A Novel Crossed Hysteresis Response Pattern of Sap Flux to Solar Radiation

Abstract The hysteresis response of tree sap flux (SF) to its main driving factor of incoming short‐wave radiation (Rsi) has been widely reported, affecting the accuracy of sap flux and transpiration estimates in forest ecosystems. The diurnal cycle of SF usually lags the Rsi cycle by certain hours, thereby generating a closed counterclockwise hysteresis pattern. However, a few studies have reported that diurnal SF cycle may advance Rsi cycle, and such a response pattern has not been fully explored. In this study, we reported a rarely seen crossed hysteresis response pattern of SF to Rsi in 1/3 trees of a young temperate pine forest. We found that the diurnal SF cycle advances Rsi cycle especially in the morning induced by the early stomatal closure, thereby generating the crossed hysteresis response of SF to Rsi. We also proposed a method to quantify the magnitude of hysteresis (A hys ) for both the crossed and closed hystereses. Our analysis suggests that a lower A hys of two time series results in (a) a larger crossing degree of hysteresis, and (b) a stronger linear correlation between the two time series. The seasonal variation of soil water content can explain the variation in A hys for the hysteresis response of SF to Rsi, and the crossed hysteresis of SF is more likely to occur under water stress conditions. This study contributes to advancing our understanding of forest transpiration and how forests may respond to drought stress, which are expected to become more frequent and longer under future climate change. Plain Language Summary Sap flux (SF) is widely used as a surrogate of transpiration in field studies. It is broadly reported that the diurnal cycle of SF lags its driving force—incoming short‐wave radiation (Rsi). Correspondingly, a closed counterclockwise loop appears when plotting SF against Rsi on a diurnal basis. Such phenomenon is widely identified as a hysteresis response of SF to Rsi. In our study, we reported that the diurnal SF cycle can advance the Rsi cycle especially in the morning, with a crossed SF‐Rsi hysteresis loop emerging. The crossed hysteresis is more complex in terms of the direction and pattern than the closed hysteresis loop; we further found the crossed hysteresis pattern contributes to reducing the magnitude of hysteresis and improving the correlation between SF and Rsi. We developed a method to quantify the magnitude for both the crossed and closed hystereses. We found that the closure of stomatal conductance drives the early decline of the diurnal SF cycle, resulting in the crossed hysteresis. We also found that soil water content plays an important role in controlling the seasonal variation of hysteresis and the crossed hysteresis is more likely to emerge in water stress conditions. Key Points A novel crossed hysteresis response of sap flux to solar radiation is reported and investigated The early stomatal closure drives the variation of the hysteresis pattern of forest transpiration Soil water stress induces the seasonal variation of the sap flux‐radiation hysteresis