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

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.