## 3 Results
### The Effects of Environmental Factors on the Correlation between PRI and LUE  
Table 2 shows the response of LUE and PRI to PAR, VPD, and Ta in the wet season (from April 2014 to September 2014) and dry season (from October 2014 to March 2015). In the dry season, LUE was affected differently by PAR, VPD, and Ta, among which PAR had the greatest impact on LUE (R\({}^{2}\) = 0.85), followed by VPD (R\({}^{2}\) = 0.26), and the effect of Ta on LUE can be ignored. In the wet season, PAR had the greatest effect on LUE (R\({}^{2}\) = 0.58). VPD and Ta had no significant difference in its affects  
\begin{table}
\begin{tabular}{c c c c} \hline
**Mon** & **R\({}^{2}\)** & **\(p\)** & **RMSE** \\ \hline April & 0.53 & \(p<0.01\) & 1.07 \\ May & 0.49 & \(p<0.01\) & 0.66 \\ June & 0.36 & \(p<0.01\) & 0.78 \\ July & 0.34 & \(p<0.01\) & 0.91 \\ Aug & 0.51 & \(p<0.01\) & 1.12 \\ Sept & 0.71 & \(p<0.01\) & 0.90 \\ Oct & 0.66 & \(p<0.01\) & 0.80 \\ Nov & 0.91 & \(p<0.01\) & 1.11 \\ Dec & 0.72 & \(p<0.01\) & 1.03 \\ Jan & 0.75 & \(p<0.01\) & 2.33 \\ Feb & 0.68 & \(p<0.01\) & 2.74 \\ March & 0.48 & \(p<0.01\) & 3.22 \\ \hline \end{tabular}
\end{table}
Table 1: The relationship between PRI and LUE from April 2014 to March 2015 for each month.  
Figure 6: The correlation between daily PRI and LUE for each day from April 2014 to March 2015.  
on LUE. The correlation of LUE and PAR was higher in the dry season than in the wet season. In the dry season, similar to the response of LUE, PRI had the greatest response to PAR (R\({}^{2}\) = 0.67), and VPD had the second greatest effect on PRI (R\({}^{2}\) = 0.24), while temperature had no effect on PRI (Table 2). In the wet season, the responses of PRI to PAR and Ta were more obvious (R\({}^{2}\) = 0.59 and R\({}^{2}\) = 0.55), while VPD had a weaker effect on PRI (R\({}^{2}\) = 0.29). Overall, LUE and PRI had similar responses to environmental factors.  
Figure 7a and b illustrate the effects of air temperature on LUE and PRI, observed by an AMSPEC spectrometer. Figure 7c reveals the change in the correlation between the PRI and the LUE as a function of temperature. During the study, the air temperature ranged from 5 \({}^{\circ}\)C to 35 \({}^{\circ}\)C. When the air temperature is less than 20 \({}^{\circ}\)C, there is no correlation between temperature and LUE and PRI (Figure 7a,b). When the temperature is higher than 20 \({}^{\circ}\)C, the LUE decreases gradually with the increase of temperature. The correlation between PRI and LUE increases and then decreases with rising air temperature. When the temperature is between 10 \({}^{\circ}\)C to 15 \({}^{\circ}\)C, the correlation between PRI and LUE is at its largest (R\({}^{2}\) = 0.78). The changes in LUE and PRI, as affected by VPD and the changes in the LUE and PRI as a function of VPD, are shown in Figure 7d-f. LUE and PRI decreased with VPD, and the correlation between PRI and LUE decreased with increasing VPD. When VPD \(<\) 0.5, the best correlation between PRI and LUE (R\({}^{2}\) = 0.22) occurred. When VPD ranges from 1.5 and 2, there is no correlation between LUE and PRI.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**R\({}^{2}\)**} & \multicolumn{2}{c}{**Dry Season**} & \multicolumn{2}{c}{**Wet Season**} \\ \cline{2-5}  & **PRI** & **LUE** & **PRI** & **LUE** \\ \hline PAR & 0.67 *** & 0.85 *** & 0.59 *** & 0.58 *** \\ VPD & 0.24 *** & 0.26 *** & 0.29 *** & 0.29 *** \\ Ta & 0.00 & 0.02 & 0.55 *** & 0.28 *** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of responses of PRI and LUE to environmental factors in the dry season and wet season.  
Figure 7: _Cont._  
Figure 8 shows the overall pattern of LUE simultaneously responding to air temperature, VPD, and PAR. Different temperature conditions may correspond to different radiation conditions, and different air humidity. Additionally, it was also established, in turn, that LUE showed a decreasing trend when radiation increases. The LUE also decreased slightly with the increase of VPD. However, LUE was not affected by air temperature.