It is important to realize that not every fire is identical to every other fire. Fires vary in their physical properties—their intensity, their rate of spread, their frequency, their flame heights, and their size. Different fires act on the same biota with different outcomes. Even two fires with similar physical parameters will yield different ecological outcomes as a function of their timing. If one fire burns in the summer and another in the winter; if one succeeds an initial fire after four years and the other after forty or four hundred; if one eliminates certain species from the site and another permits enough to survive to recolonize; if one occurs amid exotics and another does not; if one burns around a seedling, another around a juvenile pole tree, and another around an adult of the same species, the biological consequences may well differ.
Thus it is not enough to say that Eucalyptus is adapted to fire. Rather, particular eucalypts are adapted to fires of particular sorts, to fire regimes. Different species of Eucalyptus require different fires. In wet forests, severe fires, even if infrequent, are more important than mild fires. Wet eucalypt forests tend to be even-aged, triggered by episodic holocausts that prescribe the proportion of eucalypts to invading rainforest taxa. In dry forests, fires tend to be more frequent and less intense, and conflagrations, while less likely to incinerate whole stands, may cause shifts within the existing population of eucalypts.4
Nor is fire a singular event. Typically, fires occur as geographic complexes and historical cycles. Once some part of a biota burns, it influences the other parts of an ecosystem. With long-range firebrands, a fire in one site may propagate into others, and by shaping new patterns of fuels it may propagate into the future as well. Real fires do not occur in strict cycles, like returning comets; they burn in eccentric rhythms. They integrate not only seasonal and phenological cycles, but events that are unexpected, stochastic, irrepeatable, and irreversible. A site’s history is rarely wiped clean; almost always the past lingers in ways that bias the future. Once fire insinuated itself into the eucalypt environment, it was not easily expunged. Instead it spread, like a drop of acid etching new and indelible patterns on whatever it touched.
SUPPORTING SCLEROMORPHS: FIRE BY SYNERGY
Even where the eucalypts dominate as trees and control the canopy, they share the surface with other organisms, a cast of supporting scleromorphs. Within the scleroforest, all must interact—sometimes as competitors, sometimes as complements. No organism can afford to establish a special relationship to fire one-to-one in biotic isolation. Rather, its success will depend on how it responds to the spectrum of fires to which the site is subjected and which it helps to shape. If few organisms can survive without regard to the eucalypt, neither can the eucalypt ignore those scleromorphs with which it shares a site and with which it often develops a special fire synergy. In broad terms, these include grasses, shrubs, other scleromorphic trees, and a few Australian exotica such as the grass tree (Xanthorrhoea).
Gramineae—the grasses—are the most extensive fuels in Australia. They interpenetrate with most scleromorphic biotas, and they claim for themselves a great concentric ring between the central deserts and the coastal forests. In woodlands they sustain understory burning; in many drier forests they often replace eucalypt litter as a driving fuel; in deserts, they appear in the form of ephemerals after heavy rains, promoting widespread if episodic fire. Yet grasslands display few adaptations unique to fire. Their fire-hardiness derives from their adaptations to drought and grazing; 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