• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

FIRE ECOLOGY OR ADAPTATIONS:

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 [107,263]. Rocky Mountain Douglas-fir is, however, slower growing and much less fire resistant than ponderosa pine or western larch in  sapling and pole stages [137,227,262]. 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 [16,88,158,227]. 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 [227]. 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 [236]. 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 [194] 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:

Fire 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 [16,205]. Where Pacific ponderosa pine is a major associate, fires at 10-year intervals were common [160]. 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 [16,88,158]. 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 [227]. Stein [249] 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 [182]. 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 [251] 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 [140,182].

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 [162].

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]:

In 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  [169]. 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) [268]. 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]:

In 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]:

Wyoming: 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]:

Utah: 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 [274].

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]:

New 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 [251] summarize mean fire intervals in New Mexico forests between 1700-1900 as follows:

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 [233]. Swetnam and Baisan [251] 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" [181]. 

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. For further information regarding fire regimes and fire ecology of these ecosystems, see the 'Fire Ecology and Adaptation' section of the FEIS species summary for the plant community or ecosystem dominants listed below.

POSTFIRE REGENERATION STRATEGY [250]:

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