LITMIR.BIZ

Скачать книгу

      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, not—until the complex triumph of scleroforest over rainforest—a driving force. Once established, however, it was difficult to extirpate. Fire created the conditions for more fire. So long as fire persisted, there could be no biological counterrevolution, no resurgent rainforest.⁹

      Fire forced, fire stressed, fire quickened. Fire’s dynamism made it, over the short term, the most powerful of the environmental determinants shaping Old Australia. Soils changed only over geologic eons; aridity was, likewise, a product of infinitesimal change—the migration of the Australian craton into the tropics, the reformation of climates, the restructuring of storm tracks. But fire was abrupt, vigorous. Fire responded to brief bouts of drought, as well as to prolonged aridity; to storms, lightning, and winds, not just climatic change; to rapid ecological successions, environmental selections, restructured habitats, and mobilized nutrients, not merely to ponderous evolutionary coadjustments. Compared with soil degradation and climatic reform, fire was more mobile, more sensitive, more varied and malleable, more compelling. The dynamism of fire was inextricably bound to the dynamism of life.

      In the drive toward sclerophylly, fire had a paradoxical role. Often it complemented aridity. It pushed biotas to greater sclerophylly as quickly as they could, within their genetic reserves, tolerate the move. Equally, fire released precious nutrients otherwise stockpiled in dead wood or cached in forms inaccessible to biological agents. While the overall nutrient level of average soils might be degraded, fire kept the existing stock in active circulation; it made nutrient caches into rapid nutrient cycles. It also recycled organisms and whole communities. It favored those plants that were already disposed to survive as scleromorphs and it burned maladapted competitors into oblivion or herded them into fire-safe enclaves. What had existed as generic adaptations to sclerophylly now often acquired more fire-specific signatures.

      Among the scleromorphs fire constantly fine-tuned composition and dynamics, the balance between those organisms that needed more light and those that needed less, between those that reproduced by seed on mineral soil and those that propagated by vegetative sprouting or suckering, between those that needed access to surface water and those that reached deep into the water table. In the face of pressures toward geologic uniformity, fire helped inspire a biotic diversity. The many niches that had existed in the ancestral rainforest because of long stability now had, within the scleroforest, dynamic analogues, niches made possible because of frequent disturbances by fire.

      A remarkable reciprocity developed between the scleroforest and fire. If fire helped differentiate the biota, so also that biota helped particularize fire. Different communities revealed different patterns of fire starts, spread, frequency, timing, and intensity. If rainforest ecosystems could be differentiated largely on the basis of precipitation regimes, then scleroforest ecosystems could be aptly characterized by fire regimes. Fire interacted with the uniquely Australian biota in spectacular, sometimes special ways. Fire created circumstances that promoted the spread of the scleromorphs, and the scleromorphs reciprocated by promoting the spread of fire.

      Australian fire acted on and redirected those trends toward sclerophylly that preceded it. Those preadaptations gave fire an entree into Old Australia that it exploited with brilliant effect. Fire swelled into

Скачать книгу