Environmental Relationships of the Structural Types of Australian Rain Forest Vegetation

Abstract

The physiognomic—structural features used to classify Australian rain forest vegetation into 20 structural types vary along different gradients. The high correlation demonstrated between the structural types and the climatic and edaphic factors enables the identificatin of habitat types which are defined by limits of mean annual temperature and rainfall, and soil mineral status and soil drainage. The physiognomic—structural features are arranged in a hierarchical table. The primary division into vine, fern, and moss forests corresponds with tropical—subtropical, warm temperate (submontane), and cool temperate (montane) thermal regions, respectively. The temperate forms are strictly evergreen. Further subdivision of the vine forests into evergreen (including associated sclerophyll forests) and raingreen (more of less deciduous) types is correlated with differences in soil mineral status as inferred from topography, depth of soil, and parent materials. The tropical forms are differentiated from the sub—tropical forms by a higher proportion of mesophyll than notophyll leaf sizes. Further differentiation of vine forests with complex, mixed, and simple structure is correlated with inferred soil nutrient status defined as eutrophic, mesotrophic, and enriched obligotrophic. Obligotrophic soils do not support rain forest vegetation. Climatic and topographic factors tend to outweigh soil nutrient availibility at the climate extremes of the monsoonal and cool temperate regions. The structure types are finally differentiated by differences in height of canopy closure, nature of emergents, or proportion of deciduous species, and are further correlated with variations in soil moisture availability and drainage, or local exposure in montane situations. A field key to the habitat types associated with the different structural types is given. The inter—relationships thus established enable the prediction of either the type of vegetation, climate, or soil once any two of these are known. Deviations from the inferred distribution of rain forest are related to the past influence of wildfires which favor the regeneration of eucalypts and other scle sclerophylls after the destruction of the mostly fire—sensitive rain forest species. Destruction by surface fires in the tropical—subtropical region, or by running—crown fires of the mass—ignition type in the temperate regiion results respectively in abrupt ecotones, or diffuse ecotones and the virtual absence of climax rain forest vegetation. The practical implications of the ecological relationships established are briefly discussed in relation to land use and conservation.