A Transition from Mangrove Forest to Freshwater Wetland in the Monsoon Tropics of Australia

Abstract

The coastal freshwater wetlands of western Arnhem Land in the monsoon tropics of the Northern Territory of Australia are subject to new and increasing pressures from a range of land uses. Wetland history is needed as a basis for management decisions. Radiocarbon dates and pollen analyses of samples from fifty surveyed sites on the Magela floodplain show mangrove vegetation encroaching as sea level rose from about 8000 BP to 6000 BP. The extensive Rhizophora forest established at that time lasted until about 3000 BP, when the sediments built up above the upper tidal limit for these mangroves. Avicennia and other mangrove genera became more abundant in the subsequent transition phase. The floodplain has been a freshwater wetland since about 1300 BP. High resolu- tion pollen analyses of contiguous 1 cm samples through the transition at two sites show parallel sequences of vegetation changes. Large-scale spatial and temporal stability of the two mutually exclusive ecosystems, mangrove forest and freshwater wetland, contains considerable small- scale variation. Maximum diversity and variability occurred during the time of least environmental stability in the transition phase. Vegetation change was discontinuous, with each major shift followed by variation around a new mean. A rise in sea level of 0.5-1.0 m could destroy the present freshwater wetland and allow some mangroves to return to the Magela.