## 3 Results and Discussion  
### Coupling State of Airflow Crossing the Vertical Layer of the Plantation Canopy  
The ideal for ecosystem flux observations is that turbulence occurring in all canopy-subcanopy-soil profile can be adequately mixed or exchanged and that EC observations can accurately capture turbulence information and flux contributions to ensure that fluxes calculated using simplified mass balance methods are representative of the entire ecosystem ([PERSON] et al., 2016). Thus, the substratum selection for sites in the flux network is mostly low or open vegetation (e.g., grasslands, scrub, agricultural fields, and forests), and this characteristic of the substratum often allows for adequate mixing of sources and sinks of scalar fluxes in the vertical layer. The mixing intensity of vertical-layer air masses is highly susceptible to canopy structure, including factors such as canopy cover, PAI, branch extension length, and trunk space ([PERSON] et al., 2016). The coupling state of the actual vertical layer can be theoretically classified into three major scenarios according to the coupling period: (1) fully coupled during both day and night; (2) coupled during the day and decoupled during the night; and (3) decoupled during both day and night. Considering the effect of tree space, we screened \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Decoupling** & **Mixed Indicators** & **Proportion of Valid** & **Proportion of Valid** & **Proportion of Total** & **Methods** \\
**State** & & **Daytime Data (\%)** & **Data at Night (\%)** & **Valid Data (\%)** & **Methods** \\ \hline \multirow{3}{*}{-} & No index & 80.21 & 52.51 & 67.9 & 1.1 \\  & *** & 70.64 & 42.3 & 58.04 & 1.2 \\ \hline \multirow{5}{*}{Night} & Single-level u* & 80.21 & 24.06 & 55.22 & 2.1.1 \\  & *** & 70.64 & 21.01 & 53.42 & 2.1.2 \\ \cline{2-6}  & Single-level \(\sigma_{\text{w}}\) & 80.21 & 32.21 & 58.86 & 2.2.1 \\  & *** & 70.64 & 26.74 & 56.41 & 2.2.2 \\ \cline{2-6}  & Double-level u* & 80.21 & 8.95 & 48.49 & 2.3.1 \\  & *** & 70.64 & 7.58 & 47.88 & 2.3.2 \\ \cline{2-6}  & Double-level \(\sigma_{\text{w}}\) & 80.21 & 13.68 & 50.6 & 2.4.1 \\  & *** & 70.64 & 10.95 & 49.38 & 2.4.2 \\ \hline \multirow{5}{*}{Daily} & Double-level u* & 55.57 & 8.95 & 34.86 & 3.1.1 \\  & *** & 50.89 & 7.58 & 32.22 & 3.1.2 \\ \cline{1-1} \cline{2-6}  & Double-level \(\sigma_{\text{w}}\) & 71.64 & 13.68 & 45.84 & 3.2.1 \\ \cline{1-1}  & *** & 64.59 & 10.95 & 41.93 & 3.2.2 \\ \hline \hline \end{tabular} Note: There are three states: complete coupling (-), decoupling only at night, and decoupling day and night (daily). (-) indicates that the air mass is in the ideal state of complete coupling day and night. The DLEC method in the complete coupling state does not need additional indicators to analyze the coupling threshold, so ’No index’ is displayed. *** represents turbulence data quality control after diagnosis of original air mass mixing. Single-level u* and single-level \(\sigma_{\text{w}}\) mean that u* and \(\sigma_{\text{w}}\) above the canopy are used as decoupling indexes at night, respectively, and it is assumed that the vertical layer air mass of the canopy is fully coupled during the day. Double-level u* and double-level \(\sigma_{\text{w}}\) mean that u* and \(\sigma_{\text{w}}\) above and below the canopy are used to solve the threshold, respectively, considering the intraday coupling state.  
\end{table} 
Table 1: Effective data ratios for the three coupling states considering quality markers. turbulence mixing indicators for the vertical layer of the incompletely covered plantation canopy. Excluding differences in the vertical exchange of day and night scalar turbulent fluxes, \(\sigma_{\mathrm{w}}\) above the canopy was significantly linearly correlated with \(\sigma_{\mathrm{w}}\) below the canopy (y = 0.17x + 0.02; R\({}^{2}\) = 0.67), and forest sites with sparse canopies exhibit significant strong coupling behavior (Figure 1a). The main reason for this difference compared to the [PERSON] et al.