Groundwater Controls on Plant Community Structure and Species Specific Water Use Strategies in the Desert Oasis...

Desert oasis ecotones are highly sensitive, yet the mechanisms through which groundwater fluctuations regulate community structure via hydrological niche dynamics remain unclear. To address this gap, we investigated plant water use strategies and niche overlap along a natural groundwater gradient (4.8 m, 9.4 m, 11.8 m) by combining plant surveys with stable isotope analysis using MixSIAR and nicheROVER models. Species richness exhibited a unimodal response to groundwater depth, peaking at the intermediate level, whereas both shallow and deep groundwater conditions reduced richness due to salinity and water limitation, respectively. Haloxylon ammodendron displayed contrasting population structures, with strong regeneration under shallow groundwater but dominance of smaller, senescent individuals at deeper depths. Its water source shifted progressively from groundwater to unsaturated zone soil water as depth increased. Calligonum mongolicum primarily relied on shallow to mid depth soil water and exhibited moderate plasticity at intermediate depth. In contrast, Tamarix chinensis and Ephedra intermedia consistently utilized deep water sources. Niche overlap analysis revealed a nonlinear pattern: extreme water stress induced complete niche separation, moderate resource availability at the intermediate depth promoted temporary high overlap coexistence, and secondary stress coupled with competition at the shallow depth led to competitive niche sorting. Across the gradient, the dominant (H. ammodendron) and specialized (C. mongolicum) species maintained stable and differentiated ecological roles, respectively. Our findings establish a nonlinear framework linking groundwater depth, species strategies, and hydrological niche dynamics. This mechanism profoundly influences community stability, providing critical insights for predicting plant dynamics and guiding groundwater management in arid ecosystems.