grasses that survive under arid conditions and heavy browsing also survive burning. Conversely, grasses that are not palatable, that are not grazed heavily, are available as fuel for fire. Fire acts on mixed grasslands much as drought and grazing do, by shifting the floristic composition from certain species to others. Grasslands that are not grazed or burned rapidly decay in productivity.5
Other organisms show more specialized adaptations to fire in which burning stimulates reproductive success. Nearly a score of Australian vascular plants, for example, flower after a fire. The grass tree (Xanthorrhoea australis) not only floresces profusely following burning but rarely flowers without it. (Fire so stimulates the plant that a blowtorch is often applied to specimens sold at nurseries in order to improve growth and sustain them through the shock of transplanting.) A number of scleromorphic shrubs also respond to fire by flowering, though the onset of florescence may be deferred a year; in the absence of fire, the size of the flowering crops in subsequent years diminishes. Australian orchids, too, flower following burning, and in the aftermath of the Ash Wednesday fires of 1983, rare orchids carpeted whole hillsides. Whatever the proximate causes, florescence after fire leads promptly to seeding.6
Flora that rely on seed for reproduction must either protect that seed from fire or use burning as a means to stimulate germination. Some species, by means of tough coverings, shield seeds from flash fires by storing them in the crown or in the soil, where they are sheltered from fire. Others rely on intense, fast-moving fires to inaugurate seeding—to instigate seed fall or to stimulate germination. Thus many heath shrubs rely on fire to activate seed or to liberate seed from protective follicles. Banksia ornata, for example, has a dry wood fruit that fails to open unless it is scorched by flame. Hakea teretifolia initiates reproduction upon the desiccation of a parent branch, an instance in which fire replaces drought as an active agent. Those eucalypts without lignotubers—the mountain ash is probably the best known—rely on massive seed release following infrequent but intense fires to sustain their presence. Other species litter the ground with seeds over the course of many years until conditions favor their release. Among many leguminous species hard seeds are the norm and must be softened, scarified, or stripped away before germination can occur. This is true for both Acacia and Melaleuca, which compete aggressively with eucalypts in the desert and tropics, respectively. The proportion of hard to soft seed among species of Acacia seems to be related to the frequency of fire.7
Other species seem well adapted to disturbance—opportunists ready to claim niches newly shaped by a fire. A fire volatilizes organic nitrogen, so nitrogen fixers like the Leguminae are ideally positioned to seize the ashy floor. It is, for example, in this capacity that viney acacias enter into the eucalypt forests. Where Casuarina survives, it does so in part because it, unlike the eucalypts, can fix nitrogen. Some Australian species respond to fire as other species do to rain. After a fire, particularly after an intense fire, ephemerals that have not been seen since the last burn appear and flower. There are instances of species, thought extinct, that fire freed from a near-fatal dormancy.8
Accommodations by Australian flora force accommodations by Australian fauna as well. Only a few fauna show specific adaptations to fire itself, like a fly (Microsania australis) attracted to smoke, and a beetle (Melanophila acuminata) apparently steered to heat by means of infrared sensors. Equally, only the most severe fire panics animals. More common is the tendency for a fire to collect an entire food chain, from invertebrates herded in advance of the flames, to small mammals, reptiles, and insectivorous birds foraging on them and other fauna flushed out by the flames, to raptors like kites and wedge-tailed eagles who hunt in swirls through the smoke. Far from killing the ecosystem, such fires bring it to life.9
Nor does the effect end when the flames expire. Whole populations of organisms—from microbes to macropods—adjust to the new opportunities presented by fire. Fire’s immediate impact is to reduce the numbers of most species and to shift the relative proportions of those constituents which remain. Old foods and old habitats are consumed by fire; and no less than organic nitrogen, some old relationships are vaporized. But that is only half the equation. It is equally true that fire mobilizes nutrients, fashions new niches, reorganizes habitats, liberates species that were formerly suppressed, animates biochemical cycles, and recharges biophysical batteries. The site is recolonized—sometimes within as little as three to five years. What results from this sort of burning is a kind of natural swidden, a shifting mosaic of biotas that enormously enriches the species diversity of a regime.
This capacity of fire to animate and diversify is particularly critical in sluggish, apparently run-down ecosystems—heaths, tropical biotas on laterized soils, arid environments where ephemerals lie dormant until rain or fire release them. And it is particularly vital to the cavalcade of indigenous species that need the disturbed, refreshed 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,
