PATTERN AND CHANGE IN AN ACACIA-ANEURA SHRUBLAND AND TRIODIA HUMMOCK GRASSLAND MOSAIC ON ROLLING HILLS IN CENTRAL AUSTRALIA

The environmental correlates of a mosaic of Acacia aneura F.Muell. ex Benth. shrublands and Triodia hummock grasslands on rolling conglomera te hills in central Australia were explored by indirect gradient analy sis. A non-metric multidimension scaling ordination, based on the pres ence or absence of plant species, clearly separated the A. aneura shru bland from the Triodia hummock grassland; there were few intermediate quadrats. The A. aneura shrublands occurred on relatively deeper skele tal soils than the Triodia hummock grasslands. Unlike unidirection suc cessional gradients from frequently burnt, treeless vegetation to very infrequently burnt forest vegetation, the ordination presented here i s unusual because the quadrats with the greatest cover of A. aneura an d Triodia occur on the poles, and quadrats from the most recently burn t vegetation and with the least cover of A. aneura and Triodia occur i n the centre of the ordination. Interpretation of aerial photography t aken in 1950 and 1987 showed that there has been no statistically sign ificant change in the coverage of these two communities over that time period. Some quadrats that contained Triodia on the boundary of A. an eura shrublands had numerous charred A. aneura stumps which was interp reted as indicating some retreat of the shrublands. Experimental studi es are required to determine the stability of the current pattern. Bio geographic evidence, however, shows that the spatial distribution of b oth communities has fluctuated at a continental scale through geologic al time.