Stomatal ozone uptake of a Quercus serrata stand based on sap flow measurements with calibrated thermal dissipation sensors

The amount of ozone absorbed by the tree leaves is a critical factor determining the ozone effects on forest trees. Stomatal ozone uptake of a forest canopy can be estimated from the ozone concentration and canopy conductance (gc) determined by the sap-flow-based method. This method measures sap flow as a metric of crown transpiration and then derives gc. The thermal dissipation method (TDM) has been used to measure sap flow in most studies adopting this approach. However, recent studies have indicated that TDM may underestimate sap flow, especially in ring-porous tree species. In the present study, the accumulated stomatal ozone uptake (AFST) of a stand of Quercus serrata, a typical ring-porous tree species in Japan, was estimated by measuring sap flow using species-specific calibrated TDM sensors. Laboratory calibration of the TDM sensors revealed that the parameters (α and β) in an equation converting outputs from the sensors (K) to sap flux density (Fd) were substantially larger for Q. serrata than those originally proposed by Granier (1987). The Fd measured in the Q. serrata stand using calibrated TDM sensors were significantly larger than those obtained using non-calibrated sensors. The diurnal average of gc and daytime AFST (10.4 mm s−1 and 10.96 mmol O3 m−2 month−1) of the Q. serrata stand estimated by using calibrated TDM sensors in August 2020 were similar to those of forests dominated by Quercus species estimated by micrometeorological measurements in previous studies. In contrast, the gc and daytime AFST of the Q. serrata stand estimated by non-calibrated TDM sensors were remarkably lower than those estimated by micrometeorological measurements in previous studies, indicating severe underestimation. Therefore, it is strongly recommended that sap flow sensors are species-specifically calibrated when estimating the canopy conductance and ozone uptake of forests dominated by ring-porous trees based on sap flow measurements using TDM.