Burning Bush. Stephen J. Pyne

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СКАЧАТЬ on grasslands. It is likely that humans entered Old Australia when the Sahul was a grassy plain that not only defined a physical corridor by which to travel but gave entree into the critical dynamics of the emerging Australian ecosystems—a biotic corridor, a fuse for anthropogenic fire. Fire created conditions attractive to humans, and humans reciprocated by introducing more fire.

       CONCENTRATED FIRE: AUSTRALIA’S MEDITERRANEAN LANDS

      Its Mediterranean landscapes are Australia’s other special environment. Like its savannas, they are local expressions of a global feature. They come and go with climatic oscillations. They are recent, dating from the Pleistocene. And they are at least partially anthropogenic, their histories inextricably tied to humans and their biological allies. Unlike the savanna, Mediterranean lands are not corridors but collecting basins. They are less a frontier, advancing and receding, than a crossroads where many biotas converge, claim distinctive niches, and settle into a complex caste system of subbiotas. If savannas are fuses, the Mediterranean landscapes are the explosives to which they lead. Australia’s Mediterranean and peri-Mediterranean regions contain its most heavily stocked biotas, the densest populations of humans, the fullest congregation of fire practices, and the most violent fires.

      By area less than 5 percent of the Earth’s land surface, the Mediterranean-climate lands fall into five terrains—the Mediterranean Basin proper, South Africa, northern Chile, Southern California, and the southern perimeter of continental Australia. Each terrain exhibits mild wet winters and long dry summers. Their common geographies include a littoral, a seasonal pattern of cold offshore currents, mountains, and recurring disturbances. Interestingly, four of the terrains border fragments of Gondwana. Mediterranean California (ever the exception) overlies a sliver of the Pacific plate that is only coincidentally, ephemerally attached to the North American plate. Their common Gondwanic heritage helps to account for the apparent dominance of evergreen scleromorphs that trace their evolutionary origins to tropical or subtropical floras; they mingle the moist with the arid, the tropical with the temperate. Their complex physical geography makes possible, in turn, a complex geography of life, the mosaic of microclimates sustaining a mosaic of microbiotas.⁷

      Mediterranean ecosystems are chocked with new and relict species—a living palimpsest stuffed with biotic glosses and interlinear subtexts. The border between floral types is abrupt. A change in climate, land use, or fire patterns can deflect the system into any of several subbiotas. At their core are shrubby scleromorphs—chamise and chaparral in California; maquis, garrigue, macchia, and phrygana in the Mediterranean; renosterveldt and fynbos in South Africa; matorral in Chile; mallee in Australia. Each label subsumes a host of species. If precipitation increases, scrub matures into mountain forests of evergreen scleromorphs or, if they front Laurasian lands, of drought-toughened conifers. If precipitation decreases, scrub surrenders to a subdesert of low, dispersed shrubs. If soil fertility improves, grasses thrive. If fertility degenerates, heath flourishes. Each terrain pieces together a mosaic that balances, on small sites, these competing pressures.

      All share, too, a history of human manipulation. Humans are intimately implicated in the maintenance, if not the origins, of the Mediterranean lands, all of which emerged during the Pleistocene under the combined impact of climate, biotic revolutions, and humans. Mediterranean soils are either impauperate or prone to degradation; their constituent species are survivors—tough, weedy, thriving on disturbance, perfect associates for humans; and microclimates are ideal for fire. Burning can occur over a long dry season, droughts blanch the land frequently, and while large burns are possible, the rugged terrain compartmentalizes fire, giving it a specificity not unlike the fuels upon which it fed. Such an environment humans found congenial—as they cut, grazed, and fired and so altered biotic composition, induced soil erosion, and upset water regimes.

      Anthropogenic fire was everywhere present and everywhere absorbed. Yet, while formative, it was not always mandatory. In Chile, for example, lightning fire is infrequent (less than 2 percent of all starts), and it appears that the scleromorphs took over the landscape without fire driving them into ascendancy. But because these environments were so susceptible to fire, they were especially susceptible to human manipulation, and once colonized, humans indelibly branded them with anthropogenic fire, typically associated with other practices such as hunting, grazing, clearing, farming, and war. So ubiquitous has anthropogenic fire become that it is difficult to reconstruct what a fire regime without human influence might look like.⁸

      There were of course differences among the Mediterranean environments. Chile lacks the violent winds that power large fires in the cognate lands—the bora, mistral, and sirocco that sweep the Mediterranean Basin; the Santa Ana and Mono of Southern California; the desert winds and southerly burster of southeastern Australia. But large or small, fires were in Chile, as they were in the other sites. The Mediterranean Basin was notorious as a tinderbox, constantly burned in association with human land practices. The west Mediterranean and islands like Corsica and Sardinia continue to be known, because of their fires, as the “red belt.” Southern California was fired until it resembled a burnt-out case. Veldt and fynbos fires greeted the first settlers of Cape Town. The Chilean landscape continued a pattern of burning that extended from the lowland llanos to the lofty altiplano. Australia was rimmed with smoke, seared with episodic conflagrations. Whatever the natural state, the condition of the Mediterranean terrains from the time of human colonization has been one of chronic fire.

      Australia’s Mediterranean terrains possess some distinctive features. They are more influenced by intermittent summer rains, less controlled by a weaker ocean current. Their soils are among the oldest and most degraded, constantly pushing the system toward heath and scrub. With fewer mountains, the terrains display fewer niches. Their shrub component—mallee—is high, between half and three-quarters of all the biomes assimilated. They have experienced a far more intense isolation because they have been severed from large continents that could restock the site with new species. Instead they have been exceptionally dependent on flora and fauna introduced by humans, often from other Mediterranean terrains. While the number of eucalypts included among the mallee is comparable to the total number of woody species found in cognate terrains, the mallee all derive from a single genus, and its genetic variability is inherently less than the others.

      As with its savannas, however, the decisive difference between Australia’s Mediterranean terrains and those elsewhere derive from their human history. In the southeast, particularly, fire practices congregated. The Aboriginal occupation of Australia subjected the Mediterranean terrain to elaborate foraging and burning, but without farming, land clearing, herding, and the massive introduction of exotic flora and fauna that grossly complicated and magnified the effects of anthropogenic fire elsewhere. That came with European colonization. Europeans, in fact, arrived already long experienced in the settlement of Mediterranean-type terrains, and with remarkable persistence they sought out one Mediterranean landscape after another.

      And with good reason. It was in the Mediterranean Basin that Western civilization honed its agricultural techniques, selected its preferred species of plants and animals, accumulated the experience that would make possible the colonization of the remaining Earth’s Mediterranean terrains, all prime sites for human habitation in part because of the ancient bonds developed in the model region. Thus Australia quickly established important ties with South Africa, Chile, and California, and through Britain with the Mediterranean Basin as a source of crops, livestock, irrigation expertise, and human immigrants. In return, Australia exported eucalypts.

      As with savannas, so also Mediterranean lands became points of entry for human colonizers. If the Aboriginal march through Sahulian savannas recapitulated the origins of Homo, European colonization into Australia’s Mediterranean terrains recapitulated the origins of Western civilization. In both instances, those biotas influenced Earth history far beyond what their minuscule land areas suggested was possible. It was no accident that both streams of colonists sought out exactly those Australian lands most amendable to anthropogenic fire. Landscapes that had known fire apart from humans now found themselves ruled, in good measure, by human fire.

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