Abstract: Aims Analyze the characteristics of water source and hydrological ecological status of coniferous and broad-leaved pure plantation and mixed plantation in different seasons under the temperate semi-humid and semi-arid climatic conditions, and to investigate the water utilization strategy of vegetation in different types and the distribution and competition mechanism of water, so as to provide theoretical basis for the optimization and regulation of forest structure in the process of ecological restoration of the mountainous areas of Beijing, and then accelerate the restoration process of the plantation and the ecosystems. Methods In this study, we selected pure and mixed plantations of representative species of Pinus tabuliformis and Quercus variabilis in Beijing mountain area as research objects. We collected precipitation, soil samples from 0-100 cm, and samples of the xylem of P. tabuliformis and Q. variabilis, and analyzed the characteristics and dynamic changes of their δ2H and δ18O. The MixSIAR model was employed to calculate the relative contribution of each water source to the plants and Levins index and the similarity proportion index was employed to calculate the hydrological ecological niche breadth and overlap. Simultaneously, the study monitored atmospheric temperature, humidity, water content of soil, and root distribution to analyze the changes in tree water sources and their hydrological ecological niches. Important findings In the dry season (March to May), pure plantations of P. tabuliformis and Q. variabilis as well as mixed plantations of Q. variabilis mainly utilized the shallow soil water (0-40 cm); while mixed forests of P. tabuliformis mainly utilized the deep soil water (60-100 cm). In the first part of the rainy season (June to July), P. tabuliformis and Q. variabilis in pure and mixed plantations mainly utilized the middle-deep soil water. In the later part of the rainy season (August to September), P. tabuliformis and Q. variabilis in pure and mixed plantations both shifted to mainly utilize the shallow soil water (0-40 cm). During the whole study, the widths of the hydrological ecological niches of P. tabuliformis and Q. variabilis in pure plantation were both less than those in mixed plantation, and the width of the hydrological ecological niches of Q. variabilis in pure plantation and mixed plantation was slightly greater than that of P. tabuliformis. The over-lapping degree of the hydrological ecological niches of Q. variabilis and P. tabuliformis was lower in the dry season (March to May) compared in rainy season (June to September), and P. tabuliformis and Q. variabilis responded to drought stress by separating the time and space of water use, niche separation, to weaken the competition for water.

Key words: water sources, hydrogen and oxygen stable isotopes, different forest types, hydrological ecological niche characteristic