Light-acquisition and use of individuals as influenced by elevated CO2 in even-aged monospecific stands of Chenopodium album

1. We studied the effect of CO2 elevation on photosynthesis and growth of individuals in even-aged monospecific stands of an annual, Chenopodium album, established at ambient and doubled CO2 concentrations in open-top chambers. 2. The whole-plant photosynthesis of every individual in the stand was calculated from (1) the distribution of light and leaf nitrogen and (2) the relationships between photosynthetic parameters and leaf nitrogen content per area. Elevated CO2 increased light-saturated rates of photosynthesis by 10-15% and the initial slope of the light-response curve by 11%, but had no effect on the convexity of the light-response curve and dark respiration. 3. The relative rate of photosynthesis (RPR, the rate of photosynthesis per unit above-ground mass) was analysed as the product of light capture (Phi(mass), the photon flux captured per unit above-ground mass) and light-use efficiency (LUE, plant photosynthesis per unit photon capture). 4. At an early stage of stand development (33 days after germination), RPR was nearly constant and no difference was found between ambient and elevated CO2. However, CO2 elevation influenced the components of RPR such that the reduction in Phi(mass) at elevated CO2 offsets the effect of the higher LUE. Later (47 days), RPR was positively correlated with plant mass at both CO2 concentrations. When compared at an equal plant mass, RPR was lower at elevated CO2, which was caused by a reduction in Phi(mass) despite some compensation by higher LUE. 5. We conclude that elevated CO2 increases size inequality of a stand through enhanced photosynthesis and growth of dominants, which reduce the light availability for subordinates and consequently increase size inequality in the stand.