Geoscience Reference
In-Depth Information
Soils
Soils tend to be old and nutrient poor, originating from ancient bedrock of the Pre-
cambrian shield. More than 55 percent of the soils in the African rainforest are clas-
sified as either oxisols or ultisols.
Forty percent of soils found in the Tropical Rainforest Biome in Africa are oxi-
sols. Oxisols in Africa have a characteristic deep yellow or red color due to high
iron oxide content caused by heat and heavy moisture. They are concentrated in
Gabon, DRC, Rwanda, Burundi, Liberia, Sierra Leone, and eastern Madagascar.
Other common tropical soils in the Afrotropical region are ultisols. They make
up about 16 percent of African rainforest soils and are red and yellow in color.
They are somewhat well drained and typically found on slopes. In Africa, ultisols
occur in the eastern Congo region, on forested zones of Sierra Leone, as well as in
parts of Liberia, and along a thin strip from C
ote d'Ivoire to Nigeria.
The other soil types are divided among entisols, inceptisols, and alfisols. A type
of entisols called psamments covers 15 percent of the African rainforest. These are
deep, sandy soils with high acidity and very low fertility, which are susceptible to
erosion. The largest extent of these soils occurs in the western portion of the Congo
Basin in the DRC and the west Central African Republic. Inceptisols represent
12 percent of tropical soils. They are neutral soils that are less weathered than the
oxisols and ultisols and have clearly developed A, B, and C horizons. Since they
are relatively fertile, they are used for agriculture.
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Vegetation
Plant diversity is high in the tropical rainforests of Africa, but lower than in the
other two rainforest regions. This is especially evident in the limited number of
palms, lianas, and orchids and other epiphytes. The relative low species diversity is
probably due to the extensive dry periods in the geologic past that severely re-
stricted rainforest distribution.
Africa shares some dominant plant families with the Neotropical and Asian-
Pacific rainforests, including the legume (Fabaceae), euphorb (Euphorbiaceae),
soursap (Annononaceae), laurel (Lauraceae), mahogany (Meliaceae), palm
(Arecaceae), and fig (Moraceae) families. The once-connected continents of Gond-
wana influenced many of these families' evolutionary histories. Others, however,
may be modern in origin and represent more recent dispersal events.
The relationship of African plants with the Asian flora is stronger than with the
Neotropics. Africa shares 164 of its 177 families with tropical Asia, including
Malaysia. In Madagascar, 200 of 234 families are shared with Asia. Although
many trees are common to the African and Asian-Pacific rainforests,
Symphonia
globulifera
, commonly called manil or cerrillo, is the only one common to the
