Burning Bush. Stephen J. Pyne

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СКАЧАТЬ be extraordinary. Between 1950 and 1974, for example, eucalypt plantations increased worldwide (including China) from 0.7 million to 3.7 million hectares—and continue to rise.

      It is probable, too, that its alliance with humans explains the ubiquity of eucalypts in Australia. They moved overseas because humans took them into compatible environments. They did not have to transcend endless oceans or navigate through rainforest, a biota they had abandoned during their evolutionary history: they could appear directly on suitable sites. In most cases, those sites could not offer worse conditions of soil and aridity than they had known in Australia. Native organisms were kept off balance, the site chronically manipulated, by human intervention. With a few exceptions, free-burning fires were less pervasive, and when they came, they inflicted less damage than on native trees.

      Its enlarged dominion has had a cost, however. The eucalypts’ interdependence with humans could not eliminate their more ancient interdependence with fire. They continued to behave as though fire were still a principal ally, persisted in littering fuels as though surface fires routinely passed over them, acted as though fire would remain instrumental in purging the biota of competitors, in restoring conditions for regeneration, in recycling scarce nutrients. Those environmental conditions that allowed emigrant eucalypts to prosper so gloriously also encouraged an excess of unchecked fuels. Interestingly, other fire-hardened scleromorphs from Australia like Casuarina and Melaleuca have also become major fireweeds in such exotic landscapes as south Florida. But these were weeds, unwanted escapees, not assisted emigrants.¹⁰

      The environments that encourage eucalypt plantations also encourage eucalypt-dominated fire regimes. Not only commercial plantations, but other environments receptive to eucalypts have witnessed a rise in fire hazard. The transported eucalypts shed fuel as though they expected to burn. The Berkeley fire of 1923, which consumed about a fourth of the city and entered the University of California campus, was propelled in part by windfall and litter from extensive eucalypt plantings. The scene had little improved when an Australian fire specialist visited the Bay Area in the 1960s. Familiar with the intensity of eucalypt fires in their native setting, he gasped at the specter that greeted him—the intermixture of houses and giant eucalypts, branches and bark piled deep, a surreal scleroforest composed equally of Eucalyptus and houses. Shaken, he abandoned the conference tour and retired to his motel room, his head spinning with visions of holocaust.¹¹

       THE EUCALYPT AS AUSTRALIAN

      The Australian bush owes its peculiarity, more than anything else, to Eucalyptus. No other continental forest or woodland is so dominated by a single genus. Other biomes on Earth have scleromorphs, most have grasses, and few are spared wholly from fire, but none has the combination that exists in Australia and has given the bush its indelible character. Eucalyptus is not only the Universal Australian, it is the ideal Australian—versatile, tough, sardonic, contrary, self-mocking, with a deceptive complexity amid the appearance of massive homogeneity; an occupier of disturbed environments; a fire creature.

      But the ideal Australian is also the typical Australian. Its peculiar strengths delineate as well its weaknesses. The hostile environmental conditions that pushed the biota toward sclerophylly, the chronic disturbances that at once simplified and complicated the biotic ensemble, the alliance with fire that allowed a single genus to overrun a continent—all these were enormous strengths so long as those informing circumstances remained more or less in force. While the domain of fire in Australia had expanded, it ruled within an evolved order. If, however, those pressures were removed, if new biotic elements were introduced or significant portions extirpated, if the fire regime were reconstituted by new fuels or new sources of ignition, then those special traits could become liabilities.¹²

      The bush was perhaps too dominated by Eucalyptus, and Eucalyptus perhaps too closely reliant on fire and, through fire, on Homo. The eucalypt was less a pyrophyte than a pyrophiliac: fire became a near addiction with its own peculiar perils. The tendency was to create more fire, as though the biotas linked by eucalypt and fire were a kind of chain letter, a leveraged biotic buyout sustained by ever-increasing infusions of fire. Any reform in the fire regime would upset not only the status of Eucalyptus but the entire bush. And in this complex biotic chemistry lies the colossal significance of the genus Homo, advancing on Pleistocene Australia with bold firesticks.

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      Unimaginable Freaks of Fire: Profile of a Pyrophyte

      … But enough! Where are the words to paint the million shapesAnd unimaginable freaks of Fire, When holding thus its monster carnivalIn the primeval Forest all night long?

      —CHARLES HARPUR, “The Bush Fire” (1853)

      … I had a full opportunity of examining this, one of the finest sights which tropical countries display … Above us the sky was gloomy and still; all round us the far-stretching forests exposed a strange and varied pageant of darkness and fire, accompanied by the crackling of flames and the crash of falling trees.

      —“AN EMIGRANT MECHANIC,” Settlers and Convicts (1849)

      WITH GATHERING SPEED, like a flaming maelstrom, Australia spun to its destiny as a fire continent. Proliferating fires pushed scleroforest and grassland across a biotic threshold. Bushfire became a reality with which almost—but not quite—every landscape of Old Australia had to contend. Since the Pleistocene it has generally been the case that, where biotas have changed, they have moved toward a state of more fire, not less. Some species were swept aside, while some accommodated, adapted, and learned to tolerate fire. Others thrived.

      The eucalypts flourished overall. Their evolutionary history, its peculiar genetic makeup, had conditioned Eucalyptus to exploit those unsettled times. Nutrient scavengers of ravenous dimensions, woody weeds ready to colonize disturbed sites, evergreens that could adopt many growth habits and that wrapped protective coverings around critical tissues so that they could thrive in strong heat and sunlight—the eucalypt alliance amalgamated hundreds of species which were ideally predisposed to survive in an environment of increasing fire. Their scleromorphic traits were even better preadapted to fire than to drought, and the rising tide of fire soon swept them before it. Generic adaptations evolved into more fire-specific traits.

      No other genus that had so far survived the voyage from Gondwana could compete with Eucalyptus for dominance within Australian forests. Yet there remained areas from which they were excluded: eucalypts shunned the frost-ridden subalpine terrain; on chronically dry sites eucalypts gave way to spinifex, mulga, and gibber desert; on perennially wet sites, eucalypts were crowded out by rainforest or were challenged by paperbark Melaleuca. But everywhere else—wherever fire was routinely possible, even over a span of centuries—eucalypts flourished and shaped whole communities of pyrophytes. The scleroforest it dominated bloomed when burned. Without fire its biophysical engines cooled, and its biotic dynamics decayed.

       FRIENDLY FIRE

      Most eucalypts can accommodate most fires. But they do so in ways both common and diverse.

      Their defenses begin with their bark. What kills is a kind of thermal ring-barking caused by a very high temperature or a long duration of lower heating. But bark is thick, it is densest at the base where the fire burns, and it conducts heat poorly. Surface fires pass by, charring the exterior but not killing the living cambium beneath it. While it is common for heat to concentrate preferentially on one side or the other, either because fuels pile up on the uphill side of a trunk or because winds form eddies on the lee side, at worst this wounds only one side and explains why most basal cavities СКАЧАТЬ