Effects of Combined CO₂ and O₃ Exposures on Net CO₂ Assimilation and Biomass Allocation in Seedlings of the Late-Successional Fagus Crenata

We examined the effects of elevated CO₂ and elevated O₃ concentrations on net CO₂ assimilation and growth of Fagus crenata in a screen-aided free-air concentration-enrichment (FACE) system. Seedlings were exposed to ambient air (control), elevated CO₂ (550 µmol mol⁻¹ CO₂, +CO₂), elevated O₃ (double the control, +O₃), and the combination of elevated CO₂ and O₃ (+CO₂+O₃) for two growing seasons. The responses in light-saturated net CO₂ assimilation rates per leaf area (A_growth-CO2) at each ambient CO₂ concentration to the elevated CO₂ and/or O₃ treatments varied widely with leaf age. In older leaves, A_growth-CO2 was lower in the presence of +O₃ than in untreated controls, but +CO₂+O₃ treatment had no effect on A_growth-CO2 compared with the +CO₂ treatment. Total plant biomass increased under conditions of elevated CO₂ and was largest in the +CO₂+O₃ treatment. Biomass allocation to roots decreased with elevated CO₂ and with elevated O₃. Elongation of second-flush shoots also increased in the presence of elevated CO₂ and was largest in the +CO₂+O₃ treatment. Collectively, these results suggest that conditions of elevated CO₂ and O₃ contribute to enhanced plant growth; reflecting changes in biomass allocation and mitigation of the negative impacts of O₃ on net CO₂ assimilation. View Full-Text