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Biome: Tropical Rain Forest

The plant geographer A. F. W. Schimper first used the term tropical rain forest in 1898 to characterize the broad-leaved evergreen trees that dominate the wet equatorial regions of the world. Under tropical wet conditions, a structurally diverse forest occurs that is typically high stature, closed canopy, species rich, structurally diverse, and unique in the adaptations that different plants and animals have to the environmental conditions. Rain forest can be described by these general attributes and also by the unique conditions that occur across continents and within continents along environmental gradients.

The Physical Environment

The potential geographic distribution of tropical rain forest is pantropical within the equatorial latitudes of 10° N and 10° S. These forests occur on the continents of South America, Africa, and Asia, and extend into the subtropical realm in Eastern Central America and the Caribbean, southwestern India and Sri Lanka, and Northern Australia. Day lengths hardly change from 12 hours during the spring and fall equinox, noon sun angles are always high, and the sun rises and falls quickly in the horizon. Temperature conditions are always warm, above 18 °C, and warm moist equatorial air rises to support daily convective showers and constantly moist conditions. Tropical rain forests experience no frost; daily temperature differences show greater ranges than the monthly means. Monthly precipitation always exceeds water needs (potential evapotranspiration) and can average more than 100 mm (millimeters) per month or an excess of moisture. Rainfall averages 2,000 to 3,000 mm in South America, 1,500 to 2,500 mm in Africa, and >3,000 mm in southeast Asia.

Tropical rain forests extend beyond the equatorial zone where the periods of moisture deficit are very short and the rainy periods are exceedingly wet, such as those brought by the monsoons to southwest India, southeast Asia, and West Africa, and those brought by the northeast and southeast trade winds to Central America, the Eastern Caribbean, the South Pacific, and Australia. Moisture deficits, however, will eventually limit broad-leaved evergreen trees and transition tropical forests toward dominance by trees that are semievergreen or deciduous during the dry season. Tropical rain forests and tropical deciduous forests capture the broad diversity of forest types that occur across the tropical realm where temperatures are constantly warm and suitable for plant growth.

Biogeographical Patterns of Diversity

Plant and animal occurrences in the tropical rain forests show a relationship with their biogeographical distribution among tropical regions. By far the greatest land area (>50%) and species diversity occurs in the New World tropics, which include South and Central America. Tropical rain forest occurs in the Amazon River basin over a 3,000-km (kilometer) distance from the Andes to the Atlantic, along the Orinoco River in Venezuela, and in north coastal Guyana, Suriname, and French Guiana. The rain forests of the western Andes in Peru and Ecuador are among the wettest in the world, which then extend northeastward along the east coast of Central America and include the northeastern exposed Caribbean islands (e.g., Jamaica, Hispaniola, Puerto Rico, Dominica, and Trinidad). In Africa, tropical rain forests occur in the Congo River basin and along a narrow coastal zone in West Africa. More than 60% of the Democratic Republic of Congo is in rain forest, and some of the most diverse rain forests occur in Eastern Madagascar. In southeast Asia, tropical rain forests straddle the equatorial zone between Indochina, Malaysia, Indonesia, and the Philippines. One of the most famous biogeographical boundaries occurs across Indonesia, Wallace's line, distinguishing the rain forests of southeast Asia from Australia and New Guinea along a plate boundary that merged about 50 million years ago (mya). Moreover, the isolated distribution of the Atlantic forest from Recife to São Paulo in Brazil and the coastal and Eastern Arc forests of East Africa suggest a historically broader distribution of rain forests across these continents.

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