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On a world scale, the appearance of vegetations varies principally
according to climate, i.e. temperature and precipitation.
In this way, Walter and Breckle (Ecological systems of the
biogeosphere, 1985) defined biomes, which are basically the
ecological equivalents of climate zones. In the tropics (defined
as regions with little annual variation in temperature, but
considerable variation in precipitation), three biomes are
distinguished: tropical rain forest; tropical savanna and
deciduous forest; and subtropical deserts. Within each biome,
orobiomes (vertical vegetation zones on mountains) and pedobiomes
(vegetation zones linked to particular soil conditions) create
variation in vegetation.
Another well-known biome classification system is the Holdridge
life zone classification scheme, which is principally used
in the Americas. This system considers temperature and climate,
especially as they affect the physiological activity of organisms.
Although many local variations exist, the complexity and
diversity of forests decline from low latitudes with a warm,
humid and stable climate towards areas where one or several
environmental factors become limiting (dry seasons, low temperatures,
salt water etc). This holds both in terms of latitude, going
north or south, as in altitude, going up the mountain. The
following general description of forest formations is taken
from the WRI
PAGE report .
| Tropical rainforest (incl. Tropical
lower and upper montane forests) |
Low latitude (ca 10º N to 10º
S), i.e., the equatorial and tropical zones. Continuously
warm, frost-free, abundant rainfall (>1800 mm annually).
Typically found in the Amazon lowland, central lowlands
of Africa, and a belt from Sumatra, Indonesia to the islands
of the western Pacific. Some extensions found poleward
along the monsoon and trade wind coasts. |
Tall, closely set trees form continuous
canopy of foliage, dense shade. Trees usually smooth-barked
and unbranched in lower two thirds. Multilayered crowns,
with tall emergent trees above closed canopy and lower
level of smaller trees. Broadleaf evergreen foliage. Forests
contain numerous lianas and epiphytes, mosses, lichens,
and algae. Heavy shade results in little vegetation on
forest floor. High temperatures and humidity cause rapid
decomposition of organic matter and low accumulation of
litter. Many trees compensate by developing mats of horizontal
roots, to capture nutrients. At higher elevations, rainforest
structure gradually changes to montane (cloud) forest.
Cloud forest characterized by lower tree height, more
open structure, trees are gnarled instead of smooth. |
Biodiversity: richest of all terrestrial
ecosystems. Up to 3,000 tree species in a few square kilometers,
and world's highest diversity of arboreal insects and
other invertebrates. High concentrations of rare, endemic,
and endangered animal and bird species. Carbon storage:
second biggest terrestrial carbon store. Soil maintenance:
tropical forest soils leach nutrients rapidly when forest
cover is removed. Water cycling: large tracts of forest
believed to regulate local hydrology and climate. Nonwood
products: food, medicines, fibers support numerous indigenous
peoples. Ecotourism: Major popular destination for ecotourists.
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| Tropical deciduous forest (including
moist tropical, dry tropical, and monsoon forest) |
Hot lowlands outside the equatorial
zone (ca 10º - 30º latitude). Rainfall more
seasonal; dry season more pronounced, especially in dry
tropical forests. Monsoon forest found in southern Asia,
Myanmar, Thailand, and Cambodia; also south-central Africa
and South America bordering the equatorial rainforest.
Note: monsoon forests now largely logged out in SE Asia.
Dry tropical forests also heavily logged, nearly vanished
in Indonesia. |
Canopy lower and more open than equatorial
rainforest. More light penetration and more understory
vegetation. Leaves shed in dry season to conserve soil
moisture. Most luxuriant form is monsoon forest, with
large-leaved foliage and dense undergrowth rich in bamboos.
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Biodiversity: less diverse than low-latitude
rainforest, but rich in many taxa. Up to 40 tree species
in small forest tracts. Timber: monsoon forests the source
of highly valued species such as teak. Woodfuels: major
source for rural and urban populations. Carbon storage:
major global store |
| Subtropical evergreen forest |
Moist, subtropical zones, with mild
winters and ample rainfall. Often at intermediate elevation
(montane forest). Broadleaf once covered extensive areas
of southern China and southern Japan. |
Broadleaf evergreen forests differ from
tropical rainforest in having relatively few tree species,
lower tree height, less dense canopy. Often have well-developed
lower vegetation layer, including tree ferns, small palms,
bamboos, lianas, and epiphytes. Oaks, laurels, magnolias
predominate in Northern Hemisphere, Southern beeches in
Southern Hemisphere |
Soil stabilization and water flow regulation:
montane forests particularly important in hydrological
cycle. Timber: broadleaf species provide valuable hardwoods.
Carbon storage: important store. Nonwood forest products:
major source. Note: Asian forests now largely cleared
for cultivation. |
Subtropical evergreen forest Moist, subtropical zones, with
mild winters and ample rainfall. Often at intermediate elevation
(montane forest). Broadleaf once covered extensive areas of
southern China and southern Japan. Broadleaf evergreen forests
differ from tropical rainforest in having relatively few tree
species, lower tree height, less dense canopy. Often have
well-developed lower vegetation layer, including tree ferns,
small palms, bamboos, lianas, and epiphytes. Oaks, laurels,
magnolias predominate in Northern Hemisphere, Southern beeches
in Southern Hemisphere Soil stabilization and water flow regulation:
montane forests particularly important in hydrological cycle.
Timber: broadleaf species provide valuable hardwoods. Carbon
storage: important store. Nonwood forest products: major source.
Note: Asian forests now largely cleared for cultivation.
Mangroves are a specific forest formation growing in coastal
areas of the tropical rain forests and tropical deciduous
forest biomes. Their root systems are regularly inundated
with saline water. Mangroves are among the most productive
terrestrial ecosystems. Although they are structurally and
compositionally very simple, their high productivity supports
numerous forms of wildlife as well as estuarine and near-shore
fisheries.
Most attention in the media goes out to tropical rain forests,
but there are many different kinds of tropical forest that
experience dry seasons, with names like tropical dry forest,
tropical deciduous forest, thorn forest, cerrado, and caatinga.
Their tree cover varies much more strongly than wet and moist
forests, from dense dryland forest to scrublike savannas.
Tropical dry forests cover c. 238 million ha in the world,
principally in Africa. (Source)
These forests generally harbour far larger human populations
than true rain forests and provide ranging land for cattle.
70 percent of the dry tropical areas are affected by desertification.
Because such forests/woodlands occur in more densely populated
regions than tropical moist forests, their disappearance is
likely to have a more severe impact on people living near
the forests.
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