Sapwood and intermediate wood thickness variation in Japanese cedar: Impacts on sapwood area estimates

Variation in sapwood thickness (TS) around the circumference of a stem and the presence of intermediate wood can cause errors in the estimation of sapwood area (AS) and individual tree-scale transpiration (Q) based on the sap flow technique. We measured bark thickness (TB), TS, and intermediate wood thickness (TI) in 16 orthogonal directions for wood discs from 57 Japanese cedar (Cryptomeria japonica) trees, and evaluated the impact of variation in TS around the circumference of a stem and the validity of assuming a constant TI for AS estimates. The coefficient of variation of TS in the 16 directions was 5.3–36.7%. AS based on four orthogonal directions resulted in only minor errors for all trees, although this was not the case for AS based on one and two directions. The mean TI over the 16 directions was not significantly correlated to the diameter. If TI was assumed to be constant at the median value for our forest (=0.8 cm), the relative absolute errors for six of the 57 trees exceeded 30%. If these errors are unacceptable, we recommend extracting stem discs to measure TI when estimating AS and Q for trees with intermediate wood having a similar color to sapwood.