## 3 Results
### Land Cover Change during the Study Periods  
In Table 4, the bare areas are dominant among each land cover type in the ARNWC, accounting for approximately 70% of the total area, which is distributed in the Alxa Plateau, Tuha Basin, Junggar Basin, and Tarim Basin. Grassland ranked second with an area of approximately 22%, mainly distributed in the Altai Mountains, the western part of the Tacheng Prefecture, the south and north slopes of the Tianshan Mountains, the Ili Region, the Kunlun Mountains, the Qilian Mountains, and the Helan Mountains. The area of cropland accounts for approximately 7% and is mainly distributed in the Irtysh River Basin, the northern part of the Tacheng Prefecture, the Ili Region, the economic belt on the northern slope of the Tianshan Mountains, the oasis belt around the Tarim Basin, and the Qilian Mountains. The area of forestland and urban areas is the smallest, both accounting for less than 1%. Among them, forestland is mainly distributed in mountainous areas, including the Altai Mountains, the Tianshan Mountains, and the Qilian Mountains.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Year** & **Cropland** & **Forestland** & **Grassland** & **Urban Areas** & **Bare Areas** \\ \hline
2001 & 137,764.5 & 17,148.25 & 451,596.5 & 730.25 & 1,448,575 \\ Percentage & 6.70\% & 0.83\% & 21.97\% & 0.04\% & 70.46\% \\
2018 & 152,862.25 & 19,332.25 & 455,680.75 & 2615.25 & 1,425,324 \\ Percentage & 7.44\% & 0.94\% & 22.17\% & 0.13\% & 69.33\% \\ change & 15,097.75 & 2184 & 4084.25 & 1885 & \(-\)23,251 \\ Percentage & 10.96\% & 12.74\% & 0.90\% & 258.13\% & \(-\)1.61\% \\ \hline \hline \end{tabular}
\end{table}
Table 4: Area of each land cover type in the ARNWC during 2001–2018 (unit: km\({}^{2}\)).  
Figure 2: Validation of NPP estimation results. In which ** indicates that the \(p\)-value is less than 0.01.  
The spatial distribution of land cover change in the ARNWC from 2001-2018 is illustrated in Figure 3. The most obvious characteristics are mainly shown by the expansion of cropland and forestland in the central part of the ARNWC and the interconversion of grassland and bare areas in the southern and central mountainous areas, combined with Table 4 and the results of the land use transfer matrix (Table 5). The total cropland area increased by 15,097.75 km\({}^{2}\), which showed outward expansion through the occupation of bare areas and grassland on the edge of the Tarim Basin and the economic belt on the northern slope of the Tianshan Mountains. The total area of forestland increased by 2184 km\({}^{2}\), showing the expansion of forestland located in the southern part of the Ili Region to external grassland and the conversion of grassland to forestland. The net increase in the total grassland area is 4084.25 km\({}^{2}\), which is mainly the result of the conversion between bare areas and grassland, which mainly occurred in mountainous areas, including the Helan Mountains, Qilian Mountains, Kunlun Mountains, and Tianshan Mountains. The urban areas increased the least, approximately 1885 km\({}^{2}\), but the tendency toward urban expansion was more significant because the urban area in 2018 was nearly four times that of 2001. The net decrease in total bare area of 23,251 km\({}^{2}\) was mainly the result of bare areas being converted into cropland and grassland. Thus, land cover change in the ARNWC from 2001 to 2018 mainly occurred as a result of the conversion between cropland, grassland, and bare areas, and the expansion of cropland and urban areas is obvious. Due to the influence of national policies such as the Grain for Green Program, the forestland area is slowly and steadily increasing.  
Figure 3: Land cover change in the ARNWC from 2001 to 2018. (**a**) Areas where no land cover changed; (**b**) areas where land cover change has occurred. The codes for the different land cover type conversions represented in the legend can be found in Table 6.