Structure and Alpha Biodiversity of Major Plant Communities in South Africa, a Close Biogeographical Relation with Australia

Abstract

During the short season of annual shoot growth, Foliage Projective Covers (FPCs) in both overstorey and understorey strata, coupled with leaf attributes (Leaf Specific Weight, Leaf Area), are determined by aerodynamic fluxes flowing over and through the plant community. These parameters determined the Evaporative Coefficient — the monthly ratio of actual to potential evapotranspiration per mm of available soil moisture. Maximal FPC is produced in habitats supplied with optimal soil moisture during this short season of shoot growth.

Vertical growth of foliage shoots — the number of leaves per shoot and internode lengths — is determined in relation with available soil water and soil nutrients (especially phosphorus and nitrogen ions) during this seasonal growth. Ambient temperature is similarly concerned.

The Leaf Area Index (area of all leaves per hectare) determines the interception of solar radiation — and hence net photosynthesis per hectare per annum. This energy-fixation by overstorey foliage determines the yearly growth of the producer, consumer and decomposer sections of the ecosystem. In turn, the biodiversity (number of species per hectare) of producers and resident consumers is determined.

Wherever optimal soil water occurred in median-nutrient soils in the coastal subtropical region, closed-forests/scrubs developed containing 131 ± 11 species compared with 80-90 species per hectare in nearby open-forests.

Wherever optimal soil water occurred in nutrient-poor soils in the warm temperate region, closed-heathland (± tall proteoid shrubs) vegetation developed containing 90-135 species per hectare, compared with about 60 species per hectare in nearby open-heathlands.