**1. Introduction**

In desert ecosystems, water is a restrictive factor for plant survival and growth because of low and unpredictable precipitation and high evaporation [1, 2]. The ability to use rainwater in

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spring and summer is important for plant phenology and growth [3]. Additionally, the sustainable water source is necessary for plant growth especially in the drought period, such as deep soil water or ground water [4]. A stable isotope technology is often used to study the water use strategy of desert plants. Generally, there is no stable isotope fractionation during water uptake by root system or water transportation in the xylem of most plant species. Thus, the main water source can be distinguished by comparing the δD or δ18O value of xylem water with that of the potential water source, for example, rain, snow, river, lake, soil water or ground water [1]. Further, leaf δ13C value of C3 plants is positively related to their long-term water-use efficiency (WUE). The δ13C value decreased from spring to autumn in the growing season and increased in the drought period [4].

Previous studies with the stable isotope of hydrogen or oxygen indicated that woody plant species used different water sources in desert or other arid and semi-arid ecosystems. Firstly, many trees and shrubs mainly used shallow soil water recharged by rain, for example, *Artemisia ordosica* in Mu Us Sandy Land [5], *Pinus edulis* and *Juniperus osteosperma* in Utah [6], *Larix sibirica* and *Potentilla fruticosa* in the mountain forest along the Kherlen River of Mongolia [7], *Senecio filaginoides* and *Mulinum spinosum* in Patagonian steppe [8], *Nitraria tangutorum* and *Artemisia arenaria* in Badain Jaran Desert [9], *Sarcobatus vermiculatus* in San Luis Valley of Colorado [10] and *Haloxylon persicum* in Gurbantunggut Desert [11]. Secondly, some deeprooted shrubs and trees mainly used ground water, for example, *Sabina vulgaris* and *Salix matsudana* in Mu Us Sandy Land [12], *Populus fremontii* in the non-saline habitat of Colorado River Basin [13], *Ericameria nauseosa* in San Luis Valley [10], *Ulmus pumila* in Hunshandake Sandy Land [14], *Pinus sylvestris* var. *mongolica* in Horqin Sandy Land [15], *Populus euphrati‐ ca* along Heihe River [16] and *Haloxylon ammodendron* in Gurbantunggut Desert [11]. Thirdly, few shrubs or trees with dimorphic root systems used both soil water recharged by rain and deep soil water recharged by ground water and even ground water. *S. vermiculatus* used ground water during dry periods but used deep soil water large amounts of rainfall in San Luis Valley [10].

Gonghe Basin is located in the northeastern Qinghai-Tibetan Plateau, which is the ecotone from semi-arid to arid region, with the altitude varying from 2600 to 3400 m. It contains part of Gonghe County, Guinan County and Xinghai County of Qinghai Province. The dominant vegetation is steppe and desert steppe. The basin is as long as 210 km in East and West, and as wide as 60 km in South and North, with a total area of 13,800 km2 . It is one of the most vulnerable land desertification area in Qinghai Province. The area of desertified land is 3530 km2 , which accounts for 25.58% of the total area. The ecological security in the upper reaches of Yellow River has been affected significantly by land desertification in Gonghe Basin, especially Longyangxia Reservoir [17]. In order to control and prevent land desertification, large areas were planted with trees and shrubs to form the shelterbelt system in the ecotone between oasis and desert to protect farms, villages and roads in Gonghe Basin. Some trees in *Populus* were planted by transplanting inside oasis. Some shrubs in *Caragana* were planted by seeding on sand dunes. Other shrubs in *Salix* were planted by cutting on interdune.

In this chapter, the main water source of four dominant desert shrubs is compared by stable hydrogen and oxygen isotopes in the growing season (May, July and September). The water use ratio of different sources was analysed by Iso-source 1.3.1 software [18]. Two shrubs in *Caragana* on sand dunes may use shallow soil water recharged by rain and deep soil water recharged by ground water. Two shrubs in *Salix* on interdune may use shallow soil water recharged by rain, deep soil water recharged by ground water and even ground water. Further, the long-term WUE is compared using their leaf stable carbon isotope. The WUE of four shrubs may be higher in drought spring than in summer and autumn.
