Transpiration from a Cryptomeria japonica plantation, part 2: responses of canopy conductance to meteorological factors

To our knowledge, there is no information on the canopy conductance of Cryptomeria japonica plantations, although such forests are very common in Japan. We examined the canopy conductance G c of a C. japonica plantation based on 1‐year sap‐flow data using the simplified Penman–Monteith equation. After selecting data based on the criterion proposed in part 1 of this study (Komatsu et al. , 2006. Hydrological Processes 20 : this issue), we now examine G c responses to meteorological factors. G c was expressed using functions of solar radiation S (W m −2 ), the vapour pressure deficit D (kPa), and air temperature T ( °C). The resulting functions were f 1 ( S ) = [(1000 + 30) S ]/[1000( S + 30)], f 2 ( D ) = exp(−0·54 D )/exp(−0·54), and f 3 ( T ) = {[(30 − 5) + 14]( T − 5)}/{(30 − 5)[( T − 5) + 14]}, where f 1 ( S ), f 2 ( D ), and f 3 ( T ) are functions expressing the G c responses to S , D , and T respectively. The functions were characterized by comparing general functions for coniferous forests, which showed that it was necessary to consider f 3 ( T ) to reproduce the 1‐year transpiration at the site, and that f 1 ( S ) and f 2 ( D ) determined here were not very different from the general functions. This paper is the first to determine G c responses to meteorological factors for a C. japonica plantation. When predicting transpiration from a C. japonica plantation using hydrological or ecological models, the functions determined here should be used instead of the default functions of the models.