Influences of shallow groundwater on sap flow of riparian poplar plantations in northern China.

: Water use strategy in growing seasons plays a great role in the management of riparian poplar plantations in northern China. The research was conducted on a poplar (Populus × euramericana cv. '74/76') plantation located near the floodplain of Chaobai River in the northeastern su-burb of Beijing. The influencing factors of poplar sap flow (SF) were presented based on the measurements on the stem SF with thermal dissipation sap flow probe (TDP), soil volumetric water content (VWC) with time-domain-reflectometer (TDR) and environmental factors with eddy cova-riance system from June to July, 2014, to explore the impact of shallow ground water on SF. The results showed that the diurnal variation of SF presented single or double peaks depending on the varia-tion of solar radiation (R). The SF density decreased in soil water relative deficient period (RDP), and the peak time advanced to 12:30 from 14:00, while the time lag between the SF density and R showed no obvious change. The solar radiation and air vapor pressure deficit were the predominant factors of SF during soil water relative sufficient period (RSP), while soil water became the limiting factor in RDP. During the RDP, the daily water consumption of individual poplar tree was significantly negatively correlated with soil VWC at the depth of 100 cm, and significantly positively correlated with soil VWC of the other depths. The shallow ground water (at the depth of ≥150 cm where the soil water content showed little change) might move to the upper soil layer controlled by the capillary force and supply for poplar growth during the soil water-relatively deficient period.