## 3 Results  
### Variability of T\({}_{min}\), VPD, GPPEC, and Input Model Variables  
The flux-tower based GPP varied among the study sites and land use classes. The CRO sites of US-Bi2 and US-Bi1 showed the highest average GPPEC: 12.69 \(\pm\) 7.04 g C m\({}^{-2}\) d\({}^{-1}\) at US-Bi2 and 6.11 \(\pm\) 4.51g C m\({}^{-2}\) d\({}^{-1}\) at US-Bi1. The GRA, DBF and WSA sites showed a relatively lower average GPPEC: 4.72 \(\pm\) 1.88g C m\({}^{-2}\) d\({}^{-1}\) at US-KFS, 3.53 \(\pm\) 4.80 g C m\({}^{-2}\) d\({}^{-1}\) at US-WCr, and 2.54 \(\pm\) 1.88 g C m\({}^{-2}\) d\({}^{-1}\) at US-Ton. Two types of shrubland (i.e., CSH and OSH) sites, RIs and Rws, had the lowest average GPPEC of 1.68 \(\pm\) 1.59 g C m\({}^{-2}\) d\({}^{-1}\) and 1.21 \(\pm\) 0.99 g C m\({}^{-2}\) d\({}^{-1}\), respectively. The temporal dynamics in GPPEC at the seven sites is shown in Figure 2. Apparently, the seasonal change patterns in the GPPEC of alfalfa and corn are different. The corn site (US-Bi2) is a single croppingsystem, whereas the alfalfa site (US-Bi1) is a multiple cropping system. At the US-Bi2 site, GPPEC rose rapidly and peaked in early August 2017 and late July 2018, respectively. At the US-Bi1 site, the harvest cycle of alfalfa is approximately a month. Alfalfa is harvested multiple times per growing season, depending on the level of rainfall and magnitude of production. Moreover, the maximum GPPEC of corn was found to be substantially larger than that of alfalfa. For example, in 2018, the highest GPPEC of corn was 23.75 \(\pm\) 4.02 g C m\({}^{-2}\) d\({}^{-1}\), whereas alfalfa was only 16.33 \(\pm\) 3.46 g C m\({}^{-2}\) d\({}^{-1}\). The OSH and CSH presented a similar temporal variation characteristic, reaching a peak in early May 2017 and the end of May 2018, respectively. The maximum GPPEC of the CSH site (US-RIs) was slightly higher than that of the OSH site (US-Rws) in 2017 and 2018. The DBF site (US-WCr) reached maximum GPPEC in late July 2017 and in late June 2018. The maximum GPPEC in 2017 (11.87 \(\pm\) 3.02 g C m\({}^{-2}\) d\({}^{-1}\)) was lower than in 2018 (18.77 \(\pm\) 4.78 g C m\({}^{-2}\) d\({}^{-1}\)). The GPPEC in the GRA site (US-KFS) exhibited a great variation, ranging from 0.02 \(\pm\) 0.01g C m\({}^{-2}\) d\({}^{-1}\) to 21.36 \(\pm\) 4.80 g C m\({}^{-2}\) d\({}^{-1}\). The WSA site (US-Ton) reached maximum GPPEC earlier than US-WCr and US-KFS, and the maximum GPPEC was 11.42 \(\pm\) 2.73 g C m\({}^{-2}\) d\({}^{-1}\) in 2017 and 9.13 \(\pm\) 2.19 g C m\({}^{-2}\) d\({}^{-1}\) in 2018.The temporal dynamics of T\({}_{\min}\) and VPD for all sites areshown in Figure 3. Overall, T\({}_{\min}\) and VPD presented a similar variation trend. For T\({}_{\min}\), great variation was found at the two shrulands, i.e., the GRA and DBF sites. Amongst them, maximum T\({}_{\min}\) can reach 20 degC while minimum T\({}_{\min}\) was even below \(-\)15 degC. At the two croplands sites, the overall growing season T\({}_{\min}\) was higher in 2017 than 2018. At the WSA site, the T\({}_{\min}\) was approximately 0 degC and the maximum T\({}_{\min}\) was higher in 2017 than 2018. In regard to VPD, the twoshrublands and GRA sites were generally higher than other sites. Two croplands sites had a similar range during growing season. The maximum VPD was the lowest at the WSA site compared with other sites. Figure 3: Temporal dynamics of the minimum temperature (T\({}_{\text{min}}\)) and vapor pressure deficit (VPD) for all seven sites in 2017 and 2018. (a–g) represent the sites the same as Figure 2.  
Figure 2: Temporal dynamics of the daily flux tower gross primary productivity (GPP\({}_{\text{EC}}\)) at seven eddy flux tower sites in 2017 and 2018. Colored shadings denote the uncertainties. (a): alfalfa site, (b): corn site, (c): closed shrubland site, (d): open shrubland site, (e): deciduous broadleaf forest site, (f): grass site, (g): woody savanna site.