Keeping in Touch: Vegetation Prehistory on Both Sides of the Tasman

Abstract

At the end of the Cretaceous New Zealand broke away from the Australian-Antarctic continental mass and was physically isolated by the Tasman Sea. Early in the Tertiary New Zealand moved a long way north relative to Australia, but with the rapid northward movement of Australia, starting in the Eocene, Australia overtook New Zealand, so that much of the South Island of New Zealand now lies south of Tasmania. The northward and relative movements of the two blocks provide an interesting framework for comparing the development of their vegetation. In the Late Cretaceous, New Zealand and Australia were physically attached and shared a flora dominated by podocarp and araucarian conifers and deciduous angiosperms, consistent with growth in a polar latitude with periods of winter darkness. When New Zealand broke away and moved north, a typically evergreen angiosperm-dominated flora developed. This showed similarities to the extant and fossil flora of the Australian mainland. To the south, Tasmania developed a quite distinct flora often dominated by conifers. In the Early-mid Miocene, when New Zealand lay at the same latitude as south eastern Australia, a change from Nothofagus dominated rainforest to, at times, drier vegetation including wet sclerophyll with Eucalyptus, occurred in both regions. This may record the roughly synchronous effects of more northerly tracking Sub Tropical High Pressure systems. In the Late Miocene-Pliocene there was a return to Nothofagus-podocarp dominance in both Australia and New Zealand. Today, the conifer-dominated communities of Tasmania have largely retreated to montane regions where they form dwarf shrublands, and have disappeared from the Australian mainland. In New Zealand the situation has quite reversed from that of much of the Tertiary, and conifers now form a prominent part of many rainforest communities. The evidence suggests Australia and New Zealand can be thought of as a single biogeographic entity, with the vegetation in both landmasses responding principally to climate change, with relatively free exchange, at least in one direction, of plants, rather than evolving in isolation since Late Cretaceous oceanic rifting.