akar.earthRoot-to-shoot ratio: Definition
What is a root-to-shoot ratio? How it converts above-ground biomass to root biomass in forest carbon accounting, and how to pick one for Indonesia.
A root-to-shoot ratio is the ratio of below-ground root biomass to above-ground biomass for a tree or stand, expressed as a dry-weight fraction. A ratio of 0.25 means roots weigh a quarter as much as stems, branches and leaves.
Why it matters
Few projects dig up roots. Instead, they estimate below-ground biomass by multiplying above-ground biomass by this ratio. That single number can change total biomass, and therefore credits, by a fifth or more. Verifiers check that the value is sourced, matches the forest type and is applied conservatively.
How it is chosen and used
The calculation is simple: BGB = AGB × R. The hard part is choosing R. Common sources are:
- IPCC default tables in the IPCC AFOLU Guidelines, which give ratios by ecological zone and by above-ground biomass class. Ratios usually fall as stands get bigger.
- Methodology defaults or tools named in the methodology, such as CDM A/R tools.
- Published studies for a specific species or forest type, if they are peer reviewed and from comparable conditions.
Some methodologies prefer a root allometric equation instead of a fixed ratio. Either way, the result is converted to carbon and CO2 like any other biomass; see CO2 from biomass.
Indonesia angle
Indonesian projects span very different systems. Young ARR plantings on degraded land, mature lowland dipterocarp forest, mangroves and peat swamp forest all allocate biomass differently. Mangroves in particular have large root systems, so a generic terrestrial ratio is not appropriate. For smallholder coffee and cocoa agroforestry, published local ratios are scarce, so an IPCC default for the matching zone is usually the defensible choice.
Common mistakes
- Using one ratio across forest types and ages in the same project.
- Picking the highest ratio in a published range to boost credits.
- Applying the ratio to carbon or CO2 rather than to dry biomass, then converting twice.
- Not reporting the biomass class used to select the IPCC value.
Related terms
See below-ground biomass, carbon pool and allometric equation. The ARR hub shows how these factors feed a project’s removal estimate.
Frequently asked questions
What is a typical root-to-shoot ratio for tropical forest?
IPCC default ratios for most tropical forest types fall roughly between 0.2 and 0.4, depending on ecological zone and above-ground biomass class. Check the current IPCC table for the exact value.
Where do I find a root-to-shoot ratio for my project?
Use the IPCC AFOLU default tables, a value named in your methodology, or a peer-reviewed study for the same forest type and region.