## 2 Materials and Methods
### Data
#### 2.2.2 GPP and Meteorological Data  
The daily GPP and meteorological data (T\({}_{a}\), VPD, and PAR) at the four sites were acquired from the ChinaFLUX website ([[http://www.chinaflux.org/](http://www.chinaflux.org/)]([http://www.chinaflux.org/](http://www.chinaflux.org/))) [65,79]. These data represent the region of the footprint area of the flux sites (ca. 250 x 250 m). The daily GPP was calculated as the sum of the net ecosystem CO\({}_{2}\) exchange and ecosystem respiration. More information can be found at [PERSON] et al. [65] and [PERSON] et al. [78]. The daily GPP and meteorological measurements were averaged to an eight-day interval to match the temporal resolution of the MODIS data. The model comparisons of this study were conducted in the growing season only, to exclude possible spurious satellite observations during the nongrowing season [80,81]. The growing seasons for the four sites were defined by the phenology observation data at nearby meteorological stations (Figure 1). The meteorological data were obtained from the China Meteorological Data Service Center ([[http://data.cma.cn/](http://data.cma.cn/)]([http://data.cma.cn/](http://data.cma.cn/))). The years 2005, 2006, and 2009 were identified as drought years in the temperate steppe according to [PERSON] et al. [60]. [PERSON] et al. [60]  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & **Lat, Lon, Alt** & **Data Period** & **Vegation Type** & **Dominant Species** & **Land Use** & **Reference** \\  & (\(\
abla\)**N**, \({}^{\circ}\)**E, m)** & & & & & & \\ \hline \multirow{3}{*}{NM} & 43.55, 116.68, 1189 & 2004–2005 & \multirow{3}{*}{Temperature steppe} & _Lepuna chimeiss_, & & & \\  & & & & & _Stign grandis_, & & \\  & & & & & _Agorynor existatum_ & & \\ \cline{1-1} \cline{5-6} \multirow{3}{*}{XL} & 43.55, 116.67, 1250 & 2006–2011 & \multirow{3}{*}{Temperature steppe} & _Stign grandis_, & & & Overgrazed before & \\  & & & & & _Pototulla fruitosa_, & & \\ \cline{1-1} \cline{5-6} \multirow{3}{*}{GCT} & 37.67, 101.33, 3293 & 2003–2006 & \multirow{3}{*}{Alpine shrub} & _Kebresia humilis_, & & & \\  & & & & & _Festuca ocina_, & & \\ \cline{1-1} \cline{5-6} \multirow{3}{*}{SD} & 37.61, 101.33, 3160 & 2003–2006 & \multirow{3}{*}{Alpine swamp} & _Kebresia tibtica_, & & & \\  & & & & & _Poticularis longiflora_ & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: General information on the four grassland eddy flux tower sites in northern China.  
Figure 1: Geographic distribution and characteristics of the four grassland eddy flux tower sites in northern China.  
identified these drought years based on the characteristics of precipitation events (e.g., size, frequency, interval, and seasonal distribution of precipitation events). The drought years can also be identified from MODIS evapotranspiration (ET) observations (MOD16A2, 500 m, version-6) [82]. For example, the growing season ET in 2005, 2006, and 2009 was clearly lower than that in the other years for the two temperate steppe sites (NM and XL) (Figure 2). The growing season lengths were similar for the drought (176 \(\pm\) 23.6 days) and non-drought (171 \(\pm\) 19.3 days) years in the temperate sites, suggesting that the growing season length had little influence on the annual GPP.