Land Use, Land-Use Change and Forestry

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1.3. The Carbon Budget of Terrestrial Ecosystems

The carbon sequestration potential of terrestrial ecosystems depends on the type and condition of the ecosystem-that is, its species composition, structure, and (in the case of forests) age distribution. Also important are site conditions, including climate and soils, natural disturbances, and management. For the analysis of a carbon budget, the fundamental differences between GPP, NPP, NEP, and NBP must be recognized (see Figure 1-2). The justification of the quantitative global flux estimates as defined below is given in the succeeding sections of this chapter (see also Steffen et al., 1998).

Figure 1-2: Global terrestrial carbon uptake. Plant (autotrophic) respiration releases CO2 to the atmosphere, reducing GPP to NPP and resulting in short-term carbon uptake. Decomposition (heterotrophic respiration) of litter and soils in excess of that resulting from disturbance further releases CO2 to the atmosphere, reducing NPP to NEP and resulting in medium-term carbon uptake. Disturbance from both natural and anthropogenic sources (e.g., harvest) leads to further release of CO2 to the atmosphere by additional heterotrophic respiration and combustion-which, in turn, leads to long-term carbon storage (adapted from Steffen et al., 1998).

Gross Primary Production denotes the total amount of carbon fixed in the process of photosynthesis by plants in an ecosystem, such as a stand of trees. GPP is measured on photosynthetic tissues, principally leaves. Global total GPP is estimated to be about 120 Gt C yr-1.

Net Primary Production denotes the net production of organic matter by plants in an ecosystem-that is, GPP reduced by losses resulting from the respiration of the plants (autotrophic respiration). Global NPP is estimated to be about half of the GPP-that is, about 60 Gt C yr-1.

Net Ecosystem Production denotes the net accumulation of organic matter or carbon by an ecosystem; NEP is the difference between the rate of production of living organic matter (NPP) and the decomposition rate of dead organic matter (heterotrophic respiration, RH). Heterotrophic respiration includes losses by herbivory and the decomposition of organic debris by soil biota. Global NEP is estimated to about 10 Gt C yr-1. NEP can be measured in two ways: One is to measure changes in carbon stocks in vegetation and soil; the other is to integrate the fluxes of CO2 into and out of the vegetation (the net ecosystem exchange, NEE) with instrumentation placed above (Aubinet et al., 2000). The precision of both of these methods is improving.

Net Biome Production denotes the net production of organic matter in a region containing a range of ecosystems (a biome) and includes, in addition to heterotrophic respiration, other processes leading to loss of living and dead organic matter (harvest, forest clearance, and fire, etc.) (Schulze and Heimann, 1998). NBP is appropriate for the net carbon balance of large areas (100-1000 km2) and longer periods of time (several years and longer). In the past, NBP has been considered to be close to zero (Figure 1-2). Compared to the total fluxes between atmosphere and biosphere, global NBP is comparatively small; NBP for the decade 1989-1998 has been estimated to be 0.7 ± 1.0 Gt C yr-1 (Table 1-2)-about 1 percent of NPP and about 10 percent of NEP.

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