Response Of Plant Community To Stocking Rate And Precipitation Variation In The Grassland Of Northeastern Edge Of Qinghai-tibetan Plateau

Grazing is utilized to achieve the multiple functions of grassland. The alpine grassland in the Qinghai-Tibetan Plateau accounts for 34.3% grassland area in China, of which the alpine meadow has 58 million ha, ranking the top of the various types of grassland. At present, because of overgrazing and serious degradation in this area, the function of grassland ecological system has been paid close attention to the influence of the herding behavior and stocking rate on grassland. The objective of this study is to investigate the response mechanism of grasslands to a long-term fence under varying precipitation and appropriate grazing intensity in the alpine meadow ecosystem. Four stocking rates were established:no grazing, light grazing (0.75 yak unit/ha), moderate grazing (1.00 yak unit/ha) and heavy grazing (1.25 yak unit/ha) in four consecutive years (annual precipitation 295,486,526 and 252 mm). Sampling surveys were conducted in the alpine meadow and typical steppe after 28 years fence, and identified two types of grassland response mechanism after a long-term fence. We found that:1. The height, cover, density and biomass of the alpine meadow deceased with the stocking intensity. Under the moderate grazing, the most species diversity was observed, while highest proportion of edible grass and the least niche overlap value was also detected. This suggests that the alpine meadow maintained a healthy and stable state, in consistence with the intermediate disturbance hypothesis.2. Increasing stocking rate increased the diversity of alpine meadow community, and the nitrogen, phosphorus and crude fat content of two dominant species, Polygonum viviparum and Kobresia capillifolia. Under the moderate grazing, individual yak gaining weight and yak gaining weight per ha were 112% and 88%, compared with that under the heavy grazing. In summary, the alpine meadow can maintain in a healthy and sustainable development state under the moderate grazing.3. The characteristic index and diversity index of plant community in high annual precipitation year was better than that in low precipitation years in the alpine meadow. Different functional groups responded differently to the variations of precipitation. Gramineae, cyperaceae and forb biomass ratio increased with the precipitation in the community, whereas the biomass ratio of polygonaceae decreased. The niche overlap value was 0.813 under 526 mm precipitation,8.7% lower than 0.890 under 252 mm precipitation. Such a variation alleviated interspecific competition in the community, meeting the requirements for different plant species on the demand of natural resources, especially the soil moisture for the growth and reproduction of plant.4. The higher precipitation can improve the nitrogen, phosphorus and other nutrients concentration of plant community and dominant species K. capillifolia. Another dominant species P. viviparum had no obvious response to precipitation in the alpine meadow. The alpine meadow was able to adapt to annual precipitations variability, owning much wider adapting amplitude.5. In the alpine meadow, there was no obvious difference on the community characteristics compared with grazing after 28 years fence. The biomass ratio of cyperaceae and polygonaceae reduced from 82.2% to 72.2%, and the biomass ratio of forb increased from 17.8% to 21.8%. In the typical steppe, community characteristics and species diversity significantly increased, and the structure and composition of grassland ecosystem improved. The biomass ratio of gramineae and compositae increased from 48.4% to 64.7%, whereas the biomass ratio of forb reduced from 51.6% to 35.3%.6. Under grazing utilization, the nitrogen, phosphorus and crude fat content of, P. viviparum and K. capillifolia decreased. After long-term fence, in comparison, nutrient value indexes reduced, and forage quality was in degradation in the alpine meadow community and two dominant species. By contrast, the nitrogen, phosphorus and crude fat content, nutrient value indexes increased in the typical steppe community and dominant species Agropyron cristatum, as well as the forage quality was optimized.Our research systematically investigated the relationship between the alpine meadow growth and grazing intensity based on the community and species levels. We validated responses to grazing mechanism, confirmed the appropriate grazing intensity in the alpine meadow ecosystem of the northeastern edge of the Qinghai-Tibetan Plateau, and thereof provided the certain scientific basis for the appropriate grazing system and grassland management for policy maker and produce-herders. The characteristics, diversity, nutrient accumulation and distribution’s convergence, divergence and mechanism were explored in the alpine meadow and typical steppe after 28 years fence, offering scientific basis for the appropriate fence time in the alpine meadow and typical steppe.