Classification, species richness, and environmental relations of monsoon rain forest in northern Australia

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Abstract
A floristic classification for monsoon rain forest vegetation in the Northern Territory, Australia, is derived based on comprehensive floristic inventory and environmental data. Allied aims include relating the floristic classification to Australia‐wide structural and floristic schema, documenting species richness, and exploring site‐environmental relations. TWINSPAN classification and complementary DCA analysis of a data set comprising 1219 sites x 55 9 rain forest taxa yielded 16 floristic assemblages. A diagnostic floristic key to these groups is presented. Eight groups describe rain forests associated with sites of perennial moisture; eight groups are associated with seasonally dry landforms. The structural typology of Australian rain forests is found wanting when applied to relatively simple monsoon rain forest communities. Rain forest patches are mostly less than 5 ha in size; maximum species richness is ca. 135 species per patch. Two major environmental gradients are identified through indirect gradient analysis: a primary latitudinal‐moisture gradient and a subsidiary topographic‐drainage gradient. Given the demonstrated tolerance of monsoon rain forest to a broad range of environmental conditions, the question remains: why is this vegetation type so restricted in occurrence in northern Australia?