Abstract: Clarifying the effects of grazing interference on ecosystem services is an important basis for the sustainable grazing managements. The northern farming-pastoral zone traverses the Loess Plateau, for which ecological restoration on this region still survives the impacts of grazing disturbance. In this paper, the grazing intensity across the Loess Plateau was estimated by using the global livestock density data and remote sensing vegetation index. Then, the constraint effects of grazing intensity on three ecosystem services namely water yield, forage provision and soil conservation in 2000 2010 and 2020 were examined by using the constraint line extraction method. On this basis, the spatial-temporal characteristics of trade-off and synergy of ecosystem services from 2000 to 2020 in the Loess Plateau under different grazing intensity were explored. The results show that: (1) the study area was dominated by the low-level grazing intensity and moderate-level grazing intensity by 2020. The area proportion of high-level grazing intensity only accounted for 5.30%. (2) Grazing intensity showed negative constraint effects on all three ecosystem services, which however, a higher grazing intensity indicated a larger amount of water yield within a certain threshold range. (3) From 2000 to 2020, the synergies among ecosystem services in the high-level grazing area were severely weakened. Under the moderate-level grazing intensity, the synergies among ecosystem services increased on different levels, and trade-off between water yield and forage provision in 2000 had been turned into synergic relationship in 2020. However, on the low-level grazing areas, synergic relationship between water yield and forage provision with correlation coefficient of 0.29 in 2000 had been turned into trade-offs in 2020, with correlation coefficient of -0.14. These results reveal that within the 20 years, the improvement of ecosystem function benefited from policy of grazing prohibition has show an effect of diminishing marginal. The consideration on types of grazing livestock combined with the spatial heterogeneity of ecosystems is a prerequisite for developing moderate grazing in arid regions for the improvement of ecosystem functioning in drylands.

Key words: ecosystem services, grazing intensity, constraint line method, trade-off analysis, spatial-temporal characteristics, the Loess Plateau