Compensatory responses of CO2 exchange and biomass allocation and their effects on the relative growth rate of ponderosa pine in different CO2 and temperature regimes

Growth parameters, photosynthesis, respiration, and biomass allocation of Pinus ponderosa seedlings grown for 2 months in 2 x 2 factorial treatments of 350 or 650 micro bar CO2, and lO/25 deg C or 15/30 deg C night/day temperatures were measured. After 1 month in treatment conditions, total seedling biomass was higher in elevated CO2, and temperature significantly enhanced the positive CO2 effect. However, after 2 months the effect of CO2 on total biomass decreased and relative growth rates did not differ among CO2 and temperature treatments over the 2-month growth period even though photosynthetic rates increased approximately 7% in high CO2 treatments and decreased approximately 10% in high temperature treatments. Additionally, CO2 enhancement decreased root respiration and high temperatures increased shoot respiration. Based on CO2 exchange rates, CO2 fertilization should have increased relative growth rates (RGR) and high temperatures should have decreased RGR. Higher photosynthetic rates caused by CO2 fertilization appear to have been mitigated during the second month of exposure to treatment conditions by a approximately 3% decrease in allocation of biomass to leaves and a approximately 9% increase in root: shoot ratio. It was not clear why diminished photosynthetic rates and increased respiration rates at high temperatures did not result in lower RGR.