Effects of tree-to-tree and radial variations on sap flow estimates of transpiration in Japanese cedar

Abstract

To scale sap flux density ( F d) measurements of individual trees to stand level transpiration ( E), the mean stand F d ( J S) is among the most important factors, because E is frequently estimated as a product of J S and stand sapwood area. Major uncertainties in J S estimations are caused by tree-to-tree and radial variations in F d. Thus, by using the Monte Carlo sampling technique with original tree-to-tree F d data measured at several depths, we examined how many trees and radial depths need to be sampled for calculation of appropriate J S. In this study, F d was measured in 14 mature Japanese cedar ( Cryptomeria japonica) trees at several radial depths, 0–2, 2–4 and 4–6 cm, using Granier-type sensors. The results revealed that the potential error in J S with the largest available sample size was not stable even though radial profiles in F d for all sample trees were measured. They also showed that to consider radial variation in F d it might be necessary to sample at least six trees, but not necessarily more than this. The results imply that, for E estimations, there is more tree-to-tree variation in F d than in radial variation in F d.