• Partitioned nighttime transpiration & stem refilling using VPD & dendrometers. • Tracked seasonal shifts in transpiration & refilling ratios to nighttime sap flux. • Independently evaluated model with eddy covariance & leaf gas exchange measurements. • Proposed a new high-frequency baselining strategy for thermal dissipation probes. • Revealed & corrected errors in sap flux density baselining methods. Nighttime sap flux is important for understanding tree water-use dynamics in ecohydrological studies, as it represents the diurnal minimum of sap flow and often serves as the reference period for establishing thermal dissipation probe baselines. The baseline indicates simultaneous cessation of both nighttime transpiration and refilling of trees internal water stores. However, finding this moment is challenging as the baseline is dynamically changing, and the complete sap flux cessation may not even occur for boreal forests. Here, we proposed a novel framework using VPD and dendrometer measurements to partition transpiration and refilling from nighttime sap flux density, and introduced a night-by-night baselining scheme that does not rely on identifying the zero-flow moment. One conventional baselining method using the maximum probe voltage (or temperature) difference substantially underestimated sap flux density by 204.56% for nighttime, 28.30% for daytime, and 45.20% for full-day flux in P. sylvestris , and by 344.77%, 45.68%, and 54.76%, respectively, in A. glutinosa . Evaluation through leaf gas exchange and evapotranspiration data confirmed the accuracy of partitioning between nighttime transpiration and refilling, demonstrating that nighttime transpiration can drive measurable nighttime evapotranspiration responses. Seasonally, sap flux density showed an initial increase in both transpiration and refilling, followed by a decline, with transpiration dominating until mid-summer before refilling gradually took precedence.
Wang et al. (Fri,) studied this question.
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