CO₂ Removal from Below-ground Biomass (BGB)

The below-ground biomass (BGB) represents the roots and other subterranean components of mangrove 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:

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. Mangroves typically exhibit higher BGB to AGB ratios than other forest types due to their adaptation to waterlogged and saline conditions, which promote extensive root systems. The IPCC 2013 Wetlands Supplement 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 mangrove ecosystems. To achieve this:

• Tier 1 Approach: Default ratios of BGB to AGB provided in global standards, such as the IPCC Wetlands Supplement (Table 4.5), 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. For example, projects featuring dominant species like Rhizophora mucronata may use updated, regionally-relevant BGB-to-AGB ratios from peer-reviewed studies or field data, reflecting actual sequestration conditions.