Carbon in a forest sits in five accounting compartments. Almost no project measures all five, and that is normal rather than negligent, but which ones a project omits, and in which direction the omission errs, is one of the first things an assessor checks.
The Five
| Pool | What it contains | Typically measured? |
|---|---|---|
| Above-ground biomass | Stems, branches, foliage | Almost always |
| Below-ground biomass | Roots | Usually, via root:shoot ratio |
| Deadwood | Standing and fallen dead wood | Sometimes |
| Litter | Fine dead material on the floor | Rarely |
| Soil organic carbon | Carbon in the soil profile | Rarely in forestry, often in agriculture |
Above-ground biomass dominates the measurement effort because it is the largest pool that changes fastest and is the only one that can be measured directly at scale.
Below-Ground Is Estimated, Not Measured
Nobody excavates root systems for a carbon project. Below ground biomass is derived from above ground biomass using a root shoot ratio, which is a multiplier, typically between 0.2 and 0.3 for tropical forests, applied to the above ground estimate.
This has an underappreciated consequence: below-ground biomass inherits the entire uncertainty of the above-ground estimate, and adds the ratio's own uncertainty on top. It is not an independent measurement and does not behave like one in error propagation.
Using a default ratio is standard and accepted. Using a default ratio and then reporting below-ground carbon to three significant figures is not.
The Conservativeness Test
Pool selection is governed by one principle: omission must not inflate the claim.
The test is directional. Ask whether the pool is larger in the project scenario or the baseline. Omit a pool that is larger in the project and you understate your own gain, which is conservative, permitted, and requires no argument. Omit a pool that is larger in the baseline and you overstate the difference, which is not permitted without justification.
In ARR on degraded land nearly every pool is larger in the project scenario, which is why ARR projects can omit freely. In IFM and REDD+ the picture is mixed, which is why pool selection there requires an argument rather than a checkbox.
The Symmetry Rule
Whatever set of pools a project uses, it must use the same set on both sides of the subtraction.
Counting soil carbon in the project scenario but not the baseline produces a number that is not a difference at all. This sounds too obvious to require stating, and it appears in real project documents often enough that standards state it explicitly.
What This Means for Budgets
Pool selection is largely a budget allocation decision dressed as a technical one. Measuring soil carbon well costs a great deal and, in most forestry projects, adds a modest and slow-moving quantity to the total.
The defensible position for most projects: measure above-ground biomass thoroughly, derive below-ground with a documented ratio, and exclude the rest with a written conservativeness argument. Spending the saved budget on more precise above-ground estimation buys more certainty per rupiah than extending coverage to pools that barely move.
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