Impacts of temporal discrepancies between basal sap flow and canopy transpiration on estimating canopy ozone uptake...

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.