We investigated the relationship tree size and radial growth of oak in mixed oak-beech stands where beech ranged from 0.0 to 98.2 %. We analyzed 30 long-term permanent plots in Northern France with a total of 167 growth periods between 1904 and 2000. This dataset included even-aged one and two storey mixed stands. The study was based on a nonlinear segmented model of individual tree basal area increment over diameter at breast height, which is implemented in the French forest growth simulator Fagacées, that is so far parameterized for pure oak stands. We investigated the variations of the slope g from the linear branch of the segmented model. The slope g was calculated for each growth period between two measurements. We found mixture to have a significant influence on the slope g, calculated as the percentage of admixture of beech. Other influencing factors were site quality, expressed as the dominant height at 100 years and the relative stand density index ‘Rdi’ as a measure of stand density. Tree age did not have any influence. The basal area increment of oak decreased with rising admixture of beech and increased with site quality and lower stand density. Thus, the effect of mixture is that oak trees surrounded by beech grow slower in diameter than oaks in pure oak stands. The inferiority of individual oak tree basal area increment in mixtures with beech found in the present study corresponds with findings from tree physiology on belowground competition, which indicate that root growth of oak in mixed stands is reduced due to the presence of beech. Our results are also in line with studies on aboveground competition, where beech absorbs more light because of its species specific crown transparency. We conclude that resource utilization of oak as a part of the species specific competitiveness is one of the reasons for long-term species dynamics in mixed oak-beech stands.