Sustainability of inner Mongolian grasslands: Application of the Savanna model

The sustainability and resilience of an Asian typical steppe grazing ecosystem was assessed by determining thresholds and stable states with an ecosystem simulation model. This analysis used the Savanna model to simulate spatial climate, vegetation, and livestock grazing dynamics, at 14 different stocking rates (5.5-59.8 AUY km(-2)). Grazing effects on vegetation were assessed, including effects on primary production, vegetation composition, and root biomass. Simulations were run for 100 years: 50 years to examine sustainability and 50 years to examine resilience of the system. Results showed that a grazing intensity (1-g/u; g = biomass in grazed area, u = biomass in ungrazed area) of 0.49 was sustainable for this particular system. This region was resilient to grazing up to the intensity of 0.49, where the system remained dominated by herbaceous production. Grazing intensities higher than 0.49, in combination with low precipitation events, resulted in decreased herbaceous net primary production and root biomass, and increased shrub net primary production and root biomass. Herbaceous vegetation was unable to gain a competitive advantage over shrubs in areas where grazing intensities were above 0.49; consequently, the system shifted to a stable shrub-dominated state that could not return its original composition even without further grazing.