Effects of ozone on Japanese trees

Abstract

The effects of ozone (O3) on tree species in Japan have been studied since the 1970s. Based on the results from O3 fumigation studies, current ambient levels of O3 have negative impacts on the growth and physiological functions of Japanese forest tree species, although there is a big variation of O3 sensitivity between species. Stomatal O3 uptake is one of the key factors that can explain the differences in O3 sensitivity between species, and modeling of this factor has been intensively studied during the past decade. Although O3 generally induces stomatal closure, less efficient stomatal control, so-called stomatal sluggishness, is also induced by chronic exposure to O3. These opposite phenomena result in complex responses of stomata to O3. Detailed gas exchange analysis has revealed that O3- induced reductions in the photosynthetic rate of Japanese forest tree species were mainly due to a biochemical limitation in chloroplasts, but not due to stomatal closure. Risk assessments of the O3 impact on Japanese forest tree species, based on the results of experimental studies, national monitoring data of air pollutant concentrations, and vegetation surveys, indicate that the areas with high O3-induced reduction in growth do not necessarily correspond to the areas with relatively high O3 exposure. Free-air O3 fumigation systems in Japan were developed in 2011. Studies with this novel technology have clarified differences in leaf O3 sensitivities between canopy positions, and have estimated the effects of O3 on whole-canopy carbon budgets. As future perspectives, not only we need clarification of the physiological mechanisms of O3 impact, but we also need clarification of the effects of interactions between trees and other biotic factors such as diseases, herbivores, and symbiotic microbes.