## 4 Discussions  
### GPP Variations and Driving Mechanisms of GPP Variation  
Our findings revealed a significant greening trend in the study area from 2001 to 2020, with GPP increasing at an annual rate of 6.7 gC m\({}^{-2}\), consistent with previous studies [15; 38; 39; 40]. Previous studies have demonstrated that solar radiation, temperature, and precipitation significantly influence vegetation GPP variations, particularly in the context of climate change [34; 41; 42; 43]. Since the comprehensive implementation of the TNSP in 1978, ecological engineering has emerged as a pivotal factor in vegetation greening and GPP enhancement [16; 34; 44]. The increase in GPP in China from 2001 to 2020 is attributed to both climate factors and afforestation efforts, with distinct spatial impacts. Solar radiation, in particular, enhances photosynthesis in arid and semi-arid regions, thereby boosting GPP in areas such as the Inner Mongolian Plateau and Loess Plateau [15; 45]. It also promotes forest growth by providing the energy necessary for photosynthesis, particularly in the Northeast region [46]. Our study found that solar radiation is the primary driving factor for vegetation productivity in the high-mountain areas of the northeastern part of the study region, consistent with previous research [22]. Precipitation is crucial for vegetation growth in dryland regions such as the Mongolian Plateau, Loess Plateau, and the farming areas of the Northeast and North China Plain [34; 44; 47], although it may hinder vegetation productivity in northeast parts of the QTP [41; 48]. Higher temperatures can stress vegetation in arid areas by increasing evapotranspiration, thereby reducing GPP [34; 49]. However, in the cold such as the QTP, higher temperature can enhance vegetation growth by increasing water availability from ice and snow melt [11].  
A strong positive correlation between forest coverage and GPP fluctuations was observed, as indicated with a Pearson correlation coefficient of 0.9. This study emphasizes the significant impact of increased forest coverage in boosting vegetation productivity across Northern China. The effect is particularly pronounced in central regions of Northeast  
Figure 11: The relative contributions of various factors to GPP changes in the four major ecological engineering regions. Abbreviations—PAR: solar radiation; Pre: precipitation; Tem: temperature; Fcc: forest coverage changes; TNSP, Three-North Shelterbelt Program; APTM, Afforestation Program for Taihang Mountain; SPLR, Shelterbelt Program for Liaohe River; SPMRYR, Shelterbelt Program for Middle Reaches of Yellow River.  
China, the North China Plain, the Loess Plateau, the Inner Mongolia Plateau, and parts of Northwestern China [50], suggesting that GPP changes in these areas are primarily driven by variations in vegetation cover. Numerous ecological restoration initiatives, such as the conversion of cropland to forest, natural forest protection, and the extensive TNSP, are concentrated in these areas. These initiatives have substantially increased both vegetation cover and GPP [12]. These findings underscore the critical role of ecological engineering in enhancing vegetation cover [17; 18], consequently improving the overall primary productivity of the region.  
In this study, we evaluated the influence of climate factors and forest coverage changes on vegetation dynamics using ridge regression to determine their relative impacts. Our findings revealed that climate factors are the predominant contributors to GPP variations, aligning with previous research [15]. Climate factors accounted for about 61.04% of the influence on GPP, particularly in forested areas of the Northeast regions and the QTP. Conversely, changes in forest coverage contributed 38.96% to GPP growth, especially in the Loess Plateau, Inner Mongolia Plateau, and northern Xinjiang, highlighting the multifactorial influences on vegetation productivity in Northern China.