NATIONAL PARK SERVICE Research in the Parks NPS Symposium Series No. 1

INTRODUCTION

Despite efforts by the best-trained firemen in the world, coniferous forests, chaparral, and similar vegetation types are periodically going to burn (Roe et al. 1971; Wilson and Dell 1971). It therefore behooves us, as scientists, laymen, and environmentally concerned citizens to learn everything we can about the natural role of fire in our wildlands and to support intelligent management based on this knowledge. This is particularly true of our national parks and wilderness areas, where natural processes are supposed to run their course, as nearly as possible.

The impact of fires on the sequoia-mixed conifer forest ecosystem and the role of fire in maintaining natural environmental conditions in this and other vegetation types in the Sierra Nevada are my primary research interests at this time. Related studies are being carried out by a number of other investigators in government agencies and universities.

Our interests are in part academic, for we hope to learn basic truths which will help us understand the complex interrelationships of this forest ecosystem. But our studies are also aimed at gathering the facts necessary to insure that this ecosystem, with all its diversity, will be managed so as to perpetuate the dynamic processes which, in an evolutionary sense, have given us the sequoia-mixed conifer forest.

In certain higher elevation forests of Sequoia and Kings Canyon National Parks, it has been National Park Service policy since 1968 to let lightning fires burn unless human life or property are endangered (Kilgore and Briggs 1972). In our lower elevation sequoia-mixed conifer forests, however, a considerable fire hazard has built up because of the exclusion of natural fire during the past half-century (Leopold et al. 1963). Hence, a program of prescribed burning has been adopted as the technique for restoring fire to this ecosystem (Kilgore 1970). In order to carry out effectively this management objective, we must know far more than we do at present about the natural role of fire in this forest.

NATURAL HISTORY OF THE SEQUOIA-MIXED CONIFER FOREST

Within a sequoia grove, the primary species are giant sequoia (Sequoiadendron giganteum), sugar pine (Pinus lambertiana), and white fir (Abies concolor). Incense-cedar (Libocedrus decurrens) joins these three in lower elevation groves. Such species as ponderosa pine (Pinus ponderosa) and black oak (Quercus Kelloggii) are not typical associates in the mesic habitat of the giant sequoia grove, but rather they represent vegetation of xeric habitats within the mosaic of mesic and xeric sites characterizing most groves (Rundel 1969). Nevertheless, from a fire ecology standpoint, we must consider the whole range of vegetation occurring within this sequoia-mixed conifer ecosystem in that each of the somewhat more mesic or more xeric subtypes make up only a portion of the complex mosaic throughout which fires function. For example, fire originating in a slightly warmer exposure site will often move quickly into a cooler or more moist exposure involving another subtype. Hence, consideration of what fire does in a strictly mesic sequoia grove would be far removed from the on-the-ground reality of how fire operates in the whole ecosystem.

What then does fire do in the giant sequoia-mixed conifer forest? I have selected seven functions of fire which seem particularly significant. Fire in this forest (1) prepares a seedbed; (2) cycles nutrients; (3) sets back succession in certain relatively small areas; (4) provides conditions which favor wildlife; (5) creates a mosaic of age classes and vegetation types; (6) reduces numbers of trees susceptible to attack by insects and disease; and (7) reduces fire hazards.

THE ROLES OF FIRE

Seedbed Preparation

Giant sequoia. Fire in the sequoia-mixed conifer forest provides soft, friable soil on which the light-weight sequoia seeds fall and in which they are buried (Hartesveldt and Harvey 1967). By consuming the accumulation of down branches, litter, and duff, fire allows the seed to reach mineral soil. And in heating the soil, fire changes the texture in a way which allows a seed to be covered by a few millimeters of soil as a result of its fall from the tree, thus promoting germination.

Timing of the burn is important. In the Redwood Mountain Grove of sequoias in Kings Canyon National Park, one experimental burn took place in August 1969. This allowed 2 months of seed fall before winter snows came. Table 1 shows the influence of this fairly intense 1969 broadcast burn on germination of seedlings of both sequoia and a primary shrub species.

On plot 3, which burned hottest, more than 40,000 Sequoia seedlings per acre were found during the first year after burning, while about 13,000 per acre germinated on more lightly burned plots 1 and 2. The three burn plots averaged nearly 22,000 sequoia seedlings per acre during the first post-burn year. By the second year, these numbers had decreased through natural mortality factors to an average of 2614 sequoia seedlings per acre. Not a single sequoia seedling was found either year on the unburned control plot. By comparison, very few seedlings germinated after another burn on adjacent plots in late November 1970. This was true, at least in part, because snow covered the ground 2 days after the fire, and there was practically no opportunity for seeds to fall and be buried in soil and ashes. For a short period of time following burning, the soil remains loose and friable. Rain and snow, however, increase the soil density again in a way that falling seeds do not penetrate the surface by impact (Hartesveldt et al. 1967).

A possible correlation between numbers of seedlings and numbers of seed-producing trees was evident in this study (Kilgore and Biswell 1971). On burn plot 3, there were more than nine sequoia greater than 6 ft diameter at breast height (dbh) per acre compared with less than five per acre on plots 1 and 2. Thus the burn plot most productive of sequoia seedlings had both the greatest numbers of large sequoias per acre and the hottest burning conditions. It appears the rising convection column of heat, which dried out and killed sequoia needles more than 100 ft up in three trees on plot 3, may have also caused the drying and opening of sequoia cones on several of these same trees and contributed to very heavy seed fall in the area of the hottest burn. The extremely large numbers of sequoia seedlings germinating in plot 3 were probably related to both ideal seedbed conditions and heavy seed fall.

TABLE 1. Sequoia and deerbrush seedling response to 1969 prescribed burning at Redwood Mountain, Kings Canyon National Park, California.

Once seeds are buried and germination takes place, moisture in the rooting zone becomes a critical factor. Rundel (1972) concluded that giant sequoias are limited to habitats of relatively high soil moisture and noted this limiting factor acts through the ecological tolerances of the seedling stages. Stark (1968) found that partially burned giant sequoia litter held more available water (273% by weight) than unburned litter and that it formed a good seedbed. Furthermore, highest survival of sequoia seedlings has been found on very heavily burned soils, possibly because of greater soil moisture availability (Hartesveldt and Harvey 1967). Other factors such as killing of 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 (pers. comm.) of the University of California show that 5% more moisture (% by volume) was found in the top 5 ft of soil beneath a giant sequoia after clearing of undergrowth than before such clearing. Likewise, the center of a one-half acre cleared area showed from 3 to 6 inches more moisture available in this top 5 ft of soil than was found in surrounding forested areas (Kilgore 1968).

In other vegetation types, studies confirm that additional soil moisture is available after fires, but additional surface run-off and erosion often have accomplished 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. Some of these substances found in litter are driven downward in the soil by fire and condense on soil particles at lower levels, depending upon temperature gradients (DeBano 1969).

In laboratory tests of soils from giant sequoia groves, Donaghey (1969) found that temperatures high enough to destroy organic material decreased soil water-holding capacity but increased its water infiltration capacity. She found partially or wholly nonwettable soils resulted from temperatures in the 300-750°F range, but after being heated at 750°F for 4 hours or more, soils became wettable and absorbed water immediately upon contact. Temperature readings found in our prescribed burning studies currently underway at Redwood Mountain varied from no change in unburned sites and at lower soil depths to between 500 and 750°F in the first 2 inches of soil. These higher temperatures were found in a few extremely heavy fuel sites, often under sizeable logs which were completely consumed during a burning period of many hours. It would seem probable, as Donaghey suggests, that a nonwettable layer may develop in certain sites after burning under giant sequoia. However, no erosion problems have developed to date with prescribed burning on our study plots at Redwood Mountain or in our higher elevation red fir study plots (Kilgore 1971b). This may be due, in part, to the highly varied character of the burn patterns in these areas—a pattern less often found in hot brushfires studied by DeBano (1969) in Southern California chaparral, where water repellancy has led to serious soil erosion problems.

Other species. Fire also plays a role in the germination and survival of seeds of other mixed conifer species. Seedling ponderosa pine and Jeffrey pine 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 undoubtedly benefit to some extent from conditions following fire.

Various shrubs of this community are almost entirely fire-dependent, and these species have become increasingly scarce during the past 50 years, 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 case the species is stimulated to greater growth and production as a result of fire (Buchanan et al. 1966; Sweeney 1967, 1969).

Table 1 also shows the impact of the 1969 burn on germination of Ceanothtis integerrimus (deerbrush). Contrary to the results found for sequoia seedlings, between 3700 and 5600 deerbrush seedlings per acre were recorded the first post-burn year on the more lightly burned plots 1 and 2, while only 218 per acre were found on the heavily burned plot 3. The almost complete absence of living deerbrush plants before burning makes difficult any quantitative estimate of seeds available in the soil. Hence, the greater number of shrub seedlings on the less heavily burned plots must be explained by the fact that heavy burning conditions destroy seeds, while lesser temperatures crack seed coats and allow germination. Smaller numbers of other brush species, Ceanothus parvifolius, Arctostaphylos patula, and Ribes roezlii, were also found on the burn plots, while no shrub seedlings of any kind were found on the unburned control areas. There are apparently no fire-type herbaceous species associated with a conifer forest (Sweeney 1969), but several species increased 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).

Nutrient Recycling

The giant sequoia-mixed conifer forest may be a prime example of an ecosystem which will not function unless it is periodically burned (Lyon and Pengelly 1970). Here, as in other coniferous forests, fire plays an important role in returning various mineral nutrients to the soil. Mineral absorption by plants is a constant drain upon the soil (Behan 1970). A sizeable 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).

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). Light burns, however, often increase soil pH, stimulate nitrification, and improve soils chemically. The ash deposit increases available phosphorus, potassium, calcium, and magnesium (Hare 1961).

The effects of light burning on soil nitrogen are more complex. Ponderosa pine seedlings planted in soil samples taken from the top 10 inches of burned and unburned plots showed greatest growth in more hotly burned areas, suggesting greater availability of nutrients (Vlamis et al. 1956), sterilization of the soil, or both. Seedlings on burned soils showed nearly a 50% increase in weight over seedlings on unburned soils. An increase in both nitrogen and phosphorus in burned soils was found in another study (Vlamis et al. 1955). Less immediate but important chemical changes in soil can occur when fire stimulates growth of such nitrogen-fixing shrubs as Ceanothus spp.

The growth release pattern of surviving trees following fire is another indication of the quick conversion of nutrients tied up in dead plant materials to new living tissue. Weaver (1947) found the average diameter of 40-year-old ponderosa pine on a burned area was 7.4 inches compared to 1.7 inches on an adjacent unburned site. Giant sequoia have shown marked growth increases after fires (Hartesveldt 1964), indicating that nutrients and moisture formerly tied up in various kinds of litter and in certain standing dead and living trees were made available to the remaining sequoias through burning.

Impact on Succession

Before the early 1900s, frequent and widespread surface fires kept the sequoia-mixed conifer forest open and park-like (Biswell 1961). Pioneer or secondary successional stages were favored over climax forms; sun-loving species were favored over shade-tolerant forms; and fire-resistant and fire-dependent species and associations were favored over nonfire-dependent forms.

European man has caused two types of changes in fire ignition patterns which, in turn, have affected the successional stages now seen in the mixed conifer forest of the Sierra Nevada: (1) With the arrival of western man in the 1850s, burning carried out as part of Indian culture declined and was virtually eliminated by 1865 (Vankat 1970); and (2) during the last decade of the 19th century and the first decade of the 1900s, fire suppression activities were undertaken by Federal Government agencies in the Sequoia and Kings Canyon National Parks areas. Vankat (1970) reported two increases in numbers of white fir (a notoriously shade-tolerant species) in Sequoia National Park—one in the 1860s which coincides well with the elimination of Indian burning and another in the 1900-1910 period which coincides with the beginning of Federal fire suppression activities. He also found a corresponding decrease in cover and density of shrubs such as manzanita and Ceanothus because of increased competition with tree species.

Rundel (1971) notes that sequoia groves represent a fire-climax community whose stability is maintained by frequent burning of the understory. Without regular surface fires, litter accumulation limits sequoia seed germination, and the grove community becomes a "long standing seral stage in succession toward a climax overwhelmingly dominated by Abies concolor, with giant sequoia absent."

Our recent prescribed burning work at Redwood Mountain in Kings Canyon National Park has begun setting back succession in a modest way 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 as great as would have been accomplished by periodic natural fires during the past 50-70 years.

One of the results of the natural process which 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 burned through the forest, young white fir were killed when they were still part of the understory level of vegetation and fuels. Fairly mild burning conditions could still accomplish this and leave a mosaic of openings in the crown. These openings in turn allowed sunlight to reach the forest floor and permitted growth of sequoia seedlings, shrubs, and herbaceous plants which require substantial sunlight. Since fires have been suppressed, however, certain fir trees—perhaps not large in numbers yet—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.

Such small but important openings in the crown canopy did not generally result from the type of minimum treatment employed by Biswell at Whitaker's Forest in which white fir and incense-cedar less than 11 ft tall were cut, piled, and burned (Kilgore 1971a). We recorded an increase from an average of slightly less than 9% to about 12% of full sunlight on the forest floor in this work; in certain small thickets of saplings having less overstory vegetation, however, an average of 36% of sunlight was found after burning (Kilgore 1968).

A few white fir and sugar pine between 12 and 24 inches dbh were killed in the hotter prescribed burn areas at Redwood Mountain in 1969, while few trees as large as 12 inches dbh were killed in the milder 1970 burn. Hartesveldt's work with small plots did bring about openings in the crown canopy by felling snags, cutting out white fir thickets, bucking up all logs, piling all material with a bulldozer, and burning. This was, of course, a major departure from natural burning conditions and cost from $300 to $500 per acre (Hartesveldt et al. 1967).

Such intensive work would be impossible over large acreages from an economic standpoint and perhaps would be undesirable ecologically as well. However, some sizeable 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, even in these purportedly high fire hazard giant sequoia-mixed conifer forests.

Faunal Relationships

The good deer ranges which have nutritious and palatable browse are usually found in subclimax stages of plant succession. As Leopold (1966) points out, "Burned or cutover forest lands support most of the deer in 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 11 ft 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 substantial changes in cover type or set succession back severely, this degree of habitat modification resulted in relatively small changes in avifauna.

The importance and the complexity of ecosystem relationships involving fire and wildlife are highlighted by the role which a small mammal and an insect seem to play in sequoia seedling regeneration in the sequoia-mixed conifer forest. Researchers from San Jose State College, under contract to the National Park Service, have recently found that the chickaree or Douglas squirrel (Tamiasciurus douglasii) and a small cerambycid beetle play a significant role in sequoia reproduction. Outside sequoia groves, Shellhammer notes that this tree squirrel commonly feeds on seeds of sugar pine, white fir, and ponderosa pine. Within the groves, the chickaree also cuts sequoia cones, not for their tiny seeds, but instead to chew on their green, fleshy cone scales. Some 80% or more of the seeds are unharmed by the squirrel's feeding process (Hartesveldt et al. 1970).

Certain squirrels cut large numbers of cones from individual sequoias, and perhaps seeds from a few of these cones may be useful in production of seedlings. But apparently the squirrel's most important role is its feeding upon cones within the tree itself, allowing seeds to fall from considerable heights (Hartesveldt et al. 1970), thus maximizing seed impact on the soil and seed dispersal.

Age of the cones is also involved. Greatest seed viability was found in 5-year-old cones (Hartesveldt and Harvey 1967) with a gradual decrease thereafter. Chickarees seem to prefer young green cones 2-5 years of age, while older cones are subject to the workings of the larvae of the beetle Phymatodes nitidus. Stecker found that the larva of this small, long-horned beetle chews its way inside the cone and gets nourishment from the tissues (Hartesveldt et al. 1970). In so doing, it cuts vascular channelways, causing the gradual death and drying of the cone. As the cone dries, it opens, and the seeds fall from high in the trees.

The relationship between fire and the squirrel and beetle would seem to be this: Following fire, when a squirrel cuts cones and particularly when it feeds on them in the tree, the seeds or cones fall into the soft, friable seedbed of mineral soil and ash which is ideal for sequoia germination and survival. The work of the beetle causes the older cones to dry on the tree; as they dry, cones open, allowing seeds to fall, sometimes in great numbers, at a time when germination and survival possibilities are highest. Partly as a result of these two animals, heavy burning conditions under giant sequoia seem to favor sequoia regeneration over any other species (Kilgore and Biswell 1971) unless a crown fire should develop which would completely consume the seed source high on the mature trees—a highly unlikely prospect under natural conditions where periodic fires kept ground fuels and understory vegetation at low accumulations.

Formation of a Vegetative Mosaic

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 400°F to 1200°F or more with no uniformity of distribution in a given burn (Lindenmuth 1960; Sweeney and Biswell 1961). 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.

In describing this phenomenon in a ponderosa pine forest, Weaver (1967) says: "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 needle fall from large trees will not support a surface fire. Hence, until the pines are large enough to build up fuels under themselves, fires would not be 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.

Reduction in Insect Susceptible Trees

Fire also has a sanitizing effect by thinning stands or eliminating old stands or old trees before insects and disease have overtaken them (Heinselman 1970; Loope 1971). As an example, under natural fire cycles, outbreaks of spruce budworm may have been less prevalent, bark beetle epidemics may have been less common and less severe, and dwarf-mistletoe may have been held more in check.

Without fire, older trees become more susceptible to insect attack or disease (Hare 1961). Weaver (1964) believes recent heavy western pine beetle attacks resulted from excessive competition in dense stands of ponderosa pine which developed in the absence of fire. Under these circumstances, trees killed by insects leave a forest more susceptible to fire (Wellner 1970). Lyon and Pengelley (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.

In the giant sequoia-mixed conifer forest, there has been concern expressed about the role of the giant carpenter ant (Campanotus levigatus) in building nests in the heartwood of the tree (Jack Hickey pers. comm.). Large numbers of these ants were found at the points of breakage of two sequoias in the Hazelwood section of Giant Forest in 1969. There is certainly a possibility that natural fires kept numbers of this insect at a lower level from that we find today by burning out ant nests found in the bark and heartwood of the sequoia and in other living and dead woody materials in the forest. The National Park Service has contracted with the Department of Entomology and Parasitology at the University of California, Berkeley, to investigate the role of this ant in the forest. We hope to be in a better position soon to judge what role natural fire and human visitation to these groves may have in altering numbers of this insect and its nest-building activities in the giant sequoia.

Fuel Reduction

In one year an acre of forest converts solar energy into vegetative matter equivalent to 300 gallons of gasoline (Roe et al. 1971). By putting out lightning fires in the mixed conifer forest, we have been merely postponing the inevitable release of this energy stored through photosynthesis. Unless we take action soon, future wildfires will be far more destructive than those we have previously faced (Biswell and Weaver 1968).

For this reason the major current problem in management of the giant sequoia-mixed conifer forest is the high fire hazard that has built up since the turn of the century. In the absence of lightning fires and aboriginal burning, formerly open forests now have a dense understory of young trees. The bulk of these are white fir, which germinates readily in shade and survives in dense thickets in the absence of light surface fires. While virgin forests in California were once said to be uneven-aged, patchy, and broken—so much so that "a continuous crown fire is practically impossible" (Show and Kotok 1924)—such crown fire immunity has now been lost in many of our mixed conifer forests. 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 13,000 acres of brush and mixed conifer forest amid had threatened a grove of giant sequoias.

TABLE 2. Flash fuel and duff weights^a before and after burning at Redwood Mountain, Kings Canyon National Park, California.

In our first major effort at reducing such fuel hazards in the sequoia-mixed conifer forest, some 100 acres of forest were burned under prescribed conditions in late summer and early fall of 1969 on the ridge of Redwood Mountain in Kings Canyon National Park. A second burn involving research plots took place in late fall of 1970. Preburn data were collected on a variety of vegetation and weather parameters and included weight measurements of flash fuels and duff. Before burning, more than 50 tons of fuels per acre were stored in the litter and duff layers alone, without taking into account the logs and standing dead and living trees (Table 2). Following the November 1970 burn, this total had been reduced some 85% to 7.7 tons per acre. Numbers of young trees in the understory had also been greatly reduced, and while data is now being analyzed for publication, it appears that crown fire potential has been decreased substantially.

Immediately downslope from this largest grove of giant sequoias, fuel weights on the west slope of Redwood Mountain have been calculated at 20-40 tons per acre (Agee 1968). In a cut, pile, and burn program at Whitaker's Forest, mid-way on that slope, some 22 tons of fuels per acre were burned, including nearly 1000 living saplings and more than 500 dead trees per acre (Biswell et al. 1968). Costs of this manipulation ranged from $114 to $146 per acre.

The importance of fire to the fuel reduction process of the fire-dependent forest ecosystem was set forth clearly by Mount (1969):

. . . one aspect of fire that requires most study is its role in destruction. removal and recycling of forest products, especially the dead components of the vegetation. . . . Fuel reduction burning fits in as part of the natural role of fire. Nutrient recycling, patently not fully achieved by biological agencies in forests where fuels accumulate, is more efficiently carried out by the combination of storage in durable debris and periodic processing by fire. Biological agencies are just not capable of breaking down some of the complex polyphenols and terpenes.

Whether we call this process "dry ashing" or "ecological recycling by environmental pyrolysis" or, simply, "prescribed burning," the need is there in our sequoia-mixed conifer forests, and fire seems to be about the only way to get the job done efficiently and completely.

FIRE FREQUENCY

Mutch (1970) hypothesizes that, "Plant communities may be ignited accidentally or randomly, but the character of burning is not random. . . . Fire-dependent plant communities burn more readily [and more frequently] than non-fire-dependent communities because natural selection has favored development of characteristics that make them more flammable."

The giant sequoia-mixed conifer forest is such a fire-dependent community. And the National Park Service program of fire research and management in this forest type is based on the assumption that fire plays an important role in seedbed preparation, in nutrient recycling, in modifying successional patterns, in providing a mosaic of age classes and vegetation types important to wildlife, and in reducing fuel hazard in the forest. But how often did it play this role in the past? What was the natural frequency or periodicity of fire in a sequoia-mixed conifer forest, particularly in the forests at Sequoia and Kings Canyon?

To answer this question, we are currently analyzing fire dates on stumps of trees cut on adjacent national forest lands (Weaver 1951; Heinselman 1969). In detailed studies of small 7-10 acre plots, involving sugar pine, incense-cedar, white fir, and ponderosa pine, frequencies in the range of 7-9 years seem to be developing. In preliminary work on the relatively few stumps of sugar pine and ponderosa pine cut within sequoia groves during past insect control programs, we found a most interesting frequency record on three sugar pine stumps located within 100 years of each other in the Redwood Mountain Grove (Table 3). The period between fires, recorded on one or more of these stumps varied from 3 to 15 years, and averaged about 9 years. This is fairly comparable to the overall frequency of fire in Sierra Nevada forests determined by Wagener (1961) and the fire frequency for Southwestern ponderosa pine forests found by Weaver (1951). It is considerably more frequent than the 20-25 year fire periodicity which Hartesveldt and Harvey (1967) estimated for a given locality in the Mariposa Grove of sequoias in Yosemite. Our findings, however, are similar to the more frequent fire pattern noted by Presnall (1933) and Hartesveldt (1964) for fires somewhere in the 250-acre Mariposa Grove.

Fire frequency and intensity must have varied somewhat from habitat to habitat within the mixed conifer forest. The more mesic east and north slopes do not burn as readily as the more xeric west and south slopes. Because of this, when they do burn, they may burn more intensely than those that burn more frequently. A similar relationship was found in Soeriaatmadja's (1966) study of past fire frequency in ponderosa pine in the Central Cascades of Oregon. There, however, elevation was the variable which led to somewhat more mesic or more xeric conditions with consequent fire-frequency differences.

The records of fire frequency that we are gathering for Redwood Mountain and from nearby mixed conifer forests will offer concrete evidence of the periodicity which fire assumed naturally in these forests. For stump records gathered to date extend far beyond the time when European man played any role as ignition source for fire. The role of Indian burning (Stewart 1956; Reynolds 1959) as compared with lightning ignition (Taylor 1971) must still be worked out. But for purposes of restoring natural environmental conditions to the parks of the Sierra Nevada, this distinction may not be essential.

We expect to complete the first segment of these studies in 1972 and hope to publish our results soon thereafter. These frequency records can then become the basis for determining how frequently we should use prescribed burning within our sequoia-mixed conifer forests.

TABLE 3. Dates of fires and intervals between them on three neighboring sugar pines, Redwood Mountain, Kings Canyon National Park, California.

WOOD SMOKE AND PUBLIC REACTION

Concern is often expressed about the public's willingness to accept a new fire management direction (Pechanec 1970) and about their willingness to accept a reasonable explanation of the difference between the air pollution contribution of wood smoke compared with automobile exhaust (Zivnuska 1967).

The National Park Service is greatly interested in studies of wood smoke which are now underway at the University of California's experimental forest adjacent to Redwood Mountain as well as air pollution studies being carried out by the U.S. Forest Service (Barrows 1971). We will want to take advantage of the best weather conditions for burning to minimize any possible adverse influence (Roe et al. 1971). No one should forget, however, that the quality and quantity of materials released in wood smoke are not the same as those found in industrial pollutants or automotive exhaust systems. And the desire to eliminate smoke from prescribed burns must be tempered by the desire to control air pollution from the inevitable present and future wildfires (Roe et al. 1971).

Based on our experience at Sequoia and Kings Canyon, the public seems quite ready to accept the natural role of fire in the forest and our plans to restore fire to that role as nearly as possible. We take every opportunity—through the press, in community talks, and in park interpretive programs—to explain the reasons for both our natural fire program in higher elevation forest types and for the use of prescribed fire in our lower elevation forests and chaparral country. We feel confident that candor on our part will continue to enhance public acceptance of this new, exciting, and ecologically viable management of park lands.

CONCLUSION

The original conifer forests of much of North America—including the giant sequoia-mixed conifer forest—were fire-dependent ecosystems. Whether ignited by lightning or Indians, fire was the key environmental factor that initiated new successions, controlled species composition and age structure of the forest, and produced the mosaic of vegetation which supported the animal components of these communities (Heinselman 1970).

Fire appears to be essential to the life cycle of the giant sequoia. As such, it becomes essential to the whole ecosystem, involving complex interrelationships between the sequoia, white fir, the Douglas squirrel, a cerambycid beetle, the carpenter ant, and many other plant, animal, and soil components of this system. Fire appears to be the dynamic process that allows minerals and energy to cycle faster within the ecosystem's operation. In theory, similar decomposer functions are performed by fungal and bacterial action. But these processes are far slower than fire, and it is doubtful 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 communities (Hartesveldt 1964).

In all probability, the giant sequoia survives today because of the role fire plays in the ecosystem operation of a sequoia-mixed conifer forest. Fire must be restored, as nearly as possible, to that natural role if we are to continue to have giant sequoias through the next many millenniums.

Fire probably burned under sequoias at least every 10-20 years. After longer periods, there is danger that fuel buildup will allow many mature trees to be killed. Until we are sure that fires which start by natural ignition will not threaten whole groves of sequoias or human life, the National Park Service will continue a policy of prescribed burning of the understory vegetation and accumulation of down logs and litter to bring the groves back to more natural conditions.

In managing this ecosystem, we are trying to restore natural forces to the forest; when natural frequencies of fire have been determined, we will incorporate these into our burning programs. We expect that enough mineral soil will be exposed by burning to allow germination of seedling sequoias. Burning will also aid establishment of native shrubs, an important segment of the natural community required by wildlife. When a better system of judging fuel buildup and climatic variables is developed, we will utilize this system in making the best possible decisions about managing with fire.

We must approach the assignment of restoring natural environmental conditions with humility and great ecologic sensitivity. Some will feel we are arrogant when we try to second-guess the current stage of plant succession. Others may feel we are becoming gardeners instead of guardians. Our guiding principle should be that, "Above all, the maintenance of naturalness should prevail." 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.

Fig. 1. Prescribed burning in the Redwood Mountain Grove of giant sequoias, Kings Canyon National Park. Fire consumes the accumulation of forest fuels, leads to a recycling of nutrients and reduction in wildfire hazard, and prepares a seedbed for sequoias. During these early efforts, National Park Service crews used fire hoses as an added safety precaution. National Park Service photo by Bruce M. Kilgore.

Figs. 2-3. These two photos from Yosemite's Confederate Group in Upper Mariposa Grove, Yosemite National Park, taken in 1890 and 1959, illustrate the successional process which occurs in the absence of fires. Thickets of white fir have grown up which could fuel a crown fire fatal to giant sequoias. Note the large fire scar on the sequoia (left) caused by a series of lighter ground fires. The early photo is by George G. Reichel, courtesy Mrs. Dorothy Whitener; the later photograph is by A. W. Hood. Such historical documentation by National Park Service collaborators Mary and Bill Hood lends strength to both research and management efforts.

Figs. 4-5. This pair of photos was taken from the same site before and after prescribed burning at Redwood Mountain, Kings Canyon Nationsl Park. In addition to consuming the fir carcasses in the foreground, the fire burned considerable fine litter and duff, reducing litter weights from about 50 tons to less than 8 tons per acre. National Park Service photos by Bruce M. Kilgore and Dan Taylor.

Fig 6. National Park Service forestry foreman uses a drip torch in igniting forest litter under the canopy of giant sequoia. Fire will consume most of the litter and will kill some of the understory fir trees which have grown here in the absence of fire during the past 50 or more years. National Park Service photo by Bruce M. Kilgore.

Fig. 7. The frequency of natural fires before the coming of European man in the late 1800's, is clearly documented in the growth ring pattern seen on this section cut from a sugar pine stump in the Redwood Mountain Grove, Kings Canyon National Park. On this particular tree, fires were recorded on an average of every 18 years between 1778 and 1867. Fire scar records in other localities in the grove indicate more frequent fires. National Park Service photo by Dan Taylor.

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