## 2 Data and Methods
### MODIS Data  
We used the MODIS GPP (MOD17) and ET (MOD16) datasets obtained from the University of Oklahoma ([[http://www.eomf.ou.edu](http://www.eomf.ou.edu)]([http://www.eomf.ou.edu](http://www.eomf.ou.edu))). MODIS GPP was calculated based on the light use efficiency model using satellite-derived land cover and fractional photosynthetically-active radiation (FPAR) [26].  
MODIS ET was produced by the Penman-Monteith (PM) equation (Equation (1)) driven by daily meteorology (_i.e._, temperature, actual water vapor pressure, relative humidity and downward shortwave radiation) and remotely-sensed albedo, leaf area index (LAI) and FPAR [27].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sites**} & \multirow{2}{*}{**Location and Altitude**} & **Mean Annual** & **Mean Annual** & \multirow{2}{*}{**Soil Type**} & \multirow{2}{*}{**Predominant Species**} \\  & & **Temperature** & **Precipitation** & & & \\  & **(\({}^{\circ}\)C)** & **(mm)** & & & \\ \hline \multirow{4}{*}{XSBN} & 21.90\({}^{\circ}\)N, & & & & & \\  & 101.27\({}^{\circ}\)E; & 21.8 & 1493 & \begin{tabular}{c} Lateritic and red \\ lateratic soil \\ \end{tabular} & _Pometia tomentosa,_ & _Terminalia myriocarpa_ \\  & 756 m & & & & & \\  & 44.13\({}^{\circ}\)N, & & & & \\  & 116.30\({}^{\circ}\)E; & 0.4 & 350.9 & Chernozem soil & _Leymus chinensis, Stipa grandis, Koeleria cristata,_ \\  & 1189 m & & & & _Agropyron cristatum_ \\  & 37.67\({}^{\circ}\), & & & Alpine meadow soil, & _Potentilla fraticosa L., Stipa alena, Kobresia capillifolia,_ \\ \multirow{2}{*}{HBGC} & 101.33\({}^{\circ}\)E; & \multirow{2}{*}{\(-\)1.7} & \multirow{2}{*}{580} & \multirow{2}{*}{\begin{tabular}{c} Alpine meadow soil and \\ alpine scrubby \\ meadow soil and \\ swamp soil \\ \end{tabular} } & _Potentilla fraticosa L., Stipa alena, Kobresia capillifolia,_ & _Kobresia humilis_ \\  & 30\({}^{\circ}\)51’N, & & & & \\  & 91.07\({}^{\circ}\)E; & \multirow{2}{*}{\(-\)10.4} & \multirow{2}{*}{476.8} & \multirow{2}{*}{\
\begin{tabular}{c} Meadow soil with \\ sandy loam \\ \end{tabular} } & _Blysmus sinocompressus, K._ & _microglobin, [PERSON] Iittledalei K._ \\  & 4333 m & & & & \\  & 36.96\({}^{\circ}\)N, & & & \\  & 116.64\({}^{\circ}\)E; & 13.1 & 610 &
\begin{tabular}{c} Alluvial deposit of the \\ Yellow River \\ \end{tabular} & _Winter wheat and_ \\  & 28 m & & & Yellow River & _summer maize_ \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._\[ET=\frac{\mathrm{s}\times\mathrm{A}+\mathrm{p}\times\mathrm{C}_{\mathrm{p}}\times \mathrm{(e_{sat}-e)/r_{a}}}{\mathrm{s}+\gamma\times\mathrm{(1+r_{s}/r_{a})}} \tag{1}\]  
where \(s\) is the slope of the curve relating saturated water vapor pressure (\(e_{sat}\)) to temperature; \(A\) is available energy partitioned between sensible heat, latent heat and soil heat fluxes on the land surface; \(\rho\) is air density; \(C_{\mathrm{p}}\) is the specific heat capacity of air; \(\gamma\) is the psychrometric constant; \(r_{a}\) is the aerodynamic resistance; \(r_{s}\) is the surface resistance; \(e\) is the actual water vapor pressure; and the difference between \(e_{\mathrm{sat}}\) and \(e\) is the water vapor deficit (VPD).  
MODIS eight-day GPP and ET data in 3 \(\times\) 3 homogenous pixels (3 \(\times\) 3 km) around the center of eight ChinaFLUX flux towers were extracted from and MOD17 and MOD16 products from January 2003-December 2008.