Australia: The Land Where Time Began
Australian Tertiary Phytogeography - Palynoflroas
Throughout the Tertiary the continent of Australia was almost entirely forest covered, mostly by rainforest before the Middle Miocene. Forests of drier kinds came to dominate much of the continent from the close of the Middle Miocene. The forest opened up, and the herbaceous element increased, woodlands-grasslands-herbfields becoming dominant in the Late Pliocene-Pleistocene. As time passed the flora was changing almost continuously, mostly small changes, but at times there was drastic changes in vegetation. General trends in vegetation type, and the times when change took place are observed in different parts of the country, the variation seen in the vegetation was comparable with that of the present. The times of climatic changes coincide with the times of major vegetation changes. Throughout the Tertiary there was a gradient of climates, wetter coastal regions and arid interior, as is the case at the present.
The vegetation of Australia and New Zealand were similar until the Miocene, they had both been part of the same land mass, following which the Australian vegetation became progressively more different as the aridification of Australia intensified and New Zealand underwent mountain building and glaciation, resulting in very different vegetation on the 2 landmasses.
When the fossil record was compared to the accepted phytogeographical hypothesis it was found that the hypotheses were inadequate. Origins and migration routes cannot be deduced from present distributions, cytogenetics and taxonomy. The evolution of taxa is a more or less continuous process, habitats being occupied by plant communities that are adapted to that microenvironment, and selection pressures continue to improve the suitability of the component species to its habitat. Even in environments that appear unchanging over large areas, with a dominant vegetation type, there are pockets where the local environment differs in some way from the overall environment, such as in protected areas in valleys of ranges, or that differ on the side of a range that faces north (in the Southern Hemisphere), where they are exposed to direct sunlight for longer periods than those from the south facing slope of the same range. In these pockets, where the habitat differs even slightly from the large-scale habitat surrounding it, the communities can diverge in various ways and to varying degrees from the dominant community. It is from these pockets that plant communities can expand their range when the climate changes to one that better suits them, then retract back to the favoured pockets if the climate changes back again.
Not all pollen types can be used to identify plants down to the species level, most being useful to identify only to genus, family or even order level. Pollen of some taxa are fund only in inland deposits, such as the Murray Basin, but not in the Gippsland Basin on the coast. Examples of these are Alangium, fuchsia, Caesalpinoides, Guettarda, Alyxia, Malpighiaceae (Macphail & Truswell, 1989). Ranges can differ in other parts of the continent. After some time near the end of the Eocene the pollen of Anacolosa is no longer found in southeastern Australia, but is found in deposits from Queensland in the Early Miocene (Hekel, 1972). Many taxa are still present in the vegetation of the continent, but with different distributions, often dramatically changed from the time of their first appearance. Taxa such as Ilex (Martin, 1977) and Nypa (Lear & Turner, 1977) are now restricted to the north, but previously had a much wider range. Many that are now restricted to the coastal strip in the north and east where previously found in inland areas.
There is a group of gymnosperms that ranged throughput the Mesozoic. Angiosperm pollen from the Late Cretaceous has been identified from both extant and extinct taxa, indicating that these extant taxa were not the first angiosperms (Dettmann & Playford, 1969; Dettmann, 1973). It is not known if the first appearance of taxa in the Australian record is as a result of evolution in southeastern Australia or of migration.
Changes through time
The relative abundance of taxa in the vegetation is reflected in the abundance of the pollen in the record, but this abundance is skewed towards those plants producing pollen that is more widely dispersed by the wind, and those producing large quantities of pollen. The plant species that rely on pollinators, such as insects, birds or bats, usually produce pollen of larger size and in lower quantities than those producing wind-born pollen. An example of this skewing of the pollen record is given by Ladd (1979) with regard to the lack of pollen of Acacia from the forest floor, even though it was co-dominant in that forest. In the fossil record Acacia pollen is often found, but always in low concentrations, indicating it is only a minor part of the assemblage, making it impossible to make definite estimates of its actual abundance and variation in the vegetation. The Myrtaceae produce large amounts of pollen, though they are mainly pollinated by birds, bats and insects (Churchill, 1970; Crome & Irvine, 1986; McCoy, 1989), resulting in high percentages being found in the pollen record, indicating high numbers in fossil assemblages. Though the percentage of a particular pollen may not reflect the actual abundance of that plant in a fossil assemblage, if the percentages change it does indicate a change in the vegetation.
The spores of ferns and their allies don't usually show any particular variation over time, and are believed to probably reflect the ecoilogical conditions of their habitat. Only a few of the many taxa of the group may be abundant, such as the tree fern Cyathea, probably the most abundant taxon. In the Late Miocene-Pliocene sequence, it is usually represented by high spore frequencies. The Anthocerotae, a bryophyte, is indicated in the Late Pliocene-Early Pleistocene by high spore numbers, though the group composition may not be uniform spatially and through time.
The earliest Tertiary was the time when the gymnosperm group were most abundant, decreasing in the Early Eocene. Geographical variation, or a response to ecological conditions locally, is indicated by the lack of a particular pattern for the remainder of the Tertiary. Casuarinaceae pollen is found at various times during the Tertiary, which reflects local ecological conditions. Gymnostome cones and foliage, a taxon found in rainforests, can be distinguished from Casuarina-Allocasuarina, found in open forests, shrublands and woodlands, but the pollen cannot be identified below the family level (Kershaw, 1970a), the environmental significance of the presence of this family requires the presence of macrofossils in the deposit.
In the Late Cretaceous, Nothofagus first appeared in the pollen record, continuing at low percentages until the Late Eocene, at which time there was a dramatic increase in the percentage of its pollen. The high percentages continued until the Miocene, decreasing slightly in the Early Miocene and in the Middle-Late Miocene the percentages of its polled crashed. The pollen in the record is mainly from the subgenus Brassospora. There was a minor resurgence of Nothofagus pollen in the Early Pliocene, but by this time subgenus Brassospora was no longer present in the record. Nothofagus pollen is not found in the fossil record of inland regions beyond the Early Pliocene. In the highland regions it persisted until Pliocene-Pleistocene times (Mcewan Mason, 1989), still being present as relict populations in east Gippsland, the most southeasterly part of the continent. In Tasmania it is an important part of the vegetation of the present.
In the pollen record there are fossil taxa of Myrtaceae with a diverse range of ecological requirements. Myrtaceidites cucalyptoides, a pollen type, has similar morphology to the bloodwood/eucalypt/Angophora. Pollen of other eucalypt types are also present. Eucalypts are usually associated with open sclerophyllous forests and woodlands, but among the fossil pollen are some the myrtaceous types that have been indetified as Backhousia, Szygium, Tristania (Martin, 1978) decaspermum, Austromyrtus, Rhodamnia (Truswell et al., 1985), ?Acmena and Szygium-cleistocalyx (Luly et al., 1980), all being predominantly rainforest taxa. Indirect evidence of the vegetation type represented by this group is gained from the fire record, the charcoal in the deposits. The amount of charcoal parallels the Myrtacece abundance (Martin, 1987). Rainforests usually burn only when they experience exceptional drying, as in drought. (Webb, 1977; Luke & McArthur, 1978), but eucalypt regularly burn (Ashton, 1981). According to the pollen record eucalypts probably became dominant in the vegetation in about the Late Miocene.
The pollen of Proteaceae was abundant in the Palaeocene-Middle Eocene, decreasing in the Late Eocene, after which it is present in relatively low percentages for the rest of the Tertiary. The abundance of Proteaceae pollen in the record, representing up to 23 pollen types, is viewed as remarkable because the low amounts of pollen produced by Proteaceae of the present (Martin, 1978). It is believed the Proteaceae represented in the pollen record were probably rainforest trees. In modern northeastern Australian rainforests Proteaceae are relatively common, but their pollen contribution is low (Kershaw, 1970b). At about the end of the Eocene, many of the pollen species disappear from the pollen record. Another feature of the early Tertiary Proteaceous pollen species that is regarded as remarkable is their geographical diversity, whole suites being found in Western Australia (Stover & Partridge, 1982), and Queensland (Dudgeon, 1983). They have also been found in southeastern Australia. Some species are apparently restricted to the areas in which they were described, while others have been found to be common to all these areas.
In the Miocene, Asteraceae pollen first appears in the pollen record, continuing at low percentages until the Late Pliocene-Pleistocene, at which point it increased dramatically.
The Eocene saw the first appearance of the Poaceae in the pollen record, remaining rare until the Late Miocene-Pliocene, when it underwent local increases. In the Late Pliocene-Pleistocene it dramatically increased.
Throughout the early to mid-Tertiary the vegetation of the continent was rainforest or closed forest, with Nothofagus being common. The presence of wet sclerophyllous forests in the Middle-Late Miocene, which are characterised by a canopy of tall eucalypts with some rainforest taxa in the understory, and, as with other eucalypt forests, burns on a regular basis (Ashton, 1981), is believed to be indicated by the change to more abundant Myrtaceae, though some rainforest taxa remained present, and increased amounts of charcoal.
There was a minor resurgence of rainforest, that is thought to have probably only partially replaced the wet sclerophyll forests, in the Early Pliocene. This reassurance was short-lived, wet sclerophyll forests again becoming dominant in the Middle-Late Pliocene. The indications are that the vegetation was probably more open, as suggested by the increase of Poaceae and Asteraceae, and woodlands and grasslands/herbfield developed in the Late Pliocene-Pleistocene. Both temporal and spatial variation occurs within these general vegetation types.
H.A. Martin in Hill, Robert S., (ed.), 1994, History of the Australian Vegetation, Cambridge University Press.
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