Functions for below-ground biomass of Pinus sylvestris, Picea abies, Betula pendula and Betula publescens in Sweden

Accurate estimates of below-ground biomass of trees are important when quantifying the amount of carbon sequestered in forests. Allometric single-tree below-ground biomass functions were developed for Pinus sylvestris, Picea abies and Betula pendula and Betula pubescens in Sweden. The idea was to calibrate an existing comprehensive data set of about 600 trees that only covered the stump and coarse roots against a new data set that covered roots down to 2 mm diameter. The new data set consisted of about 80 trees acquired using the same design as for the existing set, but complemented with a detailed inventory of the fine root fractions remaining in the ground. Checks were made to determine whether the density properties of the two data sets were comparable and it was concluded that they were. This was a prerequisite for calibrating the older data against the new information and further for merging the two data sets. The merged data set was used to derive regression functions for below-ground biomass. For all functions the adjusted R 2 values were always higher than 0.95 and the root mean square errors were always lower than 36% for P sylvestris and P abies. Below-ground biomass predicted with the new functions was approximately 11% higher than the values obtained using the existing biomass functions.