Effects of tree size and temperature on relative growth rate and its components of Fagus sylvatica seedlings exposed to two partial pressures of atmospheric [CO2]

Growth responses of two provenances of European beech (Fagus sylvatica) wer e studied. The seedlings were grown in closed-top chambers at four temperat ure regimes (-2.9 degrees C below ambient, ambient, + 2.3 degrees C and + 4 .8 degrees C above ambient) in combination with two CO2 partial pressures ( 40 Pa and 74 Pa). Growth was followed by making destructive harvests c. eve ry 25 d from germination in early June to senescence in late September. All ocation patterns were significantly affected by the temperature regimes. Ho wever, 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 relati ve growth rate, RGR, at the beginning of the experiment. In contrast to tem perature, 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 tend ency to increase. Increases in NAR caused by elevated [CO2] were partly cou nteracted by reductions in the leaf area ratio, NAR. Reductions in LAR were caused by concomitant reductions in specific leaf area, SLA, whereas the l evel of [CO2] did not significantly affect leaf weight ratio, LWR, nor othe r dry weight ratios. The interactions between temperature and [CO2] are hig hly dependent on whether they are expressed as instantaneous values for pla nts at a common age or instantaneous values at a common size (and thereby e xtracting the effects of ontogenetic drift). When comparing instantaneous v alues at common sizes, the positive effect of [CO2] on RGR increased with p lant size in every temperature regime. This also occurred in every temperat ure 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 effec t on photosynthesis. The two provenances did not differ significantly in th e response of RGR to [CO2] which is in agreement with measurements of photo synthesis.