Effects of sample size on sap flux-based stand-scale transpiration estimates

Abstract

In this study, we aimed to assess how sample sizes affect confidence of stand-scale transpiration (E) estimates calculated from sap flux (Fd) and sapwood area (AStree) measurements of individual trees. In a Japanese cypress plantation, we measured Fd and AS-tree in all trees (n = 58) within a 20 × 20 m study plot, which was divided into four 10 × 10 subplots. We calculated E from stand AS-tree (A S-stand) and mean stand Fd (JS) values. Using Monte Carlo analyses, we examined the potential errors associated with sample sizes in E, AS-stand and JS using the original AS-tree and F d data sets. Consequently, we defined the optimal sample sizes of 10 and 15 for AS-stand and JS estimates, respectively, in the 20 × 20 m plot. Sample sizes larger than the optimal sample sizes did not decrease potential errors. The optimal sample sizes for JS changed according to plot size (e.g., 10 × 10 and 10 × 20 m), whereas the optimal sample sizes for AS-stand did not. As well, the optimal sample sizes for JS did not change in different vapor pressure deficit conditions. In terms of E estimates, these results suggest that the tree-to-tree variations in Fd vary among different plots, and that plot size to capture tree-to-tree variations in Fd is an important factor. The sample sizes determined in this study will be helpful for planning the balanced sampling designs to extrapolate stand-scale estimates to catchmentscale estimates. © The Author 2009. Published by Oxford University Press. All rights reserved.