Environmental Relationships of Woody Vegetation Patterns in the Australian Monsoon Tropics

Abstract

Indirect gradient analysis was applied to 48 vegetation samples taken from a mosaic of woody vegetation at Berry Springs in the Northern Territory of Australia. Compositional variation among the samples was effectively summarised by a two-dimensional ordination by non-metric multidimensional scaling. Subsequent rotational correlation analysis revealed marked relationships between the vegetation pattern and edaphic variables which reflect two aspects of the moisture regime: water availability during the dry season and the degree of inundation during the wet season. Moisture availability is principally determined by topographic position, through its relationship with soil texture and water table depth. Poor drainage during the wet season appears to separate Melaleuca communities from those dominated by eucalypts. Shrubby and grassy open forests appear to be differentiated by the intensity of the winter drought. The grassy understoreys, which occur on upslope positions well above the water table, die off shortly after the end of the wet season thus providing fuel for fires. A closed Carpentaria forest, located on the slopes above a spring, was found to have relatively organic-rich, fertile, fine-textured soils, possibly reflecting the superior nutrient cycling of the closed forest compared with the frequently burnt surrounding open communities. We suggest that the dense evergreen vegetation presents a barrier to fires from the open communities. This would account for the greater proportion of woody, closed forest species that regenerate exclusively from seed. The fires in the eucalypt forests are of low intensity and plants have vegetative mechanisms to recover from damage. We conclude that the edaphically determined vegetation controls fire regime rather than the converse.