## 4 Discussion
### Diverse NPP Correlations with Climate Drivers and LOS  
Vegetation NPP is influenced by a variety of factors, and the primary one is climate-related. Temperature, precipitation, and solar radiation are the main climate drivers for vegetation growth [59], but the relationships vary with spatial scale. Nevertheless, limited efforts have been made to investigate the relative roles of different climate variables in the NPP [10]. Generally, temperature has been found to be positively correlated with the NPP at a national level, which is consistent with previous studies [10; 60]. This response may in part be related to the increased activity of photosynthetic enzymes [61] and improved capacity for photosynthesis and growth [62]. However, the relationship between temperature and NPP in Northern China was negative. This may have resulted from higher temperatures leading to increased water scarcity caused by accelerated evaporation, which would work against vegetation growth, especially for arid and semi-arid areas in Northern China [49]. Additionally, the positive relationship between precipitation and NPP in Northern China illustrated this conclusion. Precipitation showed a positive correlation with NPP at both the national scale and regional scale, which may have occurred as more precipitation increases soil moisture, satisfying the water requirements for vegetation growth and productivity increase, especially in late spring and summer, which usually have higher temperatures and more evapotranspiration [63].  
Little is known about the light-related physiological mechanisms that regulate NPP due to the difficulty in distinguishing the effects of day length (i.e., photoperiod) and light intensity [18], especially when studies are extended to consider their correlations with temperature and precipitation. Therefore, in this study, solar radiation was considered as an integrated surrogate for both day length and sunlight intensity [18] to investigate the effects of solar radiation on the NPP. A positive correlation was observed between the NPP and solar radiation at both the national and regional scales, except for a negative relationship which occurred in the Tibetan Plateau. Generally, increased solar radiation provides sufficient materials and energy for vegetation photosynthesis and solar radiation is typically accompanied by warmer temperatures and sufficient sunlight intensity, both of which can enhance photosynthetic capacity and promote vegetation NPP. The negative correlations between solar radiation and NPP in the Tibetan Plateau may be partly related to larger areas of melting seasonal snow and permafrost soils caused by abundant solar radiation, leading to higher soil moisture content and creating an anaerobic soil environment within the plant root zone, thus limiting vegetation growth [64]. Alternatively, increased solar radiation may enhance surface soil evaporation and limit water availability for herbaceous plants that have shallow root systems.  
Vegetation phenology has long been regarded as an important factor that affects vegetation productivity. Consistent with previous studies in References [65; 20], our results also indicated a positive relationship between the NPP and LOS at both national and regional levels. This suggests that an extension of LOS is one of the most important factors that affect plant productivity [66].