## 3 Results
### Seasonal Variation of PRI, LUE, GPP, and Environmental Factors  
Figure 5 illustrates the continuous variations of PRI, LUE, environmental factors (air temperature (Ta), vapor pressure deficit (VPD), and PAR), and GPP on a half-hour scale. Figure 4a shows the continuous variations of the PRI and LUE. There was a gap of the PRI from 22 March 2014, to 4 April 2014, due to instrument failure. The seasonal variations of LUE and PRI are complicated. During 14 July 2014, to 28 September 2014, PRI and LUE show a steady fluctuation trend. The PRI decreases with temporal change patterns to a very low value in early October. Afterwards, the trend of PRI becomes more obvious, it increases around 28 September 2014, and begins to decrease after 8 November 2014, and then quickly decreases in 23 November 2014, and decreases to the minimum. LUE is generally steadily fluctuating; the values of LUE remain high from 27 October 2014, to 6 December 2014. The temperature reached a peak on 31 July 2014, and then continued to decrease. Until 27 November 2014, the temperature rose slightly. From January to March 2015, the temperature was low and fluctuated slightly. The VPD was lower from November 2014 to March 2015. At other times, it is basically at a higher value, about 100 hPa. The value of PAR in the whole year is relatively high from June to October, and the maximum value is concentrated at 451 J m\({}^{-2}\)s\({}^{-1}\), which was recorded on 4 July 2014. From November 2014 to March of the next year, the maximum value of PAR is 337 J m\({}^{-2}\)s\({}^{-1}\), which was recorded on 28 March 2015. From the beginning of April 2014, GPP climbed up steadily and reached the peak level during the end of the June 2014 until the middle of the July 2014. After that, GPP dropped down quickly during the end of July to the start of August 2014. While this process had a rebound in September and October 2014, the decline continued until December 2014. In the next year, it dropped to the bottom in March 2015. Definitely, the maximum value of GPP reached to 0.6 g C m\({}^{-2}\) hh\({}^{-1}\) in April, May, August, November, and December 2014, as well as in March 2015. In July, September, and October, the maximum value of GPP is about twice the value of the previous months.  
Figure 4: Comparison of the relationship between the LUE and the PRI not corrected by a BRDF model (**a**) and that between the LUE and the PRI corrected by a BRDF model (**b**) in the LUE and PRI data were measured every half-hour from April 2014 to March 2015. The red lines represent the linearly fitted functions, and the green lines represent the exponentially fitted functions.  
correlations are mostly at a lower value, which increase after 9 November 2014, and remain stable until the end of the year. The correlation value from January to March in the second year is slightly lower than the value at the end of the previous year.  
During each month, there are some differences in the relationship between PRI and LUE (Table 1). The linear correlations between PRI and LUE are the smallest from June to August. The smallest correlation occurred in July 2014 (R\({}^{2}\) = 0.34), followed by June, 2014 (R\({}^{2}\) = 0.36). However, a strong correlation occurred between September and December 2014 (R\({}^{2}\) > 0.66), with the largest correlation in November 2014 (R\({}^{2}\) = 0.91). The correlations between April and May 2014 were intermediate. PRI can be a good indicator of LUE from September and December 2014.