Section 3.4

    CO₂ Removal from Below-ground Biomass (BGB)

    The below-ground biomass (BGB) represents the roots and other subterranean components of trees that store carbon. CO₂ removal from BGB refers to the carbon dioxide absorbed from the atmosphere and subsequently stored in these below-ground components as part of the tree's natural growth and carbon cycle processes and is calculated with the following equation:

    CO2 by BGB (tCO2/year)=BGB to AGB ratio×CO2 by AGB (tCO2/year)\text{CO}_2\text{ by BGB}\ (t\text{CO}_2/\text{year}) = \text{BGB to AGB ratio} \times \text{CO}_2\text{ by AGB}\ (t\text{CO}_2/\text{year})
    (3.4)

    The BGB to AGB ratio is a proportion that indicates how much carbon is stored below ground for every unit of carbon stored above ground. This ratio varies by species, ecological zone, climate, and environmental factors. The IPCC 2019 Refinement (Table 4.4) offers default ratios categorized by region and species, serving as a Tier 1 default method. If site-specific data or species-specific studies are available (Tier 2 or 3 approaches), these should be applied for greater accuracy.

    This accurate estimation of below-ground biomass (BGB) sequestration rates is critical for quantifying carbon storage in forest ecosystems. To achieve this:

    • Tier 1 Approach: Default ratios of BGB to AGB provided in global standards, such as the IPCC 2019 Refinement (Table 4.4), are applied when specific site data is unavailable. These defaults ensure consistency and a reliable starting point.
    • Refined Approaches (Tier 2 or 3): Site-specific measurements or species-based ratios derived from recent scientific literature are prioritized when available. Regionally-relevant BGB-to-AGB ratios from peer-reviewed studies or field data, reflecting actual sequestration conditions.
    CO₂ Removal from Above-ground Biomass (AGB)CO₂ Removal from Dead Organic Matter (DOM)