Differences in sap flux-based stand transpiration between upper and lower slope positions in a Japanese cypress plantation watershed

This study investigated the difference in stand transpiration (E) between upper (UP) and lower (LP) slope plots of a Japanese cypress (Chamaecyparis obtusa) watershed to obtain catchment-scale E estimates. Sap flux-based stand-scale E estimates were conducted on the UP and LP, which had contrasting stand structures (diameter at breast height and tree height) despite being the same age. The plant area index was similar in the UP and LP, while the total sapwood area (A_{S_stand}) and the mean sap flux density (J_S) in the UP were 23% and ~30% lower, respectively, than those of the LP. Stand-scale E is the product of the A_{S_stand} and J_S in a stand. As a result, E in the UP was ~50% lower than that in the LP. This ratio was relatively constant throughout the year, which was supported by a similar decline in E, which resulted from soil water decline in the UP and LP. Canopy conductance (G_C) in the UP was ~50% smaller than that of the LP. In contrast to previous studies that showed a consistent J_S along a slope, the differences in E between the UP and LP were caused by differences in A_{S_stand} and J_S, probably because the UP exhibited a lower G_C per unit leaf area than the LP. Additionally, evapotranspiration (ET), the sum of sap flux-scaled E plus rainfall interception, was 8–14% lower than the catchment water balance ET (precipitation minus runoff), which suggests the feasibility of calculating sap flux-scaled E for the two slope positions.