# Temporal Changes in Land Use, Vegetation, and Productivity in Southwest China
## 4 Discussion
### Migration Changes in NDVI and GPP Centroid  
Our study indicated that the migration direction of the NDVI center of gravity of cropland, forest, and grassland was the same as that of GPP during 2000-2019 in southwest China (Figure 5), and the spatial distribution pattern of vegetation cover and productivity gravity center changed with a change in LULC distribution. In southwest China, the center of gravity of cropland moved westward, and there were inflection points of the center of gravity in 2001 and 2005. In 2001, the project of returning cropland to forest, ecological protection in the east, and the natural forest protection plan in the west were fully launched. Therefore, the area of cropland increased, and the area of forest decreased during 2000-2005. After 2005, ecological engineering began to achieve results, which showed that the cropland area decreased and the forest area increased, which was consistent with the results of previous studies [76]. These results indicated that the LULC change caused by the ecological restoration project was the direct cause of the change in the vegetation growth and productivity center of gravity in southwest China over the past 20 years. The inflection point times of the change in the center of gravity were consistent with the planning and implementation time of the restoration project. The grassland continued to migrate to the northwest, which may be related to the conversion of grassland to other land types in the southeast, while the northwest region had a higher altitude, less human activity, and fewer changes in land cover types [36]. The study also found that the change in GPP's gravity centers of different vegetation NDVI grades was highly similar to the migration direction of NDVI's gravity center changes of different grades (Figure 11). This indicated that the increase or decrease in vegetation productivity in southwest China was related to the restoration or degradation of vegetation; the productivity increased in the region of vegetation restoration, and the productivity decreased in the region of vegetation degradation. From 2000 to 2019, the change in the center of gravity of the lowest coverage was relatively stable, and the direction and distance of the center of gravity were not large. The center of gravity of low coverage and high coverage mainly shifted to the southeast, and the center of gravity of medium coverage and high coverage mainly shifted to the northwest. This was mainly because of the spatial distribution, composition, and climatic characteristics of LULC types of different vegetation grades at different stages, which affected the change in vegetation cover and the center of gravity of productivity. Therefore, reasonable planning of regional cover-type composition is of great significance to effectively improve regional vegetation NDVI and GPP. The shift in vegetation types in the direction of the center of gravity indicates that the expansion of vegetation in the direction of migration or the degradation of vegetation in the opposite direction, and the long-term shift in the center of gravity in a single direction, lead to the imbalance of ecosystem structure and function. For example, the southern and southwestern regions of southwest China are the main regions of returning farmland to forest. Due to the favorable ecological conditions in this region, the vegetation coverage has shifted from medium coverage to high coverage. Recent studies have also confirmed that afforestation measures have achieved good results in improving vegetation cover and promoting carbon sequestration [58; 77]. However, it was also found that the excessive growth of large regions of forest consumes surface water, resulting in a shortage of regional water resources [78; 79]. In addition, local negative effects have also occurred due to the planting of unsuitable tree species [80].