Burning Bush: A Fire History of Australiaby Stephen J. Pyne
Pyne traces the impact of fire in Australia, showing that it has been a powerful environmental determinant, shaping both social and natural histories
Pyne traces the impact of fire in Australia, showing that it has been a powerful environmental determinant, shaping both social and natural histories
Read an Excerpt
A Fire History of Australia
By Stephen J. Pyne
Henry Holt and CompanyCopyright © 1991 Stephen J. Pyne
All rights reserved.
The Universal Australian
... round the bases of the bark Were left the tracks of flying forest-fires, As you may see them on the lower bole Of 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.
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.
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.
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.
"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.
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 and mycorrhizae, that evidently improve phosphorus uptake. The scavenging eucalypt can grow where other trees starve.
Getting scarce nutrients is only half the equation. Once absorbed, eucalypts carefully, obsessively retain and recycle them. Seedlings develop lignotubers — enlarged storage organs in the roots. Here nutrients can be collected and stashed until needed. If the shoot is killed, new shoots promptly emerge. Some eucalypts retain their lignotuber into adulthood, and some can send out from it multiple stems. A lignotuber ensures that, when conditions are right for growth, the seedling will have adequate reserves of the nutrients it needs. Likewise, eucalypts store nutrients selectively within their bole. A nutrient gradient exists between inner heartwood and outer sapwood such that phosphorus, in particular, is cached where it will be most useful. If branches are destroyed, new sprouts shoot out from beneath the bark, and the nutrient reserves in the sapwood ensure that this process will be rapid. Thus not only the roots but also the crown are buffered against erratic and ephemeral changes. The effective nutrient reserve shifts from the soil alone to the tree itself and the immediate environs under its biological control. Eucalypts can thus acquire nutrients far in excess of their immediate needs, and they can cache that surplus for years, perhaps as long as a decade. When young, eucalypts prefer mechanisms of internal cycling; when more mature, cycling between the soil and the tree.
Recycling occurs as well in the crown. A eucalypt canopy is dynamic: old branches become senescent and die back, while new branches immediately spring forth from epicormic sprouts lodged just under the protective bark. The crown is thus continually reshaped for maximum efficiency, and nutrients are reabsorbed before the branch is vulnerable to breakage and loss. As an evergreen, the eucalypt retains its leaves, shedding them as infrequently as possible, tenaciously hoarding their precious supply of nutrients. Instead, eucalypts shed their impoverished bark. When leaves do fall, they are drained of vital nutrients to the fullest extent possible before deposition. And once on the ground, leachates from the crown quickly return residual nutrients to the tree through the soil.
These adaptations served Eucalyptus well during the Great Upheaval. The particular mechanisms it favored for the foraging and cycling of nutrients did double duty for water. But there were greater variances in coping with aridity; the range of responses to water stress among eucalypts exceeded their range of responses to soil degradation. In fact, Eucalyptus is not a true drought evader. Eucalypts do not close their leaf stomata, go into seasonal hibernation, or shed their leaves. Instead they tolerate drought. They search out new water sources, hoard existing reserves, shut down nonessential processes. When drought comes, they tough it out.
Like all the scleromorphs, eucalypts have hardened leaves that reduce moisture loss. (The same is true for the operculum, from which derives the name Eucalyptus — from the Greek eu, meaning "well," and kalyptos, "covered.") Their canopy drapes downward, evading excessive leaf temperatures. Their vast, plunging root systems; their lignotubers; the capacity of seedlings to reside in apparent dormancy within lignotubers for years, even decades; their ability to shrink their leaf stomata to reduce transpiration and conserve water — all ensure the survival of the eucalypt within a land that is seasonally dry or episodically blasted by drought. But eucalypts have a harder time conserving water than nutrients. Their physical geography is thus limited, in some regions, by cold and in others by water. Where aridity becomes chronic and pronounced, eucalypts surrender to grasses, scleromorphic shrubs like saltbush, and that prolific rival, Acacia.
Its acquired traits were adequate to keep Eucalyptus alive during the eons of soil impoverishment, and they were enough, within the context of the Great Upheaval, to liberate eucalypts among the emergent scleroforest. The reformation in the physical environment meant a reformation in the biotic environment as well, and organisms had to accommodate to both circumstances. The eucalypts were supreme opportunists, infiltrating sites more and more frequently disturbed. As they broadened their domain, entire biotas had to reorganize around the defining properties of Eucalyptus. Eucalypts were too effective as scavengers and as hoarders of scarce nutrients and water to ignore. They were aggressive competitors — and a vital focus for grazing by insects, mammals, and birds. They concentrated bionutrients into particular forms; their hard gum nuts, for example, were accessible to some species and not to others. They created special niches and coevolved unique associations with koalas, termites, possums, and parrots; while eucalypts covered 25 percent of the surface of Australia, they harbored some 50 percent of its avifauna. The patterns of eucalypt forests defined the structure of critical habitats; the processes of eucalypt life determined the flow of nutrients and water.
If they wished to survive, other organisms had to seek out an accommodation with the Universal Australian. But the revolution did not end with the breakup of rainforest into scleroforest. The last 20,000 years — the epoch of the eucalypt revolution — have been marked by massive biotic realignments and extinctions. Each stress inspired others. Selective aridity encouraged fire, and fire fostered another suite of conditions, both abiotic and biotic. If they wished to survive, flora and fauna had to adapt not only to fire in the abstract but to the kinds of fire their scleromorphic neighbors supported. How their associates burned and reproduced determined in no small way the kind of fire they confronted. Amid fire the eucalypts flourished.
THE EUCALYPT AS PYROPHYTE
The spread of Eucalyptus traced the spread of fire. Charcoal and eucalypt pollen march side by side in the geologic record of the late Pleistocene and Holocene. Fire proliferated across the spectrum of Old Australian biotas — in scleroforest of course, but also in the grasslands, the acacia-splattered savannas, the heaths; it rolled back the rainforest into sharply bounded sanctuaries. The environments were varied, and so, not surprisingly, were the responses even among the prolific eucalypts.
Those inherited traits for contending with deteriorating soils and unreliable water preadapted the genus to survive fire. It knew how to cope with irregular nutrient fluxes, with an erratic tempo of too much and too little. Its quest for water already plunged roots safely out of the way of surface fires. Its weedy ancestry had groomed the eucalypt into an opportunist, ready to seize disturbed, opened sites. Eucalypts could capture nutrients released by fire, could store them until another release, could in emergencies live off internal caches in heartwood and lignotuber. Bark was thick, tough, and it shed as it burned like the ablation plate of a descending spacecraft. If branches were seared off, new ones could sprout from beneath the protected layer. If the bole burned, new trunks could spring from the buried lignotuber. A eucalypt could pour old nutrients into new growth, even as it scavenged liberated minerals from freshly burned ground. Fire could, for a couple of years, purge hostile microbes from the site; it might encourage better percolation of groundwater; it opened an area to sunlight, allowing the sun-worshiping eucalypt seedlings a chance to outgrow more shade-tolerant rivals. For most eucalypts, fire was not a destroyer but a liberator.
There were differences between fire and other pressures toward sclerophylly. Fire acted on a scale of minutes or hours, not over decades or millennia or eons. It was also interdependent with life in ways that leaching and drying were not. Soils degraded regardless of vegetative cover. Droughts arrived and departed whether there was anything on the surface or not; rocks or rainforest, it mattered little, for while organisms could alter the surface concentrations of minerals and water, while they could modulate the force of climate, they could not prevent rain or drought from appearing. But fire could only thrive in the presence of organic fuels. The character of those fuels profoundly influenced the character of the fires that resulted. And those fires, in turn, shaped the kind of biotas on which the fires fed. Fire and flora entered into a process of mutual selection, of positive reinforcement, that was far more rapid, intimate, and compelling than any of the relationships that preceded it.
Excerpted from Burning Bush by Stephen J. Pyne. Copyright © 1991 Stephen J. Pyne. Excerpted by permission of Henry Holt and Company.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
What People are saying about this
and post it to your social network
Most Helpful Customer Reviews
See all customer reviews >