## 3 Results and Discussion
### Responses of Vegetation Dynamics to Climatic Drivers
#### 3.3.4. Abrupt Change in NDVI and Its Relationship to Climatic Drivers  
As discussed in Section 3.2.1, the increasing trend of national mean NDVI has decreased since 1994. To further investigate how NDVI trends have changed in different regions and understand the reasons behind this, we detected the most likely abrupt change points in NDVI and climate divers. By comparing the abrupt change points in each time series, it can be inferred that the abrupt change of NDVI could be a result of climatic jumps if the change points were close to each other. Figure 10 shows the abrupt change points of the growing season NDVI, air temperature and precipitation, as well as the difference of trends (DT) before and after the change points. The trends of the growing season NDVI changes in 1992-1994 in southern forests and changes in around 2000 in grassland of northeast Inner Mongolia, Loess Plateau and west Tibetan (Figure 10a). In 14.61% of research areas (Table 4), mainly located in forested southern and central China and Loess Plateau, NDVI has increased faster since the change points (Figure 10b). Accelerated NDVI trends in southern China can be attributed to higher temperature trends. Though the abrupt change points for NDVI and temperature in southern China were not exactly overlapping, they were close to each other and both accelerated in 1992-1997, which still helps explain the increasing trend of NDVI in southern China. 16.64% of areas have undergone slowed down NDVI trends, especially in northern grassland, which could be partly contributed by the slowed down trends of temperature in these areas since mid 1990s (Figure 10c,d). The study by [PERSON] _et al._[20] also indicated the decreasing trends of NDVI in the Mongolian Plateau since mid or late 1990s. In some arid and semi-arid areas such as southern Tibetan and northern Xinjiang, speeded up temperature increment could limit the water availability for vegetation hence led to stalled NDVI trends [55]. In northeast Inner Mongolia, both NDVI and precipitation (Figure 10e,f) changed in late 1990s. It can be inferred that the abruptly decreased precipitation has led to the stalled or reversed NDVI trends in this area. Seasonally, spring NDVI showed more abrupt change points compare to the other two seasons (see supplementary material, Figures S1a, S2a and S3a), indicating that the changes of NDVI behaviors mainly happened in spring. Both NDVI and temperature changed in mid 1990s in most areas (Figure S1a,c), despite little change in precipitation occurred (Figure S1e). Spring NDVI in forested southern and central China (24.65% of the research areas) have speeded up together with temperature while slowed down in parts of southwest and southeast China and Qinghai province (15.42% of the areas) as the increasing trend of temperature decreased (Figure S1b,d). The summer NDVI changed in the late 1990s (Figure S2a), mainly corresponding to precipitation (Figure S2e). Summer precipitation dropped in northeast Inner Mongolia, northern Xinjiang and south Tibetan (Figure S2f), where summer NDVItends slowed down as well (11.49% of the areas) (FigureS2b). The higher temperature and dropped precipitation could lead to the abrupt decrease of summer NDVI in these areas. In autumn, temperature increased faster in southern and central China since mid-1990s (Figure S3c,d) and promoted vegetation growth (Figure S3b). While higher temperature and dropped precipitation (Figure S3e,f) could have limited the vegetation growth in northeast Inner Mongolia. Figure 10: Spatial distribution of abrupt change points in the growing season mean (**a**) NDVI; The difference of trends (DT) before and after abrupt change points in the growing season mean (**b**) NDVI; (**c**) air temperature; (**d**) air temperature; (**e**) precipitation and (**f**) precipitation. DT was calculated by trends after abrupt change points minus trends before abrupt change points.