Transpiration from a Cryptomeria japonica plantation, part 1: Aerodynamic control of transpiration

To our knowledge, no one has examined the canopy conductance of a Cryptomeria japonica plantation, although such forests are very common in Japan. We plan to examine the canopy conductance of a Cryptomeria japonica plantation (canopy height 8 m) based on 1-year sap-flow data using the simplified Penman-Monteith equation. This study examines the effect of the aerodynamic control of transpiration before calculating canopy conductance using the simplified Penman-Monteith equation, because the equation assumes that the effect of aerodynamic control is negligible. To examine the effect of aerodynamic control, we examined the dependency of the relationship between the vapour pressure deficit (VPD) and transpiration on wind speed, when solar radiation, air temperature, and soil water content do not limit transpiration greatly. The relationship between VPD and the heat pulse velocity measured 11 m above the ground was dependent on wind speed only when the wind speed was <0·7 m s^-1. This suggests that the effect of aerodynamic control was negligible when the wind speed was ≥0·7 m s^-1, but that the effect was not negligible when the wind speed was <0.7 m s^-1. Therefore, we should use data from when the wind speed was ≥0·7 m s^-1 for calculating canopy conductance G_c, based on the simplified Penman-Monteith equation. Although this criterion is site specific, the methods used to develop it are applicable to other sites.