Water-Use Strategies and Plasticity of Sand-Fixing Plants Along the Tarim Desert Highway in the Taklamakan Desert

In the hyper-arid Taklamakan Desert, where water availability is extremely limited, key sand-fixing species such as Tamarix ramosissima, Calligonum arborescens, and Haloxylon ammodendron are largely maintained by saline groundwater irrigation. Understanding their water-use strategies is therefore essential for optimizing irrigation practices and species selection in shelterbelt systems. However, the water sources, long-term water-use efficiency (WUE), and interspecific water competition of these species across different establishment years remain insufficiently understood. The primary objective of this study is therefore to elucidate the water-use strategies of these three sand-fixing species and to assess how they vary with stand age and mediate interspecific coexistence. To achieve this objective, stable hydrogen, oxygen, and carbon isotope analysis (δ2H, δ18O, and, δ13C) was integrated with MixSIAR modeling and water niche assessment for stands established in 1999, 2004, and 2009. The results showed that: (1) water sources differed significantly among species and changed dynamically with stand age, with C. arborescens exhibiting the most pronounced variation in water-uptake depth and the greatest water-use plasticity; (2) long-term WUE varied significantly across species, and its relationship with water-niche breadth showed a negative trend, suggesting that broader water-niche breadth may be associated with lower WUE; and (3) interspecific water competition diminished as increasing plantation age increased. These findings demonstrate that in the extreme arid environment of the Taklamakan Desert, sand-fixing species achieve coexistence through divergent water-use strategies, thereby mitigating competition under severe water scarcity. This study provides a theoretical foundation for the sustainable management of shelterbelt ecosystems in arid regions.