landscapes that routine fire replenishes. Kangaroos, wallabies, and wombats—the grazers need the nutritious new growth that springs up after a burn. Termites may proliferate into cavities carved by fire in eucalypts. Koalas need fire to prevent other trees from crowding out eucalypt regeneration. Certain species of ground parrots (like Pezoporus wallicus) require heath of a certain height in which to nest and reproduce; a possum like Burramys parvus exists only in dense stands of even-aged snow gums; the rat kangaroo (Bettongia penicillata) prefers thickets of Casuarina for shelter—all habitats that can perpetuate themselves only through some regimen of burning.10
A pattern of fire, like a pattern of rainfall, has become an expected norm for many Australian biotas. Some species have made the expectation of fire an essential part of their strategy for reproduction and survival; and a few, within the parameters of their genetic resources and the dynamics of their resident ecosystems, have shaped themselves in ways that sustain advantageous fire patterns. The linkage between life and fire is the biomass they share—for one, part of a cycle of nutrients and habitats; for the other, fuel. But what fire considers fuel is the residue and living tissue of organisms and is subject to ecological dynamics and evolutionary selection. The kind of fuel available determines the kind of fire that burns, and the character of the fire helps shape the character of the fuel that reburns—a brilliant dialectic of fire and life. Once started, once pushed by climate and genetic predisposition, once confirmed by isolation, fire could propagate beyond its prime movers into a pervasive presence from which few residents of Old Australia were exempt.
FUELING THE FIRE
The dynamics of bushfires are thus intimately interdependent with the dynamics of their fuels. Fuel chemistry and physics determine whether fire is possible; fuel availability sets important parameters for fire frequency and intensity. Fuel links fire with biotas, for, in the broadest sense, fire and organisms compete for litter. In environments that are uniformly warm and humid, such as tropical rainforest, productivity is high but organic decomposition is equally aggressive and little litter remains as potential fuel; there are few natural fires. In cold, dry environments like the boreal forest productivity is low, but decomposition by biological agents is even more retarded; fuels build up relentlessly over long years until a fire, or cycle of fires, sweeps through. In temperate regions, the interplay between biological and physical decomposition is complex and irregular. What really matters is its mobile fraction of the fuelbed, the surface litter. Where soils are poor and the climate dry—where, that is, biological agents are few—fire becomes increasingly obligatory if that litter is to be recycled. If fire fails to decompose it, the system slows, its nutrients sitting in worthless caches, a natural economy in which scarce hard currency is stuffed into mattresses or buried in backyards.11
In natural systems, all these fuel attributes vary. There are variations within a single biota and, of course, variations between biotas. Over time fuels build up in quantity; they are rearranged; they show seasonal changes in chemistry and structure; they interact not only with fire but with storms, insects, diseases, and organic decomposers. Different biotas exhibit very different patterns of fuels, and the same biota may show radically different patterns of fuel availability according to seasons and moisture content. The rhythms of fuel availability, however, define the boundaries for fire 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.