Plant communities responding to grazing pressure by sheep in an Alpine meadow

The Chinese grassland ecosystem is an important national asset that not only impacts climate regulation, soil and water conservation, wind protection, and soil carbon and nitrogen fixation but is also an important contributor to maintaining grassland biodiversity while supporting livestock production. Grasslands are a key component contributing to the productivity of grazing animals but also provide basic food production via livestock grazing for herder survival. Grazing is the most basic means of grassland utilization but is considered one of the more important disturbance factors controllable by humans that has a universal and profound impact on the grassland ecosystem due to animal density and over grazing. For Alpine grasslands, it is not clear what grazing intensity (GI) can be achieved to improve plant biodiversity and vegetative nutritional value while improving sheep productivity. This field experiment was conducted for 7 yr comparing the impact of different GI on vegetation community characteristics, nutritional value, and sheep growth performance on the Alpine meadows of the Qinghai-Tibetan Plateau. The GI measured were: Control: 0 sheep/ha; Low: 3.7 sheep/ha; Medium: 5.3 sheep/ha; and Heavy: 7.6 sheep/ha. The grazing experiment started in 2008, but experimental data collection and analyses were collected for the final 4 yr of 2015 through 2018. All grazing intensities >0 sheep/ha reduced (P < 0.05) plant height (27%, 46%, and 48%, respectively, for 3.7, 5.3, and 7.6 sheep/ha), ground coverage (16%, 24%, and 48%), and above ground biomass (2%, 42% and 53%) of the various plant communities while increasing (P < 0.05) the grass community density (individuals/m²) compared to a nongrazed Control. With increasing GI, the community height, coverage, and above-ground biomass decreased (P < 0.05), and the plant community density increased then decreased (P < 0.05) compared to Control. As GI increased, the available community biomass nutritional quality increased (P < 0.05). Comprehensive analysis showed that the community density (quantity) and nutritional quality were the highest when the GI was 5.3 sheep/ha. The higher the GI, the greater the grass's nutritive value with lower above-ground net primary production (ANPP). When GI was the highest, the average daily gain (ADG) per hectare was the highest in the short term, but the highest GI endangers the ANPP and profitability of the grassland grazing ecosystem in the long term. Targeting a moderate GI (5.3 sheep/ha) can provide 78% of the ADG per hectare of the highest GI, which meets the requirement of maintaining a sustainable grazing grassland.