Area-based Quantification of Woody Biomass

Several terrestrial biosphere Carbon Dioxide Removal (CDR) approaches rely on the capture and storage of carbon in living woody biomass, including both aboveground biomass (AGB) and belowground biomass (BGB). The quantification of BGB is often dependent upon AGB¹, which is more directly observable. Thus, for these pathways, accurately and conservatively quantifying the gross storage of carbon in woody biomass is crucial for demonstrating net CO₂ removal.

Aboveground biomass (AGB) encompasses all living vegetation above the soil surface, including stems, branches, foliage, and bark. Woody biomass refers to plants whose structure includes lignified stems, such as bamboo, shrubs, and trees. Belowground woody biomass (BGB) encompasses the subsurface woody biomass in root systems.

Field-based measurements underpin biomass estimation, typically identifying species and employing individual plant measurements of diameter at breast height (DBH), vegetation height, and wood density. These measurements feed into allometric equations, which are empirically derived relationships between these empirical parameters and biomass. These equations are species- or biome-specific, and are often developed through extensive field- and laboratory-based research.

This Module was developed based on the current state of the art, publicly available science regarding quantification of woody aboveground biomass and long-term monitoring of terrestrial ecosystems. This Module aims to be scientifically stringent and robust. We recognize that some requirements may exceed the status quo in the market and that there will be opportunities to improve the rigor of this Module.

Additionally, this Module will be reviewed when there is an update to published scientific literature, government policies, or legal requirements which would affect net CO₂e removal quantification or the monitoring guidelines outlined in this Module, or at a minimum of every 2 years.

This Module quantifies the total CO₂ equivalent stored as aboveground woody biomass across the project area ($CO_2e_{AGB,t}$) at a given time point, t, through direct vegetation sampling and parameter estimation within survey plots and scientifically validated allometric equations. Where included as a pool, it also quantifies the total CO₂ equivalent stored as belowground woody biomass across the project area ($CO_2e_{BGB,t}$) at a given time point, t, which is derived from the directly sampled aboveground biomass.

Throughout this Module, the use of “must” indicates a requirement, whereas “should” indicates a recommendation.

This Module only quantifies living aboveground woody biomass and, where it is included as a pool, belowground woody biomass. Standing dead trees and deadwood are excluded from this Module.

$CO_2e_{AGB}$ (tonnes) can be calculated by taking sufficient field plot measurements to obtain an estimate of the mean AGB density (in tonnes CO₂e), which can then be multiplied by the project area to obtain a total AGB carbon pool:

$$CO_2e_{AGB}(t) = \bar{m}_{AGB}(t) \times A$$

(Equation 1)

Where:

• $\bar{m}_{AGB}(t)$ is average AGB density, in tonnes CO₂e/hectare.
• $A$ is the total project area, in hectares.

Estimation of $\bar{m}_{AGB}(t)$ is based on direct measurements of tree parameters (e.g., diameter at breast height (DBH) in field plots), and the use of allometric equations to convert tree parameters to biomass.

As field plots may be of unequal area, average AGB density must be calculated as an area-weighted average to ensure the resultant $\bar{m}_{AGB}(t)$ is expressed in tonnes CO₂e per hectare:

$$\bar{m}_{AGB}(t) = \frac{\sum_{i=1}^{n} AGB_{plot,i}(t) \times a_{plot,i}}{\sum_{i=1}^{n} a_{plot,i}}$$

(Equation 2)

Where:

• $AGB_{plot,i}(t)$ is the AGB density (tonnes CO₂e/hectare), in plot $i$ at time $t$.
• $a_{plot,i}$ is the area of plot $i$, in hectares.
• $n$ is the number of plots.

Biomass density within each plot at a given time point $AGB_{plot,i}(t)$ is calculated by summing the biomass of all individual woody plants in that plot:

$$AGB_{plot,i}(t) = \frac{\sum_{j=1}^{m_i} AGB_{tree,j}(t)}{a_{plot,i}}$$

(Equation 3)

Where:

• $AGB_{tree,j}(t)$ is the aboveground biomass of the $j^th$ tree in plot $i$, derived from an appropriate allometric equation, in tonnes CO₂e.
• $m_i$ is the number of trees in plot $i$.

For each tree within the plot, the aboveground biomass of each tree is determined via an allometric model (Section 4.3) which converts field measurements of tree size into a total aboveground biomass (tonnes). This value must be converted to a CO₂e basis using the appropriate carbon fraction ($CF$), which describes the fraction of woody biomass that is carbon, and then applying the mass conversion for mass CO₂ to mass C ($\frac{44}{12}$).

The carbon fraction, $CF$, must be chosen from the following hierarchy:

1. A regional and species-specific factor that is justified based on scientific literature (e.g., Doraisami et al., 2022²). This is the preferred approach to have the most accurate estimate and avoid overestimation;
2. If the above is not available, then a genus-specific or national average factor that is justified based on scientific literature can be used;
3. As a last resort, if it is proven that the above two factors are not available, then a default factor of 47%, which is a mean across species, can be used^3,4.

If the project area is stratified, the total biomass is calculated as the area-weighted sum over all the strata:

$$M_{AGB,t} = \sum_{h=1}^n \frac{A_h \times \bar{m}_{AGB,h}}{A_{project}}$$

(Equation 4)

Where:

• $A_h$ is the area of strata $h$ of $n$ total strata, in hectares
• $\bar{m}_{AGB,h}$ is the mean AGB density in strata $h$, in tonnes per ha
• $A_{project}$ is the total project area, in hectares

The total carbon stored in belowground woody biomass at time ($M_{BGB,t}$) is determined by multiplying the measured aboveground biomass for each individual by an appropriate root-to-shoot ratio ($RS$), a dimensionless parameter which describes the ratio of belowground biomass to aboveground biomass:

$$BGB_{tree,j}(t) = AGB_{tree,j}(t) \times RS$$

(Equation 5)

Where:

• $BGB_{tree,j}(t)$ is the carbon stored in belowground biomass in tree $j$ at time $t$, in tonnes CO₂e
• $RS$ is the dimensionless root-to-shoot ratio

Appropriate root-to-shoot ratios should be selected by regional and species-specific factors that are justified based on scientific literature (e.g., USFS's Component Ratio Method or similar national-level species-specific ratios). This is the preferred approach to have the most accurate estimate and avoid overestimation. If sufficient evidence is provided to demonstrate that no suitable project-specific factor can be obtained, matching to the ecological zone and continent of the project area, based on the IPCC 2019 Table 4.4³, must be used. In this case, sufficient evidence documenting the unsuccessful search for project specific factors must also be supplied. Acceptable evidence must show (1) a list of search terms used within a research database (e.g., Web of Science, Google Scholar) that encapsulate the region and species relevant to the Project, and (2) the relevant species are not included in the list of species for which root-to-shoot ratio data are available on the TRY Plant Trait Database⁵.

The uncertainty in selected $RS$ factors must be reported from the same source dataset. For example, the IPCC 2019 Chapter on Forest Land³ provides an uncertainty in the root-to-shoot ratio.

With the calculated BGB values for each measured individual tree, the same procedure for scaling from individual measurements to project-level estimates is used for BGB as described for AGB in Section 4.1.

Project Proponents should use independently published allometric equations, from the following sources, in order of preference:

1. Local peer-reviewed equations;
2. National forest inventories;
3. Chave, et al. (2014)⁶ for project areas in the Tropics;
4. IPCC generalized equations.

Projects may deviate from this hierarchy with justification in scenarios where models from lower tiers offer better quality or fit to the project scenario (e.g., larger sample sizes, applicable size range, etc.).

Where wood density is required as part of the allometric model for calculation of AGB, Project Proponents must use wood density values from the following hierarchy:

1. Species -specific values sourced from scientific literature or databases (e.g., Global Wood Density Database)
2. Genus or Family specific values
3. Ecosystem mean values

• Provide documentation of robust validation through peer-reviewed publication or an equally-rigorous scientific review.
• Include validation datasets (must be withheld from calibration) and clearly report metrics such as relative root mean square error (RMSE), relative bias, and the residual standard error and distribution. These must be reported both overall and by diameter class.
• Provide comparative analysis with established equations to demonstrate equal or improved accuracy.
• Justify suitability explicitly for the Project's specific biome and species composition.

In order to use a newly developed allometric model, the Project Proponent must demonstrate that a suitable model does not exist in the scientific literature. To demonstrate this, Project Proponents must supply a list of search terms of relevant project characteristics to be captured in the allometric model (e.g., species, setting, size class) and a list of scientific databases which were used to search for suitable allometric models in the literature. If any potential models exist, a newly developed model can only be used where the new allometric model produces more conservative estimates of biomass or where it can be justified that the new model is of demonstrably better quality (e.g., developed using larger sample sizes, lower uncertainty, etc.). 

Models and measurements of woody biomass inherently include uncertainty from the assumptions and various sources of data which are used in the calculation of biomass.

Within each plot, the species of individual trees must be recorded. Projects must provide details on the procedure used for species identification. Stems which cannot be confidently identified must be recorded as such, and the most conservative allometric model and parameter values must be applied for calculation of the biomass. 

The following data must be collected, with the sub-bullets indicating recommended additional data collection:

• Tree diameter at 1.3 m above the ground (DBH) must be recorded for all trees in the field plot with a diameter greater than 10 cm. Where allometric models use metrics other than DBH (e.g., crown area or basal stem diameter), these variables must be collected in lieu of a standard DBH for the corresponding species and the procedures for this must be clearly defined within the field protocol.

  • Recommended: DBH should be recorded for trees with a diameter greater than 5 cm.
  • Recommended: During the initial years following planting, there may not be many trees with DBH > 5 or 10 cm. However, field plot surveys should still monitor for any potential disease, ecological hazards, and/or mortality as these risks can be higher in young trees.

• G-EBVH-0GPS coordinates of a plot reference point sufficient to enable relocation must be recorded using mapping-grade receivers or higher (capable of achieving < 3 m accuracy, e.g., 500 measurements at a rate of 1 point per second).

  • Recommended: Multiple location measurements should be taken at each point of interest.

• Tree height must be collected for individuals where it is required for the allometric model. If it is not required for the allometric model then measurement of height is optional.

  • Recommended: Tree height should be recorded for a subset of trees (e.g., selected using a basal-area factor prism), even if not required for the allometric model.

Field measurements:

• Field manual that was followed for sampling
• Description of field plots, and how they were designed, allocated, and selected
• Stratum definitions and areas
• GPS coordinates, shape, size, and orientation of field plots
• G-20Q6-0Description of measurement approaches and instruments used (e.g., diameter tape or calipers for DBH, clinometer or hypsometer for tree height)

• Allometric equations used and their sources
• Any other input parameters used (e.g., wood density) and their sources)
• Assessment of major sources of uncertainty (e.g., measurement errors, equation parameterization):
  • Quantify uncertainty for key identified sources through the approved approaches outlined in the Isometric Standard.
  • Report conservative AGB estimates

For each Verification, Project Proponents must submit a description of the measurements collected and report the full tree-level field inventory data. This includes, at minimum, the plot ID, species, and DBH for each measured individual. Any changes in methods between verifications (e.g., changes in allometric models) must be reported and justified. 

Isometric would like to thank Renoster, for their extensive feedback during this Module's development.

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