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Conversion of tropical forests into pastures and agriculture have serious impacts on carbon allocation in ecosystems and releases carbon to the atmosphere, mainly as CO2 and CH4. The abandonment of these land uses typically results in secondary forest growth which can serve as a a sink for carbon. To determine the relative importance of this sink for the western portion of Brazil\'s arc of deforestation in Acre State, we measured the aboveground biomass of six secondary forests (51 to 136 Mg/ha) with ages ranging from 6 to 35 years. Based on a negative exponential model, the biomass accumulation into secondary forests in Acre is approximately 6.2* e(-0.025t) Mg ha(-1) yr(-1), of which approximately 50 percent is carbon. This simple model underestimates rates during the first ten years, but demonstrates the decreasing rate of carbon uptake with time, consistent with the classical pattern of secondary succession where slower growing tree species replace fast growing pioneer species. The time necessary to achieve half of primary forest biomass is about 30 years. Recent estimates of deforestation of Acre range between 40,000 and 50,000 ha yr(- 1). For secondary forests to compensate for the carbon loss from deforestation, 80,000 to 100,000 ha would need to be abandoned each year and left to accumulate carbon for 30 years. Pilot studies indicate that secondary forests are not increasing but rather decreasing in area in Acre. Consequently, secondary forests in Acre are insignificant sinks of carbon at present

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