Transpiration process and environmental response of poplar plantation under different irrigation conditions

Abstract

Aims Understanding the relationship between transpiration and environmental factors is critical to the establishment of efficient irrigation strategies for Populus tomentosa. Therefore, we studied transpiration and environmental responses of P. tomentosa under varied irrigation treatments. Methods This study was carried out at the triploidP. tomentosaplantation in Gaotang County, Shandong Province. The irrigation treatments were set as full irrigation (DF, irrigated when the soil water potential at 20 cm directly below the dripper reached -18 kPa), water-controlled irrigation (DC, irrigated when the soil water potential at 20 cm directly below the dripper reached -45 kPa) and no irrigation (CK, blank control). Thermal dissipation probes (TDP) were used to observe the continuous sap flow ofP. tomentosa during the growing season of 2019. The environmental factors were simultaneously monitored, including solar radiation (SR), air temperature (Ta), soil moisture content (SWC) and wind speed (WS). Important findings 1) The trend of the diurnal sap flow variation of P. tomentosa was similar among the contrasting treatments, showing a single-peak curve, and there was significant sap flow at night. Irrigation treatments did not influence the start of stem sap flow but caused differences in the timing of the stem peak flow. The peak flow arrived earlier in the DF treatment than in other treatments. Irrigation increased the transpiration ofP. tomentosa. However, the transpiration would increase with the decrease of soil moisture deficit threshold of irrigation, namely transpiration of DC followed by DF and CK in sequence. 2) There was significant time-lag between the sap flow and SR and VPD. The sap flow was ahead of the VPD and lagged behind the SR. Irrigation treatments have no significant effect on the time-lag effect between the sap flow and environmental factors. 3) Under the three treatments, transpiration has a significantly positive correlation with VPD, SR and SWC, and negatively correlated with wind speed. The transpiration responses to the environmental factors varied among irrigation treatments. In summary, irrigation could effectively mediate the hydraulic adaptation of plantation trees to the environment factors. However, increased irrigation does not necessarily lead to transpiration enhancement. Based on the differences of stand transpiration and natural rainfall, irrigation during the beginning of the growing season (April to July in this study) would benefit the hydraulic physiological activity of theP. tomentosa plantation.