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Evolution of wetland habitats and vegetation associations on a holocene coastal plain, Southwestern Australia

Semeniuk, Christine (2002) Evolution of wetland habitats and vegetation associations on a holocene coastal plain, Southwestern Australia. PhD thesis, Murdoch University.

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This study takes a multi-disciplinary approach to the investigation of a genetically related suite of wetlands, the Becher Suite, on the Swan Coastal Plain in southwestern Australia. The wetlands occur on a vegetated beachridge plain which forms the surface of an accretionary cuspate foreland, the Becher Cuspate Foreland. The Becher Cuspate Foreland is the largest sedimentary coastal deposit on the southwestern Australian coast which, by nature of its formation, contains a 7,000 year Holocene history of sea level changes, shoreline and beachridge plain development, and climate history. When swales within the beachridge plain became waterlogged or inundated by a rising water table following coastal progradation and a gradually falling sea level, it signalled the commencement of wetland development. Because of the varying geographic relationship between the ground surface and the water table, wetland initiation across the beachridge plain was staggered, commencing circa 4,500 years BP and continuing to circa 600 years BP.

Wetland basins were filled with carbonate mud and peat, and over time, sedimentologic, pedogenic, diagenetic and hydrological processes created stratified wetland sedimentary sequences. An increase in the heterogeneity of wetland fill in response to regional climatic processes progressively influenced hydrological functions in and adjacent to wetland basins. The combined effects of a local and variable wetland stratigraphy and plant uptake and release on cation concentrations down profile in sediments, interstitial waters and groundwater, resulted in very localised hydrochemical signatures specific to the type of sedimentary fills, their evolutionary stage, and to their extant vegetation association. The Becher Suite wetlands demonstrate that less than a metre of wetland sediment not only sets the wetland apart from larger scale regional processes, but establishes a physical, chemical and biological system which evolves independently.

Vegetation across the range of wetlands in the study area exhibits an increase in complexity of pattern and form with increased age of wetland. In the youngest wetlands, the vegetation is uniform sedgeland. With increasing age, the pattern becomes concentrically zoned with two, three and four zones, comprising mixed sedgeland, herbland, shrubland, or low forest, and an outer zone of closed grass tree. The fluctuations in plant communities recorded over 10 years, as well as that recorded geohistorically in the pollen record indicate expansion and contraction of vegetation assemblages in response to fluctuating hydrological conditions within the wetland basins, and in the long term, in response to some climate forcing.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Ladd, Phil
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