## 4 Discussion
Because the negative impact of O\({}_{3}\) on photosynthetic CO\({}_{2}\) uptake can indirectly increase radiative forcing [7], we evaluated the effect of O\({}_{3}\) on photosynthetic CO\({}_{2}\) uptake based on POD\({}_{0}\) in the leaves of representative Japanese deciduous broad-leaved tree species. According to the meta-analysis by [PERSON] et al. (2007) [28], the 28% of variation in the O\({}_{3}\)-induced percent changes in \(A_{\text{sat}}\) measured at a certain time was accounted for by the linear function of POD\({}_{0}\) (i.e., \(R^{2}\) value was 0.28). On the other hand, [PERSON] et al. (2013) [39] did not observe any correlations between changes in \(A_{\text{sat}}\) and POD owing to the large variability across studies. In the present study, about 60% of O\({}_{3}\)-induced percent changes in the \(A_{\text{sat}}\) was accounted for by POD\({}_{0}\) (Figure 2a-d), which is greater than [PERSON] et al. (2007) [28]. However, the POD\({}_{0}\) accounted for the O\({}_{3}\)-induced percent changes in the \(\Sigma P_{\text{n\_est}}\) rather than those in the \(A_{\text{sat}}\) (Figure 2e-h). These results suggest that the \(\Sigma P_{\text{n\_est}}\) is an appropriate dependent variable in the response function of photosynthetic CO\({}_{2}\) uptake to POD, which could result from the fact that the cumulative stomatal O\({}_{3}\) uptake corresponds to the extent of cumulative O\({}_{3}\) effect on photosynthetic CO\({}_{2}\) uptake. Therefore, the response function of relative leaf-level \(\Sigma P_{\text{n\_est}}\) to POD is useful for the quantification of the negative impact of O\({}_{3}\) on the NPP of forests. In the present analysis, there are uncertainties in the calculations of leaf-level \(\Sigma P_{\text{n\_est}}\), whole-plant-level \(\Sigma P_{\text{n\_est}}\) and POD. For the calculation of leaf-level \(\Sigma P_{\text{n\_est}}\), the calculation period was from the first day of the first measurement of \(A\)-light curve to the last day of the last measurement in each growing season. Because this period was shorter than actual accumulation period, the leaf-level \(\Sigma P_{\text{n\_est}}\) could underestimate the actual cumulative photosynthetic CO\({}_{2}\) uptake per unit LA. It is unclear that the ignorance of the effects of changes in air temperature and air humidity on the \(A\)-light curves causes overestimation or underestimation, but it would be the source of the error. Furthermore, it is also unclear the measurement of \(A\)-light curve at about one-month interval was enough time resolution to estimate the leaf-level cumulative photosynthetic CO\({}_{2}\) uptake. The greater the number of measurements during the growing season, the better for the estimation of absolute value of accumulated photosynthetic CO\({}_{2}\) uptake. In the present study, however, we focused on the extent of O\({}_{3}\)-induced reduction in leaf-level \(\Sigma P_{\text{n\_est}}\), rather than absolute value. Because there were relatively high \(R^{2}\) value in the relationship between WDM increment and the whole-plant-level \(\Sigma P_{\text{n\_est}}\) in all tree species, the monthly measurement of \(A\)-light curve could be enough time resolution for the evaluation of O\({}_{3}\) effects on cumulative photosynthetic CO\({}_{2}\) uptake. For the scaling up \(\Sigma P_{\text{n\_est}}\) from leaf-level to whole-plant-level, it is necessary to account for the difference in the light condition within the canopy and the changes in the leaf area per plant not only owing to the leaf emergence but also owing to the O\({}_{3}\)-induced leaf abscission. The calculation of whole-plant-level \(\Sigma P_{\text{n\_est}}\) did not consider the shelf-shading within the canopy and assumed that all leaves received full sunlight, which could result in the overestimation of the cumulative photosynthetic CO\({}_{2}\) uptake at whole-plant level. The estimation of changes in the leaf area per plant of tree species having flush-type leaf emergence, such as _F. creata_, \(Q\). _serrata_ and \(Q\). _mongolica_ var.