Quantifying hydrological niche segregation and water use divergence among coexisting desert plants in an extreme arid environment

Understanding the coexistence mechanisms of multiple plant species in extreme arid environments is a crucial challenge in dryland ecology. Hydrological niche segregation (HNS) has been recognized as a key driver of plant coexistence; however, its linkage with intrinsic water use efficiency (WUE i ) remains poorly understood. In this study, we combined stable isotopes (δ 2 H, δ 18 O, δ 13 C) with MixSIAR and NicheROVER models to investigate water uptake strategies (WUS), hydrological niches, and WUE i of four coexisting plant species ( Populus euphratica : Pe , Haloxylon ammodendron : Ha , Alhagi sparsifolia : As , and Halimodendron halodendron : Hh ) in the Ebinur Lake Basin, northwest China. Results showed that Pe , Ha , and As relied predominantly on groundwater (46–69%) and deep soil water (26–42%), while Hh exhibited greater flexibility, obtaining 58 ± 17% of its water from deep soil layers. In particular, Ha exhibited the highest WUEᵢ and a relatively narrow niche breadth (2.15), while As showed the narrowest niche breadth (1.93) and low WUEᵢ, coupled with a strong reliance on groundwater. In contrast, Hh possessed the broadest hydrological niche (3.40) and a moderately high WUEᵢ, whereas Pe exhibited the lowest WUEᵢ and a broad niche (3.26). Pairwise niche overlap was highly asymmetric: Pe overlapped with Hh by 55.3%, while the reciprocal overlap was only 34.5%, indicating directional competitive asymmetry. This differentiation appears to facilitate species coexistence through divergent strategies: the high WUEᵢ species ( Ha ) maintained a narrow hydrological niche, whereas Pe and Hh achieved coexistence through broader hydrological niches, and As relied on stable groundwater despite low WUEᵢ and a narrow niche. These findings enhance our understanding of the relationship between WUEᵢ and hydrological niche segregation in desert ecosystems and provide practical guidance for adaptive vegetation management in arid regions.