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Over the last several decades, riparian forests in Northwest China have been extremely vulnerable to the increasing pressures from human activity and climate change. However, it is unclear how forests correlate with these influences over time and on a wide scale. In the present study, we developed 28 tree-ring chronologies of Populus euphratica in the Ejina Oasis, in the lower reaches of Northwestern China’s Heihe River, to evaluate the oasisscale riparian forest growth variation and its correlation with the runoff regulation. We identified three hydrologic periods using regime-shift analysis: a natural runoff period from 1954 to 1989, when the oasis was sustained by natural river flows; a degradation period from 1990 to 2002, when excessive upstream withdrawals of runoff decreased flows to the oasis; and a recovery period, from 2003 to 2010, when water allocation projects restored flows. In general, stem radial growth of poplar trees in the oasis showed decreasing and increasing trends in the degradation period and recovery period, respectively, with high spatial heterogeneity in each period. Our analysis of the relationship between the chronologies and corresponding hydrologic and climatic data revealed that the runoff-recharged hyporheic groundwater depth was the major factor that limited tree stem radial growth. Thus, it is important to allocate water between the middle and lower reaches, and inside the lower reaches, to mitigate or prevent the adverse effects of low runoff. Our study also demonstrates that researchers must carefully consider the high spatial heterogeneity during dendrochronological research in an arid riparian forest.