Abstract: Stem sap flow is an important basis to characterize the transpiration of trees. Studying the dynamic characteristics of sap flow and the driving effects of environmental factors on it is of great significance to explore the physiological characteristics and water dynamic changes of trees. Populus×euramericana cv.‘74/76’ is one of the main fast-growing tree species widely cultivated and also a water-fertilizer-sensitive tree species. The amount of water directly affects its growth. Populus×euramericana cv.‘74/76’ in Luannan county forest farm in Tangshan city was taken as research object, the drip irrigation method was used to set the soil volumetric water content at 15%−20%, 10%−15% and 5%−10% respectively by controlling the irrigation time, the soil volumetric water content was respectively set as 15%−20%, 10%−15% and 5%−10% by drip irrigation, the sapwood flow rate of Populus×euramericana cv.‘74/76’ was measured by the thermal dissipation probe(TDP), and the solar radiation, air temperature, atmospheric humidity, atmospheric pressure, wind speed and other meteorological factors were simultaneously monitored by using the HOBO automatic weather monitoring station to explore the change rules of sap flow during day and night under different environmental conditions, establish a regression model between meteorological factors and the sap flow, analyze the water utilization of trees in different environments in order to provide guidance for the formulation of reasonable water irrigation strategies for fast-growing and high-yield plantation. Tharted earlier, peaked higher, lasted longer, and fluctuated more at night than cloudy and rainy de results showed that the sap flow was driven by different weather conditions, it on sunny days stays; The sap flow was also driven by soil moisture, under the condition of soil water shortage, the start time was late, the peak was low, the duration was short, and it changed steadily and increasingly at night. After time lag analysis, it is found that there is a "time lag effect" between the sap flow and meteorological factors, the sap flow lagged behind the solar radiation for 40 min, and was ahead of temperature, relative humidity and VPD (vapor pressure deficit) for 60min. There was a "hysteresis loop" relationship between sap flow and VPD, meteorological factors all had a significantly driving effect on the sap flow after the time lag dislocation, it had the greatest correlation with air temperature and stronger driving force during the day, while had the greatest correlation with VPD and stronger driving force at night. After time lag dislocation, the imitative effect of sap flow and meteorological factors was better than before. It can be seen that the environmental factors had an obvious driving effect on the sap flow, especially weather types, meteorological factors, and soil moisture, the regression model established between the weather factors and the sap flow after the time lag can better explain the relationship between the two mathematical relationship, meanwhile, sap flow can better reflect the water condition of sandy soil and the water consumption characteristics of trees, which provided a strong basis for the precise irrigation strategy of poplar fast-growing and high-yielding forests.

Key words: Sap flow, Thermal dissipation probe, Sandy land in eastern Hebei, Populus×euramericana cv.‘74/76’, Environmental factors