Model analysis of grazing effect on above-ground biomass and above-ground net primary production of a Mongolian grassland ecosystem

In this study, we have analyzed the productivity of a grassland ecosystem in Kherlenbayan-Ulaan (KBU), Mongolia under non-grazing and grazing conditions using a new simulation model, Sim-CYCLE grazing. The model was obtained by integrating the Sim-CYCLE [Ito, A., Oikawa, T., 2002. A simulation model of carbon cycle in land ecosystems (Sim-CYCLE): a description based on dry-matter production theory and plot-scale validation. Ecological Modeling, 151, pp. 143-176] and a defoliation formulation [Seligman, N.G., Cavagnaro, J.B., Horno, M.E., 1992. Simulation of defoliation effects on primary production of warm-season, semiarid perennialspecies grassland. Ecological Modelling, 60, pp. 45-61]. The results from the model have been validated against a set of field data obtained at KBU showing that both above-ground biomass (AB) and above-ground net primary production (Np,a) decrease with increasing grazing intensity. The simulated maximum AB for a year maintains a nearly constant value of 1.15 Mg DM ha^-1 under nongrazing conditions. The AB decreases and then reaches equilibrium under a stocking rate (Sr) of 0.4 sheep ha^-1 and 0.7 sheep ha^-1. The AB decreases all the time if Sr is greater than 0.7 sheep ha^-1. These results suggest that the maximum sustainable Sr is 0.7 sheep ha^-1. A similar trend is also observed for the simulated Np,a. The annual Np,a is about 1.25 Mg DM ha^-1 year^-1 and this value is also constant under non-grazing conditions. The annual Np,a decreases and then reaches equilibrium under an Sr of 0.4 sheep ha^-1 and 0.7 sheep ha^-1, but the Np,a decreases all the time when Sr is greater than 0.7 sheep ha^-1. It also indicates that the maximum sustainable Sr is 0.7 sheep ha^-1. Transpiration (ET) and evaporation (EE) rates were determined by the PenmanMonteith method. Simulated results show that ET decreases with increasing Sr, while EE increases with increasing Sr. At equilibrium, the annual mean evapotranspiration (E) is 189.11 mm year^-1 under non-grazing conditions and 187.46 mm year^-1 under an Sr of 0.7 sheep ha^-1. This indicates that the water budget of the KBU grassland ecosystem is not significantly affected by grazing.