Coupled responses of leaf water isotopes and photosynthetic physiology of Haloxylon ammodendron to microhabitat heterogeneity

Microhabitat heterogeneity plays a crucial role in shaping plant water use strategies and ecophysiological processes in arid ecosystems, yet little is known about the coupled response of leaf water isotopes and photosynthetic physiology to such fine-scale environmental variation. This study employed stable isotope tracing (δ2H and δ18O) and photosynthetic gas exchange measurements to investigate water sources, leaf water isotope dynamics, and photosynthetic traits of the dominant desert plant Haloxylon ammodendron across different microhabitats (flats and dunes) within the oasis-desert transition zone of the Hexi Corridor. In both microhabitats, H. ammodendron relies on groundwater as its primary stable water source, but the depth of soil moisture uptake shows significant differentiation. This differentiation is tightly coupled with seasonal trajectories of leaf water isotope enrichment (Δ2H, Δ18O): the flat habitat exhibits a “high in spring, low in summer” pattern, while the dune habitat shows a “low in spring, high in summer” pattern, accompanied by opposed photosynthetic physiological responses. Notably, the rate of leaf water isotope enrichment peaks at noon when stomatal conductance is at its lowest, confirming the nonlinear regulation of transpiration fractionation by stomatal behavior. Air temperature is the dominant meteorological driver of Δ18O variation, yet habitat specificity is pronounced—a positive correlation exists in flat habitats, while a negative correlation prevails in dune habitats. Hydrogen isotope enrichment, however, is not directly regulated by meteorological factors. Structural equation modeling quantified that xylem water isotopes directly govern leaf water isotopes (path coefficients 0.35–0.58), while soil water isotopes exert indirect regulation. Environmental factors primarily influence leaf isotopes by modulating soil moisture. This study elucidates the coupled regulatory mechanism by which microhabitat heterogeneity drives plant water uptake, leaf isotope enrichment, and photosynthetic physiology, providing important insights into how plants adapt to heterogeneous arid environments.