VARIATION IN THE COMPOSITION AND STRUCTURE OF TROPICAL SAVANNAS AS A FUNCTION OF RAINFALL AND SOIL TEXTURE ALONG A LARGE-SCALE CLIMATIC GRADIENT IN THE NORTHERN-TERRITORY, AUSTRALIA
Rj. Williams et al., VARIATION IN THE COMPOSITION AND STRUCTURE OF TROPICAL SAVANNAS AS A FUNCTION OF RAINFALL AND SOIL TEXTURE ALONG A LARGE-SCALE CLIMATIC GRADIENT IN THE NORTHERN-TERRITORY, AUSTRALIA, Journal of biogeography, 23(6), 1996, pp. 747-756
Variation in structural and compositional attributes of tropical savan
nas are described in relation to variation in annual rainfall and soil
texture along a subcontinental-scale gradient of rainfall in the wet-
dry tropics of the Northern Territory, Australia. Rainfall varies alon
g the gradient from over 1500 mm p.a. in the Darwin region (c. 12 degr
ees S) to less than 500 mm in the Tennant Creek region (c. 18 degrees
S). Soils are patchy, and sands, loams and clays may occur in all majo
r districts within the region. We utilized a large data set (1657 quad
ratsx291 woody species; with numerous measured and derived sample vari
ables) covering an area of 0.5 million km(2). Correlations between flo
ristic composition of woody species and environmental variables were a
ssessed using DCA ordination and vector fitting of environmental varia
bles. Vectors of annual rainfall and soil texture were highly correlat
ed with variation in species composition. Multiple regression analyses
incorporating linear and quadratic components of mean annual rainfall
and topsoil clay content were performed on three structural attribute
s (tree height, tree cover, tree basal area), and two compositional at
tributes (woody species richness, deciduous tree species richness). Tr
ee height declined with decreasing rainfall; cover, basal area, woody
species richness and deciduous species richness ail declined with decr
easing rainfall and increasing soil clay content. Regression models ac
counted for between 17% and 45% of the variation in the data sets. Var
iation in other factors such as soil depth, landscape position and rec
ent land-use practices (for which there were no data on an individual
quadrat basis) are likely to have contributed to the large residual va
riation in the data set.