## 3 Results and Discussion
### Responses of Vegetation Dynamics to Climatic Drivers
#### 3.3.2 Relationships between NDVI and Multiple Climate Dirvers  
According to the normalized multiple regression (Equation (6)), temperature had a larger relative effect on NDVI than precipitation at a national scale during the four periods (Table 2). This quantitatively illustrates that on average, vegetation growth was more affected by temperature than precipitation in China. The largest relative effect of temperature was in spring with value of 0.726, suggesting that temperature rise has greatly motivated vegetation growth in spring. Precipitation exerted larger relative effect in the growing season than any of the other three seasons. Growing seasons represent the periods when vegetations are highly active, while the other three seasons are only part of or partly included in the growing season. Hence the high correlation between growing season precipitation and NDVI highlighted the importance of precipitation during high vegetation activity periods.  
Among temperature and precipitation, the one with larger and significant relative effect to NDVI was taken as the dominant climate driver for each pixel. Figure 9 illustrates the spatial patterns of dominant climatic drivers in the four periods. In the growing season (Figure 9a), vegetation growth  
Figure 9: Spatial distribution of dominant climatic drivers in (**a**) growing season; (**b**) spring; (**c**) summer and (**d**) autumn. Pixels in red are temperature-dominated and pixels in blue are precipitation-dominated. Darker color represents larger relative effect. Light gray pixels are masked out according to land cover and dark gray pixels show insignificant coefficients.  
was strongly dominated by temperature in most of the forest-covered (including EBF, ENF and DBF) southern China (26.5% of the research area) and by precipitation in grass-covered arid or semi-arid northern China (17.4% of research area). The study by [PERSON] and [PERSON] [26] also found that precipitation has exerted more effects on NDVI than temperature in semi-arid grassland in Canada. A large portion (51.3%) of the research area was temperature-dominated in spring (Figure 9b), especially in northeastern and central China where the majority of vegetation is deciduous broadleaf forest (Figure 1). The large relative contributions of temperature highlight the crucial effect of spring temperature on vegetation growth again. In summer (Figure 9c), 10.7% of the area, mainly the grassland in northern China such as northeast Inner Mongolia, was slightly dominated by precipitation and 12.3% of the area was dominated by temperature. However, these two climatic drivers did not have significant controlling effect on NDVI in most parts of southern China during summer. In autumn (Figure 9d), there was little precipitation-dominated area, while 24.0% of the area, including deciduous broadleaf forest in northeastern and central China, grassland in eastern and southern Tibetan, west of Loess Plateau and northern Xinjiang showed increasing NDVI corresponding to temperature rise.  
Both the national scale and pixel scale regressions highlighted the strong effects of the temperature in the spring and the precipitation in the whole growing season on the NDVI variation. With the multiple regression analysis, this study is among the first to reveal that the growing season NDVI in northern grassland was mostly dominated by precipitation while that in the southern forest was mostly dominated by temperature. Nevertheless, the variation of vegetation growth cannot be fully explained by the relationship with climatic drivers, other factors such as time lag [19], precipitation intensity, altitude [35], CO\({}_{2}\) concentration and nitrogen deposition [57] can also play vital roles in trends of vegetation growth, about which further study is needed.