DAILY AND SEASONAL CO2 EXCHANGE IN SCOTS PINE GROWN UNDER ELEVATED O-3 AND CO2 - EXPERIMENT AND SIMULATION

Starting in early spring of 1994, naturally regenerated, 30-year-old S cots pine (Pinus sylvestris L.) trees were grown in open-top chambers and exposed in situ to doubled ambient O-3, doubled ambient CO2 and a combination of O-3 and CO2 from 15 April to 15 September. To investiga te daily and seasonal responses of CO2 exchange to elevated O-3 and CO 2, the CO2 exchange of shoots was measured continuously by an automati c system for measuring gas exchange during the course of one year (fro m 1 Januray to 31 December 1996). A process-based model of shoot photo synthesis was constructed to quantify modifications in the intrinsic c apacity of photosynthesis and stomatal conductance by simulating the d aily CO2 exchange data from the field. Results showed that on most day s of the year the model simulated well the daily course of shoot photo synthesis. Elevated O-3 significantly decreased photosynthetic capacit y and stomatal conductance during the whole photosynthetic period. Ele vated O-3 also led to a delay in onset of photosynthetic recovery in e arly spring and an increase in the sensitivity of photosynthesis to en vironmental stress conditions. The combination of elevated O-3 and CO2 had an effect on photosynthesis and stomatal conductance similar to t hat of elevated O-3 alone, but significantly reduced the O-3 induced d epression of photosynthesis. Elevated CO2 significantly increased the photosynthetic capacity of Scots pine during the main growing season b ut slightly decreased it in early spring and late autumn. The model ca lculation showed that, compared to the control treatment, elevated O-3 alone and the combination of elevated O-3 and CO2 decreased the annua l total of net photosynthesis per unit leaf area by 55% and 38%, respe ctively. Elevated CO2 increased the annual total of net photosynthesis by 13%.