Seasonal Forests of the Tropics

As one proceeds poleward from the equatorial rainforests, one finds a progressively longer dry season until one finally encounters truly arid climates. This latitudinal and moisture gradient is reflected in the natural vegetation. The forests poleward of the rainforest become shorter in stature and simpler in structure. The proportion of trees that are deciduous increases as the dry season becomes more pronounced. Leaves become smaller and thorns or spines more frequent. At the drier extremes of this gradient, succulents become common.

The general sequence of vegetation from about 10° to 25° of latitude is semi-evergreen forest; deciduous seasonal or dry forest; and thorn forest or thornscrub.

Tropical Semi-evergreen Forests. These are broadleaf forests, some species of which are evergreen and some of which are deciduous. Two tree layers are evident: an upper canopy of deciduous trees with large leaves and an understory of small-leafed evergreen species. Frequently stands are composed of a single-species, such as teak in Africa and Southeast Asia and miombo (Brachystegia spp. and Isoberlinia spp.) in Africa. There are many fewer tree species than in the rainforests, but still many lianas. Epiphytes are infrequent. The seasonal leaf fall produces abundant nutrients and more fertile soils than found under tropical broadleaf evergreen forests but also creates fuel for fires. The fire factor in the environment may explain the thicker bark of tree species in this formation compared to those of the rainforest. Some researchers consider teak woodlands to be a fire subclimax.

Tropical Deciduous Broadleaf Forests or Dry Forests. This forest type occurs where two of the dry season months receive less than 1 inch of rain. It is not found in Africa. Two layers are again evident. In this case both the upper story of trees 60 to 80 feet tall and the understory trees some 10 to 30 tall are small-leafed and deciduous. Bamboo (a member of the grass family) thickets are common. The forest is poor in lianas, epiphytes, and herbs. The shrub layer is dominated by members of the pea family (Leguminosae) such as Mimosa spp. and Acacia spp. Palms are also common.

Thorn Forest or Thornscrub. This vegetation type, comprised of low-growing trees, shrubs, and frequently stem-succulents, occurs where annual precipitation averages 20 to 25 inches and the dry season extends for 6 or 7 months.

The woody species are deciduous and small-leafed; many have thorns or spines. Stem succulents in the neotropics include members of the cactus family (Cactaceae) and a member of the Bombacacae, the barriguda or Brazilian bottle-tree, Chorisia glaziovii. Xerophytic palms and epiphytes (esp. genus Tillandsia, family Bromeliaceae) are common. Terrestrial bromeliads also occur.

In the neotropics, this vegetation type is represented by the caatinga in northeast Brazil and by the thornscrub communities of the Mexican state of Oaxaca.

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Created by SLW, October 1996.