Forest plantations cover large areas globally, but their climate benefits remain unclear, hampering projections of the future land carbon (C) sink. Here, we combined an unprecedented 14-years series of eddy-covariance CO2 fluxes (net ecosystem productivity, NEP) with biometric measurements to explore C balance in a commercial Eucalyptus plantation in Brazil across three rotations. The plantation exhibited a 14-year average and maximum annual NEP of 9.8 and 20.2 MgC ha-1 y-1, respectively, which ranks among the highest documented forest productivity. The amount of time before the forest recaptures as much C as was emitted after harvest was substantially shorter than previously published values: 20 and 27¿months, for first and second harvest, respectively. Time series of leaf area index, but not trunk biomass growth, were strongly related to NEP dynamics (>¿75% of explained variance at annual and monthly scales) suggesting possible amenability to remote-sensing assessment in Eucalyptus plantations. Estimates of C accumulation in litter and below-ground stocks varied from positive to negative depending on the methodology used, and showed substantial differences across rotations. This indicates that long-term C accumulation in commercial Eucalyptus plantations is not guaranteed, despite their high productivity.