Water use strategies and coexistence mechanisms of desert plants in relation to groundwater depth: evidence from stable oxygen isotope.

Understanding the mechanisms of plant coexistence and vegetation -groundwater interactions in arid regions is critical for predicting ecosystem responses to environmental change. Groundwater depth is a key factor affecting plant water availability, yet how changes in groundwater accessibility shape the water-use strategies and niche differentiation of coexisting desert species remains unclear. We investigated three typical desert species (Haloxylon persicum, Calligonum leucocladum, and Haloxylon ammodendron) in the Kalamaili National Park candidate area, Xinjiang, China. Using stable oxygen isotope (δ18O) techniques combined with a mixing model, we analyzed plant water sources and use strategies across three phenological phases (May: leaf-out; July: peak physiological stress; September: pre-senescence), and examined the effects of groundwater depth on water-use patterns. Soil water isotope composition showed pronounced seasonal variation, with depletion in shallow layers (0 -40 cm) in spring and enrichment in summer and early autumn, while deep layers (100 -300 cm) remained relatively stable. The three species exhibited markedly different water-use strategies: H. ammodendron relied predominantly on groundwater (mean contribution: 37% -73% across seasons); C. leucocladum relied mainly on shallow and middle soil water (29% in spring, 31% in summer), consistent with its shallow but laterally extensive root system; H. persicum used both deep soil water and groundwater, but with lower groundwater dependence. Groundwater use proportion of H. ammodendron showed a significant negative correlation with groundwater depth (P < 0.05), while those of the other two species were largely decoupled from groundwater. Our findings suggest that hydrological niche segregation among coexisting desert species may arise from asymmetric groundwater sensitivity --only the phreatophyte closely tracks groundwater fluctuations, whereas others compensate through flexible use of seasonally variable soil water --thereby reducing competition and stabilizing community coexistence. We propose this asymmetric response as a mechanism for transient coexistence or short-term niche differentiation in groundwater-dependent ecosystems, providing a conceptual framework for vegetation restoration and water resource management.