Below-ground biomass: Definition

Updated 2026-11-02 · 2 min read · By the akar.earth specialist team

What is below-ground biomass (BGB)? Definition, how root biomass is estimated with root-to-shoot ratios, and how carbon projects in Indonesia account for it.

Below-ground biomass (BGB) is the dry weight of all live roots of trees and other plants. Fine roots under about 2 mm diameter are often excluded because they are hard to separate from soil organic matter. BGB is a distinct carbon pool that adds a significant share on top of above-ground biomass.

Why it matters

Roots can hold a substantial fraction of a tree’s total carbon. Leaving them out understates removals in ARR projects. Including them with a poorly chosen factor can overstate them. Because roots are rarely measured directly, BGB is one of the less certain numbers in a carbon calculation, and verifiers check the source of the factor used.

How it is estimated

Direct measurement means excavating root systems, which is slow and destructive. Projects therefore estimate BGB in one of two ways:

• Root-to-shoot ratio: BGB = AGB × R, using a ratio from IPCC tables or published studies for the relevant forest type and biomass class.
• Root allometry: an equation that predicts root biomass from DBH or from above-ground biomass, such as the widely cited Mokany et al. (2006) relationship.

The resulting biomass is converted to carbon and CO2 in the same way as above-ground biomass. See CO2 from biomass for the steps.

Indonesia angle

Two Indonesian ecosystems need special care. Mangroves invest heavily in roots, including stilt and aerial roots, so generic terrestrial ratios understate them; mangrove-specific equations exist. In peat swamp forest, roots sit in waterlogged organic soil, and BGB must be kept separate from peat carbon to avoid double counting. Our guide to carbon pools covers how pools fit together.

Common mistakes

• Applying a temperate-forest ratio to tropical plantations.
• Using one ratio for all ages; young trees often allocate proportionally more to roots.
• Counting root carbon twice, once as BGB and again as soil organic carbon.
• Not citing the ratio’s source and biomass class in the PDD.

Related terms

Below-ground biomass is closely tied to above-ground biomass, root-to-shoot ratios and soil organic carbon. For how all pools combine in a planting project, see the ARR hub.