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