The biomass volume tables and the default biomass stock as well as growth rates are often estimated considering only the merchantable or commercial volume. Estimating only the commercial component of the tree biomass, which is largely the main tree trunk, may be adequate for industrial roundwood producers. However, for estimating carbon stocks and changes for climate change mitigation projects and greenhouse gas inventory estimation, all the above-ground biomass including twigs and branches and even leaves needs to be estimated. To convert merchantable tree volume into total biomass, biomass conversion and expansion factors (BCEF) are used (IPCC 2006). Biomass conversion and expansion factors expand dry weight of the merchantable volume of above-ground biomass or growing stock to account for non-merchantable components of the tree, stand and forest. Such factors can also be developed for local tree species by harvesting trees and estimating the proportion of merchantable and non-merchantable components. Default BCEF values are also available in literature (IPCC 2006). The factors can be estimated using the following broad steps at a species level or for a given forest type:
Step 1: Select tree species or the forest type for determining BCEF Step 2: Adopt the sampling method described for estimating above-ground biomass from the harvest method Step 3: Select at least 30 trees of a given species or a mixture of species for a selected forest type
Step 4: Cut the trees to the base and segregate the above-ground tree into the main tree trunk or the merchantable bole and the remaining tree components such as tapering tree stems, branches and twigs Step 5: Estimate the volume of the merchantable tree component and the remaining biomass using methods described for the harvest method (Section 17.1.1) and extrapolate the estimate obtained from the trees and sample plots to a per hectare value. Step 6: Estimate BCEF by calculating the ratio (BEF) of total volume to merchantable volume and multiplying by density:
BEF = (Total volume of trees/ha)/(merchantable volume of trees/ha) BCEF (in t/m3) = BEF x density (t/m3)
BCEF can be estimated for a given species, a forest type consisting of multiple tree species and young or mature plantations or forests.
Biomass expansion factors (BEF) could be used if a biomass equation provides the merchantable biomass (t/ha) directly. BEF expands the dry weight of the merchantable volume of the growing stock to account for non-merchantable components of trees.
Total biomass can be estimated in two ways depending on the units of merchantable biomass estimates (as volume in cubic metres or in tonnes per hectare).
Total biomass (t/ha) = (total merchantable biomass (t/ha)) x BEF
Total biomass (t/ha) = merchantable biomass in volume (m3/ha) x BCE
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