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
