Canopy CO 2 exchange of Scots pine and its seasonal variation after four-year exposure to elevated CO 2 and temperature

Abstract

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°C and 2°C above the outside ambient temperature, respectively), elevated CO 2 (550–600 μmol mol −1 from 15 April to 15 September) and a combination of elevated temperature and CO 2 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 CO 2 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 CO 2 alone, by 58% for elevated CO 2 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 CO 2 and temperature. Elevated CO 2 alone caused a depression of 7% in the annual respiration. Seasonal variations of the CO 2-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.