TYPES OF RAINFORESTS
Tropical rainforests merge into other types of forest depending on the altitude, latitude, and various soil, flooding, and climate conditions. These forest types form a mosaic of vegetation types which contribute to the overwhelming diversity of the tropics.
EQUATORIAL EVERGREEN RAINFOREST VS. MOIST FOREST
There are two major types of wet tropical forests: equatorial evergreen rainforests and moist forests, which includes monsoon forests and montane/cloud forests. Equatorial rainforests, often considered the "real rainforest," are characterized by more than 80 inches (2,000 mm) of rain annually spread evenly throughout the year. These forests have the highest biological diversity and have a well-developed canopy "tier" form of vegetation. Roughly two-thirds of the world's tropical wet forests can be considered the equatorial type. These forests are near the equator where there is very little seasonal variation and the solar day is a constant length all year round. The greatest expanses of equatorial rainforest are found in lowland Amazonia, the Congo Basin, the Southeast Asian islands of Indonesia, and Papua New Guinea.
PRIMARY VS. SECONDARY FOREST
Throughout this site, other books, and discussions about tropical rainforest, the term "primary forest" is used. Primary forest refers to untouched, pristine forest that exists in its original condition. This forest has been relatively unaffected by human activities. Primary rainforest is often characterized by a full ceiling canopy and usually several layers of understory. The ground floor is generally clear of heavy vegetation because the full canopy allows very little light, necessary for plant growth, to penetrate. Occasionally, when a canopy tree falls, a temporary "light gap" is opened in the canopy, allowing growth of floor and understory species. Primary forest is the most biologically diverse type of forest.
LOWLAND VS. MONTANE FORES
Lowland tropical rainforest refers to the majority of tropical rainforest, that is, forest which grows on flat lands at elevations generally less than 3,300 feet (1,000 m)—although elevation may vary. Lowland primary forest, often characterized by more than five forest tier levels, is usually taller and more diverse than montane forest. It has a greater diversity of fruiting trees; hence more animals specially adapted to feed on their fruits and more large mammals. Lowland rainforest is far more threatened than montane forest because of its accessibility, more suitable soils for agriculture, and more hardwoods valuable as timber. In many countries, virtually all lowland primary forest is gone, while montane forest still remains.
SEASONAL OR MONSOON FOREST
Monsoon forests are tropical moist or seasonal rainforests found primarily in Asia (India/Sri Lanka to China), West and East Africa, Northern Australia, and Eastern Brazil. In this type of forest there is a distinct cooler dry season and a distinct wet season. These forests tend to be less diverse and more dwarfed in terms of tree size in comparison to typical equatorial rainforests.
IGAPÒ FOREST
Igapò forest is rainforest that is regularly inundated for extended periods during the flood season (sometimes considered permanently flooded rainforest). The best known of such forests are found in the Amazon Basin where they make up about 2 percent of the total rainforest. Igapò forest trees are shorter than those of non-flooded forest because of the instability caused by the wet, poorly drained soils (hence it is sometimes known as "swamp forest") and characterized by certain tree species like Cecropia, Ceiba, and Mauritia palms (also known as the aguaje palm). Many igapò tree species have stilt roots and flying buttresses to lend structural support. Igapò forest is flooded (4-10 months of the year) and flooding is usually predictable. Fish play an important role in seed dispersal in this forest system.
VÀRZEA FOREST
Vàrzea forests are floodplain forests which flood seasonally. Unlike swamp forests, varzeà forests have relatively rich soils from the annual replenishment of nutrients from whitewater rivers. Because these forests are more suitable for agriculture than typical rainforest, they are some of the most threatened. Even in the Amazon where vast majority of such forests are found, vàrzea are disappearing rapidly for development.
HEATH FOREST
Heath forests are found on well-drained, sandy soils that are extremely nutrient-poor. These forests are characterized by certain tree species tolerant of the poor, acidic soil conditions and are considerably "stunted" in comparison with typical rainforests. More light reaches the forest floor making for dense tree growth. Heath forests, also known as blackwater or caatinga forests, are drained by blackwater rivers and are found primarily in the Amazon Basin (the Rio Negro drainage), but also in parts of Asia.
PEAT FOREST
Peat forest is found in small parts of Africa, northeastern South America, and large areas in southeast Asia (especially Borneo and Sumatra). These swamp forests appear in places where dead vegetation becomes waterlogged and accumulates as peat. The peat acts as a sort of sponge withholding moisture at times of little rainfall and absorbing monsoon rains. When peat swamp forests are drained for agricultural projects, they become highly susceptible to combustion. Under the dry el Niño conditions of 1997-98, thousands of fires raged in the peat swamps of Indonesia.
TERRA FIRME FOREST
Terra Firme literally means "firm earth" and refers to rainforest that is not inundated by flooded rivers. This forest is noticeably taller and more diverse (>400 species/hectare in some areas) than igapò or flooded forest. It is found only on dry, well-drained soils and is characterized by such species as Brazil nut trees, Rubber trees, and many tropical hardwood trees.
MANGROVE FOREST
Mangrove forest is found in silt-rich, saline (brackish water) habitats worldwide, generally along large river deltas, estuaries, and coastal areas. It is characterized by low tree diversity, almost exclusively mangroves, with a low broken canopy. Mangroves are evergreen trees and shrubs that are well adapted to their salty and swampy habitat by having breathing roots (pneumatophores) that emerge from the oxygen-deficient mud to absorb oxygen.
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