# Temporal Changes in Land Use, Vegetation, and Productivity in Southwest China
## 4 Discussion  
### Spatiotemporal Variation in Land Types and NDVI and GPP  
The results revealed that the rate of returning farmland to forest increased significantly in the study period (2000-2019), and cropland was the main land source for forest expansion. The area changes of LULC types were as follows: the area of cropland increased first and then decreased, while the area of forest decreased first and then increased, and the grassland continued to decrease in southwest China (Figure 2). This trend was consistent with the trend of land-use change in China [19]. In the study region, cropland was mainly distributed in the east of Sichuan, the center of Guangxi, the west of Guizhou, and the east of Yunnan (Figure 1). These regions were suitable for agricultural activities due to their flat terrain. The increase in agricultural intensification and productivity has led to the expansion of cropland in these regions, the main source of which was forest and grassland. Due to the development of ecological engineering, the phenomenon of \"returning farmland to forest\" appeared in the margin of Sichuan Basin, northwest and southwest Yunnan, southeast Guizhou, and south Guangxi, which effectively controlled the expansion of cropland. This indicates that the ecological restoration project in this region was the main driver of LULC change in most regions of southwest China and has achieved some results at this stage [55]. Second, rapid urbanization was also one of the most common causes of farmland loss [56]. At the beginning of the 21 st century, the urban area of the Sichuan Basin increased by 66,000 h\({}^{2}\), accounting for 68.31% of the decreased area of cultivated land [57]. The grassland was mainly concentrated in the northwestern part of Sichuan, where the altitude is higher, combined with a cold and dry climate. There are rivers flowing through this region, and the precipitation is relatively abundant. Under the background of climate warming, this is conducive to the growth of vegetation, and with the LULC change, in some regions, grassland has shifted to forest. The study found that the NDVI and GPP of vegetation in the growing season had shown a significant upward trend as a whole, and the growth trends had been different for different LULC types in southwest China from 2000 to 2019 (Figures 3b and 4a). This is consistent with the conclusion that the main vegetation types in China show a dynamic greening trend, which is the result of the combined effects of climate change, LULC types of distribution, and human activities (such as ecological engineering and agricultural management), among other factors [58; 59; 60]. In general, the increase in the NDVI value in southwest China was mainly due to ecological restoration caused by ecological projects such as returning farmland to forest, natural forest protection plans, and closing mountains for afforestation [55; 61]. The average NDVI and GPP of vegetation in the growing season in southwest China were high in the southeast and low in the northwest (Figures 3a and 4b), which was mainly related to the spatial distribution of LULC types. In southwest Yunnan, southeast Guangxi, and Sichuan Basin, high mean NDVI and GPP levels in the growing season were mainly distributed in forests, while northwest Sichuan and Sichuan Basin were mainly distributed in grasslands and croplands, which were lower than those of forests on the whole. Compared with 2000, most regions became greener and more productive in 2019. The conversion of a large amount of cropland or grassland to forest and the ecological restoration of most regions (low coverage shift to high coverage) resulted in significant greening of vegetation. However, at the periphery of most cities, vegetation degraded, and productivity declined, indicating that urbanization has led to the loss of vegetation [62].