Australasia
Geography
Australasia is a region of Oceania comprising Australia, New Zealand, New Guinea and neighbour- ing islands in the Pacific Ocean. Here we exclude New Guinea, as it is treated within the biogeo- graphic region known as Malesia. Relevant to the distribution of conifers, Australasia therefore includes Australia with Tasmania (8.84 million km²), New Zealand (0.27 million km²) and the fol- lowing archipelagos of small islands and islands in the Southwest Pacific: New Caledonia, Vanuatu, Solomon Islands, Santa Cruz Islands, Fiji, Norfolk Island, and Tonga. A few other island groups in this region have no conifers. Tonga is the farthest archipelago into the Pacific where conifers occur naturally. All the islands with conifers are either parts of continental plates or accreted terrains, or have emerged on sunken lands (rises) that were once connected to continents. True oceanic islands in the Pacific, i.e. volcanoes that rose above the ocean surface from mid-oceanic plates, do not have conifers. The region divides naturally into the continent of Australia, mostly a desert land mass, and the islands, of which only New Zealand is of large size, comprising two large and a number of small islands. Other islands are small or at most of medium size (the larg- est, New Caledonia, is 18,576 km²). In this chapter, we will give accounts of conifer biogeography separately for Australia, New Zealand, New Caledonia and the other Pacific Islands.
Conifers in Australasia
Families Genera Species Species + infra Countries with Area of occupancy Endemic taxa conifers in km² 5/8 26/70 119/615 122/794 7/11 42,342/9,115,000 111
Numbers after ‘/’ are global or continent totals.
Map AU-1. Distribution of conifer species in Australasia [Tonga not shown, but see map AU-115 on p. 420]
The distribution map of Australasia shows clearly that conifer species are concen- trated on islands, especially Tasmania in Australia, New Zealand, New Caledonia and Fiji (see also map AU-2 on p. 342). In Australia, concentrations occur in the margins of the continent in Queensland, New South Wales, Victoria and SW West 340 australasia
Australia. The islands are all mountainous and receive high levels of rainfall with limited dry seasons. They are situated in the tropics and in the temperate zone of the Southern Hemisphere (Norfolk Island, New Zealand and Tasmania) with the coolest climate in Tasmania and Stewart Island (47° S). Australia extends from the tropics of northern Queensland to cool temperate Tas- mania, but it has a vast interior that is arid to semi-arid, in which only a single conifer, Callitris columellaris has a scattered occurrence. Rainfall is the limiting factor for conifer occurrence here, not temperature. At the species level of taxonomy, the island archipelagos and Australia all have unique conifer floras, i.e. with very few exceptions no species occur in more than one of the geographical units defined here. At the generic level there are numerous shared distributions, but also some unique occurrences. This indicates separation of these areas for long periods of geological history. Links at species level mostly occur with Malesia, in particular with New Guinea, reflecting the more recent proximity of the two regions.
Table 4. The distribution of 26 genera of conifers occurring in Australasia across 10 areas.
The genera are arranged by family in Table 4. The first three are in Araucariaceae. This family, in particular Araucaria, had a more global distribution in the Mesozoic and its Australasian distribu- tion can largely be seen as relict. No seeds could easily disperse across wide stretches of ocean, making dispersal events unlikely as explanations for current patterns of distribution. Agathis has winged seeds, so perhaps short distance “island hopping” may have been possible within archi- pelagos. It is at any rate the more widely distributed genus in the family, here occurring in six of the 10 areas, mostly on islands. Wollemia, increasingly recognized in the fossil record of the region since living plants were discovered near Sydney, New South Wales in 1994, is an extreme relict reduced to virtually one location. The Cupressaceae are represented by genera 4 to 9 and occur in Australia, Tasmania, New Zealand and New Caledonia, with most genera (and species) in Australia. This family is the only one to occur in both the Northern and Southern Hemispheres, but at least among living genera no southern representatives cross the equator. Tasmania has two genera, Athrotaxis and Diselma, not found elsewhere. The Podocarpaceae, with genera 10–24 (some researchers also include Phyllocladus in this family) is the most diverse and widespread family in Australasia. However, most are found on the islands, with only coastal Queensland with its strip of tropical rainforest giving mainland Australia two extra genera beyond Podocarpus.