19.4.2  Pollen evidence indicating changes in vegetation

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Pollen is an enduring signature of vegetation type and is therefore a vital tool in analysis of climate change. As pollen and charcoal from fires wash into swamps and lakes, they are deposited in a time sequence that can be analysed by palynologists using cores drilled into the sediments. Fossil leaves recovered largely from surface outcrops are used to supplement these pollen data.

Figure 19.12 illustrates stark shifts in vegetation in central and western New South Wales particularly throughout the second half of the Tertiary. Increasing abundance of pollen from the families Myrtaceae, Poaceae (grasses) and Asteraceae reflect the increasing aridity and oceanic cooling which began about 15 million years ago during the Mid to Late Miocene. ‘Wet’ rainforests, dominated by Nothofagus, gave way to
myrtaceous woodlands and forests dominated by eucalypts. A relatively brief wet period about five million years ago (Late Miocene to Early Pliocene) saw the Nothofagus forests return temporarily at the expense of sclerophyllous species. These broad alignments of climatic events and vegetation mix support the view that climatic factors have driven evolution of major vegetation types throughout Australia, largely through variations in water supply and temperature.

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Figure 19.12 Summary of major changes in vegetation, fire history and precipitation over the past 40 million years in central and western New South Wales. Major pollen groups show there was a steep decline of Nothofagus forests around 15 million years ago with species of Myrtaceae (especially Eucalyptus spp.) taking their place as the dominant woody species. The wet sclerophyll forests that developed after the Mid Miocene were associated with an increasing abundance of grasses and herbs (families Poaceae and Asteraceae). Charcoal deposits became more prolific as the myrtaceous species expanded, demonstrating that fire was now a feature in the environment. A drier climate that prevailed after the Mid Miocene is believed to be responsible for a rise of sclerophylly. Note the brief return of Nothofagus forest about five million years ago during a brief wet period (Based on Martin 1986, 1998; reproduced with permission of the Linnean Society of NSW)

 

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Figure 19.13 Summarised palynological and charcoal data from Lynch's Crater in North Queensland showing major changes in vegetation types over the last 140 000 years, with a dramatic increase in charcoal deposition (thousands of particles per cubic centimetre) around 38 000 years ago. Vegetation types are: (1) rainforest conifers (e.g. araucarias); (2) other rainforest plants and (3) plants from sclerophyll forests. The arrival of Aboriginal people in the area has been cited as a reason for increased burning and development of sclerophyllous forests. Wetter rainforest returned 7000 years ago in spite of a long history of fires, possibly through climate change as the Last Glacial Maximum receded. (Based on Kershaw 1976; reproduced with permission of New Phytologist)

 

Other parts of the continent underwent the same broad trend towards sclerophylly during the Tertiary. However, climatically induced changes to flora within individual regions of Australia varied distinctly depending on the pre-existing vegetation mix and climatic range. For example, at different times until 120 000 years ago, northeast Queensland supported casuarinaceous forest (either Gymnostoma, a rainforest taxon, or species of the sclerophyllous family Casuarinaceae), araucarian rainforest (family Araucariaceae) and sclerophyllous woodland. The characteristically high rainfall of this region enabled ‘dry’ rainforest, dominated by araucarias, to persist after they vanished in drier regions and indeed sustains tropical rainforest to the present day. On the contrary, north-west Australia has been persistently drier than the northeast throughout the past 40 million years and has pollen records that reflect this history. Early Tertiary rainforests were replaced by casuarinaceous forests by the Late Miocene and more recently by grasslands and Acacia shrublands. The rarity of eucalypts throughout the past 40 million years in this part of the continent endures to the present day, with sclerophyllous shrublands rather than eucalypt woodlands dominating the arid landscape of northwest Australia. In central Australia, too, rainforest succumbed early in the Tertiary period to aridity, leading to sclerophyllous woodlands and finally in the Pleistocene to open shrubland with abundant daisies, grasses and chenopods (saltbush and bluebush).

Data relating to a more recent epoch have been collected from a number of sites in Australia including Lake George in New South Wales, Lashmar’s Lagoon in South Australia and Lynch’s Crater in Queensland (full details in Singh et al. 1981).

At Lynch’s Crater in North Queensland (Figure 19.13), conifers (e.g. araucarias and podocarps) and rainforest angiosperms became much less dominant about 40 000 years ago and were replaced by sclerophyll forests of eucalypts and casuarinas for almost 30 000 years. A return to rainforest species, this time predominantly angiosperms rather than gymnosperms, began about 10 000 years ago after the passing of the Last Glacial Maximum. Indeed, rainforests re-established on wet, fertile sites throughout southeastern Australia during this period of climate change (e.g. Tasmania, Barrington Tops in New South Wales). Pollen data also indicate increasing predominance of eucalypts in drier regions during the Holocene (see Dodson 1991), driven by climate change and associated fire regimes.

Increasing sclerophylly at Lynch’s Crater over tens of thousands of years (Figure 19.13) and in western New South Wales over tens of millions of years (Figure 19.12) both correspond with abundant deposition of charcoal. Water and temperature were certain contributors to vegetation change in both time scales but fire also played a role.

 

 

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