AbstractThe assessment of air quality standards for forest ecosystems requires proper knowledge about concentration‐effect relations of air pollutants for tree species. In this study, 2‐yr‐old Douglas fir seedlings [Pseudotsuga menziesii (Mirb.) Franco] were exposed to 23, 81, and 169 µg O3/m3 during 8 h/d, 5 d/wk for a period of 4 wk. After exposure, the trees were placed in an atmosphere containing 14CO2. Release of 14CO2 in the root/soil compartment was analyzed once a day and was regarded as an index of transport and utilization of 14C photosynthates. Respiratory release of 14C was temporarily inhibited by O3 during the first days following exposure to the air pollutant. Within 2 wk after the O3 treatment, 14CO2 in the root/soil respiration was the same in all treatments, indicating that trees were recovering from O3 injury.