This section presents the scientific understanding of the mechanisms by which land-use activities lead to changes in carbon stocks contained in terrestrial ecosystems, as well as changes in the net emissions of non-CO2 GHGs. It explains why some of the mechanisms are temporary, over what periods they can be expected to operate, and why the carbon pools that are formed differ in their capacity and permanence.
Ecosystem carbon stock changes are determined by the balance of carbon inputs-via photosynthesis and organic matter imports-and carbon losses through plant, animal, and decomposer respiration; fire; harvest; and other exports. Typically, but not always, intensive human use of an ecosystem leads to a net depletion of carbon storage relative to lightly exploited ecosystems. For example, conversion of grasslands and forests to agriculture typically causes the loss of aboveground and below-ground carbon stocks and accelerates respiration relative to photosynthesis, resulting in a decline of carbon reserves until the rates of inputs and outputs again converge (e.g., Cerri et al., 1991; Davidson and Ackerman, 1993; Balesdent et al., 1998). Conversely, a change in management that favors inputs relative to losses will elicit a gain in carbon storage until losses again equal inputs (see Figure 4-2). Such an increase in stored carbon, relative to the current trajectory, can be achieved by a shift in management or environmental conditions. Broadly, there are two ways of achieving an increase in carbon stocks:
The rate of accumulation of additional stored carbon from a change in land use or management cannot be sustained indefinitely. Eventually, input and loss rates come into balance and carbon stocks approach some new, higher plateau (Odum, 1969; Johnson, 1995). Consequently, additional carbon storage in response to any management shift is of finite magnitude and duration (Greenland, 1995; Scholes, 1999). Rates of carbon gain often are highest soon after adoption of a new practice, but they subside over time. Although the temporal pattern of carbon accumulation varies, the benefits of biological mitigation will likely be most pronounced in the first few decades after adoption of carbon-sequestering land use and management practices (Dumanski et al., 1998; IGBP, 1998; Smith et al., 1998).
|Figure 4-2: Idealized view of changes in ecosystem carbon storage. During early succession, rate of carbon input (I) exceeds rate of decomposition (D), resulting in an accumulation of stored carbon, equivalent to accumulated difference in area between the two curves. Eventually the rates converge and carbon storage approaches a maximum. Adoption of a new land use or management within a land use may alter the relative rates of I and D, resulting in either a loss of stored carbon (a) or a gain in stored stored (b). In either case, rates of I and D eventually converge, and the ecosystem approaches a new "equilibrium" carbon level (based on Odum, 1969).|
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