Alpine grasslands are sensitive to grazing and atmospheric nitrogen (N) deposition. Despite increases in N deposition, few field studies have assessed the effects of grazing in conjunction with increased N deposition on alpine grassland vegetation.

In this two-year field study, we examined the effects of overgrazing and overgrazing plus N deposition on eco-physiological processes of alpine grassland plants at the functional group level. We found that both overgrazing and overgrazing plus N deposition altered species composition and the dominance of three plant functional groups (grasses, sedges, and forbs) in an alpine meadow and alpine steppe.

In the overgrazing and overgrazing plus N deposition treatments, forbs dominated the plant community. Grass and sedge dominance decreased substantially, relative to a ungrazed treatment. The underlying eco-physiological processes that led to the forb-dominated plant community differed between the overgrazing and overgrazing plus N deposition treatments. Overgrazing plus N deposition increased forb dominance both directly by selective herbivory and indirectly by enhancing forb photosynthetic rates.

Our results suggest that overgrazing concomitant with increases in N deposition will likely shift the plant community composition of alpine grasslands on the Qinghai-Tibetan Plateau from grass/sedge communities to forb-dominated communities in the future.

高山草原对放牧和大气氮 (N) 沉积很敏感。尽管氮沉积增加，但很少有实地研究评估放牧与氮沉积增加对高山草原植被的影响。

在这项为期两年的实地研究中，我们在功能组水平上研究了过度放牧和过度放牧加氮沉积对高寒草地植物生态生理过程的影响。我们发现过度放牧和过度放牧加氮沉积都改变了高山草甸和高山草原中三种植物功能群（草、莎草和杂草）的物种组成和优势。

在过度放牧和过度放牧加氮沉积处理中，杂草在植物群落中占主导地位。相对于未放牧的处理，草和莎草的优势显着下降。导致杂草占主导地位的植物群落的潜在生态生理过程在过度放牧和过度放牧加氮沉积处理之间存在差异。过度放牧加氮沉积直接通过选择性食草和通过提高杂草光合速率间接增加杂草优势。

我们的研究结果表明，随着氮沉积的增加，过度放牧可能会在未来将青藏高原高寒草地的植物群落组成从草/莎草群落转变为以森林为主的群落。