Signatory countries of Kyoto Protocol are engaged in carrying out national inventories to quantify greenhouse gas emission and potentiality of C sinks. Forests represent the terrestrial ecosystem with the highest C sequestration capacity taking up CO2 from the atmosphere and fixing it in vegetal biomass through photosynthesis process; C stocks can be divided in aboveground and belowground ones. In inventorial processes, root biomass is empirically extrapolated from aboveground biomass using a 0.2 factor, which underestimate the real value. Some authors suggest that total underground C allocation can be assessed from the difference between annual respiration rate and litter fall. Belowground biomass can be divided in permanent biomass (structural roots) and temporary one (fine roots). Models allow a valuation of structural roots biomass from stand dendrometrical characteristics. Literature reveals that underground biomass, as fine roots than structural ones, highly varies with local conditions. The development of models that take into account these station parameters and therefore able to reproduce this variability seems to be obligatory to deal with inventory processes with an acceptable precision.