## 2 Materials and Methods
### Data
#### 2.2.2 Remote Sensing Data  
According to Equation (1), the EVI was generated using the eight-day MODIS land surface reflectance datasets (MOD09A1, Level 3, Collection 5) for 2001-2015, which were downloaded from the NASA Land Processes Distributed Active Archive Center ([[https://lpdaac.usgs.gov/](https://lpdaac.usgs.gov/)]([https://lpdaac.usgs.gov/](https://lpdaac.usgs.gov/)), accessed on 12 October 2021).  
\[EVI=G\frac{\rho_{NIR}-\rho_{red}}{\rho_{NIR}+C_{1}\times\rho_{red}-C_{2}\times \rho_{blue}+L} \tag{1}\]  
where \(\rho_{NIR}\), \(\rho_{red}\) and \(\rho_{blue}\) are the surface reflectance for the near-infrared, red, and blue bands, respectively. The coefficients \(G\), \(C_{1}\), \(C_{2}\) and \(L\) used in the EVI algorithm were 2.5, 6, 7.5, and 1, respectively [57].  
The original spatial resolution of the EVI was 500 m; it was resampled to 1 km for ease of computation. To further reduce the effects of cloud and to capture the seasonality of the  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Zone** & **Grassland Type** & **Site** & **Location** & **Elevation (m)** & **Operation Period** \\ \hline \multirow{8}{*}{TP 1} & \multirow{3}{*}{KO 3} & AR & 38.04\({}^{\circ}\)N, 100.46\({}^{\circ}\)E & 3033 & 2014 \\  & & & GL & 34.35\({}^{\circ}\)N, 100.56\({}^{\circ}\)E & 3980 & 2007, 2010–2011, 2013 \\  & & & HBKO & 37.61\({}^{\circ}\)N, 101.31\({}^{\circ}\)E & 3148 & 2003–2004 \\  & & & HBSH & 37.67\({}^{\circ}\)N, 101.33\({}^{\circ}\)E & 3293 & 2003–2012 \\  & & & & & \\  & & & & & \\  & & & & & & \\  & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & \\  & & & & & & \\  & & & & & \\  & & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline \multirow{8}{*}{IM 2} & \multirow{8}{*}{TS 8} & DL & 42.05\({}^{\circ}\)N, 116.28\({}^{\circ}\)E & 1324 & 2010–2011 \\  & & NMG & 43.53\({}^{\circ}\)N, 116.28\({}^{\circ}\)E & 1200 & 2004, 2007–2008, 2010–2011 \\  & & & XLHT & 44.13\({}^{\circ}\)N, 116.32\({}^{\circ}\)E & 1187 & 2010–2011 \\  & & YZ & 35.95\({}^{\circ}\)N, 104.13\({}^{\circ}\)E & 1968 & 2008–2009 \\ \hline \multirow{8}{*}{DS 9} & DS & 44.09\({}^{\circ}\)N, 113.57\({}^{\circ}\)E & 990 & 2008–2009 \\  & & SZWQ & 41.8\({}^{\circ}\)N, 111.9\({}^{\circ}\)E & 1438 & 2012 \\ \cline{1-1}  & & XLS & 35.77\({}^{\circ}\)N, 104.05\({}^{\circ}\)E & 2481 & 2008 \\ \hline \hline \end{tabular} \({}^{1}\) Tibetan Plateau, 2 Inner Mongolian Plateau. 3 Alpine _Kobresia_ Meadow, 4 Alpine Shrub Meadow, 5 Alpine Swamp Meadow, 6 Alpine Meadow Steppe, 7 Meadow Steppe, 8 Typical Steppe, 9 Desert Steppe.  
\end{table}
Table 1: Main characteristics of the 18 flux sites in northern China’s grasslands [34].  
EVI, a data smoothing tool providing a double logistic curve fit in the TIMESAT software was used to smooth the original time series [58].  
Digital elevation model (DEM) data was obtained from the Shuttle Radar Topographic Mission ([[https://srtm.csi.cgiar.org/srtmdata/](https://srtm.csi.cgiar.org/srtmdata/)]([https://srtm.csi.cgiar.org/srtmdata/](https://srtm.csi.cgiar.org/srtmdata/)), accessed on 10 November 2021) and the original data were resampled with a spatial resolution of 90 m to 1 km.