## 3 Results
### Correlation Analysis of NPP Influencing Factors
#### 3.2.3 Correlation Analysis of Topographic Factors  
Elevation and slope, as fundamental characteristics reflecting the terrain, closely influence the types of vegetation, development of root systems, and the distribution of plant communities. Topographic factors affect NPP by impacting vegetation growth and thus altering vegetation carbon cycle efficiency. Elevation differences can lead to diurnal temperature variations, which affect chlorophyll decomposition, light availability and photosynthetic activity and ultimately influence the carbon sequestration efficiency of vegetation. As shown in Figure 12a, the ASTER DEM data were used to produce an elevation distribution map of the Songnen Plain, from which it can be seen that the terrain slopes from the east and northeast toward the low plains, with gentle undulating hills in the east and wave-like undulations in the southeast. According to the elevation distribution characteristics of Songnen Plain, the elevation is divided into 10 levels with an interval of 50 m. Based on the land cover distribution in the Songnen Plain, after conducting statistical analysis, the mean NPP was calculated for each land cover type within each elevation range. As shown in Figure 12b, the impact of elevation on each land cover type is quite similar, with NPP showing a rising trend with increasing elevation. In the case of FOR, the  
Figure 11: (**a**) Soil moisture (SM) distribution contour. (**b**) The distribution of PCCs for NPP and SM. (**c**) Boxplot of PCCs between NPP and SM in different cases of land cover types.  
NPP tends to level off after the elevation reaches 270 m, which indicates that the effect of elevation weakens when it exceeds 270 m. At the elevation of 120-170 m, a slight decrease in NPP is observed across all land types. Within this range, the predominant landform is plains, which account for 52.25% of the entire Songnen Plain and exhibit the lowest NPP. The low NPP may be due to the gentle topography of plains surrounded by hills on three sides. Specifically, natural precipitation may be insufficient; water from high places will pool, leading to poor drainage conditions.  
The spatial differences in slope will accelerate soil water loss and create water stress on vegetation. To cope with the water deficit, plants will close their stomata to avoid excessive water consumption by transpiration, but this will lead to CO\({}_{2}\) uptake reduction and NPP decline. The slope distribution map of the Songnen Plain was produced based on ASTER DEM data and is shown in Figure 13a. The slope of the Songnen Plain is classified into six levels (0-0.5\({}^{\circ}\), 0.5-2\({}^{\circ}\), 2-5\({}^{\circ}\), 5-15\({}^{\circ}\), 15-35\({}^{\circ}\), 35-55\({}^{\circ}\)) according to the classification criteria of landform slope set by the International Geographical Union. The average NPP of different land cover types was calculated at different slopes. As shown in Figure 13b, the effect of slope on NPP shows a similar pattern across different categories. When the slope is below 15\({}^{\circ}\), most land types exhibit small changes in NPP, and their NPP gradually stabilizes with the increasing slope; however, FOR has a large NPP increase between slope of 5 and 15\({}^{\circ}\). When the slope is above 15\({}^{\circ}\), there is a significant improvement in the NPP of GRA; the NPP of DRY only increases significantly at 35-55\({}^{\circ}\). The reason for the significant NPP increases is as follows. In an area with a higher slope, the runoff is faster, and salt accumulation is reduced. This provides a favorable condition for vegetation growth, exerting a positive effect on NPP.  
Figure 12: (**a**) Spatial Distribution of elevation in the Songnen Plain. (**b**) The mean NPP for each land cover type within each elevation level.