Land Use, Land-Use Change and Forestry

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1.3.1. Carbon Stocks and Flows in Major Biomes

For the estimation of present and future carbon sequestration potential, it is necessary to consider broad vegetation types differentiated by climatic zones and water availability (i.e., tropical, temperate, and boreal regions). Table 1-1 lists the areas, current estimates of aboveground and below-ground carbon stocks, and NPP of the world's major regions or biomes. Within each biome, large additional variation exists resulting from local conditions and topography. In the tropics, for example, moist and dry forests have widely differing carbon stocks and NPP.

  • Pristine forests (e.g., in the wet tropics or boreal region) were long believed to be mostly in a state of equilibrium, such that over a period of several years their carbon balance would be neutral. This view has been challenged in more recent years by increasing evidence from sample plot studies that undisturbed areas of forests also sequester carbon (e.g., Lugo and Brown, 1993; Phillips et al., 1998, for the tropics; Schulze et al., 1999, for the Siberian boreal forest). These carbon quantities will eventually be returned to the atmosphere when patches of trees die for biological or climatic reasons, localized natural disturbance occurs, or compartments of the forest are cleared. Because of its importance to the Kyoto Protocol, carbon sequestration by managed and unmanaged forests is considered in some detail below (Section 1.3.2).
  • Grassland ecosystems store most of their carbon in soils, where turnover is relatively slow (Table 1-1). In most grassland types, below-ground NPP is at least equal to or higher than aboveground production. Carbon accumulation by combined aboveground and below-ground NPP may be as much as 3.4 t C ha-1 yr-1 in tropical humid savannas and as little as 0.7 t C ha-1 yr-1 in tropical dry savannas and 0.5 t C ha-1 yr-1 in temperate steppe (Parton et al., 1995).
  • Wetlands are important reservoirs of carbon. Undrained peatlands in high latitudes have accumulated appreciable amounts of carbon from the atmosphere since the retreat of the ice and continue to be significant CO2 sinks (0.2-0.5 t C ha-1 yr-1), but they are also sources of methane (0.03-0.3 t CH4 ha-1 yr-1). By contrast, peatlands that are drained for agriculture or for afforestation release carbon as CO2 because of accelerated decomposition of the aerobic peat (Cannell et al., 1993), although they no longer release methane in significant amounts. The quantitative balance between these two processes is poorly understood (Cannell et al., 1999), although it is important because the GWP of methane is 21 times that of CO2 (see Section 1.2). Peatlands drained for agriculture continue to be a sustained carbon source as long as any peat remains in the soil. Peatlands drained for afforestation may continue to be a source of carbon in spite of forest biomass growth (Zoltai and Martikainen, 1985), but under certain climatic conditions they can also revert to carbon sinks (Minkkinen and Laine, 1998).
  • In agricultural land, by far most of the carbon is stored below ground (see Table 1-1). Losses of carbon from terrestrial systems during the past 200 years, particularly until the middle of the 20th century, were mostly the result of the establishment of agriculture on grassland and land that was previously covered by forests. Regular plowing, planting, and harvesting led to enhanced oxidation of organic matter in the soils, which has been emitted into the atmosphere as carbon dioxide. Today, agricultural lands are major sources of CO2 in many countries as a result of past land-use changes (e.g., Cannell et al., 1999). Soil organic carbon in cultivated soils is continuing to decline in many areas of the world. The use of fertilizers, high-yielding plant varieties, residue management, and reduced tillage for erosion control has contributed to the stabilization or increase in soil organic carbon (Cole et al., 1993; Sombroek et al., 1993; Blume et al., 1998).

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