"Natural" flooding events are increasing in China as a consequence of climate change. Poplar plantations are widely established in northern China and there are large areas located in easily flooded lowlands. To investigate the response of poplar plantations' water use to flooding, we measured xylem sap flow in irrigated and non-irrigated poplar plantations, covering both before flooding and during flood periods in the growing season. Stem xylem sap flow, soil moisture, and meteorological variables were measured using thermal dissipation sensors, time-domain reflectometry, and a weather station, respectively. The results showed that the daily courses of sap flux density (SFD) followed the patterns of solar radiation (R (s)), vapor pressure deficit (VPD) and reference evapotranspiration (ET0). Under similar meteorological conditions expressed by the same daily values of R (s) or VPD, the daily averages of SFD were lower in the flooding period than those in the before flooding period, and the percent attenuation of SFD caused by flooding was lower in irrigated trees than non-irrigated trees. In addition, the nocturnal SFD for both irrigated and non-irrigated trees were higher in the flooding period than those in the before flooding period, and the difference in trees with irrigating management was significant. It is concluded that: (1) SFD in poplar was reduced by flooding and that the effect was lower in irrigated trees than in non-irrigated trees; and (2) nocturnal SFD increased in response to flooding and the increases were significant in irrigated trees, suggesting that irrigation decreases the vulnerability of poplar plantations to flooding.
