On a global basis, the conversion of tropical forests and the expansion of crop- and pasturelands on former forestlands account for about 20 to 25 percent of carbon dioxide emissions and 25 percent of the total radiative effect of greenhouse gas emissions (Dale et al., Appendix, this volume; Houghton, 1990a).

In terms of potential impact on climate change, the most important feature of land use in the humid tropics is the net release of carbon that occurs as a result of forest conversion. The carbon release represents the difference between the pre- and postconversion levels of carbon stocks. This figure can range widely, depending on the nature of the original forest, the degree and rate of conversion, and the subsequent land use. Permanent agriculture based on annual crops, for example, reduces by more than 90 percent the amount of carbon stored in the original vegetation, while the loss from selective logging can be as small as 10 percent (Dale et al., Appendix, this volume). (Tropical vegetation and soils can also naturally release greenhouse gases, such as nitrous oxide and methane.) As secondary forests regrow, or are replaced by forest fallows, plantations, agroforestry systems, or other agricultural land uses, carbon is sequestered again within the biomass and soil (Wisniewski and Lugo, 1992).

These differential releases and accumulations become important in weighing the land use options described in Chapter 2. Some activities, such as logging, might allow a virgin forest landscape to actually accumulate and store more carbon than it would if it was left as virgin forest, where the storage and release of carbon are in balance. In logging, the sawn boards are not destroyed but used for long periods of time. Hence, carbon remains stored in the harvested wood and, meanwhile, carbon continually accumulates through vegetation growth in the open spaces left after cutting. If the forest is not treated carefully, or the sawn wood is not put to wise long-term uses, even logged forests could act as sources, instead of collectors of carbon.