In none of these attributes was there anything to account for its extraordinary ubiquity or its supremacy within the scleroforest. What made the eucalypt special was its extraordinary opportunism, a relationship reinforced by fire. Eucalypts accepted wretched soils and tolerated drought, but they thrived amid fire.
A eucalypt forest became a fire forest. The alliance between Eucalyptus and fire compelled other organisms to respond likewise to fire—and not just to any and every fire, but to fires occurring at certain seasons and across a specified range of frequencies and intensities, a fire ensemble to a considerable extent dictated by the burning properties of the eucalypt and its scleromorphic associates. No organism could survive in Quaternary Australia because of its fire-hardiness alone; but it became equally, increasingly true that generic sclerophyllous traits were by themselves inadequate. Fire was too sudden, too powerful. Fire could even allow the eucalypt, within limits, to defy climatic oscillations, to preserve a relatively dry environment against pressures to restore elements of rainforest or araucarian forest. This apparently explains the otherwise anomalous persistence of Eucalyptus and scleroforest pollen at Lake George on the Atherton Tableland during the wet cycle of the last glaciation. The growing prevalence of fire revolutionized the internal relations among the scleromorphs.7
As Pleistocene inflected into Holocene, Eucalyptus was primed for a biological explosion. Once torched, the burning bush resembled a spiral nebula, its fuels and fires like paired arms locked into an accelerating vortex. Anything that altered the bush altered the regime of fire. Any change in fire behavior, timing, or frequency rippled throughout the entire biota. One encouraged the other. Unlike many organisms—Acacia, for example—Eucalyptus did not mold microenvironments unfavorable to fire, or shape fuel complexes unlikely to burn routinely, or inhibit those environmental parameters that supported free-burning fire. It burned readily, greedily, and gratefully. A fire weed had discovered a fire continent.
The Universal Australian became the archetypal fire colonizer of Australia. Granted a certain abundance of water, its range was limited by fire, and fire, by the prevalence of ignition. The Pleistocene revolution dramatically expanded those sources of ignition. With fire the genus Eucalyptus and the genus Homo had common cause and shared a common future.
THE EUCALYPT AS EMIGRANT
Eucalyptus is almost, but not quite, confined to Australia, and the exceptions are revealing. A few eucalypts have crossed the Torres Strait (or its land bridge, the Sahul) northward, and an extraordinary quantity of eucalypts have, through human efforts, become established throughout the world. The contrast between the natural and the anthropogenic—the extra-Australian eucalypts and the emigrant eucalypts—is expressive.8
Some ten species of eucalypts have infiltrated northward, half of which belong to the Corymbia subgenus, and half to the Symphyomyrtus—the later branches of the grand Eucalyptus alliance to emerge. They represent, that is, Australian indigenes that have attempted to occupy extra-Australian sites. Some probably crossed the Sahul, the shallow plains that, from time to time, have joined northern Australia to Papua New Guinea. Others may have colonized afterward, a product of catastrophic windstorms that biologically bridged the strait. Of the ten, four are still found in Australia, and only two exist outside the provenance of Gondwanic Australia before it sutured with the Sunda arc.
The New Guinea eucalypts claim drier sites, outliers of rainforest. In effect, they have rediscovered more ancient niches, not unlike those that species of Eucalyptus occupied during the early tremors of the Great Upheaval. They are minor, marginal constituents of New Guinean rainforest. All but E. deglupta live in monsoonal climates where fire is seasonally important. Two species, however, continued their move away from Australia. E. urophylla entered Timor, the Lesser Sunda Islands, and New Caledonia. It claims seasonally dry or disturbed sites, but it shows few of the typical eucalypt traits, and it competes poorly with other scleromorphs of the Myrtaceae family such as Melaleuca.
By contrast, E. deglupta penetrated into New Britain, New Ireland, the Celebes, and the Philippines—the only eucalypt to cross the classic Wallace Line. In the process it shed most of its Australian traits and entered the rainforest, thronging gregariously onto wet, lowland sites. No longer did it seek out niches thrown up by disturbance or seasonal aridity; it chose another, alternative path. The farther it distanced itself from Australia, the further it sloughed off those attributes that accounted for the dominance of Eucalyptus within Australia. As a result, neither E. urophylla nor E. deglupta survived in Australia, and may, in fact, have emerged outside Australia altogether. E. deglupta, in particular, lost completely any affiliation for fire. Fire destroyed it.
Clearly the special status of Eucalyptus within Australia depended on its special circumstances—its isolation, its pattern of disturbance and aridity. Yet, paradoxically, this most indigenous of Australian flora has been successfully transplanted not only throughout ancient Gondwana but into Eurasia, Africa, and North and South America. In the early years the French more than the British actively promoted eucalypts (most often E. globulus, the blue gum). It was hoped that gums would become a valuable hardwood, provide fuelwood, assist agriculture by establishing windbreaks and shelterwoods, and decorate the countryside with attractive ornamentals. Eucalypt oil was promoted for medicinal and commercial purposes, as an antimalarial agent and as a chemical base for perfumes. French explorers to Australia returned to Paris with seeds, and from the Jardin des Plantes eucalypts were propagated throughout the Mediterranean littoral. Britain soon followed suit, moving Eucalyptus out of the category of an ornamental for estate aboriculture and extending it through Kew Gardens into its African and Asian colonies. Italy, in turn, became another center for export, largely into North Africa. The Trappist monks of the Tre Fontane monastery inaugurated perhaps the most celebrated experiment when, in 1868, they planted thousands of eucalypts in the Pontine Marshes in the pious hope that the mysterious, aromatic “gums” would purge the miasmic swamps of malaria. Only with the accession of the indefatigable Ferdinand von Müller to the status of colonial botanist in Victoria did Australia become an active distributor.
About twenty to thirty species define the emigrant eucalypts. Their chief liability is their intolerance to extreme cold—a reason why their range in central and alpine Australia is restricted, and another legacy of their Gondwanic heritage in a rainforest and of Australia’s migration toward the equator. But in Mediterranean climates, especially, Eucalyptus has proliferated. It thrives in Spain, Portugal, Italy, Turkey, Israel, and such islands as Corsica and Cyprus. It has been widely planted in Africa—the northern littoral of the Mediterranean, the southern and eastern veldt, the Ethiopian plateau, the Congo basin. Eucalypt plantations are extensive in Brazil, and they have reclaimed otherwise denuded plains in Chile, Ecuador, and Argentina. Gums are grown in China and India, where they are praised as fast-growing fuelwood and cursed as thirsty aliens. Eucalypts clothe patches of the Coast Ranges of California, where they were promoted by various interests (including the Southern Pacific Railroad), often as a surrogate for that enduring California passion, real-estate speculation. Eucalyptus has successfully transplanted to a panoply of islands from Madagascar to Mauritius, from Sri Lanka to the Seychelles, from Easter Island to Alcatraz. Eucalypts grace the royal palaces at Katmandu and Addis Ababa. Where Australian soldiers have fallen on foreign battlefields, local gravesites are framed with eucalypts. (Poignantly, they refuse to grow on the cold flanks of Gallipoli.)9
The reasons for the success of the emigrant eucalypts are several. They flourish in climates similar to those they knew in Australia. They were often unpalatable to local browsers (even goats). No less, there exists a strong fertility gradient between Australia and most other lands such that it is often easier to export indigenous plants from Australia than to import exotics into it. Only a special suite of organisms could thrive under Australian conditions, and then primarily if the native biotas were upset beyond their normal tolerances. But transplanting flora like eucalypts that had evolved in a nutrient-deprived and droughty environment into relatively rich, well-water sites was a formula for successful
