## 1 Introduction  
Grassland ecosystems are one of the most important parts of the global land ecosystem, covering nearly 25% of the land surface of the Earth, but they are sensitive to global change [1, 2]. Grassland ecosystems also play an important role in global carbon cycling, holding 10% of the global carbon stocks [1, 3, 4]. Grasslands also provide a huge material basis for human survival and support the livelihoods of over 1 billion people [5, 6]. However, due to the global population growth, many grasslands present a degradation trend [5, 7]. China is a country with rich grassland resources, which account for about 40% of the total land area in China [8, 9]. Inner Mongolia has the second largest area of grassland and the largest area of pasture in China, in which approximately 66% of the total area is cove by grassland [8; 10; 11]. The Inner Mongolia grassland is acknowledged as an important barrier for the ecological environment in northern China [12]. However, grasslands are fragile ecosystems, especially in arid and semi-arid locations [13; 14; 15]. The grassland degradation in Inner Mongolia is now very serious, including both area reduction and growth decline [11; 16]. Two of the most important factors causing the grassland degradation are climate change and human activities [11; 13; 17]. The study found that Inner Mongolia had become warmer and drier during 1961-2012, with the rate of the global warming twice as fast [18]. It was reported droughts usually occurred in spring when strong winds blow away the surface soil easily [11], causing the degradation of grasslands. Moreover, many studies concluded that the impact of human activities (e.g., overgrazing, reclamation of grassland, mining, etc.) on desertification of grasslands intensified [19; 20]. To slow down the trend of environmental deterioration in northern China, the Chinese government has launched several environmental restoration projects since 1999 [21], such as Grain to Green Program (1999-2015), Rangeland Law (2003), Return Grazing Land to Grassland Program (first round during 1999-2013, second round during 2014 to now), Beijing-Tianjin Sand Source Control Engineering Project (first round during 2001-2010, second round during 2013-2022), Payments for Environmental Services (2001-2020), Subsidy and Incentive System for Grassland Conservation (2011-2021), etc. [14; 21; 22; 23]. With the repaid development of remote sensing technology and the gradual abundance of data, remote sensing has been indicated as a powerful way to study the spatiotemporal changes of earth ecosystems and their causes [24; 25; 26; 27]. Net primary production (NPP) and land-use/cover change (LUCC), as calculated based on remote sensing technology, have been widely applied to analyze the structural and functional health of different ecosystems [28; 29]. NPP refers to the net amount of new carbon absorbed by plants per unit of time and space, which can reflect the plant growth and ecosystem health status [17; 30]. Several evaluation models for NPP based on remote sensing data have been developed, such as the Carnegie-Ames-Stanford approach (CASA) [31], the global production efficiency model (GLO-PEM) [32], and the vegetation photosynthesis model (VPM) [33]. The CASA model is the most popular approach, and has been widely applied [31; 34]. [PERSON] et al. [35] analyzed the trend of grassland NPP changes in Northern Tibet from 1981 to 2004 and found that the impact of regional climate change on NPP was more detrimental than positive. [PERSON] et al. [36] evaluated the grassland NPP in the Eurasian Steppe for 2000-2014 and found that the main cause of degradation of grassland was climate change in China, whereas human activities in Mongolia, while human activities promoted the grassland recovery in Kazakh steppe regions. [PERSON] et al. [37] analyzed the influencing factors for grassland NPP in Inner Mongolia, and their results showed that ecological restoration projects have had a positive impact on grassland NPP.