## 5 Results
### Carbon sequestration  
The model is used to calculate GPP, NPP and NEE at a spatial resolution of 1 km\({}^{2}\) and a temporal resolution of ten days. The basis for this modelling is a time series of the described SPOT-Vegetation data set. From 10-day values we calculated growing season products of GPP, NPP, \(R_{e}\) and NEE for the year 2004. Figure 3 shows final maps of these products. We also disaggregated our estimates on the basis of land cover classes (Table 3). Class totals of GPP, NPP, \(R_{e}\) and NEE are given, as well as their means per category and standard deviations. The maps of the growing season GPP and NPP reveal clear spatial patterns in their distribution over the study area (Figure 3a,b). The growing season GPP and NPP was significantly higher in the areas covered by the steppe grassland. This difference was primarily a reflection of differences in the \begin{table}
\begin{tabular}{|l|l|l|l|l|l|} \hline
**Cover type** & **Area (km\({}^{2}\))** & \begin{tabular}{l} **Modelled** \\ **variables** \\ \end{tabular} & \
\begin{tabular}{l} **Mean (g** \\ **C/m\({}^{2}\)/year**) \\ \end{tabular} &
\begin{tabular}{l} **Standard** \\ **deviation** \\ \end{tabular} & **Total (g C\(\times\)10\({}^{6}\))** \\ \hline Short & 20,028 & GPP & 211.03 & 51.42 & 4,226,509 \\ grassland & & NPP & 131.30 & 44.29 & 2,629,877 \\  & & \(R_{e}\) & 187.05 & 35.12 & 3,446,237 \\  & & NEE & 13.17 & 32.03 & 263,768 \\ \hline Steppe & 6,955 & GPP & 243.70 & 59.50 & 1,694,934 \\ grassland & & NPP & 145.61 & 54.22 & 1,012,648 \\  & & \(R_{e}\) & 243.79 & 44.47 & 1,695,351 \\  & & NEE & -0.09 & 39.56 & -625 \\ \hline Total & 26,983 & GPP & 227.35 & 55.46 & 5,921,442 \\  & & NPP & 138.9 & 49.25 & 3,642,525 \\  & & \(R_{e}\) & 215.4 & 44.9 & 5,658,300 \\  & & NEE & 6.54 & 35.80 & 263,142 \\ \hline \end{tabular}  
*Positive NEE indicates that site was a sink for carbon, and negative NEP indicates the sites were a source for carbon to the atmosphere.  
\end{table} 
Table 3: Modelled GPP, NPP and NEE* by cover type. productive potential of the vegetation in the two land cover types. As the steppe grassland occupies territories with significantly higher precipitation amount, the conditions for vegetation growth are much better. It is well known that, because precipitation is the major limiting factor for vegetation growth in drylands, it controls strongly the spatial patterns in vegetation [35,58,59]. Within the short grassland, there is a distinct gradient in the distribution of GPP and NPP from South to North. The southern areas show low biomass production with values of 12-150 g C/m\({}^{2}\) for GPP and 100-120 g C/m\({}^{2}\) for NPP, while the northern areas of the short grassland demonstrate much higher values of GPP and NPP, 200-250 g C/m\({}^{2}\) and 130-160 g C/m\({}^{2}\), respectively. This pattern was driven primarily by the South-North precipitation gradient clearly presented in the flatland areas, which are occupied mainly by short grassland. The mean GPP was 243.7 g C/m\({}^{2}\)/year for the steppe grassland and 211.03 g C/m\({}^{2}\)/year for the short grassland (Table 2). The mean NPP was 145.6 g C/m\({}^{2}\)/year for the steppe grassland compared to 131.31 for the short grassland. The modelled NEE ranges throughout the study area from highly negative to positive values (about -100 to more than 100 g C/m\({}^{2}\)/year) (Figure 3d). Spatial pattern in the NEE is different from those of GPP and NPP. The spatial distribution of the net ecosystem production, NEP, was strongly imposed by the patterns in the ecosystem respiration, \(R_{e}\) (Figure 3c). There is a strong gradient in the distribution of NEE from SE to NW. Most of the areas with positive values of NEE are located in the western and southern parts of the study area, while the north-eastern part is dominated by low or negative values of NEE. This gradient is more consistent with the temperature gradient than with the precipitation gradient.