## 4 Discussion
[15] reported that the contribution of the transboundary transport of O\({}_{3}\) from the Asian continent to its concentration in Japan seemed to be relatively small except for that in the Kyushu region. According to [PERSON] et al. [27], the contribution to the O\({}_{3}\) concentrations in Japan from China and the Korean Peninsula were estimated to be approximately 10% and 4%, respectively, although the domestic contribution was approximately 42% in the Japanese summer. Furthermore, O\({}_{3}\) is recognized as a short-lived climate pollutant (SLCP). These results indicate the necessity and effectiveness of domestic countermeasures against O\({}_{3}\) formulation for avoiding the O\({}_{3}\)-induced reduction in CO\({}_{2}\) uptake by Japanese forest tree species. The concept of critical level of O\({}_{3}\) has been discussed in Europe to prevent its damage to plants. The critical levels of O\({}_{3}\) for European beech and European birch were defined as 5 ppm h of daylight AOT40 to diminish the growth reduction to a value of 5% [28]. In Japan, a provisional critical level based on AOT40 is suggested for the tree species sensitive to O\({}_{3}\), namely 8-15 ppm h (6 months) corresponding to a 10% growth reduction [29]. However, the critical level based on stomatal O\({}_{3}\) uptake has been developed because the O\({}_{3}\) absorbed through stomata is more physiologically related to the O\({}_{3}\) damage to tree growth [30]. Critical levels of the two European trees were set at 5.2 mmol m\({}^{-2}\) of POD\({}_{1}\) (phototoxic O\({}_{3}\) dose above the threshold of 1 mmol m\({}^{-2}\) s\({}^{-1}\)) corresponding to a 4% growth reduction [28]. Although this concept of the critical level of O\({}_{3}\) is more scientifically plausible, how much O\({}_{3}\) should be reduced is unclear. While the stomatal O\({}_{3}\) uptake concept is significantly useful to assess the impact of O\({}_{3}\) on plants, to create and implement mitigation measures against O\({}_{3}\) damage to plants, determining the atmospheric O\({}_{3}\) concentration is needed. To the best of our knowledge, the present study is the first to evaluate the impact of the mitigation of O\({}_{3}\) absorbed via stomata on the annual photosynthetic CO\({}_{2}\) uptake. As a result, we clarified that O\({}_{3}\) concentrations should be reduced by 28% (2011-2015 range: 19-34%), 20% (11-23%), 17% (8-21%), and 49% (46-51%) for _F. crenata, Q. serrata, Q. Mongolia var. crispula, and B. platyphylla var. japonica, respectively, to diminish the O\({}_{3}\)-induced reduction to a value of 5% (Figure 7). Because the domestic contribution of O\({}_{3}\) formation is 42% in the Japanese summer [27], it would be impossible to prevent the O\({}_{3}\) damage by 5% for _B. platyphylla_ var. _japonica_ by domestic control of the emission of O\({}_{3}\) precursors. In contrast, such a mitigation measure may be effective for the other tree species: _F. crenata, Q. serrata_, and _Q. mongolica_ var. crispula. We provisionally propose that the O\({}_{3}\) concentration should be reduced by approximately 30% to decrease its damage on CO\({}_{2}\) uptake below 5% in some Japanese forest tree species. We hope our results will contribute to creating and implementing mitigation measures against the detrimental impact of O\({}_{3}\) on forest tree species in Japan and to enhancing their photosynthetic CO\({}_{2}\) uptake. There are some uncertainties and future issues in the assessment on the impact of O\({}_{3}\) on the photosynthetic CO\({}_{2}\) uptake by forest tree species, including estimation accuracies of the stomatal conductance and errors of COU induced by the interpolation or extrapolation by kriging. One of the major issues is whether the parameterization results of the stomatal conductance model and the responses of CO\({}_{2}\) uptake to COU obtained in the particular environment of the experimental studies by [PERSON] et al. [19] and [PERSON] et al. [18] could be applicable to the whole Japan in 2011-2015. [PERSON] et al.