Year

1999

Degree Name

Doctor of Philosophy

Department

School of Geosciences

Abstract

Palaeoecological records for semi-arid and arid environments of Australia are limited to poor preservation of material in this environmental setting. A s a consequence, a Holocene vegetation and climatic record for a large part of the continent is incomplete. Leporillus spp. (stick-nest rat) middens provide a wealth of palaeoecological information for Holocene environments in areas where such records are rare. Eighteen middens from three key sites in the Flinders Ranges (Arkaroola-Mount Painter Sanctuary, Mount Chambers Gorge and Brachina Gorge), were investigated in this project to provide a thorough spatial and temporal coverage of palaeoecological sites. Issues of midden taphonomy, temporal resolution of pollen and macrofossil evidence, refining the interpretation of palaeoecological records from middens, and reconstructing palaeovegetation and climates during the Holocene, are dominant themes of this research.

Modern pollen rain at study sites was investigated to provide the foundation for interpretation of palaeoecological evidence from the stick-nest rat middens. A study of the regional modern pollen rain along a west-east transect in the central and northern Flinders Ranges reflected the high spatial variability in vegetation communities from semi-arid rocky upland environments. Local environmental conditions at individual midden sites (aspect and degree of exposure at cave sites and local vegetation cover) were significant factors in determining the composition of the modern pollen rain at individual midden sites.

Accelerator Mass Spectrometry (AMS) radiocarbon dating was used to examine the taphonomy of middens and refine the temporal resolution of these deposits. The majority of middens were found to have been deposited rapidly. Temporal resolution of middens can be complex, however in most instances, there were contemporaneous ages of different components in the middens (leaves, pollen concentrates and faecal pellets). AMS dating provided reliable ages for individual middens.

Vegetation taxa were better represented by pollen rather than macrofossils in the midden assemblages as evidenced by MacrofossihPollen Index values. Macrofossils are regarded as a secondary line of evidence in these midden records. They can verify the local occurrence of a taxon, whose source (local or regional) may not be distinguished solely in the pollen record.

Pollen and macrofossil records suggest that woodland and shrubland communities with an understorey of herbaceous taxa and grasses were dominant around 7 000-5 000 yrs B.P in the northern ranges, and shrublands with an understorey of herbaceous taxa and chenopods were dominant in the central ranges. This is indicative of wetter and warmer conditions than present during this part of the Holocene. Shrubland communities declined in the central ranges while persisting in the north from 4 000-2 000 yrs B.P, to be replaced by chenopod shrublands with a less diverse herbaceous component in the understorey with increasing aridity into the Late Holocene. Chenopod shrublands continued to increase from 1 000 yrs B.P to the present in the central ranges. Midden records have provided evidence for a shift between transitional shrubland communities to chenopod shrublands in the central ranges. T o the north, woodland and shrublands remained throughout the Holocene. Topographically buffered vegetation in the northern ranges is more resilient than central ranges vegetation to climate change.

Change in vegetation communities was more visible in the central ranges, as a result of different environmental conditions more sensitive to changes in effective precipitation. This highlights the complexity of factors that affect plant distributions in semi-arid environments. Underlying the climatically driven response is the role of biogeographic parameters that influence the diversity and structure of different vegetation communities. In sheltered rugged topography, in the northern ranges, plant communities are more stable as evidenced by the occurrence of relict species and maintenance of woodland and shrubland communities throughout the Holocene even as conditions were becoming increasingly arid. The same climatic scenario of increasing aridity in the central ranges resulted in less stable vegetation communities that responded to cooler and drier conditions by shifting from dominantly shrublands to an extensive cover of chenopod shrublands. Present spatial variability in the vegetation being a feature of the last 1 000 yrs (and possibly longer in the central ranges), compared with more homogeneous conditions across the ranges from 7 000-5 000 yrs B.P, is certainly consistent with the argument that climatic parameters such as ENSO have become more variable during the Late Holocene.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.