## 4 Discussion
### Impact of Climatic Factors on NPP  
Due to its special geographical location, the research region is vulnerable and sensitive to climate change, as it is affected by both sea-land thermal differences and seasonal variations in circulation [12; 59; 60]. Some studies have shown that the growing season in the research region has experienced a dramatic warming and drying trend in the past 20 years [60]. However, the area of the research region is very large (358,000 km\({}^{2}\), Figure 1), and the difference in elevation and climate are relatively significant within the region, e.g., the spatial distribution of temperature, annual precipitation, and sunshine hours are significantly different. This is reflected in the significant spatial heterogeneity of Pearson correlation coefficients of NPP with mean annual temperature, annual precipitation, and sunshine hours.  
Previous studies have shown that NPP increases with temperature in many regions [14; 18]. However, this finding does not completely hold true in the research region. The Pearson correlation analysis results for annual average temperature and NPP showed that more than 48.88% of the regions exhibited a negative correlation between NPP and annual average temperature, and about 51.12% of the regions showed a positive correlation between NPP and annual average temperature (Figure 9a). The results of these analyses  
Figure 12: Interaction of various driving factors on NPP. “**” represents two-factor enhancement, and “**” represents non-linear enhancement.  
show that the warming of nearly half of the research region does not directly contribute to the increase in NPP. This was likely due to the fact that the research is in the southern region of China, which belongs to the subtropical monsoon climate, so the main limiting factor of vegetation growth is not temperature [12]. However, increased temperature may increase soil evapotranspiration, cause soil moisture stress, reduce soil water content, and thus affect the vegetation growth process [61; 62; 63].  
Annual precipitation was the most dominant positive factor for NPP changes (Figure 9b), with more than 78% of the area positively correlated with annual precipitation (Figure 8b), which indicated that an increase in annual precipitation, in most areas of the study region, would contribute to an increase in NPP. Our results are consistent with previous studies [12; 60]. In addition, the areas negatively correlated with annual precipitation (about 21.12%) are mainly located in the higher elevation mountains and hills in the southern part of the research, which may be due to the higher vegetation cover in this region, and the vegetation has the functions of water conservation, precipitation retention, runoff regulation, and evapotranspiration reduction [64]. Thus, the increase in precipitation does not result in the increase in NPP in this region.  
Most plants have their own biorhythmic clocks, which can sense changes in the external light environment. Suitable sunshine hours are the basis for effective photoenergy absorption, transfer, and conversion, which affect the growth of plants [65]. Photosynthesis of vegetation increased with the increase in sunshine hours. In contrast to annual precipitation, vegetation in the southern part of the study area was positively correlated with sunshine hours (Figure 9c), and increasing sunshine hours in this region could increase NPP. Moreover, NPP was negatively correlated with sunshine hours in the northern part of the study area, and this phenomenon was also observed in the Pearl River Delta region [6]. The main vegetated land type in the northern part of the study area is cultivated land (Figure 1), where the main crops are wheat and corn, and the sunshine hours in the area may have met their growth requirements [66].