Types of Forest

TYPES OF RAINFORESTS

Rainforests are found throughout the world, not only in tropical regions, but also temperature regions like Canada, the United States, and the former Soviet Union. These forests, like tropical rainforests, receive abundant, year-round rainfall, and are characterized by an enclosed canopy and high species diversity, but lack the year-round warmth and sunlight associated with tropical rainforest. However this book focuses on tropical rainforests, and these are the only forest forms discussed here.

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 forest: equatorial evergreen rainforest and moist forest, which includes monsoon forest and montane/cloud forest. Equatorial rainforests, often considered the "real rainforest" is characterized by more than 6.5 feet 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.

Tropical moist forests are found at a greater distance from the equator where rainfall and day length vary seasonally. These forests get "only" 4-12 feet of rain annually and are markedly distinguished from equatorial rainforest by a cooler dry season. During this dry season, many trees shed some or even all their leaves, creating a seasonal reduction of canopy cover and allowing more sunlight to reach the forest floor. The increased sunlight reaching the forest floor allows the growth of vigorous understory vegetation not found in lowland equatorial forest. Such moist forest is found in parts of South America, the Caribbean, West Africa, and Southeast Asia especially Thailand, Burma, Vietnam, and Sri Lanka.

PRIMARY VS. SECONDARY FOREST

Throughout this book, 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 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.

Secondary forest is rainforest that has been disturbed in someway, naturally or unnaturally. Secondary forest can be created in a number of ways, from degraded forest recovering from selective logging, to areas cleared by slash and burn agriculture that have been reclaimed by forest. Generally, secondary forest is characterized (depending on its level of degradation) by a less developed canopy structure, smaller trees, and less diversity. Due to the lack of a full canopy, more light will reach the floor, supporting vigorous ground vegetation. "Jungle" is the term often applied to secondary forest with dense ground growth, but it is also applied to some tropical moist forests where seasonal variations permit thick ground growth.

This chart shows primary forest cover versus total forest cover for selected countries.

LOWLAND VS. MONTANE FOREST

Lowland tropical rainforest refers to the majority of tropical rainforest, that is, forest which grows on flat lands at elevations less than 3300 feet. 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 is 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.

Montane tropical rainforest is forest that grows on mountains and above an altitude of 3300 feet. Montane forest is often manifested as "cloud forest," forest that receives the majority of its precipitation from mist or fog that passes up from the moist, humid lowlands. The trees of cloud forests are typically shorter than those of lowland forest resulting in a less developed canopy. Nevertheless, cloud forest trees are heavily burdened with epiphytes that thrive with the abundance of moisture from the passing fog. Trees in places like the lower elevations of the Andes in Ecuador, Peru, Colombia, and Venezuela; Central America (Monteverde in Costa Rica in particular); Borneo (Mount Kinabalu); and Africa (Ethiopia, Kenya, Rwanda, Zaire, Uganda), are frequently green with dense moss and beautiful, often rare, orchids.

Patches of cloud forests tend to have many endemic species, because they are often isolated from other patches of cloud forest by valleys and ridges. These species are prevented from migration to other forest patches by these obstacles to the sides, by the lowland forest below, and by steep cliffs above. Cloud forests are home to an abundance of hummingbirds, frogs, and orchids, many of which are endemic to a single locality like the Golden toad of Monteverde, Costa Rica. Cloud forests generally lack an abundance of large-bodied mammals due to the small number of fruiting trees.

OTHER TYPES OF FOREST

SEASONAL OR MONSOON FOREST
Monsoon forest is tropical moist or seasonal rainforest 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 rainforest.

In West Africa, where over 90% of the coastal rainforest and the monsoon forest has been cleared, seasonal monsoon rains are plummeting in the drought-prone interior African nations like Mali and Niger. Monsoon forests are highly threatened worldwide by clearing for cultivation and hydroelectric projects.

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 forest is found in the Amazon Basin where it makes up about 2% 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. 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.

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. Fires in peat swamps are extraordinarily difficult to extinguish because the conflagration continues in the deeper layers of peat.

VÀRZEA FOREST

Vàrzea forests are floodplain forests which flood seasonally. Unlike swamp forest, 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 rainforest. 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.

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.

Mangrove swamps are home to numerous bizarre amphibious fish species like the mudskippers of eastern Africa to Australia and Anableps, the so-called four-eyed fish of the New World. Mudskippers are renowned for their preference for terrestrial haunts over aquatic realms. These fish spend more time on floating debris, tree toots, and plants than they do in the water where they only go to escape predators. Watching a group of Mudskippers reminds the observer of what our ancestors must have looked like when they first left the ocean for life on land. Mudskippers are highly intelligent fish that feed primarily on insects and crustacean.

A second amphibious species found in mangrove forests is the Anableps. a species widespread in the New World from Central America to northern South America. Most notable about its physical features is its double-lobed eyes which allow it to see both above and below the water line as it swims along the water surface. The Anableps too, regularly leaves the water to perch on tree roots and rocks.

Unfortunately, mangrove forests are some of the most threatened ecosystems on the planet because of their proximity to the ocean (prime resort/development property) and the tendency to see them as useless swamps full of all sorts of threatening creatures. Over the past twenty years, great swaths of mangrove forest throughout Southeast Asia has been cleared to create commercial shrimp and prawn hatcheries. Ironically this form of aquaculture has come at the expense of the natural fish and shrimp hatchery. Once mangrove forests are damaged, recovery can be very slow. For example, seismic lines only a few meters wide in the mangrove forest of Nigeria were still visible by air a decade after they were cut.

The destruction of mangrove forest has dire implications for the fisheries industry since these forests provide an important spawning ground and serve as a nursery for many commercially important species. In addition, mangrove forest protects coastal regions against storm damage and erosion.

Tropical Rainforests of the World
Types of Rainforest

Biogeographical Forest Realms