Stable isotope insights into water use sources and adaptation strategies of Tamarix Chinensis in desert ecotone of arid regions.

Water availability is a critical determinant of ecosystem stability and vegetation persistence in arid environments. Understanding the water-use strategies of xerophytic shrubs under diverse hydrological conditions is essential for elucidating plant-water interactions and informing ecological restoration practices. This study focuses on Tamarix chinensis communities across the desert-steppe ecotone in northwestern China. By analyzing the stable isotopic compositions of hydrogen and oxygen (δ²H and δ¹⁸O) in precipitation, soil water, xylem water, and groundwater, and applying a Bayesian mixing model (MixSIAR), this study quantitatively assessed the sources and controlling factors of water uptake under varying groundwater depths. The results reveal that Tamarix chinensis employs flexible water-use strategies that vary with habitat conditions. Rainfall contributed only (10% ± 2%) to total water uptake, while groundwater (24% ± 3%), mid-soil water (22% ± 3%), and deep-soil water (22% ± 3%) were the predominant sources. Under conditions of shallow groundwater and low salinity, the species accessed deep-soil water (27%) and groundwater (29%). As groundwater levels declined, reliance on groundwater decreased, and uptake shifted toward mid- (25%) and deep-soil layers (26%). Additionally, in areas with reduced vegetation richness, the contribution from shallow-soil water increased significantly, reaching 24.8%. These results demonstrate the strong ecological plasticity of Tamarix chinensis, which adjusts its water-use strategy in response to variation in groundwater depth, soil salinity, and community structure. The study provides critical insights for vegetation management and ecological restoration in arid and transitional desert regions.