Hydrological niche segregation and water-use strategies of coexisting plant species evidenced by water-stable isotopes in a hyper-arid endorheic basin

Better understanding of the mechanisms of plant species' coexistence and close interactions between vegetation and groundwater in drylands is indispensable for predicting the potential consequences of environmental changes. These interrelationships, however, remain an understudied topic. In this study, 262 sample plots along water table depth (WTD) gradients (0 to 5 m, 5 to 10 m, 10 to 20 m, and > 20 m) were investigated in a hyper-arid endorheic basin (the Shule River Basin, SRB) in northwestern China. Widespread coexistence of plant species, as well diverse compositions of plant communities, were observed. Using the MixSIAR and nicheROVER models based on water-stable isotope (WSI) data, dynamic hydrological niche segregation (HNS) and diverse water-use strategies were detected among coexisting plant species during both high-water-level (mean WTD = 4 m) and low-water-level (mean WTD = 4.3 m) periods. Five plant species that co-occurred most frequently (shrubs Nitraria tangutorum, Lycium ruthenicum, Tamarix ramosissima; a semi-shrub, Alhagi camelorum; and an herb: Phragmites communis), as well one species of tree (Populus euphratica) were analyzed. Overall, groundwater contributed 9–71 % (31 % on average) and 17–75 % (53 % on average) to the water use of the plant species, and the mean probabilistic niche overlap metrics among the six plant species were 52 % and 68 % during the high-water-level and low-water-level periods, respectively. These plants showed adaptive responses to WTD variations, correlated with diverse root plasticities and hydrological niche widths, demonstrating diverse patterns of HNS and water-use strategies—characteristics that enable their coexistence in drylands. Species-specific adaptive management that considers the HNS and water-use strategies of prioritized plant species, and better reflects their varied susceptibility to WTD fluctuations—especially during low-water-level periods—could therefore help in developing sustainable management strategies for groundwater and for the plant species it supports in drylands.