## 3 Results and Discussion
### Biophysical Controls of Carbon Fluxes  
Figure 4 shows the results of the relative importance of eight factors to carbon fluxes by the random forest method. GPP and Re are the two most important components in the carbon cycle, which determines whether the ecosystem is a CO\({}_{2}\) source or sink. The influences of environmental factors on carbon exchange are complex since they independently and interactively influence GPP and Re (Figure 4). Among the variables, the PI, Ra, and RH were the three dominant controls of NEE (Figure 4a). In fact, the impact of biophysical controls on NEE were due to their balanced impact on GPP and Re.  
The daily GPP was mainly controlled by PI, Ra, and LAI (Figure 4b). Ra represents the intensity of light energy, which is converted into chemical energy for organic carbon production in photosynthetic reactions [12]. LAI was closely linked to the carbon uptake, as it indicates the amount of foliage that intercepts light and gas exchange through the stomata [42]. Increasing Ra and LAI promoted GPP (Figure 5a,b), since both factors determine how much plants intercept and convert solar radiation into biomass [43; 44]. A previous study also showed that canopy gross photosynthesis increased with LAI [45], which corresponds to the results of this study (Figure 4b). PI was also an important factor since it represents the vegetation density in the EC source area. In addition, high VPD could reduce stomatal conductance under strong atmospheric vapor stress and downregulate GPP in mangroves [13; 46; 47].  
P1 had the highest impact on Re (Figure 4c), which indicated the importance of autotrophic respiration through the canopy. This corresponds to a previous study showing that the canopy respiration dominates the total ecosystem respiration (canopy, water, and soil respiration) [48; 49]. Re increased quasi-exponentially with air temperature (Figure 5c), which is consistent with the experiments that both autotrophic respiration and soil microbial respiration increase exponentially with mean temperature before reaching an optimum [50; 51; 52; 53]. In addition, the temperature fluctuations can also influence the soil microbial respiration [54]. Therefore, the random forest analysis indicated that air temperature was the most important predictor of Re (Figure 4c). Since the air temperature also affected GPP (Figure 4b), the trade-off between the effect on GPP and Re reduced the relative importance on NEE (Figure 4a).  
CH4 fluxes from coastal wetlands are determined by the processes of CH4 production, oxidation, and transportation [55]. Our measurements indicated that the temperature was the strongest predictor of CH4 (Figure 4d). This was because temperature has a strong impact on methanogenesis and methane oxidation due to controlling the activities of methanogens and methanotrophs [56; 57; 58]. Higher temperatures can also induce drought and lower water tables by increasing evapotranspiration, which decreases CH4 emissions but increases CO2 emissions [59], although this may be uncommon in always flooded mangroves. In this study, an increasing temperature facilitated CH4 emissions (Figure 5d), which indicated that methanogenesis was more sensitive to warming. In addition, compared with freshwater wetlands, mangroves have relatively high salinity, which has been confirmed to suppress CH4 production [60]. This is because the presence of sulfate in coastal wetland soils allows sulfate-reducing bacteria to outcompete methanogens for energy sources, consequently inhibiting methane production [61; 62]. Our analysis also identified salinity as an important predictor of CH4 fluxes (Figure 4d), which corresponds to the results of previous studies.  
Figure 4: The relative importance of explanatory variables for the daily **(a)** NEE, **(b)** GPP, **(c)** Re, and **(d)** CH4 flux. The results are presented as 200-time averages.