At the moment, there are three primary ways to estimate forest carbon: Jenkins equations (Jenkins), Component Ratio Method (CRM) and National Scale Volume and Biomass equations (NSVB). All three are currently being used and the choice often depends on what a particular market requires.
Assisi Forest Management Software has incorporated all three as options...
Jenkins Equations
The Jenkins biomass equations were published in 2003 (Jenkins, et. al., 2003) and updated in 2013 (Chojnacky et. al., 2013). The goal of Jenkins was to create a set of equations that could be applied throughout the United States so that every region could calculate biomass in the same way. Prior to Jenkins, published biomass models differed by species, region and size classes.
The Jenkins approach used models from studies across the US to calculate the biomass in standard units of above ground and component biomass of as many species, regions and size classes as possible. This calculated dataset, called "pseudodata" became the input data for development of the Jenkins equations.
Jenkins equations are based on an allometric scaling theory approach (Chojnacky, 2002). Allometry relates the size of an organism to the structure of its energy use. In the Jenkins case, DBH is assumed to be a good indicator of the cellular ability to create biomass . DBH is also universally measured which aids in the goal of a national scale biomass model.
The Jenkins study of 2003 resulted in a set of 10 equations that cover all species in the US. Although this seems like a small amount of equations, many species were found to have very similar results even while their growing regions were diverse. For example, although pine grows in ecoregions as diverse as warm and humid to cold and dry, results indicated that all pine could be grouped into a single biomass equation.
The small number of groupings was a concern however, and a 2013 update to Jenkins addressed concerns about how species were grouped in the original publication. The original publication grouped species based on statistical significance alone. The update added wood specific gravity and taxonomy as grouping factors as suggested by allometric scaling theory. The results was an expanded list of 35 species groupings.
The advantage to Jenkins is that biomass is estimated in a consistent method for all species regardless of region. Ie, the same tree in different regions will calculate the same amount of biomass. The concern is that basing biomass on DBH alone leaves out more accurate volume estimates that include height and taper. Also, the form of the Jenkins equation results in an increasing rate of biomass versus DBH when this rate is expected to decrease as DBH rises and trees reach their maximum height.
Assisi Jenkins Implementation
The Jenkins approach first calculates the biomass of all above ground parts of a tree. This total is then proportioned to bole, bark, foliage and roots using component equations. Bole is defined from a 1' stump to a 4" top leaving the stump, top and branch (both bark and wood) biomass to be calculated as the difference from total above ground minus the other components.
In Assisi, Jenkins is implemented as both internal equations ('Jenkins') or as parsable text equations ('Parse'). The internal option requires Jenkins coefficients be set. The parsed option requires the text for biomass and components be entered.
The Jenkins equation setup for Douglas Fir shown in Assisi's Calibration Editor. Two options for Jenkins are shown: internal equation where coefficients TB0 - RB1 are required and Jenkins as a parsable text equation. Only one option is needed but both are shown for clarity.
Component Ratio Method
The component ratio method (CRM) calculates biomass as a function of tree volume (Heath et al. 2009; Woodall et al. 2011). The volume of the merchantable bole is calculated using volume equations then converted into biomass using wood specific gravity factors. The biomass of other components (stump, top, branch, foliage & root) are calculated using various ratios of the Jenkins component equations to bole biomass. Bark biomass is a percentage of bole biomass using a separate bark specific gravity.
The advantage to CRM is that starting with the biomass of the merchantable section of the main stem allows the use of well vetted volume equations with commonly measured values. The bole volume, being the largest component of biomass, should therefore be more accurately estimated and other components being based on the bole should also benefit.
The main concerns of CRM are that methods of volume calculation differ across regions. This means a biomass estimate of the same tree in different regions will differ simply because volume is calculated differently. Additionally, the region boundaries are delineated by FIA administrative boundaries rather than ecologically based ranges.
Assisi CRM Implementation
The CRM approach is coded in the National Biomass Estimation Library (NBEL) component of the USFS Forest Vegetation System (FVS). NBEL is available as a DLL executable that can be incorporated into third party applications like Assisi Forest. Calculating carbon using the NBEL software requires setting the Region, Forest and District codes for each species. NBEL then internally calculates volume and biomass according to district standards. Note that the NBEL internal volume methods are separate from Assisi's volume methods.
The NBEL setup for Douglas Fir. NBEL requires Region, Forest and District settings. Additional settings convert biomass to carbon, CO2 units and scale.
National Scale Volume and Biomass
The National Scale Volume and Biomass approach (NSVB) is a set of new equations to predict component biomass as well as total above ground biomass directly from both DBH and total height (Westfall et. al., 2024). Direct component calculation removes potential errors in the Jenkins component allocation approach and their use as ratios in CRM. Direct calculation also removes the need for volume to biomass conversion along with the different volume calculation methods in different FIA divisions. NSVB also introduces the use of ecodivisions (Cleland, 2007) as a criteria for grouping species. Ecodivisions replace the formal FIA division boundaries. Finally, NSVB is designed to allow calculation of biomass on any section of the main tree bole allowing product level tracking of carbon pools.
NSVB ecoregion codes. (Fig 1. Westfall et al., 2024)
Assisi NSVB Implementation
NSVB has been coded into the National Volume Estimation Library (NVEL) of the USFS Forest Vegetation System (FVS). As with NBEL, NVEL available as a DLL for incorporation into third party applications. Using NVEL within Assisi requires single coded designation that includes species and ecoregion.
The NVEL setup for Douglas Fir. NVEL requires an equation that describes describes both Eco-region and Species (NVBM260202). Additional settings convert biomass to carbon, CO2 units and scale.
Citations
Cleland, D.T.; Freeouf, J.A.; Keys, J.E., Jr.; Nowacki, G.J.; Carpenter, C.A.; McNab, W.H. 2007.
Ecological subregions: sections and subsections for the conterminous United States. [1:3,500,000]. Gen. Tech. Rep. WO-76D. U.S. Department of Agriculture, Forest Service.
Chojnacky, D.C. 2002. Allometric scaling theory applied to FIA biomass estimation. In Proceedings of Third Annual Forest Inventory and Analysis Symposium, 2001. General Technical Report NC-230. US Department of Agriculture, Forest Service, North Central Forest Experiment Station
Chojnacky, D.C.; Heath L.S.; Jenkins J.C. 2013. Updated generalized biomass equations for North American tree species.
Heath, L.S.; Hansen, M.H.; Smith, J.E.; Miles, P.D.; Smith, B.W. 2009. Investigation into calculating tree biomass and carbon in the FIADB using a biomass expansion factor approach. FIA Proceedings RMRS-P-56CD. US Department of Agriculture, Forest Service, Rocky Mountain Research Station
Jenkins, J.C.; Chojnacky, D.C.; Heath, L.S.; Birdsey, R.A. 2003. National-scale biomass estimation for United States tree species. For. Sci. 49
Westfall, J.A.; Coulston, J.W.; Gray, A.N.; Shaw, J.D.; Radtke, P. J.; Walker, D.M.; Weiskittel, AR.; MacFarlane, D.W.; Affleck, D.L.R.; Zhao, D.; Temesgen, H.; Poudel, K.P.; Frank, J.M.; Prisley, S.P.; Wang, Y.; Sánchez Meador, A.J.; Auty, D.; Domke, G.M. 2024. A National-Scale Tree Volume, Biomass, and Carbon Modeling System for the United States. Gen. Tech. Rep. WO-104. U.S. Department of Agriculture, Forest Service.
Woodall, C.W.; Heath, L.S.; Domke, G.M.; Nichols, M.C. 2011. Methods and Equations for Estimating Aboveground Volume, Biomass, and Carbon for Trees in the U.S. Forest Inventory, 2010. General Technical Report NRS-88. US Department of Agriculture, Forest Service, Northern Research Station
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