Effect of Experimental Warming on Forage Nutritive Value and Storage in Alpine Meadows at Three Different Altitudes of Nianqing Tanggula Mountain, Northern Tibet: A Long-Term Experience

Effects of climate warming on nutrition quality and storage of alpine grasslands are still controversial, which is not conducive to the management and utilization of alpine grasslands. A long-term warming experiment (with open-top chambers used to elevate temperature) was conducted at three elevations (relatively low, mid-, and high elevations with 4313, 4513, and 4693 m) of Northern Tibet in 2010 to compare the differences in forage nutritional quality and storage response to warming among three elevations and to explore the relationships between forage nutritional quality and production. In 2019, community surveys, observations of forage biomass and nutrition quality, and soil physicochemical properties were carried out. Forage nutrition quality included crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), ether extract (EE), crude ash (Ash), and water-soluble carbohydrate (WSC) content. Warming did not affect community aboveground biomass (AGB) at the three elevations. Warming improved community nutrition quality by increasing community CP content by 25.80% and decreasing community NDF content by 15.51% at the low elevation. In contrast, warming reduced community nutrition quality by increasing community CP, ADF, and NDF contents by 13.45%, 23.68%, and 17.43%, respectively, and decreasing Ash content by 39.50% at the high elevation. Warming did not affect community CP, ADF, NDF, EE, Ash, or WSC contents at the mid-elevation. Warming increased community nutrition storage by increasing community CP, ADF, and NDF storges by 74.69%, 88.18%, and 79.71%, respectively, at the high elevation. Warming did not affect community nutrition storages at the low or mid-elevations. Overall, forbs had higher CP, EE, Ash, and WSC contents and lower ADF and NDF contents compared with graminoids. Community EE content increased with community AGB, but community CP, ADF, NDF, EE, Ash, and WSC contents were not related to community AGB. Therefore, from the low to high elevation, the effects of warming on forage nutrition quality gradually changed from improving to inhibiting. Warming altered rangeland quality by affecting forage nutrition quality rather than forage production. There were no trade-offs between forage nutrition quality and forage production.