## 4 Results
### Estimates of the Maximum LUE (\(\varepsilon_{0}\)) during Phenological Stages  
The phenology-based maximum light use efficiency (\(\varepsilon_{0}\)) at the seven flux sites is shown in Table 3. Phenological stage 1 (PS1) refers to the period from the first day in the DOY to the SOS, phenological stage 2 (PS2) refers to the second period from the SOS to the EOS, and phenological stage 3 (PS3) refers to the third period from the EOS to the last day in the DOY. At the US-Ho1 site from 2013 to 2015, the maximum \(\varepsilon_{0}\) appeared in the PS3 and varied from 0.042 to 0.049 umol CO\({}_{2}\)/umol PPFD, while the minimum \(\varepsilon_{0}\) appeared in the PS1 and differed from 0.030 to 0.036 umol CO\({}_{2}\)/umol PPFD. For the DBF site, \(\varepsilon_{0}\) showed considerable ranges from 0.037 to 0.062 umol CO\({}_{2}\)/umol PPFD at US-Ha1 and 0.013-0.040 umol CO\({}_{2}\)/umol PPFD at US-MMS. In regard to the cropland site of US-Ne2 and US-Twt, the maximum \(\varepsilon_{0}\) was synchronous in the PS2 with the value of 0.065 in the dryland and 0.045 in the paddy field. The \(\varepsilon_{0}\) of the wetland (US-Myb site) in PS2 was relatively higher than that in PS1 and PS3, with the maximum value being approximate to 0.38 from 2012 to 2014. For the grassland site of US-Var during 2012-2014, the \(\varepsilon_{0}\) showed substantial variations among the PS1, PS2, and PS3, especially the minimum value of 0.01 in PS1 and the maximum value of 0.084 in PS2 in the year of 2014. The maximum \(\varepsilon_{0}\) ranged from 0.042 umol CO\({}_{2}\)/umol PPFD to 0.084 umol CO\({}_{2}\)/umol PPFD in the PS2 of the three years. Comparison of GPP Estimates with Phenology-Regulated \(\varepsilon_{0}\) and Different FAPAR Proxies  
A total of 21 site-years GPP values were simulated with phenology-based \(\varepsilon_{0}\) and four FAPAR proxies. For one site-year GPP simulation, the specific parameters used to generate five GPP estimates are shown in Table 4. GPP5 and GPP\({}_{\text{EC}}\) represent VPM GPP and observed GPP estimated from each flux site. Figure 3: Two examples of the validation of SOS and EOS at forest site and wetland site. (**a**) Photographs used to visually examine SOS and EOS extraction results (US-MMS in 2014); (**b**) photographs used to visually examine SOS and EOS extraction results (US-Myb in 2013). SOS and EOS could not be extracted with the GCC_DLF method for the US-MMS sites in 2014, so the comparisons were missed. Meanwhile, SOS and EOS estimated from NDVI_DLF and GCC_DLF failed, resulting in unavailable comparisons at US-Myb in 2013. The temporal dynamics of five GPP products are plotted against the temporal dynamics of \(\mathrm{GPP_{EC}}\) in Figure 4. At US-Ho1 and US-Ha1, GPP2, GPP3, and GPP5 accurately tracked both the seasonal dynamics and inter-annual variation of \(\mathrm{GPP_{EC}}\), while GPP1 and GPP4 somewhat to moderately overestimated \(\mathrm{GPP_{EC}}\) during these years. At US-MMS, five GPP products overestimated the observed GPP from 2012; however, in 2013 and 2014, GPP2 and GPP3 traced the variability of \(\mathrm{GPP_{EC}}\) on seasonal and inter-annual scales well. At US-Ne2, five GPP products followed \(\mathrm{GPP_{EC}}\) in 2010 and 2011 well, whereas these modeled GPP values slightly lagged behind the \(\mathrm{GPP_{EC}}\) in 2012. The same situation occurred at US-Twt in 2013. In 2012 and 2014, GPP4 slightly overestimated \(\mathrm{GPP_{EC}}\). GPP1, GPP2, GPP3, and GPP5 overall fitted the observed GPP in 2012, and GPP2, GPP3, and GPP5 somewhat underestimated \(\mathrm{GPP_{EC}}\) in 2014. From 2012 to 2014, GPP1 and GPP4 were greatly larger than GPP2 at US-Myb. GPP2, GPP3, and GPP5 generally properly matched with \(\mathrm{GPP_{EC}}\) in 2013 and 2014, while the three GPP products were moderately smaller than the observed GPP in 2012. At US-Var, GPP1 and GPP4 significantly overestimated \(\mathrm{GPP_{EC}}\) from 2012 to 2014. In particular, in 2013, the five GPP products moderately lagged behind the observed GPP.