Biomass expansion factors (BEFs) for Scots pine, Norway spruce and birch according to stand age for boreal forests

Assessments of forest resource in national inventories provide a firm basis for quantifying forest biomass and carbon stock. National statistics on forest resources provide estimates of forest area, timber volume, and growth of timber by age classes with known precision. Estimates of carbon stock are, however, obtained by expanding the total stemwood volume to total biomass with simple conversion factors. The objective of this study was to improve the accuracy and reliability of the biomass expansion factors (BEFs) and to develop expansion factors that are dependent on stand age and dominant tree species. For development of BEFs, we applied volume and biomass equations to describe the allometry of single trees and a systematic network of forest inventory data to determine variation in stand structure. The results of this study indicate that the proportions of most biomass components vary considerably during the rotation. We conclude that the reliability of the national carbon stock inventory could be improved by applying these age-dependent BEFs, which are formulated on the basis of representative data and which include an estimate of uncertainty.