Burning Bush. Stephen J. Pyne

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СКАЧАТЬ 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 СКАЧАТЬ