Hysteretic response of sap flow in Scots pine (Pinus sylvestris) to meteorological forcing in a humid low-energy headwater catchment

Transpiration mediated by physiology is driven by the meteorological variables. Understanding their relationships is crucial to advancing terrestrial ecohydrological studies. Here, we used sap flow measurements in Scots pine to examine the temporal responses of transpiration to temperature (Ta), vapour pressure deficit (VPD), and net radiation (Rn). Seasonal variations and the hysteresis between sap flux density (Js) and meteorological variables were investigated. The results show that VPD and Rn were dominant factors influencing hourly Js. Daily peaks of Js lagged behind peaks of Ta, VPD, and Rn, by ~2-4 hr. Anticlockwise hysteresis loops between Js and Ta and VPD were observed, and the size of loops change with prevailing weather conditions (i.e., rainy or rainless) and seasons. Longer time lags were found in April and September than in other months. The hysteresis is partly related to the low energy, humid nature of the site which resulted in limited moisture stress to the trees, and thus the trees did not necessarily need a conservation mechanism to restrict water loss as proposed by other studies in semi-arid areas. Release and refill of trunk water storage could plausibly contribute to the hysteresis, although the effect may be entwined with atmospheric demand making it difficult to separate without sophisticated measurements. This study highlights the diversity in plant water relations across temporal scales and seasons and calls for more work across geographic locations and climates.