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Unimaginable Freaks of Fire: Profile of a Pyrophyte

… But enough! Where are the words to paint the million shapesAnd unimaginable freaks of Fire, When holding thus its monster carnivalIn the primeval Forest all night long?

—CHARLES HARPUR, “The Bush Fire” (1853)

… I had a full opportunity of examining this, one of the finest sights which tropical countries display … Above us the sky was gloomy and still; all round us the far-stretching forests exposed a strange and varied pageant of darkness and fire, accompanied by the crackling of flames and the crash of falling trees.

—“AN EMIGRANT MECHANIC,” Settlers and Convicts (1849)

WITH GATHERING SPEED, like a flaming maelstrom, Australia spun to its destiny as a fire continent. Proliferating fires pushed scleroforest and grassland across a biotic threshold. Bushfire became a reality with which almost—but not quite—every landscape of Old Australia had to contend. Since the Pleistocene it has generally been the case that, where biotas have changed, they have moved toward a state of more fire, not less. Some species were swept aside, while some accommodated, adapted, and learned to tolerate fire. Others thrived.

The eucalypts flourished overall. Their evolutionary history, its peculiar genetic makeup, had conditioned Eucalyptus to exploit those unsettled times. Nutrient scavengers of ravenous dimensions, woody weeds ready to colonize disturbed sites, evergreens that could adopt many growth habits and that wrapped protective coverings around critical tissues so that they could thrive in strong heat and sunlight—the eucalypt alliance amalgamated hundreds of species which were ideally predisposed to survive in an environment of increasing fire. Their scleromorphic traits were even better preadapted to fire than to drought, and the rising tide of fire soon swept them before it. Generic adaptations evolved into more fire-specific traits.

No other genus that had so far survived the voyage from Gondwana could compete with Eucalyptus for dominance within Australian forests. Yet there remained areas from which they were excluded: eucalypts shunned the frost-ridden subalpine terrain; on chronically dry sites eucalypts gave way to spinifex, mulga, and gibber desert; on perennially wet sites, eucalypts were crowded out by rainforest or were challenged by paperbark Melaleuca. But everywhere else—wherever fire was routinely possible, even over a span of centuries—eucalypts flourished and shaped whole communities of pyrophytes. The scleroforest it dominated bloomed when burned. Without fire its biophysical engines cooled, and its biotic dynamics decayed.

FRIENDLY FIRE

Most eucalypts can accommodate most fires. But they do so in ways both common and diverse.

Their defenses begin with their bark. What kills is a kind of thermal ring-barking caused by a very high temperature or a long duration of lower heating. But bark is thick, it is densest at the base where the fire burns, and it conducts heat poorly. Surface fires pass by, charring the exterior but not killing the living cambium beneath it. While it is common for heat to concentrate preferentially on one side or the other, either because fuels pile up on the uphill side of a trunk or because winds form eddies on the lee side, at worst this wounds only one side and explains why most basal cavities develop uphill or downwind.¹

The thick bark, too, protects epicormic buds buried beneath it. When branches die, new buds are liberated and shoot out. Even if fire torches the crown, a new canopy rapidly emerges and clumps of epicormic sprouts clothe the bole and major branches like moss. Canopy-depleting fires, however, are abnormal in most scleroforests. Once past a juvenile stage, eucalypts shed their lower branches. Between the forest litter, which sustains the fire, and the living canopy, which maintains the tree, there is a considerable gap in fuels that is difficult to bridge by flame unless the surface fire burns with extraordinary intensity, a pilot light in a forest furnace. Even if the canopy is burned off or irredeemably scorched, the fatal fire is only a flash burn. It does not consume the live tissue or the woody fruits encased in tough, nutty caps. Eventually some sprouts become dominant and shape a renewed canopy, and seeds rain down to the waiting ashes.²

Something analogous happens below the surface as well. All but twelve or fifteen eucalypts develop a lignotuber. In place of bark, these subsurface tissues are protected by soil and the simple physics of heat transfer. Probably 95 percent (or more) of the heat released by a fire dissipates upward through radiation and convection; the remainder enters the soil, but it cannot penetrate far since soil is a poor conductor of heat and a few centimeters is ample to shield roots and microbes. An intense surface fire could well consume or lethally scorch a seedling; but if a young tree existed in an environment that burned—if, that is, adequate litter was piled around it—then it was probably not thriving anyway. Regardless, the seedling had already stored in its lignotuber most of the critical nutrients it required. A new shoot, or multiple shoots, punches through the ashy crust; within a few years one stem becomes dominant and rapidly evolves into a new tree; the lignotuber matures. In many species, even if the entire bole is destroyed, new sprouts appear. In the mallee habit, the process is so well developed that Eucalyptus grows naturally as a coppice.

The lignotuber is particularly important because eucalypt seed is not long-lived. A tree holds its seeds for one or two years and in exceptional cases for as many as four. After a fire, seed predation is heavy, Germination is typically poor unless the seed is buried in mineral soil or in an environment free from competition for scarce water and nutrients in the critical first years. But a fire, paradoxically, can produce ideal circumstances for germination. Seed virtually rains down from the charred canopy, overwhelming the capacity of those invertebrate animals that normally feed upon it. The fluffy ash accepts the falling seed, buries it, encases it in an environment full of mineralized biochemicals and temporarily purged of antagonistic microorganisms.

The ashbed effect is multiple, complex. The fire temporarily sweeps competition away. It sterilizes the soil of microflora and microfauna, most of which resided in the combustible litter. It may burn away or cripple other woody species, thus permitting greater access to the site resources by the phoenix eucalypts. It mobilizes vital trace nutrients like molybdenum that are never more accessible for biological intake than in their disintegrated forms after a fire. It volatilizes leachates in the litter, some of which are packed with inhibitory chemicals. A moderate or severe fire restructures the canopies of forest and scrub to permit greater sunlight and to restrict toxic leaching from rain drip. A burn scours out fuels, permitting a few years of fire-free existence. Although the biochemical details are not altogether understood, the outcome for most eucalypts is incontestable: it is essentially only in such a context that new seedlings emerge, and it is through successive burns that the resprouting lignotuber allows eucalypt seedlings to triumph over less vigorous competitors. While various scenarios exist for regeneration, almost all depend, at some stage, on an intense fire.³

These are generic traits, common to most eucalypts, and it is important to recognize that an extraordinary variation exists within the alliance. Eucalyptus had, over its evolutionary history, acquired a suite of traits to cope with a suite of environmental stresses. Particular adaptations to fire were, after a fashion, grafted on to already existing traits. Defoliation by fire might differ little from defoliation by insects; the decapitation of a seedling by burning, from decapitation by browsing; branch loss by fire, from branch failure by wind; temporary nutrient losses by fire, from soil paupery or drought. Some eucalypts favor seed production; others, vegetative propagation. Some have enormous lignotubers, while others feature lignotubers that seem almost vestigial or persist only through certain stages in their life cycle. Some eucalypt forests tolerate surface fires; others thrive on stand-replacing fires. E. regnans, the mountain ash, is highly sensitive to surface fires but seeds prolifically after a conflagration with the result that the towering mountain ash forests are even-aged. Even within one species, there are variations according to the pattern of fire to which they are subjected.

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 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, 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. 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 upslope fire would spread more rapidly. Exactly the same principles govern the interaction of flame and wind. A wind-driven fire acquires a head; the stronger the wind (or steeper the slope) the more rapid the spread; the more rapid the rate of spread, the narrower is the ellipse that traces the flaming front. The combustion properties of the heading fire differ from those of the backing fire. Where fuels are light, the backing fire may be simply snuffed out.

The flaming part of the fire is its perimeter, and it assumes a shape that integrates the combined effects of both fuel and wind. The more fuel, the more vigorous the fire; the more wind, the more rapid its spread; and the two in combination define the fireline intensity, a measurement that correlates roughly with flame height. These interactions are complicated, however, by terrain, by the fuelbed, and by the flaming front itself. Terrain directs and deflects general air flow. Fuelbeds, particularly in the case of forests, greatly modify the flow of air across the fire. It is normal for wind speed to be far less at the surface than at the canopy. The fire itself, by generating gases as a result of pyrolysis and combustion, produces a convective flow upward. This gaseous outflow interacts with the ambient winds and can engender special phenomena during high-intensity fires.

Horizontal vortices may roll alongside the flanks of fast-moving fires like mobile levees. If eddies develop within the combustion zone, firewhirls may appear. Perhaps no less dramatic is the phenomenon of long-distance spotting. Particles of burning fuel are lofted above the canopy—perhaps through a torching tree, or by a firewhirl, or simply by the overall convective vigor of the flaming front—and then enter the main winds to be carried away and ultimately deposited, still burning, far from the fire. Through long-distance spotting, a single ignition multiplies into many; a chain reaction begins that scatters fire like broadcast seed through a host of environments. If the convective flow is stronger than the ambient winds, then the fire may collapse its spread and intensify its burning rates into a mass fire, but this is rare in nature, and probably rarer still in Australia. The great fires of Australia are the product of great winds.

Those winds are patterned, roughly predictable as to place and time. They are the product of local airflows between valley and mountain, land and sea, and of large-scale weather systems expressed as monsoons or fronts. They flow at particular seasons and in particular directions. Exposed ridges rich in fine fuels can develop into “fire paths,” preferential routes for fast-moving fires, while complex terrain may exhibit “fire shadows” in which fires are commonly confined to the windward side of slopes. Frontal weather systems complicate that scenario with accelerations and wind shifts, but again winds must interact with curing, drying, and ignition sources to mold particular fire behaviors and fire regimes. The geography of wind helps to shape a geography of fire.

There is one pattern, however, that dominates the typology of Australian fires as much as Eucalyptus dominates the composition of Australian forests. It transforms the southeastern quadrant of the island continent into a veritable fire flume. Here the climate is broadly Mediterranean, culminating in a prolonged summer drought. Gradually, storm tracks migrate northward and cold fronts, sweeping west to east, brush the southern border of Australia. Ahead, they draft air from the north, and from the Nullarbor Plain to Tasmania this means desert air from the interior—hot air, dry as tinder, violent as a dust storm. No Mediterranean Sea mitigates this Australian sirocco. What it passes over it parches. Clouds may roll ahead of it, a squall line of dust and, often, of ash.

As the front approaches a site, the northerly winds accelerate. Air streams out of the Red Centre in violent gusts, a dusty avalanche. If a high-pressure cell stagnates over the Tasman Sea, frontal progress may slow and desiccation, by desert winds, prolong. But once the front passes, there is an equally violent shift in wind direction from north and northwest to south and southwest. What had been blistered by hot winds is now swept by equally ferocious cold winds. It is a deadly one-two punch, calculated to knock down by fire anything still standing after drought. As often as not flames ride the winds like froth on a surf. When the wind shift comes, it instantly punches new heads out of what had been a fire flank. Fires double, triple in size. Renewed, they rage on with irrepressible vigor.¹⁴

This combination—desert blast followed by southerly burster—concentrates in the southeastern quadrant, the fertile crescent of Australia, where the best soils, the best-watered landscapes, the greatest fuel loads are found, where the continent gathers itself together into a great funnel with its spout at Tasmania. Here reside the fires that give Australia its special notoriety, not merely as a continent of fire but as a place of vicious, unquenchable conflagrations. In the fire flume lurk the great, the irresistible fires of Australia.

PYRIC DOUBLES: MALLEE AND BRIGALOW

It was not inevitable that the eucalypts should dominate Australian woodlands or that fire should pervade Australia with the singularity it has enjoyed. There were alternative biotic candidates and alternative fire histories. Isolation and aridity explain only part of the mystery; other fragments of Gondwana, outfitted with a similar biotic stock, moved into the tropics, seasonal drought, and fire, yet did not come so ruthlessly under the spell of one genus or one process. Acacia, not Eucalyptus, was the great arid woodland species of the Gondwana commonwealth. Yet to compare these two genera in Australia is to trace a contrapuntal history. In particular, the mallee (a eucalypt) and the brigalow (an acacia)—outwardly similar in their multistemmed growth habits—record fire histories so different that they may be considered as biotic doubles, the one a pyrophyte, the other a pyrophobe.

In Old Australia neither accepted fire on an annual basis. But mallee assumed that fire would repeat according to some quasi-regular rhythm, and brigalow, that if fire happened once it might never occur again. The mallee withstood fire, even defied it. It managed to thrive across the maw of the southeastern fire flume; probably it needed fire to favor it against those potential competitors, hovering around it like raptors, whose powers of postfire recuperation were far less vigorous. Brigalow ignored fire, shunned fire, created an environment in which fire was, under natural conditions, almost impossible. It squeezed out the eucalypts. Their different fire regimes reflect not only diverse biologies, but different wind regimes and vastly different fuel histories.

The expression “mallee” describes a place, a growth habit, and the conglomeration of eucalypt species which exhibit that habit—multiple stems and relatively short canopies (three to nine meters) that make mallee woodlands resemble a large woody shrubland. Geographically, mallee claims the most inland and arid of the eucalypt-dominated woodlands, clustering in both the southeast and the southwest quadrants of Australia. More than a hundred eucalypts are mallees, of which seventy-one are endemic to Western Australia and another twenty-one are shared between southeast and southwest. What makes this coppicelike growth possible is an enormous lignotuber, some of which actually hold free water. (The largest on record has measured ten meters across, out of which branched 301 stems.) But what makes the mallee so flammable is the complex of associated pyrophytes.¹⁵

Mallee eucalypts share a diverse understory with grasses like Stipa and spinifex (Triodia), with scleromorphs like the shrubby chenopods and the casuarinas, and with ephemerals that blossom after major storms. Between them the mallee complex can generate fuel loads for which only fire appears competent to decompose. Mallee litter reaches a quasi-steady state at 10 tons/hectare^–1, and further flammability depends on a rain-flushed understory. Thus, under routine conditions, lightning fires fail to spread beyond a clump or two; under exceptional conditions, however, the outcome is a conflagration. One by one, the pieces come together, like an old cannon readied to fire—heavy winter rains, which cause bare ground to burst with ephemeral grasses and forbs; an outbreak of desert winds, washing over the landscape like blown sand across a dune; the steady, year-by-year growth of resinous spinifex and of eucalypt litter, which hangs in seductive streamers from the branching stems, ready to fly like flaming chaff to new sites. Fuels are continuous, deep, crackling with a pyric chemistry that requires only a spark to explode.

No such fire could occur more than once unless the biota that sustained it could recover. In fact, it appears that episodic fires are essential to the perpetuation of the mallee community, especially in the southeast. Seeds germinate poorly, almost always confined to burned sites. But the primary mechanism of regeneration is resprouting from the giant lignotubers. Within six months after a fire as many as seventy shoots may be present; after seven years, some twenty to thirty may remain; and after a century, somewhat fewer than ten. The capacity to resprout after a fire is astonishing, though sensitive to seasonal timing. Studies suggest that two or three successive autumn fires may prove fatal, but that a twelve- to eighteen-month-old eucalypt mallee can be completely defoliated twenty-six times before exhaustion culminates in death. At less frequent rates, fire stimulates productivity. Mallee eucalypts can continue to increase biomass production in the canopy at a rate of 6 to 9 percent per year for up to thirty-five years following a burn. By favoring vegetative regrowth over seeds, fire is actually a conservative event, perpetuating the existing over the experimental, promoting the inevitable return of fuel and fire.

What is true for the mallee eucalypts holds equally for their understory. Where spinifex underlies mallee, species diversity after a fire can, in places, increase from eighteen species of vascular plants to sixty-three species, of which twenty-six are annuals. The shrubs experience an efflorescence in species and an acceleration in growth rates. Fire prunes back and stimulates spinifex, assuring a spectrum of habitats as the hummocks grow back over many years to their prefire dimensions. Overall productivity does not peak in the mallee biota for probably fifteen years, with a plateau for canopy fuels at thirty years. Without fire the understory lapses into decadence. Once established, so long as ignition and winds persist, fire assumes the status of a normal event, like earthquakes rupturing episodically along a fault line.

The contrast with brigalow is profound, rendered more enigmatic by the overall success of Acacia within the commonwealth of Gondwana. Outside Australia many acacias thrive in seasonally arid grasslands that burn routinely, even annually. A canopy well above the flame heights of grass fires, bark sufficiently insulating to survive the brief spurt of flame, hard seeds that must be stripped of their coat in order to germinate—these generic traits help make Acacia the great tree of the Gondwana savannas. Within Australia Acacia copes with water stress better than Eucalyptus, its many species challenge the eucalypts in truly arid landscapes, and it successfully interpenetrates with them in wetter environs. As a legume it can fix nitrogen, whereas the eucalypt cannot. In sheer variety it outnumbers Eucalyptus by more than a hundred species. What most differentiates their geography is their response to fire.¹⁶

The brigalow belt spans mostly the interior flank of the Great Dividing Range where it crosses the Tropic of Capricorn. It amounts to a dead zone of fire. In its penumbra what Acacia species dominate and what growth habits predominate depend largely on rainfall. Brigalow proper is the domain of A. harpophylla, which grades from shrubland to woodland as rainfall improves. Following a disturbance, root suckering leads to a relatively low, branching expression not unlike mallee. The associated flora—wilga, bottlebrush, yellow wood—have more in common with rainforest than with scleroforest. Where conditions grade into further aridity, brigalow degenerates into a shrubland characterized by gidgee (A. cambagei). Regardless, the Acacia dominants suppress grasses and forbs.

Fuel chemistry is marginal for burning, and fuel loads, feeble. The shape of acacia leaves makes for flat, poorly aerated fuelbeds. The leaves themselves are barely flammable. Bark is not shed. Associated species like yellow wood (Terminalia oblongata) burn only under protest, a heat sink. Although the biomass may total over 250 tons/hectare, without shrubs, without grasses, its surface fuelbed limited to leaf litter, the available fuel for combustion is less than 0.1 percent of the biomass, or under 1 ton/hectare. Leaf fall, moreover, is keyed to intermittent rains rather than to seasons, complicating the timing of kindling to spark routine fire. The microclimate of the forest floor discourages drying and promotes biological decomposition. For practical purposes, combustible litter does not exist.¹⁷

There is little to compel the brigalow belt to accept fire. The dominant acacias are not by and large capable of long-distance dispersal; they do not demand disturbance to perpetuate themselves; they do not require fire. Except where they share sites with eucalypts and grasses, there are no associated scleromorphs. The belt resides outside the major fire tracks—south of the monsoonal winds, north of the temperate storms, inland and on the mountain lee from the Pacific trades. It lies outside the fire flume. Its fuel dynamics made natural fire improbable in much the same way that mallee made fire all but inevitable. Whatever ecological tilting the biota received in the past, here it could right itself without catalyzing a future of endless fire cycles, without committing to an evolutionary path down which the pyrophytes like the mallee were forcibly marched, then seized as their own.

Like the relict rainforest, the brigalow persisted as a kind of stubborn refugia—or more correctly, as a kind of alternative future, a path not taken generally in Old Australia. Along the fringes of the brigalow belt, and occasionally interpenetrating with it, are scleromorphs like Casuarina and several species of Eucalyptus; spinifex crowds the margins, and, where severe disturbances have temporarily destroyed the acacias, grasses carpet the surface. A greater frequency of catastrophic disturbance, more fire under extreme conditions, a different local history during the eucalypt revolution—and the brigalow belt might have evolved toward something like the mallee. But it did not. Disturbance was irregular. Fire did not lead to more fire. Along with rainforest, brigalow became instead the only real forest type not dominated or codominated by eucalypts, the only one for which fire does not appear to be essential. Like the eye of a hurricane, the brigalow belt flourished in the skewed calm center of a fiery vortex.