## 4 Discussion  
### Spatiotemporal Variations in \(\text{FCO}_{2}\) and Its Driving Factors  
We conducted the field \(\text{FCO}_{2}\) measurements at two study areas in a typical subtropical forest system during the dry season for investigating the temporal and spatial variations of \(\text{FCO}_{2}\). Our results found that \(\text{FCO}_{2}\) showed a decreasing trend in the dry season (Figure 2), which is consistent with the previous studies in the similar subtropical forests of Guangdong province, southern China [20; 24; 79]. Moreover, our findings indicated that the \(\text{FCO}_{2}\) in the dry season is significantly and positively correlated with soil temperature (ST) in the subtropical forest (\(p<0.001\)), and the ST decline decreased the \(\text{FCO}_{2}\) over the study period (Figures 2 and 3). Many convincible and agreeable conclusions from previous studies have confirmed that the ST is the dominant driving force in controlling \(\text{FCO}_{2}\) variation at the different time and space scales [5; 8; 20; 67; 39]. Moreover, our study observed that soil moisture (SM) experienced a downward trend in the dry season (Figure 3a,c), and was significantly positively correlated with \(\text{FCO}_{2}\) (\(p<0.001\)) (Figure 4b) as reported in similar subtropical forests [20; 79; 80]. Although SM is the key driving factor for determining the variation of \(\text{FCO}_{2}\) during the dry season in the subtropical forest, the relationship between \(\text{FCO}_{2}\) and SM is more complex compared with ST [67]. For example, [PERSON], et al. [8] reported that the \(\text{FCO}_{2}\) increases as the water stress condition decreases. Nevertheless, oversaturated soil can result in low \(\text{FCO}_{2}\) values in the same forest ecosystem because excessive SM will limit the diffusion of \(\text{O}_{2}\) in the soil, resulting in the weakening of the activity of plant roots and aerobic microorganisms and ultimately the reduction of the soil \(\text{CO}_{2}\) production [81]. In more detail, previous studies focusing on similar subtropical forested areas reported a significant positive correlation between \(\text{FCO}_{2}\) and SM when SM is below 50% [79; 82; 83]. In this study, SM averages 18.10% and 20.99% in the CHD and GY areas during the dry season (Figure 2a,c), which is suitable for soil respiration as demonstrated in [PERSON], et al. [79]. This is why the \(\text{FCO}_{2}\) is positively correlated with SM and explains around 60% of variations of \(\text{FCO}_{2}\) in our two areas (Figure 4b). In this study, the mean values of \(\text{FCO}_{2}\) in CHD and GY areas are 9.05 g C m\({}^{-2}\) day\({}^{-1}\) and 5.03 g C m\({}^{-2}\) day\({}^{-1}\), individually, during the dry season. Moreover, for the whole forested basin, the regional average value is about 8.19 g C m\({}^{-2}\) day\({}^{-1}\). [PERSON], et al. [84] found that the \(\text{FCO}_{2}\) of a temperate forest in northern China was nearly 1.88 g C m\({}^{-2}\) day\({}^{-1}\). As expected, our results showed that the \(\text{FCO}_{2}\) in the subtropical forest is higher than that in the temperate forest. For subtropical forests that are similar to our study area. [PERSON], et al. [20] reported that the \(\text{FCO}_{2}\) was about 14.10 g C m\({}^{-2}\) day\({}^{-1}\). [PERSON], et al. [79] showed that the mean value of \(\text{FCO}_{2}\) for three different forests was approximately 9.30 g C m\({}^{-2}\) day\({}^{-1}\). [PERSON], et al. [85] also found that the average \(\text{FCO}_{2}\) of the three subtropical forests was approximately 9.36 g C m\({}^{-2}\) day\({}^{-1}\) and 16.41 g C m\({}^{-2}\) day\({}^{-1}\) in 2009 and 2010, respectively. However, the magnitude of \(\text{FCO}_{2}\) that we measured during the dry season is relatively lower than those reported in similar forest ecosystems characterized by the relatively humid dry seasons with higher precipitation. During our study period, there was almost no precipitation during the entire study period (i.e., 2019-2020) (Figure 10), which caused the season to be drier than normal.