Australasian and Oceanian flora

The bottlebrush Callistemon macroponctatus speckled yellow by its elongated stamens. It is the appearance of these elongated stamens, protruding beyond the petals, that led to this flowering shrub's common name.

The relationships between the plant species of Australia, New Zealand, and the Pacific Islands are very complex. Some 260 species have been recorded as common to Australia and New Zealand, while more than 50 species are shared between Australia and New Caledonia. Australia is estimated to possess some 1,500 genera and over 12,000 species.

The most distinctive feature of the Australian flora is the important gum-tree genus Eucalyptus (Myrtaceae), which is thought to contain over 440 species. Not all of these are confined to Australia. E. deglupta, for example, is known to occur naturally in the Philippines, New Britain, and New Guinea.

Eucalyptus species In Australia, species of Eucalyptus occupy a very wide range of habitats, with shrubby species, the branches of which originate on an underground tuberous stem, forming mallee scrub and trees completely dominating the forests of the winter-rain regions. Jarrah forest, so characteristic of areas with a Mediterranean-type climate (650–1,250 mm rainfall per year, with a summer drought), is the realm of E. marginata. In the wetter, southern regions, E. diversicolor forms karri forest, while in the drier Wandoo zone (500–625 mm rainfall per year), E. redunca is important. Where the rainfall is less than 300 mm per year, eucalypts are not usually found, although in areas with 300–500 mm of rainfall many species are found scattered through the landscape. The arid heart of Australia is characterized by members of the Chenopodiaceae, such as salt bush (Atriplex vesicaria) and the blue bush (Kochia sedifolia).

Other Australian species Next to Eucalyptus, the most distinctive Australian plants are the wattles, which belong to the genus Acacia, some members of the Proteaceae, the woody genus Casuarina (flowering plants with trees that resemble conifers), and the grass-trees of the Xanthorrhoeaceae.

The flora of New Zealand is much smaller than that of Australia, with about 1,725 species in some 345 genera. Plants that originated in New Zealand and have been introduced as garden plants elsewhere include Veronica (Hebe) species, Senecio greyii, and Fuschia procumbens. Perhaps the outstanding feature of New Zealand's vegetation is the southern beech forest, dominated by the southern beech genus Nothofagus. In the very humid fjord country of the southwest, these magnificent forests are similar to those formed by Nothofagus in Chile.

Another particularly interesting aspect of New Zealand's flora is the widespread naturalization of plants. Over 600 species are estimated to have been introduced to New Zealand, where they have since become more or less wild. Most of these introductions are common European plants, such as the cat's ear Hypochoeris radicata, and the daisy Bellis perennis. One North American plant, the Californian poppy (Eschscholzia californica), has also spread widely.

The small island of New Caledonia has no less than three endemic families, over 100 endemic genera, and some 400 endemic species, including endemic species of figs (Ficus).

The flora of Australasia, with its distinctive distribution patterns within the southern hemisphere, has for a long time attracted the attentions of plant geographers. The genus Nothofagus, for example, is a key genus in plant geography. Its present distribution, from island Southeast Asia through Australasia to the southern tip of South America, with a former distribution, according to the fossil record, that included Antarctica, is of vital importance in assessing the relative merits of continental drift, land bridges, and plant dispersal as explanations of disjunct or broken distributions in the flowering plants.