4.2. Processes and Time Scales
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
4.2.1. Land Use, Management, and Ecosystem Carbon Balances
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:
- A change in land use to one with higher carbon stock potential, usually
revealed by a change in land cover-for example, conversion of cropland to
grassland (e.g., Robles and Burke, 1998; Conant et al., 2000).
- A change in management within a land use that does not lead to a qualitative
change in land cover-for example, introduction of more productive species
in grasslands (Fisher et al., 1994), reduction in tillage intensity
in croplands (Paustian et al., 1997a; Rosell and Galantini, 1998),
or forest regeneration (Ravindranath et al., 1999).
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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