Biomass equations have become a vital estimation tool and a prerequisite for studies on forest productivity, nutrient cycling and carbon sequestration. In this paper a new set of biomass equations were fitted for Eucalyptus globulus in Northern Spain. These equations allow us to estimate the total biomass and above- and below-ground fractions from the basal area and the height of the tree. A dummy variable was included in the model to calculate the root fraction of planted versus coppice stands. A descriptive study of the root system was also carried out to complete the information about this component. Root fraction plays an important role in forest structure, but is often omitted in carbon sequestration estimates due to the difficulties and cost associated with measurement. Our results indicated that root biomass accounted for 15% and 35% of total biomass in planted and coppice stands, respectively, at a shoot age equal to 9 years. We also found that the stand type and plantation age influenced the number of roots per root system, the volume of the root system and root length. This paper brings to light how coppice stands accumulate significant amounts of carbon in their root systems from the time a plantation is established. Such information makes it possible to orient ecosystem management towards potential for C fixation. (C) 2014 Elsevier Ltd. All rights reserved.