18.104.22.168. Scenarios in Which Forest Degradation or Aggradation Create ARD Land
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
22.214.171.124. Land- and Activity-Based Carbon Accounting
Section 3.3.2 introduces three possible approaches to
1) A land-based approach in which an activity creates ARD land and the carbon
stock change on that land is reported for the entire commitment period (land-based
2) A land-based approach in which an activity creates ARD land and, if the
activity occurred during the commitment period, the carbon stock change on
that land is reported from the start of the activity to the end of the commitment
period (land-based approach II).
3) An activity-based approach in which each activity is assigned a carbon stock
change, and the reported carbon stock change of a landscape is calculated
as the area of each activity times the stock change assigned to each activity.
Only stock changes resulting from an ARD activity are accounted. There can
be carbon stock changes on ARD land that do not result from an ARD activity
and therefore are not accounted. This approach introduces a series of complications-such
as multiple activities on the same land area, verification of ARD land area,
verification of carbon stock changes, and so forth. This approach could be
implemented without reference to a specific land area, but such implementation
would aggravate such complications.
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