The potential for carbon sequestration through reforestation of abandoned tropical agricultural and pasture lands

Approximately half of the tropical biome is in some stage of recovery from past human disturbance, most of which is in secondary forests growing on ab andoned agricultural lands and pastures. Reforestation of these abandoned l ands, both natural and managed, has been proposed as a means to help offset increasing carbon emissions to the atmosphere. In this paper we discuss th e potential of these forests to serve as sinks for atmospheric carbon dioxi de in aboveground biomass and soils. A review of literature data shows that aboveground biomass increases at a rate of 6.2 Mg ha(-1) yr(-1) during the first 20 years of succession, and at a rate of 2.9 Mg ha(-1) yr(-1) over t he first 80 years of regrowth. During the first 20 years of regrowth, fores ts in wet life zones have the fastest rate of aboveground carbon accumulati on with reforestation, followed by dry and moist forests. Soil carbon accum ulated at a rate of 0.41 Mg ha(-1) yr(-1) over a 100-year period, and at fa ster rates during the first 20 years (1.30 Mg carbon ha(-1) yr(-1)). Past l and use affects the rate of both above- and belowground carbon sequestratio n. Forests growing on abandoned agricultural land accumulate biomass faster than other past land uses, while soil carbon accumulates faster on sites t hat were cleared but not developed, and on pasture sites. Our results indic ate that tropical reforestation has the potential to serve as a carbon offs et mechanism both above- and belowground for at least 40 to 80 years, and p ossibly much longer. More research is needed to determine the potential for longer-term carbon sequestration for mitigation of atmospheric CO2 emissio ns.