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Ecological restoration efforts to combat soil loss often fail to deliver the expected benefits despite enormous investments. Better understanding the close relationships between climate, vegetation, and non-linear ecosystem dynamics can help restoration activities meet their intended goals. The severely eroded Loess Plateau (LP) was selected for a case study to investigate the underlying drivers of soil retention change and their non-linear constraint impacts. The soil retention function was substantially enhanced during 2000-2015, while the water yield function was weakened. Both the favourable changes in climate conditions (i.e., increasing precipitation and declining evapotranspiration) and governmental policies for soil and water conservation and ecological restoration promoted vegetation restoration and expansion, thereby strengthening the soil retention function. Consequently, water and sediment yields declined significantly. The relationship between water yield and soil retention is non-linear and shows that the two ecosystem functions can be either in synergy or in trade-off according to the level of vegetation cover. We observed that the vegetation cover had an upper threshold (i.e., 50%) for controlling the soil erosion because of the limited water condition (rainfall), which implies that the effect of vegetation on reducing the water erosion reached its maximum capacity when the plant cover was 50%. Considering the limited water conditions in drylands, to minimize the trade-off between the water yield and soil retention and to maximize the effectiveness of ecological restoration programs, it is recommended that the overall vegetation cover in the LP be controlled at approximately 30-40%, i.e., without exceeding 50%. Currently, the non-linear constraint effects of the water condition and vegetation cover on the soil retention are yet to be thoroughly understood. Moreover, practical policy recommendations require further field-based observations and experimental studies.