To scale up from stand-level to landscape-level impacts of the definitional scenarios, a simulation model was used to assess nine cases of various human activities in a hypothetical landscape. The actual carbon stock changes, as simulated by the model, were compared to those reported for each definitional scenario.
The area of ARD land in the first commitment period is much less than the total area in the landscape. The ARD area is greater in definitional scenarios in which the harvest/regeneration cycle creates ARD land but is still less than the total area of forests, except where harvest cycles are shorter than 20 years.
Definitional scenarios that limit ARD land to areas with forest to non-forest change and in which the harvest/regeneration cycle does not create ARD land (IPCC, Land Use, Flexible, and Biome) report very similar amounts of carbon stock changes for the same amount of forest to non-forest change. These definitional scenarios are insensitive, however, to actual changes in the landscape-level carbon stock. Thus, although the same carbon stock change is reported for the ARD land area, the actual carbon stock change in the landscape could be positive, unchanged, or negative.
In two definitional scenarios (FAO and Land Cover), ARD land is created during the harvest/regeneration cycle. In the FAO scenario, the regeneration activity-not the harvest-creates ARD land. If land-based accounting approach I is used, regeneration following harvest that occurs during the commitment period establishes ARD land, and a carbon stock change over the commitment period is reported that includes the effects of harvesting. If land-based approach II or the activity-based accounting approach is used, the carbon stock change would start from the regeneration activity; because it would cover less than the 5-year commitment period, it would not account for the impacts of harvest on aboveground biomass.
These two definitional scenarios (i.e., FAO with land-based I accounting of
stock changes, and Land Cover) report debits for ARD land in the early commitment
periods even if the stocks in the entire landscape are not changing because
regrowth on lands harvested prior to 1990 is not counted. In the FAO scenario
with land-based II carbon accounting approach-and if dead organic matter and
soil are included-there is still a carbon debit in the first commitment period,
but this debit is 70 percent lower than with the land-based I accounting approach.
If only aboveground biomass carbon is reported, small credits would be recorded
as the regenerating seedlings accumulate carbon. Over longer time periods (decades
to a century), large credits would be reported even if stocks on the landscape
are unchanged because reforestation is credited whereas harvesting is not debited
if the land-based II or activity-based accounting approach is applied to the
If the intent is to report carbon stock changes that are accurate reflections of actual carbon stock changes in a landscape, only full carbon accounting will suffice. If the intent is the most accurate accounting of carbon stock changes in the ARD land area, a definitional scenario that includes only forest to non-forest transitions is best.
Differences between actual and reported carbon stock changes can be reduced further if forest management activities that alter landscape-level carbon stock are included using the mechanisms defined in Article 3.4.
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