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

摘要

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.