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The Ecological Role of Fire in Sierran Conifer Forests: Its Application to National Park Management
By: Bruce M. Kilgore
Originally published in Journal Quaternary Research, Volume 3, Number 3, October 1973
The impact of fire on the environment of the various Sierran conifer forests varies with intensity and frequency. Generally, however, fire (1) prepares a seedbed; (2) cycles nutrients within the system; (3) adjusts the successional pattern; (4) modifies conditions affecting wildlife; (5) influences the mosaic of age classes and vegetation types; (6) alters the numbers of trees susceptible to disease and insects; and (7) both reduces and creates fire hazards. Natural fire frequency apparently coincides with levels of fuel accumulation that result in burns of relatively low intensity at frequent intervals. This may average 8 yr in mixed conifer forests, although frequencies from 4 to 20 yr or more are found in particular sites.
In all probability, giant sequoia and various pines of the Sierra survive today because of the role fire plays in the various forest types. National Park Service management policies are aimed at restoring fire, as nearly as possible, to its natural role in Sierran conifer forests. This is being accomplished by prescribed burning at lower and middle elevation types and by allowing lightning fires to burn in higher elevation forests.
Fire in the California pine region has been portrayed as the great destroyer of forests. In 1924, Show and Kotok maintained that this force has led to "the present unsatisfactory condition of our forest property, a condition in itself the outcome of centuries of repeated fires." Thus, the authors were well aware of the long history of periodic fires in the pine forests of the Sierra Nevada, but they believed these natural forces had been adverse to some arbitrarily determined set of desirable conditions. Only in recent years has convincing evidence been published that records the essential ecological roles of fire in the conifer forests of the Sierra Nevada (Weaver, 1947, 1951, 1964; Biswell, 1961, 1967; Hartesveldt, 1964; Hartesveldt and Harvey, 1967; Kilgore, 1971b, 1972a). This paper will attempt to review and synthesize the factual evidence to date dealing with fire in the coniferous forests of the Sierra. Sierran forests vary in density and size from north to south, with denser forests and trees of shorter stature in the north and more open forests of larger trees in the south, culminating in the groves of giant sequoias (Sequoiadendron giganteum). Coniferous vegetation ranges from the ponderosa pine (Pinus ponderosa) forest, just above the chaparral zone, through middle-elevation, mixed-conifer forest to forests of red fir (Abies magnifica) and lodgepole pine (Pinus contorta). Just below timberline are found the subalpine forests of western white pine (Pinus monticola), foxtail pine (P. balfouriana), and white-bark pine (P. albicaulis).
The most heterogeneous of these types is the mixed-conifer forest, made up of several subtypes comprised of various combinations of sugar pine (Pinus lambertiana), white fir (Abies concolor), Douglas fir (Pseudotsuga menziesii), giant sequoia, ponderosa pine, and incense-cedar (Libocedrus decurrens). Together with the pure ponderosa pine type, the mixed-conifer forest constitutes the bulk of the commercial timber lands of the Sierra and provides the major middle-elevation sites for recreational use in national parks and national forests. It is thus a highly important type, and this discussion will compare the better-known role of fire in the ponderosa pine and mixed-conifer forest with fire's role in higher-elevation coniferous forests.
There are seven functions of fire in a mixed-conifer forest that seem particularly significant. Fire (1) prepares a seedbed, (2) cycles nutrients within the system, (3) adjusts the successional pattern in various ways, (4) moderates conditions that favor wildlife, (5) influences the mosaic of age classes and vegetation types, (6) alters numbers of trees susceptible to attack by insects and disease, and (7) both reduces and creates fire hazards. Each of these roles is affected by fire intensity and frequency. Before a discussion of the various functions of fire, it is important to consider the relationship between fire intensity (the rate of energy release) and the impact of fire.
THE FOREST AS FUEL
Any attempt to conceptualize the "ecological role of fire in Sierran conifer forests" can be misleading if it conveys a particular dichotomous series of positive or negative roles. For the impact of fire on a forest environment varies tremendously with frequency, duration, and intensity of burn, and fire intensity can vary more than 1000-fold (Van Wagner, 1965). These factors, in turn, vary with forest type, depending upon (1) quantity, quality, and arrangement of fuels that accumulate in different strata of the forest and (2) topographic and weather variables. Physical characteristics such as ratio of surface area to volume, fuel loading, and fuel-bed porosity all influence how a fire will burn (Countryman, 1969); the chemical composition of various species establishes the energy base for fire intensity and influences the rate of energy release (Mutch, 1970). Other complex relationships involving mineral content of plants are now being studied (Philpot, 1968); these factors apparently are important in influencing combustibility of forest types. Hence, only in a long-term sense - under natural conditions of fuel build-up and unsuppressed lightning fires - is there a fairly consistent set of functions performed by fire in Sierran conifer forests. This long-term ecological role will be stressed in this discussion.
Fire intensity in Sierra Nevada Forests
Forest fires vary from the relatively mild surface fires found under natural conditions in ponderosa pine (Weaver, 1943; Biswell, 1967), sequoia-mixed conifer (Kilgore, 1972b), red fir (Kilgore, 1971b), and higher-elevation types (Kilgore and Briggs, 1972) to the extremely intense fires found in logged-over country at the brush/timber boundary such as the Harlow Fire of 1961, which burned 8094 hectares (20,000 acres) west of Yosemite National Park in less than two hours and covered a total of 16,673 hectares (41,200 acres) of grass, brush, and timber.
In the mixed-conifer forest of the Sierra, crowning may occur in individual trees or may involve part of a stand. But prior to intensive fire suppression in the early 1900s, crown fires over sizable segments of the forest were almost unknown. Show and Kotok in 1924 explain the reason for this:
The virgin forest is uneven-aged, or at best even-aged by small groups, and is patchy and broken; hence it is fairly immune from extensive, devastating crown fires. Extensive crown fires, though common in the forests of the western white pine region, are almost unknown in the California pine region. Local crown fires may extend over a few hundred acres, but the stands in general are so uneven-aged and broken and have such a varied cover type that a continuous crown fire is practically impossible.
This immunity from crown fires, however, does not extend to the second-growth forests that have come in after logging nor to forests having an understory of shade-tolerant species that have established themselves in the absence of regular surface fires during the past half century or more. Hence, both the role of the more usual surface fires and the present and future role of occasional crown fires must be considered for Sierran conifer forests.
During a relatively light surface burn in November under a sequoia-mixed-conifer forest, great variations in temperature and energy output were found over short vertical and horizontal distances (Kilgore, 1972b). Fuel loading was a major factor here. Temperature measurements a short distance above the ground in understory canopy appeared to be a better index to probable mortality of sapling white fir and sugar pine than surface bark temperatures at the base of the tree. When air temperatures 2 m above the ground reached 121° C (250° F), often above heavy fuels, saplings less than 30 cm (12. in.) dbh (diameter at breast height) were usually killed; generally, of course, these trees also recorded temperatures more than 260°C (500°F) at their base. Total energy output reaching the measuring devices varied from 0 to more than 1860 kg cal over very short distances (Kilgore, 1972b). Generally, temperatures higher than 93 ° C (200°F) did not penetrate below the top 5 cm of soil; but under heavy fuel conditions a reading of 398°C (750°F) was recorded at a soil depth of 4 cm and 93°C was measured at 17 cm.
Fire intensity (heat production per second per unit length of fire front) has not been described in quantitative terms for most Sierran forest types and particularly for the high intensities found in crown fires. In southeastern pine forests, Hough (1968) indicates an acceptable range of surface fire intensities from 20 to 90 BTU per second per foot; fuel consumption was poor below 20, and the timber stand was damaged above 90. Hodgson (1968) noted severe damage to forest values in Australian eucalyptus forest with intensities above 500 BTU per second per foot. Van Wagner (1968) measured crown fire intensity experimentally in a uniform red pine plantation in Canada and found values as high as 6500 BTU per second per foot.
As an important step in the program of quantifying field measurements of fire behavior in Sierran forests, van Wagtendonk (1972) studied spring surface fires in Yosemite's ponderosa pine and incense-cedar forest type. He measured available fuel energy, rate of spread, fire intensity, and scorch height at different fuel moisture levels in four understory fuel types and found that intensity increased as fuel moisture decreased from 19 to 10%. The intensities involved were mild, however, with a maximum intensity of 75 BTU per second per foot. Unlike fall burning in a giant sequoia-mixed-conifer forest, the fires had little impact on heavy fuels and on weathered and decomposed needle and duff layers. Additional quantitative measurements of fire intensity are needed for Sierran forests to permit correlation of such intensities with impact on various aspects of an ecosystem.
SOME FUNDAMENTAL ROLES OF FIRE
In considering the various roles of fire, an intensity comparable to a "moderate surface burn" will be assumed except where crown fire impact is specifically mentioned. Most previous studies specify intensity only in such subjective terms; hence it is not possible at this time to relate more sophisticated intensity measurements to specific changes in the ecosystem. Part of past disagreements about the role of fire in a given forest can be attributed to this lack of refinement in data gathered.
In a sequoia-mixed-conifer forest, moderate to heavy surface fires provide soft, friable, ashy soil on which the lightweight sequoia seeds fall and in which they are buried (Hartesveldt and Harvey, 1967). By consuming the accumulation of downed branches, litter, and duff, fire allows the seed to reach mineral soil. And in heating the soil, fire changes soil structure in a way that allows seeds to be covered by a few millimeters of soil as a result of their fall from the tree, thus promoting germination. Fire's role in other Sierran forests is apparently similar, although not always so crucial.
Large numbers of sequoia and ceanothus seedlings germinated following fairly intense surface fire in 1969 in the Redwood Mountain Grove of sequoias in Kings Canyon National Park (Table 1). The first year after burning, about 54,000 sequoias seedlings per hectare (22,000 per acre) were found; through natural mortality, this decreased to 6459 per hectare the second year and dropped further to 1077 per hectare by the third year. A similar pattern was found with deerbrush (Ceanothns integerrimus) seedling germination which increased to 7939 per hectare the first year after burning and gradually decreased to 494 per hectare by the third year. By comparison, however, not a single seedling of either species has been found on control plots at any time during the study.
The most intense fire yielded the largest number of sequoia seedlings and least number of shrubs, while lower-intensity burn areas gave more shrubs and fewer sequoia seedlings (Kilgore and Biswell, 1971). By the third year, however, numbers were similar on the three plots, although the surviving and most vigorous seedlings were located primarily in the larger, sunny openings and where the most intense burns occurred.
Once seeds are buried and germination takes place, moisture in the rooting zone becomes a critical factor. Rundel (1972) concludes that giant sequoias are limited to habitats of relatively high soil moisture and notes that this limiting factor acts through the ecological tolerances of the seedling stages. Stark (1968) found that partially burned giant sequoia litter holds more available water (273% by weight) than unburned litter and that it forms a good seedbed. Furthermore, highest survival of sequoia seedlings has been found on very heavily burned soils, possibly because of more available soil moisture (Hartesveldt and Harvey, 1967). Other factors such as killing fungi (Davidson, 1971) and elimination of competition, however, are probably equally significant. Fire usually causes a decrease in fungal populations and an increase in soil bacteria and actinomycetes (Wright and Tarrant, 1957; Roe et al., 1971).
Unpublished studies at Whitaker's Forest by Paul J. Zinke (personal communication) of the University of California show that 5% more moisture (by volume) occurs in the top 1 ½ m of soil beneath a giant sequoia after clearing of undergrowth than before such clearing. Likewise, the center of a cleared area 0.2 hectare (one-half acre) in size shows from 7.5 to 15 cm more moisture available in this top 1 ½ m of soil than was found in surrounding forested areas (Kilgore, 1968).
* Trees more than 1.8 m diam. at height of 1.4 m.
In other vegetation types, studies confirm that additional soil moisture is available after fires, but additional surface run-off and erosion often have accompanied this moisture increase (Ahlgren and Ahlgren, 1960). Such run-off and erosion are sometimes related to the phenomenon of water-repellent soils, apparently resulting from naturally occurring organic substances having hydrophobic properties. In the chaparral country of Southern California, some of these substances found in litter are driven downward into the soil by fire and condense on soil particles at, lower levels, depending upon temperature gradients (DeBano, 1969). A nonwettable layer may develop in certain sites after burning under giant sequoia, but no erosion problems have materialized thus far with prescribed burning on our study plots at Redwood Mountain or in our higher-elevation red fir study plots (Kilgore, 1971b). This may result in part from (1) soil differences between the Sierra and Southern California, (2) lower-intensity fires on our study plots, and (3) highly varied character of the burn patterns in these areas - a pattern less often found in hot brush fires studied by DeBano (1969) in Southern California chaparral, where water repellency has led to serious soil-erosion problems.
Fire also plays a role in the germination and survival of seeds of other mixed-conifer species. Ponderosa pine and Jeffrey pine (Pinus jeffreyi) seedlings are favored by seedbed conditions after burning (Vlamis et al., 1956; Bock and Bock, 1969). Sugar pine is somewhat more shade-tolerant, but its seedlings appear to benefit from conditions following fire. White fir germination also seems favored by fire (Agee and Biswell, 1969), although fir also does well without fire.
The importance of fire to seedbed preparation in other Sierran forest types is apparently similar, although perhaps not so critical as for sequoias. Lodgepole pine (Kilgore, 1971b) and presumably the subalpine pines are favored by mineral-soil seedbeds, although little work has been done on the latter.
Shrubs of the various coniferous forest types, including most species of Arctostaphylos and Ceanothus, are almost entirely fire-dependent, and these species have become increasingly scarce during the past 50 yr, thus reducing the value of these areas for deer and other wildlife. Many such shrubs have hard seed coats, which prevent germination unless cracked by fire. Others will sprout following fire, but in either ease the species is stimulated to greater growth and production as a result of fire (Buchanan et al., 1966; Sweeney, 1967).
There are apparently no fire-type herbaceous species associated with a conifer forest in California (Sweeney, 1969), but several species increase in coverage or frequency following burning in the giant sequoia-mixed-conifer forest, perhaps in part because of the increase in sunlight reaching the forest floor (Kilgore, 1971a; Hartesveldt et al., 1967; Rundel, 1971).
Mineral absorption by plants is a constant drain upon the soil (Behan, 1970). A sizable quantity of minerals is incorporated in living and dead tree trunks and retained for many years, while needles and small twigs are dropped annually as litter. Minerals are gradually returned to the soil from this litter by leaching and by the relatively slow action of decomposer organisms. The nitrogen and potassium tied up in litter represent a fair drain on the soil's reservoir of these nutrients (Cole et al., 1967).
Fire plays a significant role in returning various mineral nutrients to the soil in all Sierran conifer forests and particularly in the giant sequoia-mixed-conifer forest. While nutrient capital may be depleted when hot fires volatilize nitrogen and potassium or when soil and dissolved minerals are lost in run-off from rains following fires (Behan, 1970), only a small percentage of Sierran conifer forest fires are of this type. Light burns often increase soil pH, stimulate nitrification, and improve soils chemically. The ash deposit increases available phosphorus, potassium, calcium, and magnesium (Hare, 1961). The growth-release pattern of surviving trees following fire is an indication of the quick conversion of nutrients tied up in dead plant materials to new living tissue (Weaver, 1947; Hartesveldt, 1964).
The effects of burning on soil nitrogen is more complex; some studies show a loss of nitrogen from the forest floor (Knight, 1966), while others report a net gain of nitrogen (Klemmedson et al., 1963). Greater seedling survival in heavy burned spots may be due to more nutrients, more moisture, sterilization of soil, or some combination of these (Vlamis et al., 1956). Less immediate but important chemical changes in soil can occur when fire stimulates growth of such nitrogen-fixing shrubs as Ceanothus Spp. (Delwiche et al. 1965), as fire does in most Sierran forests.
Current studies are aimed at assessing fire's effect on the nutrients available to sequoia seedlings in a mixed-conifer forest (Ted St. John, personal communication). Another study seeks to evaluate physical and hydrological effects of fire on soil, litter, and duff of the same ecosystem (James Agee, personal communication).
Impact on Succession
Before the early 1900s, frequent and widespread surface fires kept the ponderosa pine and sequoia-mixed-conifer forests open and park like (Biswell, 1959, 1961). Pioneer or secondary successional stages were favored over the traditional "climatic climax" forms; species requiring sunlight, such as pines and sequoia, were favored over shade-tolerant forms such as white fir and incense-cedar; and fire-resistant and fire-dependent species and associations were favored over nonfire-dependent forms. The influence of fire on secession in red fir and subalpine forests is less clear and probably less significant (Kilgore, 1971b). While the impact of fire on the successional stages of lodgepole pine forests in Yellowstone is strong (Taylor, 1969), the size of individual trees and age structure of lodgepole stands of the Sierra are considerably different; hence, careful field work in the Sierra lodgepole type is needed before findings from Rocky Mountain lodgepole forests can be applied to Sierra lodgepole stands.
European man caused two types of change in fire-ignition patterns, which in turn affected the successionsl stages now seen in the ponderosa pine and mixed-conifer forests of the Sierra Nevada. Initially he eliminated ignition by Indian people, who had inhabited the mountains for centuries and were known to have employed fire as a management tool (Reynolds, 1959; Driver, 1937). Subsequently, active fire-suppression efforts by white settlers and Federal Government land-management agencies resulted in the virtual elimination of the light surface fire and hence its influence in keeping the forest open (Vankat, 1970).
Recent prescribed burns at Redwood Mountain in Kings Canyon National Park have brought about an adjustment in the successional pattern by killing many young white fir seedlings and saplings, which have become numerous beneath the giant sequoia and sugar pine. To date, however, the changes are not so great as would have been accomplished by periodic natural fires during the past 50-70 yr (Kilgore, 1972b).
One of the results of the natural process that may be most difficult to duplicate at the present stage of plant succession is fire's role in making openings in the crown canopy. When periodic light fires burn through the forest, young white fir are killed while they are still part of the understory level of vegetation and fuels. Fairly mild burning conditions can still accomplish this and leave a mosaic of openings in the crown. These openings in turn allow sunlight to reach the forest floor and permit growth of sequoia seedlings, shrubs, and herbaceous plants that require substantial sunlight. Since fires have been suppressed, however, certain fir trees - perhaps not yet many - have continued to grow in height and size in a way that (1) they have become part of the crown canopy, or at least the lower levels of this canopy, and hence fire moving into this canopy can more readily threaten crowns of other trees, including giant sequoia; and (2) they are much less likely to be killed by moderate fires because the trees are much larger and have much thicker bark.
Two or more burns in the same area over a period of years may be necessary to reduce understory vegetation and to make some openings in the crown canopy. Some sizable white fir may need to be cut and burned in certain high-value sequoia groves, or we may have to accept some fairly hot burning conditions to restore the system to what we believe were more natural environmental conditions. Once this is achieved, we would hope to perpetuate equilibrium conditions by allowing natural forces, including lightning fires, to play their original role as nearly as possible.
Formation of a Vegetative Mosaic
In Sierran conifer forests, fire often burns in a highly variable pattern. It may burn hot in one site, lightly nearby, and not at all in another site. Surface temperatures can vary from 204 °C (400 °F) to 648 °C (1200 °F) or more, with no uniformity of distribution in a given burn (Lindenmuth, 1960; Sweeney and Biswell, 1961). In a red fir forest (Kilgore, 1971b), the fire burned irregular-shaped areas with varying spread rates, possibly related to the variations in fuel-type pattern and fuel-moisture content, as hypothesized by Kourtz and 0'Regan (1971) and van Wagtendonk (1972), as well as to local topographic variations and weather conditions. The result is that over the years fire - in combination with other factors such as exposure, slope, soil type, insects, and disease - brings about the development of a mosaic of age classes and vegetation types. This is particularly true in the ponderosa pine and mixed-conifer forests.
In deseribing this phenomenon in a ponderosa pine forest, Weaver (1967) states: "Periodic burning causes development of uneven-aged stands, comprised of even-aged groups of trees of various age classes." This system operates because fire kills small pines under the canopies of larger trees but not in openings. It does so because heavy accumulations of flammable needles, cones, and bark scales build up under these larger trees and carry a surface fire. But here and there throughout the forest, single mature trees or groups of trees have been killed by insects, disease, lightning, or windthrow. These dead trees are gradually reduced to ashes in subsequent burns. An opening develops within which young pine can both germinate and survive, because the small accumulation of needles in the openings will not support a surface fire. Hence, until the pines are large enough to build up fuels under themselves, fires are not intense enough to kill them; and by the time they do create such heavy fuels, many of them are also large enough to withstand the surface fires.
The influence of fire on wildlife in various Sierran conifer forests is largely related to fire's role in (1) stimulating germination or sprouting of species of shrubs, herbaceous plants, or trees that are useful to mammals or birds for food or cover; or (2) making openings in the forest understory or canopy that favor wildlife. Leopold (1966) has noted that burned or cutover forest lands "support most of the deer on the continent." In a cut, pile, and burn program in a giant sequoia-mixed conifer forest, Lawrence and Biswell (1972) found that browse and forage were more abundant, more heavily hedged by deer, and more nutritious on the burned areas than on untreated controls. Such results are found in many forest types (Ahlgren and Ahlgren, 1960) and relate to the processes of shrub regeneration referred to earlier.
Bird populations have also increased in numbers or biomass following fire in various. vegetation types (Marshall, 1963; Lawrence, 1966; Bock and Lynch, 1970). In a second-growth giant sequoia forest, however, elimination of saplings less than 3.5 m tall did not make major changes in species composition of a breeding-bird population (Kilgore, 1971a). Compared with results from areas where wildfires or logging operations have made major changes in cover type and modified succession processes substantially, this degree of habitat modification resulted in relatively small changes in avifauna. While few studies involving fire and wildlife seem to have been made in higher-elevation forest types of the Sierra, similar results would be expected wherever shrub species are involved.
The importance and complexity of ecosystem relationships involving fire and wildlife are indicated by the interrelated roles played by a small mammal and cerambycid beetle in sequoia seedling regeneration in a sequoia-mixed-conifer forest (Hartesveldt et al., 1970). The squirrel cuts sequoia cones, and in feeding on them it allows seeds to fall into the ideal seedbed created by fire. The work of the beetle causes cones to dry on the tree and drop seeds. Partly as a result of these two animals, heavy surface burning under giant sequoias seems to favor sequoia seedling germination over any other species in the mixed-conifer forest (Kilgore, 1972a). Under fairly heavy burning conditions, however, the rising convection column of heat may be most important in drying and opening of sequoia cones following fire (Kilgore and Biswell, 1971).
Influence on Insect-Susceptible Trees
Fire in northern forests apparently has a sanitizing effect by thinning stands or eliminating old stands or old trees before insects and disease have overtaken them (Heinselman, 1970a; Loope, 1971). Without fire, older-age trees become mare susceptible to insect attack or disease (Hare, 1961). Weaver (1964) believes recent heavy western pine beetle (Dendroctonus brevicomus) attacks resulted from excessive competition in dense stands of ponderosa pine that developed in the absence of fire. Under these circumstances, trees killed by insects leave a forest more susceptible to fire. Lyon and Pengelly (1970) point out that insects and disease are also vital components of the dynamic forest ecosystem. Their role may be related to increasing forest fuel accumulations and, hence, the probability of fire following their own activities. Some trees wounded by fire are in turn attacked by insects and disease and may die, again building up more fuel.
A study is now in progress on the role of carpenter ants in building nests in the bark and heartwood of the giant sequoia. There is some possibility that natural fires burned out ant nests found in bark and decayed heartwood of the sequoia and in other living and dead woody materials in the forest, thus keeping populations of ants lower than those found in sequoia groves today.
Influence on Fuel and Fire Hazard
The climatic pattern of the Sierra Nevada, involving warm, dry summers and wet, cool or cold winters leads to maximum net accumulations of dead fuels or stored energy that may be released by wildfires (Dodge, 1972). Litter on a mixed-conifer forest area contributed annually by the various species was found to be about 2 ½ tons per acre on ponderosa pine areas, from 2 to 3 tons under sequoia, slightly less than 2 tons under sugar pine, and between 1 and 1 ½ tons per acre for incense-cedar and white fir (Biswell et al., 1966). Fuels from higher-elevation species may tend to be somewhat less because of shorter growing seasons, although decomposition processes may also be slower. The normal annual increment of dead fuel may be increased by events that kill many trees in any area - whether blowdown, insects and disease, or wildfire. Foresters have pointed out the tremendous wildfire hazards compounding themselves as a result of the accumulation of both dead and living fuels (Wilson and Dell, 1971), and some believe that the worst enemy of a fire-suppression agency in this regard may be its own efficiency because "the longer forests go without burning, the greater the fuel accumulation and the greater the hazard" (Towell, 1969).
Studies of fire in a red fir forest (Kilgore, 197lb) indicate that fire hazards in these higher elevation areas were considerably less than those found in the middle-elevation mixed-conifer forest. Partly on the basis of this experimental burning project and partly on the basis of the behavior of natural fires at this elevation, the National Park Service began a policy in 1968 of letting lightning fires burn in certain higher-elevation forests of Sequoia and Kings Canyon National Parks, unless human life or property would be endangered (Kilgore and Briggs, 1972).
In the lower-elevation sequoia-mixed-conifer and ponderosa pine forests, however, a considerable fire hazard has built up because of the exclusion of natural fire during the past half century or more (Leopold et al., 1963; Biswell et al., 1968). What may have once been natural crown-fire immunity has now been lost. A wildfire in 1955 swept up from the chaparral country below the Grant Grove of giant sequoias in Kings Canyon National Park. In a short time, it had devastated more than 5,200 hectares (13,000 acres) of brush and mixed-conifer forest and had threatened a grove of giant sequoias.
The National Park Service is now attempting to reduce fuel hazards in the sequoia-mixed-conifer forest. In 1969 some 40 hectares (100 acres) of forest were burned under prescribed conditions in late summer and early fall on the ridge of Redwood Mountain in Kings Canyon National Park. A second burn involving research plots took place in late fall of 1970. The results of these programs, reported previously (Kilgore, 1970, 1972b), were that more than 123 tons of fuels per hectare (50 tons per acre) were stored in the litter and duff layers alone before burning, without taking into account the logs and standing dead and living trees. Following the November 1970 burn, this total had been reduced some 85 ½ to 19 tons per hectare (7.7 tons per acre), and numbers of young trees in the understory were greatly reduced. While additional data are now being analyzed for publication, it appears that crown-fire potential has also been decreased substantially.
Calculation of fire intensity requires that available fuel weight be known. Under the same weather parameters, doubling the available fuel seems to double the rate of spread and increase the intensity fourfold (Hodgson, 1968). It would be both time-consuming and difficult to determine available fuel energy for the many different combinations of fuel and weather found in the Sierra, but it could be done and would provide a basis for an objective fuel-classification system (Kiil, 1968). A fuel model to be used in estimating hazard has been worked out for the National Fire Danger Rating System (model G) which would be reasonably applicable to the Sierran mixed-conifer forests. This model is described as, "... dense conifer stands where a heavy buildup of downed tree material has accumulated. Natural breakup of over-mature stands, insect and disease damage... create the heavy amounts of fuel which typify this model. The canopies of these stands are usually closed, but large openings, the result of the downing of timber, are common. Deep litter and very high loading of dead fuels larger than 1-inch in diameter are characteristic. The amount of undergrowth may be quite varied" (Deeming et al., 1972).
While this description is fairly accurate for the mixed-conifer forest, and perhaps the ponderosa pine forest as well, it would overestimate fuel buildup in higher-elevation red fir, lodgepole pine, and subalpine forest types. To accurately predict fire behavior in these types, separate and specific fuel models would need to be developed. From a fire hazard standpoint, the role of fire in these higher-elevation forests appears to be less significant than its role in the taller, denser, faster-growing mixed-conifer forest.
The National Park Service is committed to restoration of natural processes to the coniferous forest areaas of the three principal Sierran national parks: Sequoia, Kings Canyon, and Yosemite. Inasmuch as lightning and periodic fires are integral elements among these processes, determination of natural fire frequency and intensity becomes imperative. It is known that crown fires were practically nonexistent in the pristine Sierran forest (Show and Kotok, 1924). It is therefore probable that fire frequency and intensity were determined in large part by the inherent characteristics of fire-dependent plants that make them more flammable, and indeed more fire-attracting, than nonflammable plants (Mutch, 1970). Natural fire frequency must have coincided with levels of fuel accumulation that result in burns of relatively low intensity at frequent intervals rather than high intensity burns at infrequent intervals. The exact timing would presumably depend upon the simultaneous chance occurrence of an ignition source and suitable weather conditions (van Wagtendonk, 1972).
To obtain some positive data on fires in the sequoia-mixed-conifer forests, studies are currently in progress that analyze fire-scar dates on stumps of trees cut within or adjacent to Sequoia and Kings Canyon National Parks. Preliminary findings reveal minimum frequencies average 7 - 9 yr, although frequencies range from 4 to 20 yr in particular areas. An interesting frequency record was found on three sugar pine stumps located within 100 m of each other in the Redwood Mountain Grove (Table 2). The period between fires recorded on one or more of these stumps ranges from 4 to 15 yr, averaging about 9 yr. This is fairly comparable to the overall 8-yr frequency of fire in Sierra Nevada forests reviewed by Wagener (1961) and a similar fire frequency for Southwestern ponderosa pine forests recorded by Weaver (1951).
DATES OF FIRES AND INTERVALS BETWEEN THEM ON THREE NEIGHBORING SUGAR PINES, REDWOOD MOUNTAIN, KINGS CANYON NATIONAL PARK, CALIFORNIA
*Mean interval = 8.84 yr
MANAGEMENT IMPLICATIONS OF FIRE'S ROLE
Man's manipulation of the forest resources and the processes that would normally have operated in the various forest types of the Sierra Nevada during the past century have created a problem for him - most urgently a fire-hazard problem. A number of solutions seem available from a theoretical standpoint. Heinselman (1970b) suggested six options for forested wilderness areas and national parks; presumably mechanical and chemical treatments or weather modification (Fuquay, 1967) might also be considered on lands devoted to commercial forestry interests. Heinselman's options for wilderness and park forests include (1) continue fire exclusion with its inevitable change in plant and animal communities and its buildup in fire hazard; (2) let "safe" lightning fires burn, but try to put out the rest; (3) allow "safe" lightning fires to burn, allow for some wildfires which cannot be controlled, and prescribe enough more fires to simulate a natural fire regime; (4) suppress all wildfires, if possible, and use prescribed burning to simulate a natural fire regime; (5) allow all wildfires to burn unchecked, unless life or property are threatened, and hope a natural regime will result; and (6) abandon natural ecosystems and turn to full-scale vegetational and environmental manipulation by various means.
In Sequoia and Kings Canyon National Parks, a policy approaching option number 5 is being employed in the higher-elevation forest types of the parks, covering about 70% of the acreage of the parks (Kilgore and Briggs, 1972); no problems have developed during the first 5 yrs of its use. For lower elevation sequoia-mixed-conifer and ponderosa pine forests, a policy approaching option number 4 is in use, but detailed plans and prescriptions are still being developed (Kilgore, 1972b). Similar programs are underway in Yosemite National Park.
It may be that once the abnormal fuel accumulation has been removed from the sequoia-mixed-conifer forests in national parks, the intervals between subsequent burns and the timing of the burns could be set by nature or at least could follow patterns of intervals known to exist in times before the advent of European man. Lightning-caused fires would then no longer be regarded as a threat to this particular forest type.
But during the interim period, while prescribed burning is being used to reduce fire hazard and abnormal buildup of understory fuels, certain additional facts could help improve our approach to fire-management. Fundamental questions that Hal E. Anderson (personal communication) suggests need to be answered are: (1) How do fuel loads change with time in major types? (2) What fire potentials exist in various types and how do they change with time? (8) Are there periods of high fire potential at particular ages? and (4) What is the fuel situation on a site recently burned by plan - how much dead and living fuels are left on the ground, in the understory, and in the crown? Answers to these questions would in turn help the manager know (1) How often should an area be burned? (2) What prescription is appropriate? (3) How much fuel accumulation indicates the need to prescribe another burn? and (4) What management actions can best simulate "naturalness" and at the same time minimize smoke contribution to adjacent communities.
At the same time answers are being sought to these questions by carefully controlled laboratory and field studies, good information can also be gathered by observing actual experimental fires and wildfires. Researchers can take advantage of the techniques used by Van Wagner (1968) and van Wagtendonk (1972) to gather behavior data on actual fires under various conditions of fuel moisture and weather. Such data will offer considerable help to forest managers in developing practical guides to fire management in Sierran forests.
The original conifer forests of the Sierra Nevada were dependent on fire. Fire was a key environmental factor that initiated new successions, controlled species composition and age structure of the forest, and produced (in concert with other natural agents) the mosaic of vegetation that supported the animal components of these communities.
Fire is the dynamic process that allows minerals and energy to recycle faster within the various forest ecosystems. In theory, similar decomposer functions are performed by fungal and bacterial action. But these processes are far slower than fire, and it is doubtful in the temperate Sierran forests whether these organisms have ever played the complete decomposition role without fire. Through our fire-suppression programs, we have slowed this cycle and allowed the buildup of perhaps the highest degree of fire hazard ever observed in sequoia-mixed-conifer and ponderosa pine forests. The higher-elevation forests do not yet have this same hazard, in part because of shorter growing seasons, so lightning fires can again be allowed to burn in national park and wilderness forests at these elevations.
In all probability, the giant sequoia and various pines of the Sierra survive today because of the role fire plays in the various forest types. Fire must be restored, as nearly as possible, to that natural role if we are to continue to have these species through the millennia that follow.
In managing the sequoia-mixed-conifer forest and the other forest types in the National Parks of the Sierra, the National Park Service is trying to restore natural forces to the forest; when natural frequencies of fire have been determined, we will incorporate these into our fire-management programs. We expect that enough mineral soil will be exposed by burning to allow germination of seedling sequoias and other intolerant species.
In National Parks and wilderness areas we must approach the assignment of restoring natural environmental conditions with humility and great ecologic sensitivity. And whenever and wherever possible, the best way to restore a vignette of primitive America may be to let natural forces run their own course.
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Did You Know?
The Sierra Nevada is still growing today. The mountains gain height during earthquakes on the east side of the range. But the mountains are being shortened by erosion almost as quickly as they grow. This erosion has deposited sediments thousands of feet thick on the floor of the San Joaquin Valley.