The numerical values selected for all aspects of carbon dynamics in the hypothetical landscape are comparable to those of a temperate ecosystem. The actual values are not important here, nor are the absolute numerical values of the results. The purpose of these examples is to demonstrate qualitatively how, for each definitional scenario, the reported carbon stock changes in the ARD land portion of the landscape compare to the actual carbon stock changes in the simulated landscape. Although the simulations extend over several commitment periods, for clarity only the results of the first commitment period are reported here.
The area of ARD land in each commitment period is a function of the human activities simulated in each of the nine cases, as well as the definitional scenarios that specify which activities create ARD land (see Table 3-10). In all cases, the area of ARD land in the first commitment period is much less than the total area in the landscape. For cases A through F, no ARD land is created for the definitional scenarios that require a forest change (Figure 3-5). For these scenarios, only cases G, H, and I (i.e., those with a true forest/non-forest change) create ARD land at the end of the first commitment period. The Degradation/ Aggradation definitional scenario also reports ARD land for cases E and F, in which the potential forest cover at maturity is changed for some areas.
|Figure 3-5: Area of ARD land (in kilo ha = 1000 ha) at the end of the first commitment period (2012) for each of the nine cases (A-I) and for various definitional scenarios: FAO, Forest Change [includes all scenarios requiring a forest/non-forest conversion (i.e., IPCC, Land Use, Flexible, and Biome)], Land Cover, and Degradation/Aggradation (Deg/Agg). Missing bars indicate zero area. FAO options 1, 2, 3 refers to the three approaches for accounting carbon stock changes.|
The definitional scenarios in which a harvest/regeneration cycle creates ARD land (i.e., FAO and Land Cover) report large areas of ARD land. In case A, the area of ARD land is 23 kha for both the FAO and the Land Cover scenarios. In case B, the area naturally regenerated in the FAO scenario does not create ARD land. In cases H and I, the afforested area is not included in the Land Cover scenario because the planted areas have not grown past the forest cover threshold at the end of the first commitment period. In case I, all deforested and afforested area contributes 9.2 kha of ARD land at the end of the first commitment period-except in the Land Cover scenario, in which the afforested area again has not yet grown past the forest cover threshold.
Figure 3-6 summarizes the results for the nine cases defined in Table 3-10 for the first commitment period and for three carbon pools: aboveground biomass; dead organic matter, including soil carbon; and total ecosystem carbon, which is the sum of aboveground and below-ground biomass and dead organic matter. Below-ground biomass is not shown separately because it is proportional to aboveground biomass. All carbon stock changes are reported in kilo tons over the 5-year commitment period. The actual stock change is that observed in the hypothetical landscape.
|Figure 3-6: Changes in carbon stock during the first commitment period for each of the nine simulated cases of human activities for aboveground (top), dead organic matter including soil (middle), and total ecosystem (bottom) carbon.|
In most combinations of definitional scenarios and simulated cases, Figure 3-6 shows large discrepancies between the actual carbon stock change in the landscape and that reported for the ARD land. In cases A and B, the actual carbon stock change in the landscape is zero. The scenarios that limit the creation of ARD land to only forest change activities report no ARD land, therefore no carbon stock changes for cases A-F. These definitional scenarios fail to account for increases (case D) or decreases (case C) in actual carbon stocks resulting from accelerated or decelerated harvesting rates and from changes in forest productivity (cases E and F). These scenarios do capture the land-use change activities of cases G, H, and I and represent the correct carbon stock increase in case H, as well as the decrease in cases G and I.
Definitional scenarios in which the harvest/regeneration cycle creates ARD land (FAO, Land Cover) report a large decrease in carbon stock in the ARD land, except with an activity-based accounting approach in the FAO scenario. In cases in which the actual landscape carbon stock increases (cases D, F, and H) or decreases (cases C, E, G, and I), the reported stock change becomes somewhat larger or smaller relative to case A, respectively. Despite these small differences between cases, the reported change is always negative, even if the actual carbon stock increases in the landscape (e.g., cases D and F).
The choice between the three carbon accounting approaches (land-based I and II and activity-based) for the FAO definitional scenario does not affect the area of ARD land. The approaches do report different carbon stock changes for the ARD land, however. The stock change in land-based approach I is reported for the 5-year commitment period. For land-based approach II, the stock change on the land is reported from the time of the activity to the end of the commitment period. In the activity-based approach, stock changes that do not result from the reforestation activity but happen on the land after reforestation has taken place are excluded from the accounting.
The impacts of the three accounting approaches differ between carbon pools. For aboveground biomass, the FAO definitional scenario with land-based approach I (FAO opt 1 in Figure 3-6) always yields a large negative carbon stock change because this option indirectly accounts for harvesting during the commitment period. Land-based approach II (FAO opt 2) reports a small positive carbon stock change in aboveground biomass, resulting from the growth of reforested seedlings. This increase in aboveground biomass is more than offset by decreases in dead organic matter carbon stocks. Land-based approach II does not account for the increase in dead organic matter from the input of logging slash and root biomass during harvest. It does account for the decay of all dead organic matter from the reforestation activity to the end of the commitment period. Thus, land-based approach II reports a large negative carbon stock change for the dead organic matter pools. Total ecosystem carbon stock change under land-based approach II is negative-albeit only about one-third of the values reported for land-based approach I. The results for the activity-based approach are the same as for land-based approach II, except that the negative stock change from decaying slash between reforestation and 2012 is excluded from the accounting because it does not result from the reforestation activity. Therefore, the FAO scenario with activity-based accounting results in carbon credits in the first commitment period; that is, only the aboveground biomass increase under FAO (option 2) would be reported.
The definitional scenario that is designed to capture degrading or aggrading carbon stocks (the Degradation/Aggradation scenario) correctly reports no change in carbon stocks in cases A and B. By design, it does not capture carbon stock changes associated with altered harvest rates (cases C and D) because they do not change the potential cover of carbon density at maturity. This scenario fails to accurately report the actual carbon stock changes in cases E and F, which represent degrading and aggrading forest conditions. As before, the changes at the landscape level are different from those reported for the ARD land. Moreover, even when the carbon stock at the landscape level is increasing (case F), the reported carbon stock change is still negative.
The decision about which pools to include will also affect the differences between the actual carbon stock changes in the landscape and those reported for the ARD land. For aboveground biomass, below-ground biomass (not shown), and total ecosystem carbon, the differences are in the same direction but of different magnitude (Figure 3-6). The differences can be of opposite direction for the dead organic matter pool alone, but it is not likely that carbon stock changes for only this pool will be reported. For definitional scenarios in which the harvest/regeneration cycle creates ARD land, the dead organic matter pool can increase when biomass pools are decreasing because logging slash-including stumps and root biomass-is added to the dead organic matter pool following harvesting.
We have conducted additional simulation experiments as sensitivity analyses. We systematically altered the rates of land-use change and harvest to cause landscape-level carbon stocks to either increase, remain approximately unchanged, or decrease. It can be demonstrated that combining one rate of land-use change with three different rates of harvesting, or vice versa, creates very different reported carbon stock changes for the three groups of definitional scenarios.
We drew the following conclusions from the analyses of the simulations:
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