## 4 Discussion  
### The Synergistic Effect of \(O_{3}\) and \(Co_{2}\)  
Based on the previous statistical results, we found that there is a significant difference between [CO\({}_{2}\)] and [O\({}_{3}\)] on woodland GPP singly and in combination. Woodland GPP showed a significant upward trend under the synergistic action of climate and [CO\({}_{2}\)]. This is because the promotion of vegetation photosynthesis by [CO\({}_{2}\)] increases forest GPP, and this effect continues to increase with increasing CO\({}_{2}\)[62, 63, 64]. The effect of CO\({}_{2}\) fertilization increases GPP by 1-5% over 20 years, which is close to the results of [PERSON] et al. (2013) [62] based on multiple model comparisons. In contrast, observations based on global carbon flux sites (0.138 \(\pm\) 0.007% ppm\({}^{-1}\); percentile per rising ppm of CO\({}_{2}\)) were slightly higher than those in our study [65]. This may be because the input LAI recorded the increase in vegetation leaf area caused by the CO\({}_{2}\) fertilization effect. Without considering the elevated [CO\({}_{2}\)], the GPP loss caused by [O\({}_{3}\)] and climate change showed a slow increasing trend, while the loss rate showed a slow decrease. This is consistent with the findings of [PERSON] (2007) [50] and [PERSON] (2018) [51]. O\({}_{3}\) concentrations have increased over the past 20 years, but mainly in temperate regions of China. Woodland in temperate areas may be stressed by drought, O\({}_{3}\) and other factors at the same time, and the decrease in stomatal conductance caused by drought stress may limit ozone uptake by vegetation [66, 42]. In contrast, the synergistic effect of [CO\({}_{2}\)] and [O\({}_{3}\)] on GPP decreased more slowly. After 2006, and the loss of GPP caused by [O\({}_{3}\)] alone exceeded the loss of GPP caused by the synergistic action of [O\({}_{3}\)] and [CO\({}_{2}\)] (Figure 4). Therefore, we compared the mean ozone uptake by vegetation stomata under single [O\({}_{3}\)] and [CO\({}_{2}\) + O\({}_{3}\)] scenarios (Figure S8a). With the increase in CO\({}_{2}\) concentration, ozone uptake by vegetation stomata gradually decreased (Figure S8b). This indicates that the synergistic effect of [CO\({}_{2}\)] and [O\({}_{3}\)] may cause a decrease in canopy conductance and prevent O\({}_{3}\) uptake by vegetation stomata [20, 21, 36]. Field observation experiments have proven the adaptability of vegetation photosynthesis to the increase in CO\({}_{2}\), and the change in stomatal conductance cannot affect the intercellular CO\({}_{2}\) concentration of vegetation, but the adaptation of stomata to [CO\({}_{2}\)] has not been found [19, 67]. This result is supported by [PERSON] et al. (2018) [51], whose estimates of GPP in European ecosystems demonstrate that elevated [CO\({}_{2}\)] offsets [O\({}_{3}\)] damage to GPP. The studies of [PERSON] (2017) and [PERSON] (2021) [36, 37] on crop yield also reached a similar conclusion. Although the damage of [O\({}_{3}\)] on ecosystem GPP was greater than the gain of the CO\({}_{2}\) fertilization effect on the ecosystem, CO\({}_{2}\) gradually weakened the effect of ozone. The results of causal analysis also reveal the synergistic effect of O\({}_{3}\) and CO\({}_{2}\), as shown in Figure 5. Most forest areas show a synergistic effect of O\({}_{3}\) and CO\({}_{2}\), some areas show a redundant effect of CO\({}_{2}\) and O\({}_{3}\), and only a few grids show uniqueness. This indicates that among the combination of meteorological factors that have the greatest impact on GPP, CO\({}_{2}\) and O\({}_{3}\) play a major role. Although subtropical regions are affected by ozone pollution, the better water and heat conditions result in the fact that the impact of ozone on GPP is not obvious. Therefore, the redundant effects of O\({}_{3}\) and CO\({}_{2}\) on GPP are mainly concentrated in subtropical regions. With the changes in water and heat conditions, O\({}_{3}\) and CO\({}_{2}\) show obvious synergistic regulatory effects in temperate regions.