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.
These two photos, taken eighty years apart in the confederate Group, Mariposa Grove, yosemite National Park, illustrate the successional process which occurs in the absence of fires.
Note how the thicket of white fir has grown up obscuring all but the fire-scarred sequoia on the left. Such thickets provice fuel which could support a crown fire fatal to even mature sequoias.
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.
BEHAN, M. J. 1970. The cycle of minerals in forest ecosystem. In Role of Fire in the Intermountain West Symp. Proc. 11-29.
BISWELL, H. H. 1961. The big trees and fires. Nat. Parks Mag., 35: 11- 14.
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).
LEOPOLD, A. S. 1966. Adaptability of animals to habitat change. In Future Environment of North America, Edited by F. F. Darling and J. P. Milton. Nat. Hist. Press, N. Y. 66-75.
LOOPE, L . L. 1971. Dynamics of forest communities in Grand Teton National Park. Naturalist 22(1):39-47.
LYON, L. J. AND W. L. PENGELLY. 1970. Commentary on the natural role of fire. In Role of Fire in the Intermountain West Symp. Proc. 81-84:
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.
VANKAT, J. L. 1970. Vegetation change in Sequoia National Park, California. Ph.D. Thesis, Univ. of' Calif., Davis. 197 pp.
WEAVER, H. 1967. Fire and its relationship to ponderosa pine. Proc. Tall Timbers Fire Ecol. Conf. 7:127-149.
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).
Last updated: May 20, 2017