Display omitted •Soil organic carbon (SOC) stock changes of upland forest sites were studied in 1995–2016.•Mean annual SOC stock increase was 36 g m−2 yr−1 in the organic layer and mineral soil (0–40 cm).•An opposite trend was observed on harvested plots or plots where part of the standing tree biomass was lost due to disturbances.•Total forest vegetation biomass correlated positively and soil C/N ratio negatively with the annual SOC stock increase.•Top mineral soil layer played an important role in annual SOC stock increase. This study’s aim was to evaluate the soil organic carbon (SOC) stock changes of the upland forest sites (ICP Forests, Level II network) in relation to factors affecting the stocks in the long run, such as soil and stand properties. The SOC stock changes were based on repeated soil surveys carried out in 1995, 2006 and 2016 on ten plots on managed boreal forest ecosystems, and they represent typical mature forest stands in Finland. In addition, two plots in old-growth forests and two with a short monitoring history were studied. SOC stocks (organic layer, mineral soil 0–40 cm depth) were significantly higher in the spruce stands than in pine stands due to the higher site fertility of the spruce stands, and the biomass in the litter layer was positively correlated with the SOC stocks. The mean annual increase in the SOC stock was 36 g m−2 yr−1 for ten of the plots, which were monitored for the period between 1995 and 2016. An opposite trend was observed on two of the monitored plots, which were on harvested stands or plots where part of the standing tree biomass was lost due to disturbances before 2016. Total forest vegetation biomass was positively correlated, and the soil C/N ratio negatively, with the SOC stock’s annual increase. The top mineral soil layer (0–20 cm) played an important role in the SOC stock’s average annual increase.