6,000 mm or more are not uncommon. In general, seasons in the humid tropics are determined by variations in rainfall, not temperature. Most areas experience no more than 4 months with less than 200 mm of precipitation per year.

About 60 countries, with a total population of 2 billion, are located partly or entirely within the humid tropics (Table 1-1). About 45 percent of the world's humid tropics are found in the Americas (essentially Latin America), 30 percent in Africa, and 25 percent in Asia. Small portions of the humid tropics can be found in other areas such as Hawaii and portions of the northeastern coast of Australia.

The typical vegetation for the humid tropics consists of moist, wet, and rain forests in the lowlands and in the hill and montane uplands. Estimates of their extent vary. The most current effort to provide reliable and globally consistent information on tropical forest cover, deforestation, and degradation is by the Forest Resources Assessment 1990 Project of the Food and Agriculture Organization (FAO) of the United Nations, using remote sensing imagery and national survey data as part of its methodology (Forest Resources Assessment 1990 Project, 1992). It defines forests as ecological systems with a minimum of 10 percent crown cover of trees (minimum height 5 m) and/or bamboos, generally associated with wild flora, fauna, and natural soil conditions, and not subject to agricultural practices.

The project estimates that forests cover 1.46 billion ha, or 48 percent of the land area (3.02 billion ha) in the tropical rain forest, moist deciduous forest, and hill and montane forest zones. These forests constitute 30 percent of the land area within the tropical region (4.82 billion ha) and 86 percent of the total tropical forest area (1.7 billion ha). Although they cover only 10 percent of the land area of the world (15 billion ha), they contain one-third of the world's plant matter. Nearly two-thirds of the world's humid forests are found in Latin America, with the remainder split between Africa and Asia.

The soils of the humid tropics are highly variable. Table 1-2 shows the geographical distribution of soil orders and major suborders based on the soil classification system developed in the United States. Oxisols and Ultisols are the most abundant soils in the humid tropics, together covering almost two-thirds of the region. Oxisols, found mostly in tropical Africa and South America, are deep, generally well-drained red or yellowish soils, with excellent granular structure and little contrast between horizon layers. As a result of extreme weathering and resultant chemical processes, however, Oxisols are acidic, low in phosphorus, nitrogen, and other nutrients, and limited in their ability to store nutrients, but have relatively high soil organic matter content. Ultisols are the most abundant soils of tropical Asia,