Australia: The Land Where Time Began

A biography of the Australian continent 

Australian Plant Communities – Early Eocene

At this time megathermal rainforests are present in the coastal/lowland regions in southeast Australia, which contrasts with the situation in the the Palaeocene. Evidence of this is seen in the diversity of the angiosperms, the pollen of which dominate the vegetation. Among the plant types that are represented are mangroves, a plant type that at the present is found only in the tropics. In this vegetation Nothofagus spp. is rare. According to Macphail, the evolution of these communities at this time was apparently a response to global warming, with humidity remaining high throughout the year. He also suggests that the repeated marine inundation of low-lying coastal plains was at least partly responsible for their evolution. The sea surface temperatures along the coasts of northwest and northeastern Australia are believed to have been relatively low, with maximum temperature of less than 18^o C at this time, Macphail suggesting that in spite of these lower temperatures similar rainforest types extended along these coast.

Further inland the rainforest communities are believed to have been of mesothermal type but with many species that are also present in the megathermal rainforests.  The pollen record indicates that at this time there were strong differences between the pollen dominance at inland sites. Macphail suggests these differences can be seen as early manifestations of the ecological differentiation that is characteristic of the vegetation from the later Cainozoic.

Based on evidence available at the present it is believed communities dominated by Nothofagus were mostly restricted to Tasmania, and there may possibly have been some in the Southeastern Highlands. Macphail suggests it is premature to propose that these areas were refugia for Brassospora spp. during the thermal maximum of the Early Eocene.

In central Australia the vegetation is believed to have been a mosaic of rainforest communities with dominance varying between gymnosperms, such as Cupressaceae and/or Taxodiaceae, Casuarinaceae, Cunoniaceae and Myrtaceae. At this time it has been suggested some species of distinctive Proteaceae possibly evolved, migrating subsequently into southeast Australia. While this process may have been associated with the more extreme climate of the central parts of the continent well away from the coast, eustacy has also been suggested to be involved in the way the flora of the Bassian region diversified progressively. The rate of change in both instances is considered to be too low to result in the development of vegetation types that were low and open, as is seen in sedgeland, though such taxa as Poaceae and Restionaceae were present.

Macphail suggests the Early Eocene could be regarded as the period in which the Australian nonseasonal rainforests achieved their maximum development.

Sources and Further Reading

1.      Macphail, M.K., in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.