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Flowering trees in the rainforest canopy of Peru. (Photo by R. Butler)
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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
| | Total land area | | Total forest cover
2005 | | Primary forest cover
2005 | | Total
deforestation
1990-2005 | | Loss of
primary forest
1990-2005 |
| Country | | (1000 ha) | | (1000 ha) | | % of total
land area | | (1000 ha) | | % of total
land area | | %
of 1990
forest cover | | % of 1990
primary
forest cover |
| | | | | | | | | | | | | | |
| Bolivia | | 109,858 | | 58,740 | | 54.2 | | 29,360 | | 26.7 | | -6.5 | | -6.5 |
| Cambodia | | 18,104 | | 10,447 | | 59.2 | | 322 | | 1.8 | | -19.3 | | -58.0 |
| Congo | | 34,200 | | 22,471 | | 65.8 | | 7,464 | | 21.8 | | -1.1 | | -1.1 |
| Costa Rica | | 5,110 | | 2,391 | | 46.8 | | 180 | | 3.5 | | -6.7 | | -29.4 |
| Ghana | | 23,854 | | 5,517 | | 24.2 | | 353 | | 1.5 | | -25.9 | | 0.0 |
| Guatemala | | 10,889 | | 3,938 | | 36.3 | | 1,957 | | 18.0 | | -17.1 | | -17.0 |
| Malawi | | 11,848 | | 3,402 | | 36.2 | | 1,132 | | 9.6 | | -12.7 | | -34.5 |
| Malaysia | | 32,975 | | 20,890 | | 63.6 | | 3,820 | | 11.6 | | -6.6 | | 0.0 |
| Nigeria | | 92,377 | | 11,089 | | 12.2 | | 326 | | 0.4 | | -35.7 | | -79.0 |
| Senegal | | 19,672 | | 8,673 | | 45 | | 1,598 | | 8.1 | | -7.2 | | -9.2 |
| Thailand | | 51,312 | | 14,520 | | 28.4 | | 6,451 | | 12.6 | | -9.1 | | 0.0 |
| Brazil | | 851,488 | | 477,698 | | 57.2 | | 415,890 | | 48.8 | | -8.1 | | -9.7 |
| Colombia | | 113,891 | | 60,728 | | 58.5 | | 53,062 | | 46.6 | | -1.2 | | -1.5 |
| Côte d'Ivoire | | 32,246 | | 10,405 | | 32.7 | | 625 | | 1.9 | | 1.8 | | 0.0 |
| Liberia | | 11,137 | | 3,154 | | 32.7 | | 129 | | 1.2 | | -22.3 | | 0.0 |
| Madagascar | | 58,704 | | 12,838 | | 22.1 | | 10,347 | | 17.6 | | -6.2 | | -1.5 |
| Mexico | | 195,820 | | 64,238 | | 33.7 | | 32,850 | | 16.8 | | -6.9 | | -15.3 |
| Panama | | 7,552 | | 4,294 | | 57.7 | | 3,023 | | 40.0 | | -1.9 | | -18.4 |
| Papua New Guinea | | 46,284 | | 29,437 | | 65 | | 25,211 | | 54.5 | | -6.6 | | -13.7 |
| Peru | | 128,522 | | 68,742 | | 53.7 | | 61,065 | | 47.5 | | -2.0 | | -2.9 |
| Philippines | | 30,000 | | 7,162 | | 24 | | 829 | | 2.8 | | -32.3 | | 0.0 |
| Sri Lanka | | 6,561 | | 1,933 | | 29.9 | | 167 | | 2.5 | | -17.7 | | -35.0 |
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 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.
Chart showing percentage cover of world forests. Rainforests are a subsection of "Tropical forests".
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OTHER TYPES OF FOREST
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.
IGAPÒ FOREST
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Flooded forest in Brazil | more
Photo by R. Butler
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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.
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 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 [news and information on peatlands]
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 [news and information on peatlands]
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.
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.
Review questions:
- What is the difference between primary and secondary forest?
- True or false—Cloud forest is found in mountainous areas.
- True or false—flooding is common in the Amazon rainforest.
- Why are mangrove forests important?
- Why are mangrove forests being destroyed?
[print version | spanish | french | portuguese
| chinese | japanese]
Continued: Structure of the tropical rainforest
This article was written by Rhett A. Butler [bibliographic citation for this page] and was last updated on the most recent date listed in the column on the right side.
Other pages in this section:
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