This study investigated changes in biomass and carbon stocks in Quercus robur trees in Galicia (NW Spain). Individual tree biomass equations that use diameter at breast height as the sole independent variable were developed. A system of additive biomass equations by component was fitted simultaneously. Heteroscedasticity and inherent correlations between biomass components were also taken into account in the GMM (generalized method of moments) fitting process. The number of trees per diameter class was obtained from the four national forest inventories (NFI) carried out to date. Carbon storage was estimated using the average carbon contents in the different tree components. Although pure stands of Q. robur in the study region have declined, the number of trees always increased in the period considered (1972-2009). The biomass also increased in the same period, from 12,209,663 Mg in 1972 to 40,333,571 Mg in 2009 (equivalent to 5,878,971 Mg and 19,415,955 Mg of carbon stocks, respectively).