California Air Resources Board

Offsets 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 reductions and removal enhancements on a per acre basis within the boundary of an offset project. To accomplish this, ARB has approved the use of volume and biomass equations generated through the United States Department of Agriculture (USDA) forest Service Forest Inventory and Analysis (FIA) National Program. These equations are 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. Equations vary by region and by species.

Estimating Volume and Biomass using Methods and Equations for Estimating Aboveground Volume, Biomass, and Carbon for Trees in the U.S. Forest Inventory, 2010 (Woodall et al. 2011) and the Component Ratio Method

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

Offset Project Operators (OPO) or Authorized Project Designees (APDs) must estimate gross and sound cubic foot volume by region and by species for all trees having a DBH ≥ 5 inches. Woodland species are excluded and must be estimated following a different approach¹. Methods and Equations for Estimating Aboveground Volume, Biomass, and Carbon for Trees in the U.S. Forest Inventory, 2010 (Woodall et al. 2011). Download the Coefficients Database  which includes coefficients necessary to calculate gross volume for each region and species and sound volume for some regions. Volume equations for all states except California, Oregon, and Washington can be found in the document.

1. Species code: In the Coefficients Database, open the REF_SPECIES workbook and find the species code, identified in the database as "SPCD", and wood and bark specific gravity, identified in the database as "SPGR" values for each species in the project area.

2. Gross cubic-foot volume equation by region and species: Use the appropriate table in Appendix A within Woodall (2011) based on the project's location to identify the gross cubic-foot volume (VOLCFGRS) equations for a compliance offset project.

• Table 1  provides VOLCRGRS equations for the Northern Region of the United States, which includes IA, IL, IN, MO, KS, ND, NE, SE, MI, MN, WI, OH, WV, MD, DE, PA, NJ, CT,RI, MA, NY, NH, VT, and ME.
• Table 2  provides VOLCFGRS equations for the Southern Region, which includes TX, OK, AR, LA, MS, AL, GA, FL, SC, NC, VA, KY, TN, and VA.
• Table 3  provides VOLCFGRS equations for the Rocky Mountain Region, which includes NM, AZ, UT, CO, WY, MT, NV, and ID².
• Table 4  should not be used. Follow the instructions for California, Washington and Oregon found here. 

3. Find the appropriate VOLCFGRS coefficients: In the Coefficients Database, open the Gross Cubic Foot Volume Equation Coefficients workbook.  Using the correct "config" worksheet and the "SPCD" determined in step 1 locate the correct species in the "SPECIES_NUM" column to identify the correct coefficient table (COEF_TABLE") worksheet and corresponding species number ("COEF_TBL_SP") that will be used to identify the correct coefficients for the volme equation. The coefficient table species number may be different than the "SPCD" determined in step 1. Then use the appropriate coefficient table and coefficient table species number to obtain the coefficients for the VOLCFGRS equation identified in step 2.

4. Calculate VOLCFGRS: Use the identified VOLCFGRS equation and the correct coefficients to estimate gross volume for live and dead standing trees. Repeat for each species within project area. 

Note:

• If a species is not listed in the Coefficients Database it cannot be included in the project. Substitutions with coefficients from other regions or with similar species are not permitted.

• A project is only permitted to use a genus-only species code when a species binomial (i.e., including both genus and species names) is not listed in the "REF_SPECIES" workbook.

• Volume models are not used for trees with DBH <5". The Component Ratio Method may be used directly to estimate biomass for trees with DBH <5".

5. Sound cubic-foot volume equations by region and species: For projects located in the 45 contiguous states except CA, OR, and WA, sound volume (VOLCFSND) is calculated for both standing live and standing dead trees by deducting the volume of wood that is rotten or missing from the VOLCFGRS. Deductions are based on the percentage of missing or rotten volume estimated or directly measured in the field during the inventory. VOLCFSND equations can be found in Appendix A, Table 5 of Woodall (2011).

5a.  For projects located in IA, IL, IN, KS, MI, MO, MN, ND, NE, SD AND WI: The VOLCFSND equations incorporate standard volume deduction coefficients that must be obtained from the coefficients database.  Use Appendix A, table 5 of Woodall (2011) to determine which equation is used for projects located in these states. There are separate sound cubic foot volume equations for projects located in KS, MI, MN, ND, NE, SD, WI and for projects located in IA, IL, IN, MO.

5b.  For projects located in states other than CA, OR, WA, IA, IL, IN, KS, MI, MO, MN, ND, NE, SD and WI: OPO/APDs must estimate the percentage of rotten or missing volume, no coefficients are needed.

6. For projects requiring a coefficient for sound volume: In the Coefficients Database, open the "Sound Cubic Foot Volume Equation Coefficients" workbook. Using the correct "config" worksheet and the "SPCD" determined in step 1 locate the correct species in the "SPECIES_NUM" column to identify the correct coefficient table ("COEF_TABLE") worksheet and corresponding species number (COEF_TBL_SP") that will be used to identify the correct coefficients for the volume equation.  The coefficient table species number may be different than the SPCD determined in step 1. Open the correct species coefficient worksheet and identify the rows corresponding to the coefficient table species number. The coefficients vary depending on the tree class code (Treeclcd).

For all regions, OPO/APDs must assign a Treeclcd (2-3-31³ or other) to the inventory based on the definitions in the table below. Tree class codes must be determined from field observations for new inventories⁴. For inventories conducted prior to January 1, 2015, OPO/APDs may use field notes from the existing inventory to assign tree class codes if the code assignments are based on documented soundness and decay notes taken during the inventory's data collection. OPO/APDs must be able to demonstrate when and how decay clases were determined.

    Table: Tree class code ("Treeclcd")

7. Calculate VOLCFSND: Use the identified VOLCFSND equation and the correct coefficients to estimate sound volume for live and dead standing trees. Repeat for each tree in project area.

8. Adjust standing dead tree carbon pool: The standing dead tree carbon pool must be adjusted for density reduction and structural loss using the approach established in Accounting for density reduction and structural loss in standing dead trees: Implications for forest biomass and carbon stock estimates in the United States (Domke et al. 2011).  OPO/APDs will need to assign decay class categories 1-5, density reductions factors (DRFs), and structural loss adjustment (SLA) values for each tree in an inventory in order to adjust standing dead tree biomass.  

DRFs are found by Decay Class in Appendix B Differences Between Standing and Downed Dead Tree Wood Density Reduction Factors: A Comparison Across Decay Classes and Tree Species (Harmon et al 2011). Use appropriate values from the standing dead tree "SD" column in Appendix B. OPOs/APDs should cite the source (e.g. document and appendix name) of the DRF for ease of verification. If a species is not identified in Appendix B, OPO/APDs should first see if a species within the same genus exists in Appendix B for the appropriate density class. Appendix B provides density reduction factors for species that have been directly sampled⁵. If a species does not have an associated DRF for the appropriate decay class within Appendix B, look up the species in Appendix D. If the species does not have an associated DRF in Appendix D, identify a related species within the same genus in Appendix D and use the associated DRF for the same decay class (1-4). If a related species within the same genus cannot be identified, use a DRF default value listed in Table 6 by hardwood or softwood in Table 6 of Harmon (2011). SLA values are found in Table 2 of Domke (2011) repeated below.  These SLA values are applicable to all species in the 45 states. Adjustments are applied and integrated during the Component Ratio Method calculations.

Structural loss adjustment (SLA) factors (Domke 2011)

9. Estimate biomass using the Component Ratio Method: The Biomass Estimation Component Ratio Method (CRM) document is an excerpt from the FIA Database (FIADB) User's Manual and should be used to calculate biomass for all projects outside  California, Oregon, and Washington. It provides a nationally consistent method of estimating tree biomass. The CRM involves calculating the dry weight of individual components before estimating the total aboveground or belowground biomass.

The CRM document describes the equations used to estimate various components of tree biomass (stem wood, top and branches, bark, stump, course roots). Most of these components are estimated through a series of ratio equations. The companion document Biomass Coefficients for Use with Component Ratio Method  provides the coefficients for common species.  Stem wood biomass is calculated directly from the sound cubic-foot volume of the tree bole, percentage of bark on the bole, and specific gravities of both wood and bark.

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. Below ground biomass is estimated for all trees greater than or equal to 1 inch.
 

Trees with DBH < 5 inches extimate biomass using the Component Ratio Method directly with DBH only. No cubic foot volume estimation is required for saplings.

^[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.)
^[2]Table 3 includes equations for  easten and western portions of the states listed. The OPO must identify which portion the project falls into, using the map, to determine the equations.
^[3]Due to a transcription error, please note that the tree class code "31" in the Woodall coefficients database is the same as "4" in the FIA Users Manuel.
^[4]A "tree class code" is defined in the FIA Database (version 6.0.1) as a code "indicating the general quality of the tree. In annual inventory, this is the tree class for both live and dead trees at the time of current measurement. In periodic inventory, for cut and dead trees, this is the tree class of the tree at the time it died or was cut. Therefore, cut and dead trees collected in periodic inventory can be coded as growing-stock.
^[5]Appendix D is a larger compilation of DRFs by species that include species found in Appendix B that have been directly sampled (identified by code "A"), DRFs that have been estimated for a species based on the genus sampled (identified by code "B"), and DRFs for species that were estimated based upon available literature (identified by code "c").


 

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