Tree Physiology Advance Access published online on October 12, 2009
Tree Physiology, doi:10.1093/treephys/tpp074
Effects of sample size on sap flux-based stand-scale transpiration estimates
1 School of Forestry and Resource Conservation, National Taiwan University, Taipei 106-17, Taiwan
2 Corresponding author (kumett{at}ntu.edu.tw)
3 Kasuya Research Forest, Kyushu University, Sasaguri, Fukuoka 811-2415, Japan
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Abstract |
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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 (AS_tree) measurements of individual trees. In a Japanese cypress plantation, we measured Fd and AS_tree in all trees (n = 58) within a 20 x 20 m study plot, which was divided into four 10 x 10 subplots. We calculated E from stand AS_tree (AS_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 Fd data sets. Consequently, we defined the optimal sample sizes of 10 and 15 for AS_stand and JS estimates, respectively, in the 20 x 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 x 10 and 10 x 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 catchment-scale estimates.
Keywords: granier-type sensor, Monte Carlo sampling, sap flow, sapwood area, scaling procedures
Received December 7, 2008; Accepted August 8, 2009