## 1 Introduction  
Grassland degradation is one of the most important issues closely related to biodiversity conservation, ecological functionality and sustainable development in a rapidly changing world [1; 2; 3]. Alpine pasture degradation on the Tibetan Plateau is mainly attributed to overgrazing under the ongoing climate change in the last decades [4]. Grassland degradation on this plateau has been increasingly claimed to not only threaten the livelihood and culture of local residents, but also to widely affect water security in East China and South Asia [5; 6; 7]. However, the mechanism, the direction and the magnitude of the relative influences of climatic and anthropogenic drivers are still unknown and under debate, especially in regard to the policy-making at a broader geospatial scale [8; 9; 10]. Climate change is believed to primarily impact functions and services of various ecosystems on the Tibetan Plateau. Total precipitation during the plant growing season (GSP) controls both temporal and spatial variabilities in vegetation phenology [9] and biomass production [11] in the context of global warming. Compared with fenced pastures, aboveground biomass and vegetation coverage are reduced by livestock grazing [12; 13], so overgrazing by unfenced livestock is very likely to be the most important anthropogenic driver for pasture degradation on this plateau [4]. Furthermore, grazing exclusion with fencing is assumed to be a necessary trajectory for pasture restoration in the heavily degraded grasslands. However, the reduction in aboveground biomass reported in field surveys does not mean a decline in net primary productivity (NPP) because the proportion consumed by large herbivores is unknown. According to the intermediate disturbance hypothesis [14; 15], a reasonable stocking rate might be better for maintaining stability in community structure and ecosystem functionality. Therefore, identifying the direction of vegetation dynamics, restoration or degradation, is the keystone of policy-making for alpine pasture conservation [16]. The direct and indirect impacts of either climate change or grazing management, especially on the potential and actual capacity of livestock, are still not clear. It is not necessary for vegetation to linearly respond to either climate change or grazing disturbance [17; 18; 19]. Therefore, disentangling and assessing their relative contributions to grassland degradation or restoration is still a challenge but increasingly required [8; 20]. In addition, the difference between potential net primary productivity (NPP\({}_{\text{P}}\)) and actual net primary productivity (NPP\({}_{\text{A}}\)) [8; 20] should be regarded as the proportion of grassland productivity that can be potentially consumed by livestock (NPP\({}_{\text{P}}\)) rather than the proportion of grassland productivity that has been actually consumed by livestock (NPP\({}_{\text{A}}\)) (Table 1). This is because NPP\({}_{\text{A}}\) includes the plant's regrowth after grazing in the same season. In a recent study, [PERSON] et al. [21] proposed a modified framework for assessing the relative impacts of climate and grazing on vegetation productivity at local small villages, and reported a method for estimating the actually consumed productivity by livestock (NPP\({}_{\text{a}}\)). At a coarser spatial scale, the field investigations generally cost more time and money, while remote sensing provides a more economical and effective data source for assessing historical vegetation dynamics. To assist regionally specific policy-making concerning livestock regulation and pasture conservation in the future, we hypothesized that the NPP\({}_{\text{gap}}\) (defined as NPP\({}_{\text{P}}\) - NPP\({}_{\text{a}}\), Table 1) can effectively indicate the direction of grassland change over a defined period, a positive trend for restoration while a negative one for degradation.