Forest Land, Grassland, Settlements, and other land-use categories could be potentially converted to Cropland which, in general will have little or no dead wood or litter, with the exception of agroforestry systems. Methods are provided for two types of dead organic matter pools: 1) dead wood, and 2) litter. Chapter 1of this report provides detailed definitions of these pools.
Dead wood is a diverse pool which is difficult to measure, with associated uncertainties about rates of transfer to litter, soil, or emissions to the atmosphere.
Litter accumulation depends on litterfall, which includes all leaves, twigs and small branches, fruits, flowers, and bark, minus the rate of decomposition. The litter mass is also influenced by the time since the last disturbance, and the type of disturbance. During the early stages of cropland development, litter increases rapidly. Management such as vegetation harvesting and burning dramatically alter litter stocks, but there are very few studies clearly documenting the effects of management on litter carbon.
In general, croplands will have little or no dead wood or litter, and therefore these pools can often be assumed to approach zero after conversion, the exception being agroforestry systems which may be accounted either under Cropland or Forest Land, depending upon definitions adopted by countries for reporting. It is likely that the same will be true of many land uses prior to conversion, so that corresponding carbon pools prior to conversion can also be assumed to be zero. The exceptions are forest, agro-forests, and wetlands converted to Cropland, which could have significant carbon in DOM pools, as well as forest areas around settlements that may have been defined as Settlements based on nearby use rather than land cover.
Estimating change in carbon stocks in DOM for lands converted to Cropland under higher tiers requires a two-phase approach. During the first phase, there is often an abrupt change in DOM associated with the land-use change, particularly then the change is deliberate and associated with land preparation operations (e.g., clearing and burning). The second phase accounts for decay and accumulation processes during a transition period to a new steady-state system. At some point in time, the cropland ecosystem should reach an equilibrium at which time it can be considered Cropland Remaining Cropland and accounted for under that category. The transition period should be 20 years, but some countries can determine the appropriate transition period more accurately at higher tiers.
To account for the transition period, lands converted to Cropland should be treated as annual cohorts. That is, land converted in a given year should be accounted for with Phase 1 methods in the year of conversion, and with Phase 2 methods for the subsequent 19 years. At the end of the 20 year period, the land area for that given year is added to the land area being accounted under the Cropland Remaining Cropland category.
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