## 4 Discussion
### Relationship between Affecting Factors and NPP  
The carbon sequestration of ecosystems is intricately linked to vegetation, and the growth of vegetation is influenced by diverse factors, including anthropogenic actions and climate variability. Climate change contributes significantly to the variation in NPP, accounting for 40.01% of the relative contribution and thus serving as a crucial factor affecting vegetation productivity changes. In the period from 2003 to 2020, the YRB experienced a warming and humidifying trend. The effects of temperature, precipitation, and solar radiation on vegetation NPP differ noticeably. Temperature and precipitation are positively correlated with NPP, accounting for 76.20% and 58.44% of the regions, respectively, while solar radiation is negatively correlated, covering 67.07% of the regions. However, the overall areas showing significant correlations are relatively small, considering that this is due to not accounting for the lag effects of vegetation on climate [64].  
Solar radiation is a necessary condition for photosynthesis in vegetation, and solar radiation is directly proportional to the altitude. Due to its lower altitude, the Sichuan Basin consistently experiences lower solar radiation throughout the year compared to other regions [65]. In this region, solar radiation is significantly negatively correlated with NPP. Ge reached the same conclusion [66]. Additionally, when compared to other regions at the same latitude, the vegetation NPP in this area is noticeably lower.  
The precipitation in the YRB increases from west to east, with a positive correlation area of 58.44%. However, the significantly positively correlated area is only 4.68%. This is due to the uneven distribution of annual precipitation in the YRB, where frequent heavy rainfall events occur, but the area and intensity of these heavy rainfall events vary greatly. Previous studies have mentioned [67] that, from 2016 to 2020, the YRB experienced different numbers, intensities, and distribution areas of heavy rainfall events in June and July, with rainfall events numbering 7, 4, 8, 9, and 9, respectively.  
In general, the temperature in the YRB basin gradually decreases from east to west. The Jinsha River and Minto River basins, starting from the northwest of the Hengduan Mountains, exhibit more pronounced NPP stage changes. The NPP values vary from 200 to 400 gC/m\({}^{2}\) in the area from the Hengduan Mountains to the source of the YRB, while the NPP variation in the source of the YRB ranges from 0 to 200 gC/m\({}^{2}\). The average annual temperature at the source of the YRB is below 0 \({}^{\circ}\)C [68], and excessively low temperatures often suppress the vegetation's growth, leading to reduced vegetation productivity.  
The relative contribution of human activity change is 59.28%, serving as the primary driver for the increase in NPP productivity in the YRB. Starting in 2003, the implementation of the \"Grain for Green Program\" marked a historic transition from deforestation and land cultivation toward afforestation and reforestation. The YRB played a pivotal role in this transformation, serving as the primary battleground for reforestation efforts. Over the past two decades, the Chinese government has implemented the \"Grain for Green Program\", covering more than 80 million mu (about 5.33 million hectares) in the YRB, with a yearly average reduction of 3.45% in land desertification, primarily in the southwestern regions. Compared to the urbanization process, the \"Grain for Green Program\" has played a more significant role in promoting the increase in NPP in forested areas [69]. Although urbanization has led to a continuous expansion of urban areas, there has been a greening trend in some core urban areas' surroundings [70]. Similar conclusions have been drawn by researchers such as [PERSON] [71] and [PERSON] [72] for regions where the NPP has shown a significant increase, such as the YRB source area, Sichuan Basin, and the upper reaches of the Han River.