## 3 Results
### Variability of NDVI, EVI, and SIF in Driving Factors
#### 3.2.2 Effect of Land Use Type on NDVI, EVI, and SIF  
Figures 8 and 9 display the spatial and temporal distribution of land use types from 2000 to 2019. Land use types in China primarily consist of grassland, cropland, forest land, and wasteland. The spatial distribution of land use types (vegetation) shows an increasing trend from the northwest to the southeast region, consistent with the spatial distribution of the NDVI, EVI, and SIF (Figure 3A-C). A large amount of wasteland and grassland exists in the northwest, and a large amount of forest land and arable land exists in the southeast. China's overall trend related to arable land has been decreasing in recent years. However, the arable land in the east-central area has been increasing, and these regions have implemented the policy of changing from \"nomadic\" to \"semi-agricultural and semi-pastoral\" areas. This is inconsistent with the fact that the total arable land in China  
Figure 7: Spatial distribution of mean air temperature, precipitation, and relative humidity in China from 2000 to 2019 and their correlation with NDVI, EVI, and SIF. **(A)**: the partial correlation coefficient between NDVI and temperature; **(B)**: the partial correlation coefficient between EVI and temperature; **(C)**: the partial correlation coefficient between SIF and temperature; **(D)**: the partial correlation coefficient between NDVI and precipitation; **(E)**: the partial correlation coefficient between EVI and precipitation; **(F)**: the partial correlation coefficient between SIF and precipitation; **(G)**: the partial correlation coefficient between NDVI and relative humidity; **(H)**: the partial correlation coefficient between EVI and relative humidity; **(I)**: the partial correlation coefficient between SIF and relative humidity. decreased by 50,529 Km2 from 2000 to 2019 (Figure 9A), and we believe that many other arable lands have been converted into lands with other vegetation types. Arable land is followed by grassland and woodland, with the three types accounting for 73.2% of the total regional area. Woodland-grassland is mainly distributed in the southeast. The land types closely associated with grassland conversion are cropland, woodland, and unused land. Nevertheless, grassland areas have decreased by 46,877 Km2 in the last 20 years (Figure 9D), followed by a decrease in wasteland (Figure 9G) and wetland (Figure 9I). Although the total water area of China's inland water bodies has increased significantly by 13,598 Km2 over the past 20 years (Figure 9E), the situation is not optimistic, as the increase in the total water area of natural lakes is dominated by the rapid expansion of brackish lakes on the Qiangtang Plateau, while the area of natural freshwater lakes is shrinking due to human activities. In the past 20 years, although China's forest land has mainly been increasing (Figure 9B), this is primarily due to the reforestation of cultivated land and grassland and afforestation activities. According to Table 2, China's three vegetation remote sensing products showed some correlations with land use types over time. The NDVI, EVI, and SIF showed negative correlations with cropland, shrubs, grassland, wasteland, and wetland; additionally, they showed positive correlations with woodland, water bodies, impervious surface, and snow/ice. Although wetland types had high negative correlations with the NDVI, EVI, and SIF, the small and more dispersed areas of wetlands relative to other land use types had a weaker effect on vegetation. There are some differences in the sensitivity of land use types to the NDVI, EVI, and SIF [19]. The sensitivity of the SIF is stronger than that of the NDVI and EVI when the land use type is green vegetation (woodland, shrubs, grassland, and wasteland) and more robust than the SIF when the land use type is another type (snow/ice, water bodies, and wetlands).