Monsoon forests in Northwestern Australia I. Vegetation classification and the environmental control of tree species

An indirect gradient analysis was performed on woody species basal area data collected from 144 monsoon forests situated throughout northewestern Australia. In order to eliminate edge effects each monsoon forest was differentiated from surrounding vegetation by inspection of sorted lists of tree species recorded from quadrats $(10 \times 20 m)$ placed on a transect that traversed each forest boundary. On each transect the monsoon forest quadrats were amalgamated to yield a single averaged sample of tree species basal area and environmental characteristics to represent each fo the 144 sampled forests. Surface soil physical and chemical characteristics were related to compositional patterns of tree species abundance data in one-, two-and three-dimensional Hybrid Multi-dimensional Scaling ordination solutions. The unidimensional ordination recovered a major environmental gradient where dry fertile soils, often on rocky slopes occur at one extreme and infertile waterlogged soils charcterize the other. Mean basal area of the fifty species with the highest importance values showed a continuously varying sequence of unimodal curves over the unidimensional ordination axis, although individual species distribution and abundance showed considerable variation on the gradient. Forest biomass, as measured by mean canopy height and mean total basal area, increased linearly over the gradient. With the exception of the dry extreme, stem density was relatively uniform across the gradient. Both the wet and dry extremes of the gradient have lower species richness per $200 m^2$ and fewer species with peaks of abundance. There appears to be a relatively sharp environmental and physiognomic discontinuity along the gradient as indicated by surface soil moisture content, and the cover of ferns and palm regeneration. This beak is consistent with a primary dichotomy of wet and dry forest formations as derived by other floristic and structural classifications of this vegetation. Of the common tree species (149), 53% of the dry forest species (125) only occurred in that community, while 29% of the wet forest species (83) were restricted to the wet forest.