Enhancing sustainability of grassland ecosystems through ecological restoration and grazing management in an era of climate change on Qinghai-Tibetan Plateau

The grassland ecosystems cover above 60% of Qinghai-Tibetan Plateau (QTP) and provide important ecosystem services and functions at local and regional scales. However, both anthropogenic and non-anthropogenic disturbances are challenging the sustainability of the QTP's grassland ecosystems in the era of global change. In this review paper, we summarized the causes and effects of grassland degradation, restoration practices used on degraded grasslands, impacts of climate change and nitrogen deposition on plants and soils, adaptation strategies to climate changes, impacts of livestock grazing on plants and soils, and sustainable grazing management on the QTP. We believed that the integration of internal drivers of ecosystem fragility with external disturbances such as overgrazing are the major causes of grassland degradations on the QTP. The quantitative evaluation systems for grassland ecosystem health including plant, soil and livestock indicators are theoretically feasible and technically reliable. The major impacts of grassland degradation on the QTP are loss of biodiversity, increased water erosion, reduced carbon sequestration, decrease pastoral productivity and reduced local human well-being. The current restoration practices are mostly revegetation of "Bare Land" severely degraded grasslands using cultivated grasses. Other restoration practices such as grazing fallow and grassland fencing can be applied to restore the lightly or moderately degraded grasslands. We believed that the side effects from soil drying on plant productivity and diversity in manipulative warming experiments could be the main cause of controversial findings about the effects of climate change on plant productivity, diversity, phenology, soil physics, biochemistry and organisms. Many scholars argued the advances and delays in greening-up date, lengthening and shortening of the growth period in the phenological responses of alpine grassland plants to a warming climate. Multiple sources of remote sensing data and diverse interpretive approaches should be applied and ground-based observations and surveys should be performed to improve the accuracy and reliability of model predictions on plant phenology. Inconsistent conclusions of climate change impacts on soil biochemistry and organisms except soil acidification and eutrophication have been widely observed in most manipulated studies of nitrogen deposition. Adaptive strategies should be employed to promote grassland system's sustainability of the QTP. Rotational grazing regime with moderate grazing intensity in the warm season may retain or promote plant height, cover, productivity and biodiversity, improve soil structure, fertility and microbial biomass, enhance carbon and nitrogen storage and reduce greenhouse gas emissions in the alpine grassland ecosystems.