Burning Bush. Stephen J. Pyne
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