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Incorporation of trees on farms affects carbon stocks differently than cropland or forest management. For example, trees on farms provide tighter coupling of key processes such as nutrient cycling and weed control than in croplands; trees in agroforestry are harvested more frequently than under forest management. One accounting option for agroforestry is the time-averaged carbon sequestration rate (Palm et al., 2000), which takes into account periodic woody biomass harvests based on the "average storage method" of Schroeder (1992). Figures 4-10 and 4-11 show the application of this method for two broad practices. All estimates of changes in carbon stocks and carbon sequestration rates are based on time-averaged carbon stocks.
The potential land area suitable for agroforestry in Africa, Asia, and the Americas is 585-1215 Mha (Dixon, 1996). This estimate is a compilation of several estimates (Unruh et al., 1993; Dixon et al., 1994; Dixon, 1995). The current area in agroforestry is on the order of 400 Mha, of which 300 Mha are "arable land" and 100 Mha are "forest lands" in the FAO database. For example, the 14 Mha of agroforestry in China are classified as agricultural land (Xu, 1999). It is estimated that an additional 630 Mha of current croplands and grasslands could be converted into agroforestry, primarily in the tropics.
This chapter considers two kinds of agroforestry activities: land conversion and improved land use. Land conversion includes transformation of degraded cropland and grasslands, including those from slash-and-burn agriculture, into new agroforests. The potential area of new agroforests from land-use change could be on the order of 400 Mha during the next 25 years. Improved use of current agroforestry systems with interventions that result in increased carbon is analogous to "improved cropland"-but with trees on the farm. Both kinds of activities increase carbon stocks, and many also prevent carbon losses in adjacent forests and woodlands by avoiding further deforestation or land degradation. Leakage can reverse agroforestry's effect on avoiding deforestation (Fearnside 1995, 1997), however, particularly if new practices are capital-intensive rather than labor-intensive (Kaimowitz and Angelsen, 1998). This section discusses only the effects on carbon stocks in agroforests because Chapter 3 discusses avoidance of further deforestation. Two practices involving land-use change are discussed: one from slash-and-burn agriculture and another that involves converting degraded cropland in Africa.
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