California Air Resources Board

Offset Projects Operators with projects located in all states except California, Oregon, and Washington must estimate volume and biomass and convert biomass to tons of CO₂e emission reduction and removal enhancements on a per acre basis within the boundary of an offset project.  To accomplish this, ARB has approved the use of biomass equations generated from the United States Forest Service Forest Inventory and Analysis (FIA) National Program to estimate biomass.  These equations are the same equations used to generate the Common Practice estimate for each Assessment Area.  Offset Project Operators must use these same equations when modeling growth and yield for their projects.  The form of equations varies by region.

Using the Component Ratio Method

Projects in all states except California, Oregon, and Washington must utilize FIA regional cubic foot volume equations and incorporate the United States Forest Service’s Component Ratio Method (CRM) to estimate the volume in the non-bole (everything except trunk or main stem) portions of the tree.  The bole portion of the tree is estimated directly from inventory field measurements (DBH, height, and species).  The CRM involves calculating the dry weight of individual components before estimating the total above-ground or below-ground biomass.

Equations for estimating cubic foot volume are organized by Supersection and provided by species for all trees having a DBH ≥ 5 inches, excluding woodland species¹.  These equations can be found in the document “A. Cubic Foot Volume Equations for the U.S. Outside of CA-OR-WA.”  Use this document in conjunction with the table in “B. Volume Equation References and Coefficients by Species for Supersections Outside of CA-OR-WA” to determine which equations to use and to retrieve the appropriate coefficients for your project.  

The OPO must first calculate gross cubic foot volume (VOLCFGRS) for each tree, living and dead.  Gross cubic foot volume must then be converted to sound cubic foot volume (VOLCFSND) by subtracting rotten and missing cull volume.  Biomass is then calculated for individual tree components for each tree, with DBH and sound cubic foot volume as inputs.  The OPO/APD must then use document “C. Biomass Estimation Component Ratio Method” to calculate biomass.  Trees with DBH less than 5 inches do not require cubic foot volume calculations to estimate biomass using the Component Ratio Method.  Biomass for trees less than 5 inches in DBH is calculated using “C. Biomass Estimation Component Ratio Method” with DBH only.  If an equation is needed for a species that is not listed in your Supersection, contact ARB or an approved Offset Project Registry for guidance.
 

Instructions for Estimating Volume and Biomass for Projects in All States Outside California, Oregon, and Washington

To implement the Component Ratio Method an OPO/APD must follow these steps:

1. Identify the Supersection your project is located in from the Supersection Maps in the table on the Compliance Offset Protocol U.S. Forest Offset Projects webpage.
2. Identify the volume equation reference for each species in your project by Supersection by referring to the document “B. Volume Equation References and Coefficients by Species for Supersections Outside of CA-OR-WA.”  Coefficients are found in this document as well. Volume equations are found in “A. Cubic Foot Volume Equations for the U.S. Outside of CA-OR-WA.”
3. Calculate gross cubic foot volume (VOLCFGRS) of each tree, living and dead, using the equations provided, from a 1 foot stump to a 4 inch top.  This will yield the volume for VOLCFGRS.  
4. Convert the VOLCFGRS to sound cubic foot volume by subtracting rotten and missing cull volume in the central stem of each tree.
5. Download the document “C. Biomass Estimation Component Ratio Method” from the ARB Forest Resource webpage and follow the instructions to estimate biomass for each tree using VOLCFSND from Steps 3 and 4 above and DBH.  You will need to refer to the “D. Biomass Coefficients for Use with Component Ratio Method” document to obtain the coefficients for estimating biomass using CRM.

^[1] Note that component equations are not available for woodland tree species or for saplings because saplings have no volume in FIADB.  Because of this, only total aboveground biomass is estimated for saplings (trees from 1 to 4.9 inches in diameter) and woodland species [trees where diameter is measured at the root collar (DRC)].  The individual component biomass values for bole, top, and stump are not available in FIADB.  Volume equations for woodland species include all wood and bark from ground to tip.  When converted to biomass, the result is total aboveground biomass excluding foliage for these species.  Belowground biomass is estimated for all trees greater than or equal to 1 inch. (Draft FIA Database Users Manual for Phase 2 Version 4, Rev. 2.  2009.  USDA, Forest Service.  Page 362.)   

For questions or comments regarding the U.S. Forest Protocol, contact Barbara Bamberger at 916-324-2303.