Abstract: In this study, a mature riparian Populus euphratica forest in Ejina Banner in the lower reaches of the Heihe River was selected to analyze the relationship between the annual tree ring width series of P. euphratica with different ages and the environmental factors on different time scales. The results showed that: ①On the annual scale, temperature was the main factor affecting the radial growth of P. euphratica, but the response of P. euphratica with different ages to temperature was different; ②On the seasonal scale, the higher the temperature during the dormancy was, the better the radial growth of adult P. euphratica would be, however, the higher the temperature in growing season was, the worse the radial growth of old P. euphratica would be; ③On the monthly scale, the higher the average temperature in March of previous year and in September every year was, the more favorable the conditions to the radial growth of adult P. euphratica would be; the higher the average temperature in October every year, the more disadvantageous the conditions to the radial growth of old P. euphratica would be. The increase of runoff volume in February every year was beneficial to the radial growth of adult P. euphratica, while that in July of previous year was beneficial to the radial growth of old P. euphratica. In summary, on relatively long time scales, such as seasonal and annual scales, temperature was the main factor affecting the radial growth of P. euphratica. With the contraction of time scale, the response of P. euphratica to environmental factors became more detailed, and a certain hysteresis occurred. As an important environmental factor, runoff volume was prominent just on the monthly scale, the runoff recharge in July of previous year and February every year was very important for the radial growth of P. euphratica. Therefore, In the process of unified dispatching and management of the Heihe River, an appropriate flood discharge in February and July could meet the water demand of the radial growth of P. euphratica.

Key words: tree age, tree ring width, time scale, temperature; runoff volume, Ejina Banner