## 4 Discussion  
The present study firstly evaluated the impact of the current levels of O\({}_{3}\) absorbed via stomata on the annual cumulative photosynthetic CO\({}_{2}\) uptake per unit leaf area of four Japanese forest tree  
Figure 6: Ozone-induced reduction in annual CO\({}_{2}\) uptake per unit leaf area (%) of _F. creenta_, _Q. serrata_, _Q. mongolica_ var. _crispula_, and _B. platphylphylla_ var. _japonica_ in 2011–2015. Horizontal solid lines show the median, boxes show the first through third interquartile ranges, and whiskers show the minimum and maximum reductions in CO\({}_{2}\) uptake. Figure 7: Effects of O\({}_{3}\) mitigation for the O\({}_{3}\)-induced reduction in annual CO\({}_{2}\) uptake per unit leaf area (%) of _F. creenta_, _Q. serrata_, _Q. mongolica_ var. _crispula_, and _B. platphylphylla_ var. _japonica_ in Japan in 2011–2015. The range of the O\({}_{3}\)-induced reduction in annual CO\({}_{2}\) uptake in 2011–2015 is represented in gray, and the means are indicated by the solid black lines. species. The O\({}_{3}\)-induced reduction in annual CO\({}_{2}\) uptake averaged across Japan was 14.0%, 10.6%, 8.6%, and 15.4% in _F. crenata_, _Q. serrata_, _Q. mongolica_ var. _crispula_, and _B. platyphyllla_ var. _japonica_, respectively (Figures 5 and 6). This result indicated that the current atmospheric O\({}_{3}\) concentrations have the potential to reduce the capacity of CO\({}_{2}\) uptake by Japanese forest tree species. [PERSON] et al. [19] indicated that when the four tree species were grown in the same environmental conditions, the COU was ranked as follows: _B. platyphyllla_ var. _japonica_\(>\)_F. crenata_\(>\)_Q. mongolica_ var. _crispula_\(>\)_Q. serrata_. However, the tree distributions and environmental growth conditions greatly differ between the four tree species. _Bettula platyphyllla_ var. _japonica_ is mainly distributed in the Hokkaido region, where the stomatal O\({}_{3}\) uptake was relatively low (Figure 3). As a result, the COU of _B. platyphyllla_ var. _japonica_ averaged across Japan was lower than that of _F. crenata_ and _Q. mongolica_ var. _crispula_ (Figures 3 and 4). Nevertheless, the impact of O\({}_{3}\) on the CO\({}_{2}\) uptake of _B. platyphyllla_ var. _japonica_ was more remarkable than that of the other tree species (Figures 5 and 6). This is due to the relatively higher sensitivity of photosynthetic CO\({}_{2}\) uptake to COU in _B. platyphyllla_ var. _japonica_ (Figure 2). Several studies have assessed the effects of O\({}_{3}\) absorbed via stomata on gross or net primary production (GPP or NPP, respectively) in forest ecosystems [9; 26]. [PERSON] et al. [26] estimated the O\({}_{3}\)-induced reduction in GPP targeted at global forest based on the relationships between COU and maximum carboxylation rate and between COU and stomatal conductance derived in a specific tree species, the tulip poplar. [PERSON] et al. [9] evaluated the impact of O\({}_{3}\) on the GPP of five plant functional types, namely broad-leaved trees, conifers, C\({}_{3}\) grasses, C\({}_{4}\) grasses, and shrubs, considering different sensitivities of the net photosynthetic rate to O\({}_{3}\) among the plant functional types. However, the sensitivity of the photosynthetic capacity to O\({}_{3}\) is extremely different between tree species possibly because of the differences in the defensive ability to O\({}_{3}\)[18]. The advantage of our impact assessment is that we considered the species-specific responses of photosynthetic CO\({}_{2}\) uptake to COU, even though there was uncertainty in the evaluation of O\({}_{3}\)-induced reduction rates by 3.1%, 3.6%, 3.8%, and 6.2% in _F. crenata_, _Q. serrata_, _Q. mongolica_ var. _crispula_, and _B. platyphyllla_ var. _japonica_, respectively (Figure 2). In the present study, COU and O\({}_{3}\)-induced reduction rates of CO\({}_{2}\) uptake were relatively high in Kanto, Chubu, Kinki, and Chugoku regions (Figures 3-6). [PERSON] et al.