Biome: Tropical Deciduous Forest

Tropical deciduous forest is a major vegetation type that occurs in association with seasonally wet and dry or monsoon climates. These forests vary significantly in their composition and structure in relation to their geographic location in the tropical realm, rainfall and soil conditions, and history of disturbances. The transition from evergreen to deciduous forests is gradual in response to the occurrence of a water deficit, the length of the dry season, and the amount of rain received during the rainy season. Tropical deciduous forest, in contrast to tropical rain forest, includes trees that lose their leaves synchronously and remain without leaves for varying amounts of time. In contrast to tropical savanna, tropical deciduous forests maintain a greater dominance by trees (>50%) across the landscape.

Generalized attributes of tropical deciduous forests include a closed to open tree canopy that may be tall (>20 m [meters]) with a single trunk or low in stature (<10 m) with a coppiced trunk that produces multiple stems close to the ground surface. Leaves are broad but often smaller and thicker than those typical of tropical rain forest, and the forest type varies greatly in the adaptations these trees have to the seasonal climatic conditions. Semievergreen forests have a high forest canopy with deciduous trees that remain dormant for a short period of time and an understory that remains evergreen (see the first photo). These forests can support a high diversity of lianas and understory vegetation. Dry deciduous forests occur as low (<8 m) closed-to-open woodland with trees that lose their leaves and remain dormant during long dry seasons of 3 to 6 mos. (months; see the second photo). Evergreen tropical rain forest can maintain itself only if the drought period is very short, 1 to 2.5 mos., and rainfall is very high, 2,500 to 3,000 mm (millimeters). Tropical deciduous forest transitions to tropical thorn scrub under conditions of very low rainfall (<500 mm) and a dry season that exceeds 6 to 8 mos.

Tropical deciduous forests occur between approximately 5° and 20° north and south of the equator, and extend into the subtropics where rainfall is distinctly seasonal and there is only a slight chance of frost. Seasonal rainfall patterns coincide with the migration of the sun's direct rays between the tropics of Cancer and Capricorn, bringing precipitation under high sun (summer) conditions. In South and Central America, they are most diverse in Mexico and Bolivia and along the Pacific Coast of Central America, but “dry” forests also occur between the Cerrado savannas and Caatinga dry scrublands of Northeast Brazil, and on Caribbean islands where mountains (e.g., the Blue Mountains in Jamaica) or other islands block the receipt of rainfall from the Northeast trades. Seasonal monsoon forests occur in southwest India, Sri Lanka, and Indochina. A small region in Northern Australia is distinctly seasonal with summer rainfall conditions, but evergreen Eucalyptus species dominate on that continent and maintain an evergreen forest canopy. Tropical deciduous forests are particularly important as a vegetation type in Africa, where they occur under two wet and dry seasons in equatorial East Africa, under one wet and dry season south of the equator from Tanzania to Southern Africa, and in Western Madagascar. These forests differ in their tree species composition, including the Acacia-Commiphora bushland of East Africa, the Brachystegia-dominated [Page 231]miombo woodlands of Tanzania and south-central Africa, and the mopane (Colophospher-mum mopane) woodlands of South Africa. Whereas one species of baobob (Adansonia digitata) occurs in tropical deciduous forest on the continent of Africa, at least seven species are recorded in the dry forests of Western Madagascar (the third photo). Tree genera with very broad geographic ranges show high diversity in different woodland types.

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