## 4 Results and Analysis
### Characteristics of Spatiotemporal Variation in NPP
#### 4.3.1 Trends in Climate Change and their Impact on Vegetation NPP  
Over the studied period, there was an overall upward trend in the average annual temperature in the research region (Figure 13), with a rate of increase of 0.004 \({}^{\circ}\)C. However, a downward trend was evident from 2007 onwards, dropping to a minimum of 2.04 \({}^{\circ}\)C in 2012 before turning into a significant upward trend. The annual average solar radiation as a whole shows a decreasing trend with a rate of \(-\)1.7791 MJ/m\({}^{2}\), with significant decreasing trends in 2009-2010, 2013-2016, and 2019-2020, especially from 2019 to 2020, with a decrease of 17.40 MJ/m\({}^{2}\). The average annual precipitation fluctuated relatively smoothly, with the lowest average annual precipitation in 2011 (63.82 mm) and the highest in 2010 (75.20 mm). The impacts of temperature, precipitation, and radiation on vegetation NPP were analyzed separately, eliminating the impacts of additional elements. The main positive correlation was among vegetation NPP and temperature, precipitation, and radiation in the research region (Figure 14d). The areas where NPP showed a positive correlation with the temperature accounted for 62.19% of the total, primarily in East China and South China, the southern part of Central China, the eastern and southern parts of North and Northeast China, the northern and eastern parts of Northwest China, and the western and eastern parts of Southwest China (Figure 14a). The areas where NPP was positively correlated with precipitation were as high as 79.65%, mainly in Northeast and North China, the northern part of East China, and the northern and eastern parts of Northwest China (Figure 14b). The precipitation in certain regions was negatively correlated with the NPP process, mainly in southern China, central China, and southern East China. The areas where NPP was positively correlated with solar radiation accounted for 69.86% and were mainly located in North China and Central China, the eastern part of Southwest China, most of Northwest China, North China, and East China (Figure 14c). Figure 12: Significance graphs. (**a**) Mann-Kendall trend test results; (**b**) Theil-Sen median trend analysis and Mann-Kendall trend test superimposed analysis results. The part of the research region in which NPP was positively correlated with precipitation, temperature, and radiation for only twenty years was larger than the area occupied by a negative correlation (Figure 15). The percentage of area with a significant positive correlation between temperature and NPP was 10.15%, and the percentage of area with a significant negative correlation was 3.53%. The precipitation and NPP had a significant positive correlation between the areas with a percentage of 31.82%, and only 1.34% of the area had a significant negative correlation. The percentage of areas with a significant positive correlation between the radiation and NPP was 13.79%, and the percentage of areas with a significant negative correlation was 1.82%. Figure 13: Trends in temperature (**a**), precipitation (**b**), and radiation (**c**) over the studied period of 2001–2020. The combined investigation of the impacts of the temperature and precipitation factors on the vegetation NPP demonstrated that the multiple correlation coefficients of temperature, precipitation, and vegetation NPP ranged from 0 to 1 (Figure 16a). The  
Figure 14: Plots of the bias correlation coefficients with NPP for (**a**) temperature, (**b**) precipitation, and (**c**) solar radiation; (**d**) Histograms of the bias correlation coefficients for all three factors. Figure 15: Significance test plots: (**a**) temperature, (**b**) precipitation, (**c**) radiation and NPP bias correlation coefficient test; (**d**) significance histograms. correlation between temperature, precipitation, and vegetation NPP was predominantly non-significant by the F-significance test (Figure 16b).