Sap flow characteristics and their response to environmental variables in a desert riparian forest along lower Heihe River Basin, Northwest China

Hysteresis, related to tree sap flow and associated environmental variables, plays a critical ecological role in the comprehensive understanding of forest water use dynamics. Nevertheless, only limited researches related to this unique ecological phenomenon have been conducted to date in desert riparian forests under extreme arid regions. Populus euphratica Oliv sap flow velocity (V-S) was measured during the 2012 growing season using the heat ratio method, at the same time as environmental variables, such as photosynthetically active radiation (PAR), vapor pressure deficit (VPD), and leaf water potential. We found clockwise patterns of hysteresis between V-S and VPD but anticlockwise patterns between V-S and PAR. Pronounced hysteretic V-S lag time, a function of PAR and VPD, was approximately 1.0 similar to 1.5 and -0.5 h, respectively. Hysteresis was primarily caused by the biophysical declining in canopy conductance. Sigmoid response of V-S to synthetic meteorological variables was enhanced by approximately 56 % after hysteresis calibration to sunny days. Consequently, hysteresis can be seen as a protection mechanism for plants to avoid the overlapping of peak V-S and environmental variables. Furthermore, the consistent presence of hysteresis suggested that estimating of plant water use in large temporal and spatial models may require certain provisions to different V-S responses to variables between morning and afternoon and between seasons.