## 1 Introduction  
Climate change, including rising air temperatures and changes in precipitation, is recognized globally as a serious environmental problem [1]. Atmospheric carbon dioxide (CO\({}_{2}\)) is one of the major factors causing climate change [1]. The CO\({}_{2}\) concentration has increased to approximately 400 umol mol\({}^{-1}\) (ppm) from 280 ppm in 1750 with the increasing consumption of fossil fuels and burning of biomass [2; 3]. In general, forest trees absorb atmospheric CO\({}_{2}\) by leaf photosynthesis, which is expected to attenuate climate change [4; 5; 6].  
Tropospheric ozone (O\({}_{3}\)) exhibits direct radiative forcing for climate change [1]. O\({}_{3}\) also has a potential for indirect radiative forcing, as it reduces the photosynthetic CO\({}_{2}\) uptake because of its high phytotoxicity [7; 8]. [PERSON] et al. [9] suggested that indirect radiative forcing, which is the O\({}_{3}\)-induced reduction in the photosynthetic CO\({}_{2}\) fixation, could also contribute to global warming. Thus, an impact assessment of O\({}_{3}\) on photosynthetic CO\({}_{2}\) uptake is needed to support countermeasures, such as adaptation and mitigation, against the detrimental impact of O\({}_{3}\).  
Accumulated exposure over a threshold of 40 nmol mol\({}^{-1}\) (AOT40) has been utilized to evaluate the effects of O\({}_{3}\) on tree species [10; 11]. Although such an exposure-based O\({}_{3}\) index is useful for impact assessment of O\({}_{3}\) on plants because of the ease in calculation and fewer required parameters, stomatal O\({}_{3}\) uptake is more related to leaf O\({}_{3}\) damage than the exposure-based O\({}_{3}\) index [7; 12]. In Europe, the impact of O\({}_{3}\) on trees has been assessed based on cumulative stomatal O\({}_{3}\) uptake (COU) [13]. However, the impact assessment on trees based on stomatal O\({}_{3}\) uptake is extremely limited in Asia. The tropospheric O\({}_{3}\) concentration in Asia is relatively high in the world [14], indicating remarkable damage of O\({}_{3}\) on plants grown in Asia. Therefore, it is necessary to assess the impact of O\({}_{3}\) on the photosynthetic CO\({}_{2}\) uptake of forest tree species in Asia.  
In Japan, the tropospheric O\({}_{3}\) concentration has been increasing although the concentration of its precursors, such as nitrogen oxides (NO\({}_{\rm x}\)) and volatile organic compounds (VOCs), has decreased [15]. The increasing trend of O\({}_{3}\) concentration is mainly caused by the reduction in the titration effect of nitric oxide (NO) and partly increasing transboundary transportation of O\({}_{3}\) and/or its precursors from the Asian continent [15]. Current levels of O\({}_{3}\) are related to growth inhibition of plants and forest decline in Japan [16; 17]. Thus, countermeasures may need to be implemented against O\({}_{3}\) damage to plants. Mitigation, which refers to the reduction of the atmospheric O\({}_{3}\) concentration here, is a fundamental and effective countermeasure against the damage of O\({}_{3}\) to plants. To date, however, limited information is available on the effects of the mitigation of O\({}_{3}\) on plants and on how much surface O\({}_{3}\) concentration should be reduced.  
Recently, [PERSON] et al. [18] evaluated the species-specific responses of cumulative photosynthetic CO\({}_{2}\) uptake to COU of the leaves in four Japanese forest tree species: _F. crenta_, _Q. serrata_, _Q. mongolica_ var. _crispula_, and _B. platyphylla_ var. _japonica_. Using these species-specific responses, we aimed (1) to evaluate the effects of the current levels of O\({}_{3}\) absorbed into the leaves via stomata on the photosynthetic CO\({}_{2}\) uptake by the four representative Japanese deciduous broad-leaved tree species and (2) to assess the impact of the mitigation of O\({}_{3}\), i.e., reducing surface O\({}_{3}\) concentration, on the CO\({}_{2}\) uptake by the tree species.