## 3 Results and Discussion
### Responses of Vegetation Dynamics to Climatic Drivers
#### 3.3.5. Temporal Variation in NDVI in Typical Regions  
According to the spatial distribution of the growing season NDVI trends (Figure 6a), NDVI in some regions sharply increased or decreased, such as those in northeast Inner Mongolia, Loess Plateau and central China. Since trends cannot completely reflect the variation process of NDVI time series, we took these areas as typical regions (Figure 6a) to analyze the relationships between the variation process of NDVI and climatic drivers as well as the influence of other external factors that can help explain the reason behind long term NDVI trends. (1) Northeast Inner Mongolia  
Northeast Inner Mongolia (\(42.5^{\circ}\)N\(-52.5^{\circ}\)N, \(112.5^{\circ}\)E\(-122.5^{\circ}\)E), is located in a semi-arid temperate climate zone and mainly covered by grass. The mean growing season NDVI in this area decreased significantly at rate of \(-0.98\times 10^{-3}/\)year on average from 1982 to 2011(Figure 11a). However, previous study showed that NDVI in this area slightly increased from 1982 to 1999 [36]. According to the Pettitt test, 1999 is an abrupt change point. The trend reversed from \(1.51\times 10^{-3}/\)year to \(-1.16\times 10^{-3}/\)year and NDVI mean values decreased from 0.478 to 0.453 before and after 1999. As for climatic drivers, the temperature time series was stable (Figure 11b) while the precipitation series also significantly changed at 1999 (Figure 11c), before and after which the mean value dropped from 321.5 mm to 250.4 mm. The research by [PERSON] _et al._[60] also revealed that the mean values of precipitation from 1999 to 2011 in northeastern China sharply decreased compared to that from 1982 to 1998. Seasonally, according to the detected abrupt change points (Figures S1f, S2f and S3f), it was mainly in summer and autumn when precipitation dropped. Though it is difficult to separate the effects of precipitation and grazing [26], the correlation and corresponding abrupt change points in NDVI and precipitation partly provide explanations for the decreasing NDVI in this area. Less precipitation limited the growth of grass \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Change Direction** & **Growing Season** & **Spring** & **Summer** & **Autumn** \\ \hline Slowed down & 16.64 & 15.42 & 11.49 & 8.05 \\ Speeded up & 14.61 & 24.65 & 5.96 & 14.52 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Proportion of areas with slowed down or speeded up NDVI trends since abrupt change points (%). or even partly destroyed the grassland ecosystem with low ecological resilience, and meanwhile rising temperature could have aggravated water limitation. This suggests that the decrease of growing season NDVI in northeast Inner Mongolia occurred after 1999, as a result of decreased precipitation, consistent with the result in Section 3.2.2 that the vegetation growth in this area was dominated by precipitation (Figure 9a). (2) Loess Plateau  
Loess Plateau (\(35^{\circ}\)N\(-42.5^{\circ}\)N, \(107.5^{\circ}\)E\(-115^{\circ}\)E), where the majority vegetation types are grasses and shrubs, belongs to semi-arid warm temperate zone. Growing season NDVI in this area increased at an extremely high rate of \(2.35\times 10^{-3}\)/year on average over the past 30 years. The NDVI series significantly changed at 2001 (Figure 12a), before and after which the increasing trends were \(1.59\times 10^{-3}\)/year and \(7.78\times 10^{-3}\)/year respectively. In spite of the stable precipitation series, the growing season temperature in Loess Plateau also abruptly changed in 1998, with the mean value increasing by 1.3 \({}^{\circ}\)C at 1998, which promoted vegetation growth (Figure 12c,b). However, the increasing trend of NDVI in Loess Plateau  
Figure 11: (**a**) Mean NDVI; (**b**) mean temperature and (**c**) accumulate precipitation in the growing season in northeast Inner Mongolia from 1982 to 2010.