frequency and fire intensity. Grasslands may burn annually; wet scleroforest, on a cycle of several hundred years.
It is a simple fact, often overlooked, that not all biomass is available as fuel. Here, again, natural biotas differ dramatically in how much of their above-surface biomass they offer as fuel. In grassland, this includes virtually everything; in heath, approximately 93 percent of its biomass; in eucalypt forest, less than 5 percent; in brigalow (Acacia aneura), barely 0.1 percent. These proportions reflect not only the relative frequency of burning within the respective biotas but something of the biological significance of fire to them. The forest figures are especially low because so much biomass is locked into the living trunks and branches of trees, which may char but will not be consumed by even the most intense fire.
Nor is all the potentially available fuel always accessible to a fire. What drives a fire are its surface fuels, and what drives a surface fire are its fine fuels with their large surface-to-volume ratios that render leaves, needles, and bark stringers so receptive to radiant heat and so sensitive to wetting and drying. In eucalypt forests, surface fuels vary along a gamut that runs from open grasses to dense scrub. Eucalypts influence the understory by regulating sunlight, by dripping leachates from their crown, by depositing litter in the form of leaves, bark, and branchwood that is at once both nutrient and fuel. This influence varies considerably according to the supporting sclermorphs with which eucalypts share the biota.
Where grass dominates—such as in semiarid savannas, the wet-dry tropics, blade-grass coastal forests—bushfires are really grass fires. Eucalypts contribute litter and shade beneath the thinning woodland, but the dynamics of fire follow the dynamics of the primary fine fuel, grass. Such biotas typically burn annually or biennally. Without fire, the grasses become decadent, some species after only one or two seasons. Fires are frequent, and if intense, fast moving.
Dry scleroforests, while they feature some grasses, obey the dynamics of eucalypt litter. On the average, it takes about three to five years for litter to build up sufficiently in quantity and coverage to sustain a fire, and somewhat longer for litter accumulation to reach a steady state through organic decomposition. Depending on forest type, 34 to 84 percent of the litter consists of leaves. Eucalypts shed perhaps a third to half of their leaves annually, with a peak drop during late spring and summer when new growth flushes the canopy. Other contributors to the litter are twigs and branches, and of course there is the celebrated eucalypt bark, also dry and nutrient-poor and prone to endless shedding.
These fuels burn well when dry, and on the open, sun-immersed floor they dry quickly. Interestingly, eucalypt leaves are flammable in the canopy because of their high heat content (due to their oils) but are flammable in the litter because of their low mineral content, which allows combustion to flame vigorously. The phenological cycle is thus perfect for fire. Dry scleroforests burn on a three-to-twelve-year rhythm. The lower limit is set by minimum fuel needs; the upper, by the opportunity for ignition. In addition, about 150 species of Eucalyptus feature stringybark or candlebark, filigree strips that not only add to litter but carry fire up the bole and, during intense burning, can break free as firebrands and ignite new fires as far as ten to thirty kilometers away. A fire in a eucalypt forest is rarely self-limiting—or put differently, eucalypts help to enlarge their sphere of fire influence far beyond the sites they inhabit.12
Wet scleroforests are more efficient at biological decomposition, but they compensate by supporting scleromorphic shrubs that effectively enlarge the surface layer available for burning. The litter layer proper needs only to support enough fire to ignite the shrubs, nearly all of which are available as fuel. The combined combustion of litter and shrubs enormously inflates the flaming zone and multiplies—“accelerates,” in Australian parlance—the heat output of the advancing front. The shrubs are a fuel threshold that, once crossed, powers a fire to a state of uncontrollable fury. If the litter and shrub zone is large—if they have not burned for many years, if the moisture content of the fuelbeds is low—the flaming zone may expand further to include the canopy. In the oil-rich canopy, a crown fire is a flash fire.
Actual fuel accumulation is complex. Surface fuels increase rapidly then approach a quasi-steady state. Grasses slow their growth after a few years unless cropped or burned. Eucalypt litter mechanically breaks down into smaller, more compact portions; some biological agents support outright decomposition; and growth rates, after the postfire flush, decay. What controls the variability of the fuel load is the low layer of shrubs, grasses, and herbs, entangled with tree-shed litter, that extends up to thirty centimeters above the forest floor. Its size and arrangement vary widely, but the time since the last fire is a critical parameter. In scrub-prone environments, the longer the interval between fires, the more fuel builds up and the more vigorous a subsequent fire; and the more intense the surface fire, the more likely it is to involve the canopy. While there exists in some scleroforests a scenario by which a maturing, fire-free forest will suppress by shade and leachates a scrubby understory, this assumes a condition of stability that is almost unknown in contemporary Australia. Besides, episodic and sometimes catastrophic events—windstorms, insect invasions, major infestations of diseases—can quickly superimpose enormous quantities of fuel onto a site. The best means to counter massive fuel deposition is by equally massive decomposition—fire.
Not all of the fuels can burn all of the time. A fuel complex’s true availability depends on the moisture content of the dead and living components. Fine dead fuels like grass require only a few hours to dry adequately enough to burn. Large-diameter fuels such as logs demand a season, perhaps a drought. Intermediary-sized fuels dry out and wet at different rates, and a single large-diameter fuel particle will likely have imbalances within itself—a dry surface and a moist interior at the start of summer after a wet winter, or a moist surface and a dry interior as a result of light rain at the end of a blistering summer. Thus the fuel complex is far from uniform; fires reflect this heterogeneity.
Burning is patchy, combustion incomplete, and the more complex the fuel, the more complicated the fire. Under normal circumstances fires will burn more fiercely in summer than in winter, along exposed ridges better than within sheltered ravines, in open forests more vigorously than in closed. Only during times of severe drought—when all fuels, living and dead, small and large, are drained of moisture—can a fire burn with relative disregard for local nuances of fuel moisture. Under such conditions everything burns, and fire intensity correlates closely with fuel quantity. Where the climate makes fire routinely possible, where ignition is abundant and reliable, fire history follows fuel history.
This was almost certainly the case with Old Australia. Fires followed from fuels, but fuels reflected, in part, a history of past fires. Fire worked on selective species, tilting the biotic balance, priming the scleromorphs. A source of new ignition could result in an explosion; new fuels could expand or contract the realm of that detonation. Then intruders violated the isolation of Old Australia. With firestick and later with new biological allies—weeds and domesticated fauna—Homo could break down and reorganize the boundaries imposed on Australian fire regimes by climate and genetic inheritance. Humans could attack the surface litter, alter the frequency and timing of fire, and restructure fire regimes. By revising fuel history, humans could rewrite fire history.
FIRE WINDS, FIRE FLUME
Combustion chemistry requires oxygen as well as fuel, and combustion physics makes fire a flaming boundary between a fuel array and its surrounding air mass. The fires of Old Australia inscribed patterns that balanced biotas with winds. Old Australia’s fire regimes integrated not only the variability of fuels but the variability of air flows. Each, however, had been disciplined over eons into certain patterns and they interacted in predictable forms. Together they defined a typology of Australian fires.13
In the absence of wind a fire would assume a shape according to the available fuels. A perfect distribution of fuels would result in a perfect circle of expanding flame. If fire burned on a hillside, the flames that burned upslope would be closer to fresh fuel than flames backing down the hill, and the
