As a result of rising atmospheric CO2 concentrations, numerous studies have been aimed at determining and clarifying the response of photosynthesising plants to this phenomenon. Despite the significant role of trees and forests in the carbon cycle, most of the CO2 research carried out so far has been focused on studying single plants grown in conh·olled environments for relatively short periods. This study intends to bridge this gap in knowledge and it is a part of European Free Air Carbon Enrichment Experiment on Poplar Plantations (POPFACE). Specifically, the objective of this research is to characterise the effects of elevated CO2 on root biomass production and associated mycorrhizal activity and functioning of the following tlU'ee Populus species: P. alba, P. nigra and P. euramericana (I-214). The h·ees were grown in a 1 x lm spaced plantation under ambient (350ppm) and elevated (550ppm) atmospheric CO2 conditions provided by a FACE system. After tlu-ee growing seasons, it was shown that elevated CO2 increases below-ground allocation of biomass in all three species examined. This enhancement of root production was approximately twofold when compared to the increase of aboveground biomass induced by FACE, when standing root biomass was increased by 47% to 76%. Similarly, fine root biomass present in the soil increased by 35% to 84% as a result of elevated CO2. FACE treatment resulted in 55% faster fine root turnover in P. alba and a 27% increase in turnover of roots of P. nigra and P. euramericana. P. alba and P. nigra invested more root biomass into deeper soil horizon under elevated CO2. Response of the mycorrhizal community to elevated CO2 was more varied, the rate of infection increased only in P. alba for both ECM and AM. The roots of P. nigra showed greater infection only by AM and the colonisation of the root system of P. euramericana was not affected FACE treatment. The results suggest that elevated atmospheric CO2 conditions induce greater belowground biomass investment, which could lead to accumulation of assimilated C in the soil profile. This might have further implications for C sequestration and must be taken into account when considering long-term C storage in the soil.