Biomass expansion factor (BEF)
A multiplier that converts measurable merchantable stem biomass into total above-ground biomass, including branches and foliage.
Field campaigns can weigh or estimate stems with reasonable confidence, but branches, bark, foliage and tops are laborious to measure. A biomass expansion factor bridges that gap by scaling stem biomass up to total above-ground biomass.
Expansion factors vary with species, age, site quality and stand density. Young trees allocate a larger fraction of biomass to foliage and branches; mature stands concentrate it in stemwood. Applying a single default factor across a heterogeneous landscape therefore introduces systematic bias.
Good practice is to use locally derived or species-specific factors where available, document their source, and propagate the uncertainty they add. Where factors are borrowed from other regions, treat that transfer as an explicit assumption rather than a hidden convenience.
Frequently asked questions
Is BEF the same as a root-to-shoot ratio?
No. BEF expands stem biomass to total above-ground biomass; the root-to-shoot ratio then extends above-ground to below-ground biomass.
Why do defaults cause trouble?
Because a default encodes an average of other forests. Used across a different mix of species and ages, it produces a bias that no amount of careful measurement downstream can remove.