## 3 Results and Discussion
### Influence of Climate on NPP  
The correlation between NPP and the climate was the main focus of previous research [46; 47]. This study focused on analyzing the influence of climate factors on NPP (Figure 6a-d). To exclude the interference of other variables on NPP, this study adopted the second-order partial correlation analysis method to analyze the correlation between NPP and precipitation and LST (Figure 6a,c). Firstly, the ET and LST were used as control variables to analyze the partial correlation between NPP and the target variable (precipitation). Secondly, precipitation and EP were used as control variables to analyze the partial correlation between NPP and target variable (LST). Table 2 shows the statistical results of the second-order partial correlation between NPP, precipitation, and LST for the 17 provinces. A significance test was used to determine the correlation (Figure 6b,d). The partial correlation coefficient between NPP and precipitation was between \(-0.96\) and \(0.95\), and the positive and negative correlation areas accounted for \(39.24\%\) and \(60.76\%\) of the study area, respectively (Table 2). This indicates that there was a negative correlation between NPP and precipitation in the study area. The regions with a positive correlation between NPP and precipitation were concentrated in eastern and western Inner Mongolia, western Jilin, and Liaoning, as well as most of Hebei and central Qinghai, which have experienced severe water shortage and are distant from the ocean. The abovementioned areas are severely affected by drought due to the lack of precipitation and high rate of evaporation. Relevant studies show that the trend of warming and drying in the inland areas of northwest China, especially the Loess Plateau, is strengthening [48]. An upward trend of temperature promoted an increase in soil moisture evaporation, further aggravating water shortage in these regions. Therefore, water resources are the main limiting factor for vegetation growth. This is why NPP and precipitation have a positive correlation in arid and semiarid regions. This positive correlation between NPP and precipitation in the arid and rainless steppe region of Inner Mongolia is consistent with the conclusion obtained by [PERSON] et al. [49] through a correlation analysis between NPP and meteorological factors that were based on data measured over the last 30 years in the Hulinbei  
Figure 6: Spatial distribution and the T significance test of partial correlations coefficient between annual NPP and precipitation (**a**,**b**) and NPP and Land surface temperature (**c**,**d**) between 2000 and 2015.  
forest and grassland transition zone. The negative correlation was concentrated on the border between Qinghai and Tibet, the northeastern part of Inner Mongolia, and the border between Inner Mongolia and Liaoning. Water resources in the above areas, particularly the southeastern edge of Tibetan Plateau, are relatively abundant. Precipitation decreases at higher altitude. This, in turn, reduces regional temperatures and limits the NPP. The partial correlation coefficient of NPP and precipitation passed the significance test (\(p<0.01\)) over an area of \(11.12\times 10^{4}\) km\({}^{2}\), accounting for 3.67% of the study area. The area with a positive correlation for 1.39% of the total and was mainly distributed in Jilin and Liaoning. Regions with a negative correlation accounted for 2.28% of the total area, mainly distributed at the Qinghai and Tibet border. This is consistent with the conclusion that the NPP was negatively correlated with temperature, precipitation, and solar radiation in relevant studies [50]. The partial correlation coefficient between NPP and LST was between \(-0.94\) and \(0.96\), and the positive and negative correlation regions accounted for 52.64% and 47.36% of the total study area, respectively (Table 2).