Abstract The exchange of CO 2 between the atmosphere and a beech forest near Sorø, Denmark, was measured continuously over 14 years (1996–2009). The simultaneous measurement of many parameters that influence CO 2 uptake makes it possible to relate the CO 2 exchange to recent changes in e.g. temperature and atmospheric CO 2 concentration. The net CO 2 exchange (NEE) was measured by the eddy covariance method. Ecosystem respiration (RE) was estimated from nighttime values and gross ecosystem exchange (GEE) was calculated as the sum of RE and NEE. Over the years the beech forest acted as a sink of on average of 157 g C m −2 yr −1. In one of the years only, the forest acted as a small source. During 1996–2009 a significant increase in annual NEE was observed. A significant increase in GEE and a smaller and not significant increase in RE was also found. Thus the increased NEE was mainly attributed to an increase in GEE. The overall trend in NEE was significant with an average increase in uptake of 23 g C m −2 yr −2. The carbon uptake period (i.e. the period with daily net CO 2 gain) increased by 1.9 days per year, whereas there was a non significant tendency of increase of the leafed period. This means that the leaves stayed active longer. The analysis of CO 2 uptake by the forest by use of light response curves, revealed that the maximum rate of photosynthetic assimilation increased by 15% during the 14-year period. We conclude that the increase in the overall CO 2 uptake of the forest is due to a combination of increased growing season length and increased uptake capacity. We also conclude that long time series of flux measurements are necessary to reveal trends in the data because of the substantial inter-annual variation in the flux.