Environmental correlates of tree species diversity in Stage III of Kakadu National Park, northern Australia

A quadrat based survey that sampled across the environmental range of the geologically and topographically diverse Stage III of Kakadu National Park, Northern Territory, revealed that the region has a tree flora dominated by the family Myrtaceae, and the genus Eucalyptus in particular. Principal components analysis (PCA) defined three axes of environmental variation: site rockiness, site hydrology and surface soil clay content. The three PCA axes were divided into halves and a 2x2x2 matrix was created to classify eight environments; however, there were quadrats in only seven of the eight possible environments. Kruskal-Wallis one-way ANOVAs showed that there was significant variation of the following variables between the 7 environments: total basal area, tree species richness, proportion tree richness composed of eucalypt species, and proportion of eucalypt richness composed of the four subgenera Blakella, Corymbia, Eudesmia and Symphyomyrtus. Of the most abundant 25 tree species and other common eucalypts only five species (Allosyncarpia ternata, Eucalyptus bigalerita, E. clavigera, E. foelscheana and E. jacobsiana) did not have significant differences in their mean basal area between the seven environments. The above patterns are interpreted as evidence that the savanna is a highly evolved system rather than the product of geologically recent disturbance to a once widespread monsoon rainforest.