Tropical and Subtropical Dry Forests are found in southern Mexico, southeastern Africa, the Lesser Sundas, central India, Indochina, Madagascar, New Caledonia, eastern Bolivia and central Brazil, the Caribbean, valleys of the northern Andes, and along the coasts of Ecuador and Peru.
Though these forests occur in climates that are warm year-round, and may receive several hundred centimeters or rain per year, they deal with long dry seasons which last several months and vary with geographic location. These seasonal droughts have great impact on all living things in the forest.
Deciduous trees predominate these forests, and during the drought a leafless period occurs, which varies with species type. Because trees lose moisture though their leaves, the shedding of leaves allows trees such as teak and mountain ebony to conserve water during dry periods.
The newly bare trees open up the canopy layer, enabling sunlight to reach ground level and facilitate the growth of thick underbrush. Though less biologically diverse than rainforests, tropical dry forests are still home to a wide variety of wildlife including monkeys, large cats, parrots, various rodents, and ground dwelling birds. Many of these species display extraordinary adaptations to the difficult climate.
The most diverse dry forests in the world occur in southern Mexico and in the Bolivian lowlands. The dry forests of the Pacific Coast of northwestern South America support a wealth of unique species due to their isolation. The subtropical forests of Maputoland-Pondoland in southeastern Africa are diverse and support many endemics. The dry forests of central India and Indochina are notable for their diverse large vertebrate faunas. Dry forests of Madagascar and New Caledonia are also highly distinctive (pronounced endemism and a large number of relictual taxa) for a wide range of taxa and at higher taxonomic levels.
Species tend to have wider ranges than moist forest species, although in some regions many species do display highly restricted ranges; most dry forest species are restricted to tropical dry forests, particularly in plants; beta diversity and alpha diversity high but typically lower than adjacent moist forests.
Large natural areas are required to maintain larger predators and other vertebrates; large areas are also needed to buffer sensitive species from hunting pressure; the persistence of riparian forests and water sources is critical for many dry forest species; periodic fires require larger blocks of intact forest to be able to aborb occassional large events.
Sensitivity to Disturbance
Dry forests are highly sensitive to excessive burning and deforestation; overgrazing and exotic species can also quickly alter natural communities; restoration is possible but challenging, particulary if degradation has been intense and persistent.
Tropical Dry Forest
Temperatures are high all year, but there is a better-developed dry season than in the tropical rain forest. Evapotranspiration exceeds precipitation for enough of the year to have a significant effect on the vegetation. Edaphic conditions (dryer, better-drained soil) may produce this vegetation type in the rain-forest zone.
Soils are essentially like those of tropical rain forests, with the same processes.
The deciduousness of most tree species is a significant difference from the tropical rain forest. Many evergreen tree species of the rain forest become deciduous in this zone. Growing conditions are not so optimal, thus the tree canopy is lower (10-30m) than in the tropical rain forest and the trees less dense where drought is more extreme. The undergrowth is often dense and tangled because of greater light penetration. Lianas are much less common than in the rain forest, not such an important growth form where light is less limiting and also perhaps highly susceptible to desiccation. Drought-resistant epiphytes (orchids, bromeliads and cacti) may be abundant. The trees have thicker, more ridged, bark; deeper roots without buttresses; much more variable leaves, including many compound-leaved legumes; and more species with thorns.
Species diversity is invariably lower than in nearby tropical rain forests. Environmental stress increases with instability (seasonality) of the environment, and fewer plants and animals can generate homeostatic mechanisms (for internal stability) to cope. There is still relatively high diversity on a world scale, but most of the taxonomic groups in the dry forest are less diverse than in the rain forest. Dry forest is important as habitat for migratory birds in their nonbreeding season (Central America, India).
Trees have thicker bark (antifire adaptation), thicker and smaller leaves (antidesiccation adaptation), thorns (antiherbivore adaptation), longer roots (to reach deeper water table), and other features along a gradient toward the well-developed drought adaptations of woody plants of the savanna and desert zones (which see).
With more spaces between trees, larger mammals are more prominent in this environment. There is more seasonality in reproductive cycles, timed with rains in most groups. In motile species, migration may occur in the dry season to wetter environments, including nearby rain forest, gallery forest, and wet bottomlands.
The high productivity during the rainy season, coupled with relief from rains during the dry season, makes this a favorable environment for humans and domestic stock, so much of the zone has been cleared and developed for pastureland as well as agriculture. Dry forests vary from largely extirpated to still extensive, depending on the geographic region, but in some regions they are more endangered than rain forests.