DOES LOW-TEMPERATURE CAUSE THE DOMINANCE OF ACACIA ON THE CENTRAL AUSTRALIAN MOUNTAINS - EVIDENCE FROM A LATITUDINAL GRADIENT FROM 11-DEGREES TO 26-DEGREES SOUTH IN THE NORTHERN-TERRITORY, AUSTRALIA

A latitudinal gradient, from the central north coast of the Northern T erritory (11 degrees S) to the South Australian State border (26 degre es S), was defined to subsample a large 20 m x 20 m quadrat data set ( N>2000 quadrats) collected during the course of the Northern Territory 1:10(6) vegetation mapping programme. The mean and standard error of a range of environmental data, and structural and floristic variables pertaining to woody species were calculated for fifteen cells (3.5 deg rees E wide and 1 degrees S long) on the transect using a total of 105 0 quadrats. It was found that the interrelated measures of mean canopy height, mean canopy cover and mean total basal area steadily declined from the north coast to reach their minimum levels at 18.5 degrees S. There was little variation in these variables south of this latitude. This pattern is probably controlled by precipitation given that there is a highly significant (r(2) = 98%) negative exponential decay of me an annual rainfall with latitude for ten meteorological stations on or near the transect, and that the southern limit of the summer monsoon rains is at about 18 degrees S. The mean percentage of the woody speci es quadrat richness attributable to Acacia species was found to increa se at around 18.5 degrees S. However, the mean Acacia basal area and t he percentage of the total basal area composed of Acacia basal area in creased at 21.5 degrees S. At this latitude the mean Eucalyptus basal area, the percentage of the total basal area made up of Eucalyptus spe cies, and the mean percentage of woody species quadrat richness compos ed of Eucalyptus species all decreased to minimum levels. South of 21. 5 degrees S mean landscape elevation ranges between 400 and 700 m abov e sea level while north of this latitude mean landscape elevation rang es 10-300 m above sea level. The combined effects of continentality an d environmental lapse rates result in a highly significant (r(2) = 82% ) negative exponential decay of mean July (winter) minimum temperature with latitude for the five climate stations on the transect for which data are available. Mean slope angle, rock cover, surface soil gravel content and surface soil clay content were unrelated to any of the ab ove patterns. It is suggested that the sharp change-over in Eucalyptus to Acacia dominance at 21.5 degrees S is related to minimum temperatu re, but this hypothesis requires testing with detailed ecophysiologica l studies. None the less, the local dominance of Acacia shilleyi at 16 .5 degrees S suggests that environmental history also may have a stron g influence on the contemporary latitudinal distribution of Acacia and Eucalyptus in the northern half of the Australian continent. Regretta bly, few data are available to evaluate critically the importance of l ong-term environmental change on current distributional patterns of Ac acia and Eucalyptus.