Canopy CO2 exchange of Scots pine and its seasonal variation after four-year exposure to elevated CO2 and temperature

Single Scots pines (Pinus sylvestris L.), aged 20-25 years, have been grown in open-topped chambers with elevated temperature (during winter and summer, the mean temperature was 5-20 degrees C and 2 degrees C above the outside ambient temperature, respectively), elevated CO2 (550-600 mu mol mol(-1) from 15 April to 15 September) and a combination of elevated temperature and CO2 for four years. The vertical and seasonal variations of key physiological parameters concerning photosynthesis and stomatal conductance were measured. The annual canopy photosynthesis and respiration were predicted by using a multilayer model in which the profile of the canopy properties and the microclimate data through a whole year (1994) were used as inputs to the model. The results indicate that during the main growing season (day number 121 to 243), the elevated CO2 increased the maximum apparent quantum yield by 24% and the mean light-saturated rate of assimilation by 41%, and decreased the mean light-saturated stomatal conductance by 13%. However, elevated temperature had no significant effect on these parameters. During early spring and late autumn, elevated temperature increased significantly the apparent quantum yield, the light-saturated rate of assimilation and stomatal conductance. The predicted annual net photosynthesis increased by 40% for elevated CO2 alone, by 58% for elevated CO2 and temperature, and by 10% for elevated temperature alone. The annual sum of respiration increased by 39% for elevated temperature alone and by 29% for elevated CO2 and temperature. Elevated CO2 alone caused a depression of 7% in the annual respiration. Seasonal variations of the CO2-exchange rate between treatments were evident, and they can be largely attributed to changes in the apparent quantum yield, the light-saturated rate of assimilation, leaf area index, and the ability to adapt to environmental stress conditions.