## 3 Results  
### Spatial-Temporal Variations of NPP in the Songnen Plain  
#### 3.1.1 Characteristics of Interannual Variability  
Using the MOD17 data of Songnen Plain from 2001 to 2020, the characteristics of annual mean NPP were statistically analyzed, the NPP range was divided into five levels equidistantly according to the distribution of NPP values, and the distribution of each interval was analyzed. It was demonstrated that the spatial distribution of NPP varied significantly in the study area, showing an increasing trend from southwest to northeast (Figure 3). The average value of NPP in the unit pixel is between 34.32 and 649.01 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\). The NPP low-value area (34.32-300 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)) accounts for 58.22% of the total area, mainly concentrated in the southwestern central part of the study area. The NPP high-value area (400-649 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)) occupies only 2.27% of the total area, centered in the northeastern part.  
Figure 3: Spatial distribution of NPP in the Songnen Plain.  
To investigate the temporal variation characteristics of NPP for various land cover types, this study conducted an overlay analysis of national vegetation coverage data and land use data. NPP was sorted by type of land cover, and the arithmetic mean NPP within each land cover category per year was calculated. Figure 4 shows the interannual variation trends of NPP on FOR, GRA, SWA, DRY, and PAD from 2001 to 2020. It can be seen that the annual mean values of NPP differ significantly among the five land cover types, with FOR having the largest value (364.02 gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\)) and the rest being DRY (312.66 gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\)), PAD (304.79 gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\)), and SWA (304.14 gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\)) sequentially. The lowest annual mean NPP is found in the GRA area, only 280.78 gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\). The temporal changing characteristics of NPP for various land cover types show a consistently increasing trend, with the peak occurring in 2014. Among them, the most significant increasing trend of NPP is observed in the FOR region with a growth rate of 8.47 gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\), and the slowest growth of NPP is seen in the DRY region with a growth rate of 5.47 gC\(\cdot\)m\({}^{-2}\)-a\({}^{-1}\).  
To examine the spatial variation of NPP, MOD17 data from 2001 to 2020 in the Songnen Plain were employed, the trends of NPP were analyzed using the Theil-Sen median, and the significance of these trends was tested using the Mann-Kendall trend test. \(\alpha\) is defined as the level of the significance test. \(\alpha\) = 0.1, 0.05, and 0.01 correspond to \(|Z|\geq\) 1.645, 1.96, and 2.576, indicating that the study series pass 90%, 95%, and 99% confidence tests of significance, respectively. \(|Z|\leq\) 1.645 suggests that the study series do not pass the significance test with a 90% confidence level. The spatial distribution trends of NPP from 2001 to 2020 (Figure 5) show that 98.46% of the study area has an increasing trend of NPP, and only 1.53% exhibits a decreasing trend, concentrated in the eastern and northwestern part of the study area. Notably, 83.96% of the region with an upward trend in NPP passes the significance test level of \(\alpha\) = 0.01, showing a highly significant upward trend.  
Figure 4: NPP interannual variation of different land cover types from 2001 to 2020.