## 3 Results
### Seasonal Patterns of Bioclimatic Factors, Productivity, PRI, and LUIE  
Figure 2 illustrates the seasonal variations of the bioclimatic parameters of productivity, PRI, and LUE. A data gap from day 228 to 229 was caused by instrumental failure. Ta, VPD, PAR, CI, and GPP exhibited mostly similar seasonal patterns (Figure 2a,b), while PRI and LUE showed opposite trends (Figure 2c). According to the temporal pattern of precipitation, the seasonal dynamics can be characterized by two phases (Figure 2a): the rainy season from day 101 to 167 and the dry season from day 168 to 275. In the rainy season, SM (at 5 cm) was high. The dry season was characterized by less precipitation, low SM, and continuously high temperature.  
VPD, PAR, and CI varied synchronously across the season. In the dry season, VPD was high (up to 50 hPa sometimes) due to high temperatures. Meanwhile, SM decreased quickly due to high evapotranspiration consumption. During the period from day 218 to 225, daily maximum temperatures were continuously above 35 \({}^{\circ}\)C. Such a long-term heat wave was second only to that which occurred in 2003 during the years from 2003 to 2013 with flux measurements available. During the period from day 196 to 226, rainfall was low and temperature was high. We defined this period as a summer depression period (highlighted by a red rectangle in Figure 2).  
GPP exhibited an overall seasonal variation similar to air temperature (Figure 2a). However, GPP declined during the summer depression period. Previous studies also reported that high temperatures and low soil water content might cause GPP to decline in this forest [64, 65].  
The seasonal patterns of LUE and PRI appeared to be complicated. LUE generally varied inversely with PAR, and reached the lowest level during the summer depression period. PRI could track variations of LUE to some extent, especially in the dry season. Although PRI and LUE were  
Figure 1: Variability of PRI with different view angles at different times. (**a**) and (**b**) used data obtained from 10:45 a.m. to 11 a.m. on day 218, while (**c**) and (**d**) used data obtained from 14:45 to 15:00; (**a**) and (**c**) showed variability of PRI with different view azimuth angles (VAA) and view zenith angles (VZA); (**b**) and (**d**) illustrated PRI variations in relation to the angle between sun and viewer (\(\theta_{r}\)).  
correlated, the variability of PRI was greater than that of LUE. In the rainy season, the fraction of diffuse radiation was high, which might induce high LUE [23, 42]. Overall, PRI inversely varied with PAR. However, the change of PRI with PAR was not obvious on rainy or overcast days. PRI was more obviously correlated with PAR and CI in the dry season than in the rainy season. Moreover, PRI, GPP, and LUE all apparently decreased during the depression period with continuously high temperature.  
As presented in Figure 3, PAR could explain 52% of LUE variance (Figure 3b), followed by VPD (Figure 3a) and Ta (Figure 3c). LUE decreased significantly with increases of PAR, Ta, and VPD (\(p<0.05\)). PRI also decreased with increases of PAR, Ta, and VPD. Both PAR (Figure 3e) and VPD (Figure 3d) could explain more than 20% of PRI variance. The correlations between PRI and Ta are weak (Figure 3f).  
Figure 2: Seasonal variations of (**a**) SM (at 5 cm), Ta, VPD; (**b**) GPP, CI, PAR; and (**c**) PRI, LUE measured every half hour from 7 a.m. to 17:30 when the solar zenith angles were less than 75\({}^{\circ}\). The solid lines indicated 50 samples’ moving averages. The red rectangle indicated the depression period.