In Europe, both forest area and growing stock have increased since the 1950s, and European forests have acted as a carbon sink during the last six decades. However, the contribution of different factors affecting the sink is not yet clear. In this study, historical inventory data were combined with land-use modelling data to reconstruct the development of forest area and age-structure between 1950 and 2010 without afforestation in two case study countries, Finland and the Czech Republic. These reconstructions were then used in a scenario analysis to assess the effects of afforestation, development of mean growing stock volume and age structure of forests on the forest biomass carbon stock. The results show that afforestation has affected the development of the mean age of forests, but has not changed its trend. There have been large increases in the mean volume of growing stock over the study period in both countries; the increase has occurred both in younger and older age-classes, and in both coniferous and broadleaved species. As not many countries have sufficiently detailed inventory data available for such analysis, the presented case studies are valuable in demonstrating that these changes occurred under very different circumstances. In both countries, the increase in the mean volume of growing stock has been the dominant factor explaining the increase in the forest biomass carbon stock compared with the effect of afforestation.