Choice of methods

This section provides guidance on methods for calculating carbon stock change in biomass due to the conversion of land from natural conditions and other uses to Cropland, including deforestation and conversion of pasture and grazing lands to Cropland. The methods require estimates of carbon in biomass stocks prior to and following conversion, based on estimates of the areas of lands converted during the period between land-use surveys. As a result of conversion to Cropland, it is assumed (in Tier 1) that the dominant vegetation is removed entirely leading to emissions, resulting in near zero amounts of carbon remaining in biomass. Some type of cropping system is planted soon thereafter increasing the amount of carbon stored in biomass. The difference between initial and final biomass carbon pools is used to calculate carbon stock change from land-use conversion; and in subsequent years accumulations and losses in perennial woody biomass in Cropland are counted using methods in Section 5.2.1 (CroplandRemaining Cropland).

It is good practice to consider all carbon pools (i.e., above ground and below ground biomass, dead organic matter, and soils) in estimating changes in carbon stocks in Land Converted to Cropland. Currently, there is insufficient information to provide a default approach with default parameters to estimate carbon stock change in dead organic matter (DOM) pools2. DOM is unlikely to be important except in the year of conversion. It is assumed that there will be no DOM in Cropland. In addition, the methodology below considers only carbon stock change in above-ground biomass since limited data are available on below-ground carbon stocks in perennial Cropland.

The IPCC Guidelines describe increasingly sophisticated alternatives that incorporate greater detail on the areas of land converted, carbon stocks on lands, and loss of carbon resulting from land conversions. It is good practice to adopt the appropriate tier depending on key source analysis, data availability and national circumstances. All countries should strive for improving inventory and reporting approaches by advancing to the highest tier possible given national circumstances. It is good practice for countries to use a Tier 2 or Tier 3 approach if carbon emissions and removals in Land Converted to Cropland is a key category and if the sub-category of biomass is considered significant based on principles outlined in Volume 1, Chapter 4. Countries should use the decision tree in Figure 1.3 to help with the choice of method. Land Converted to Cropland is likely to be a key category for many countries and further biomass is likely to be a key source.

Tier 1

The Tier 1 method follows the approach in Chapter 4 (Forest Land) where the amount of biomass that is cleared for cropland is estimated by multiplying the area converted in one year by the average carbon stock in biomass in the Forest Land or Grassland prior to conversion. It is good practice to account completely for all land conversions to Cropland. Thus, this section elaborates on the method such that it includes different initial uses, including but not limited to forests.

Equation 2.15 in Chapter 2 summarises the major elements of a first-order estimation of carbon stock change from land-use conversion to Cropland. Average carbon stock change on a per hectare basis is estimated for each type of conversion. The average carbon stock change is equal to the carbon stock change due to the removal of biomass from the initial land use (i.e., carbon in biomass immediately after conversion minus the carbon in biomass prior to conversion), plus carbon stocks from one year of growth in Cropland following conversion. It is necessary to account only for any woody vegetation that replaces the vegetation that was cleared during land-use conversion. The GPG-LULUCF combines carbon in biomass after conversion and carbon in biomass that grows on the land following conversion into a single term. In this method, they are separated into two terms, BAFTER and ACg to increase transparency.

At Tier 1, carbon stocks in biomass immediately after conversion (BAFTER) are assumed to be zero, since the land is cleared of all vegetation before planting crops. Average carbon stock change per hectare for a given land-use conversion is multiplied by the estimated area of lands undergoing such a conversion in a given year. In subsequent years, change in biomass of annual crops is considered zero because carbon gains in biomass from annual growth are offset by losses from harvesting. Changes in biomass of perennial woody crops are counted following the methodology in Section 2.3.1.1 (Change in carbon stocks in biomass in land remaining in a land-use category).

2 Any litter and dead wood pools (estimated using the methods described in Chapter 2, Section 2.3.2) should be assumed oxidized following land conversion.

The default assumption for Tier 1 is that all carbon in biomass removed is lost to the atmosphere through burning or decay processes either on-site or off-site. Tier 1 calculations do not differentiate immediate emissions from burning and other conversion related losses.

Tier 2

The Tier 2 calculations are structurally similar to Tier 1, with the following distinctions. First, Tier 2 relies largely on country-specific estimates of the carbon stocks in initial and final land uses rather than the default data. Area estimates for Land Converted to Cropland are disaggregated according to original vegetation (e.g., from Forest Land or Grassland) at finer spatial scales to capture regional and crop systems variations in country-specific carbon stocks values.

Second, Tier 2 may modify the assumption that carbon stocks immediately following conversion are zero. This enables countries to take into account land-use transitions where some, but not all, vegetation from the original land use is removed.

Third, under Tier 2, it is good practice to apportion carbon losses to burning and decay processes if applicable. Emissions of carbon dioxide occur as a result of burning and decay in land-use conversions. Further, non-CO2 trace gas emissions occur as a result of burning. By partitioning losses to burning and decay, countries can also calculate non-CO2 trace gas emissions from burning (Section 5.3.4).

The immediate impacts of land conversion activities on the five carbon stocks can be summarized in a disturbance matrix, which describes the retention, transfers and releases of carbon in the pools in the original ecosystem following conversion to Cropland. A disturbance matrix defines for each pool the proportion that remains in that pool and the proportion that is transferred to other pools. A small number of transfers are possible, and are outlined in a disturbance matrix in Table 5.7. The disturbance matrix ensures consistency of the accounting of all carbon pools.

Biomass transfers to dead wood and litter can be estimated using Equation 2.20. Tier 3

The Tier 3 method is similar to Tier 2, with the following distinctions: i) rather than relying on average annual rates of conversion, countries can use direct estimates of spatially disaggregated areas converted annually for each initial and final land use; ii) carbon densities and soil carbon stock change are based on locally specific information, which makes possible a dynamic link between biomass and soil; and iii) biomass volumes are based on actual inventories. The transfer of biomass, to dead wood and litter following land-use conversion can be estimated using Equation 2.20.

Table 5.7

Example of a simple disturbance matrix (Tier 2) for the impacts of land conversion activities on carbon pools

Harvested wood products

Atmosphere

Sum of row (must equal 1)

Table 5.7

Example of a simple disturbance matrix (Tier 2) for the impacts of land conversion activities on carbon pools

Harvested wood products

Atmosphere

Sum of row (must equal 1)

Enter the proportion of each pool on the left side of the matrix that is transferred to the pool at the top of each column. All of the pools on the left side of the matrix must be fully accounted, so the values in each row must sum to 1. Impossible transitions are blacked out.

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