There are numerous environments on the Pacific lands, each with its plant associations, that have become adapted to the natural conditions. Most widespread are the strandline or seashore association of trees, shrubs, vines, and herbs, some of which bear seeds or fruits capable of floating far in salt water without losing the power of germination. Other seeds have been carried by the wind and by birds. The strand flora occurs as a band along most shores and is the only type on low coral islands, but the density of the vegetation varies with the rainfall. For example, on rainy Palmyra Island it forms a jungle, but on dry Canton Island there are only a few coconut trees and bushy thickets. Trees growing along coasts include the mangrove on the tide flats and on the drier land the coconut (mostly artificially planted), screw pine (Pandanus tectorius), ironwood or she-oak (Casuarina equisetifolia), India barringtonia (Barringtonia asiatica), Indian almond (Terminalia catappa), the primitive palmlike cycad (Cycas circinalis), the tree heliotrope (Messerschmidia argentea), and the linden hibiscus (Hibiscus tiliaceus). The nipa palm (Nipa fruticans) grows naturally only in the western Pacific. Other common plants are shrubs, especially the half-flower (Scaevola frutescens), creeping vines like the beach morning glory (Ipomoea pes-caprae), and herbs. In all about fifty families are represented in the seashore flora, with the greatest number in the western Pacific and the fewest families on the more remote Oceanic islands. The paucity of the flora on low atolls is largely the result of there being only one environment. However, it is partly due to the salinity of the ground water and soil, which prevents the survival of any introduced plants except salt-tolerant ones. On the low islands and strandline of the high islands the coconut is easily the most valuable plant to man.
The high islands possess several plant zones above that of the shoreline, which usually include valley bottoms and low coastal plains, rainforests, grassy uplands, semi-arid leeward slopes, and, if high enough, summit bogs and rocky peaks, each of which has plant associations adapted to the conditions. The high oceanic islands may have many endemic plants that have evolved under special conditions and never have spread from their place of origin. For example, 80 per cent of the 2500 species of flowering plants in New Caledonia and 68 per cent of the 1000 species of New Zealand are endemic. About half of some 9000 flowering plants on the continent of Australia are endemic.
The virgin rainforest has a great mixture of species with some huge, dominant trees with flaring buttressed trunks, many smaller trees barely reaching the branches of the big trees, and a ground growth of seedlings, palms, bamboo thickets, rattan vines, pandanus, various shrubs, tree ferns, and herbs. On the forest margins, and along the stream banks, are such dense screens of verdure that they are difficult to penetrate, but travel through the forest itself is somewhat easier. The forest floor is in twilight, and here only shade-tolerant ground plants can grow well. There are many orchids and other epiphytes in the western Pacific area, but the orchids are few on the Oceanic islands. When cleared for primitive agriculture and then abandoned, a secondary forest springs up that is of jungle character. Repeated burnings of the forest may transform some areas to savannas covered with tall grasses. In the highest mountains the trees disappear at 8000 to 10,000 feet, and only grasses, stunted shrubs, and herbs continue to the summits or the snowline if such is present as on a few New Guinea peaks. It should be noted that much greater differences occur between the floras of the upper forests on the high islands than between the plants of the strandline, most of which are widely dispersed.