Effects of grazing by large herbivores on plant diversity and productivity of semi-arid alpine steppe on the Qinghai-Tibetan Plateau

Grazing by large herbivores may have a strong impact on plant diversity and productivity, but the effects are expected to vary with grazing pressure. The changes in productivity and species diversity of Stipa purpurea-dominated semi-arid alpine steppe grassland were measured under four different stocking rates of Tibetan sheep [no grazing, light (2.4 sheep units ha(-1)), moderate (3.6 sheep units ha(-1)), and heavy (6.0 sheep units ha(-1)) grazing] in a 5-year (2006-2010) grazing experiment on the Qinghai-Tibetan Plateau, China. Herbage mass and aboveground net primary productivity of alpine steppe declined significantly with increasing stocking rate (P < 0.05). Over the 5 years of the experiment, the proportion of forbs and sedges increased significantly under light and moderate grazing; the proportion of grasses decreased significantly, whereas the proportion of S. purpurea did not change compared with the no grazing treatment. Species diversity was highest under moderate grazing and was significantly higher than the no grazing treatment in 2 years (2008 and 2010). Moderate grazing enhanced the species diversity of the plant community due to an increase in the proportion of forbs. There were significant positive linear correlations between herbage mass and species diversity under no and light grazing. Species diversity was not related to productivity under moderate and heavy grazing. In conclusion, grazing by sheep reduced plant productivity; plant diversity increased under low and moderate grazing, and was as a result of changes in the composition of the plant community of a semi-arid alpine steppe.