Differences in water-use strategies along an aridity gradient between two coexisting desert shrubs (Reaumuria soongorica and Nitraria sphaerocarpa): isotopic approaches with physiological evidence

Understanding the responses of different plant species to changes in available water sources is critical for accurately modeling and predicting species dynamics. Our study aimed to explore whether there were differences in water-use strategies between the two coexisting shrubs (Reaumuria soongorica and Nitraria sphaerocarpa) in response to different amounts of summer precipitation. We conducted 3 years of field observations at three sites along an aridity gradient from the middle to lower reaches of the Heihe River basin, northwestern China. Stable oxygen composition (delta O-18) in plant xylem water, soil water and groundwater were analyzed concurrently with ecophysiological measurements at monthly intervals during the growing seasons. Water source for coexisting R. soongorica and N. sphaerocarpa did not differ at the sites with high precipitation, but significantly differed in more arid locations. The N. sphaerocarpa was more sensitive to summer precipitation than R. soongorica in terms of predawn water potential, stomatal conductance and foliage carbon-isotope discrimination. The plants relying on groundwater maintained consistent water use strategies, but not plants that took up precipitation-derived water. We also found that the difference in water source uptake between the coexisting species was more apparent in more arid locations.