|Types of Forest|
TYPES OF RAINFORESTS
Rainforests are found throughout the world, not only in tropical regions, but also in temperate 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 rainforests. 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 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.
Tropical moist forests are found at a greater distance from the equator where rainfall and day length vary seasonally. These forests get "only" 50 inches (1,270 mm) of rain annually and are markedly distinguished from equatorial rainforests 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 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.
Secondary forest is rainforest that has been disturbed in some way, 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.
Primary versus total forest cover for selected tropical countries
The Food and Agriculture Organization of the United Nations estimates that primary forests now account for 36 percent of total forest area, but are being lost or modified at a rate of 6 million hectares a year through deforestation or selective logging. Selective logging, where only one or two valuable tree species are harvested from an area, was recently found to be degrading forests in the Amazon twice as fast as deforestation figures indicate.
Scientists do not know how long it takes for secondary forest to attain the structure and levels of biodiversity of primary forest. A recent study by conducted as part of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) determined that trees in the Central Amazon may, on average, be several hundred years old, suggesting that primary forests take a long time to develop.
LOWLAND VS. MONTANE FOREST
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.
Tropical montane rainforest is forest that grows on mountains and above an altitude of 3,300 feet. High montane forest, above 6,600-10,000 feet (2,500-3,000 meters) in elevation, 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 sections of cloud forest by valleys and ridges. These species are prevented from migrating to other forest areas 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 epiphytes like orchids, bromeliads, and mosses. Many of these species are endemic to a single locality, like the Golden toad of Monteverde, Costa Rica, a species which is now believed to be extinct. Cloud forests generally lack an abundance of large-bodied mammals due to the small number of fruiting trees.
Tropical montane forests are among the most threatened of all tropical vegetation—especially in the South American Andean region, where much of the forest has been cleared for agriculture. Of the continent's endangered species, a disproportionate number of those are found in yungas, the regional name for tropical montane forests in the Andes. These forests have also been little studied.
Above 10,000 feet (3,300 m), cloud forest may give way to sub-alpine and alpine forest. These habitats have less rain, fewer trees, and reduced biodiversity compared with lower elevation forests.
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.
Monsoon forests are highly threatened worldwide by clearing for cultivation, especially in West Africa, where over 90 percent of the coastal rainforests and the monsoon forests have been cleared.
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.
Floodplain forests, especially those located on river banks and islands, are often short-lived due to the meandering nature of tropical lowland rivers which eat away at the forests' base. According to Amazon Headwaters, a book by Michael Goulding and his colleagues, research in Peru suggests that most floodplain forests are rarely older than 200 years and may have turnover rates exceeding 1.6 percent, implying an average tree life of 63 years. For this reason, floodplain forests are nearly always in some stage of succession with pioneer species like Cecropia being replaced with Kapok (Ceiba) and fig trees further away from the river.
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 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.
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 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.
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.
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 for local people and governments to undervalue the services they provide. A recent study by the Food and Agriculture Organization of the United Nations found that 20 percent of the world's mangrove forests have disappeared since 1980, mostly due to farming, harvesting for timber and charcoal, freshwater diversion, real estate development, and conversion for tourism.
According to the Environmental Justice Foundation, about 38 percent of global mangrove deforestation is linked to shrimp farm development. Mangrove clearing for commercial shrimp and prawn hatcheries is particularly prevalent in Southeast Asia. Ironically this form of aquaculture has come at the expense of the natural fish and shrimp hatchery.
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. Research conducted following the 2004 tsunami in Asia found that areas forested with mangroves suffered considerably less damage than areas without tree vegetation.
Mangrove forests are slow to recover from clearing and degradation. 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.
[full photo version]
Continued: Structure of the tropical rainforest
Bibliographic citation for this page
Other pages in this section:
|what's new | rainforests home | help support the site | madagascar | search | about | contact|
Copyright Rhett Butler 1994-2005