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Wildlife, Animals, and Plants |
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FIRE EFFECTS
IMMEDIATE FIRE EFFECT ON PLANT:Fire mortality in Rocky Mountain Douglas-fir can occur via cambial damage, root damage, or crown scorch [3,107,225]. These damage indices may be highly variable across the landscape, and root damage is difficult to quantify [107,203]. Thus causal determination is limited because, by necessity, most mortality predictions or studies are based on aboveground characteristics [107,203]. 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) [203]. 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 [192]. 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 [225]. Shallow lateral roots can be damaged if the organic layer burns [225], 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 °C) for 1 hour, 140 degrees Fahrenheit (60 °C) for 1 minute, and 158 degrees Fahrenheit (70 °C) for 1 second [222,223]. Saplings are often killed by surface fires because their thin bark offers little protection from damage [4,263]. 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 [269]. 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 [269]. 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 [215]. 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 [266]. 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 [9,197]. 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 [15,266]. Trees with brooms may increase fire spotting [9].
DISCUSSION AND QUALIFICATION OF FIRE EFFECT:There have been a number of models to predict succession, fuel consumption, or mortality in forests that include Rocky Mountain Douglas-fir; these include CLIMACS, NONAME, and FIRESUM [3,46]. A model of mortality was developed by Ryan [222] for Rocky Mountain Douglas-fir and Pacific ponderosa pine on Lolo National Forest, Montana. Peterson and Ryan [204] modeled fire mortality in Rocky Mountain Douglas-fir for the northern Rocky Mountains for trees 15.5 m tall with diameter of 20 cm; time to cambial kill was 3.2 minutes in late summer. For a 40 cm diameter, 24.4 m tall tree, critical time to cambial kill was 13.4 minutes for the same conditions. The critical time for seedling mortality at any temperature has been modeled with the following equation (where T is temperature C°, and t is time in minutes): T= 59.44- 2.291 log e t [222]. The FIRESUM (FIRE SUccession Model) was applied to a Rocky Mountain Douglas-fir/ninebark habitat type of western Montana. With a 10-year fire return interval, predicted fireline intensities were approximately 50 to 100 kW/m; with a 20 year return interval, predicted fireline intensities were 80 to 150 kW/m; predicted fireline intensities with a 50 year return interval were 300 to 1,200 kW/m [143]. Cambial and root damage or postfire insect damage may be partial causes of the inability of crown scorch-based models to consistently accurately predict mortality [107].
PLANT RESPONSE TO FIRE: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 [5,208]. 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 [3,107,119,225]. 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 [208]. Postfire bark beetle infestation occurs when the phloem is not too damaged (hardened or scorched) as this condition inhibits feeding [72,129,224]. 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 [224]. 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 [72,224]; 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" [72,224]. After a large stand-replacement fire on Shoshone National Forest, Wyoming Pasek and others [198] 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 [224]. 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 [72,224]. Cumulative percentages of insect infestation and mortality for 4 postfire years (n=125) [224]:
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 [214]. 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) [203]:
Rocky 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 [71,235]. 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].
DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:No entry FIRE MANAGEMENT CONSIDERATIONS:Prescribed 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 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 [202]. 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 [214]:
Fuel 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 [219] 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 [219]. 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% [193]. Norum [192] 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 [14,273]. 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 [103,202]; 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 [262]. 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 [202]. Heavy grazing, however, can have the opposite effect in some cases; if unpalatable species become more dominant, probability of fire increases [275]. 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.
Related categories for SPECIES: Pseudotsuga menziesii var. glauca | Rocky Mountain Douglas-Fir |
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