Differences in diurnal pattern of transpiration between young sugi trees (Cryptomeria japonica) located at the ridge and the bottom of a slope

On a mountainous slope, sap flow velocities in sugi trees (Cryptomeria japonica) located at the ridge and the bottom of the slope were measured using heat pulse sensors. Meteorological factors were also observed at the ridge and the bottom of the slope. Time lag in sap flow velocities was observed between trees at the ridge and the bottom of the slope when transpiration began to occur after sunrise or rainfall. Usually, transpiration occurred earlier at the ridge of the slope than at the bottom. The average value of the time lag was 58.5 min. Additionally, differences in meteorological factors between the ridge and bottom of the slope were observed. The average and variation of the time lag were larger after sunrise than when rain ceased in afternoon. The time lag was highly correlated with differences in both vapor pressure deficit and shortwave radiation between the ridge and the bottom of the slope. Thus, we suspected that differences in both vapor pressure deficit and shortwave radiation between the ridge and the bottom of the slope caused differences in the time lag after sunrise.