12,500 acres (5,000 ha) [51]. In ponderosa pine/bunchgrass types in north Kamloops, mean fire interval was 7 years "before the suppression era"; in similar communities in southern Kamloops mean fire interval was 10 years (ranging from 3 to 42 years). In the driest Rocky Mountain Douglas-fir communities in Kamloops mean fire interval was also 10 years, ranging from 2 to 32 years [163].
Eastern Washington and eastern Oregon: In the Okanogan Highlands, approximately in the Rocky Mountain Douglas-fir and grand fir zones, mean moderate severity (moderate meaning generally surface fire with some areas of crown fire) fire interval was 22 years, with a range of 12 to 52 years. Moderate fires generally occurred after long fire-free periods with herbaceous growth and fuel accumulation. Where low-severity fire is more common western larch, Pacific ponderosa pine, and, to a lesser degree, Rocky Mountain Douglas-fir are favored over grand fir because of grand fir's canopy's proximity to the ground [5]. Western larch occurs in Rocky Mountain Douglas-fir communities that have experienced moderate- to high-severity fire that expose mineral soil and increase light penetration [3]. In the Blue Mountains fire return intervals (historically) ranged from 3 to 30 years in ponderosa pine stands. These stands now support Rocky Mountain Douglas-fir 6-12 inches (15 to 30 cm) in diameter as a result of fire exclusion [6]. Estimates for the "historic" fire return interval in eastern Washington in the Rocky Mountain Douglas-fir habitat type series include 7 to 11 years and 8 to 18 years [4]. Fires were generally large-scale, sometimes greater than 15,000 acres (6,000 ha), with variable intensities [5,131]. In the Blue Mountains of eastern Oregon and extreme southeastern Washington, fires in the Rocky Mountain Douglas-fir series are now moderate or high-severity because of fuel accumulation [4].
In the grand fir habitat type series (generally mixed conifer composition) mean fire interval was about 47 years, ranging from 25 to 100 years [3,4]. Fires in this type were variable but "moderate" severity [5]. At slightly higher elevations where grand fir and Rocky Mountain Douglas-fir are codominant, fire return intervals varied form 10 to 25 years with low-severity fire. In these forest types low-severity surface fires occurred as well as stand-replacing fires; these created openings for Rocky Mountain lodgepole pine or western larch [6].
Idaho: Northern aspects in Idaho are more likely to experience stand-replacing fires than northern aspects in Montana, because they are generally dominated by western hemlock, western redcedar, or grand fir rather than Rocky Mountain Douglas-fir [15]. On the Clearwater National Forest near Pierce, Idaho, fires in Rocky Mountain Douglas-fir stands on relatively warm aspects and grand fir-Rocky Mountain Douglas-fir stands on cool aspects were large and often severe, with few surviving overstory trees. The middle elevation mixed-conifer forests on south and west aspects have the highest fire severity in the area [30]. Arno and Davis [18] report that in stands (between 2,500 and 5,000 feet) now dominated by western hemlock and western redcedar in the Salmon-Challis National-Priest River Experimental forests, mean fire return intervals historically ranged from 50 to 150 years. Western hemlock and western redcedar are dominant on these sites where fire has been excluded. Pre-1900 forests included these climax species mixed with western white pine, western larch, Rocky Mountain Douglas-fir, grand fir, Rocky Mountain lodgepole pine, paper birch, Engelmann spruce, and Pacific ponderosa pine with the climax species. In some western white pine forests of northern Idaho, however, Rocky Mountain Douglas-fir and grand fir have increased as a result of white pine blister rust and drought. In these stands, bark beetles and root diseases in Rocky Mountain Douglas-fir have increased concomitantly [55]. Frequencies of stand-replacing and understory fires in different Rocky Mountain Douglas-fir communities of the Selway Bitterroot Wilderness Area are listed below [45]:
Forest type Elevation range (m) AspectsDates of earliest and latest fires
Fire intervals
SeverityStand replacing
Understory and mixed
Earliest Latest mean number mean number Pacific ponderosa pine/Rocky Mountain Douglas-fir 366 to 1,365 SW, S, SE 1528 1934 ---- ---- 22 127 nonlethal Shrubfield/conifer 750 W 1880 1934 54 1 ---- ---- lethal Rocky Mountain Douglas-fir/grand fir 1,250 to 1,798 NE, SW, W, NW 1580 1919 119 13 ---- ---- lethal and mixed Engelmann spruce-Rocky Mountain Douglas-fir-subalpine fir 1,481 to 1,999 N, NE, NW 1589 1919 166 9 ---- ---- lethalIn dry areas of Idaho, Rocky Mountain Douglas-fir has invaded grasslands as a result of decreasing fire frequency, climate change, and grazing pressure [19,26,54]. This was observed on big sagebrush grasslands in the Lemhi Mountains of Idaho [54] and in southwestern Montana; on these sites Rocky Mountain Douglas-fir is successional to big sagebrush. Mean fire intervals were 35 to 40 years in 1910, but have been less frequent since [19].
Montana: Recently (1900 until late 1970s) in western larch/ Rocky Mountain Douglas-fir stands of Washington, Idaho, and Montana, fire intervals have been approximately 25 to 75 years-- generally longer than historic fire intervals [170]. Accumulation of Rocky Mountain Douglas-fir has increased fire danger in ponderosa pine habitats [15]. In lower-elevation foothill Rocky Mountain Douglas-fir stands in Bighorn Canyon National Recreation Area, southeastern Montana, fire regime was mixed. Mean return interval for surface fire was 7 years and canopy fire mean interval was 31 years (since approximately 1630) [270]. In southwestern Montana, on Rocky Mountain Douglas-fir/ pinegrass habitat types, fire intervals ranged from 22 to 58 years (mean=41, n=6). In Rocky Mountain Douglas-fir/Idaho fescue habitat types, mean fire intervals ranged from 31 to 60, with a mean of 45 years. The sites had been dominated by big sagebrush, but because no fires had occurred since 1902, Rocky Mountain Douglas-fir was able to invade [19].
On 3 study sites in the Bitterroot National Forest in Montana, the Rocky Mountain Douglas-fir/bluebunch wheatgrass habitat type was dominated by Pacific ponderosa pine before 1900. Mean fire intervals were 6 years, ranging from 2 to 20 years; 11 years, ranging from 2 to 18 years; and 10 years, ranging from 2 to 18 years. At the same sites, in the Rocky Mountain Douglas-fir/ninebark habitat type (which supported ponderosa pine, Rocky Mountain Douglas-fir, Rocky Mountain lodgepole pine, and western larch before 1900), historic mean fire intervals were 7 years, ranging from 2 to 28 years; 16 years, ranging from 4 to 29; and 19 years, ranging from 2 to 28 years. Grand fir habitat types in the Bitterroot Mountains, in which western larch, Rocky Mountain lodgepole pine, and Rocky Mountain Douglas-fir had been dominant (western larch is now sole dominant), had a mean fire interval of 17 years, ranging from 3 to 32 years [22]. On wetter grand fir mixed-conifer types, fire return interval is estimated to be 17 years in western Montana with stand-replacing fire occurring approximately every 100 to 200 years [3]. Fire intervals in Rocky Mountain Douglas-fir communities are listed below [24]:
Sample area, plot Habitat type Aspect, slope inclination Site moistureOld growth composition
Historic (1600-1900) mean (range) fire intervals in years Stand replacement fires detected Ponderosa pine Western larch Bitterroot 1,2,3 Rocky Mountain Douglas-fir/pinegrass SW, >40% Very dry present absent 49 (19 to 97) No Lolo 1,2 Rocky Mountain Douglas-fir/pinegrass SW, >40% Very dry present absent 32 (17 to 47) No Lolo 3 Rocky Mountain Douglas-fir/big huckleberry and pinegrass SW, >40% Moderately dry present absent 26 (7 to 51) No Lolo 4 Rocky Mountain Douglas-fir/big huckleberry WNW, >40% Moderate present present 27 (17 to 35) Yes Flathead 1 Rocky Mountain Douglas-fir/dwarf huckleberry Flat Moderate present absent 31 (8 to 66) Yes Flathead 2 Rocky Mountain Douglas-fir/dwarf huckleberry Flat Moderate present present 25 to 30 Yes Bitterroot 4 Grand fir/twinflower E, gentle Moderately moist present absent 13 (5 to 41) No Lolo 5 Subalpine fir/queencup beadlily Flat Moist absent present 24 (9 to 42) NoIn Glacier National Park, 8 sites studied in Rocky Mountain lodgepole pine, western larch, Pacific ponderosa pine, and Rocky Mountain Douglas-fir communities had mean fire intervals between 28 and 52 years (fire intervals ranged from 4 to 70 on one site to 16 to 113 years on another). Where Rocky Mountain lodgepole pine was present, stand-replacing fires had occurred at intervals between 79 and 147 years [34]. In Coram Experimental Forest subalpine fir habitat types Rocky Mountain Douglas-fir is a co-climax; before 1910 Rocky Mountain lodgepole pine was dominant but is not currently prominent because of decreased fire frequency. Fire intervals prior to 1910 for these stands are described below [69]:
Landscape position Elevation Mean (range) fire intervals Valleys 1,000 to 1,140 m >117 years, ranging from 21 to 175 years Montane slopes 1,200 to 1,650 m 121 years, ranging from 6 to 173 years Lower subalpine slopes 1,575 to 1,800 m 146 years, ranging from 47 to 132 years Upper subalpine slopes 1,800 to 1,910 m longer than 146 years, ranging from 47 to at least 175 yearsWyoming: In northwestern Wyoming cool, dry Rocky Mountain Douglas-fir likely burned every 50 to 100 years; fires were generally "thinning" surface fires. Adjacent big sagebrush communities' fire return intervals were probably shorter historically. Rocky Mountain Douglas-fir stands with seral quaking aspen burned approximately every 25 to 100 years [43]. For low-elevation sites where limber pine is seral, fire return interval was estimated at 50 to 100 years for the Yellowstone area [67]. Near Andesite Mountain in Yellowstone National Park, Rocky Mountain Douglas-fir/common snowberry and Rocky Mountain Douglas-fir/pinegrass habitat types occur in many-aged (up to 500 years) stands adjacent to grasslands. This table summarizes 3 such stands' fire histories [32]:
Elevation (m) Max age (years) Time period studied Number of fires Interval range (years) Current interval (yr) Mean fire interval 2,066 259 1766-1870 4 17-44 121 35 2,096 506 1534-1988 11 14-110 3 45 2,243 367 1756-1940 3 70-114 51 92Utah: A survey of fire histories in Bryce Canyon National Park showed that interior ponderosa pine savannas and mixed-conifer forests (of which Rocky Mountain Douglas-fir was a major component) burned "at least once every decade and probably more often" [48]. In mixed-conifer communities in Bryce Canyon National Park there has been a decrease in fire frequency from a mean return interval of 7.5 years to 45 years since 1900; there has been a concomitant increase in white fir and Rocky Mountain Douglas-fir as well as a 200% increase in fuel accumulation. In dry Rocky Mountain Douglas-fir communities where interior ponderosa pine is potentially dominant, a fire return interval of greater than 50 years favors Rocky Mountain Douglas-fir; after 50 years fires have generally been stand-replacing [44,67]. Cooler and/or wetter Rocky Mountain Douglas-fir/Rocky Mountain lodgepole pine stands have more variable FIRE REGIMES [44]. Cool and dry Rocky Mountain Douglas-fir habitat types in central and southern Utah do not experience frequent, low-severity surface fires characteristic of the northern Rockies. These habitats are drier and typically have discontinuous ground fuels and poor grass cover that hamper fire spread [277].
Colorado: FIRE REGIMES in Rocky Mountain Douglas-fir/interior ponderosa forest types below 8,200 feet (2,500 m) were historically likely "mixed and variable" with fires historically larger than 3.6 square miles (10 km2) occurring 50 to 60 years apart; stands were not even-aged on a landscape scale [140]. "Passive" crown fire (where crown fire occurs in a stand but does not spread to adjacent ones) was more common than "active" crown fire (where crown fire occurs and spreads from a stand) which, if it occurred, was usually very localized and confined to younger stands. When crown fire occurred it created openings. Tree recruitment thereafter was episodic and influenced by moisture [139]. Kaufman and others [139] modeled stand conditions prior to fire exclusion in the mid-elevation forests of Cheesman Lake: interior ponderosa pine (pure) patches were 35-50% of area (not as much on north slopes), interior ponderosa pine/Rocky Mountain Douglas-fir (>10% Douglas-fir canopy cover, 20% of trees are Douglas-fir) patches were 20-30% of the area, and 25% were very open (<10% canopy cover). The fire regime and tree recruitment patterns that create this variable forest structure were [139]:
Process Mean interval (years with standard errors) Range (years) Fires >5 km2 in 35 km2 landscape between 1496 and 1880 42.7 (12.7) 27-65 Fires in 0.5 to 2 km2 areas, 1496 to 1880 50.0 (17.2) 29-83 Tree recruitment, 1588 to 1885 45.3 (23.5) 18-82New Mexico: In most vegetation types supporting Rocky Mountain Douglas-fir, fire is more frequent and regular (and thus lower severity) in the southern Rockies than in the central and northern Rockies. Swetnam and Baisan [252] summarize mean fire intervals in New Mexico forests between 1700-1900 as follows:
Region Stand type Mean fire interval (years with ranges) Southeastern New Mexico Mixed conifer 9.16 (2-38), 6 (1-21) Arizona pine/ mixed conifer 2.98 (1-15),12.29 (1-31), 7.42 (1-21), 2.93 (1-15) Arizona pine 3.47 (1-10), 4.51 (1-18), 4.54 (1-9), 5.35 (1-16), 5.52 (1-23), 5.9 (2-19), 7.38 (1-33), 11.25 (2-33), 4.85 (1-21) Arizona pine, pinyon, juniper, oak 6.27 (1-34) East-central New Mexico (near Arizona border) Interior ponderosa pine 13.14 (1-30), 5.63 (1-12), 5.33 (1-12), 9.32 (1-25), 12 (2-31), 16.5 (3-55), 7.3 (2-21), 8.96 (2-30), 5.86 (1-17) Interior ponderosa pine, pinyon, juniper 9.1 (2-22) Central New Mexico (Santa Fe area) Arizona pine, pinyon, juniper 8.26 (1-25) Mixed conifer 25.17 (1-89), 4.54 (1-12), 15.75 (1-33), 12 (3-32) Interior ponderosa pine, mixed conifer 6.79 (1-24 4.75 (1-17) Interior ponderosa pine 8 (1-24), 9.45 (1-21), 19.5 (4-52), 6.21 (1-21), 10.05 (2-29) 9.45 (1-21), 5.84 (1-24), 17.1 (1-46) 5.59 (1-13), 7.8 (1-28), 14.36 (4-28), 5.57 (1-12)Arizona: Surface fires have been quite frequent in most Rocky Mountain Douglas-fir communities in Arizona. The Arizona ponderosa pine stands had fire return intervals of 2 to 10 years. A mixed conifer stand in the White Mountains burned at 22 year intervals (small fires occurred between) prior to 1900. In the Rincon Mountains there were approximately 80 fires between 1937 and 1986 in mixed-conifer stands (Rocky Mountain Douglas-fir and white fir) between 7,940 and 8,760 feet (2420 and 2670 m). Mean fire return interval for the mixed-conifer zone was 9.9 years: where fire was more frequent in this zone, quaking aspen was dominant [27]. In 2 Apache pine, Chihuahua pine, Arizona pine, Rocky Mountain Douglas-fir communities in the Chiricahua Mountains, mean fire interval between 1637 and 1876 was 3 to 4 years [136]. A site in the Chiricahua Mountains with Rocky Mountain Douglas-fir dominant and lesser amounts of southwestern white pine, interior ponderosa pine, white fir, and quaking aspen had a mean fire interval of 3 years between 1700 and 1900. Most fires occurred in late winter or spring; the documentary record of the area suggests that many were set by Native Americans [234]. Swetnam and Baisan [252] offer an extensive fire history record for sites throughout Arizona and New Mexico.
Texas: In mixed-conifer stands composed of interior ponderosa pine, southwestern white pine, Rocky Mountain Douglas-fir, and Colorado pinyon in the Guadalupe Mountains, mean fire return interval was 4.7 years, with a maximum of 30 years, probably result of livestock grazing. Most fires were low severity surface fires [7]. In the Chisos Mountains in the Big Bend area, Rocky Mountain Douglas-fir grows with Arizona cypress, interior ponderosa pine, Mexican pinyon, bigtooth maple, junipers, gray oak, and Graves' oak; the communities have a mean fire return interval of 70 years (between 1770 and 1940) but its range is "wide" [182].
Mexico: Open structure, mixed-conifer forests (Durango-fir, Arizona pine, piño blanco, Durango pine, Apache pine, Chihuahua pine, piño triste, madrone, and junipers) in Durango and Chihuahua, Mexico have had an increase in Rocky Mountain Douglas-fir with fire exclusion in most areas [93]. In some areas, however, such as northern Sonora and northwestern Durango, frequent fires (mean interval of 4 years) occurred into the 1970s [91]. In the 7,000-ha La Michilia Biosphere Reserve, between 1779 and 1945, fire return intervals ranged from 2 to 37 years with a mean of 9.77 years; most fires burnt over 60% of the 7,000-ha area. Highest fire frequency was in low-elevation forests rather than mixed conifer sites. Here fire exclusion has reduced fire frequency for 30 to 50 years resulting in increased fuel loading, increased density of young trees, and increased density of less fire resistant fir and Rocky Mountain Douglas-fir [93].
FIRE REGIMES for plant communities and ecosystems in which Rocky Mountain Douglas-fir occurs are summarized below. Where this table provides information on plant communities described above, the text is generally more location-specific and more precise than the table. Find further fire regime information for the plant communities in which this taxon may occur by entering the plant name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) grand fir Abies grandis 35-200 [17] sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 [201] mountain big sagebrush Artemisia tridentata var. vaseyana 15-40 [19,52,180] Wyoming big sagebrush Artemisia tridentata var. wyomingensis 10-70 (40**) [260,272] Arizona cypress Cupressus arizonica western juniper Juniperus occidentalis 20-70 Rocky Mountain juniper Juniperus scopulorum 201] western larch Larix occidentalis 25-100 Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to > 200 blue spruce* Picea pungens 35-200 [17] Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-300+ [16,221,226] western white pine* Pinus monticola 50-200 Pacific ponderosa pine* Pinus ponderosa var. ponderosa 1-47 interior ponderosa pine* Pinus ponderosa var. scopulorum 2-10 Arizona pine Pinus ponderosa var. arizonica 2-10 [17] quaking aspen (west of the Great Plains) Populus tremuloides 7-120 [17,102,177] mountain grasslands Pseudoroegneria spicata 3-40 (10**) [16,17] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [17,19,23] western redcedar-western hemlock Thuja plicata-Tsuga heterophylla > 200 mountain hemlock* Tsuga mertensiana 35 to > 200 [17] *fire return interval varies widely; trends in variation are noted in the species summaryPrescribed burning: Prescribed fire can be used for reducing fuel loadings, understory conifer reduction, or when thinning is impractical or in conflict with other uses [108]. The likelihood of ladder fuels allowing ponderosa pine mortality raises concerns about wildlife habitat and biodiversity [23]. Fire management can increase the variety of stand types and densities and reduce risk of severe fire [15]. Prescribed burning has been used to limit invasion of Rocky Mountain Douglas-fir in bunchgrass habitat types [103], and for site preparation, fuel reduction, and habitat improvement in increasingly crowded forests of the Intermountain West [149]. Low-severity surface fires generally lessen fuel loading, stimulate shrub and herbaceous growth, kill saplings, and increase plant-available nutrients in soil [21].
A prescribed fire in a Rocky Mountain Douglas-fir stand in Yellowstone National Park. USDI, National Park Service photo.A first step to reducing Rocky Mountain Douglas-fir cover on sites that were historically open savannas often is a "low thinning" treatment. This process mechanically removes some understory Rocky Mountain Douglas-fir as well as suppressed members of the overstory and thus reduces the likelihood of canopy fire destroying desired overstory trees [21]. When burning understory in ponderosa pine-western larch-grand fir forests, Rocky Mountain Douglas-fir leave trees should be larger than 16 inches (40 cm) in diameter when fuels exceed 30 tons/acre (73 t/ha). Heavy fuels within 6 feet (1.8 m) of the base of leave trees should be removed [107]. Damage to desired Pacific ponderosa pine, western larch, or Rocky Mountain Douglas-fir can be minimized by moving fuel from bases of trees or prescribing fire under moist conditions [107,108]. Late summer and fall fires damage Rocky Mountain Douglas-fir foliage less than spring fires; accordingly, fires designed to eliminate encroaching saplings are often prescribed in the spring, weather permitting [203]. Predicted mortality, fuel reduction, and smoke production in a Pacific ponderosa pine-Douglas-fir stand in the Bitterroot National Forest, Montana during hypothetical low-severity prescribed fire and severe wildfire are as follows [215]:
Fuel consumption (tons per acre) Prescribed understory fire WildfireDuff
2.0 5.1Small woody (0-3" diameter)
3.5 3.5Large woody (3"+)
3.7 4.1Canopy fuels
1.0 5.4Particulate matter (less than 10 microns) emission (pounds per acre)
271 450Tree mortality (%) all species by diameter
0-4"
91 964.1-8"
63 968.1-12"
40 7912.1-16"
27 8816.1"
23 80Fuel reduction: In ponderosa pine/Rocky Mountain Douglas-fir stands, fire severity can be controlled by selecting burn conditions/days that eliminate most fine fuels but do not burn large fuels or all duff [108]. Complete duff consumption can allow excessive erosion and is thus usually avoided. Robichaud and others [220] burned a mixed western hemlock, grand fir, western white pine, western larch, and Rocky Mountain Douglas-fir stand with relatively moist conditions in late April. This reduced fuel loading and fire hazard and improved regeneration conditions by removing 50% of litter and only 22% of humus while protecting mineral soil from erosion [220]. An experimental burn in the Bitterroot National Forest provides an example of conditions that allow fuel reduction while protecting soil from erosion: fine fuel moisture was 9%, duff moisture was 50%, large woody fuel moisture was 90%; 65% of litter and "small woody fuels" was consumed and duff was reduced 20% [108]. In western larch-Rocky Mountain Douglas-fir forests in western Montana, broadcast burning in clearcuts or in standing timber can be controlled and practical when small diameter fuel (less than 4 inches (10 cm)) moisture content is between 10 and 17% [194]. Norum [193] offers "when to burn" guidelines that include the combined effects of moisture content and dead fuel loading for minimizing crown fire risk in western larch Rocky Mountain Douglas-fir stands. Fresh, cured coniferous logging slash is generally very flammable because of its loose arrangement and high percentage of needles and twigs. Flammability decreases with time, particularly as needles are compacted by winter snow. In experimental burns with 32.5 tons of slash per acre (80 t/ha) and relative humidities of 52 to 70%, the rate of fireline spread in fresh, cured Rocky Mountain Douglas-fir logging slash was 20.7 seconds/foot, while the rate of spread in 1-year-old slash was 70 seconds/foot [81].
Insect outbreaks: The duration and intensity, but not the frequency, of western spruce budworm epidemics have increased since 1910 [14]. Douglas-fir beetle populations and Douglas-fir dwarf mistletoe infestation have also increased [5,8]. Insect epidemics, though "naturally occurring," have been exacerbated by the presence of other insect or disease outbreaks, past high-grading timber extraction, and fire exclusion [5]. Low thinning and surface fire prescriptions that favor ponderosa pine will likely reduce the frequency and/or duration of insect outbreaks [14]. Douglas-fir dwarf mistletoe is controlled by fire [14,276]. Alexander and Hawksworth [9] state that high-severity fire controls Douglas-fir dwarf mistletoe because canopy elimination "sanitizes" the areas and trees recolonize burned sites faster than the parasite.
Invasion of grasslands and fire: To control Rocky Mountain Douglas-fir invasion of sagebrush-bunchgrass communities, spring fires are best to kill young Rocky Mountain Douglas-fir [103,203]; the primary disadvantage to spring burning is that sometimes fuels do not dry sufficiently during this short period [103]. Gruell and others [103] provide much information on prescription specifics for sagebrush-grasslands at different degrees of Rocky Mountain Douglas-fir invasion. Grazing can reduce fire danger by reducing fuels, and this decrease in fire frequency is in part responsible for Rocky Mountain Douglas-fir's invasion of these communities [19,54]. Fire and grazing history greatly influence the fuel buildup. In northern Idaho, Rocky Mountain Douglas-fir was more susceptible to fire damage in stands subjected to years of livestock grazing than in ungrazed stands [264]. Ungrazed stands remained open and parklike, and had a nearly continuous distribution of small fuels that carried fire well. Prescribed fires had flame lengths up to 36 inches (91 cm), but spread rapidly and only scorched the lower crowns of large trees. On grazed sites open stands were converted to dense pole stands with sparse understories and numerous sapling thickets. These stands had a greater accumulation of duff and large woody fuels that contributed little to fire spread. This resulted in a less intense but slow-spreading fire that was more damaging to trees, probably because of the long residence time [203]. Heavy grazing, however, can have the opposite effect in some cases; if unpalatable species become more dominant, probability of fire increases [278]. Published guides outline prescribed burning objectives and techniques for killing invading Rocky Mountain Douglas-fir in bunchgrass habitat types [103].
Soils: Effects of fire on soil nitrogen are variable [135]. Use of "cool" prescribed fire in moist conditions in a 250-year-old Rocky Mountain Douglas-fir, western larch, subalpine fir, Engelmann spruce stand resulted in a temporary increase in available nitrogen [135]. In 1976, Debyle [70] found that soil nitrogen decreased after prescribed fire in a clearcut Rocky Mountain Douglas-fir-western larch site; Jurgensen and others [135] stated that this was the result of the fire's high severity and surface fuel consumption. It is important to note that even where available nitrogen decreases, nitrogen fixation and other inputs compensate for this over the development of the stand [135]. Harvey and others [110] found that broadcast burning of slash (rather than "intensive removal") significantly (p<0.05) reduced the number of active ectomycorrhizal tips per tree. They suggest that when site preparation is used for natural or planted regeneration, organic layers that are less disturbed benefit the ectomycorrhizal symbiosis and nutrient uptake.Rocky Mountain Douglas-fir grows on a variety of sites across its wide geographic range [64,117,249]. 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 [64,73,249].
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 [80,247]. 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 [277]. 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 [159].
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) [267].
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 [174].
Fire mortality in Rocky Mountain Douglas-fir can occur via cambial damage, root damage, or crown scorch [3,107,226]. These damage indices may be highly variable across the landscape, and root damage is difficult to quantify [107,204]. Thus causal determination is limited because, by necessity, most mortality predictions or studies are based on aboveground characteristics [107,204]. In addition, postfire insect infestation of individual trees is correlated with bark and crown damage parameters [13]. An investigation of fire caused mortality in eastern Idaho and Yellowstone National Park encountered extensive variability in mortality and damage parameters: statistically, crown scorch was the best predictor of postfire mortality, but it explained very little variation (r=-.028, p<0.01) [204]. Agee [3] states that in Montana, Wyoming, and Idaho Rocky Mountain Douglas-fir is most commonly killed by crown destruction in fire and mortality is a function of both crown scorch and postfire insect damage [3]. Generally Rocky Mountain Douglas fir with greater than 60% crown scorch do not survive [193]. However, on Lubrecht Experimental Forest, mortality of Rocky Mountain Douglas-fir 8 years after a "light" surface fire in a Rocky Mountain Douglas-fir stand was best predicted by the number of quadrants of the bole with dead cambium. Secondarily, crown volume scorch was a better predictor than height of lethal scorch [226]. Shallow lateral roots can be damaged if the organic layer burns [226], but this type of damage is seldom quantified or included in mortality models [107].
The effects of fire on Rocky Mountain Douglas-fir vary with fire severity and tree size. Seedlings are most susceptible to fire damage but can live through 122 degrees Fahrenheit (50 ðC) for 1 hour, 140 degrees Fahrenheit (60 ðC) for 1 minute, and 158 degrees Fahrenheit (70 ðC) for 1 second [223,224]. Saplings are often killed by surface fires because their thin bark offers little protection from damage [4,265]. Photosynthetically active bark, resin blisters, closely spaced flammable needles, and thin twigs and bud scales are additional characteristics that make saplings more vulnerable to all fires [44,88,107]. Surface fires intense enough to kill saplings by girdling them often also scorch the entire crown [271].
Chance of survival generally increases with tree size [4,107]. Because larger trees have thicker bark and larger crowns, they can withstand proportionally greater bole and crown damage than small trees. Following a low- to moderate-severity surface fire in an open mixed-conifer stand in Colorado, 64 out of 103 Rocky Mountain Douglas-fir trees died within 2 years. Live trees averaged 9.5 inches (24 cm) in diameter and 32 feet (9.8 m) in height, while fire-killed trees averaged 5.6 inches (14.3 cm) in diameter and 22.6 feet (6.9 m) in height [271]. Fire resistant bark develops by about age 40, but branching habit and stand density can offset this fire resistance. If branches grow (or are dead and retained) along the entire bole, as is common when the tree is open-grown, fire can climb into the crown [44,107]. If regeneration is dense and crowns overlap, the potential for canopy fire is even greater [107]. In the Yellowstone fires of 1988 Rocky Mountain Douglas-fir types had little stand replacing fire even though many fires started. Most fires started prior to curing of surface fuels: the fuel arrangement did not allow crown fire to start but carried surface fire in adjacent stands [216].
Fuel type and arrangement, and related fire behavior, vary greatly in dry Douglas-fir habitat types. Where surface fuels are discontinuous, many trees survive burning [268]. If there are heavy fuel accumulations around bases of trees, severe cambial damage can occur from surface fires that otherwise burn primarily in the litter. Trees infested with Douglas-fir dwarf mistletoe, rust fungi (Chrysomyxa or Melampsorella), and/or needle cast fungus (Elytroderma deformans) commonly have suppressed growth and large accumulations of dead, fallen "brooms" around their base [9,198]. The branches of the brooms have a higher than normal proportion of compression wood, decreasing their susceptibility to decay and increasing the length of time that they are a fire hazard [9]. When ignited, this fine debris burns hot, girdling the bole and/or providing a fuel ladder to torch the crown [15,268]. Trees with brooms may increase fire spotting [9].
Livestock: Rocky Mountain Douglas-fir is poor livestock browse [78]. Rocky Mountain Douglas-fir stands have variable forage productivity; the bunchgrass habitat types, particularly the rough fescue type, are relatively productive. Generally, forage production is greatest in early succession and decreases with stand development and canopy closure. Steep topography limits livestock use in many stands [206]. Forage production in Rocky Mountain Douglas-fir habitats is as follows [132]:
Big game: In general, big game use is greatest in early to mid-successional Rocky Mountain Douglas-fir stands because valuable shrub forage species' cover is reduced by shading in late successional stands [206]. Elk browsing (or lack thereof) can greatly influence shrub development in Rocky Mountain Douglas-fir stands [127]. In spring and winter (in British Columbia, Idaho, and Montana) elk use south- and southwest-facing Rocky Mountain Douglas-fir and Pacific ponderosa pine stands, particularly when shrubs and/or grasses are productive [40,206,249]. 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 (>75%) [40]. Elk use of Rocky Mountain Douglas-fir is generally low if preferred species are present [95,153,154]. 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 [147].
Low-elevation and south-facing open-structure Rocky Mountain Douglas-fir types are often important winter range for white-tailed and mule deer [206,249]. In a Rocky Mountain Douglas-fir/ninebark community in the Selway Bitterroot Wilderness of Idaho, white-tailed deer preferred sites that had short average distance to cover; mule deer used more open areas. White-tailed deer preferred unburned habitat in these communities except in February when they preferred unburned Rocky Mountain Douglas-fir bunchgrass communities. Mule deer more frequently used burned portions of ninebark and bunchgrass communities [145]. Mule deer in north-central Washington showed a preference for Rocky Mountain Douglas-fir dominated conifer forests; these had an availability of 0.5% but a summer use of 9.5% and winter use of 12.0% [59]. Big game typically browse Rocky Mountain Douglas-fir in the winter or early spring when other preferred forage is lacking. Mule deer browse it more than elk do [95,153,154].
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 [99]. Rocky Mountain Douglas-fir may make up a small (up to 2%) portion of bighorn sheep winter diets [144].
Small mammals: Chipmunks, mice, voles, and shrews eat large quantities of conifer seeds from the forest floor [105], 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 [87,105]. In the Blue Mountains of Oregon, American martens frequently use "brooms" created by rust fungi and Douglas-fir dwarf mistletoe (43%), cavities in trees (23%), and burrows in snow or hollow logs (23%) for resting sites. American marten commonly den in hollow logs. The removal of trees with brooms and other management practices aimed at reducing fuels may negatively impact marten habitat [50].
Birds: 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-winged crossbill, white-winged crossbill, dark-eyed junco, and pine siskin [105,151,239]. Migrating flocks of dark-eyed juncos may decimate seeds and freshly germinated seedlings [151,239]. Approximately 26 species of birds feed on western spruce budworm associated with Rocky Mountain Douglas-fir [68]. Woodpeckers commonly feed in the bark of Rocky Mountain Douglas-fir [175]. Blue grouse forage on needles and buds in winter [56,169]; they and other birds rely heavily on Rocky Mountain Douglas-fir communities for cover (see below).
Indirect postfire mortality: Douglas-fir beetle, wood borers, Douglas-fir tussock moth and western spruce budworm cause significant postfire mortality, particularly as some insect populations have increased as a result of fire suppression [5,209]. Small-scale outbreaks of Douglas-fir engraver beetles sometimes occur after "light ground fires," and root rot interacting with fire damage may also cause mortality [3,107,119,226]. On sites surveyed in Yellowstone National Park after 1988, postfire mortality was 31.7%: 18.5% from fire, 12.6% from interaction of fire, bark beetle, and wood borer, and 0.6% unidentified [209]. Postfire bark beetle infestation occurs when the phloem is not too damaged (hardened or scorched) as this condition inhibits feeding [72,129,225]. Thus the highest probability of significant postfire outbreak is in stands where most vegetation is scorched but few trunks are blackened. Bark beetles must utilize injured trees before the phloem becomes too dry for feeding [72]. Bark beetles usually used larger fire-injured trees [225]. After the Yellowstone fires of 1988 Douglas-fir beetle infestation was highest in the trees where the percentage of basal circumference killed by fire was highest [72,225]; 77% of Rocky Mountain Douglas-fir with bark beetle infestations were at least 50% girdled by fire [13]. Infestation was also more common in trees with "ample green phloem and less than 75% crown scorch" [72,225]. After a large stand-replacement fire on Shoshone National Forest, Wyoming Pasek and others [199] noted that most areas of large-diameter Douglas-fir adjacent to burned areas "likely were infested" by Douglas-fir beetle in 1990. In another postfire study in Yellowstone 83% of dead Rocky Mountain Douglas-fir were infested with wood borers and Douglas-fir beetles; 34% of living trees were infested [225]. Bark beetle populations in fire-injured trees in Yellowstone caused increased infestation of residual trees that were not fire-injured [72]. The most severely damaged trees were generally utilized in the 1st year; in following years trees with less severe damage were utilized [72,225]. Cumulative percentages of insect infestation and mortality for 4 postfire years (n=125) [225]:
Year
1989 1990 1991 1992 Infested 24% 62% 76% 79% Dead 12% 37% 52% 77%Postfire growth: Though thinning via fire can increase growth of residual trees, radial growth can be greatly reduced for up to 4 years following fire [3]. At Lubrecht Experimental Forest, western Montana, in a Rocky Mountain Douglas-fir/big huckleberry habitat type, Rocky Mountain Douglas-fir had similar growth on sites that had prescribed understory fire and those that did not [215]. On sites on the Salmon-Challis National Forest of central Idaho, Bitterroot National Forest, and Yellowstone National Park, 75% of Rocky Mountain Douglas-fir trees showed a decline in mean basal area increment over the 1st 4 postfire years (wildfires with no description given). In Rocky Mountain lodgepole pine/Rocky Mountain Douglas-fir mixed stands, postfire growth always declined when crown scorch exceeded 50% in Rocky Mountain Douglas-fir. At these sites surviving burned Rocky Mountain Douglas-fir had the following characteristics (n=135) [204]:
Mean Standard deviation Minimum Maximum Diameter (cm) 35.9 15.1 13.9 109.0 Height (m) 18.1 15.1 9.0 47.0 Bark thickness (cm) 1.9 0.8 0.3 4.7 Scorch height (m) 9.7 4.8 2.5 23.0 Crown scorch (%) 40.1 26.8 0 100 Basal scorch (%) 84.0 27.5 0 100 Bark char (cm) upslope 1.00 0.78 0 5.40 Bark char (cm) downslope 0.60 0.60 0 2.10 Bark char ratio 0.45 0.31 0 1.50Rocky Mountain Douglas-fir seedling establishment following fire is dependent on the spacing and number of surviving seed trees. Following large, stand-replacing fires, Rocky Mountain Douglas-fir seedling establishment is slow. Seedlings are restricted to the burn edge or near surviving trees within the main burn [66]. Germination of artificially sown seed was about 60% on burned seedbeds but only 10% on unburned duff [42]. On logged sites Rocky Mountain Douglas-fir establishes after slash burning, particularly where Douglas-fir is a seral species, such as in grand fir or subalpine fir habitat types, on north- and east-facing slopes [71,236]. On dry, south- and west-facing slopes some shade is often needed for seedlings to survive [112]. Many tree associates are more dependent on mineral soil for seedling establishment than Douglas-fir is. Thus burning may increase the percentage of associates such as Pacific ponderosa pine, Rocky Mountain lodgepole pine, and western larch [71].
Breeding system: Rocky Mountain Douglas-fir is monoecious [53].
Pollination: 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].
Cone of Rocky Mountain Douglas-fir. Image used with permission of Dave Powell, USDA Forest Service (retired), Bugwood.org.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" [197]. 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 [197].
Seed dispersal: Douglas-fir has winged seeds that are dispersed primarily by wind and gravity [197]. Rocky Mountain Douglas-fir seeds are disseminated about twice as far as seeds of ponderosa pine [228]. In western Montana clearcuts, Rocky Mountain Douglas-fir 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 [237]. 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 [115,239]. Clark's nutcrackers also disperse Rocky Mountain Douglas-fir seeds. Unretrieved seeds in Clark's nutcracker caches may have a better change of establishment than wind-dispersed seed [155,256].
Seed banking: 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 [115,239]. 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 [156]. In northwestern Montana, seed "soundness" averaged 39% to 43% during good seed crop years but was only 11% during poor years [237]. 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 [197]. 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 [53,227,228]. 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 [112,230]. 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:
Seed source and seedbed type Emergence (%) Emergence rate (%/day) Mortality (%) Longevity (days) Length (cm) Weight (mg/seedling) Seed source 1Mineral soil
48.4 (7.6) 3.23 (0.47) 49.5 (5.8) 31.5 (1.7) 12.5 (1.2) 19.5 (2.5)Douglas-fir
12.5 (2.8) 0.29 (0.07 61.9 (13.2) 15.9 (4.0) 10.5 (1.0) 13.7 (2.6)Ponderosa pine
55.6 (2.6) 2.28 (0.29) 58.5 (10.6) 26.8 (2.7) 10.3 (0.9) 17.1 (1.0)Sagebrush
41.3 (5.9) 1.31 (0.22) 37.3 (4.7) 11.9 (2.0) 8.2 (0.6) 17.1 (1.5)Fescue
75.9 (4.3) 3.19 (0.22) 43.1 (7.2) 26.8 (2.1) 10.5 (0.4) 17.5 (1.8)Manure
82.2 (2.3) 4.62 (0.23) 29.3 (4.7) 35.6 (5.2) 12.4 (0.3) 20.0 (0.7) Seed source 2Mineral soil
47.8 (4.1) 3.12 (0.29) 50.1 (5.6) 29.4 (3.0) 12.4 (0.8) 19.0 (1.3)Douglas-fir
6.6 (2.1) 0.14 (0.04) 87.5 (6.6) 14.0 (3.3) 8.6 (0.4) 12.7 (3.0)Ponderosa pine
59.1 (2.6) 2.42 (0.15) 60.0 (5.6) 24.7 (2.2) 10.1 (0.7) 15.8 (0.8)Sagebrush
31.6 (7.0) 1.04 (0.21) 57.8 (10.2) 16.5 (2.0) 7.9 (0.9) 11.4 (2.1)Fescue
77.2 (2.1) 3.50 (0.16) 47.6 (3.8) 27.2 (2.3) 9.7 (0.4) 15.9 (0.7)Manure
84.6 (2.4) 5.15 (0.17) 34.7 (5.8) 32.6 (2.0) 11.8 (0.6) 18.8 (0.4)Seedling growth during the 1st 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 [151,227]. 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 [53,134,229]. 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 [206,243].
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 1st 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 [178]. 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 [168].
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].
Rocky Mountain Douglas-fir is a shade-tolerant climax species in dry to moist lower and middle elevation forests but is (relatively) shade intolerant in wetter forests [64,117]. In the absence of disturbance it tends to replace interior ponderosa pine, Rocky Mountain lodgepole pine, and western larch in the northern Rockies [3,24,206]; interior ponderosa pine, Rocky Mountain lodgepole pine, limber pine, and quaking aspen in the central Rockies [24,172]; and ponderosa pine, southwestern white pine, quaking aspen, and Gambel oak in the southern Rockies [73,183]. On moist sites west of the Continental Divide western redcedar, western hemlock, spruces, and true firs replace Rocky Mountain Douglas-fir [115]. It is often a persistent seral species in grand fir and subalpine fir habitat types in the northern Rockies, in subalpine fir habitat types in the central Rockies, and in white fir habitat types in the southern Rockies [73,206,249,277].
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 [228]. 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 [26,140,228,250,252]. 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 [164]. 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:
Species Change in forest composition (% basal area) between 4,500 and 5,800 feet Change in forest composition (% basal area) between 5,800 and 6,900 feet Change in forest composition (% basal area) between 6,900 and 7,500 feet 1900 1995 1900 1995 1900 1995 Rocky Mountain Douglas-fir 19% 55% 24% 24% 10% 4% Ponderosa pine 52% 26% 3% 1% ---- ----The currently accepted scientific name of Douglas-fir is Pseudotsuga menziesii
(Mirbel) Franco (Pinaceae) [90,138,267]. This FEIS summary focuses Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca
(Beissn.) Mayr)
[138,267]. Coast Douglas-fir (Pseudotsuga menziesii var. menziesii)
is the other recognized variety [120,138,267]. Intermediate forms and "clinal variation" in
some characteristics exist where the varieties' ranges overlap [197]. There is also
much variation within
Rocky Mountain
Douglas-fir [53], but analyses of variation in volatile leaf oils [222] and at enzyme loci have shown that splitting of varieties
is in accord with the species' phylogeny. However, 1 population in Mexico
did not cluster with either variety in the genetic analysis at enzyme loci [156].
Information presented in this species summary pertains to Rocky
Mountain Douglas-fir, and the variety will be referred to by its full common name.
When information pertains to the species as a whole, the common name Douglas-fir
is used.
Rocky Mountain Douglas-fir is planted for erosion control, particularly after fire [241]. Genetic variability in seed sources maximizes successful establishment. Same-site collection is useful; "supertree" seed will likely increase incidence of disease [157]. Also, planting low-elevation seed at high elevations can increase risk of frost damage [228]. Rehfeldt [211] recommended that in the northern Rockies, seed be used within 300 feet (91 m) of its source at elevations below 4,600 feet (1,400 m), within 410 feet (125 m) at elevations between 4,600 and 6,550 feet (1,400-2,000 m), and to within 650 feet (170 m) at elevations above 6,550 feet (2,000 m). Myers and Howe [190] successfully propagated Rocky Mountain Douglas-fir cuttings using hormone treatment and nursery rooting
Kilns are preferable for drying cones but air-drying also works. When dry, cones may be kept for 3 or 4 months without affecting viability. A seed extraction process includes tumbling dry cones, removing scales and debris, removing seed wings, and winnowing away hollow seeds and debris. Seed sowing for nurseries is most commonly done in spring; fall sowing allows stratification but also increases losses to birds and rodents. Stratification in nurseries is commonly done at 33 to 41 degrees Fahrenheit (1 to 5 ðC) for 21 to 60 days [197]. More recently, Wells [266] recommended 39 degrees Fahrenheit (4 ðC) for 17 weeks in a controlled environment for stratification (or 20 weeks post hard frost in Moscow, Idaho climate). In many potential Rocky Mountain Douglas-fir sites growth and survival are potentially limited by competition for water from rhizomatous grasses [206,243].
Pseudotsuga menziesii var. glauca, or Rocky Mountain Douglas-fir, 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.[2] 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 northern 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), and to the south, it is replaced by Mexican Douglas-fir in high mountains as far south as Oaxaca. Some botanists have grouped Mexican Douglas-fir with P. menziesii var. glauca,[3] but genetic[4] and morphological[5] evidence suggest that Mexican populations should be considered a different variety (Pseudotsuga menziesii var. lindleyana).[6]
Rocky Mountain Douglas-fir is most commonly treated as a variety (Pseudotsuga menziesii var. glauca),[3][7] but has also been called a subspecies (Pseudotsuga menziesii subsp. glauca)[8] or more rarely (mainly in the past) a distinct species (Pseudotsuga glauca).[9] The strong ecological and genetic differentiation with intergradation limited primarily to postglacial contact zones in British Columbia[10][11] supports infraspecific groupings. Some botanists have further split Rocky Mountain Douglas-fir into two varieties,[12] but these are not widely acknowledged and have only limited support from genetic testing.[8][10][11]
Rocky Mountain Douglas-fir is a large tree, typically reaching 35–45 m (115–148 ft) in height and 1 m (3 ft 3 in) in diameter, with exceptional specimens known to 67 m (220 ft) tall, and 2 m (6 ft 7 in) in 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, 1+1⁄4–2+1⁄4 in), 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 (1⁄8–1⁄4 in) 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 (3⁄4–1+1⁄4 in) 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 (3⁄4–1+1⁄4 in) 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 (1+1⁄2–2+3⁄4 in) long, 2 cm (3⁄4 in) broad when closed, opening to 3–4 cm (1+1⁄4–1+1⁄2 in) 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 (3⁄16–1⁄4 in) long and 3–4 mm (1⁄8–3⁄16 in) broad, with a 12–15 mm (1⁄2–9⁄16 in) 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.
Rocky Mountain Douglas-fir grows more slowly than coast Douglas-fir and is also much more cold tolerant. Tolerance of different environmental conditions varies among populations of Rocky Mountain Douglas-fir, especially among populations from the northern and southern Rockies.[13] However, even nearby populations can differ in cold hardiness.[14]
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 (820 ft) uphill from their source, but seedfall between 180–250 m (590–820 ft) was only 7% of that found in uncut stands. Other studies determined that seedfall in clearcuts beyond 80 m (260 ft) 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 (0.62–1.24 mi) distant, but most Douglas-fir seeds fall within 100 m (330 ft) 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.
The oldest accurately-dated Rocky Mountain Douglas-fir, 1275 years old, is in New Mexico. This longevity is apparently uncommon; 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.
In the dry-belt forests of central British Columbia, ages can exceed 500 years on sites normal for the region. The oldest accurately-dated growth ring available for the region is 1475; dates in the 1500s and 1600s are more common for remnant patches that have escaped logging, fire, and other disturbances.
It is affected by the diseases Phaeolus schweinitzii, Armillaria, Phellinus weirii, Fomes annosus, dwarf mistletoe, Dasyscypha canker and Rhabdocline needle cast.[15]
It attracts the Douglas fir beetle, western budworm and tussock moth.[15]
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 (3.3–4.9 ft) 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.
The Douglas-fir is vulnerable to infestation by a woolly aphid, Adelges cooleyi that also infects the Engelmann spruce to complete its lifecycle.
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, railroad ties, mine timbers, log cabins, posts and poles, fencing, and firewood. Other uses listed include "machine-stress-rated lumber", glued-laminated (Glulam) beams, pallets, furniture, cabinets, doors, flooring, window frames, and other miscellaneous woodwork and millwork. Rocky Mountain Douglas-firs are also cut and sold as Christmas trees.
Pseudotsuga menziesii var. glauca, or Rocky Mountain Douglas-fir, 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 northern 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), and to the south, it is replaced by Mexican Douglas-fir in high mountains as far south as Oaxaca. Some botanists have grouped Mexican Douglas-fir with P. menziesii var. glauca, but genetic and morphological evidence suggest that Mexican populations should be considered a different variety (Pseudotsuga menziesii var. lindleyana).
Rocky Mountain Douglas-fir is most commonly treated as a variety (Pseudotsuga menziesii var. glauca), but has also been called a subspecies (Pseudotsuga menziesii subsp. glauca) or more rarely (mainly in the past) a distinct species (Pseudotsuga glauca). The strong ecological and genetic differentiation with intergradation limited primarily to postglacial contact zones in British Columbia supports infraspecific groupings. Some botanists have further split Rocky Mountain Douglas-fir into two varieties, but these are not widely acknowledged and have only limited support from genetic testing.
