Burning Bush. Stephen J. Pyne

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СКАЧАТЬ are endemic, nearly 90 percent of all plant species; Victoria alone has a flora twice as great as that of Britain. But such figures fail to convey the utter, continental-scale domination of the landscape by the scleromorphs. The revolution was comprehensive. With the new flora came new fauna and new patterns of interaction between sclerophyllous plants and sclerophyllous animals. Birds and mammals, not insects, typically pollinated the flowering scleromorphs. Placental mammals and reptiles repeated the radiation of specialized plants. Two genera, Eucalyptus and Acacia—gums and wattles—virtually tyrannized every forest and woodland biota, excepting only the relict rainforest. Two genera of grasses, Triodia and Astrebla—the hummocks and the tussocks—similarly dominated the grasslands. Scleromorphs invaded and reshaped forests, woodlands, grasslands, deserts. They penetrated every ecological niche—the canopy, the understory, the surface. They claimed relatively dry sites and relatively wet and those areas that were, on an annual cycle, both wet and dry. The rainforest eroded away like the plateaus of ancient Gondwana. What began as a Gondwanic ark ended as an island continent, Old Australia, that only remotely resembled anyplace else.⁴

      The final expulsion of rainforest came relatively late. By the onset of the last glaciation (80,000 years ago) a rough balance still existed between forests consisting of scleromorphic angiosperms like the casuarinas and those composed of ancient gymnosperms like the araucarias. By 38,000 years ago, however, the araucarian forest had all but vanished. Aridity had decided the contest between rainforest and scleroforest, but as aridity settled in to an enduring presence, it became more complex and found new allies. During the final, near extinction of rainforest another biotic revolution broke out, this time within the scleroforest. Casuarinas receded, eucalypts advanced, and charcoal saturated the landscape. This second upheaval was decided by the renaissance of a new, vastly more complicated stress—fire.

      WHAT ESCALATED THE GREAT UPHEAVAL was not the simple fact of aridity, but its rate of growth, the frequency of its oscillations, the way it introduced routine disturbances. Wet periods gave way to dry, and dry returned to wet, like a two-cycle engine. With the onset of the Quaternary era the frequency of oscillations increased. A gradual change could have been met with gradual adaptations, but rapid, frequent flux encouraged organisms that could respond with equal vigor and speed, that thrived amid disturbance. It encouraged the tough, the opportunistic. It promoted the weeds among the Gondwana greenery. What began as a tendency stiffened into a trend as Australia began to burn.⁵

      There had been some fire in the past. Coal seams preserved, as pyrofusinite, the charcoal of Carboniferous- and Tertiary-era fires. Brown coals from the Yallourn-Morwell district of Victoria reveal ample evidence of burning, probably in the late Tertiary or early Pleistocene times. Where coal seams had been exposed as outcrops, they also ignited from surface fires. Burning Mountain in New South Wales, already smoldering when Europeans arrived, is a celebrated example. But smoldering coal and fiery basalt flows could not become a selective force of continental proportions. Lightning fire could—and did.⁶

      What is required is not lightning per se, but the interaction of lightning with appropriate fuels properly cured and dried. The scleromorphs and grasses offered ideal fuels, and a pattern of seasonal aridity and lightning storms stirred the right mixture of fire and water. The storms had to arrive when the vegetation was cured, massed, and dried. Too much rain dampened the fuels; too few storms reduced the probability of ignition; and too prolonged aridity not only dried but killed the vegetation and starved the fire of fuel. During the Great Upheaval, however, the proper, improbable combination of conditions appeared and persisted.

      Those circumstances are difficult to reconstruct in any detail. As scleromorphs emerged from the morass of rainforest taxa and as aridity evolved into seasonal or secular patterns, it is likely that fires appeared where they had not been present before, or became more active where they had gained footholds. For fire to be biologically effective, it need not occur annually, only at critical times within the life cycle of the prime species. For rainforest these cycles may involve decades, centuries, perhaps millennia. Elsewhere in what endures of Gondwana rainforest, there is evidence of fire. Thick lenses of charcoal of uncertain origin underly sites in Amazonia. The rainforest of East Kalimantan, Borneo, burns in long, relentless stringers from surface coal seams that act as a slow match, ready to kindle the surrounding terrain at times of severe drought. In the early phases of the Great Upheaval the fossil record suggests a pattern of swamp fires within a landscape of closed rainforest. The frontier between scleroforest and rainforest was almost certainly etched with fire.⁷

      Current statistics furnish some insight into the potential power of lightning fire in Old Australia. A 1961 lightning barrage in the Australian Alps impressed fire professionals that “there is no doubt that if all the fires … had spread unhindered by firefighters, they would have burnt over most of the Snowy Mountains area before winter rain put them out.” During the 1970s in Victoria lightning was responsible for 24 percent of all fire starts. In terms of area burned it accounted for 60 percent of the acreage. A single storm in 1972 ignited thirty-nine fires in rugged terrain. While most fires began in the eastern mountains, the largest fires raced through the more interior grasslands. Some 80 percent of fires in western Queensland, it is estimated, originate with lightning. About 20 percent of fires in southeastern Australia and perhaps 12 percent of fires in the forests of the southwest, plus potentially large fractions in the north and center begin, on the average, from lightning. Perhaps 60 percent of the fires in Victoria’s Big Desert originate with lightning, as do about 12 percent of starts in the national parks of Western Australia. The onset of the tropical monsoon is a time of storm and sun, even of dry lightning—ideal conditions for fire starting. In the Mitchell grasslands of the subtropical north, where anthropogenic fire is infrequent, lightning remains an important cause. The capacity of lightning to kindle fires in the desert interior is largely limited by fuels, a product of rains. Many of the worst fire complexes of recent decades include multiple starts from lightning—the 1951–52 fires in New South Wales, the 1961 fires in Western Australia, the 1977 fires of the southeast, and the gargantuan fires of the central deserts in 1974–75. Perhaps 97 percent of the area burned in 1974–75—about 15 percent of all Australia—is attributed to lightning. (Lightning starts in Tasmania, however, appear to be negligible.) Such statistics, however, are allusive rather than conclusive. Millennia of human intervention have so distorted natural fire regimes that it is difficult to assign reasonable values. That lightning is most prominent where humans are least present is no accident. Yet the numbers do testify to the power of lightning to kindle fire in nearly every environment, and that is enough.⁸

      Initially, fire reinforced the trend toward aridity. It is possible that fires dried out landscapes, further favoring scleromorphs and shaping microclimates that made future fires more likely: increases in fossil charcoal parallel increases in scleromorph pollen. Then, as its domain expanded and it established reciprocity with critical components of the biota, fire began to redirect the evolution of the Australian scene. Almost certainly fires are implicated in the emergence of sclerophylly, in the astonishing ascendancy of the scleromorphs from their obscurity within the ancestral rainforest, and in the rapidity of overall environmental change. It was no longer sufficient on the Australian ark to adapt to soil paupery and aridity; to thrive, organisms had to adapt also to a regimen of fire. Fire set to boil the whole biological billy that was Old Australia.

       INFORMING FIRE

      When it first appeared, fire was a minor phenomenon, and it supported minor elements of the biota. The rainforest thrived under a regimen of rain and stability. It adapted to soil degradation, tolerated minor disturbances, closely resembled its Gondwanic cognates. If fire infiltrated that environment, it did so marginally or episodically. With or without it the rainforest continued.

      The advent of aridity expanded enormously fire’s potential habitats. It made available new fuels and served new environments that mingled wet and dry, the rain that flushed the landscape with fuels and the spark that kindled them. Yet fire remained one process among many that rallied around aridity, that drove Gondwana greenery toward sclerophylly. It was a catalyst, an accelerant, СКАЧАТЬ