Quantifying the Radial Profile of Stem Sap Flux Density in a Cool Temperate Forest of Japan: Tree-To-Tree Variability Outweighs Seasonal and Species-Specific Effects

Abstract

Although quantifying the radial profile of stem sap flux density (SFD) is crucial for accurately estimating tree and stand transpiration, limited information exists regarding species-specific and seasonal variations in SFD profiles, particularly in humid forests. To address this knowledge gap, we measured SFD at four specific depths across different tree species (Acer nipponicum, Acer shirasawanum, Betula grossa, and Fagus crenata) over the growing seasons in a cool temperate forest in Japan. Our findings revealed divergent patterns in radial profiles of SFD among trees, with no distinct species-specific profiles due to significant tree-to-tree variation. Environmental responses of SFD were uniform during the full-leaf period (June–September) regardless of species and individuals. However, significant seasonal variations in the radial profile were observed in F. crenata, where post-summer declines in outermost layers exceeded those in inner layers. Ignoring these seasonal variations resulted in a mean error of 7.4 ± 3.3 (standard deviation) % for F. crenata and 3.5% ± 2.0% for the other species in annual transpiration estimates, whereas neglecting tree-to-tree variability caused up to errors of 29.0% ± 13.0%. Therefore, considering seasonal variations in the radial profiles of SFD is important for accurate transpiration estimation, but accounting for tree-to-tree variations is even more critical.