Fire's Role in a Sequoia Forest
By BRUCE M. KILGORE
Originally published in Naturalist 23 (1): 26-37, Spring 1972
Despite efforts by the best trained firemen in the world, coniferous forests, chaparral, and similar vegetation will periodically burn. It behooves us, therefore, as scientists, laymen, and environmentally concerned citizens to learn all 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.
As Research Biologist at Sequoia-Kings Canyon National Parks, my primary research interests are 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. Related studies are being carried out by 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 so managed to 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 will be endangered. 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. Hence a program of prescribed burning has been adopted as the technique for restoring fire to this ecosystem. In order to effectively carry out this management objective, we must know far more than we do at present about the natural role of fire in this forest.
THE ROLES OF FIRE
Within a sequoia grove, the primary species are giant sequoia, sugar pine, and white fir. Incense-cedar joins these three in lower elevation groves. Such species as ponderosa pine and black oak are not typical associates in the moist (mesic) habitat of the giant sequoia grove, but rather they represent vegetation of the drier (xeric) habitats within the mosaic of sites in the grove (Rundel, 1969). Nevertheless, from a fire ecology standpoint, we must consider the whole range of vegetation occurring within this ecosystem in that each of the somewhat more mesic or more xeric subtypes make up only a part of the complex mosaic throughout which fires function.
What then does fire do in the giant sequoia-mixed conifer forest? Seven functions of fire 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) provides 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.
As a first and important role, fire in the sequoia-mixed conifer forest provides soft, friable soil on which the lightweight 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 Sequoia Grove in Kings Canyon National Park, one experimental burn took place in August, 1969. This allowed two months of seed fall before winter snows came. 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 lighter burned plots 1 and 2. The three burn plots averaged nearly 22,000 sequoia seedlings per acre the first year after burning.Not a single sequoia seedling was found 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 two days after the fire, and there was little chance for seeds to fall and be buried in soil and ashes.
A possible correlation between. the numbers of seedlings and numbers of seed-producing trees was evident in this study. On plot 3, there were more than nine sequoia greater than 6 feet in diameter 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. The rising convection column of heat which dried out and killed needles more than 100 feet up in three trees on plot 3 may have also helped dry out sequoia cones and contributed to heavy seed fall. So the extreme numbers of seedlings on plot 3 were probably related to both ideal seedbed conditions and heavy seedfall.
Once seeds are buried and germination takes place, moisture in the rooting zone become critical. Furthermore, highest survival of sequoia seedlings has been found on very heavily burned soils, possibly because of more available soil moisture. But killing of fungi and elimination of competition are probably equally significant.
In our prescribed burning studies at Redwood Mountain, soil temperatures varied from no change to 750° F. These higher temperatures were found in a few extremely heavy fuel sites (often under sizeable logs which burned for many hours.) While some non-wettable soils may have developed in certain sites after burning under sequoia, no erosion problems have developed to date as they have in Southern California chaparral.
Various shrubs of this community are almost entirely fire-dependent, and these species have become increasingly scarce during the past 50 years or more, thus reducing the value of these areas for deer and other wildlife. Many such shrubs have hard seed coats that prevent germination unless cracked by fire.
Contrary to the results found for sequoia seedlings, we found that between 3,700 and 5,600 deerbrush seedlings per acre germinated on the lighter burned plots, while only 218 per acre germinated on the heavy burned plot 3. The greater number of shrub seedlings on the less heavily burned plots can probably be explained by the fact that heavy burning conditions destroy seeds, while lesser temperatures crack seed coats and allow germination.
A second main function of fire is its role in recycling nutrients. The giant sequoia-mixed conifer forest may be a prime example of an ecosystem which will not function unless it is periodically burned. 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 the relatively slow action of decomposer organisms. The nitrogen and potassium tied up in litter, for example, represents a fair drain on the soil's reservoir of these nutrients. While hot fires may volatilize some nutrients, light burns often increase soil pH, stimulate nitrification and generally improve soils chemically. The ash deposit increases available phosphorus, potassium, calcium, and magnesium (Hare, 1961).
Impact on Succession
Before the early 1900's, frequent and widespread surface fires kept the sequoia-mixed conifer forest open and park-like (Biswell, 1961). Secondary successional stages were favored over climax forms; sun-loving species were favored over shade-tolerant forms; and fire-resistant and fire-dependent communities were favored over non-fire-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:
Vankat (1970) reported two increases in numbers of the shade-tolerant white fir in Sequoia National - one in the 1860's, which coincides with the end of Indian burning, and another in the 1900 to 1910 which coincides with the start of Federal fire suppression activities.
Rundel (1971) notes that sequoia groves represented a fire-climax community whose stability is maintained by frequent burning of the understory. Without regular surface fires, the grove community becomes a "long-standing seral (transitional) stage in succession toward a climax overwhelmingly dominated by white fir, with sequoia absent."
Our recent prescribed burning at Redwood Mountain has begun setting back succession in a modest way by killing many young white fir seedlings and saplings which have become numerous beneath the sequoia and sugar pine. To date, however, changes are not as great as would have been accomplished by periodic natural fires during the past 50 to 70 years.
One of the results of the natural process most difficult to duplicate at this stage of succession is fire's role in making openings in the crown canopy. In the past, young fir were killed while they were quite small. Mild burning conditions could do this and leave a mosaic of openings in the crown. Now, however, some fir have, (1) become a part of at least the lower levels of the crown canopy and hence fire moving into them can threaten crowns of the giant sequoia; and (2) these larger fir are less likely to be killed by moderate fires because of thicker bark.
In the hotter burn plots at Redwood Mountain, some fir and pine between 12 and 24 inches diameter were killed, but few trees as large as 12 inches diameter were killed in the milder 1970 burn. Hartesveldt's work with small plots brought about some openings in the crown by felling dead 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. Such intensive and expensive work would be impossible over large acreages from an economic standpoint and perhaps would be undesirable ecologically as well. However, some sizeable 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 more natural environmental conditions. Once achieved, we would hope to perpetuate equilibrium conditions by allowing natural forces including lightning fires to play their original role as nearly as possible in the sequoia-mixed conifer forests.
The good deer ranges which have nutritious and palatable browse are usually found in subclimax stages of plant succession (Leopold, 1966). In a cut, pile, and burn program under giant sequoias, Lawrence and Biswell (1972) found that browse and forage were more abundant, more heavily hedged by deer, and more nutritious on burned areas than on untreated controls. Bird populations have also increased in numbers and biomass following fire in various vegetation types. In a second-growth giant sequoia forest, however, elimination of tree saplings less than 11 feet tall did not make major changes in species composition of the birdlife.
The importance and 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. Researchers from San Jose State College, under contract to the Park Service, have recently found that the Douglas squirrel and a small cerambycid beetle play a significant role in sequoia
reproduction. Outside sequoia groves. Shellhammer finds that this tree squirrel commonly feeds on seeds of sugar pine, white fir, and ponderosa pine (Hartesveldt, et al, 1970). Within the groves, however, the squirrel also cuts sequoia cones, not for their tiny seeds, but instead to chew on their green, fleshy cone scales. Most of the seeds are not harmed by this feeding process.
Apparently the squirrel's most important role is feeding on cones within the tree, thus allowing seeds to fall from great heights. The squirrel prefers young green cones, while older cones are subject to the working of the beetle Phymatodes nitidus. Stecker (ibid) has found that the larva of this small, long-horned beetle chews its way inside the cone and gets nourishment from the tissues. 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 and feeds on cones, the seeds or cones fall into soft, friable soil which is ideal for germination and survival. The work of the beetle causes the older cones to dry on the tree. As they dry, cones open, and seeds fall sometimes in great numbers at a time when germination and survival possibilities are highest.
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. 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 the way in which such a mosaic is formed within a ponderosa pine forest, Weaver (1967) said, "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 pine under canopies of larger trees, but not in openings. Young pines can both germinate and survive in openings because the small accumulation of needle fall from somewhat distant large trees will not support a surface fire. Hence until pines are large enough to create heavy fuels under themselves, fires would not be intense enough to kill them; and by the time they create heavy fuels, many are large enough to survive such surface fires.
Reduction in Insect Susceptible Trees
A perhaps somewhat controversial role of fire is the sanitizing effect it has by thinning stands or eliminating old stands or trees before insects and disease overtake 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.
Lyon and Pengelly ( 1970) point out that insects and disease are vital components of the dynamic forest ecosystem, and that their role may be related to increasing forest fuel accumulations and, hence, the probability of fire following their activities. Some trees wounded by fire, of course, are in turn attacked by insects and disease and may die, again building up more fuel.
In the sequoia-mixed conifer forest, concern has been expressed about the role of the giant carpenter ant which builds nests in the heartwood of the tree. Hickey (personal communication) feels that natural fires may have kept numbers of this insect at a lower level from that we find today by burning out ant nests. The National Park Service has contracted with the Entomology Department at the University of California to investigate the role of this ant in the forest. We soon hope to be in a better position to judge what role natural fire may have had in the life cycle of this ant and any possible management implications of that natural role.
Experimental plot before and after prescribed burning at Redwood Mountain Grove in Kings Canyon National Park.
The fire consumed the down fir in the foreground and killed a number of white fir saplings, reducing the fire hazard.
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. While virgin forests in California were once said to be uneven-aged, patchy, and broken so much so that a continuous crown fire was practically impossible, 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 burned out more than 13,000 acres of brush and mixed conifer forest and had threatened a grove of giant sequoias.
ln 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. A second burn (involving research plots) took place in late fall of 1970. We collected pre-burn data on a variety of vegetation and weather variables. This included weight measurements of flash fuels and duff.
Before burning, more than 50 ton 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. Following the November, 1970 burn, this total had been reduced some 85% to 7.7 ton per acre. Numbers of young trees in the understory had also been greatly reduced, and it appears that crown fire potential has been decreased substantially.
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.
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. But how often did fire play this role in the past? What was the natural frequency or periodicity of fire in the sequoia forest?
To answer this question, we are currently analyzing fire dates on stumps of trees cut on adjacent national forest lands. In detailed studies of small 7 to 10-acre plots involving sugar pine, incense-cedar, white fir, and ponderosa pine, frequencies in the range of 7 to 9 years seem to be developing. In preliminary work on a few pine stumps cut in the Park during past insect control programs, we found a most interesting frequency record on 3 sugar pine stumps located within 100 yards of each other in the Redwood Mountain Grove. 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 comparable to overall frequencies found in Sierra forests, but is somewhat more frequent than previously estimated for sequoias.
The frequency of natural fires in the late 1880's is clearly documentd in the growth rings from a sugar pine (Pinus lambertiana) stump in the Redwood Mountain Grove. Fires were recorded on an average of every 18 years between 1778 and 1867.
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. So we are watching this as we gather our data, and we soon hope to have concrete evidence of the periodicity of fire naturally in these forests which can form the basis for how frequently we should prescribe burn here.
WOOD SMOKE AND PUBLIC REACTION
Concern is sometimes expressed about the public's willingness to accept fire in the forest. The National Park Service is greatly interested in studies of wood smoke now being undertaken by the University of California and the Forest Service. We try to take advantage of the best possible weather conditions for burning to minimize any possible negative effects. But no one should forget the difference in quality and quantity of materials released in wood smoke as compared with those found in industrial pollutants or automotive exhaust. For example, automotive exhausts and many industrial discharges contain much larger percentages of sulfur and nitrogen oxides and lead. These differences are large and environmentally important. And the desire to eliminate wood smoke from prescribed burns must be tempered by the desire to control smoke from inevitable wildfires of the present and future.
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. We take every opportunity to explain reasons for our "let burn" program in higher elevation forest types and for the use of prescribed fire in our lower elevation forests. 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.
The original conifer forests of much of North America - including the giant sequoia-mixed conifer forest - were dependent on fire. 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.
Fire appears to be essential to the life cycle of the giant sequoia, and as such, to the whole ecosystem. Fire is the dynamic process that allows minerals and energy to recycle 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. Fire must be restored, as nearly as possible, to that natural role if we are to continue to have sequoias through the next many millenniums.
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.
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.
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HARE, R. C. 1961. Heat effects on living plants. Southern Forest Expt. Sta. Occasional Paper 183. U. S. Forest Service. 32 pp.
HARTESVELDT, R. J. 1964. Fire ecology of the giant sequoias: controlled fires may be one solution to survival of the species. Nat. Hist. Mag. 73(10):12-19.
HARTESVELDT AND H. T. HARVEY. 1967. The fire ecology of sequoia regeneration. Tall Timbers Fire Ecol. Conf. 7 :65-77.
HARTESVELDT AND H. T. HARVEY, H. S. SHELLHAMMER, AND R. E. STECKER. 1970. Giant sequoia ecology. Final Contract Report. National Park Service. 48 pp. ditto.
HEINSELMAN, M. L. 1970. The natural role of fire in northern conifer forests. Naturalist 21(4): 14-23.
LAWRENCE, G. AND H. H. BISWELL. 1972. Some effects of forest manipulation on deer habitat in a grove of giant sequoia. Jour. Wildlife Mngt. (In press).
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MUTCH, R. W. 1970. Wildland fire and ecosystems - a hypothesis. Ecol. 51(6):1046-1051.
RUNDEL, P. W. 1969. The distribution and ecology of the giant Sequoia ecosystem in the Sierra Nevada, California. Ph.D. Thesis, Duke Univ. 204 pp.
RUNDEL, P. W. 1971. Community structure and stability in the giant sequoia groves of the Sierra Nevada, California. Amer. Midl. Nat. 85(2):478-492.
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This article was adapted from a paper presented as part of the AAAS Symposium in Philadelphia, December, 1971, on Research in the National Parks. The original paper will appear in a publication of the American Association for the Advancement of Science and the National Park Service. Complete reference citations can be found in Kilgore, B. M. 1972. The role of fire in a giant sequoia-mixed conifer forest. AAAS Symposium on Research in the National Parks. (In press).