## 3 Results
### Driving Factors of Carbon Fluxes
#### 3.3.3 Driving Factors of Daily GPP, NEE, and \(\mathrm{R_{e}}\)  
The primary driving factors of daily carbon fluxes were nearly comparable to those of average diurnal carbon fluxes (Figure 4c,d). The results indicated that PAR was the determinant of daily carbon fluxes, with Pearson correlation coefficients of 0.79 for GPP, 0.72 for \(\mathrm{R_{e}}\) and 0.64 (\(p<0.01\)) for NEE, respectively. Temperature terms (\(\mathrm{T_{a}}\) and \(\mathrm{T_{s}}\)) and VPD were also closely positively related to GPP and \(\mathrm{R_{e}}\), and the Pearson correlation coefficients between those factors and GPP and \(\mathrm{R_{e}}\) were higher than that of NEE. \(\mathrm{CCO_{2}}\) and RH were the two factors negatively correlated to GPP, \(\mathrm{R_{e}}\), and NEE, and the effect of SWC on carbon fluxes was almost negligible. In addition to environmental factors, the LAI exerted a considerable influence on GPP, evidenced by a correlation coefficient of 0.59, 0.58, and 0.43 (\(p<0.01\)) for GPP, \(\mathrm{R_{e}}\), and NEE in 2018, while the value was relatively lower in 2019. The correlation analysis showed that the environmental factors during the vigorous growing period are generally less prominent than daily carbon flux variations compared to that of  
Figure 6: Standardized direct and indirect path coefficients of environmental factors and leaf area index (LAI) on diurnal (**a–f**) and daily (**g–l**) gross primary productivity (GPP), ecosystem respiration (\(\mathrm{R_{e}}\)), and net ecosystem exchange (NEE) for 2018–2019. Environmental factors are vapor pressure deficit (VPD), air carbon dioxide concentration (\(\mathrm{CCO_{2}}\)), atmospheric water vapor concentration (\(\mathrm{CH_{2}O}\)), air temperature (\(\mathrm{T_{a}}\)), air relative humidity (RH), photosynthetically active radiation (PAR), soil temperature (\(\mathrm{T_{s}}\)) at 5 cm depth, and soil water content (SWC) at 20 cm depth. Note: \({}^{\star}\) and \({}^{\star\star}\) represent a significance level of \(p<0.05\) and \(p<0.01\), respectively. non-vigorous periods (Figure 5). However, the LAI had a more positive impact on GPP and R\({}_{\rm e}\) during the vigorous growing period, with a correlation coefficient of 0.27 and 0.43 (\(p<\) 0.01), respectively. The path analysis showed that the variance explained by the environmental factors on daily GPP, R\({}_{\rm e}\), and NEE was 74%, 89%, and 53% in 2018, and 78%, 86%, and 57% in 2019 (Figure 6g-l). The effect of PAR on daily GPP and NEE was observed, primarily through its direct influence, with direct path coefficients of 0.56 and 0.78 (\(p<\) 0.01), respectively. T\({}_{\rm s}\) exerted a significant indirect effect on carbon fluxes through T\({}_{\rm a}\) and LAI, with indirect path coefficients of 0.91 for GPP, 0.54 for R\({}_{\rm e}\), and 0.92 for NEE. T\({}_{\rm a}\) played a significant role in the daily variation in carbon fluxes, exhibiting substantial variability. In particular, its cumulative positive indirect influence exceeded its negative direct influence in 2019, which resulted in a net total positive impact. Biophysical factors predominantly affected R\({}_{\rm e}\) indirectly, whereas VPD played a significant role in direct influence, with a direct path coefficient of 0.42 (\(p<\) 0.01). As illustrated in the path diagrams in Figure 7, the direct effects on GPP and NEE during the vigorous growth period originated from PAR, T\({}_{\rm a}\), T\({}_{\rm s}\), SWC, and LAI, while the indirect effects, mediated by T\({}_{\rm a}\), were mainly attributed to VPD and CH\({}_{2}\)O. Regarding R\({}_{\rm e}\) during the vigorous growth period, the direct effects arose from T\({}_{\rm a}\), CCO\({}_{2}\), and SWC, with indirect effects being derived from most factors influencing T\({}_{\rm a}\), especially for T\({}_{\rm s}\) and CH\({}_{2}\)O. Compared to the vigorous growth period, direct effects during the non-vigorous growth period were primarily from PAR and VPD, while RH exerted a main indirect influence on carbon fluxes through PAR.