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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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