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

Abstract

Variation in sapwood thickness (T_S) around the circumference of a stem and the presence of intermediate wood can cause errors in the estimation of sapwood area (A_S) and individual tree-scale transpiration (Q) based on the sap flow technique. We measured bark thickness (T_B), T_S, and intermediate wood thickness (T_I) in 16 orthogonal directions for wood discs from 57 Japanese cedar (Cryptomeria japonica) trees, and evaluated the impact of variation in T_S around the circumference of a stem and the validity of assuming a constant T_I for A_S estimates. The coefficient of variation of T_S in the 16 directions was 5.3–36.7%. A_S based on four orthogonal directions resulted in only minor errors for all trees, although this was not the case for A_S based on one and two directions. The mean T_I over the 16 directions was not significantly correlated to the diameter. If T_I 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 T_I when estimating A_S and Q for trees with intermediate wood having a similar color to sapwood.