Vegetation structure and biodiversity along the eucalypt forest to rainforest continuum on the serpentinite soil catena in a subhumid area of Central Queensland, Australia

The deep lateritic earths that cap the serpentinite outcrop in the Rockhampton - Marlborough area on the Tropic of Capricorn in Central Queensland have been eroded to expose the underlying ultramafic rock. Water-holding capacity of these nutrient-poor soils increases in a gradient from the skeletal soils to the deep lateritic earths and results in a continuum of structural formations from open-woodland to woodland to open-forest. A couple of closed-forest (rainforest) stands have developed where seepage into Marlborough Creek occurs throughout the year. Aerodynamic fluxes (frictional, thermal and evaporative) in the atmosphere as it flows over and through the vegetation influence the annual foliage growth in all strata in the continuum from skeletal soils to deep lateritic earths. The lateral growth of each plant is abraded so that the sum of the foliage projective covers of overstorey (FPCo) and understorey (FPCu) strata - that is [Sigma](FPCo [+] FPCu) - remains constant throughout the serpentinite soil catena. As more water becomes available in the soil catena, the mineral nutrient levels in overstorey leaves increase, making developing leaves more vulnerable to insect attack. Although the number of leaves produced annually on each vertical foliage shoot in the overstorey increases along the soil-water gradient, [Sigma](FPCo [+] FPCu) remains constant in all stands. The carbon isotope ratios (a measure of stomatal resistance) and leaf specific weights (LSWs) (a measure of the proportion of structural to cytoplasmic content in a leaf) of overstorey and understorey strata, however, are constant throughout the continuum. The well-watered rainforest pockets - where seepage occurs - form the end point of this serpentinite continuum. LSWs and carbon isotope ratios of the canopy trees are similar to those in the sheltered understorey in the eucalypt communities. A gradient of foliage attributes is observed from evergreen canopy trees (12 m) to subshrubs (2 m) in the sunlit life forms that compose the complex structure of the rainforest stands in the humid to subhumid climate of Central Queensland. As alpha diversity (number of species per hectare) is correlated with annual shoot growth per hectare, species richness along the serpentinite continuum is almost half that of nearby plant communities on medium-nutrient soils. The one to two eucalypt species per hectare are about a tenth of the number recorded on adjacent medium-nutrient soils.