Narrowly separated to the north, the island of New Guinea contrasts with most of Australia in many respects — it has a moist tropical climate leading to a predominance of closed forests. Except for extensive plains in the south, it is much more mountainous and the mountains are steep and high enough, even at such low latitudes, to rise above treeline.

Lowland Zone

Along estuaries mangrove forests are well developed and contain many species. Further inland on the plains, swamp forests are widespread, and in the south, where there is a distinct dry season, monsoon forests characterised by a proportion of deciduous trees occupy better drained page 227sites. In the driest areas fire is an important factor and an open forest of Australian aspect and relationships covers the landscape. Species of Eucalyptus and other genera of the Myrtaceae dominate in these forests. However, the most widespread forest type up to 700 m is a species-rich, evergreen rain forest. Strangling figs may occur as emergents over a main canopy of buttressed trees up to 45 m in height. Vines and epiphytes are common, the latter including ferns and many orchids. New Zealand shares only a few genera with these forests, for example, Agathis, Elaeocarpus, Dysoxylum and Beilschmiedia.

Montane Zone

The montane zone ranges from about 700 to 3200 m altitude. The lower montane forests reach up to an average altitude of 1700 m, are 20–25 m in height and are generally without emergents. Palms and tree ferns are uncommon.

Some of the tree genera shared with New Zealand are Elaeocarpus, Litsea, Weinmannia, Pittosporum and Schefflera. Shared vine genera are Parsonsia and Clematis.

In places dense stands of Araucaria cunninghamii and A. hunsteinii are emergent over lower montane forest species. The Araucarias establish in forest openings, but eventually give way to broadleaves in the course of forest succession.

'Oak forests', which are also found in the lower montane zone, generally on ridge crests, are dominated by relatively small-leaved, evergreen species of the oak-related genera Castanopsis and Lithocarpus. Oak forests are usually fairly open with few epiphytes and lianes. The rather sparse subcanopy includes tree ferns, Phyllocladus hypophyllus and species of Rhododendron and Vaccinium.

Mid montane forests, from about 1700 m to 2800 m, are moister than those at lower montane levels and have an abundance of terrestrial and epiphytic ferns, mosses and lichens. The commonest forest type at this level can be termed conifer broadleaf as conifers are frequent. They include species of Podocarpus, Dacrycarpus, Papuacedrus (related to Libocedrus) and Phyllocladus. Among broadleaf genera, species of Weinmannia are particularly frequent.

Species of Nothofagus of the 'N. brassii group' are an important component in these forests, particularly on ridge crests where pure or nearly pure stands of Nothofagus occur.

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We also recognise an upper montane conifer broadleaf forest in which liverworts and lichens are abundant on trunks and branches. Neither Weinmannia nor Nothojagus are present here.

Subalpine Zone

Unlike in New Zealand, Nothofagus does not extend up to the treeline. The subalpine zone from about 3200 to 3800 m is occupied by a low forest about 10 m high with emergents (mostly conifers) to 15 m. Again there is a mixture of conifers (Dacrycarpus, Papuacedrus and Phyllocladus) and broadleaves among which species of Tittosporum, Myrsine, Schefflera, Rhododendron and Vaccinium are prominent.

Alpine Zone

Above the subalpine forest and in frosty hollows within it are tussock grasslands similar in appearance to those of the New Zealand mountains. However, no species of the snowgrass genus (Chionochloa) are present. Instead the tussocks belong to such grass genera as Poa, Deschampsia and Deyeuxia, also represented in New Zealand and many other parts of the world. Scattered through the tussock grassland are several shrubs including species of Coprosma, Gaultheria and Parahebe and small trees of Olearia spectabilis. In places clumps of the fern Papuapteris linedris intermingle with the tussocks and at lower elevations a distinctive species of tree fern (Cyathea atrox) is scattered throughout. Above about 4000 m there is a short grassland with species of Festuca and Poa as well as a number of dwarf shrubs. In poorly drained places there are swamps and bogs: the swamps dominated by grasses and sedges; the bogs by cushion plants and small shrubs. The cushion plants include Astelia papuana, Oreobolus pumilio and Carpha alpina. The last species also occurs in Australia and New Zealand.

Above about 4200 m species of Styphelia, Parahebe and Drapetes form close mats on rocky sites with small grasses and species of Ranunculus, Parahebe and Potentilla on level areas and in hollows.

The alpine flora of New Guinea is not rich in species and has links with both north and south temperate regions. An unusually high proportion, 30 per cent, of the species are found outside New Guinea.