Sources of error in estimating stand transpiration using allometric relationships between stem diameter and sapwood area for Cryptomeria japonica and Chamaecyparis obtusa

Estimations of stand sapwood area (AS_stand) are among the most critical factors for determining stand transpiration (E) using sap flux measurements. AS_stand is generally estimated using stem diameter (DBH) data measured for all trees in a stand and an allometric equation that describes the relationship between DBH and sapwood area (AS_tree) based on available allometric data. To examine the errors in E that are generated when using the allometric equation with a small sample size, the Monte Carlo sampling technique was used with an original allometric data set. In this study, all trees (921 Cryptomeria japonica and 1226 Chamaecyparis obtusa) in the studied stands provided the original allometric data (DBH and AS_tree). Monte Carlo simulations allow the number of sample trees required for an appropriate E estimate to be derived. The results revealed that potential errors in the E estimate were almost stable with a sample size of over 20 for both Cr. japonica and Ch. obtusa. This implies that at least, but not more than, 20 trees are necessary for the allometric equation when estimating E for both species.