Abstract: Shrub encroachment occurs in degraded grasslands worldwide. Most hypotheses about shrub encroachment suggest that the differences of water use strategies between shrubs and herbaceous species are important, but with limited empirical evidence. Based on filed survey of four different succession stages (Agriophyllum squarrosum community in shifting sandy land, Artemisia ordosica community in semi-fixed sandy land, A. ordosica community in fixed sandy land, and Stipa bungeana community in fixed sandy land) on the Ordos Plateau, we examined the changes of water use characters of dominant species in different succession stages by determining water source through δ¹⁸O analysis. In different succession stages, herbaceous plants mainly used water from shallow soil layer (0-20 cm), whereas shrubs predominantly used water from middle or deep soil layers (50-200 cm). The plasticity of water use by shrubs was stronger than that of herbaceous plants. When suffering water stress, shrubs can change their water use sources to different soil layers according to water availability. These findings improve our understanding of water use strategy of the main species at different successional stages on the Ordos Plateau. The results suggest that it is crucial to reduce human disturbances such as overgrazing, to improve soil structure and increase soil moisture in shallow soil, and consequently to restore zonal S. bungeana steppe from A. ordosica encroached grassland on the Ordos Plateau.

Key words: biodiversity, ecosystem functions, ecosystem management, global change, ecosystem services