Equatorial rainforest is a response to an equable, moist climate, which is continuously warm, frost-free, and has abundant precipitation in all months of the year (or, at most, only one or two dry months). A large water surplus characterizes the annual water budget, so that soil moisture is adequate at all times and the export of large amounts of stream runoff allows permanent removal of bases and silica from the soils of the region. In the absence of a cold or dry season, plant growth goes on continuously throughout the year. Individual species have their own seasons of leaf shedding, possibly caused by slight changes in the light period.
Variations in the equatorial rainforest structure are found in specialized habitats and where man has disturbed the vegetation. Where the forest has been cleared by cutting and burning (as for small plot agriculture or highways), the returning plant growth is low and dense and may be described as jungle. Jungle can consists of a tangled growth of lianas, bamboo, scrub, thorny palms, and thickly branching shrubs, constituting an impenetrable barrier to travel, in contrast to the open floor of the climax rainforest.
Coastal vegetation in areas of equatorial rainforest is highly specialized. Coasts, which receive suspended sediment (mud) from river mouths, and where water depths are shallow, typically have mangrove swamp forest, consisting of stilted trees. The mangrove prop-roots serve to trap sediment from ebb and flood tidal currents so that the land is gradually extended seaward. Mangroves commonly consist of several shoreward belts of the red (Rhizophora), the back (Avicennia), and the white (Laguncularia) mangrove. Another common coastal salt-marsh plant of the humid tropics is the screw-pine (Pandanus). Typical of recently formed coastal deposits are belts of palms, such as the cocoanut palm (Cocos nucifera). The principal world areas are: the Amazon lowland of South America; the Congo lowland of Africa and a coastal zone extending westward from Nigeria to Guinea; and the East Indian region, from Sumatra on the west to the islands of the western Pacific on the east. Poleward borders of these equatorial rainforest regions are, of course, transitional into rainforest of higher latitudes, particularly that found on tropical windward coasts and that of coastal monsoon rainforest belts of south and Southeast Asia.
Tropical rainforest is in many respects structurally similar to equatorial rainforest but has distinct difference imposed upon it by its location, which is on windward coasts, roughly from 10° latitude to the tropics of Cancer and Capricorn. Epiphytes are abundant because of continued exposure to humid air and cloudiness of the maritime tropical air masses, which impinge upon the coastal hill and mountain slopes.
In terms of global distribution, the tropical rainforest is represented on the world map by those areas of rainforest which lie between 10° and 25° latitude. The Caribbean lands represent one important area of tropical rainforest, although the rainforest is predominantly limited to windward locations.
In southern and southeastern Asia tropical rainforest is extensive in coastal zones and highlands, which have heavy monsoon rainfall and a very short dry season. The Western Ghats of India and the coastal zone of Burma have tropical rainforest supported by orographic rains of the southwest monsoon. In the zone of combined northeast trades and Asiatic summer monsoon are rainforests of coast of Vietnam and the Philippine Islands. In the southern hemisphere belts of tropical rainforest extend down the eastern Brazilian coast, the Madagascar coast, and the coast of northeastern Australia.
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