Use of 2H and 18O stable isotopes to investigate water sources for different ages of Populus euphratica along the lower Heihe River

Investigation of the water sources used by trees of different ages is essential to formulate a conservation strategy for the riparian tree, P. euphratica. This study addressed the contributions of different potential water sources to P. euphratica based on levels of stable oxygen and hydrogen isotopes (delta O-18, delta H-2) in the xylem of different aged P. euphratica, as well as in soil water and groundwater along the lower Heihe River. We found significant differences in delta O-18 values in the xylem of different aged P. euphratica. Specifically, the delta O-18 values of young, mature and over-mature forests were -5.368(+/- 0.252) parts per thousand, -6.033(+/- 0.185) parts per thousand and -6.924 (+/- 0.166) parts per thousand, respectively, reflecting the reliance of older trees on deeper sources of water with a delta O-18 value closer to that of groundwater. Different aged P. euphratica used different water sources, with young forests rarely using groundwater (mean < 15 %) and instead primarily relying on soil water from a depth of 0-50 cm (mean > 45 %), and mature and over-mature forests using water from deeper than 100 cm derived primarily from groundwater.