Geoscience Reference
In-Depth Information
forests are less fertile and dominated by oxisols in the Neotropics, with more fertile
soils of more recent origin occurring in the Asian-Pacific region. African soils lie
somewhere in between the two regions. Specific soil types associated with large
expanses of dry forest such as the Chaco and caatinga of South America are dis-
cussed in Chapter 5.
Nutrient Cycling and Decomposition
Tropical soils contribute little to the cycling of nutrients within the Tropical Sea-
sonal Forest Biome. However, some soils provide needed phosphorus, magnesium,
and nitrogen. The larger contributor of nutrients is the forest vegetation and organ-
isms within the layer directly above the soil. Intense organic activity occurs in the
decaying material dropped from plants and dead organisms. The decomposing
layer rapidly increases at the start of the dry season when trees lose their leaves.
This layer decomposes slowly until the wet season, when decomposing becomes
rapid. The decomposition of dead plants and animals is undertaken by many
organisms, including insects, aerobic and anaerobic bacteria, and fungi. They facil-
itate the conversion of unusable organic and inorganic compounds into useable
nutrients available for plants.
The uptake of this decomposed matter is facilitated by roots and their accompa-
nying beneficial fungi. The roots of many tropical seasonal forest trees are more
extensive than their rainforest counterparts. Most of the root biomass occurs as fine
roots found near the surface and at depths. They form networks with the fungi that
rapidly absorb the nutrients and make them available to plants. The fine roots
decrease during the dry season and grow again when the rain begins.
Tropical soils of the seasonal forest are complex and varied. Some are acidic,
older soils with low fertility shaped by millions of years of constant heat and rainfall,
others are infertile alluvial soils, and still others are fertile, younger soils created by
years of sedimentation or volcanic activity. These richer soils have largely been used
for agriculture, supporting the large human populations present in these regions.
Vegetation
Tropical seasonal forests vary by type and location. A diversity of forest structures,
including variations in height, canopy layers, and density of trees are evident
within the biome. Seasonal forests closer to the Equator are closed forests of semi-
evergreen or largely deciduous trees. Other seasonal forests may be smaller and
simpler in structure than these deciduous forests, with fewer canopy layers and more
drought-resistant evergreen trees. Forest stratification, seasonal rainfall, and patchy
distribution of soil moisture influence the type of forest and plant diversity in the
Tropical Seasonal Forest Biome.
As the amount of annual rainfall in tropical areas fall below 78 in (2,000
mm), the number of woody deciduous plants increases. Deciduousness is a
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