Investigation into calculating tree biomass and carbon in the FIADB using a biomass expansion factor approach

The official U.S. forest carbon inventories (U.S. EPA 2008) have relied on tree biomass estimates that utilize diameter based prediction equations from Jenkins and others (2003), coupled with U.S. Forest Service, Forest Inventory and Analysis (FIA) sample tree measurements and forest area estimates. However, these biomass prediction equations are not the equations used in the current public national FIA dataset (FIADB3), which utilizes regionally specific prediction equations, nor are they based on current FIA volume estimates. We describe and investigate an approach that is proposed for biomass estimates in the FIADB version 4 (FIADB4), due to be released in April, 2009, and that would produce national-level biomass and carbon estimates consistent with FIA volume estimates at the tree-level. The approach, called the component ratio method (CRM), is based on: 1) converting the sound volume of wood in the bole to biomass using a compiled set of wood specific gravities; 2) calculating the biomass of bark on the bole using a compiled set of percent bark and bark specific gravities; 3) calculating the biomass of tops and limbs as a proportion of the bole biomass based on component proportions from Jenkins and others (2003); 4) calculating the biomass of the stump based on equations in Raile (1982); and 5) summing the parts to obtain a total aboveground live biomass. Root biomass is also available as a proportion of the bole biomass based on component proportions from Jenkins and others (2003). The CRM approach is based on assumptions that the definition of bole in the volume prediction equations is equivalent to the bole in Jenkins and others (2003), and that the Jenkins and others (2003) component ratios accurately apply.We compare results between estimates calculated using equations in Jenkins and others (2003), current regional FIA equations, and this approach. The CRM approach is promising because the estimates are congruent with FIA volumes and compiled specific gravities. However, because FIA units currently use different volume equations the resulting estimates are not nationally consistent (that is, biomass of the same diameter and species tree will differ between regions). Because a number of volume equations are currently used by FIA, this approach can be complex for those wanting to take their own tree data and estimate biomass with FIA prediction equations especially when data cross regional boundaries. In the long-term, a planned and coordinated research study, as well as an accompanying operational implementation plan, for volume and biomass estimation methods would greatly add to the credibility of these estimates in the publicly available national FIA dataset.