Rain Forest Sites on the Australian Plate

The rumblings of a massive earthquake shattered the cool misty silence of the vast primeval rainforest of the great southern continent of Gondwana. The land was torn asunder. So began the birth of the new continent of Australia. Or so we might well imagine, for there were no human witnesses to the event, nor were any of our primate ancestors present at the birth, estimated to have been around 60 million years ago.

The earthquakes would have continued for perhaps millions of years as the Australian Tectonic Plate was wrenched and rifted from Antarctica, the parental core of the once enormous supercontinent that we now call ‘Gondwana’. Australia was the last of the sibling continents to which Gondwana gave birth, preceded by Africa, South America and the smaller but faster-moving India. The new continents rafted their way northwards through the Tethys Sea, eventually colliding with parts of the northern supercontinent of Laurasia.

A dramatic consequence of the rifting of the Australian Plate from the Antarctic Plate was the development of a sea passage between the continents, facilitating development of a strong circumpolar ocean current. No longer would warm tropical ocean currents be forced southwards to create a huge heat-exchange engine to keep the Gondwanan climate warm and balmy. Instead, Antarctica became enveloped in constantly cold ocean currents, ‘freeze-drying’ the whole landmass, pushing the Gondwanan terrestrial plants and animals to extinction.

A cargo of the accumulated rich biota that had evolved on Gondwana did not enjoy a constant and compatible climate aboard the ‘Great Southern Ark’. In the course of time, the continent was transformed from a warm and wet land clothed in rainforest, to the largely hot, arid land that it is today.

For perhaps 30 million years of its northerly drift, the ‘Great Southern Ark’ enjoyed a ‘splendid isolation’; biologically cut off from the rest of the world. The combination of mountains, rich volcanic soils and reliable rainfall along the eastern seaboard of the plate provided critically important climatic refugia for survival of the moisture-loving Gondwanan rainforest biota.

Many new species of plants and animals evolved in response to the warming and drying climatic changes, giving rise to a biota unique to the Australian Plate – the Austral-Gondwanan or ‘autochthonous’ biota. The typically ‘Australian’ biota is characterized by the marsupial-dominated mammal fauna and the eucalypt- and acacia-dominated drought-tolerant forests and woodlands.

By the time the Australian ark collided with the Asian Plate, not only was it carrying a cargo of little-changed Gondwanan rainforest biota, it also carried the uniquely Australian biota. Approaching the Asian Plate, opportunities arose for an exchange of some plants and animals with the Malesian (South-East Asian) realm, particularly of flying species and those capable of surviving a drift across the intervening sea. Even during the much lower sea levels of the last Ice Age, there remained a sea barrier to the exchange of larger animals between the islands of Bali and Lombok, known as ‘Wallace’s line’ after the nineteenth-century British naturalist who proposed this hypothetical boundary.

Less than 8,000 years ago, rising sea levels inundated part of the Australian Plate, creating the large ‘island’ of New Guinea by inserting a sea barrier. This very recent severing of the land surfaces is apparent in the many species and families of plants and animals shared by the two.

The Australian Plate, including the new subcontinent of New Guinea, is therefore a veritable Noah’s ark of original Gondwanan biota, the newly evolved Austral-Gondwanan biota together with more recently invading Malesian biota. But unlike Africa, India and South America, all of which have had land bridges to Laurasia for millions of years, sea barriers have selectively filtered the arrival of Malesian species on the Australian ark. For example, Australia and New Guinea have remained almost devoid of the larger placental mammals that are a dominant part of the fauna of the northern continents.

The picture we have, then, is one of a late-departing southern continental ark, laden with a cargo of the most recently evolved Gondwanan plants and animals, including some that would have been typical of Antarctica. During the long passage, the distinctive and unique Australian or Austral-Gondwanan biota evolved. As the ark approached the Asian Tectonic Plate, a third element was selectively added to the cargo; many Malesian plants and animals invaded the still-drifting ark.

RAINFORESTS ON THE AUSTRALIAN PLATE TODAY

Today’s rainforests on the Australian continent represent just a shadow of the Amazonian-scale rainforest of the distant past. At the time of European arrival in Australia, rainforests had shrunk to just 1 per cent of the land surface, almost exclusively associated with climatic refugia in the mountains and coastal lowlands along the eastern margins of the continent. By contrast, rainforest blanketed most of the high-rainfall lands of the island of New Guinea.

An archipelago of thousands of ‘islands’ now represents the rainforests of the Australian continent; each island being surrounded by the classic Australian eucalypt-dominated drier forest. In New Guinea, the pattern is reversed with rainforest being almost ubiquitous and the dry ‘Australian’ vegetation being the exception. The Gondwanan-derived rainforest archipelago stretches full 40° of latitude, from around 4° south in the highlands of New Guinea (Lorentz) to 44° south in Tasmania down the eastern margins of the Australian Plate.

The more southern rainforest ‘islands’ in Tasmania are almost entirely dominated by species of plants and animals with a clear Gondwanan origin. Further north, many of the Gondwanan species persist but with an increasingly greater proportion of both plants and animals of Malesian origin, especially on the hotter lowlands.

Further north, the Gondwanan biota adapted to a warm, wet climate was forced higher into cooler mountains to compensate for climate warming. Operating in reverse, the species of tropical Malesian origin, already adapted to hot, wet lowlands, tended to invade lowlands. One consequence of this mixing of the Gondwanan and Malesian biotas is that in various places along the eastern seaboard of Australia and in New Guinea, the typically Malesian species tend to be more prevalent in the lower, warmer regions and the relict Gondwanan species survive only in the cooler mountains.

WORLD HERITAGE RAINFORESTS ON THE AUSTRALIAN PLATE

Four great World Heritage listed rainforest sites on the Australian Plate collectively tell the story of the ‘Great Southern Ark’ and its northward drift, the survival of the ancient Gondwanan rainforest biota, evolution of the modern Australian biota ‘on board’, the collision of the ark with the Asian Plate and the boarding of the ark by the Malesian biota. These sites are:

§ Tasmanian Wilderness (Australia);

§ Central Eastern Rainforest Reserves (Australia);

§ Wet Tropics of Queensland (Australia);

§ Lorentz National Park (Indonesia).

The altitudes of these four great reserves range from sea level to over 5,000 m, including the ice cap and glaciers of Lorentz. Collectively they provide habitat for all forms of rainforest of both Gondwanan and Malesian origin.

Tasmanian Wilderness

Mountainous wilderness and wild coast characterize the 1.38 million ha Tasmanian Wilderness site. Cool temperate rainforest is extensive, although the site also has many other spectacular and important natural and cultural values.

The Tasmanian Wilderness site is the most southern of all rainforest sites on the Australian Plate and makes a unique contribution to the suite of four major World Heritage rainforest sites.

The cool temperate rainforests of the Tasmanian Wilderness are probably as close in composition and structure as any forest on the Australian Plate to that of the ancient Gondwanan forests, including those of Antarctica before it went into deep freeze. They are dense, dark and cool rainforests, often with only one or two tree species present. Understorey, vines and epiphytic ferns are rare or inconspicuous. The dominant tree species, including the myrtle beech (Nothofagus cunninghamii) and sassafras (Atherosperma moschatum), reveal their Gondwanan ancestry.

Many of the species found in the Tasmanian rainforest are little changed from their fossil counterparts dating back millions of years, some to the time of Gondwana. The famous Huon pine (Lagarostrobus franklinii) is extraordinarily similar to fossils dating from the Tertiary, making it truly a ‘living fossil’. The last significant stands of this threatened species are a feature of the site.

Bird species in the rainforest are few and mammals rare. The Tasmanian rainforests are aesthetically very beautiful, but lack the profusion of wildlife to be found in the northern tropical rainforests, an eerie facsimile of the ancient forests of Gondwana.

The dense Gondwanan rainforests appear to have been of little interest or value to the Tasmanian Aboriginal people, with very few edible plants and animals. Aboriginal archaeological sites in the rainforest appear to predate the latest expansion of rainforest since the last glacial period.

Timber, mining and energy development interests have over many years hotly contested protection of the Tasmanian Wilderness, but the area is now well protected and mostly managed as wilderness. Many of the past disputes have engaged people throughout the wider community, giving the site a high profile in the Australian community. Tourism is now very much a feature of the site, although most of the area remains accessible only to those seeking a high-quality adventure.

Central Eastern Rainforest Reserves (CERRA)

Comprising a selection of larger ‘islands’ of rainforests – each ‘island’ surrounded by drier eucalypt-dominated landscapes – the CERRA World Heritage site is a serial site of 366,000 ha extending over some 400 km along the coast and escarpment of central eastern Australia. The rainforests are associated with mountains, escarpments and a number of extinct volcanoes, which, combined with abundant rainfall, have provided the critically important climatic refugia for the rainforests to survive climate warming and drying.

Altitudinal differentiation is evident in the rainforest ‘islands’, subtropical rainforests in the lowlands giving way to warm temperate rainforests at higher elevations, with cool temperate rainforests of Nothofagus moorei on cool plateaux.

Dense stands of moss-covered Nothofagus forests on misty mountain tops and plateaux are testimony to the ancient Gondwanan linkages – and links to the Nothofagus forests in the high mountains of Lorentz to the north and the Tasmanian Wilderness to the south. Indeed, the story does not stop there, for Nothofagus species are found in those other ancient offspring of Gondwana, New Zealand and South America. Nothofagus is therefore one of the more obvious indicators of a direct Gondwanan rainforest heritage.

Many of the plants and animals inhabiting the CERRA rainforests have counterpart species in the Wet Tropics to the north and the Tasmanian Wilderness to the south, and in just a few cases there are shared species, confirming a past continuum of rainforest, at least up the eastern part of the continent, since truncated by climate warming and drying.

Initiatives in the 1980s to protect the rainforests and associated tall eucalypt forests attracted passionate opposing views between conservation and timber exploitation interests. Major community support for conservation propelled the State Government of New South Wales to champion the forest protection cause and to promote world heritage listing of the rainforests.

Aboriginal people occupied and utilized much of the CERRA rainforests, especially the more productive lowland forests. Although the traditional owners are no longer dependent on their rainforests, there is a considerable resurgence of interest in the culture.

Wet Tropics of Queensland

The Wet Tropics World Heritage site is right on my doorstep in Cairns in Tropical North Queensland. Stretching for more than 400 km along the coast, it mainly comprises a single large tract of 900,000 ha of tropical forest on lowlands, escarpment, mountains and plateaux.

The Wet Tropics rainforests are a treasure trove of species with Gondwanan, Austral-Gondwanan (autochthonous) and Malesian lineage. It has the greatest biodiversity of any forest region on the Australian continent. Although the site shares species with the Lorentz and CERRA sites to the north and south respectively, many plants and animals are endemic to the region.

Missing are some of the cool temperate species, such as the Nothofagus beech forests, but this absence is more than compensated for by an impressive concentration of other species of plants and animals with a Gondwanan ancestry, especially tree species of the Proteaceae family in the cooler mountain forests.

Plants

Two species of the ancient araucarian conifers are present, one looking amazingly similar to a closely related species found in Argentina and another in New Guinea – another clue to the shared Gondwanan origin of rainforests on the eastern side of the Australian Plate, Antarctica and South America. Similarly the related Kauri conifers (Agathis sp.), an ancient conifer genus shared with New Zealand, the CERRA rainforests, New Guinea and a number of islands in Malesia, are well represented in the Wet Tropics.

Animals

The animals of the Wet Tropics include many marsupial mammals such as possums and tree kangaroos, both groups of which are shared with Lorentz. The large flightless bird, the cassowary, is similarly shared between these two tropical sites. The rodent (rats and mice) group associated with the rainforests of the Australian Plate is especially common in Lorentz and the Wet Tropics, having evolved from relatively recent arrivals from the Malesian realm.

The Wet Tropics World Heritage site is the only part of Australia where tree kangaroos are found. They are almost exclusively rainforest inhabitants but are found in their greatest diversity on the island of New Guinea.

Fire

On the drier inland side of the Wet Tropics rainforests, the rainforest is in dynamic interaction with the distinctly Australian eucalypt-dominated forest. Fire tends to drive this dynamic relationship. Fire tends to favour the advance of fire-prone eucalypts into sensitive rainforest damaged or destroyed by fire encroachment or cyclonic winds. Climatic change in the past is known to have pushed the rainforest/eucalypt interface to-and-fro across the landscape as rainfall, temperature, wind and other climatic factors influenced the drought and fire conditions.

Aboriginal use of fire has played an important role in the interaction between the rainforests and eucalypts, at least in the past 40,000 years of Aboriginal occupation.

The exceptionally diverse tropical rainforest of the Wet Tropics provided many life-support resources for the Aboriginal people who inhabited the forests. Today calling themselves the Bama, these people developed a distinctive rainforest-based culture. Much of their cultural knowledge and languages have survived, although the Bama are no longer dependent on the rainforest. They are becoming increasingly involved in asserting their ownership of the rainforests, their culture and in the management of the site.

Extreme controversy preceded protection of the wet tropical rainforests in the 1980s, a move strongly contested by timber interests. However, in little more than a decade after listing in 1988, the Wet Tropics World Heritage site has become the centre of attention of a major tourism industry, generating far greater economic benefits than the timber industry that it displaced. The site is a demonstration of the economic benefits that can flow from regulated tourism integrated with heritage conservation.

Lorentz National Park

Lorentz National Park is one of the most amazing protected areas in the world. The continuous 2.5 million ha tract of tropical wilderness extends from the highest ice-capped mountains on the Australian Plate (Gunung Jaya at over 5,000 m), down the precipitous southern mountain slopes to the extensive tropical coastal lowlands and into the Arafura Sea. No other site in the world protects a continuous tract of wild lands from a tropical ice cap to a tropical sea. There could never be any doubt about its global significance.

The young, actively rising mountains of Lorentz are the dramatic results of the northward-thrusting Australian Plate against the Asian Plate. Part of the Gondwanan cargo of plants and animals found refuge in the cooler mountains of this otherwise equatorial region. By contrast, the hot wet lowlands are occupied by many species of plants and animals that have invaded from Malesia. Although none of the large mammals have crossed ‘Wallace’s Line’ from Asia, at least without human assistance, many flying birds and smaller mammals arrived in New Guinea and ‘Malesian’ plants are common in the vast flooded lowlands.

When visiting Lorentz on the UNESCO world heritage evaluation in 1999 I was delighted to see the beautiful mossy forests of Nothofagus in the highlands above the Baliem Valley – so incredibly similar in appearance to their cousins in Central Eastern Australia, Tasmania and New Zealand with which I was familiar. The Nothofagus forests are the most obvious clue to the Gondwanan links to other forests on the Australian Plate. Other plants and animals found in the cooler mountains have a recognizable Gondwanan ancestry.

Some of the more distinctive animals to be found in Lorentz are recognizably related to distinctly ‘Australian’ species. Australia is renowned for its kangaroo species, but New Guinea also has many species of kangaroo – mostly tree-climbing kangaroos. The cassowary is shared between New Guinea and the wet tropical rainforests of Australia, evidence of past land bridges between the two.

Local indigenous people live in a series of discrete areas within the site – in small agricultural-based settlements in the highlands and in the lower reaches of lowland rivers where there is a dependence on fish from the rivers and sago palms in the forests. Extensive areas of lowlands are uninhabited.

Lorentz remains one of the remotest wild lands on the planet and the visitors from outside are but a few intrepid adventurers.

CONCLUSIONS

The World Heritage listing in 1999 of Lorentz National Park in Indonesian West Papua, on the island of New Guinea, completed the suite of natural heritage Gondwanan rainforests on the Australian Plate. The rainforest sites embrace the full latitudinal and altitudinal range of the contiguous lands of the Australian Plate – from the very tropical 4° latitude of the Lorentz site down to the cool temperate Tasmanian Wilderness at 44°– a full 40° of latitude; and an altitudinal range from an ice-capped 5,000 m in Lorentz to sea level in all four sites.

The four sites beautifully represent the full latitudinal and altitudinal range of the relics of the ancient rainforests of Gondwana that survived the 60-million-year northward drift on the Australian Plate. A mosaic of the newly evolved Gondwanan-derived and distinctive Austral-Gondwanan biota complements the obvious Gondwanan survivors and the more recently arrived Malesian biota from the Asian Plate.

Collectively, these four sites represent a complementary suite of outstanding rainforests that survived continental drift, collision of tectonic plates and associated dramatic climate change on the ‘Great Southern Ark’. They are perhaps the most important relict rainforest ‘islands’ in the great archipelago of rainforests remaining on the Australian Plate. Collectively they are of far greater global significance than the individual sites themselves, four rainforest gems – truly World Heritage.

AUTHOR: PETER HITCHCOCK, heritage consultant