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

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.