Minor families included the myrtles, the grasses, casuarinas, chenopods, and xanthorrhoeas; important genera included Banksia, Hakea, Melaleuca, Eucalyptus. Much the same paleoflora characterized large portions of Antarctica, South America, and New Zealand. In all these lands the Gondwanic rainforest was sustained by a persistent, year-round moisture regime. The relentless rains leached and degraded soils, but the process occurred so slowly that the biota kept pace and adapted. The minor flora claimed special niches; many probably scavenged along the margins of rainforest, better adapted to disturbances, occasional dryness, a more fractured biotic environment.
When Old Australia broke away, about 30 million years ago, the rupture was remarkably final. There would be some late contamination along the north from Indo-Malayan biotas, though these would be restricted by the high mountains thrown up along the New Guinea border and by the deep waters, between islands, that only select species could cross. Together mountain and sea presented an effective biological filter. Greater Australia differ entiated into the Australian mainland, New Guinea, and Tasmania, to be reconnected and sundered from time to time with the geologic tides of a rising and falling global ocean. Not until historical times would there be a further, significant contamination of the biota.
The move to the tropics, while slow, induced climatic change, a new biotic force. Aridity did to the Gondwanic rainforest what tectonic stress did to the Gondwanic supercontinent. The ancestral rainforest fractured and multiplied, cleaving along biotic planes of weakness that divided those species that required uniform moisture from those that could accommodate dryness and change. The onset of aridity did not simply replace one enduring condition with another; it made regular and sporadic change a fundamental part of the biological calendar.
Australian aridity was seasonal, episodic, and chronic. It became, in places, part of an annual cycle of wet and dry seasons. In the tropics, the seasons followed the monsoonal winds, wet in the summer and dry in the winter. In the Mediterranean-like climates of the southeast and southwest, aridity took the form of a prolonged, parching summer, with moisture mostly a product of winter storms. Elsewhere aridity manifested itself as drought, extending regionally over several years. In the enormous center of the continent, aridity became a relentless presence, crowding moisture regimes to the coastal fringe and assaulting the littoral with desiccating winds. The southeast trades and the Great Dividing Range combined to raise moisture along the eastern seaboard, but the interior deserts, like a stony ice sheet expanding and contracting, defined the frontier. At times, like a red giant exploding among the stars, the desert core threatened to engulf the continent.
This transformation—the Great Upheaval—occurred over the course of the Tertiary period. It commenced with Greater Australia’s segregation from Gondwana during the Eocene epoch, and acquired a signature rhythm, long but emphatic, during the Oligocene. The Earth cooled and, overall, dried; Antarctica acquired an ice sheet; Australia continued its tread toward the equator; new circulation patterns established around the Southern Ocean and within and around the Australian continent; global changes in sea level catastrophically flooded then reexposed vast portions of the continental plains, reshaping the interactive meteorology of ocean and continent. By the Pliocene and Pleistocene epochs—over the past 5 million years—the trend became all but inevitable. Aridity became the norm and humidity the exception. The area of stony desert came roughly to equal the area of true forest. During the last glacial epochs, the transfiguration could be called irreversible. Australia’s low latitudes and low relief confined glaciation to Tasmania; there were no loess plains blown downwind, no fresh rocks exposed to weathering, no transfer of nutrients and sediments from mountain to plain. Where rainforest taxa reemerged, they bloomed like ephemerals after a desert storm. The Great Upheaval ended with a Great Inversion of the Australian biota.
In this biotic revolution once-minor constituents, now hardened and shaped by drought and disturbance, became dominant. The survivors evolved into scleromorphs (or sclerophylls)—literally, “hard leaves,” referring to the small, tough evergreen leaves that hoarded nutrients and resisted the transpiration of precious water. The scleromorphs adapted not only to soil impoverishment but to aridity—and, in fact, to disturbances of many kinds. By the mid-Miocene epoch (c. 15 million years ago), as the continental interior acquired its imperishable dryness, the relatively homogeneous biota of ancestral Australia began to differentiate.
What had been a more or less uniform cover of closed Gondwanic rainforest splintered into new, peculiarly Australian biotas. The hermetic forest became open; woodland surrendered to savanna, shrub and heathland, grasses, or outright sand and stony desert. The ancestral rainforest dominated by Nothofagus and Podocarpus gradually retreated before aridity like leaves before a blower. In its place emerged a scleroforest. Casuarinas succeeded araucarias. Tough grasses and scrubby scleromorphs seized understories formerly softened by fern, moss, and fungi. About 34 million years ago the eucalypts appeared, quiet and unannounced. Sometime around 25 million years ago acacias arrived, probably by sea from elsewhere in Gondwana. Thereafter the biotic isolation of the island continent was nearly total. By the time of European discovery the ancestral rainforest had retreated to minor enclaves in the Great Divide, where they occupied probably less than 1 percent of the total land surface of Australia.
The Great Upheaval had all but replaced a pan-Gondwanic biota with a marvelously endemic suite of biotas. About one-third of all Australian plant genera are endemic, nearly 90 percent of all plant species; Victoria alone has a flora twice as great as that of Britain. But such figures fail to convey the utter, continental-scale domination of the landscape by the scleromorphs. The revolution was comprehensive. With the new flora came new fauna and new patterns of interaction between sclerophyllous plants and sclerophyllous animals. Birds and mammals, not insects, typically pollinated the flowering scleromorphs. Placental mammals and reptiles repeated the radiation of specialized plants. Two genera, Eucalyptus and Acacia—gums and wattles—virtually tyrannized every forest and woodland biota, excepting only the relict rainforest. Two genera of grasses, Triodia and Astrebla—the hummocks and the tussocks—similarly dominated the grasslands. Scleromorphs invaded and reshaped forests, woodlands, grasslands, deserts. They penetrated every ecological niche—the canopy, the understory, the surface. They claimed relatively dry sites and relatively wet and those areas that were, on an annual cycle, both wet and dry. The rainforest eroded away like the plateaus of ancient Gondwana. What began as a Gondwanic ark ended as an island continent, Old Australia, that only remotely resembled anyplace else.4
The final expulsion of rainforest came relatively late. By the onset of the last glaciation (80,000 years ago) a rough balance still existed between forests consisting of scleromorphic angiosperms like the casuarinas and those composed of ancient gymnosperms like the araucarias. By 38,000 years ago, however, the araucarian forest had all but vanished. Aridity had decided the contest between rainforest and scleroforest, but as aridity settled in to an enduring presence, it became more complex and found new allies. During the final, near extinction of rainforest another biotic revolution broke out, this time within the scleroforest. Casuarinas receded, eucalypts advanced, and charcoal saturated the landscape. This second upheaval was decided by the renaissance of a new, vastly more complicated stress—fire.
WHAT ESCALATED THE GREAT UPHEAVAL was not the simple fact of aridity, but its rate of growth, the frequency of its oscillations, the way it introduced routine disturbances. Wet periods gave way to dry, and dry returned to wet, like a two-cycle engine. With the onset of the Quaternary era the frequency of oscillations increased. A gradual change could have been met with gradual adaptations, but rapid, frequent flux encouraged organisms that could respond with equal vigor and speed, that thrived amid disturbance. It encouraged the tough, the opportunistic. It promoted the weeds among the Gondwana greenery. What began as a tendency stiffened into a trend as Australia began to burn.5
There had been some fire in the past. Coal seams preserved, as pyrofusinite, the charcoal of Carboniferous- and Tertiary-era fires. Brown coals from the Yallourn-Morwell district of Victoria reveal ample evidence of burning, probably in the late Tertiary or early Pleistocene times. Where coal seams had been exposed as outcrops, they also ignited from surface fires. Burning Mountain in New South Wales, already smoldering when Europeans arrived, is a celebrated example. But smoldering coal and fiery basalt flows could not become a selective force of continental proportions. Lightning fire could—and