## 3 Results
### Spatiotemporal Variations in the Different Land Cover Change Types  
To explore the spatiotemporal variations in the different land cover types, we counted transfer matrices from 2000 to 2020 (Table 3). The areas of the different unchanged and changed land cover types were calculated via spatial overlay analysis based on the land cover maps for 2000 (Figure 8a) and 2020 (Figure 8b). In 2000, the area of the cropland was \(209.73\times 10^{4}\) km\({}^{2}\), but it decreased to \(204.63\times 10^{4}\) km\({}^{2}\) in 2020, with a relative change of \(-2.43\%\). The dominant change types causing cropland shrinkage were the conversion of cropland to forest and grassland. These areas were mainly scattered in SW, NC, and NW. In addition, cropland was eliminated by artificial surface construction (\(15.61\times 10^{4}\) km\({}^{2}\)), and such areas were mainly distributed in EC, NC, and CC (Figure 8c). From 2000 to 2020, the net decrease in the grassland area was \(26.16\times 10^{4}\) km\({}^{2}\), and the relative change was up to \(-9\%\). The conversion of grassland to bare land (\(29.54\times 10^{4}\) km\({}^{2}\)) was mostly responsible for the decrease in grassland area, which mostly occurred in NW, SW, and NC. Figure 7: Provincial-level (the primary axis) and regional-level (the secondary axis) net changes in the total carbon sequestration from 2000 to 2018. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**2000** & **2020** & **Cropland** & **Forest** & **Grassland** & **Shrubland** & **Bare** & **Artificial** & **Others** & **Decrease** \\ \hline Cropland & 157.60 & 20.08 & 12.56 & 0.37 & 0.45 & 15.61 & 3.05 & 52.12 \\ Forest & 20.79 & 17.58 & 14.76 & 1.23 & 0.26 & 1.29 & 1.39 & 39.71 \\ Grassland & 15.71 & 17.30 & 12.17.06 & 2.68 & 29.54 & 1.57 & 6.63 & 73.43 \\ Shrubland & 0.45 & 1.15 & 1.90 & 14.41 & 0.57 & 0.03 & 0.20 & 4.31 \\ Bare land & 1.72 & 0.20 & 15.34 & 0.46 & 16.89 & 0.30 & 2.76 & 20.78 \\ Artificial surfaces & 5.72 & 0.31 & 0.35 & 0.01 & 0.03 & 7.24 & 0.18 & 6.61 \\ Others & 2.63 & 1.02 & 2.36 & 0.05 & 2.92 & 0.58 & 18.08 & 9.56 \\ Increase & 47.02 & 40.08 & 47.27 & 4.80 & 33.77 & 19.38 & 14.21 & 206.52 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Transfer matrix of the different land cover types during the study period (\(10^{4}\) km\({}^{2}\)). In contrast, the areas of the forest, shrubland, bare land, artificial surfaces, and other types increased from 2000 to 2020. The area of artificial surfaces significantly increased from 13.85 \(\times\) 10\({}^{4}\) km\({}^{2}\) in 2000 to 26.62 \(\times\) 10\({}^{4}\) km\({}^{2}\) in 2020, with the largest net increase rate of 92.3% among the five increased land cover types. This indicates that the significant urbanization process gradually has occupied the cropland in China during the last two decades (Figure 8a-c). The expansion of bare land was also primarily attributed to land cover conversion from grassland to bare land. Although the increases in the forest (0.37 \(\times\) 10\({}^{4}\) km\({}^{2}\)) and shrubland (0.49 \(\times\) 10\({}^{4}\) km\({}^{2}\)) offset part of the decrease in the grassland area, the land covered by natural vegetation still exhibited a decreasing trend during 2000-2020, which deserves the attention of local governments and policymakers. We reclassified the 49 types of land cover change into two unchanged and four changed types (Table 4, where the different colors represent different categories). The spatial patterns of these six main land cover categories are shown in Figure 8d. The results reveal that the land use pattern in China changed significantly from 2000 to 2020. The self-rehabilitation of forest-grassland vegetation reached 5.59 \(\times\) 10\({}^{6}\) km\({}^{2}\), accounting for 58.96%, making this the most extensive covering layer, which was scattered in NW, NC, and SW. The area of cropland maintenance was 1.58 \(\times\) 10\({}^{6}\) km\({}^{2}\), accounting for 16.61%, and it was mainly distributed in EC, NE, and CC.