aged ecosystems. But now, with the increasing rate of change in human activity across the face of the land, the earth itself must be viewed as a managed ecosystem.

Timing is critical. What is not managed is at risk of being lost.

Technically, the humid tropics is a bioclimatic region of the world characterized by consistently high temperatures, abundant precipitation, and high relative humidity. Gradients of temperature, rainfall, soils, and slope of the land contribute to variations in vegetation. Tropical lowland vegetation constitutes about 80 percent of the vegetation in the humid tropics. Although a variety of distinct plant associations and forest formations exist in the region, the forests of the humid tropics are often referred to as tropical rain forests. Collectively, however, lowland, premontane, and montane forest formations that include moist, wet, and rain forests can be generally referred to as humid tropic or tropical moist forests.

Humid tropic conditions are found over nearly 50 percent of the tropical land mass and 20 percent of the earth's total land surface—an area of about 3 billion ha. This total is distributed among three principal regions. Tropical Central and South America contain about 45 percent of the world' s humid tropics, Africa about 30 percent, and Asia about 25 percent. As many as 62 countries are located partly or entirely within the humid tropics.

Forest conversion is defined as the alteration of forest cover and forest conditions through human intervention. Deforestation is a conversion extreme that reduces crown cover to less than 10 percent. Available data suggest that the annual rate of deforestation in the (primarily humid) tropics increased from 9.2 million ha per year in the late 1970s to an average of 16.8 million ha per year in the 1980s. Deforestation currently affects about 1.2 percent of the total tropical forest area annually. Forest degradation—changes in forest structure and function of sufficient magnitude to have long-term negative effects on the forest's productive potential—also affects a large area.

The leading direct causes of forest loss and degradation include large-scale commercial logging and timber extraction, the advance-

Front Matter (R1-R16)

Executive Summary (1-18)

Part One (19-20)

1 Agriculture and the Environment in the Humid Tropics (21-65)

2 Sustainable Land Use Options (66-137)

3 Technological Imperatives for Change (138-158)

4 Policy-Related Imperatives for Change (159-191)

References (192-214)

Appendix: Emissions of Greenhouse Gases from Tropical Deforestation and Subsequent Uses of the Land (215-260)

Part Two: Country Profiles (261-264)

Brazil (265-351)

C te d (352-392)

Indonesia (393-439)

Malaysia (440-482)

Mexico (483-548)

The Philippines (549-624)

Zaire (625-658)

Glossary (659-674)

Authors (675-678)

Index (679-704)