Dmjs. Bowman, DIVERSITY PATTERNS OF WOODY SPECIES ON A LATITUDINAL TRANSECT FROM THE MONSOON TROPICS TO DESERT IN THE NORTHERN-TERRITORY, AUSTRALIA, Australian Journal of Botany, 44(5), 1996, pp. 571-580
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.