## 3 Results and Discussion
### Coupling State of Airflow Crossing the Vertical Layer of the Plantation Canopy
study [23] is that the reduction in canopy depression leads to an increase in canopy vertical profile connectivity, which further enhances the proportion and depth of the canopy above which eddies can penetrate deeper into the subcanopy. Considering the different thermal and kinetic differences of the diurnal airflow, we distinguished between day and night to analyze the coupling state of the airflow in the vertical layer of the canopy. We found that \(\sigma_{\mathrm{w}}\) above and below the canopy showed a significant linear correlation during daytime (y = 0.17x + 0.03, R\({}^{2}\) = 0.62), while the nighttime showed a significant nonlinear relationship (y = 0.0655 - 0.1665x + 0.4984x\({}^{2}\) - 0.2112x\({}^{3}\), R\({}^{2}\) = 0.76). In both cases, we seem to obtain a piece of knowledge about the coupled state artifact. Even for the sparse canopy, there is a clear decoupling phenomenon at night (Figure 1b,c), and the sparse canopy cannot be simply interpreted as fully coupled at the daily scale without any distinction because the daytime will confuse the decoupling phenomenon of the vertical layer at night. The cause of this state at night is mainly the subcanopy inversion process. For undepressed stands, the standard deviation of frictional and vertical wind speeds above the canopy and in the subcanopy showed consistent characteristics of variation both during the day and at night. During the night, when the wind was weak, the subcanopy turbulence intensity was in an independent state from that above the canopy, but after the airflow above the canopy exceeded the thresholds of u* = 0.3 m s\({}^{-1}\) and \(\sigma_{\mathrm{w}}\) = 0.45 m s\({}^{-1}\), respectively. The turbulence intensity above and below the canopy showed a linear relationship; even during the daytime when the wind was stronger, it showed the same behavior, with thresholds of u* = 0.3 m s\({}^{-1}\) and \(\sigma_{\mathrm{w}}\) = 0.4 m s\({}^{-1}\) (slightly smaller) for the transition from the independent state to the linear relationship, respectively. The independence of the intensity of subcanopy turbulence from the intensity of overcanopy turbulence suggests either different independent mechanisms of turbulence action and disruption, that airflow is largely decoupled above and below the canopy, or that both play a role [23]. Subcanopy airflow at weak turbulence intensities exhibit convergence to a constant value behavior, and the sparse canopy structure preventing airflow from reaching the subcanopy deep inside the canopy does not seem to be the main reason for the independence of airflow above and below the canopy at the MQ site. In other words, overcanopy airflow and wind shear are the main sources of subcanopy turbulence. The subcanopy equivalent is greater during the day than at night for a given overcanopy turbulence intensity condition. Figure 1: Diagnosis of coupled states during whole day, nighttime, and daytime on the half-hourly scales. (**a–c**) represents three different time of data: all data; Daily; Night. Note: The x and y axes in graphs are the standard deviation of vertical wind speeds above (\(\sigma_{\mathrm{w\_up}}\)) and below (\(\sigma_{\mathrm{w\_down}}\)) the canopy, respectively. Wind shear similarly enhances the mixing of air masses between layers and further appears to increase the intensity of turbulence in the subcanopy layer under a weak overcanopy turbulence intensity. The wind speed above and below the canopy shows a consistent trend but tends to vary linearly above the canopy, while the subcanopy shows a nonlinear trend. An interesting phenomenon is that the air masses above and below the canopy rise staggeringly at wind speeds above the canopy of 2.3 m s\({}^{-1}\) < Utop < 3.7 m s\({}^{-1}\), while in the rest of the interval, the air masses are significantly stronger during the day than at night (Figure 2c).