Humid tropical environments - Fundamentals of the Physical Environment - page 683

and leaching mean that the more soluble constituents are
totally removed. Iron and even silica may be mobilized.
These soils have a low cation exchange capacity and a
limited supply of bases such as calcium and potassium (i.e.
a low base status). They possess high iron oxide contents
which give red soil colours. In the FAO-UNESCO soil
classification scheme these soils are mainly ferralsols and
acrisols. They also include the ferricretes and laterites,
which are frequently defined as tropical forest soils.
Tropical rain forest vegetation is typically diverse
both in species composition and in structure. Whereas
temperate forests may contain only three or four tree
species per hectare, tropical forests often include as many
as 100 ( Figure 27.4 ). Studies in Amazonian Ecuador found
307 tree species within one hectare, though 76 per cent
of the species present had only one or two specimens.
In general, there is a positive relationship between mean
annual rainfall and the number of species. This has
advantages and disadvantages when exploitation of
tropical forests takes place. Most species are evergreen, and
those that are leafless for any period shed their leaves at
irregular intervals. There is no autumn in the sense of the
leaf-fall period of deciduous forests. Intense competition
and the diversity of plants lead to complex structuring of
the forest, with five or more strata recognizable and
a significant epiphytic component ( Figure 27.5 ). Species

300

Yanamono, Peru

250

200

Bukit Lagong,
Western Malaysia

150

Papua

Kade,
Ghana

100

Usonigbe,
Nigeria

50

0

2.0

0 0.5 1.0 1.5

Area (ha)

Figure 27.4 Species/area curves for tropical forest tree
species with a minimum diameter of 0.1 m in selected
locations.

Nutrient
cycling

Structure Micro-habitat features

Nutrient
rain

Exposed to full sunlight,
wind and marked diurnal
temperature change
Animal life almost
wholly birds and
insects

m

50

Emergent species
level

45

40

35

Epiphytic ferns
and flowers

Living plant
biomass

30

25

Dead plant
biomass in
branches

Canopy level
almost continuous cover

Main habitat of forest
animals

Some 70 - 80% of
incident light absorbed

Animals rarely descend
to forest floor

20

Herbivores

15

Many species bear
fruit and flowers
directly on trunk
and branches

Most animals descend
to forest floor for
short periods

Carnivores

10

Soil
fauna

5

Fungi

Small diurnal changes,
little wind, poor light,
slow growth

0

Figure 27.5 Vertical stratification of a tropical rain forest.
Source: After Collinson (1977)