放牧是草地生态系统变化的主要原因，优势种植物对放牧的响应和适应机制，是理解草地群落和生态系统过程的关键。本项目拟在内蒙古锡林郭勒温带典型草原通过设置不同梯度(不放牧、轻度、中度、重度，极重度)放牧实验，采用化学分析、液相色谱、同位素标记等方法，结合光合生理以及光合固定过程中一系列酶和色素的变化与激素调节，从形态功能、光合生理、能量分配、水分利用、碳分配等方面，系统研究优势种植物羊草、大针茅和糙隐子草对放牧的响应及适应机理，探讨其生理补偿机制，寻找有利于光合生理补偿的适度放牧阈值。利用碳同位素技术，研究放牧对碳在植物体内的重新分配及利用的影响；根据植物、土壤和地下水氢氧同位素的特征，研究优势种植物水分利用的来源，植物水分关系和水源利用之间的联系以及不断变化的资源对光合作用的影响。本项研究是草原生态系统研究的重要组成部分，对深刻认识草原生产力衰减机制，探索天然草原合理放牧利用途径具有重要意义。

Grazing is the main reason of grassland ecosystem change. the response mechanism of dominant species under different land use types is key to understanding the grassland community and ecosystem processes. This project intends to select five different grazing gradient experiments including (un-grazed, lightly grazed, moderately grazed, heavily grazed and Extreme heavily grazed) in the Xilin River Basin, Inner Mongolia, China, to study the response mechanism of dominant species of Leymus chinensis and Stipa grandis and Cleistogenes squarrosa to grazing intensity, and evaluate its physiological compensation mechanism, looking for the moderate grazing threshold for photosynthetic compensation by examining the responses of their functional morphology, gas exchange, and photosynthetic apparatus, energy distribution, water uses, carbon distribution and transformation. We use chemical analysis, HPLC and isotope labelling methods, combined with the photosynthetic physiological measurements and a series of enzymes activity of photosynthesis, pigments and hormonal regulation related to photosynthesis. Also, the study is designed to make clear the process of carbon migration, transformation and allocation using carbon isotope labeling; using the stable isotopic signature of water in the plant, soil and groundwater, we explore the sources of plant water use, the relation between the plant water relations and water use and the effect of changing resource availability on photosynthesis. This study is an important part of the grassland ecological system, and it is important for comprehensive understanding of the mechanism of grassland productivity attenuation and reasonable grazing utilization of natural grassland.