The tropical forests are found in a broad belt encircling the world at the equator, broken only by oceans and mountains. Their distribution coincides with the band of low-pressure areas that occurs where rising tropical air is replaced by moist air flowing in from the north and south to form a system of converging winds.
The rain forest is the floral product of great heat and copious moisture. At all times the average temperature must be between about 21�C and 32�C and the annual rainfall in excess of 150 centimetres. As the sun is roughly overhead throughout the year, the climatic conditions have a constancy found in no other habitat.
Tropical forests are often associated with great rivers, which carry away the copious rainfall. Such rivers are found in the South American island continent, the African sub-continent and the sub-continent of Australia.
Despite the constant fall of discarded leaves the soils of the rain forests are very thin. The conditions are so favourable for decomposition that humus does not have a chance to form. The tropical rain washes the clay minerals out of the soil, preventing important nutrients such as nitrates, phosphates, potassium, sodium and calcium from being retained as they are in temperate soils. The only nutrients found in tropical soils are contained in the decomposing plants themselves.
There are many variations on the basic form of tropical forest resulting both from climatic and local environmental differences. Gallery forest is found where the forest comes to an abrupt halt, as at the edge of a broad river. Here the branches and leaves form a dense wall of vegetation reaching to the ground, to take advantage of light coming in from the side. Less luxuriant monsoon forests exist in regions where there is a distinct dry season. They are found at the edge of continental areas, where the prevailing winds blow from the dry interior at one particular time of year, and are typical of the Indian peninsula and parts of the Australian sub-continent. Mangrove forest is found in saline swamp areas along muddy shorelines and the mouths of rivers.
There are no dominant species of trees in the tropical forest as there are in other forest habitats. This is because there are no seasons and therefore the insect population does not fluctuate; the insects that feed on a particular species of tree are always present and will destroy the seeds and seedlings of that tree if they are sown nearby. Therefore the only seeds that flourish are those that are transported some distance away from their parent and its permanent insect population. In this way stands of particular tree species are prevented from forming.
The area of tropical forest has increased considerably since the Age of Man. In the past a great deal of damage was done to the habitat by man's agricultural practices. Primitive societies cut down areas of trees and farmed the clearings for a few years until the thin soil became exhausted, compelling them to move on to another area. In the cleared areas the original forest did not immediately re-establish itself and it was many thousands of years after man's extinction before the tropical forest belt returned to anything like its natural condition.