## 3 Results
### Mechanism of the NPP Change  
#### 3.2.1 Impact of Climate Change on NPP  
Characteristics of Interannual Changes in Meteorological Factors  
By analyzing the interannual changes in NPP, precipitation, and average temperature in northwest Liaoning from 2001 to 2021, we established a pattern of long-term change (Figure 6). During the 21-year period, annual precipitation in northwest Liaoning fluctuated in the range of 407.4 to 687.3 mm, with the lowest value occurring in 2009 and the highest in 2010. The multi-year average precipitation was 520.7 mm, and there were large differences between years. Except for some years, the mean NPP and precipitation showed similar volatility, indicating that the amount of precipitation in the region directly affected vegetation growth, and to a certain extent influenced the mean NPP trend. The annual mean temperature generally demonstrated a pattern of rising, then falling, and rising again, within a range of 7.6-9.3 \({}^{\circ}\)C. The minimum value was recorded in 2012 and the maximum in 2007, with an average value over multiple years of 8.6 \({}^{\circ}\)C. In 2021, both temperature and precipitation had high values in northwest Liaoning, so the annual mean NPP was also at its highest level with 441.3 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\). Other years with higher NPP values also had a relatively good combination of hydrothermal conditions. In 2012, the annual average temperature was reduced. Nevertheless, employing the monthly average temperature, the average temperature was higher throughout the vegetation growing period, from April to September. In addition, the precipitation was ample and sufficient during the critical period of vegetation water demand, so the annual mean NPP value was also high. Figure 5: Trend and significance test of NPP changes in northwest Liaoning Province from 2001 to 2021. (**a**) Theil–Sen Median Analysis of NPP, (**b**) Mann–Kendall Significance Test of NPP. Meteorological factors usually affect the physiological structure and growth process of vegetation through differences in hydrothermal conditions, and thus play an important role in NPP changes. Image-by-image meta-partial correlation analysis and significance tests were carried out between the annual mean NPP and two meteorological factors (precipitation and air temperature) in northwest Liaoning (Figures 7 and 8). Partial correlation coefficients for annual average NPP and yearly precipitation varied from \(-0.7\) to \(0.9\); however, they mainly showed high and moderate positive correlations. Measured by area, the research area exhibiting a negative correlation constituted just \(1.1\%\) of the overall area, primarily located in the eastern part of Changtu County. An area that comprised \(54.1\%\) of the complete area possessed partial correlation coefficients in the range of \(0.6\) to \(0.8\) and was found mainly in Chaoyang City, Fuxin City, central and northern Huludao City, northwestern Jinzhou City, and western Kangping County. These areas are relatively arid, with a relatively low annual precipitation, and, thus, their vegetation growth is sensitive to precipitation. A rise in rainfall can significantly reduce water stress and enhance the accumulation of dry matter, which in turn affects NPP changes. The partial correlation coefficients between the annual average NPP and the yearly average temperature ranged from \(-0.8\) to \(0.8\). The geographical distribution of NPP demonstrated a low to moderate negative correlation in the eastern areas, and a mild to moderate positive correlation in the western and northern regions. Chaoyang City, Fuxin City, Huludao City, and the western part of Jinzhou City predominantly observed positive correlations in an area that comprised \(69.7\%\) of the entire area.