Responses of two desert riparian species to fluctuating groundwater depths in hyperarid areas of Northwest China

In the hyperarid region of Northwest China, frequent variations in hydrological environments present challenges to the persistence of riparian plants. The main objective of this study was to determine whether two desert riparian species (Populus euphratica and Tamarix ramosissima) differed in their water uptake patterns and ecophysiological responses to fluctuating groundwater depths (GWDs). This study was conducted in typical desert riparian ecosystems in the downstream Heihe River basin, Northwestern China, where the GWD continuously increases during growing season. Stable oxygen composition (delta O-18) in xylem water, soil water, and groundwater, as well as leaf water potential and gas exchange were monitored. Results showed that P. euphratica used a higher ratio of soil water, whereas T. ramosissima relied more on groundwater and deep soil water. As the GWD increased during the growing season, both species modified their water use patterns, but they did so differently, P. euphratica extracted an increasing proportion of deep soil water and groundwater, whereas T. ramosissima took an increasing ratio of groundwater at critical growth stages. P. euphratica exhibited decreases in its daily maximum photosynthetic rate (A(max)) and stomatal conductance (g(max)) as the GWD increased, whereas those of T. ramosissima changed little. In summary, both species shift to use greater ratio of more reliable water sources with the increasing GWD, but the switching of water sources could not sufficiently compensate for the impact of drought stress on gas exchange for P. euphratica.