Global response of different types of grasslands to precipitation and grazing, especially belowground biomass

Precipitation and grazing both modify the structure and function of grassland. Although the effects of site-scale precipitation and grazing on vegetation communities have been investigated, the differences between different grassland types at the global scale are still not clear. Therefore, we conducted a global-scale meta-analysis synthesizing 107 field studies with different grassland types (savanna grassland, temperate grassland, cold grassland, and alpine grassland) to quantify the effects of global changes in precipitation and grazing on community structure and function of grassland types. Results showed that regardless of grassland type, increased precipitation increased species richness by 7.8%, decreased belowground biomass (BGB) by 20.0%, and increased aboveground biomass (AGB) by 22.9%, but interestingly, decreased precipitation increased BGB by 17.7%, decreased AGB by 22.8%, and decreased species richness by 13.7%. Species richness of global grassland vegetation was positively correlated with grazing intensity. The total vegetation community biomass (TB) increased by 11.7% under moderate grazing intensity, while the species diversity of global grassland vegetation increased by 10.7% under heavy grazing intensity. Increased precipitation had negatively impacted BGB in temperate grassland, while increased precipitation significantly increased cool grassland BGB. In savanna grasslands, vegetation AGB decreased with increasing precipitation. Our results highlight the impact of altered precipitation on grassland vegetation under different climate types. We suggest that future studies of grassland management should consider incorporating grazing intensity and climate type, which will help to protect grassland vegetation species diversity and predict future carbon dynamics.