The Humid Tropical Ecoregions
8
8.1
400 Humid Tropical Domain
Equatorial and tropical air masses largely control
the humid tropical group of climates found at low
latitudes. Every month of the year has an average
temperature above 18
C, and no winter season.
In these tropical systems, the primary periodic
energy flux is diurnal: the temperature variation
from day to night is greater than from season to
season (see Fig. 4.2, p. 29). Average annual rainfall is heavy and exceeds annual evaporation but
varies in amount, season, and distribution.
Two types of climates are differentiated on
the basis of the seasonal distribution of precipitation. Figure 8.1 shows the global distribution
of these two types and Fig. 8.2 shows the
climographs for two stations in humid tropical
climates. Tropical wet (or rainforest) climate has
ample rainfall through 10 or more months of the
year. Tropical wet-and-dry, or savanna, climate
has a dry season more than 2 months long.
The circulation of the atmosphere controls the
temporal pattern of precipitation in the tropics.
Near the equator the trade winds of both
hemispheres converge to form a low-pressure
trough with a gentle upward drift of air. As mentioned before, this convergence zone is often
referred to as the intertropical convergence
zone, or ITC (see Appendix, Fig. 1, p. 142), as
it represents the zone in which the trade winds
from the north and south of the equator converge.
Where the converging air has a trajectory over
the ocean, it contains large amounts of moisture,
and cloudiness and frequent precipitation are
common. Daily thunderstorms and torrential
downpours predominate. Poleward of the zone,
the air subsides near the tropics of Cancer and
Capricorn and aridity results. The ITC shifts
north and south following the migration of the
vertical rays of the solar energy. This migration
produces the seasonal pattern of precipitation
which characterizes so much of the tropical
region.
The soils of the tropics include many large
areas that cannot sustain continued crop cultivation and midlatitude soils. Temperature and
moisture availability are high, with rapid chemical weathering. The weathered regolith is thus
quite deep. Soil profiles are often 3 m or more,
and evidence of chemical weathering has been
found as deep as 70 m.
Under these conditions, a process of soil
development called laterization takes place. In
the process iron, aluminum, and manganese form
soluble hydroxides which tend to concentrate in
the topsoil (Fig. 8.3). Highly enriched layers
of iron and aluminum hydroxides, known as
laterite, stain the soils reddish. Due to rapid
chemical decomposition and solution, the soils
are low in mineral nutrients. The topsoil contains
little of essential elements for plant growth. The
soils are also low in humus, because litter
decomposes quickly. Without fertilizers, these
soils can sustain crops on freshly cleared areas
for only 2 or 3 years, before the nutrients are
exhausted and the plot abandoned. This kind of
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_8, # Springer Science+Media, LLC 2014
81
8
8.1
400 Humid Tropical Domain
Equatorial and tropical air masses largely control
the humid tropical group of climates found at low
latitudes. Every month of the year has an average
temperature above 18
C, and no winter season.
In these tropical systems, the primary periodic
energy flux is diurnal: the temperature variation
from day to night is greater than from season to
season (see Fig. 4.2, p. 29). Average annual rainfall is heavy and exceeds annual evaporation but
varies in amount, season, and distribution.
Two types of climates are differentiated on
the basis of the seasonal distribution of precipitation. Figure 8.1 shows the global distribution
of these two types and Fig. 8.2 shows the
climographs for two stations in humid tropical
climates. Tropical wet (or rainforest) climate has
ample rainfall through 10 or more months of the
year. Tropical wet-and-dry, or savanna, climate
has a dry season more than 2 months long.
The circulation of the atmosphere controls the
temporal pattern of precipitation in the tropics.
Near the equator the trade winds of both
hemispheres converge to form a low-pressure
trough with a gentle upward drift of air. As mentioned before, this convergence zone is often
referred to as the intertropical convergence
zone, or ITC (see Appendix, Fig. 1, p. 142), as
it represents the zone in which the trade winds
from the north and south of the equator converge.
Where the converging air has a trajectory over
the ocean, it contains large amounts of moisture,
and cloudiness and frequent precipitation are
common. Daily thunderstorms and torrential
downpours predominate. Poleward of the zone,
the air subsides near the tropics of Cancer and
Capricorn and aridity results. The ITC shifts
north and south following the migration of the
vertical rays of the solar energy. This migration
produces the seasonal pattern of precipitation
which characterizes so much of the tropical
region.
The soils of the tropics include many large
areas that cannot sustain continued crop cultivation and midlatitude soils. Temperature and
moisture availability are high, with rapid chemical weathering. The weathered regolith is thus
quite deep. Soil profiles are often 3 m or more,
and evidence of chemical weathering has been
found as deep as 70 m.
Under these conditions, a process of soil
development called laterization takes place. In
the process iron, aluminum, and manganese form
soluble hydroxides which tend to concentrate in
the topsoil (Fig. 8.3). Highly enriched layers
of iron and aluminum hydroxides, known as
laterite, stain the soils reddish. Due to rapid
chemical decomposition and solution, the soils
are low in mineral nutrients. The topsoil contains
little of essential elements for plant growth. The
soils are also low in humus, because litter
decomposes quickly. Without fertilizers, these
soils can sustain crops on freshly cleared areas
for only 2 or 3 years, before the nutrients are
exhausted and the plot abandoned. This kind of
R.G. Bailey, Ecoregions, DOI 10.1007/978-1-4939-0524-9_8, # Springer Science+Media, LLC 2014
81