Only in the Degradation/Aggradation definitional scenario is ARD land created when human activities aggrade or degrade forests (i.e., they change the potential carbon stock at maturity). In this scenario, human activities that result in land-use change also create ARD land. Forest harvesting followed by planting does not create ARD land if it does not alter the potential carbon stock at maturity.
Section 3.3.2 introduces three possible approaches to carbon accounting:
For the landscape-level analyses, we apply the carbon accounting rules of the three foregoing options to the FAO definitional scenario in which reforestation (and not harvest) creates ARD land. The difference between the three accounting rules becomes obvious when reforestation follows harvest during the commitment period. Under option 1, the carbon stock change from the start to the end of the commitment period is reported, thereby including the stock change effect of harvesting even though the subsequent reforestation creates ARD land. Under option 2, the carbon stock change from reforestation to the end of the commitment period is reported, including the loss of post-harvest slash. This reporting period includes less than the five-year commitment period. Under option 3, the carbon stock change is also reported from the reforestation to the end of the commitment period, but the carbon release from harvest slash and dead roots is not accounted because this loss does not result from the reforestation activity. This carbon accounting approach is approximated here by estimates of aboveground biomass changes obtained from option 2 and by ignoring all changes in soil and dead organic matter pools.
To conduct the quantitative analyses with the hypothetical landscape, we developed an analytical framework that employs a very simple model of ecosystem carbon dynamics and applies the definitions and accounting rules of the seven scenarios. The ecosystem model is driven by three curves that describe the aboveground biomass dynamics as a function of age for the high- and low-productivity forests and the agricultural land (Figure 3-4). These curves are applied to a landscape that contains 1,500 parcels of 100 ha each. Note that in the model, the agricultural land contains 5 t C ha-1 in aboveground biomass. A set of companion curves defines canopy cover as a function of stand age for the two forest types.
|Figure 3-4: Aboveground biomass carbon simulated in the model as a function of age for productive and degraded forest and for agricultural land.|
In the model, below-ground (i.e., living root) biomass is calculated as a proportion of aboveground biomass. Each year a proportion of the aboveground and below-ground biomass is transferred to a single dead organic matter and soil carbon pool. Carbon is released from this pool through decomposition. Harvest transfers carbon from biomass to the dead organic matter pool and to a pool of harvested material (which is not included in this model). The model accounts for annual growth and decomposition and for changes in cover type and land use associated with human activities.
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