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Extensive research has found that nighttime transpiration (E (n)) is positively correlated to the vapour pressure deficit (VPD), that suggested E (n) was highest during the night under high temperatures and low humidity along with high soil water availability, typically for the riparian forest in the extreme arid region of China. This study used the heat ratio method to measure sap velocity (V (s)) for mature and saplings Populus euphratica Oliv., and then E (n) was conservatively calculated as total nocturnal sap flow (F (s), the product of V (s) and sapwood area A (s)) between 01:00 to 06:00. A gas exchange system was used to measure the leaf transpiration rate (T (r)) and stomatal conductance (g (s)) of saplings. For mature trees, nighttime V (s) was extensive and logarithmic correlated to VPD (similar to daytime). For saplings, g (s) and T (r) was extensive in different months, and also a strong logarithmic relationship was found between V (s) and VPD for both daytime and nighttime periods. Both of stem sap flow and leaf gas exchange suggusted the occurrence of E (n), whether mature or sapling trees. E (n) contribution to daily transpiration (E (d)) was high just as expected for P. euphratica, which was confirmed by proportional E (n) to E (d) (E (n)/E (d)) means taken in 2012 (24.99%) and 2013 (34.08%). Compared to mature trees, E (n)/E (d) of saplings in 2013 was lower with means of 12.06%, that supported further by the shorter duration times and less T (r,n) (16.64%) and g (s,n) (26.45%) of leaf, suggesting that E (n) magnitude is associated to individual the tree size, that effect to stored water of individual trees, although this hypothesis requires further research.