Carrying capacity of high-altitude grassland in warm seasons in the middle section of Tianshan Mountain from 2000 to 2018

Abstract: Grassland is an important vegetation type in Xinjiang of western China, particularly on the coordinated development of ecological protection and animal husbandry. It is of great significance to rapidly and accurately evaluate the productivity of regional grasslands, thereby revealing the carrying capacity of herbivores in the warm season of high-altitude grasslands, especially for the balanced management of regional grasslands and herbivorous livestock. However, there are only a few reports on the temporal and spatial changes of grassland Aboveground Biomass (AGB) in high-altitude regions using long-term series. It is necessary to effectively evaluate the carrying capacity considering some limiting factors. Taking the Hejing County of Xinjiang as the research area, this study aims to explore the spatiotemporal dynamics and carrying capacity of the grassland AGB using MODIS 17A3H/NPP (Net Primary Productivity, NPP) data products. An estimation model was also proposed to improve the traditional field sampling, in terms of data availability, multi-year monitoring continuity, and regional coverage. The natural and management factors were considered to further modify the model, according to the grassland type, landform, and township. The results showed that: 1) The Aboveground Net Primary Productivity (ANPP) of grassland presented a Root Mean Square Error (RMSE) value of 3.13-11.89 g/(m2·a), indicating a higher accuracy. 2) The AGB average value was 1 252.34 kg/ hm2 during the study period (2000-2018), indicating a fluctuating upward trend. Specifically, the linear growth rate was 7.78 kg/(hm2·a) (P<0.01), and the overall AGB of grassland was a slight upward trend, where the proportion of grassland with the AGB growth reached 75.01%. The degradation was distributed mostly in the areas with a slope of more than 30%. 3) The annual average aboveground hay was 150 100 t. Among them, the alpine grassland and meadow over 2 500 m produced the largest hay, accounting for 24.20% and 57.46%, respectively. The available hay was reduced to 86 200 t after revision. 4) The average theoretical carrying capacity of grassland in the study area was 242 600 to 319 400 heads. The theoretical carrying capacity of standard sheep without considering natural and management factors was 319 400 heads, the underload of grassland is 2.99%; after revised the carrying capacity was 242 600 heads, and the grass was overloaded by 27.82%, it further illustrated that there was a certain overload risk in the development of grassland animal husbandry in the study area. The research results could provide references for the formulation of regional land management and animal husbandry planning policies, and played a positive role in promoting the sustainable economic development of regional natural resources. The average theoretical carrying capacity of grassland was 242 600 to 319 400 heads. The theoretical carrying capacity of standard sheep was 319 400 heads without considering the natural and management factors, where the underload of grassland was 2.99%. After that, the carrying capacity was 242 600 heads, and the grass was overloaded by 27.82%. Consequently, there was a certain overload risk in the development of grassland and animal husbandry in the study area. The findings can provide a strong reference for the decision-making on the land and animal husbandry, thereby promoting the sustainable development of regional natural resources.