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 f₁ (S) = [(1000 + 30)S]/[1000(S + 30)], f₂(D) = exp(-0.54D)/exp (-0.54), and f₃(T) = {[(30 - 5) + 14](T - 5)}/1{(30 - 5) [(T - 5) + 14]}, where f₁(S), f₂(D), and f₃(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 f₃(T) to reproduce the 1-year transpiration at the site, and that f₁(S) and f₂(D) determined here were not very different from the general functions. This paper is the first to determine G_c responses to meteorological fa ctors 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.