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Many Australians take it for granted that gum trees, waratahs and wattles have always been part of the Australian environment.
Although hard-leaved, or sclerophyllous, vegetation dominates the continent today, the ‘bush’ is not Australia’s original vegetation. In fact, 60 million years ago, eucalypts and acacias as we know them probably did not exist.
Millions of years before the familiar Australian vegetation evolved, much of the continent was covered with moist forests similar to the rainforests of today. Because of its dense canopy of foliage, such vegetation is termed a ‘closed forest’.
The origins of Australia’s unique rainforests have been a matter of dispute in botanical circles for over a century. Until the 1980s, many botanists believed that our rainforests had migrated to Australia in the recent geological past from South-East Asia.
They presumed that the ‘alien’ rainforests invaded the typical Australian vegetation and colonised the few areas that offered suitable living conditions.
This certainly is true for a small percentage of Australian rainforest species that have relatives in Asia.
Not only did Australia’s rainforest plants originate largely in the southern hemisphere but some of the groups that evolved in the southern hemisphere have now spread northwards to other lands.
Why were rainforests apparently so widespread in what is now the world’s second most arid continent, after Antarctica?
Evidence from many sources clearly indicates that Australia’s climate was once a lot wetter and cooler than it is today.
Over millions of years, a combination of complex climatic and global geomorphological events led to Australia’s transformation into a ‘sunburnt’ country. It was this drying out of the continent that led to the retreat of the ancient moisture-requiring rainforests.
The drying out of Australia was largely the result of a global process known as continental drift. One hundred and thirty five million years ago, today’s maps of the world would have been of little use to extraterrestrial beings intent on continent hopping.
The future continents of the southern hemisphere were all joined together in a massive supercontinent known as Gondwana. But, after millions of years of unity, deep-seated tensions below the earth’s crust began to split the giant land mass into separate pieces.
These pieces were slowly moved apart by convection currents under the earth’s crust at the rate of 5 to 10 cm a year. Over millions of years, this rate of drift caused a substantial separation of the once adjacent ‘continents’. As the continental ‘rafts’ or plates drifted through varying latitudes they encountered different climatic conditions, induced by changes in global ocean and wind circulations.
The new conditions placed stresses on the plant and animal species, including the rainforests.
The end result was the evolution of those species that were better able to survive in the changing environment.
When flowering plants first appeared about 135 million years ago, the earth’s vegetation was dominated by cycads and conifers. The first flowering plants may have been shrubs of dry or fluctuating environments but, before long, flowering plants were mixing with the conifers to form rainforests adapted to the cool, moist climates that then prevailed.
Around 45 million years ago, Australia finally broke away from Antarctica and began its separate drift northwards. At the time, rainforests covered large areas of the continent, since substantial rainfall penetrated well into inland areas.
During the course of Australia’s drift, the temperature of surrounding ocean currents changed, resulting in altered climatic patterns and lower rainfall over the land.
Although there are no fossil records of eucalypts older than 38 million years, it is probable that the first hard-leaved plants made an appearance not long after Australia’s separation from Antarctica.
Australia’s northward journey through 27 degrees of latitude slowed down when its leading edge rammed into the Asian continental plate. The once widespread rainforests had begun their long retreat, unable to survive the increasingly arid conditions.
The drying out resulted from a combination of factors. These included the rain-shadow effect of the Great Dividing Range which caught the moisture-bearing winds travelling inland, Australia’s movement towards the equator, and a worldwide change in atmospheric and oceanic circulation patterns.
Around the turn of the century, various theories were suggested to explain how continents separated by thousands of kilometres of oceans could each have closely allied plants and animals.
For example, the now widely separated continents of Antarctica, Africa, South America and Australia all had in common a 280-million-year-old fossil known as Glossopteris.
After the theory of continental drift, or plate tectonics, was finally accepted in the 1960s on the basis of palaeomagnetic evidence and the matching of once adjacent continents, the puzzling patterns started to make sense.
Living reminders of ancient continental links can be found in the rainforests of the world since rainforests were dominant over much of the super continent while the break-up of Gondwana was in progress.
Thus, the Southern Beech genus (Nothofagus) occurs in Australia, New Guinea, New Zealand, New Caledonia, South America and, in fossil form, in Antarctica.
Similarly, close relatives of Australia’ Macadamia nut occur in South America and Southern Africa.
If rainforests were widespread in Australia 50 million years ago, where and when did the typically Australian vegetation come from?
To the layperson, it may seem unlikely that moisture loving, fire-sensitive rainforest species could be the ‘parents’ of fire-resistant and drought-proof plants such as eucalypts and banksias.
In appearance, habits and habitat preferences, the two types of vegetation are very different. Research, however, shows that hard-leaved plants did evolve from ancient rainforest ancestors.
The key to this apparent puzzle lies in soil fertility. Australia’s hard leaved plants have evolved in response to a combination of factors including increasing aridity and low soil fertility.
Evidence for this can be seen where one species of a genus, largely confined to rainforest, shows adaptations to dry conditions. Leopardwood (Flindersia maculosa) grows in a drier inland environment than its relative, the rainforest-dwelling Australian Teak (Flindersia australis).
Adaptation to lower soil fertility and to lower and less reliable rainfall has influenced, in different regions, the evolution of the different types of plants that make up most of Australia’s native vegetation.
We have looked at the retreat of the rainforest as though it was a one-way process. However, there is plenty of evidence from the past two million years to suggest that the rainforests can reverse the trend and actually advance into neighbouring eucalypt woodlands if the conditions are suitable.
In the recent past, large-scale fluctuations in Australia’s rainforest distribution have been triggered by the onset and cessation of periods of glaciation.
Though Australian rainforests may seem to be diminishing today, the situation was even more critical 26,000 to 13,000 years ago on Queensland’s Atherton Tableland. In this region, a picture of the past distribution of rainforests has been worked out from the proportion of fossil pollen grains of various plant species in soil profiles stretching back 120,000 years. The pollen record indicates that 26,000 to 13,000 years ago local rainforests all but disappeared because of periods of extreme aridity associated with ice ages. Once the aridity eased, the rainforest remnants were able to expand, perhaps with the assistance of seed-dispersing animals and birds that migrated between patches of local rainforest.
Long before humans arrived in Australia, climatic pressures allowed the remnant rainforests to survive only in those places providing suitable habitats. As a result, rainforests today are scattered along the northern and eastern coastlines of the continent in a discontinuous string. They cover areas ranging in size from tiny gullies to extensive valleys, ranges and tablelands. Because of their distinctive colour and canopy, it is relatively easy to pick out from the air the mosaic of rainforest pockets in the midst of eucalypt woodlands.
Their distribution can invariably be related to a number of vital environmental factors which determine the presence or absence of a rainforest. The essential ‘ingredients’ include adequate moisture, shelter, soils, absence from fire and a suitable source of rainforest seeds. There is no rigid formula for the exact quantities required of each. In some cases, more than adequate quantities of one factor, such as fertile soils or shelter, can compensate for other deficiencies, such as moisture.
Rainforests naturally occur in locations receiving ample rainfall relatively evenly distributed throughout the year. In eastern Australia, the Great Dividing Range provides suitable habitats for rainforests as the mountains force moisture-bearing onshore winds to rise and condense, guaranteeing reliable rain or mists along its length.
The amount of rainfall necessary to support a rainforest depends on several factors, including latitude and altitude.For example, northern Queensland rainforests may receive 2400 mm annual rainfall, while 1600 mm is adequate in cooler climates further south because of lower evapotranspiration rates. Simpler types of rainforests, consisting of hardier species, can survive on a yearly average of 900 mm. Mists and fogs also provide moisture to high-altitude rainforests.
Rainforests are particularly sensitive to the drying winds that blow from the inland. The Great Dividing Range provides plenty of shelter from these winds in leeward-facing gullies and slopes. So important is shelter to a rainforest’s survival that it is quite usual to see an open eucalypt woodland growing on the windward side of a mountain while less than 200 metres away on the sheltered leeward side there is a luxuriant rainforest community. A variation on the theme of shelter occurs along the east coast of the continent. Behind fore dunes of sand or tough thickets of sclerophyllous vegetation is an unusual type of rainforest which varies between a stunted community of shrubs and a tall rainforest stand. For this Littoral Rainforest to occur, the basic element of shelter is essential to intercept seaspray and coastal breezes. The salt kills shoots on the seaward side and so prunes the canopy of plants exposed to the full brunt of the spray, causing a lopsided wind-shear pattern to develop.
Although bushfires rarely burn through a rainforest because of high humidity and moist leaf litter, scorching usually kills off trees on the edge, pushing back the boundaries of the forest. Unlike hard-leaved vegetation, rainforest species generally lack adaptations to fire that would allow them to produce new shoots from the base or trunk. Adapted to fire, the dormant seeds of hard-leaved plants rapidly respond to the increased light reaching the floor and soon take the place of dead rainforest plants.
By contrast, in the absence of fire, rainforest species are able to advance into neighbouring eucalypt forests. Seeds of rainforest pioneers carried by wind, gravity, birds or other animals are able to germinate and grow beneath sclerophyllous plants such as Brush Box (Lophostemon confertus) or Turpentine (Syncarpia glomulifera). These seedlings usually grow into dense stands that inhibit the germination of plants whose seeds cannot tolerate shady conditions. Eventually the rainforest trees grow up around ‘captive’ sclerophyllous trees which sometimes stand out above the canopy as emergent trees.
Though rainforests may be the world’s richest expression of plant life, they are not always supported by the richest soils. The more fertile the soil, the more complex the type of rainforest it is likely to support. Soils that are derived from alluvium or phosphorus-rich, volcanic parent material are more likely to support rainforest than sandy acidic soils, if other conditions are suitable.
Because most of the nutrients in a rainforest are contained in the plants, the underlying soil may be relatively nutrient-poor.
While most rainforest needs an initially nutrient-rich soil to become established, once the recycling of nutrients stored in the leaves is established the fertility of the soil becomes less important.
The ‘right’ combination of moisture, shelter, freedom from fire and suitable soils does not always guarantee a rainforest. Seemingly suitable sites in the south of Western Australia lack rainforest even though fossil evidence shows that they were present in the past. Unable to contend with extreme climatic fluctuations, these ancient rainforests were pushed into extinction.
Although environmental conditions are now suitable for rainforest growth in a number of places, there is no source of rainforest seed in the vicinity to allow natural re-establishment to take place. Even if a source of rainforest seeds became available, other species already established would provide strong competition.
Between these suitable sites in Western Australia and the remnant rainforests of the east coast are thousands of kilometres of arid interior.
Not all Australian rainforests occur in hot and humid regions. Queensland’s Tropical Rainforests are just one end of the spectrum. The Cool-temperate Rainforests of New South Wales, Victoria and Tasmania, which receive some of their precipitation as snow, are at the other.
Rainforests have often been branded as a ‘conservative’ form of vegetation, particularly when compared with the drought-resistant, hard-leaved plant communities of the bush, scrub and heathlands. But, contrary to opinion, a wide variety of local types has evolved to survive in a diverse range of climatic and geographic habitats from the tip of Cape York to Tasmania.
Despite quite obvious differences in their overall appearance and constituent species, most rainforests have certain features in common. Of these features, the complex layers of tree canopies and diversity of species and growth forms are best expressed in the Tropical and Subtropical Rainforests and may be absent or rare in the simpler rainforests of cooler regions.
Because different types of rainforests grade into each other, it can be difficult to determine with confidence which type of rainforest is which.
Australian botanists use several different approaches to categorise rainforests. A recent approach by Dr Len Webb, while working in CSIRO, divides rainforests into more than 20 different groups on the basis of structural components. These include the number of tree layers, leaf size, the height of the canopy, proportion of deciduous trees, and the presence or absence of particular species. Another way to classify the forests is into broad climatic categories: Tropical, Subtropical, Warm-temperate, Cool-temperate and Dry.
There is no simple correlation between latitude and rainforest type. Cool-temperate Rainforests are not confined to Tasmania and Victoria. Nor are Sub-tropical Rainforests restricted to the New South Wales-Queensland border area. Altitude and soil fertility can also affect the type of forest. As altitude increases and climates become cooler, rainforests adopt a simpler form. For this reason it is possible to find Subtropical, Warm-temperate and Cool-temperate Rainforests within a relatively small area such as Dorrigo in New South Wales As you climb higher, the rainforests become structurally and floristically less complex in response to cooler temperatures, changing nutrient levels and precipitation.
More on Australian rainforests