The history of the vegetation and climate in southern Tasmania since the late Pleistocene (ca. 13. 000 - 0 BP
Creators: name: MacPhail, Michael Keith
Publisher: name: University Of Tasmania
Year: 2023

description: This thesis contains the results and conclusions of the first research in Tasmania using pollen analysis to elucidate the postglacial history of the vegetation, hence changes in the climate, of Tasmania since the late Pleistocene. In contrast with mainland Australia, latitude, size and insular nature, topography, climate and vegetation combine to make the State highly suitable for pollen analysis, but also difficult as regards practical implementation of the technique. Published surveys of Tasmanian biogeography are inadequate. Hence. the original data are preceded by reviews of the present-day Tasmanian climate, physiography and plant ecology and prehistory in addition to a discussion of the limitations of pollen analysis in palaeoenvironmental research in the Australian phytogeographic context. The conclusions from previous geomorphic studies of Tasmanian Pleistocene landforms have been tested using the fossil pollen data from southern Tasmania. To enable as objective an interpretation as possible of this fossil pollen data, the present-day pollen rains across Tasmania were studied via surface sampling and pollen trapping programmes. Despite vandalism of most pollen traps, sufficient data were gained to determine (i) the approximate representivity of phytosociologically important taxa, (ii) seasonal changes in the composition of the pollen rains, (iii) changes in the composition of modern pollen spectra with changes in the distance or composition of source vegetation types, and, (iv) the identification of a regional pollen rain. Fossil pollen sequences from Frenchmans Cap, Mt. Field National Park, Adamsons Peak and Lake Tiberias were found to sensitively record the postglacial vegetational history of western, central southern, far south-eastern and eastern Tasmania respectively. The individual plant successions recorded are ecologically consistent with known directions of change in the modern vegetation continuum dictated by gradients in temperature, precipitation and fire-pressure. Despite differences in altitude and geographic location of the core sites, regional parallelism exists between all pollen sequences. Geographic patterns in the Tasmanian vegetation and their relationships in time (based on eleven radiocarbon dates) since the late Pleistocene are broadly consistent with gradients in the environment at least sub-parallel to the modern, largely unidirectional, gradients in climate. It is concluded that late Pleistocene climates were markedly colder and probably drier than at present. A progressive reduction precipitation eastwards across the State may have resulted in 'glacial - arid' climates in eastern Tasmania. Late Pleistocene vegetation was virtually herbaceous: grasslands in eastern and central Tasmania, and either grasslands or sclerophyll heath and sedgelands in western and far southern Tasmania. A major rise in temperature (recorded globally) between ca. 12,000 - 10,000 BP, accompanied by rising precipitation totals, resulted in the expansion of arboreal taxa across Tasmania and upslope onto the mountains. Precipitation limitations in eastern Tasmania delayed the development of forests here until after ca. 9500 BP. There is no pollen evidence for any major shift in climate in southern Tasmania since this rapid rise in temperature. During the Holocene, plant communities on the mountains studied have developed from alpine sedgeland / fell-field, through alpine coniferous and sclerophyll heath or scrub, to subalpine forests dominated successively by Eucalyptus, Phyllociadus; Nothofagus cunninghamii and again Eucalyptus . (mixed forests of rainforest and sclerophyll spp. in western and far southern Tasmania). Eucalyptus has remained the forest dominant in eastern Tasmania and eastern regions of the Central Plateau. Both the individual vegetation histories and regional differences in vegetation imply, with varying degrees of probability, due to the possibility - chat the changes observed were due to Aboriginal fires, initially rapfd increases in precipitation and temperature towards, and a slower reversal from, an 'optimum' in which ‚Äöclimates were nor-! equable or slightly wetter and warmer than modern climatic averages, between ca. 8000 - 5000 BP. There is no definite evidence that the climatic timberline in Tasmania has been at altitudes above its present-day level. Structural changes in the Tasmanian vegetation since the late Pleistocene closely parallel those recorded at latitudes equivalent to Tasmania in New Zealand and Chile. This suggests that postglacial events in Tasmanian vegetation reflect changes in environment common to the middle latitudes of the southern hemisphere as a whole, rather than merely the parochial effects of the Tasmania Aboriginals.