Formation of, and succession on, Atriplex cinerea-induced dune ridges in the Entrance Point Scientific Reference Area, Wilsons Promontory National Park, Victoria

摘要

The Entrance Point Scientific Reference Area (lat 38 degrees 48 degrees S long 146 degrees 38' E) in the north-eastern corner of Wilsons Promontory, Victoria, is notable for numerous parallel sandy dune ridges covered with dense Leptospermum laevigatum scrub. As a result of some demanding field work in a relatively remote area, together with aerial photos, I have been able to observe the development of this dune topography and follow the vegetation succession from 1981 to 2000.After a period of erosion early in the 20(th) century, shore progradation and ridge formation resumed in the central section of the beach between Entrance and Hunter Points and later extended southward. The parallel ridges were found to be the result of successive berm and pool formation on the upper beach of the prograding shoreline. Berm overwash during spring and storm tides fills the depression behind the berm and after the tide has gone out leaves behind a long narrow pool. Buoyant Atriplex cinerea fruits present in the flotsam strand on either or both of the pool margins, depending on prevailing wind directions. After fruits have germinated, seedlings provide nuclei for sand accumulation. If they are sufficiently numerous, a dune ridge builds up along the outer pool margin while plants on the inner margin contribute to the consolidation of the previous ridge. As long as progradation continues, this process is repeated at intervals and results in the formation of series of parallel dune ridges.A small number of other species establish in the lee of the new ridge. Acacia sophorae with its fast growth and spreading habit, together with Olearia glutinosa contribute to stabilising the ridge. They provide protection for Leptospermum laevigatum, Myoporum insulare and Banksia integrifolia during their early growth. After about 20 years, Leptospermum laevigatum becomes the dominant element of the scrub as Acacia sophorae diminishes in vigour and eventually dies. Virtually no evidence, even on the oldest ridges, was found of candidate species to continue the succession and hence, Leptospermum laevigatum thicket needs to be regarded as the end of the succession on young dune ridges.Once the pool depressions are cut off from flooding by seawater, the groundwater freshens and a sward of Isolepis cernua and Samolus repens, often with Juncus kraussii and Isolepis nodosa on somewhat higher areas, becomes established. Flooding occurs during heavy rain and water may stay above the surface for several days.At the same time, the older ridges of the central section were attacked by erosion which left large amounts of tree debris on the upper beach. This erosion resulted in the formation of a spit which, in time, carried a series of successively younger ridges built on flotsam lines. Atriplex cinerea was again the dominant pioneer species, but in the 1990s the influence of *Thinopyrum junceiforme had become equally important. Succession on these ridges was similar to that of the ridges on the prograding beach, but scrub height remained somewhat lower. Saltmarsh vegetation, dominated by Sarcocornia quinqueflora, gradually established in the lower sections between the ridges. However, the impact of erosion which had provided the sediment for building the spit, gradually shifted northward and began to destroy what had come into existence only decades earlier.Beach progradation along the western entrance to Corner Inlet is fortuitous and linked to changing channel locations in the tidal delta, while the dominant role of Atriplex cinerea in incipient dune formation is a consequence of the relatively sheltered environment at the entrance. It could well be that this combination of environmental factors is rarely if ever duplicated elsewhere in Australia.