Tree Volume, Biomass, and Carbon Models

The Forest Inventory and Analysis (FIA) program published a new modeling system in September 2023 for predicting tree cubic-foot volume, biomass, and carbon attributes, completing a goal of the 2015 FIA Strategic Plan. This system, termed ‘National Scale Volume and Biomass Estimators’ (NSVB), provides a more consistent and accurate accounting of structural components of trees (see Figure 1) across the US for total tree cubic-foot volume, biomass, and carbon. The NSVB models were released on September 30, 2023 and are available in FIADB.

The NSVB is the result of research conducted by Forest Service scientists and partners, bringing in extensive new data and contemporary model building techniques to assess and monitor the status and trends of biomass and carbon in the US more accurately. The new methods replace the former ‘Component Ratio Method’ (CRM) and regionally applied cubic-foot volume estimates. The General Technical Report (GTR) detailing methods and equations is available. Differences in estimates rendered by NSVB and the former models are summarized in Figures 2 and 3 and further discussed in the FAQ section of this page. More information is also available at Research Overview with State and National Forest Regions Reports.

Release of the cubic-foot volume, biomass, and carbon models provides additional Forest Inventory and Analysis methodologies for its surveys and is a supplement to the Sampling and estimation documentation for the Enhanced Forest Inventory and Analysis Program: 2022.  
 

Figure 2. Total above ground biomass estimates based on current and new modeling systems by regional FIA unit. The blue ‘CRM’ bars refer to the legacy Component Ratio Method (CRM) and the orange ‘NSVB (new)’ bars refer to the updated National Scale Volume and Biomass (NSVB) estimators.

Figure 3. Percent increase in wood volume and biomass by component due to updated NSVB estimators relative to the Component Ratio Method estimates and regionally applied cubic-foot volumn estimates. To further explore differences by State or National Forest or more information about the models see: Research Overview with State and National Forest Regions Reports

Frequently Asked Questions

1. Why is this effort important and why is it being done now?

• The incorporation of substantial new data into building NSVB models provides more scientifically accurate and consistent predictions of tree volume, biomass, and carbon on US forests. 
• The 2014 Farm Bill recognized the need to …” report on renewable biomass supplies and carbon stocks at the local, state, regional, and national level.” resulting in prioritizing this modeling in the FIA Strategic Plan (2015) and this is a realization of that priority.
• Forest land, harvested wood products (HWP), woodlands, and urban trees within the land sector collectively represent the largest net carbon sink in the United States. Accurate accounting of forest carbon stocks and sequestration is fundamental for guiding policies and actions to meaningfully address climate change.
• Although the CRM methods provided estimates for forest attributes across the United States, there were inconsistencies in regional or state volume models, which could be detected when analyzing data that spanned Forest Inventory and Analysis regions. The NSVB offers a nationally consistent framework and biomass estimates for all tree components. 

2. What is the difference between tree volume and biomass?

Volume refers to dimensions of the stem - how much space the tree takes up. Biomass refers to how much the stem and branches weigh which is a function of both volume and wood density. 

3. Why create national volume and biomass equations? Who do you expect to use the resulting estimates?

• FIA data on tree volume, biomass, and carbon stocks are used for US annual reporting of greenhouse gas inventories in the forestry sector to the United Nations Framework Convention on Climate Change (UNFCCC), state volume and carbon estimates, and forest carbon projects participating in voluntary or regulatory carbon markets. 
• FIA data increasingly serve as the foundation of research on topics including carbon, old growth, and forest sustainability. The NSVB models also provide data to inform implementation of the Wildfire Crisis Strategy.

4. Why is NSVB an improvement over CRM models?

• CRM biomass models for the non-merchantable portion of the tree (e.g., branches), were not based on equations created from actual tree measurements. They predicted non-merchantable biomass based on estimates of the merchantable bole of the tree. In contrast, the NSVB biomass and carbon equations were developed from detailed tree measurements, allowing for direct modeling of non-merchantable components.
• The utilization of tree measurements and raw empirical data for the development of NSVB models allows for more accurate quantification of model uncertainty so that users can assess the reliability of the predictions. 
• The new modeling framework is based on whole stem volume equations that are additive across the components: stump, merchantable bole, and non-merchantable top (see Figure 1). For biomass, these components, along with tree branches, are additive to equal total aboveground biomass. 
• NSVB eliminates administrative boundaries in favor of ecological definitions of spatial differences, better reflecting environmental gradients that influence tree size, form, and growth. 
• The NSVB update converts biomass to carbon content using species-specific carbon fractions, which is more precise than the standard 0.5 multiplier applied across all tree species in the CRM. 
• The new framework was developed with the participation of all four FIA regions, seven universities, and industry stakeholders, and was independently reviewed. 

5. How different are the results of new NSVB models compared to the former CRM models?

• Differences between the estimates produced by NSVB and former models vary across tree species and geographic areas. Except for Washington and Nebraska, all states show biomass increases above 5%, with Oklahoma, Indiana, Illinois, Missouri and Michigan increasing at least 25%. 
• From a national perspective, there are only very minor differences in total merchantable bole volume and biomass but increases in other components such as top and limb biomass contribute to increases in total aboveground biomass. 
• Species-level differences also show wide variation with some species having 15-20% increases in aboveground biomass while others decreased. 
• Total changes in state biomass estimates also depend on the amount of forested area within a state (i.e., states with large forest areas will likely see larger increases even if the new equations have a modest effect on individual tree estimates). 
• To explore the differences between the CRM and NSVB models, information by State and National Forest Region, see: Research Overview with Station and National Forest Region Reports.

6. How much do these biomass models change the total above ground tree carbon in U.S. Forests?

• The NSVB produces an estimate of 34.71 billion tons of biomass in U.S. forests, compared to 30.28 billion tons using previous (CRM) methods -- an increase of 14.6% in aboveground tree biomass. 
• National estimates of carbon storage in U.S. forests of 16.89 billion tons have increased 11.6% from 15.14 billion tons from the previous method. 
• See Figure 3.

7. How does this affect previous FIA reports?

• FIA will not modify reports created before the NSVB release in September 2023. Estimates of the number of stems, forest area, or board-foot volume are not affected by the release of NSVB so they can be compared over time. However cubic-foot volume, total aboveground biomass, biomass of tree components, and carbon pool estimates will change for most species which means comparison between NSVB estimates and former estimates cannot be done. 
• The CRM attributes will be available in a snapshot of the FIA database and housed in the Research Data Archive. They will also be available in CSV format so users can make comparisons of estimates between both models. All data collected under the “annual inventory” will be available in the FIA public database (FIADB) when the NSVB models are released. 

8. How often does the Forest Inventory and Analysis update their protocols and models and are there plans for future updates?

• The national FIA program was developed from four distinct regional programs.  Over the last two decades, FIA has been prioritizing a more consistent national program which has led to various updates to data collection protocols, data warehousing, and analysis tools.  
• All updates to the program are well documented, with formal change management processes. As new technology is developed (e.g., LiDAR), the models for estimating volume, biomass, and carbon may be updated to incorporate new information and technology resulting in more accurate estimates.
• While there are no further updates planned at this time, changes in technology and scientific advancements will continue to drive FIA program updates. 

9. Are the National Forests going to adopt this set of volume equations for Timber Sales?

• National Forests may decide to adopt these equations. However, they may also apply more localized equations as well. 
• These equations will be available in the National Volume Estimator Library and the National Biomass Estimator Library maintained by the Forest Management Service Center. As such, they will be available within the Forest Vegetation Simulator for use in growth and yield modeling.

10. Will the adoption of the NSVB models impact FIA analyses of regional and national GHG emissions and carbon sequestration?

• The new models are more accurate in their estimates of above ground tree biomass and carbon. Initial analysis shows that carbon stocks and annual stock exchanges will increase relative to past inventories.  

11.How does this affect research and applications that applied the previous models?

• The FIA database is a living resource and subject to periodic changes in methods of data collection and compilation processes. While this results in a robust FIA database in the long run, there are invariable disruptions for FIA’s user base. 
• Data are available to compare estimates from the 2 methods (see question 7 and 9).

12. How many trees were sampled in developing this new framework?

The models are based on measurements from over 232,000 sampled trees. FIA destructively sampled about 3,200 trees for this work, additional tree data came from reports and studies developed by the National Forest System, the forest products industry, and research by universities and others. Most of this information is available at LegacyTreeData: A repository of individual tree measurements of volume, weight, and physical properties. (vt.edu)

Additional Links

General Technical Report

• A national-scale tree volume, biomass, and carbon modeling system for the United States [post-print]
  • Supplemental Data Files

State and National Forest Region Comparisons:

• Research Overview with State and National Forest Regions Reports

Raw data used for fitting the NSVB models:

• www.legacytreedata.org

Archived Data:

• This previous cubic-foot volume, biomass, and carbon models used by FIA, is in SQLite and Excel spreadsheet format. This data can be used to generate comparisons between old and new methodologies.