Abstract: Mongolian pine (Pinus sylvestris var. mongolica) has been widely planted on the sandy land in arid and semi-arid areas of North China since the 1950s, but some Mongolian pine forests planted in the 1950s are subject to decline. To examine the variations of habitat water supply and the water utilization strategies of the forests is of significance to understand the mechanisms of the decline. By means of stable isotope analysis, this paper studied the water sources and their seasonal utilization dynamics of Mongolian pine forests planted in the early 1980s in an inter-dune lowland of stabilized sand dune located in southeastern edge of Horqin Sandy Land, analyzed the effects of the variations of precipitation and soil moisture on the water utilization of the forests, and discussed the similarities and differences of Mongolian pine and its companion understory shrubs (e.g., Salix gordejevii) in their water utilization sources. Obvious seasonal variations were observed in the water stable ¹⁸O isotope composition (δ¹⁸O) of the branches of Mongolian pine and its companion understory shrub S. gordejevii during the observation period from May to October 2008. The water utilization sources of Mongolian pine mainly came from 20-40 cm soil layer or even much deeper soil layers, and there existed strong competition for soil water between Mongolian pine and S. gordejevii. S. gordejevii could earlier utilize the late strong precipitation (>10 mm), giving impact on the water source supply of Mongolian pine. This study had significance for better understanding the decline of Mongolian pine in relation to its water utilization strategy on sandy land.

Key words: Shelterbelt, Imitating normal forest, Sustainable management, Spatio-temporal pattern, Ecological services