## 1 Introduction
Due to the differences in natural factors, along with human production and lifestyle, in different regions, NPP shows regional variability and exhibits a region-specific spatial distribution pattern over a certain time frame. NPP in China was found to have a changing and growing tendency over the previous 20 years, with values generally increasing in the southeastern regions and decreasing in the northwestern areas [26; 27]. Within the northeastern region, it seems that the NPP decreased from northeast to southwest [28; 29]. NPP is greatly influenced by climatic factors and exhibits different seasonal and interannual patterns of change. Researchers have recently examined the causes of NPP in several parts of China and proposed a correlation between variations in NPP and weather factors such as temperature and rainfall [30; 31], and indicated that the NPP influenced by the temperature and precipitation combination in different regions. However, except from the temperature and precipitation, changes in land use type, especially those induced by human activity, will also affect NPP changes. The detrimental effects of human activity on NPP mainly occur through transforming arable land into urbanized areas by grazing and economic activities [32], or changing highly productive land use categories into less productive ones [8]. A positive impact mainly occurs through the initiative to reforest and restore grasslands from previously cultivated land, prohibiting cutting and grazing, reducing soil erosion, and through other ecosystem conservation measures [33]. However, the specific changes in NPP caused by land use transformation are relatively less studied. In addition to the effects of climate and human activities, studies have shown that different soil types also have varying effects on plant growth conditions due to their fertility conditions, but the specific degree of influence is also relatively less studied. Northwest Liaoning sits on the southern border of the Horqin Sandy Land, which is China's key area of ecological sensitivity in the sandy northern regions of western Liaoning and a national key soil erosion control area. This area also is an important pilot study for ecological civilization in Liaoning Province, grassland sandy management, and ecological restoration. The plains of Northeast China, North China, and other areas all have their ecological security successfully safeguarded by this significant ecological barrier [34; 35]. In view of this, we firstly used the R programming language to conduct a Theil-Sen median trend analysis and a Mann-Kendall test on the MOD17A3 HGF NPP data from 2001 to 2021. Then, we explored the features of NPP's spatial and temporal variations in northwest Liaoning over this period. Finally, we discussed the effects of human actions and environmental factors on NPP in northwest Liaoning, using the correlation and linear regression analysis. This will help to strengthen our understanding of the long-term trends in NPP changes in the northwest region of Liaoning, further clarify the mechanisms of the impacts of NPP differences in different districts and counties, and focus on the key roles of local climate, soil, and land use changes in vegetation growth processes.