Sap flow estimates of stand transpiration at two slope positions in a Japanese cedar forest watershed

We determined the amount of information needed to estimate watershed-scale transpiration in a Japanese cedar (Cryptomeria japonica D. Don) forest from sap flow measurements of individual trees. Measurements of tree biometrics (diameter at breast height (DBH) and tree sapwood area (A_{S_tree})), and tree-to-tree and radial variations in xylem sap flux density (F_d) were made in two stand plots, an upper slope plot (UP) and a lower slope plot (LP), during a growing season characterized by significant variations in environmental factors. We then investigated how mean stand sap flux density (J_S) and a tree stem allometric relationship (DBH-A_{S_tree}) varied between the stands. Appropriate sample sizes for estimating representative J_S values were determined. Both a unique and a general function allowed description of the allometric relationship along the slope, but the data for its formulation was required for both the UP and LP. Values of J_S in the UP and LP were similar during the study period despite differences in tree density and size between the plots, implying that J _S measured in a partial stand in a watershed is a reasonable estimate of J_S in other stands in the watershed, and that stand sapwood area calculated from A_{S_tree} is a strong determinant of water use in a forest watershed. To estimate J_S in both the UP and LP, it was necessary to sample at least 10 trees in each plot.