The rise and fall of the Podocarpaceae in Australia - a physiological explanation

This chapter outlines the history of the Podocarpaceae in Australia. The chapter also discusses changes in the distribution and diversity of this family. The Podocarpaceae are one of the oldest of the extant conifer families, with several Early Triassic species reported. The Australian fossil record suggests that, although conifers have been a major component of the southern hemisphere flora since the early Cretaceous, it was not until the Palaeogene, more than 80 million years later, that conifer diversity reached its peak in southern Australia. The most common conifer shoots from Australian Cretaceous deposits are narrow, imbricate foliage from genera within the Araucariaceae, Cupressaceae, and Podocarpaceae. The first appearance of podocarps with significantly bilaterally flattened shoots is in southern Australia during the early Cenozoic and it is probable that the podocarp genera with bilateral shoots first evolved under the low solar angle conditions experienced during the Cretaceous/Palaeogene. The morphological plasticity of these broad shoots allowed efficient light harvesting in the understorey as well as the ability to adopt a three-dimensional helical leaf arrangement in the canopy that is suited to harvesting high light intensities. The foliar convergence between broad-shoot podocarps and angiosperm broad-leaves appears to have enabled the persistence, and possibly the radiation, of conifer taxa bearing this shoot morphology as the Cenozoic forest structure in southern Australia apparently shifted from high latitude conifer/deciduous to evergreen broad-leaf forest.