Burning Bush. Stephen J. Pyne
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,