Carbon from Dry Biomass (and CO₂ Equivalent)

Roughly half of dry plant matter is carbon, so a carbon stock is a biomass multiplied by a fraction. The IPCC's 2006 Guidelines give 0.47 as the default for above-ground forest biomass, and it is used in national inventories worldwide. Multiply the carbon by $44/12 = 3.667$ — the mass of a CO₂ molecule over the mass of the carbon atom in it — and you have the carbon dioxide equivalent that carbon markets and greenhouse-gas inventories actually trade in. Using precise IUPAC masses gives 3.6641 instead, a difference of 0.07 %, which is nothing beside every other uncertainty on this page; 44/12 is the convention reports are audited against.

0.47 is a default precisely because the true fraction varies, and the variation is systematic rather than random. Measured carbon fractions run from about 0.43 to 0.55. Conifers sit higher than broadleaves — roughly 0.50 to 0.51 against 0.47 to 0.48 — because of their greater lignin and resin content, and lignin is a more reduced, more carbon-rich molecule than cellulose. Tissues differ within a single tree: bark, heartwood, sapwood, branch and foliage are not interchangeable, so a whole-tree fraction is a mass-weighted average and a stemwood-only fraction applied to whole-tree biomass is a silent error.

There is also a known method bias. The standard preparation dries a sample at 105 °C, which drives off volatile carbon compounds along with the water; the carbon that leaves with them is never counted. Work by Thomas and Martin and others puts the resulting understatement at roughly one to two percentage points, which means the conventional fractions are all slightly low in a consistent direction. Choosing 0.50 instead of 0.47 moves a national inventory by 6 % — larger than most inventories' stated precision, and a reminder that a default is a policy choice as much as a measurement.

But the fraction is rarely the biggest error. Moisture is. The $B$ in this equation must be oven-dry mass. Freshly felled wood is 30 to 60 % water by green weight depending on species and season, with sapwood far wetter than heartwood; air-dried wood still holds 12 to 18 %. Treating a green weight as a dry one roughly halves the apparent carbon fraction and doubles the apparent stock, and no amount of care over the third decimal of $f_C$ recovers from it. Every serious protocol specifies oven-dry at a stated temperature for this reason.

Running the relation backwards makes the scale of forest carbon concrete in a way the abstractions do not. One tonne of CO₂ is 0.273 t of carbon, which at $f_C = 0.47$ is about 0.58 t of oven-dry biomass; at a wood density of 450 kg/m³ that is roughly 1.3 cubic metres of solid wood. So offsetting a single transatlantic return flight — call it a tonne of CO₂ per passenger — means growing, and then permanently keeping, something like a cubic metre and a third of new wood. The word doing the work in that sentence is permanently: carbon in biomass is a stock on loan, and the litter and decomposition pages on this site describe exactly how fast it goes back.