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 degree; of latitude), was established from the monsoon tropics on the central north coast of the Northern Temtory (11 degrees S) to the central Australian desert on the South Australian border (26 degrees S). On this transect, mean annual rainfall was found to have a negative exponential decay with latitude, with arid conditions commencing at around 18"s-the limit of the monsoonal rains. The mean elevation of each cell steadily increased from the north coast to reach a maximum average elevation of 700 m at around 23 degrees S. The mean alphadiversity (quadrat species richness), and mean beta-diversity (turnover of species along an environmental gradient) was determined for each of the 15 cells by sub-sampling a large 20 X 20 m quadrat data set (N > 2000) collected during the course of the Northern Territory 1:10 to the power of 6 vegetation mapping program. It was found that there was little within-cell variation of beta-diversity of woody species which occurred in at least five quadrats, as approximated by the first axis of a detrended correspondence analysis (DCA) of these data. The mean first axis DCA scores were strongly correlated with latitude (r = 0.99); thus, there is no evidence for a floristic disjunction in the composition of common woody species between the monsoon tropics and desert. Mean alpha-diversity had a bimodal distribution on the latitudinal transect, with the maximum mean quadrat richness in the monsoon tropics and a second smaller peak occurred in central Australia, with the lowest 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-Wiener diversity of 1:10 to the power of 6 vegetation map units on the transect. It was found that 81% of the variance of mean alpha-diversity was explained by mean annual rainfall and mean elevation for the 15 cells. The increase in mean alpha-diversity in central Australia appears to be related 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 unknown whether the woody species diversity patterns are in equilibrium with the prevailing climate. More data on the palaeo-environments of the Northern Territory are required to answer this question.