Biomass expansion factors and root : shoot ratios for coniferous tree species in Great Britain

Biomass expansion factors (the ratio of the total above-ground tree biomass to the biomass of the merchantable timber, B-ef) and root : shoot ratios (R) are commonly used in converting standing volumes of timber into total carbon stocks, for the purpose of national inventories of greenhouse gas emissions and sequestration. However, much of the uncertainty in these estimates of carbon stocks is attributable to uncertainty in the B-ef and R parameters, as they are not measured in standard forestry practice and are rarely measured on a large scale in research work. Here, we present measurements which represent the largest available data set on which estimates of B-ef and R can be based for coniferous species in Great Britain. B-ef varied between 1.04 and 2.32, with a mean of 1.43 +/- 0.008, whilst R varied from 0.12 to 1.21, with a mean of 0.36 +/- 0.008. Tree height accounted for 45 per cent of the variance in B-ef in a logarithmic regression. Differences between species accounted for 27 per cent of the remaining variance. Equations to estimate the value of B-ef for a given species and tree height are provided. No continuous variable accounted for >5 per cent of the variance in R. Differences between sites accounted for 25 per cent of the variance in R, and the effects of species, soil type and cultivation method were of lesser significance. Values of R used in some recent UK studies are lower than our values. If our results are representative, more carbon is present in UK conifer forests than is estimated in these studies, and the R terms should be increased by up to 0.2.