DIVERSITY PATTERNS OF WOODY SPECIES ON A LATITUDINAL TRANSECT FROM THE MONSOON TROPICS TO DESERT IN THE NORTHERN-TERRITORY, AUSTRALIA

A belt transect, made up of 15 contiguous cells (width, 3.5 degrees of longitude; length, 1 degrees of latitude), was established from the m onsoon tropics on the central north coast of the Northern Territory (1 1 degrees S) to the central Australian desert on the South Australian border (26 degrees S). On this transect, mean annual rainfall was foun d to have a negative exponential decay with latitude, with arid condit ions commencing at around 18 degrees S-the limit of the monsoonal rain s. The mean elevation of each cell steadily increased from the north c oast to reach a maximum average elevation of 700 m at around 23 degree s S. The mean alpha-diversity (quadrat species richness), and mean bet a-diversity (turnover of species along an environmental gradient) was determined for each of the 15 cells by sub-sampling a large 20x20 m qu adrat data set (N >2000) collected during the course of the Northern T erritory 1:10(6) vegetation mapping program. It was found that there w as little within-cell variation of beta-diversity of woody species whi ch occurred in at least five quadrats, as approximated by the first ax is of a detrended correspondence analysis (DCA) of these data. The mea n first axis DCA scores were strongly correlated with latitude (r=0.99 ); thus, there is no evidence for a floristic disjunction in the compo sition of common woody species between the monsoon tropics and desert. Mean alpha-diversity had a bimodal distribution on the latitudinal tr ansect, with the maximum mean quadrat richness in the monsoon tropics and a second smaller peak occurred in central Australia, with the lowe st levels of alpha-diversity to the south of the limit of the monsoon rains. This pattern was mirrored by the mean number and mean Shannon-W iener diversity of 1:10(6) vegetation map units on the transect. It wa s found that 81% of the variance of mean alpha-diversity was explained by mean annual rainfall and mean elevation for the 15 cells. The incr ease in mean alpha-diversity in central Australia appears to be relate d to environmental heterogeneity associated with mountainous terrain. It is possible that the central Australian mountains are a refuge for plants that were more widespread during the last ice-age. It is unknow n whether the woody species diversity patterns are in equilibrium with the prevailing climate. More data on the palaeo-environments of the N orthern Territory are required to answer this question.