Forest plantations can sequester large amounts of carbon dioxide from the atmosphere while producing commercial wood products. Quantification of carbon fluxes usually depends on the eddy covariance (EC) method, which may be biased, especially in the presence of a dense canopy. We aimed to test estimates of canopy photosynthesis, or gross primary production (GPP), derived from EC against an independent method based on transpiration (xylem sap-flux) and isotopic (tree ring δ¹³C) estimates of photosynthetic water-use efficiency. The addition of tree-ring δ¹³C to existing sap-flux data made it possible to infer GPP in the past, including the drought year of 2018 and the years surrounding it. Previously published EC based estimates at our research site, the Central Forest at Skogaryd Research Catchment in south-western Sweden, had been unable to detect a response of annual GPP to the 2018 drought. A closer look at the daily EC data revealed a decrease of about 13 % over the drought period. However, using the sap-flux and isotopes, we found a decrease in GPP of 31 %, which continued for several weeks after the drought. The loss in GPP occurred primarily because water stress reduced sap flux and it occurred despite an increase in water-use efficiency. Stem biomass growth decreased in 2018, but the ratio of stem biomass growth to GPP (17 %) was within the range of published values. The discrepancies highlight method differences during the severe drought and during the spring of some years, but the long periods of agreement tend to confirm the accuracy of both methods over most of the study period.