Stand transpiration and canopy conductance dynamics of Populus popularis under varying water availability in an arid area

As a tree species of shelterbelts, Populus popularis maintains significant ecological functions in arid and semiarid areas. However, stand transpiration (T) and canopy conductance (g_c) dynamics of P. popularis are unclear in arid irrigated areas with shallow groundwater fluctuations. To better understand the responses of T and g_c to meteorological factors, soil water, and shallow groundwater in arid areas, we observed the environmental conditions and sap flow of P. popularis, and quantified T and g_c in three growing seasons of 2018-2020 in a typical arid area of China. Results showed T and g_c ranged from 0.18 to 6.11 mm day^-1 and 2.26-12.54 mm s^-1 in 2018-2020, respectively. Solar radiation and vapor pressure deficit (VPD) were major drivers of T at daily scales. It was consistently found that T exponentially decreased with increasing groundwater table depth (GTD) and decreasing reference evapotranspiration in three years. g_c is primarily influenced by VPD and is positively related to soil water content in 0-30 cm soil layer (SWC_{0-30 cm}). Moreover, low SWC_{0-30 cm} and deepening GTD jointly decreased T and g_c by 22.45 % and 30.41 %, respectively. The response of g_c to VPD was susceptible to groundwater fluctuations, and the synergistic influences of VPD and GTD on g_c could be well described by the logarithmic function, especially in 2019. The sensitivity of g_c to VPD and its variations under different environmental conditions suggested that a flexible stomatal regulation of transpiration occurred in the observed P. popularis with the arid climate and shallow groundwater. These findings provided the essential basis for the water use strategy of P. popularis and stand water resources management in arid regions.