Ten-year-old pastures in Brazil and 13-year-old Imperata grasslands in Indonesia (common practices after tropical deforestation) contained 201 t C ha-1 less than the original forest that was cleared (Palm et al, 2000). In contrast, agroforests that are established immediately after slash-and-burn by planting trees along with food crops contained 150 t C ha-1 less than the original forest (Table 4-7). Agroforestry systems can regain 35 percent of the original carbon stock of the forest; croplands and pastures can regain 12 percent. Through the establishment of tree-based systems in degraded pastures, croplands, and grasslands, the time-averaged carbon stock in the vegetation increases by 50 t C ha-1 in 20-25 years, whereas that in the soil increases by 7 t C ha-1 (Palm et al., 2000). Agroforestry practices therefore permit the sequestration of an additional 57 t C ha-1-three times as much as croplands or grasslands can sequester (Figure 4-10). This finding indicates the key contribution of agroforestry to increasing carbon stocks at the margins of the humid tropics.
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| Figure 4-10: Time course of system carbon stocks (biomass and soil, solid lines) and time-averaged carbon stocks (dotted lines) in agroforestry systems vs. crops followed by grasslands at margins of humid tropical forest. Based on Table 4-6 and additional data from Palm et al. (2000). |
The area that could be converted to this practice is 10.5 Mha annually if enabling government policies were in place (Fay et al., 1998; Tomich et al., 1998). The area calculation is based on two assumptions: 20 percent of the 15 Mha that is annually deforested (3 Mha) is put into agroforestry every year, and 3 percent of the 250 Mha of degraded lands at the forest margins (Sanchez et al., 1994) is converted into agroforests every year (7.5 Mha). The difference in time-averaged carbon density from this land-use change is 35-90 t C ha-1, with a modal value of 57 t C ha-1. We further assume that the annual deforestation rate will stay constant for the next 10 years. If this happens, the global contribution of this practice to carbon sequestration would be on the order of 0.105-0.525 Gt C yr-1, with a modal value of 0.315 Gt C yr-1.
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