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.