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 CO 2 and elevated O 3 concentrations on net CO 2 assimilation and growth of Fagus crenata in a screen-aided free-air concentration-enrichment (FACE) system. Seedlings were exposed to ambient air (control), elevated CO 2 (550 μmol mol ？1 CO 2, +CO 2), elevated O 3 (double the control, +O 3), and the combination of elevated CO 2 and O 3 (+CO 2+O 3) for two growing seasons. The responses in light-saturated net CO 2 assimilation rates per leaf area ( A growth-CO2) at each ambient CO 2 concentration to the elevated CO 2 and/or O 3 treatments varied widely with leaf age. In older leaves, A growth-CO2 was lower in the presence of +O 3 than in untreated controls, but +CO 2+O 3 treatment had no effect on A growth-CO2 compared with the +CO 2 treatment. Total plant biomass increased under conditions of elevated CO 2 and was largest in the +CO 2+O 3 treatment. Biomass allocation to roots decreased with elevated CO 2 and with elevated O 3. Elongation of second-flush shoots also increased in the presence of elevated CO 2 and was largest in the +CO 2+O 3 treatment. Collectively, these results suggest that conditions of elevated CO 2 and O 3 contribute to enhanced plant growth; reflecting changes in biomass allocation and mitigation of the negative impacts of O 3 on net CO 2 assimilation. View Full-Tex