The differences in organic matter decomposition on the forest floor are poorly understood and result in a variety of surface runoff conditions. Many parameters of water yield from a catchment may be studied by stream flow measurements, but scientific criteria for calibration and treatment must be met. Generalizations on the effects of forest on water yield are drawn from all the successful catchment experiments in six tropical regions. In the first year after clearing forest, stream flow increased by a maximum of 6 mm for each of of the area cleared. More experiments are needed in tropical areas to elucidate the causes of the observed effects of forest clearance.
Forest occupies regions without too marked a seasonal drought.
In West Africa the minimum range of annual rainfall required to support forest is 1,200 -1,500 mm depending on the nature of the regolith. But in Latin America, numerous forests grow with annual rainfalls of only 1,000-1,100 mm. The problem of delimiting the humid tropical regions where the natural or climax vegetation is the tropical rain forest is a delicate one. Furthermore, although the forest gradually changes, it extends beyond the tropics. The subtropical and montane rain forests of these regions, and those of South Chile and South Africa together with the more seasonal types of evergreen forests found in areas with a somewhat marked dry season, are included in the region of tropical forest for the purpose of the present report.