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Red Centre: Fire Regimes of Old Australia

“There’s different kinds of bushfires. There’s grass fires, like you seen. They don’t mean much at all, just burn off your fodder, maybe catch your stock if you don’t move ‘em fast enough. Then you can have slow scrub fires, ones where there ain’t no wind and the fire don’t get up above the bush. You can belt that out easy. And then there’s a scrub fire that gets along at a pretty good lick. You get that when things are as dry as this.”

“Yes?” said Venneker. “There’s another one. That’s the one you’re scared of.”

—JON CLEARY, The Sundowners (1952)

THE NATURAL FIRE REGIMES of Old Australia can only be guessed at. Too much happened at the transition to the Holocene for relict biotas to have survived intact. There were wild oscillations in climate and oceanic transgressions. There were wholesale extinctions and repopulations of which the eucalypt revolution is the most dramatic expression. And there was the introduction of a limited number of floral and faunal exotics, among them the fire-brandishing Homo. The revolution was rapid and almost universal. The ancien régime collapsed, utterly reformed or driven into remote refuges.

Yet coarse patterns endured and prescribed then, as now, the parameters of the continent’s fire regimes. Each complex had to integrate fuel, spark, and wind. As a general rule, each of these phenomena increased from the center outward, a kind of centrifugal outflow. The farther from the center, the greater the fuels, the more vigorous the winds, the larger the interaction with humans. Old Australia resembled a centrifugal pump, with its axis at its desert core and its discharge in the southeast, pointing toward Tasmania.

Along with those increases there followed an increase in the intensity of resident fires. Beyond the arid core, moisture improved and became more regular, thus amplifying fuels. With consistent storms came consistent outbursts of lightning, timed to kindle cured grasses, drought-blasted scrub, or desiccated litter. The great wind systems, too, flowed from desert interior to watered littoral. Across the northern third of the continent, the Asian monsoon powered a seasonal tide of winds, southerly in the summer, northerly in the winter. Across the southern perimeter frontal systems smashed cold air masses into warm, a migrating whirlpool of winds that hurled desert dust across coastal biotas. The particulars of each circumstance converged into patterns by which life and fire shaped distinctive regimes. But all in some way referred back to the dry core of the continent, its fabled Red Centre.

MULGA MÉLANGE

Like a fat boomerang, Australia arcs northward far enough to share the monsoonal rains and dips its pendulous ends sufficiently southward to capture the belt of temperate storms. Between these two zones—summer rains and winter rains—precipitation is less reliable. Fuels are sparse, scattered, typically confined to swales, peaks, and ephemeral watercourses that can augment, even marginally, a precarious supply of water or are restricted to those exceptional times and places in which storms penetrate in force. Terrain mixes relentless plains with mountains that bubble upward like lithic mud geysers—the Hamersley Range, the Macdonnell and Musgrave Ranges, and of course the coast-spanning Great Dividing Range. But the added rainfall the mountains attract is countered by a broken landscape of rocky ridges and canyons. The interior deserts are a source for fire winds, not a recipient of them. In the Red Centre fires dance to an atonal beat.

The characteristic trees are acacias. From the perspective of fire dynamics, one part can stand for the whole, however, so consider the representative case of Acacia aneura, the mulga, whose visibility lends a certain unity to what is otherwise a mélange of biotas, starved by wretched soils, choked by aridity, and largely uninformed by fire. The mulga is typical of the constituent species—dispersed or strung into intergroved strips, suppressing grasses around it, contributing indifferently to fuel loads. It cannot by itself sustain a fire nor propagate flame beyond isolated patches. Large fires require surface fuels in continuous carpets, but among the mulga mélange, those fuels sprout in sufficient profusion only after extraordinary rains. When that happens perennial grasses and shrubs put forth extra growth; ephemerals fill up the interstitial spaces; the mélange acquires a fuelbed capable of supporting fire. Its fires follow its rains.¹

The irregular rhythm of rainfall and drought determines the tempo of bushfire. Normal years are dry, with a growing season restricted to a period of less than five weeks. To this comes drought, roughly one year in four. If rains materialize, they cluster around high peaks or accompany outbreaks of exceptional weather that send the moist monsoons deep into the interior or that warp storm tracks and fling cold fronts, like stray asteroids, far to the north. It is estimated that for the moister eastern environs large wildfires occur between two and five times a century. In the drier center and west, incomplete records suggest major fires every fifty years or so, with known outbreaks documented for 1921 and 1974–75. The 1974–75 fires consumed an estimated 117 million hectares in a colossal swath through central Australia. Here lightning is a competent source of ignition. It kindled most of the 1974–75 fires, and in 1984–85 a lightning storm in Cobar Shire (western New South Wales) burned some 620,000 hectares in December and another 770,000 hectares in January; one fire alone burned 101,290 hectares.²

But while enormous, such fires are too infrequent to drive a biota. What rain and fire momentarily unite disintegrates after the emergency passes; the fires are sustained by exceptional, not normal, flora; when those annuals and ephemerals no longer bind the rest of the biota together, the separate species return to their prior existence. Mulga is a type case. Easily killed by fire, it seeds profusely after a burn with seed that can remain viable for over sixty years, or until sufficient rains germinate it. If, however, one burn rapidly succeeds another, if one wet year and its fires follow hard on the heels of another, the fires may consume mulga seedlings and destroy the prospects for replenishment. There is, however, no other tree to claim the niche vacated by the mulga. Granted enough undisturbed decades, the mulga will eventually return. Fire is not so much essential as tolerated, accommodated rather than encouraged.

The point is reinforced by considering the chenopod shrublands like saltbush (Atriplex) with which mulga sometimes collates in drier environments. Fire devastates saltbush with incomparable thoroughness; recovery is painful and tedious. It is likely that the shrubs originally evolved in a littoral environment for which fire occurred as an exceedingly rare event that did not really rejuvenate the biota but simply restarted a replacement cycle. In response, Atriplex makes fire as unlikely as possible. Shrubs grow in strongly patterned clumps; interstitial grasses and forbs are normally suppressed so that bare ground prevents fire spread; leaves are both succulent and rich in salts, a fire retardant. As an individual fuel particle and as a fuelbed both, saltbush burns poorly, rendering it an ideal understory for mulga. Only when abnormal precipitation carpets the landscape with ephemerals and annuals does the scene carry fire. Atriplex restoration is possible because the site lacks alternative colonizers. The salty shrubs complement mulga well.³

The mulga environs are not informed by fire, and this perhaps helps to explain why, in contrast to so many other Australian biotas, they constitute a mélange rather than a fire regime, strictly speaking. No Australian pyrophyte has taken over the habitat. Conditions are too arid for tropical grasses, spinifex, or eucalypts to grow, and ignition is too unpredictable to force a selective pattern of fire. In fact, the primary inhabitants like mulga and saltbush inhibit rather than promote fire. In its eastern terrain, mulga merges into the unburnable brigalow.

Pyrophytes are most effective in those environments that are capable of supporting any one of several biotas. In such a context, fire can be a selective, driving process; it can perpetuate one species over another, direct the energy dynamics of an ecosystem, or restructure habitats. In return, the pyrophytes assure that fire has adequate fuels, both in amount and in arrangement, and that such fuels are available at times when ignition is probable. In a biological sense they make fire predictable and undeniable. Only in selected places, however, do eucalypts and spinifex interpenetrate with the mulga. Instead the mulga mélange hosts a biotic corroboree, a massing of multiple species that dance around a central fire which illuminates but does not inform.

THE CENTRAL FIRES

The enduring central fires follow the perennial grasses—the hummocks called spinifex. Collectively, they comprise the dominant flora for 22 percent of the continent; for arid Australia, their prevalence is even greater (41 percent). They crowd the mulga mélange and in places interpenetrate with it as rain and fire allow. Classically, uniquely Australian plants, they claim as their special dominion the infertile soils of the ancient craton; they flourish on the interior steppes, even those devoid of surface waters; and they burn, regularly and hugely. In their breadth and interdependence with fire, they resemble grassy equivalents of the eucalypt.⁴

The provenance of spinifex spans the tropical savanna to the north and the stony deserts and acacia-fringed mountains of the south. Monsoon rains are too scant to support a subtropical biota, yet too abundant to allow the mulga mélange or gibber deserts to thrive without competition. The ragged boundary of the monsoon rains—like the berm thrown up by storm waves—defines its flexible frontier. The consistent rains assure consistent growth, which assures a consistency of fire.

To a remarkable extent, fuels follow the life cycle of the principal spinifex grasses, Triodia and Plechtrechne. Perennials, they grow in spiny bunches—known, when young, as tussocks; when mature, as hummocks; and in some local vernaculars, as porcupine grass. Each tuft grows outward, sometimes assuming the shape of a living ring around a dead center. In the better-watered north, maturity may come within ten years; in the less reliable south, in perhaps twice that. In most environments, spinifex requires three to five years to develop sufficiently to support fire, but its distinctive growth habit demands that fire propagate from one hummock to the next. One solution is to combine large clumps and high winds, the one to flare into huge flames and the other to drive those flames across bare ground to a receptive hummock. The other solution is to flood the interstitial voids with grasses and forbs, and this requires exceptional rains. Fire history thus synthesizes two rhythms—the regular beat of spinifex growth and the irregular rainstorm or rainy season. Too many wet seasons encourage too many fires, which prevents the spinifex from recovering as an important fuel. Too many dry seasons prevent the eruption of ephemerals that carry fire from one coughing hummock to the next.

Still, spinifex makes an extraordinary fuel. For three years after burning, spinifex sprouts are palatable and nutritious, with up to 6 percent crude protein. Then its dietary value collapses and, with less than 3 percent protein, it becomes all but inedible. Its decomposition becomes almost wholly restricted to fire; virtually all new growth is available fuel. The hummocks do what they can to encourage combustion: live stems are rich in resins that burn fiercely, dead stems are typically dry, and all of the hummock is thoroughly ventilated. Spinifex and its associated flora weave a powerful fuelbed; a mature community may groan under a load of 3–8 tons/hectare in available fuels. Once begun, there is little in the steppe lands that spinifex favors to break down a major conflagration. Only changes in winds or surges of moisture or the past history of fires, as recorded in large-scale fuel mosaics, can modulate a free-burning blaze.

The fire history of spinifex shapes the biodiversity of the entire ecosystem. Spinifex pervades 20 to 80 percent of the total ground cover, and it erects a structural matrix for the remaining flora and fauna. In the Simpson Desert hummock grasslands support 180 species of plants; in the central Australian sandplain, 154 species. Combustion releases species—fire ephemerals—that lie otherwise dormant. In regions of little variability, spinifex’s peculiar growth habits thus provide a diversity of habitats. Its microniches testify to a history of disturbance, which is largely a history of fire. Without fire the hummocks become decadent, the landscape uniform, and the cycling of nutrients feeble. Even the hardest-seed ephemerals spoil.

Remove fire and watch mulga, Callitris, and other fire-sensitive refugees timidly reconquer a site. But, paradoxically, remove reliable rains, even precipitation as meager as that in the spinifex, and watch fire retreat before stony desert or a renewed mulga mélange. Free-burning fire requires a full-flushed biota. What powers fire in the spinifex is the mingling of seasonal rains with seasonal drought. That pattern makes spinifex into a kind of central fire, and a model for the two-cycle engines that drove the centrifugal pump that was Old Australia.

THE WET AND THE DRY

Northward, a hesitant, spotty monsoon—its southern fringe brushing against deserts—hardens with grim finality into summer and winter, the Wet and the Dry. Spinifex grades into savanna, hummock grasses into tropical swards; the Australian tropics ripen into a voluminous grassland studded with trees. Sorghums dominate the north, kangaroo grass (Themeda) the eastern steppes, and black spear grass (Heterogon) the coastal woodlands. Eucalypts are universal. On wetter sites, they share dominance with Melaleuca; in more arid lands, with Acacia. Where they can be shielded from fire, mangrove swamps cling like barnacles to the tidewater streams, enclaves of Callitris blossom, and patchy rainforests endure. But there is little protection from fire.⁵

The climate of the wet-dry tropics makes routine fire possible; its biology makes it inevitable. Soils are heavily laterized, and even by Australian standards, nutrient-drained. The monsoon and the biochemical cycles it brings to life give the nutrient flow a strongly seasonal dimension—captured into biomass during the Wet, released by fire during the Dry. The expansive grasses focus the action.

Those dominant grasses are large in biomass but small in nutritional content. Their quality, not their quantity, limits their harvesting by consumers. With the onset of the Wet, grasses spring to life and by the end of the growing season yield standing biomass on the average of 375–625 tons/hectare in central Queensland or 227 tons/hectare around Katherine in the Northern Territory, the difference between the Australian llanos and the Australian sahel. But after an early flush, the protein content decays to an abyssmal 2.5–3.0 percent by the end of the season, and grasses typically transfer important nutrients, including organic nitrogen, to underground storage. Few fauna consume the dead stalks. Without removal, without recirculating the precious nutrients, future growth falls off rapidly. Seed regeneration falters, finer grasses supersede the coarse sorghums, and woody shrubs suppress the grasses altogether.

Thus the life cycle and the fire cycle of the tropical grasses converge with machined precision. Burning stimulates biomass production (5–10 percent over that of unburned sites) and enriches its crude protein content by a factor of four or five. Fire brings the biota to life. Whole food chains—from invertebrates to raptors—collect around a moving fire. Grazers rush to the green pick that pokes through the ashes soon afterward. Even termites preferentially invade trees on burned sites rather than members of the same species in rainforest. By the end of the Dry, fire has readied the savannas for new growth, and it has even burned lowlands that, at the height of the Wet, are flooded. Fresh rains act on cleared sites and mobilized nutrients to turn black to green.

Its grasses establish the fire regime. The wetter sites are burned annually; the drier, once every two or three years. The exceedingly low nutrient reservoir and high fire frequency affect everything in the system. Unlike Eucalyptus elsewhere, the tropical eucalypts do not thrive on this fire environment. They survive. They are stunted, marginal; they can barely capture sufficient nutrients to sustain themselves; they coexist in uneasy equilibrium between rainforest and savanna. Although fire rolled back the rainforest sufficiently for eucalypts to transgress into the region, it now promotes the tropical grasslands with a fire frequency that has left the eucalypts living on the margin. While the eucalypts and paperbarks can survive the fires, they cannot compete as aggressively for the liberated nutrients, for annual fires constantly short-circuit the cycles that the trees demand. Even minor changes in fire frequency and the scale of burning can shift the balance of power. Biennial firing leaves the mosaic of woods and grasses stable; annual firing pushes it toward the grasses. The biota is poised on a knife-edge, sensitive to any variation in fire frequency.

This sensitivity makes it difficult to reconstruct the fire history of Old Australia. In the interior, routine fire is impossible because of limited fuels, which reflect uncertain rains. Outside that barren core, however, fuels are generally ample, and the patterns of fire history reflect the patterns of ignition. In Old Australia the early storms of the Wet brought lightning, forking like a lizard’s tongue. The old grasses—once tall, now laid low by monsoon winds—readily kindled. The drier sites burned first. A texture of burns resulted, a mosaic of black soot and yellow grass, as dry, unburned patches took fire. The Australian savanna—like those in tropical Africa and South America—formed a belt between rainforest and desert. As the monsoon transgressed and regressed, that savanna expanded and contracted, marched south and retreated north.

It is difficult to trace exactly these dimensions because, during the Pleistocene, a new ignition source appeared, the Aborigine. From that moment onward human uses, not natural sources, dictated fire frequency. The fire regime of the wet-dry tropics dates from that event as firesticks imposed new fire frequencies and timings onto different portions of the biota. Rainforest retired to special enclaves, more or less deliberately spared from fire. The anomalies that presently exist—such as the status of the eucalypts—date from changes in human fire use that accompanied European settlement. Thus, in this exfoliating geography of Australian fire, the increase in fire from the center outward reflects also an increase in human ignition. Humans sought out fire, added to it, and through it reshaped Old Australia.

THE SOUTHWEST ENCLAVE

In the southwest the pattern of seasonal wet and dry became Mediterranean, a cycle of winter rains followed by a long, thirsty summer. Its biotas are complex, syncretic; they form an easy enclave, the product of a double isolation—the first as Australia departed Gondwana, and the second as encroaching seas and later deserts divided the emerging Australian scleroforest into east and west. Eucalyptus rules the forest; scleromorphs fluff the understory. Toward the interior, woody savanna grades into spinifex, the mulga mélange, and unburned desert. On some sandplains heaths flourish, and on many arid ridges, mallee. Endemism is extraordinarily high, and fire is everywhere.⁶

But southwest fire, like its landscape, is syncretic, muted, balanced. No single fuel drives the regime; no single ignition source, no single wind, no single topographic feature, no singular climatic phenomenon imposes a domineering pattern onto the southwest ensemble. Instead a suite of biotas balances a suite of stresses, such that fire is one force among several, so routine as to be unexceptional. Fire is endemic, not demonic. It shapes and stirs the biotas rather than overturns them. If it is inexpungable, it is also less inclined to be catastrophic. It subjects the southwest to a low-intensity simmering, not a violent boiling; while there are seasons for fire, they are broad and accommodating. While there are few sites notorious for their fires, neither are there many refugia spared fire.⁷

The major terrain feature is a corrugated plateau, shallow yet sufficiently elevated to capture moisture enough to support scleroforest. The dry scleroforest is renowned for jarrah (E. marginata), a tree of extraordinary tenacity, almost impervious to fire. Its counterpart in the wet scleroforest is karri (E. diversicolor), a towering gum that resists fire and regenerates vigorously after even intense burns. Existing species tend to be self-perpetuating, immediately reclaiming their sites rather than emerging into dominance after a period of decades. The scleroforest suites appear secure, a stable compound of rain and fire; there is enough fire to ward off any resurgent rainforest, yet not so much (or so vicious) fire as to degrade scleroforest into grass or scrub. The regimes flourish amid abundant fire, yet can survive, within limits, in its absence.

Overall, the southwest represents an enclave of fire, a province small and a fire regime mild by Australian standards. Big fires do occur, of course; normally they accompany frontal passages as winds accelerate and shift, and as atmospheric instability encourages strong convection above burns. But storms approach from the Indian Ocean, with winds moderated by seas, not desiccated by deserts. Inland, on the lee side of the Darling Range, precipitation declines and desert winds stir. Here there is wind to drive fire but too little fuel to flame. Conversely, where ample fuel exists, winds are much less violent; only 5 percent of fires in the southwest, for example, have recorded winds that met or exceeded a strength of six on the Beaufort scale. (In Victoria and Tasmania, the percentage is 25; in South Australia, 30; in New South Wales, 35.) Moreover, during the summer fire season, high-pressure cells tend to be less intense. In the southwest enclave bushfires burn under more diverse conditions, few of which favor conflagrations.

There are two exceptions. One scenario requires that a stationary High draft desert air over the region, a kind of muted chinook splashing dry winds across the Darlings. The other involves infrequent hurricanes that churn out of the Indian Ocean. Monster storms not only hurl trees to the earth, piling abnormal fuels; they can, in the right circumstances, suck huge desert air masses into their vortex, like a black hole capturing streamers of stars from a passing galaxy. The fuel loads and fire winds of Cyclone Alby (1978) testified vividly to what, in Old Australia, must have been a rare but inevitable event.⁸

If so it was one the southwest enclave accepted and contained, even as it absorbed the cavalcade of Aboriginal firesticks that imprinted routine fire ever more firmly on the landscape. It is likely that the latter held the former in check. In recent years, lightning has accounted for approximately 12 percent of all fires in Western Australia, roughly equivalent to the frequency of lightning fire in the United States. In Old Australia the mountains would have experienced more starts; the open grasslands, the larger fires. All this only readied the southwest, however, for the profound reformation kindled by Aboriginal firesticks. Not every locale experienced those new fires with equal intensity, but where they burned more fires meant reduced fuels, which meant a smaller domain for natural burning. Anthropogenic fire branded the landscape with a seasonal regularity no less pronounced, and far more pervasive, than the meteorological minuet of lightning and cyclone. The firestick initiated the fissioning of Old Australia into New.⁹

FIRE FLUME: THE SOUTHEAST SUITE

Three points—Eyre Peninsula, Botany Bay, Port Phillip Bay—inscribe the great fire triangle of Australia. Here the centrifugal pump discharges all it has gathered up, as though every attribute that propels fire throughout the continent were hurled across this region, a running stream of fire. Everything within this colossal fire flume must accommodate big fires—not merely large in area or frequent in recurrence, but high-intensity holocaust, a fire possessed, vicious, unquenchable, final.¹⁰

Its geography prescribes an ideal formula for conflagrations. A Mediterranean climate reigns over a vast area, one of varied and elevated terrain that enriches the seasonal rains. Soils are newer overall, precipitation fuller, fuel loads abundant, ignition almost constant. Migrating storm tracks draft volumes of desiccating desert winds from the north, the desert interior, then shift abruptly to cold blasts from the south that drive fire flanks before them like a flaming blizzard. Those winds rush over the most rugged terrain in Australia, mountains that act less as baffles than as wings to accelerate and channel the most violent air on the continent. Over the course of the twentieth century, climatologists have noted that “potentially bad fire seasons” tend to occur in the southeast about once every three years, “bad fires” every six or seven years, and “very bad fires” every thirteen years. While these figures reflect ignition patterns (a largely human product), they do outline a rough proportion of fire severity relative to fire frequency.¹¹

That severity is something with which every component of the southeast suite must live. Its fuels are varied—interior savannas, scleroforests, heaths, mallee, buttongrass moor, and rainforest. Each biota modifies the coarse geography set by mountains, climate, and winds into distinctive regimes with their own frequency and typology of fires. There are fires of all sorts, a kind of background count of chronic combustion, but each landscape must in some way also survive the holocaust that rushes down the fire flume, for that is the trying, the distinctively Australian fire.¹²

Fortunately, the Australian sirocco must interact with fuels readied by drought and kindled by a well-timed spark. Considered in this way, those varied regimes are out of sync. The ideal formula for grass fires calls for an exceptionally wet winter followed by an exceptionally dry summer—the one builds up fuel loads beyond what fauna can consume, the other cures and desiccates it. For scleroforest, however, a dry summer may not be sufficient. The larger fuels and deeper taproots drain only after true drought, or following several seasons of cumulative water deficits. Thus, the circumstances that favor grass-driven fires do not promote equally vigorous fires in scleroforest fuels, while, conversely, the prolonged drought necessary to prepare scleroforest strangles the heavy growth necessary to propel grassy fires. In any year large fires affect either the lowland savannas or the mountain forests but not both simultaneously. The intermediate scleroforest varies between those extremes in timing, fire frequency, and fire intensity, its fuels meshing with its biology in rough, sometimes uncanny synchronization.

In dry scleroforest eucalypt litter sets the fuel loads (65–70 percent of the total), with minor contributions from herbs and grasses. Fires are frequent, light, syncopated with fuel buildup. Surface fuels can reach levels of 15 tons/hectare within ten years, 22 tons/hectare after twenty years, and 27 tons/hectare after thirty. But fires typically burn between three and twelve years, between the time at which fuels are adequate to support sustained combustion and the time beyond which it is unlikely that a site can escape ignition. For several years after a burn, a dry scleroforest lacks the fuel to reburn, and this pause allows the woody scleromorphs to maintain their position. More rapid burning would degrade the biota to pyrophytic grasses, if any are present, or choke the forest with coppicing eucalypts and scleromorphic scrub.¹³

By contrast, wet scleroforest cultivates an understory rich in shrubs, a higher overall productivity, and a greater proportion of available fuels. Where dry scleroforest encourages easy ignition and spread, however, wet scleroforest discourages it with a microclimate that inhibits drying and a dense canopy that shelters the surface from wind. It claims niches protected from desert winds, sites often swathed in maritime moisture. Within their understory are often found the flora of an embryonic rainforest. Short-rhythm fires are unlikely, yet long-rhythm fires essential. When they come they burn with catastrophic fury, the fuels thick and kiln-dried by intemperate drought; fire razes whole forests, from standing eucalypts to lush understory. In what has been described as a near miracle of timing, fire even rejuvenates the mountain ash (E. regnans).¹⁴

When surrounded by parched litter and shrubs, mountain ash not only burns vigorously but carries fire briskly to the canopy and hurls smoldering bark far in advance of the flaming front. Unusually thin-barked, lacking a lignotuber, even a modest fire will kill the bole. Its canopy fire, however, scarifies and liberates seeds stored in the crown—as many as 14 million seeds per hectare by some estimates. The tough tissues around the seeds can just hold off, and in fact need, the explosive heat created by a torching canopy. The thick ash left by the surface burn creates an ideal seedbed. After a fierce fire at Noojee, Victoria, nearly 2.5 million seedlings per hectare carpeted the burned lands. Patiently, the wet scleroforest begins its rejuvenation, a renewal dependent on a holocaust fire every two or three centuries. The ages of the wet scleroforests chronicle the ages of their great fires.

Too many fires—the rapid, forced succession of a holocaust by repeated surface burns—and the site will degrade to scrub, perhaps to heath. Too few fires and the rainforest taxa latent in fire-sheltered niches will mature, strangle the scleroforest like parasitic vines, and restore a lost world of green Gondwana. The fire flume ensures that the reconquest is improbable. Only in sodden gulleys, on leeward ridges flanked by castellated rocks, in isolated mountain fire shadows can the relict rainforest persist. Elsewhere the fire flume sweeps it away in floods of flame.

SCLEROSCRUB: OLD AUSTRALIA IN MINIATURE

For nearly all the major Australian biotas there exists a type of heath. There are tropical heaths, alpine heaths, mallee heaths, temperate heaths, dry heaths, and wet heaths. The kwongan—the heath of the sandplain—claims sites intermediate between the mulga and the mallee. The wallum—the heath of the Queensland coast—is a diminutive wet scleroforest. Large swaths of heath dot the Kimberleys and Arnhemland, a distilled rainforest. Patches of heath on the Cape York Peninsula concentrate the flora of the tropical savanna. Throughout the southeast a kaleidoscope of heaths compresses the biomes of the scleroforest. Its heaths form a kind of scleroscrub, Old Australia in miniature.¹⁵

If their distribution distills the biogeography of Australian life, their history recapitulates its evolution. Heaths claim sites with wretchedly impauperate soils. Even slight alterations in the nutrient flux—a sudden surge, a steady loss—can destroy the delicate balances that sustain the heath flora. Next, they accommodate water stress. Too much or too little, waterlogged or desiccated—either way, the imbalance can lead to scleroscrub, a wet heath in one case, a dry heath in the other. But these conditions only intensify circumstances to which Old Australia had long adapted in its transformation from rainforest to scleroforest. The final stress was fire.

As fire regimes, heaths resemble the dynamics of those biotas that surround them and of which they are distillations—they concentrate those properties. Their propensity for burning is extraordinary. Leaves are rich in oils, low in moisture, lean in mineral content—particularly in phosphorus—all of which makes for brilliant, flaming combustion. Fuel loads build rapidly. For three to five years surface fuels increase quickly, then taper off while shrubs proliferate, their growth rates peaking between five and ten years but never really ceasing. Biomass can reach 10 tons/hectare within five years. In Banksia-rich heath at Jervis Bay, fuels measured 50 tons/hectare after fourteen years. Virtually all the new biomass is available as fuel. On average, fuel loads in mature heath range between 20 and 35 tons/hectare, values comparable to wet scleroforest. Its open nature, its chronic aridity, its sweeping winds—all make heath fuels ideally available for burning. A heath requires only a few years of recovery from one fire to prepare for another.

Not only fuels but ignition dictates the frequency of fire and the relative proportion of high- to low-intensity burns. Most heaths reflect the fire frequency of their adjacent regimes, and, claiming less than 1 percent of Australia’s land surface, many fires necessarily enter heath from the outside. They accept such fires readily and with tremendous resilience. They accommodate a range of fires, and while those fires influence the internal dynamics of the heath, they do not alter its status relative to surrounding biotas. Fire neither perpetuates nor destroys: nothing else can claim the site. With or without fire, the heath will persist.

If not obligatory, fire is nevertheless frequent, welcome, useful, and inescapable. It modulates the scleroscrub, miniaturizing and intensifying the impacts it makes on scleroforest. Overall, heath fires are conservative; they can recycle and release precious nutrients, seeds, and access to water, but they cannot by themselves augment or diminish the total quantity of those minerals or waters present. Still, there is much to influence, for the floristic richness of heath is legendary. Australian heaths admit about 3,700 typical species, embracing representatives from nearly every prominent Australian family including many of the more flamboyant floras—the grass trees (Xanthorrhea), the ground orchids, the nectar-rich Banksia, the stilt plants Dryandra and Hakea. The kwongan contains 70 percent of the species found in the southwest enclave. This variety is fundamental. Not a dominant species, but their scrubby growth habit is what defines heathland.

There is, accordingly, plenty of flex in the system by which to accommodate fires. Because of their fuel history, even high-intensity fires can occur at almost any time, and floristic composition reflects shifts in fire frequency and seasonal timing. More-frequent fire favors those flora that propagate by sprouting; less-frequent fire, those that reproduce by seed or that aspire to control the canopy. Too frequent fire can remove some flora by not allowing them to mature into seed-producing states. Too occasional fire pushes other flora into senescence, beyond their ability to regenerate from lignotubers or beyond the capacity of their seed to remain viable. Fire early in the summer selects some species; fire in the autumn, others. Fires twist the heath like a kind of biotic kaleidoscope, the pieces always there but constantly reorganized into new patterns. The heath, in turn, testifies to the universality, ease, and subtlety of fire in Old Australia.

RELICT RAINFOREST

That leaves the rainforest.

It endured—secure from swelling drought and blasts of wind from the desert core, preserved in sheltered grottos around the watered seaboard. Like heath it was multiple, a distillation of many biotas. Unlike scleroforest, it was a relict. It claimed only a fraction of the land surface it once held and which it was yet capable of reclaiming under existing conditions of soils and climate—except, that is, for fire. Bushfire surged against rainforest, etching its geographical and historical boundaries. Insinuating itself into a mixed flora, fire selected for and shaped scleroforest and scleroscrub. It crowded rainforest to the margin of the island continent until it resembled a chain of coral reefs, a biotic atoll subsiding in a sea of scleromorphs. Fire seized the Red Centre.

There were many rainforests. Tropical rainforest revolved around the araucarias, supported by a rich understory of vines. Temperate rainforests favored Nothofagus and an understory blanket of ferns and mosses. By stratifying into gallery forests these Gondwanic relicts packed an enormous number of species into small domains. In all their variants, they showed tolerance to degraded soils and often to the peculiarities of eccentric sites. Their tenacity derived from a stability of climate made possible by reliable rainfall, by a precipitation regime that was not simply high but consistent across the seasons; a biotic stability that left true rainforest almost empty of scleromorphs; a stability of nondisturbance, the relative freedom of a site from routine disruptions, or if disturbed, by the capability to repair itself without surrendering to another biota. Together they spared rainforest from fire.¹⁶

Australian rainforest shows some tolerance for aridity—it has to—but it does not accommodate prolonged drought or seasonal dryness. Its layered canopies and dense understories work avidly to conserve moisture and preserve a fetid microclimate; dry spells have to work against this powerful moisture gradient. Likewise, few Australian rainforest types exclude completely scleromorphic species. The tropical rainforest is the most successful, being virtually free of scleroforest elements—no pyrophytic grasses; no scleromorphic scrubs; no eucalypts, casuarinas, or melaleucas. The temperate rainforest shows some intermixing, and merges uneasily into wet scleroforest. Thus the border between rainforest and its competitors—savanna or scleroforest—is, in the tropics, sharply etched and, in more temperate climates, blurred. Inevitably, too, there are disturbances. Windstorms, for example, can level tracts of rainforest far in excess of what organic agents can decompose, a shifting cultivation of rainforest by the slashing of cyclones and the burning of lightning fires.

Its fire history sums up the rainforest’s various stabilities. In tropical rainforest leaves make up probably 80 percent of the surface fuels and biological agents quickly decompose them. What persists is sparse, perennially wet, and sheltered from dry winds. When fires reach the rainforest border they flash against a green wall, gasp helplessly for fuel, and expire. If rainforest advances or retreats, it does so as a unit in response to broad shifts in climate. Changes in the dry season—greater moisture, fewer fires—allow the rainforest to advance into scleroforest and savanna. The reverse allows fire to eat a little farther into the frontier. Blowdowns along the border open the canopy, dry out surface fuels, and allow fires that normally flame out along the perimeter to enter into the interior according to a scenario that may become irreversible. The burn alters microclimates; fireweeds and pyrophytes invade the site; the new fuels carry additional fires over the burn and incinerate those slow-growing rainforest flora that demand shade and moisture.

In temperate regions, succession into and out of rainforest is less sharply defined. The borders blur; rainforest intercollates with wet scleroforest. Undisturbed, wet scleroforest will pass, over the course of several centuries, into rainforest. Disturbed—burned—it reverts to scleroforest or scleroscrub. What actually exists on any particular site reflects the unique sequencing of its past fires. Small sites are especially susceptible to irreversible change. Once initiated, a rhythm of frequent fire encourages soil erosion and the loss of nutrient capital. This encourages scleromorphs, which accept more fire. In such a scenario wet scleroforest degenerates into dry scleroforest, scleroscrub, or savanna, and rainforest no longer possesses the means to reclaim the site. Without refugia, rainforest vanishes.¹⁷

If its heath miniaturizes Australia’s fire geography, its rainforest miniaturizes its fire history. The uniformity of the Gondwana rainforest fractured under the impress of isolation, aridity, and disturbances. Increasingly, the principal disturbance and the great integrator of new conditions—fire—shaped those new regimes. The fragmentation continued, like rocks spalling off a heated cliff. From it came the unique species and unexampled biotas of Old Australia. With the relict rainforest, the story comes full circle. The flames licking at the wet flanks of tropical vine forest or overturning wet scleroforest on a rhythm of centuries are recapitulating a cycle of fire begun with the Great Upheaval.