Tropical Forests

forests distributed in the equatorial, subequatorial, and tropical zones (that is, between 25° N lat. and 30° S lat.).

Tropical rain forests have the greatest diversity of plant species and consist of extremely tall trees (heights to 60–70 or even 80 m). There is an abundance of lianas and epiphytes and an absence of a dominant species. Tropical rain forests occur in regions where the precipitation rate (2,000–7,000 mm annually) and temperatures (23°–32°C) remain uniform year-round.

Semideciduous tropical forests also develop in a moist climate, with an 80–90 percent relative air humidity and no less than 1,500–2,000 mm of precipitation annually. The forest canopy is lower than that of a rain forest, but there is a very diverse species composition, including lianas and epiphytes. Often there are marked seasonal changes, such as leaf fall and renewal. Semideciduous tropical forests are characteristic of the West Indies (Trinidad), Central America (Costa Rica), and South and Southeast Asia (India, Burma). In western Africa the upper story of such forests is made up of deciduous trees, especially Triplochiton scleroxylon.

In the tropics of Asia semideciduous forests are known as monsoon forests (seeMONSOON FORESTS). The forests are made up of evergreen trees and deciduous trees, including Xylia xylocarpa and Tetrameles nudiflora. Tropical monsoon forests include the relatively sparse sal forests of India, in which the sal tree (Shorea robusta) predominates.

Seasonal leaf fall in tropical forests is particularly marked near the boundary of the tropical zone, even if the climate there is very wet. In arid regions (such as the forests of western Sri Lanka) tropical forests are dense and evergreen but consist of low-growing xeromorphic species. Tropical regions of Indochina having a marked dry season have light deciduous forests of Dipterocarpus tuberculatus, Pentacme siamensis, and Terminalia tomentosa. Pine forests are found under similar conditions but on poor soils. While resembling the pine forests of the north, they consist of particular pine species, for example, Pinus insularis in South and Southeast Asia and P. caribaea, P. tropicalis, and P. occidentalis in the West Indies.

Mountain tropical forests, usually consisting of evergreen species and characterized by the presence of treelike ferns and some conifers, are found in the mountains at elevations above 800 m above sea level (the upper boundary of rain forests). Coniferous trees include species of Araucaria in New Guinea and species of Agathis, Podocarpus, and Dacrydium in Southeast Asia. Higher elevations—about 2,000 m above sea level—are characterized by mossy and low-growing forests made up of species of the families Lauraceae, Fagaceae, and Ericaceae. The forests of the Himalayas and China have many treelike rhododendrons and epiphytes; epiphytic mosses and a mossy soil covering are characteristic. The floristic composition includes species that are purely tropical and species characteristic of forests in temperate latitudes. For this reason, mountain tropical forests are often considered subtropical.