Seasonally Dry Tropical Forests

Introduction: This biome includes a number of forest and woodland types that occur in lowland tropical areas with distinct dry seasons. They include semievergreen, semideciduous, and deciduous tree-dominated communities that occur more or less along an environmental gradient of increasing aridity, although soil conditions also play a role in some habitats. The major response to aridity is the dropping of leaves during the dry season. With an increase in the length of the dry season, the frequency of the deciduous habit among plants increases. The driest limits of the biome are characterized by a thornscrub in which evergreen species and succulent species become common. Scientific study  of the seasonal and dry forests of the tropics has been largely neglected until recently due in part to overwhelming interest in the towering, species-rich tropical rainforests. Much of the biome has already been destroyed; and the remaining seasonal forests, woodlands, and thornscrub are vulnerable to loss due to expanding agriculture and dense human settlement, sometimes over millennia. In Central America, such dry forests are the most threatened of all lowland forest types.

Climate:  Seasonally dry tropical forests generally occur in the same tropical wet and dry climate type (Koeppen’s Aw) associated with tropical savannas. In Asia, the forest regions are under the influence of the monsoon, One long dry season lasts from November to April in the tropical monsoon climate type (Am). Evergreen seasonal forests typically receive 98 in or more of rain each year, but experience a dry period of 6 or more consecutive months with less than 4 in. Dry forests in which most trees lose their leaves in the dry season receive 32-71 in of rain a year, with 5 consecutive months averaging less than 4 inches each. Monthly temperatures average above 64.4° F all year long.

Vegetation: The geographic variability in the forests, woodlands, and thornscrub that comprise this biome make generalizations difficult. Most have closed canopies that are deciduous during the dry season. Importantly, the closed canopy suppresses grasses and hence fires–factors that make them distinct from the tropical savannas that occur in the same climate regions.

Forest classified as dry evergreen or semievergreen have three tree layers, only the uppermost being deciduous. Actually, the uppermost or A layer contains trees so widely spaced that in essence only two layers exist and both are evergreen. Trees differ from tropical rainforest species in that they have smaller, thicker leaves with leathery texture and thick cuticles. Bark may be thin, but usually thicker than on rainforest trees.

Forests deemed semideciduous or deciduous tend to have only two tree layers. In the former, the upper layer is deciduous and the lower evergreen. In the latter, both layers contain only deciduous trees; the leafless season and commonly thin canopy may allow for a dense understory and discontinuous ground layer of forbs and grasses.

In Central and South America (where most studies accessible to English-language readers have been conducted), this biome has reportedly a very high species richness with as many and possibly more plant species than the Amazon rainforest.

Growthforms: Seasonally dry tropical forests tend to have many fewer epiphytes than tropical rainforests, but lianas and other woody plants can be abundant and often share dominance with tree species. Epiphytes in the American forests are often cacti and bromeliads. Flowers of trees are frequently large and brightly colored, and seeds are wind-dispersed. Flowering typically happens at the beginning of the dry season, especially among bird-pollinated species. Insect-pollinated species tend to bloom during the wet season. Cauliflory occurs on trees such as figs dependent upon bats and larger animals (e.g., monkeys) to disperse their seeds.

Large trees that store water in their trunks are found in many dry forests around the world. These include baobabs in Africa and Madagascar and related Ceibas and Cavanillesias (Brazilian bottle trees or barrigudas) in South America.

In drier areas where forests and woodlands are of low stature, succulents are conspicuous elements of the vegetation. In the Americas, these are cacti and some terrestrial bromeliads; in Africa these are euphorbias. Xerophytic palms may be common in the understory.

Soils:  No one soil type is associated with the tropical seasonal forests and woodlands. In Africa, soils are often infertile and acidic, iron-rich laterites or oxisols developed on ancient plateau surfaces. In the Americas dry forests generally occur on soils more fertile and less acidic than those supporting tropical savannas. In Asia, soils tend to be ultisols.

Fauna: Animal diversity in high in seasonally dry forests, second only to tropical rainforests. Mammals, birds, reptiles, and amphibians are all well represented. Ants and termites are also highly diverse. Among mammals are a number of primates, some large rodents in the Neotropics, and such insectivorous animals as anteaters and armadillos (Americas), pangolins and aardvarks (Africa), and certain lemurs (Madagascar). Sloths (Americas) and langurs (Asia) have evolved ways to subsist on diets of leaves. Large cats such as jaguars (Americas) and tigers (Asia) inhabit some of these forests, as do smaller carnivores such as fossa (Madagascar) and mongooses (Asia). Large herbivores such as elephants and rhinoceroses forage and seek shelter in seasonal forests in Africa and Asia.

Adaptations that permit animals to thrive in these highly seasonal environments include local and regional migrations, seasonal storage of fat or of food, changes in diet, and changes in activity patterns and timing of reproduction.

Distribution:  Seasonal forests generally are found flanking Tropical Rainforests poleward to about 20° latitude. In South America they occur in southern and southwestern Amazonia, including Cocha Cashu in Manu Parque Nacional in Peru. The Chiquitania region of eastern Bolivia, which extends into Brazil’s Pantanal, has mostly semideciduous forest. Dry Andean valleys also support seasonal forests. The largest blocks of seasonally dry forest occur in northeastern Brazil (The Caatinga) and in the Gran Chaco in Argentina, Bolivia, and Paraguay. In North and Central America, dry forests extend along the west coast from Chamela, Mexico, (19° N) to Parque Nacional Santa Rosa (11° N) in Costa Rica. Seasonal forest extends from Barro Colorado Island, Panama (9° N) north into Belize (18° N). Dry forests also occur on the Greater Antilles.

In Africa, seasonal forests occur around Makokou, Gabon; in Korup National Park, Cameroon; and in the Ituri Forest, Democratic Republic of the Congo. The coastal forests of Ghana, Cote d’Ivoire, and Liberia are also seasonal.

Dry forest is the natural vegetation of western Madagascar at elevations above 3,330 ft asl. Endemism is high among plants and animals at species, genus, and family levels. Seven species of baobab unique to the island occur in this forest, as are eight species of lemur. The highly endangered Angonoka tortoise is restricted to this rapidly disappearing forest type, an estimated 97 percent of which has been lost to fire and clearing for shifting agriculture, livestock rearing, firewood production, and charcoal making.

In Asia, evergreen seasonal forest is found in the Western Ghats of India between 13°-15° N, as well as on the Deccan Plateau and parts of Bangladesh. In Southeast Asia tropical seasonal forest occurs in Thailand, Cambodia, Laos, Myanmar, and Vietnam. A deciduous seasonal forest extends from approximately the Thailand-Malaysian border into the Isthmus of Kra. Most of lowland Java was once covered by seasonal forest.

Although the tree-dominated, the seasonally dry diptocarp forests of continental Southeast Asia and the “miombo” or Brachystegia woodlands of southern Africa are regarded as savannas because their open canopies result in continuous grass layers which support regular fires.

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