Plant compensatory growth and optimal grazing intensity of grasslands in northern China: A meta-analysis of grazing experiments

Grassland ecosystems cover a total area of 4 million square kilometers and account for approximately 41% of the total land area in China, providing multiple ecosystem functions and services, such as primary production, carbon sequestration, forage and food production, soil erosion control, and climate change mitigation. About 3/4 of China's grasslands are distributed in the northern areas, which is an important ecological security barrier in northern China. Grazing is the most dominant land use form in grasslands worldwide; however, overgrazing has resulted in widespread degradation of ecosystem structure and functions. Previous studies indicate compensatory growth is a key adaptive strategy for grassland plants in response to herbivory. Moderate grazing can stimulate plant compensatory growth and enhance primary production and ecosystem stability. Therefore, reasonable grazing intensity is the basis for optimizing ecological and production functions for sustaining the long-term utilization of grasslands in this region. Based on data from 118 studies on field grazing experiments in grasslands of northern China, we conducted a meta-analysis assessing plant compensatory growth and optimal grazing intensity of five dominant grassland types: Alpine meadow, alpine steppe, meadow steppe, typical steppe, and desert steppe. We collected the data of aboveground biomass, belowground biomass, and plant species richness in grazed and ungrazed grasslands. Here, we converted the grazing intensity into forage consumption rate (i.e., percent removal aboveground biomass by domestic animals) for each study to facilitate data analysis and comparisons across different grassland types. Furthermore, we examined the relationships of plant compensatory growth with climatic conditions, soil fertility, and changes in belowground biomass and plant species diversity. Overall, our results showed the under-compensation in plant community biomass production across the northern China grassland and that compensatory growth in plant aboveground biomass frequently occurred at the expense of decreaced belowground biomass. Specifically, plant under-compensations were found in alpine meadow, meadow steppe, and desert steppe, while plant equivalent compensations were found in alpine steppe and typical steppe. The magnitude of plant compensatory growth positively related to mean annual precipitation and plant species richness across the northern China grassland, although the driving factors differed among grassland types. Our study indicates that the sustainable utilization of the northern China grassland requires the equivalent- and/or over-compensation of plant aboveground biomass and, simultaneously, with no substantial reductions in belowground biomass. To achieve these goals, our results suggest that the optimal forage consumption rate is 50% for alpine meadow, 48% for meadow steppe, 40% for typical steppe, 37% for alpine steppe, and 31% for desert steppe. Our findings have important implications for the ecology and management of grassland ecosystems in northern China and beyond.