Download Which features make tropical rainforests unique among

Tropical rainforests
Which features make tropical rainforests unique among ecosystems? Why are most tropical
soils red?
Metres
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A Discontinuous canopy of tree
crowns of the tallest trees
(called emergents)
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B Continuous layer of the main
canopy formed by the crowns of
the many tall trees
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C Discontinuous under-canopy of
trees between 10m and 20m high
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understanding GCSE GeoGraphy
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D Layer of shrubs and young trees
E Herb layer with ferns 6m or
more high
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Figure 1
Tropical rainforest – forest structure.
Characteristic features
A
The vegetation mass is the greatest of all ecosystems. Despite the sheer
amount of vegetation present, and the way in which climbing plants and
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creepers, known as lianas, run from tree to tree in a chaotic manner,
it is possible to recognise five distinctive forest layers (Figure 1). The
canopy provides a habitat for monkeys and numerous birds such as
macaws; on the ground are some larger animals such as jaguars, tapirs
and anteaters in the Amazon Basin.
The tall trees are deciduous, but they shed their leaves at different times
and for only six to eight weeks each year, so that the forests always look
green. The emergents (tallest trees), by reaching up to 50 metres high,
stand head and shoulders above the forest canopy; these are hardwoods
and include types such as mahogany and ironwood. They have long
trunks without any branches until their rounded crowns extend out over
the canopy. Their leaves are oval in shape with extended points known
as drip tips, and they have dark green and leathery upper surfaces. The
smooth bark is thin. Their shallow roots, which mainly extend sideways
below the ground surface, extend above the ground as buttress roots.
Epiphytes, parasitic plants growing on trees and tree branches, increase
the abundance of vegetation.
B
Adaptations to climate
The tropical rainforest’s biodiversity is a response to climate. There are
constant high temperatures, with a mean monthly average above 27°C,
accompanied by high solar light intensity. Rainfall is regular and high,
with more than 2000 millimetres falling during the year, which creates
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Figure 2
Tropical rainforest kapok tree
A: Above the canopy with many
epiphytes and lianas. B: Forest
floor showing buttress roots.
Plant communities are fiercely competitive. There
is ‘survival of the tallest’ as the tall trees are drawn
upwards by the heat and light, which is why leaf
growth is concentrated in the canopy. The leathery
upper surfaces of the trees’ leaves are necessary to
withstand the great power of the Sun’s rays. The
drip tips help the leaves to shed water during the
heavy rains.
In the lower layers of the forest, sunlight is in short
supply. Ferns are adapted to life on the forest floor
by having leaves that intercept a high proportion of
the light that reaches them. The shrub layer is sparse
because of lack of light, although shrubs quickly take
advantage of any gap in the forest canopy.
Soils
Figure 3 shows a latosol, which is the name given
to soils that form under tropical rainforest. They are
red or yellowish-red in colour throughout. They are
very deep soils, often 20–30 metres deep, compared
with the 2–3 metres for brown earths (page 68). The
black humus layer at the top is a narrow horizon of
organic material. The red and yellow colours below
it come from the oxides of iron and aluminium,
which remain in the soil after other minerals have
been washed out by leaching.
Figure 4
Figure 3
A latosol.
information
Biodiversity in Ecuador
A typical 5-hectare patch of rainforest contains:
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750 species of trees
1500 species of flowering plants
400 species of birds
150 species of butterflies
100 species of reptiles
The importance of tropical rainforests
It is their biodiversity that makes tropical
rainforests unique among ecosystems. This term
refers to the great number and variety of living
species, plants and animals; it is estimated that 50
per cent of the world’s 10 million species live in
tropical rainforests (Figure 4). The Earth’s genes,
species and ecosystems have evolved through
3000 million years and form the basis for human
survival. Wild varieties of plants are the basis for
new seeds for farmers to use; many of today’s
drugs, such as aspirin, are derived from plants.
When new combinations of genes are sought in
the future for new food crops or cures for diseases,
without the rainforests and their biodiversity, the
number of potentially useful species from which to
choose will be reduced.
Some environmentalists
emphasise the importance
of tropical rainforests as
suppliers of oxygen (the
‘lungs of the world’) and
carbon dioxide stores
(against the enhanced
‘greenhouse effect’).
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living world
humid conditions. There is no more favourable
climate on Earth than this for plant growth.
Some species of flowering plants even grow 40 metres
above the ground in the crowns of emergent trees.
Looking at the density and diversity of the vegetation
cover, you might think that it is growing from the
world’s most fertile soil. Nothing could be further
from the truth. Rainfall is much higher than evapotranspiration; even with the protection of the forest
canopy, there is the downward movement of rainwater
through the soil. Leaching washes organic material
and silica downwards and then out of the soil. The
only fertile part of a latosol is the narrow organic layer,
which is why the trees have shallow roots.
Activities
1 (a)Draw a labelled diagram to show the main
characteristics of tropical rainforest vegetation.
(b)Explain the great importance of climate in
determining these characteristics.
2 (a)Looking at Figure 3, draw a sketch of a latosol
and label three main characteristics.
(b) Are latosols fertile soils? Explain your answer.
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Tropical rainforests – sustainable uses
and management
Which groups make a living from the rainforests? Which groups want to destroy the forests? How
can the rainforests be conserved?
While natural deciduous forests have been cleared
almost to extinction in Europe and North America,
large areas of tropical rainforest remain in other
parts of the world, mainly because of obstacles such
as difficult access and infertile soils. Rainforests are
mainly located in the world’s poorer, less developed
countries (Figure 1, page 64). However, many of
these forests are now under threat as governments
view their untapped resources as a passport to
economic development and a way to reduce their
international debts.
Traditional uses
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Human settlement in rainforests is long established,
but often of low density. Indigenous (native) peoples,
living in tribes or groups, either collected and hunted
to use the food naturally available in the forest, or
practised ‘slash and burn’ and grew crops such as
manioc, or did both. With slash and burn, the clearing
was small and the group cultivated only for as long
as the soil retained its fertility (probably two or three
years). They then moved to another part of the forest,
hence the alternative label of ‘shifting cultivation’.
Indigenous tribes, low both in numbers and levels of
technology, barely left a mark on the forest, which
re-invaded within a few years as if there had been no
human presence. This is an example of sustainable
human use of rainforests.
Uses after forest clearance
Indigenous peoples are pushed back into smaller and
smaller areas of forest by the advances of loggers,
Figure 1
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Banana plants being grown in an area
cleared by indigenous people. How
great is the risk to the environment?
Figure 2
miners and farmers, each group supported by
superior modern technology. Direct contact results
in the spread of diseases to which they have no
resistance, and in the destruction of their traditional
culture and ways of life. Unlike them, outsiders
engage in activities that involve forest destruction.
The first forest clearances are usually associated
with road building. Roads attract farmers, loggers
and miners, enabling them to open up wider
areas of forest away from the roadsides. Farming
by outsiders is more likely to be cattle ranching
(Figure 2). The ranchers are interested in only one
thing – replacing forest with pastures. Often, they
do not even save and sell the valuable hardwood
timber and it is just burned (Figure 3).
Logging companies are only interested in certain
types of tree, but one of the characteristics of
rainforests is that individual species of tree are
widely dispersed. In order to reach the trees they
want, all the other trees are felled and cleared. Oil,
gas, iron ore, bauxite (for aluminium), nickel and
gold are just a few of the natural resources that have
attracted mining companies to rainforest regions.
Despite all the problems with access, once a mineral
deposit of commercial size is discovered, then
roads, railways and pipelines are built with little or
no thought for the forests or their inhabitants. In
remote locations, without government supervision
or environmental controls, mining operations,
disposal of untreated waste, leaks and spillages
cause land and water pollution.
Farming on land previously covered
by rainforest in Costa Rica (Central
America). How great is the risk to
the environment?
Figure 3
Timber from rainforest clearances
piled up and burned in Brazil. How
great is the risk to the environment?