Water-use strategies of coexisting shrub species in the Yellow River Delta, China

In coastal ecosystems, water availability is limited because of the high soil salinity influenced by sea water intrusion, high soil noncapillary porosity, and significant seasonal fluctuation of precipitation. Therefore, water availability is a key determinant of plant growth and distribution in coastal ecosystems. Tamarix chinensis Lour. and Ziziphus jujuba var. spinosa Hu are two coexisting shrub species growing on Chenier Island in the Yellow River Delta (YRD), China. Our aim was to investigate how the water-use strategies of the two species respond to variations in soil moisture to improve understand of their adaptations to drought stress and their coexistence mechanism. During the growing season, the oxygen stable isotope signatures (delta O-18) were measured for soil water in different soil depths (0-20, 20-40, 40-60, and 60-100 an), shallow groundwater, and xylem water. The proportional contributions of potential water sources for the two species were calculated by using the IsoSource mixing model. The results showed that the delta O-18 values of the two species showed a clear seasonal difference. When soil moisture was high and air temperature was low, T. chinensis mainly used water from soil depths of 60-100 cm, while Z. jujuba mainly used water from soil depths of 0-40 cm. When soil moisture was low or air temperature was high, T. chinensis mainly used the saline shallow groundwater, while Z. jujuba mainly used water from soil depths of 20-100 cm. When there was a large amount of precipitation, both T. chinensis and Z. jujube mainly absorbed water from soil depths of 20-40 cm. Tamarix chinensis and Z. jujuba had different water-use patterns during the growing season, which reflected their adaptations to seasonal fluctuations in soil water content within a water-limited coastal ecosystem, while the niche differentiation in water use of the two species clarifies their coexistence mechanism.