Rainforest history the Australian story

Millions of years ago much of Australia was covered with closed moist forests similar to the rain forests which now cover a mere two million hectares, equal to about a third of the area of Tasmania. Though their present extent is so small, the rain forests of Australia are of exceptional interest and the history of how they have waxed and waned, as well as the communities to which they have given rise, is fascinating. The answers to many of the questions concerning the nature and origin of tropical rain forests lie in the geological past. Some 280 million years ago (Ma) the fossil tree Glossopteris existed in what are now Antarctica, Africa, South America and Australia, suggesting a similar climate. Moreover the southern beech genus Nothofagus is found as a fossil in Antarctica and various species grow in Australia, New Guinea, New Zealand, New Caledonia and South America. Why did the climate of Australia change to that of the most arid of the unfrozen continents, and what accounts for these tree distributions?

The answer to these questions lies in the theory of continental drift, which was first put forward by Alfred Wegener in 1912. This drift, caused by plate tectonics, is driven by convection currents in the mantle below the tectonic plates of the Earth's crust, of which the continents form part (see Section 9.1.1). When flowering plants first appeared some 135 Ma, in a period when the world's vegetation was dominated by cycads and conifers, Australia formed part of the massive southern supercontinent Gondwanaland. This former huge southern hemisphere landmass was at that time made up of what are now Africa, South America, India, Madagascar, Arabia, Australia and Antarctica. Around 45 Ma, Australia finally broke free from Antarctica and began its separate drift 27° northwards, finally slowing when it hit the Asian continental plate some 15 Ma. Australia then became very much drier, largely because of a worldwide change in atmospheric and oceanic circulation patterns. The more recent history of the Atherton Tableland, Queensland, has been worked out from the proportions of fossil pollen grains of various species found in soil profiles going back 120 000 years. These enable us to discover, in a period interspersed by ice ages, the proportions of land dominated by rain forest as opposed to sclerophyll vegetation (see Section 1.6.1), and show that rain forests can advance into neighbouring eucalypt forests when conditions are suitable. Around 26 000 years ago rain forest almost disappeared on the Atherton Tableland, only to begin picking up again around 13 000 years ago.

Many ecologists had thought that the current flora and fauna of Australian rain forests were the result of migration from south-east Asia in the geologically recent past. Though this is true for a small percentage of Australian rain-forest species with Asian relatives, it is now clear that the southern beeches Nothofagus spp. and many other rain-forest plants originated in Australia when it was much cooler and wetter than it is now, before migrating northwards to what are now other continents. Australian vegetation is now dominated by hard-leaved (sclerophyllous) species such as gum trees Eucalyptus, waratahs Telopea and wattles Acacia. The earliest fossil records of eucalypts (see Sections 1.3.1 and 1.3.2) are no older than 38 Ma, but it is now considered that the first sclerophyllous species evolved from rain-forest plants soon after the separation from Antarctica 45 Ma. The junction between rain forest and woodland dominated by eucalypts is often remarkably sharp (although sometimes there is a transitional belt). A number of reasons have been put forward, including fire which burns readily through the eucalypt woodland but not into the rain forest (Bowman, 2000), but soil also seems to play a part. Rain forests are mostly restricted to relatively fertile soils since the rain-forest trees have a higher nutrient requirement, especially for phosphorus, than the eucalypts. But the precise boundaries are known to have moved in the past, either by rain forest invading the woodland, or cyclones opening up rain-forest areas to eucalypt invasion, so other factors apart from soil are obviously important.

Present-day Australian rain forests are arranged in a discontinuous string along the northern and eastern coasts of Australia from near Cooktown, Queensland southwards to New South Wales and Victoria. This discontinuity is as much to do with variations in rainfall caused by topography as it is to human clearance. These rain forests can be classified into many groups on the basis of their structural components, but it is simpler to place them into the broad climatic categories of Tropical, Subtropical, Warm-temperate, Cool-temperate and Dry. Tropical rain forests with an uneven, multi-species canopy of at least three layers (although these are not always discrete and apparent) occur in Northern Queensland. In the north they often receive 2400 mm annual rainfall, although 1600 mm is adequate for rain forests in cooler areas further south where evapo-transpiration rates are lower (Cameron-Smith, 1991). This rainfall is seasonal, so these forests are all part of the moist or humid tropical rain forests (Section 1.6.1). Subtropical rain forests, which have many species in the tree canopy, but are somewhat simpler than tropical rain forests, can survive on a yearly average as low as 900 mm. They are found from the cooler uplands of northern Queensland to the coastal lowlands of New South Wales. Warm-temperate rain forests grow in cooler climates, at higher altitudes, and on less fertile soils. Their composition is less diverse and more uniform with 3-15 tree species in the canopy; they also lack the buttress roots characteristic of tropical and subtropical rain forests.

Cool-temperate forests are simpler still, often with only one species - frequently of a southern beech Nothofagus - in the upper canopy and few species in the lower layer. These commonly high-altitude, cool forests with very high rainfall are often cloaked with mist, and their dominant trees usually have a thick coat of mosses, liverworts, ferns and lichens. Tree ferns and thorny bushes are often common. Tree leaves are small and simple, frequently with toothed margins. Palms and stranglers are absent, as are root buttresses, although trunk bases are sometimes massive. Such forests are found from the McPherson Range on the New South Wales-Queensland border to Tasmania. Small-leaved southern beech, notably antarctic beech N. moorei, abound in the northern cool-temperate rain forests. Pinkwood Eucryphia moorei then dominates rain forests down to the border with Victoria. Further south, myrtle beech N. cunninghamii extends into Tasmania where deciduous beech N. gunnii also occurs (Cameron-Smith, 1991).

Tiny remnants of the strangely named dry rain forests occur in regions of Australia with distinct wet and dry seasons. The number of tree species in the low to medium canopy layer varies, but may be large. Scattered larger trees known as emergents rise above this layer. The ability to shed leaves helps some species to survive temporary water shortages. Rain forests sheltered behind coastal dunes or headlands are unusual, but such littoral rain forests, often with a wind-sheared upper canopy, do occur on Australia's eastern coast. Related to subtropical rain forests, they grow on a wide range of parent materials including slate, basalt and deep beds of sand. Where shelter is insufficient, salt spray kills exposed leaves and branches, leaving dense thicket.

Rain-forest communities and their distributions have changed enormously over time. There is now considerable concern for them on a world basis and this is discussed in Section 11.1.2.

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