The seasonal trend of plant carbon dioxide (CO2) sequestration is related to the photosynthetic activity, which in turn changes in response to environmental conditions. Great interest has turned to the CO2 sequestration (CS) potential of temperate forests which play an important role in global carbon (C) cycle contributing to the lowering of atmospheric CO2 concentration. In such context, the CS of an unmanaged old broad-leaf deciduous forest developing inside a Strict Nature Reserve, and its variations during the year were analyzed considering the monthly variations of leaf area index (LAI) and net photosynthetic rates (N-p). Overall, the total yearly CS of the forest was 141 Mg CO2 ha(-1) year(-1) with the highest CS value monitored in June (405 Mg CO2 month(-1)) due to the highest LAI (5.0 +/- 0.8 m(2) m(-2)) and a high N-p in all the broadleaf species. The first CS decline was observed in August due to the more stressful climatic conditions that constrained N-p rates. Overall, the total CS of the forest reflects the good ecological health of the ecosystem due to its conservative management.