Resource Management and Geography - Theses
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ItemA geomorphological study of Spartina Townsendii (Sensu Lato) marshlands in Australia( 1981)The halophytic grass Spartina townsendii (s.l.) was widely introduced from England to Australia in the late 1920s and early 1930s, but has established successfully only in estuaries and lagoons in the south east of the country. It now occupies 620 ha, the major stations being the Tamar River, Tasmania, where the fertile form of the grass was planted in 1947, and Andersons Inlet, Victoria, where the same vigorous species was introduced as recently as 1962. Because of their comparative youth, Australian Spartina marshes are in a much earlier stage of development than many marshes in Britain, where S. townsendii (s.l.) was first collected in 1870. It has therefore been possible to examine in this country the initial and early stages of Spartina marsh formation. Particular attention has been given to Andersons Inlet, where the growth of Spartina and the accompanying transformation from mudflat to marshland have been observed from the time of marsh inception. By 1980, Spartina in Andersons Inlet occupied 63.6 ha. In favourable environments, Spartina cover in sample quadrats was approaching 100 per cent after 14 years from initial establishment of seedlings and young plants. Colonisation commenced within a narrow vertical range of 0.61 m to 0.83 m above Inverloch datum, but Spartina quickly expanded both seawards and landwards to its present vertical limits of 0.25 m and 1.13 m. The seaward limit of Spartina is now regularly submerged by both spring and neap tides for more than 6 hrs, while the landward limit is submerged by mean spring tides. Spartina has similar submergence limits at other Australian stations, and is most abundant in sheltered conditions regularly inundated by water of less than 18%o salinity. At many sites the spread of Spartina has been accompanied by rapid accretion of sediment, which is trapped and retained by plant foliage. Sediment deposition in Spartina at Andersons Inlet is generally greater than 2 cm per annum, and rates of up to 7 cm per annum have been recorded. The role of Spartina in promoting accretion may be apparent even before sward formation is completed, as indicated by the development of microtopographic highs coincident with Spartina clones and clumps. As a result of high rates of accretion, previously unvegetated mudflats are rapidly transformed into depositional terraces, of which six major types may be recognised. Their upward growth is accompanied by a change from non-channelised to partly channelised tidal flow, as creeks develop between Spartina tussocks and within continuous sward. Pans may form on the surface of the marsh as a result of deposition of tidal wrack, impeded drainage, or restriction of light beneath mangroves. Spartina thus acts as a geomorphological agent, by greatly accelerating the processes of marsh formation.