Diversity Patterns of Woody Species on a Latitudinal Transect From the Monsoon Tropics to Desert in the Northern Territory, Australia

Abstract

A belt transect, made up of 15 contiguous cells (width, 3.5° of longitude; length, 1° of latitude), was established from the monsoon tropics on the central north coast of the Northern Temtory (11°S) to the central Australian desert on the South Australian border (26°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°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 × 20 m quadrat data set (N > 2000) collected during the course of the Northern Territory 1:10⁶ 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 1:10⁶ 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.