Burning Bush. Stephen J. Pyne

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      The Universal Australian

      … round the bases of the barkWere left the tracks of flying forest-fires, As you may see them on the lower boleOf every elder of the native woods.

      —HENRY KENDALL, “A Death in the Bush”

      The extreme uniformity of the vegetation is the most remarkable feature in the landscape of the greater part of New South Wales … In the whole country I scarcely saw a place without the marks of a fire; whether these had been more or less recent—whether the stumps were more or less black, was the greatest change which varied the uniformity, so wearisome to the traveller’s eye.

      —CHARLES DARWIN, The Voyage of the Beagle (1845)

      IT IS NOT CLEAR just when the first eucalypt emerged out of the welter of ancient rainforest taxa. The earliest definite pollen appears in the Oligocene, around 34 million years ago, long after Australia had separated from the bulk of Gondwana. Nor is it obvious whether the genus developed from a single protoeucalypt or from several related forms.¹

      What is incontestable is the degree to which the genus Eucalyptus is endemic to Australia, the extent to which, by Holocene times, it came to dominate the forest and woodland environments of Australia, and the peculiar attributes to which it owes its evolutionary triumph. Its successful coup within the scleroforest, in particular, came from a powerful set of alliances, a triumvirate that Eucalyptus formed with fire and the genus Homo. Found virtually nowhere outside Australia, but within Australia found nearly everywhere, the eucalypt became the Universal Australian.

       THE EUCALYPT AS COLONIZER

      Amid the Great Upheaval, the family Myrtaceae—flowering trees and shrubs with fleshy or dry fruits—emerged as one of the scleromorphic winners. Although it probably originated in Australasia, Myrtaceae saturated all of Gondwana, a minor element in the ancestral rainforest. When Gondwana divided, so did Myrtaceae. Its fleshy-fruited genera concentrated in South (and Central) America, and its dry-fruited genera in the eastern cratons including Greater Australia. In Australasia the family Myrtaceae featured ninety-five genera, ninety-three of which were endemic. Australia contained sixty-nine genera, of which forty-five were endemic, among them Leptospermum, Melaleuca, Callistemon, Baeckea, Verticordia, and Eucalyptus. By the time Eucalyptus appeared in the fossil record, Myrtaceae had experienced perhaps 30 million years of evolutionary history.²

      The Tertiary upheaval completely reformed the status of Eucalyptus. Its genetic inheritance included as a matter of course generalized Myrtaceaen traits and scleromorphic tendencies. Probably it appeared along the margins of rainforest, a weed searching out disturbed sites at least momentarily free of an obscuring canopy. Interbreeding was common; hybridization, frequent. As the Australian ark floated into the Pacific and experienced upheaval, a genus that thrived amid disturbance found itself on an increasingly disturbed continent. Quickly Eucalyptus began to diversify, to radiate into the new niches that blinked from a disintegrating rainforest, and to reshape those environments in its own image. Southeastern and southwestern Australia divided into biotic subcontinents, segregated first by intervening seas, then by different soils, and finally by endemic biotas. As the Australian plate threw up an arc of mountains to the north, a few eucalypts crossed the Torres Strait and found a marginal existence in drier, unsettled sites of New Guinea and beyond. The remaining genera discovered plenty of opportunity within Australia, first as scleroforest replaced rainforest and then as the proliferating eucalypts seized dominance over the scleroforest.³

      The scleroforest revolution concluded between 38,000 and 26,000 years ago as the scleromorphs, led by Casuarina, completed their abrupt, all but catastrophic, expulsion of the araucarias. But almost as suddenly, between 20,000 and 7,500 years ago, Eucalyptus did the same to Casuarina. By the time of European discovery forests and woodlands comprised about 25 percent of the Australian land surface; perhaps 70 percent of those lands could be classified as pure eucalypt forest. Eucalypts claimed about 16 percent of the tropical eucalypt and paperbark biomes, and an estimated 11 percent of the cypress pine biome. Across Old Australia eucalypts comprised some 95 percent of the constituent tree species. They thrived almost everywhere—at the snow line of the Australian Alps, along the saltwater tide of tropical mangroves, along desert watercourses, on monadnocks; in relatively wet climates and in relatively dry, on impoverished sites and on more enriched; in Mediterranean climates, in true deserts, in wet-dry tropics, along the margins of rainforest and interpenetrating grasslands. They were absent only in the true, the relict rainforest. With minor exceptions, Eucalyptus dominated Australian forests. Every other organism had to accommodate that fact.⁴

      “The remarkable plurality of the Eucalypts,” as Ferdinand von Müller called it—what staggered Charles Darwin as the “never-failing Eucalyptus family”—prevailed over the Australian continent to an extent unrivaled by any other genus on any other continent. Eucalyptus had exploded so widely that it is considered by some authorities as less a genus than an alliance composed of three suballiances, ten subgenera, and over six hundred species. The plasticity of the genus is extraordinary. Hybrids are common within subgenera, juvenile habits persist into adulthood, and even phantom species (apparently hybrid populations that now exist in the vicinity of only one parent) have been identified.

      The eucalypt conveyed to Australia a special character. Marcus Clarke gave it poetic expression as “Weird Melancholy.” Here, where “flourishes a vegetation long dead in other lands,” is found the “Grotesque, the Weird, the strange scribblings of nature learning how to write,” a “phantasmagoria of that wild dreamland termed the Bush.” Others described or cursed it in more prosaic language, but no one could deny that Australia was different and that the eucalypt was to a large extent both cause and symbol of that difference. But if the eucalypt animated the bush, fire animated the eucalypt. The abrupt, smothering rise in Eucalyptus pollen that accompanied the scleroforest revolution paralleled an equally sudden increase in charcoal.⁵

       THE EUCALYPT AS SCLEROMORPH

      Eucalyptus was first a scleromorph and then a pyrophyte. Of the three suballiances that comprise the genus, Monocalyptus shows the greatest adaptation to impoverished soils but displays limited tolerance for drought or hostile soil microorganisms. By contrast, Symphyomyrtus avoids the worst soils, but shows considerable tolerance toward drought and microbes. Corymbia falls somewhere in between, and was probably intermediate in the evolution of the alliance. But degraded soils were something to which most members of the Gondwanic rainforest had to adapt. Eucalyptus, however, elevated nutrient scavenging and hoarding to an art form.

      The eucalypts typically developed extensive, deep roots, capable of foraging widely. Rather than target particular nutrient niches, rather than hone their search with exquisite refinement, the eucalypts processed soil catchments in volume, partially compensating for the relative poverty of soil at a restricted site. In addition, eucalypts evolved chemical and biological aids to improve access to those nutrient reservoirs, particularly phosphorus, that did exist. Through various biochemical mechanisms, probably involving phosphataze enzymes or organic exudates, eucalypts could extract phosphorus from iron and aluminum compounds. Similarly, it appears that leachates from leaves and litter of some eucalypt species can percolate into the soil and mobilize phosphorus compounds that are otherwise inaccessible. And then there are the biological allies of Eucalyptus, soil microbes СКАЧАТЬ