## Data and Methods
### Vegetation Indices (VIs)  
We used the following four vegetation indices: NDVI, EVI, EVI2, and NIRv in this study. NDVI is perhaps the most widely used vegetation index. NDVI captures the difference between maximum reflection of radiation in the near-infrared band due to the leaf cellular structure and the maximum absorption of radiation in the red band due to the chlorophyll pigments. NDVI can be expressed as follows:  
\[\mathrm{NDVI}=\frac{\mathrm{N-R}}{\mathrm{N+R}} \tag{1}\]  
where N and R are the reflectance in the near-infrared (NIR) and red bands, respectively. Although NDVI has been widely used to approximate photosynthetic activity, this index has some limitations. NDVI is sensitive to soil background brightness and is influenced by soil texture and moisture conditions [13].  
To overcome the saturation effects of NDVI, weight factors were introduced to the NIR reflectance term in the NDVI formulation to adjust the relative contributions of the red and NIR reflectance. The resulting VI-EVI can be expressed as follows:  
\[\mathrm{EVI}=\mathrm{G}\frac{\mathrm{N-R}}{\mathrm{N+C_{1}R-C_{2}B+L}} \tag{2}\]  
where B is the reflectance in blue band; G is a gain factor; C1 and C2 are the coefficients of the aerosol resistance term that are used to correct for aerosol affects in the red band with the blue band; L functions as the soil adjustment factor. The coefficients adopted in the EVI algorithm are as follows: G = 2.5, C1  
Figure 1: Location and distribution of the 121 eddy covariance (EC) flux sites used in our study. The site description of these FLUXNET sites is provided in Table S1 in the Supplementary Material. The base map is the MODIS land cover map.  
= 6, C\({}_{2}\) = 7.5 and L = 1. EVI has been widely used as proxies of vegetation productivity or to estimate GPP [35; 36].  
The visible bands are highly related with each other, and the blue reflectance can be expressed as a function of the red reflectance [37]. A two-band EVI index, EVI2, was developed assuming the blue-band reflectance is a function of the red-band reflectance (B = c\({}^{*}\)R) and using the L, C\({}_{1}\) and C\({}_{2}\) values in the EVI formula [22]. With the optimal C\({}_{1}\), C\({}_{2}\) and G values, EVI2 can be expressed as follows:  
\[\text{EVI2}=2.5\frac{\text{N}-\text{R}}{\text{N}+2.4\text{R}+1} \tag{3}\]  
EVI2 not only gained its heritage from EVI but also improved the linearity with vegetation biophysical parameters [22].  
A more recently developed index, NIR\({}_{\text{V}}\), is the product of the BRDF-corrected reflectance in the near-infrared band and the BRDF-corrected NDVI. For the calculation of NIR\({}_{\text{V}}\), 0.08 is subtracted from NDVI\({}_{\text{BRDF}}\) to partially reduce the effects of the bare soil on NDVI. NIR\({}_{\text{V}}\), highlighting the BRDF-corrected NIR reflectance, is said to be less sensitive to background contamination and is well related to GPP [32]. NIR\({}_{\text{V}}\) can be written as:  
\[\text{NIR}_{\text{V}}=(\text{NDVI}_{\text{BRDF}}-0.08)\times\text{NIR}_{ \text{BRDF}} \tag{4}\]  
In this study, we used the following four VIs: two classical and most widely used indices - NDVI and EVI, a relatively new and improved index - EVI2, and a more recently developed index - NIR\({}_{\text{V}}\). We calculated NDVI, EVI, and EVI2 for each of the 121 sites using MODIS surface reflectance both with and without BRDF correction as described below, and the resulting VIs are NDVI, NDVI\({}_{\text{BRDF}}\), EVI, EVI\({}_{\text{BRDF}}\), EVI, and EVI\({}_{\text{BRDF}}\). For comparison purposes, NIR\({}_{\text{V}}\) was also calculated using surface reflectance both with and without BRDF correction, and the resulting two VIs - NIR\({}_{\text{V}}\) and NIR\({}_{\text{V}}\), BRDF were used in the following analyses.