Variation in sap flux density and its effect on stand transpiration estimates of Korean pine stands

Accurate estimates of stand transpiration (E) require the consideration of three types of variation in sap flux density (J (S)): radial, azimuthal, and tree-to-tree variation. In this study, the J (S) variation of 50-year-old Korean pine (Pinus koraiensis) trees and its effect on E estimates was evaluated using Granier-type heat dissipation sensors. The value of J (S) decreased exponentially with the radial depth from cambium to pith, and the coefficient of variation (CV) for radial variation was 124.3 %. Regarding the azimuthal variation, the value of J (S) differed significantly among aspects and the average CV was 23.6 %. The average CV for tree-to-tree variation was 34.0 %, and the daily CV increased with increasing vapor pressure deficit (D). The error in the E estimates caused by ignoring the radial variation was the largest (109.2 %), followed by those caused by ignoring the tree-to-tree and azimuthal variations (24.3 and 12.6 %, respectively). While the contribution of the azimuthal variation to the E estimates was minimal in comparison to the other variations, the azimuthal variation among aspects was significant, and the usage of the north aspect measurement did not generate substantial error in the E estimates (0.6 %). Our results suggest that the variation, particularly the species- and site-specific radial variation, must be considered when accurately calculating E estimates.