in soils. Tropical savannas store about one third as much C in vegetation as do tropical forests. However, savannas also have large C stocks in soils, similar to those of temperate grasslands. Croplands worldwide have the smallest C stocks in vegetation, with intermediate values for soils.
Because of the importance of forests and forest soils as a sink for carbon, scientists are beginning to agree that forests must be preserved or re-established. Forestry-based carbon sequestration is based on two approaches: active absorption in new vegetation and preservation of existing vegetation. The first approach includes any activity that involves planting new trees (such as afforestation, reforestation, or agroforestry) or increases the growth of existing forests (such as improved silvicultural practices). The second approach involves preventing the release of existing carbon stocks through the prevention or reduction of deforestation and land-use change, or reduction of damage to existing forests. This may involve forest conservation or indirect methods such as increasing the production efficiency of swidden agriculture. Improved logging practices and forest fire prevention are other examples of actions that protect existing carbon stocks.
In 1997, the Kyoto protocol affirmed reforestation and additional incorporation of carbon into agriculture as potential substitutes for reducing the C02 emissions from fossil fuels (FAO 2001a). Using reforestation in the tropics as a method for mitigating C02 emissions had been a topic of discussion long before 1997. In deciding on the best strategy for addressing these issues, it has been suggested that choices include focusing efforts on protecting primary tropical forests, allowing regrowth of secondary forest in areas that have been cleared, establishing plantations in cleared areas, and encouraging agroforestry on land cleared for agriculture (Cairns and Meganck 1994). Carbon conservation is regarded as having the greatest potential for slowing the rate of climate change. In contrast, carbon sequestration is a slow process.
Recovery of a tropical forest with maximum carbon content can take hundreds of years (Montagnini and Nair 2004).
Some tropical countries have recently started programs of incentives to encourage tree plantation development to help offset C emissions. Since 1966, Costa Rica has contributed payments for environmental services (ES) such as promoting forest conservation, sustainable forest management, and tree plantations through the assignment of differential incentives for each of these systems. Funding for these incentives comes from a special tax on gasoline and from external sources (Campos and Ortiz 1999). In 2003, agroforestry systems were added to the list of systems receiving incentives in Costa Rica, while forest management was eliminated from the list, due to lack of funding to support all incentives and to pressure from environmentalist groups. In several other tropical countries economic incentives are given for the establishment of agroforestry systems in the form of carbon credits (Dixon 1995). For example, the Dutch government is engaged in a 25-year program to finance reforestation projects covering 2,500 km2 in South America, in order to offset carbon emissions from coal-fired stations in the Netherlands (Myers 1996). As the concept of "carbon credits" being paid by fossil fuel emitters to projects that sequester or reduce carbon output becomes more common, many nations and organizations will seek to find inventive ways to sequester carbon.
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