Tropical Dry Forests


Nature Gallery (Eco Regions)
Tropical Dry Forests and Savannahs
Many people imagine the tropics to be a realm of jungles, deserts, and palm-fringed beaches. Much more typical are the deciduous woodlands and savannahs that occupy those regions of the tropics, in which a rainy season alternates with a dry one. Just as trees in temperate zone forests cast their leaves during the drought imposed by the cold winter, deciduous trees in the tropics shed theirs during the protracted dry season. As a consequence, the same tropical forest that appears so lush and green during the rains assumes a sere and lifeless aspect during the months of drought. If the amount and pattern of rainfall were the only determinants of the vegetation, a continuous cover of deciduous forests would cloak the terrain that lies between the rainforests and the deserts of the tropics. In fact, the closed forests are often replaced by open woodlands, or by savannahs in which the trees are spaced even more widely over a carpet of grass. In some places, the trees are confined to strips along the course of dry streams, and the country resembles pure grassland.

The situations in which grasses predominate over trees are, paradoxically, those in which water is in short supply and those in which there is a glut of moisture. This is because trees typically grow roots that probe widely and deeply into the ground, whereas grasses develop thick, fine root systems that intercept moisture near the surface. If rainfall is scanty, grasses absorb it before any can percolate down into the soil where trees might tap it. If the dry season is long and severe, trees die of dehydration, because even in its leafless condition, a tree loses water from its twigs and buds. On the other hand, if rainfall is ample but drainage is retarded by an impermeable crust under the surface, as is the case in many parts of the tropics, the soil becomes waterlogged, and grasses are again favored. When rainfall is adequate and drainage is good, however, deep soils stay moist through the dry season. This tips the balance in favor of trees, which can then thrive and cast the shade that prevents the development of grasses. The savannahs and woodlands of Africa are renowned for the large and diverse herds of hoofed mammals they support. The comparatively moist Guinea savannahs and woodlands, with their tall grasses and many trees, surround the rainforests on the north, east, and south. With increasing aridity, these savannahs give way to the Sudan savannahs and woodlands of the Sahel, the Horn of Africa, and the Kalahari Desert, parts of which qualify as semi-deserts. In Asia, deciduous monsoon forests cover much of India and Indochina. Northern Australia contains similar woodlands and savannahs. Deciduous forests line much of the Pacific coast of Mexico and Central America. Similar forests cover most of Cuba, the Yucatán Peninsula, and western Ecuador. The llanos of Venezuela and Colombia are a region of extensive savannah grasslands, parts of which flood during the rainy season. Enormous tracts of central Brazil, the campos cerrados, are covered by tropical woodlands. Farther to the south are the vast, marshy pantanal and the savannahs of the Gran Chaco, which Brazil shares with Argentina, Bolivia, and Paraguay.