Growth responses of two provenances of European beech (Fagus sylvatica) were studied. The seedlings were grown in closed-top chambers at four temperature regimes (−2.9 °C below ambient, ambient, +2.3 °C and +4.8 °C above ambient) in combination with two CO2 partial pressures (40 Pa and 74 Pa). Growth was followed by making destructive harvests c. every 25 d from germination in early June to senescence in late September. Allocation patterns were significantly affected by the temperature regimes. However, changes in dry matter allocation and morphology associated with the different treatments at a given time were mostly a result of differences in tree size. Temperature regimes only had a significant effect on the relative growth rate, RGR, at the beginning of the experiment. In contrast to temperature, high [CO2] increased RGR throughout the experiment when compared with plants of equal size. As the trees increased in size net assimilation rate, NAR, decreased but the effect of [CO2] on both NAR and RGR had a tendency to increase. Increases in NAR caused by elevated [CO2] were partly counteracted by reductions in the leaf area ratio, LAR. Reductions in LAR were caused by concomitant reductions in specific leaf area, SLA, whereas the level of [CO2] did not significantly affect leaf weight ratio, LWR, nor other dry weight ratios. The interactions between temperature and [CO2] are highly dependent on whether they are expressed as instantaneous values for plants at a common age or instantaneous values at a common size (and thereby extracting the effects of ontogenetic drift). When comparing instantaneous values at common sizes, the positive effect of [CO2] on RGR increased with plant size in every temperature regime. This also occurred in every temperature regime when comparing plants of equal age but the response to [CO2] was less. The effect of [CO2] on RGR was dependent on growth temperature. The positive effects of elevated [CO2] on RGR were less than the positive effect on photosynthesis. The two provenances did not differ significantly in the response of RGR to [CO2] which is in agreement with measurements of photosynthesis.