Nocturnal sap flow is mainly caused by stem refilling rather than nocturnal transpiration for Acer truncatum in urban environment

Nocturnal sap flow is an important part of plant sap flow, while it is usually regarded as negligible. Consequently, the aim of this study was to measure nocturnal stem sap flow of Acer truncatum and to determine its driving force in an urban green space. Stomatal aperture was measured using surface impression method in a typical clear day of August. Sap flux density was monitored by a thermal diffusive probe system from July to October. Meanwhile, wind speed (WS), relative humidity, air temperature, precipitation and total solar radiation were monitored by an automatic weather station. We found nocturnal stomatal aperture was about 32% of midmorning, and nocturnal sap flux density was significantly and positively affected by WS and vapor pressure deficit (VPD) (P < 0.001), while nocturnal sap flow volume was significantly and positively related to diurnal sap flow volume (R-2 = 0.30, P < 0.001). The explanation of VPD and WS to nocturnal sap flux density was less than that of diurnal sap flow. The relationship between nocturnal sap flux density and VPD separated into two distinct models. When VPD >= 0.10 kPa, nocturnal sap flux density was significantly and positively correlated with VPD (R-2 = 0.53, P < 0.001), while there was a significant negative correlation between sap flux density and VPD when VPD < 0.10 kPa (R-2 = 0.18, P < 0.001). The variation of sap flux density showed different patterns in different weather conditions. During a clear night, sap flux density showed a decreasing trend with VPD and WS, while sap flux density only showed a similar trend to precipitation during rainfall. We suggest nocturnal sap flow of Acer truncatum is regulated by nocturnal transpiration and stem refilling, and stem refilling is the fundamental force while transpiration promotes sap flow, and the volume is restricted by soil moisture, stem storage water volume, and the water status of the previous day.