Sample size estimation for achieving the desired uncertainty for estimates of tree fine root trait parameters
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ORIGINAL ARTICLE
Sample size estimation for achieving the desired uncertainty for estimates of tree fine root trait parameters Benye Xi1 · Nan Di1 · Mark Bloomberg2 · Elena Moltchanova2 Received: 23 January 2020 / Accepted: 8 September 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Key message This study investigates efficient strategies for fine tree root sampling, in terms of estimating root trait parameters with desired confidence intervals for least effort and cost. Abstract Sampling tree roots is difficult and costly with high variation among samples and wide confidence intervals for parameter estimates. Efficient strategies for fine tree root sampling will estimate root trait parameters with the desired confidence interval for least effort and cost. We compared alternative strategies to sample and estimate fine root surface area density; (1) collecting samples at intervals of 10 cm to a depth of 150 cm for entire tree root systems versus (2) independently taking samples from different randomly-selected 10-cm depth intervals around different trees. We also quantified the pilot sample size needed to reliably estimate the number of samples that would achieve the desired confidence interval. Efficiency of sampling entire tree root systems versus independent samples depended on the structure of the sample data. Pilot sample sizes > 5 per 10-cm soil depth can give reliable estimates of sample sizes required to achieve a 95% confidence interval of ± 10% of the sample mean. The statistical strategies in this paper are not particularly novel or difficult, but are seldom applied to root studies. We contend that they should be used, both to guide efficiency in sampling design and also to assess how realistic it is to expect that estimated sample means will be reliable, in the sense of having confidence intervals of the required width. Keywords Root traits · Root area density · Study design · Pilot study design · Sample size calculation
Introduction Fine roots of trees play a large role in biomass production, plant-soil interactions and biogeochemical cycling (Chen et al. 2004; Addo-Danso et al. 2016; Berhongaray et al. 2013). Fine roots are usually defined as roots ≤ 2.0 mm in diameter (Addo-Danso et al. 2016). Communicated by K. Noguchi. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00468-020-02032-4) contains supplementary material, which is available to authorized users. * Elena Moltchanova [email protected] 1
Ministry of Education Key Laboratory of Silviculture and Conservation, Beijing Forestry University, Beijing, China
University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand
2
Sampling and measuring roots is difficult, and the difficulties and limitations of measuring roots in the soil have been well discussed in the literature (Addo-Danso et al. 2016; Bengough et al. 2000; Berhongaray et al. 2013; Metcalfe et al. 2007; Taylor et al. 2013; Yuen et al. 2013). This is particularly the
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