SPECIES RICHNESS AND CANOPY PRODUCTIVITY OF AUSTRALIAN PLANT-COMMUNITIES

The species richness (number of vascular plants per hectare) of Australian plant communities (containing a mosaic of gap, regeneration, maturation and senescent phases) is correlated with the annual biomass productivity of the overstorey canopy. The annual production of leaves and stem in the canopy of the plant community is shown to be limited by the requirements of photosynthesis (particularly light and the availability of water) and the length of the growing season. The species richness of Australian plant communities is the product of the balance between the dominance of the overstorey and the response of the understorey to the shading of the overstorey. For all climatic regions and zones the species richness of the overstorey of the plant community is shown to be exponentially related to the annual shoot growth of the overstorey canopy, until the latitudinal or altitudinal tree line is reached. With latitudinal increase outside the tropics, overstorey canopies of forest communities absorb increasingly more of the incident solar radiation, markedly reducing the species richness of the understorey strata. In contrast, in these latitudes the overstorey of plant communities with widely spaced trees or tall shrubs will absorb far less solar radiation, thus enabling the species richness of the understorey to be maintained.