Effects of elevated CO 2 on the size structure in even‐aged monospecific stands of Chenopodium album

To investigate the effect of elevated CO 2 on the size inequality and size structure, even‐aged monospecific stands of an annual, Chenopodium album , were established at ambient and doubled CO 2 with high and low nutrient availabilities in open top chambers. The growth of individual plants was monitored non‐destructively every week until flowering. Elevated CO 2 significantly enhanced plant growth at high nutrients, but did not at low nutrients. The size inequality expressed as the coefficient of variation tended to increase at elevated CO 2 . Size structure of the stands was analyzed by the cumulative frequency distribution of plant size. At early stages of plant growth, CO 2 elevation benefited all individuals and shifted the whole size distribution of the stand to large size classes. At later stages, dominant individuals were still larger at elevated than at ambient CO 2 , but the difference in small subordinate individuals between two CO 2 levels became smaller. Although these tendencies were found at both nutrient availabilities, difference in size distribution between CO 2 levels was larger at high nutrients. The CO 2 elevation did not significantly enhance the growth rate as a function of plant size except for the high nutrient stand at the earliest stage, indicating that the higher biomass at elevated CO 2 at later stages in the high nutrient stand was caused by the larger size of individuals at the earliest stage. Thus the effect of elevated CO 2 on stand structure and size inequality strongly depended on the growth stage and nutrient availabilities.