## 4 Discussion
_crisula_, could be relatively easy. However, changes in the leaf area of _B. platyphyllla_ var. _japonica_ having heterophyllous leaf emergence type is not easy to estimate, because it is difficult to identify the emergence day of each leaf, which could result in the higher variability than the other three tree species. The source of error in the calculation of POD could be estimation of \(g_{\text{sw}}\) in addition to the difficulty in the definition of leaf emergence day (i.e., start day of accumulation period) especially in _B. platyphyllla_ var. _japonica_. In the present study, the accuracy of estimation of \(g_{\text{sw}}\) was relatively low, especially for 2 nd flush leaves of \(Q\). _mongolica_ var. _crisula_ and early and late leaves of _B. platyphyllla_ var. _japonica_. On the other hand, mean bias error (MBE) in the relationship between estimated and measured \(g_{\rm sw}\) were higher than 0 in all tree species (Table 3), indicating the overestimation of \(g_{\rm sw}\) and thus resulting in overestimation of POD. Although we did not try adjusting other parameters to get a better fit, higher accuracy in the estimation of \(g_{\rm sw}\) is foundational to calculating the POD. Further study is required to improve the accuracy of the estimation of \(g_{\rm sw}\) by modification of the stomatal conductance model, considering the other parameters such as O\({}_{3}\)-induced stomatal sluggishness (e.g., [PERSON] et al., 2014 [42]). [PERSON] et al. (2007) [7] quantitatively evaluated the O\({}_{3}\)-induced reduction in global gross primary production (GPP) using the sensitivity to O\({}_{3}\) of the whole-plant growth of European beech, birch and oak and Norway spruce for forest trees. In their report, there was considerable difference in the O\({}_{3}\)-induced reduction in GPP between the estimations considering a low or high plant O\({}_{3}\) sensitivity. Thus, an uncertainty in the estimation of the O\({}_{3}\)-induced reduction in GPP could result from different sensitivities to O\({}_{3}\) among tree species. There are many forest tree species with species-specific O\({}_{3}\) sensitivity in Asia [32; 33; 43]. In the present study, the species-specific responses (i.e., O\({}_{3}\) sensitivities) of cumulative photosynthetic CO\({}_{2}\) uptake to stomatal O\({}_{3}\) uptake were clearly demonstrated (Figure 2e-h). In addition, the flux threshold appropriate for the linear regression of the responses of relative \(\Sigma P_{\rm n,est}\) to POD\({}_{\rm Y}\) was also species-specific (Table 4). Therefore, the differences in the O\({}_{3}\)-sensitivity among the representative temperate forest tree species should be taken into account for more accurate and quantitative evaluations of O\({}_{3}\) effects on photosynthetic CO\({}_{2}\) uptake. According to a meta-analysis by [PERSON] et al. (2007) [28], the O\({}_{3}\) sensitivity of \(A_{\rm sat}\) in the leaves of trees as assessed by the POD was 0.21% per mmol O\({}_{3}\) m\({}^{-2}\) LA. To compare the POD-based O\({}_{3}\) sensitivities, we conducted a linear regression analysis of the relationship between POD\({}_{0}\) and relative \(A_{\rm sat}\). The slopes of the regression lines for each tree species were \(-\)0.69, \(-\)0.95, \(-\)0.66 and \(-\)0.98% per mmol O\({}_{3}\) m\({}^{-2}\) LA for _F. crenata_, _Q. serrata_, _Q. mongolica_ var. _crispula_ and _B. plattyphylla_ var. _japonica_, respectively. These results indicate that the POD-based O\({}_{3}\) sensitivity of the \(A_{\rm sat}\) of Japanese deciduous broad-leaved forest tree species is high compared with those of the tree species reported by [PERSON] et al. (2007) [28], although the POD-based O\({}_{3}\) sensitivity reported by [PERSON] et al. (2007) [28] varied greatly owing to differences in species, genotypes, locations and experimental conditions.