# The Impacts of Climate and Human Activities on Grassland Productivity Variation in China
## 1 Introduction  
Grasslands, as an irreplaceable ecosystem, account for approximately 25% of the global terrestrial area and are universally located in arid, semi-arid, and semi-humid areas [1, 2]. Grasslands have invaluable ecological functions and economic value in terms of protecting biodiversity, conserving water resources, maintaining ecosystem balance, and promoting regional economies [3, 4, 5]. With global warming and unreasonable anthropogenic behaviors (overgrazing, over-reclamation, and overexploitation), grassland degradation is widespread and increasing globally. To date, more than 45% of grassland areas have shown different extents of degradation, and this degradation has seriously impacted the livelihood of the local population and threatened sustainable ecological development in the region [6, 7]. Therefore, monitoring grassland dynamics and quantifying driving factors are significant for grassland restoration and ensuring local residents' well-being. Attributing the long-term, large-scale, and high-resolution characteristics of remote sensing, terrestrial ecosystem research can be realized at regional, continental, and evenglobal scales. Gross primary productivity (GPP), as a physical measure of the rate of photosynthesis, reflects the amount of carbon dioxide converted into organic carbon via plant photosynthesis, and this indicator is widely used for grassland dynamic monitoring [8; 9]. Many researchers have chosen GPP to detect grassland variation. [PERSON] [4] indicated that the GPP of grassland had a significant upward trend globally at annual and seasonal scales during 1982-2011. In China, [PERSON] [10], based on model simulations and remote sensing satellites, reported that more than 80% of the area in Chinese grassland had a sharply increasing trend. However, although a consistent growth trend has emerged, the phenomenon of grassland degradation is still hidden in the context of overall restoration. Nearly 23% of the area of grassland in China has experienced significant grassland degradation [11], especially in Inner Mongolia [10; 12]. Grassland variation is mainly affected by climate change and human activities [13; 14; 15]. In previous studies, statistical methods [16; 17], model simulations [18; 19], and observational experiments [20] have been used to assess how climate and human factors affect grassland ecosystems. According to previous research results, grassland variation generally has positive or negative correlations with precipitation (PRE) and temperature (TEM) in most areas of China [14; 21]. In addition, human activities, including overgrazing, socioeconomics, and regional policy, have profound impacts on the sustainable development and biodiversity of grassland ecosystems [22; 23; 24]. Nevertheless, as climate exhibits spatiotemporal heterogeneity and human activities are immeasurable, their impact on grassland dynamics still needs further clarification. In China, grasslands cover an area of approximately 400 million ha and occupy more than 40% of the national land area [25; 26]. Grasslands are the main ecosystem, especially in North China, and play an irreplaceable role in windbreaks and sand fixation, forage supply, economic development, and ecological security. The sharply increasing population and unreasonable land use have been accompanied by the serious degradation of grasslands in China in recent years [27]. To curb grassland degradation and promote grassland restoration, many grazing managerial actions have been effectively implemented to increase the sustainability of grassland ecosystems worldwide [6]. The Chinese government has invested in policies that support ecosystem services and the recovery of degraded grassland since 2000, such as the \"returning grazing land to grassland\" policy, the Beijing and Tianjin Sandstorm Source Control Program, and \"Grassland Transfers\" [28; 29]. Additionally, the use of grazing intensity management as a significant government measure (e.g., yearlong continuous and seasonal grazing exclusion) has been extensively implemented [30].