Every forest carbon calculation contains a step where dry biomass is converted to carbon by multiplying by a fraction, which is almost always 0.47, the IPCC default. It appears in every project document, is rarely discussed, and is one of the better behaved parameters in the whole chain. It is also not universally 0.47.
Where the Number Comes From
Dry plant biomass is roughly half carbon by mass, the remainder being oxygen, hydrogen and small quantities of nutrients. Measured across many species, the carbon content of woody biomass clusters tightly around 47–50%.
IPCC settled on 0.47 for tropical and subtropical forests as a central value from that body of measurement. Earlier guidance used 0.50, and some older project documents still do, resulting in a difference of about 6% in the final carbon figure, which is not nothing.
| Value | Where it appears | Note |
|---|---|---|
| 0.50 | Older literature and some national inventories | Slightly high for most tropical species |
| 0.47 | IPCC 2006 default, tropical/subtropical | The current standard choice |
| Species-specific (0.44–0.52) | Measured values | Justifiable with local lab data |
Why It Is Low-Risk
Compared with the other parameters in the chain, the carbon fraction is well constrained. Wood density varies by a factor of three across tropical species; allometric equations can differ by tens of percent; the carbon fraction varies within a narrow band around a well-established central value.
That means it is a poor target for improvement effort. A project agonising over 0.47 versus 0.48 while using a generic pan-tropical allometric equation is optimising a parameter that contributes little uncertainty and ignoring one that contributes a great deal.
Where It Does Differ
Two situations justify moving off the default.
Non-woody components. Foliage and fine material have different carbon content from stemwood. Where a project accounts for these separately, which is uncommon but occurs in agroforestry systems with substantial non woody biomass, the fraction should reflect the component.
Documented species-specific values. If reliable local measurements exist for the dominant species, using them is more accurate and permitted. The requirement is documentation of the analytical method, not just a citation to a number.
The Rule That Actually Matters
Whatever value is chosen, it is a fixed ex-ante parameter. It must be the same in the baseline and the project scenario, and the same across every monitoring event.
Changing it mid project, for instance by adopting a newly published local value in year seven, changes the reported carbon independently of anything the forest did. If the change is worth making, it has to be applied retrospectively across all periods, with the restatement documented.
That is the whole of the carbon fraction's practical significance: pick the standard value, write down which one, and never quietly change it.
Turn uncertainty into a plan
Sampling design and growth assumptions decide how much measured carbon survives conservative deduction. TREEO Carbon Simulator models sequestration scenarios and biomass growth before the field season.
