Impacts of temporal discrepancies between basal sap flow and canopy transpiration on estimating canopy ozone uptake in a warm temperate coniferous forest

Sap flow measured at basal trunk ( F d_base ) has been used to estimate canopy transpiration and ozone uptake in forests. Although diurnal patterns of F d_base do not always correspond to those of canopy transpiration, little is known about the impact of such discrepancy on the estimation of canopy ozone uptake. We investigated the discrepancy in diurnal patterns between F d_base and canopy transpiration, and its impact on estimating canopy ozone uptake in a Cryptomeria japonica stand. The F d_base and sap flow near the top of the trunk ( F d_top , a proxy for canopy transpiration rate) were simultaneously measured. The F d_top increased prior to the F d_base in the morning and declined almost at the same time with F d_base in the evening. Then the diurnal patterns of F d_base were corrected to match those of F d_top by two methods: one was the conventional ‘shifting correction’ ( F d_shifted ) which shifts the F d_base by the time lag between F d_top and F d_base , the other was the novel ‘tracing correction’ ( F d_traced ) which traces the diurnal patterns of F d_top . These corrections for F d_base did not affect the estimation of daily accumulated ozone uptake of the stand. However, canopy conductance ( g c ) derived from F d_base or F d_shifted was lower than that derived from F d_traced in the morning. Consequently, the hourly g c derived from F d_traced significantly responded to vapour pressure deficit, while that derived from F d_base or F d_shifted did not. We concluded that temporal discrepancies between F d_base and canopy transpiration should be carefully considered toward a more accurate modelling of g c and canopy ozone uptake.