# Exploring the Spatiotemporal Alterations in China's GPP Based on the DTEC Model
## 4 Discussion  
### The Influence of Climate Change on China's Dynamic Changes in GPP  
The relationships between GPP and climate factors in China show significant spatial heterogeneity. At higher elevations and latitudes (e.g., the TP) (Figure 6d), temperature was positively correlated with GPP. Because the temperature in these areas is low, it affects the activity of vegetation photosynthesis-related enzymes, making temperature the main factor limiting the photosynthetic rate of vegetation [31]. In contrast, solar radiation had an inhibitory effect on vegetation in the TP region [32] (Figure 6b). This result may be due to sufficient solar radiation causing snow and permafrost to melt, increasing the water content of vegetation roots and keeping the soil in an anaerobic state, thus reducing vegetation productivity [33]. In the southern part of the humid region (SWC, CC, and SEC) (Figure 6c), an increase in precipitation reduced temperature and radiation, which led to precipitation suppression of vegetation growth, making GPP significantly negatively correlated with precipitation [34]. In arid and semiarid regions (IM and NWC), GPP was more sensitive to precipitation and had a significant positive correlation with precipitation; precipitation was the main factor affecting GPP in northern China [35]. This was because with the increase in global warming, the water demand of vegetation in arid regions has increased significantly compared to that in non-arid regions [36]. [PERSON] [37] also noted that vegetation water constraints are associated with greening trends, which leads to vegetation becoming increasingly sensitive to precipitation.  
In recent years, it has been concluded that aerosol optical depth (AOD) can enhance scattered radiation by weakening direct radiation and total radiation, thereby promoting or inhibiting vegetation photosynthesis [38]. This has attracted increasing attention from scholars to the impact of AOD on GPP [39]. At the regional scale, GPP was significantly positively correlated with AOD in most of the regions. These regions were significantly negatively correlated with solar radiation and vice versa (Figure 6a,e). This phenomenon indicates that when vegetation is undergoing photosynthesis, the high canopy of vegetation can absorb both direct and scattered radiation and easily reach light saturation, after which photosynthesis is inhibited with increasing solar radiation [40; 41]. In contrast, as AOD increases, direct radiation is attenuated, and the proportion of scattered radiation increases, resulting in an increase in radiation absorbed by shady lower canopy leaves that have not yet reached light saturation, thereby improving productivity [42]. In the northwestern region of NWC, AOD had a significant positive correlation with GPP, which may be related  
Figure 9: Relative contributions of climate change and human activities to GPP in nine eco-regions of China, 2001–2020.  
to the vegetation types of grassland, ENF and desert in this region [43]. In the SWC and SC regions, GPP had a large and significant negative correlation with AOD. This phenomenon was due to the reduction in total radiation, which decreased the productivity of more complex ecosystems, such as forests, at a level greater than the compensatory effect of scattered radiation, thus reducing overall productivity [43]. GPP was also significantly negatively correlated with AOD in IM and its surrounding areas. As the vegetation type in the zone was mostly grassland, where AOD leads to a reduction in direct radiation, the canopy structure of grassland is simpler, so it cannot sufficiently absorb scattered radiation [44].