Abstract
Yield estimates from single stands in short-rotation willow (Salix spp.) have been reported to differ substantially, depending on the estimation methods used. The magnitude of differences in estimates resulting from different methods may vary among clones on the same site. This indicates that some assumptions, implicitly made by using a certain method, cannot be generalised for all clones. To assess why estimates of different methods may differ and why the magnitude of the differences may be clone-specific for a given site, a number of assumptions underlying destructive and non-destructive protocols for the estimation of aboveground willow biomass were tested. Apart from general problems in satisfying the demands of representative sampling and meeting the assumptions that underlie the proper use of statistical models, it was found that basic assumptions with regard to the physical structure and phenology of willow may lead to the observed differences. For a given clone, the moisture content of willow shoots may vary over time and may be dependent on shoot size. For a given shoot, the moisture content of basal, apical and central stem parts may differ, with the magnitude of differences being dependent on the clone. Stem taper and curvature may be clone-specific and can also underlie differences in biomass estimates obtained by different methods. To improve estimates of aboveground biomass in short-rotation willow, it is important to state explicitly the assumptions underlying the methods used and, wherever possible, to test the assumptions and evaluate the effects of their violation on the resulting estimates.
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Acknowledgments
The authors thank the Swedish Energy Agency and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning for funding this research, Vehbo Hot, Sigvard Lunderqvist-Nilsson, Nils-Erik Nordh and Monika Welc for technical assistance, Johannes Forkman, Jan-Eric Englund and Anneli Lundkvist for valuable advice on statistical issues and Inger Åhman for giving valuable comments on the manuscript. The constructive comments of two anonymous reviewers also are highly valued.
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The authors hereby declare that they have no conflict of interest and that the reported research complies with the required ethical standards.
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Verwijst, T., Albertsson, J. Assumptions Made in Protocols for Shoot Biomass Estimation of Short-Rotation Willow Clones Underlie Differences in Results Between Destructive and Non-destructive Methods. Bioenerg. Res. 8, 1424–1432 (2015). https://doi.org/10.1007/s12155-015-9607-0
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DOI: https://doi.org/10.1007/s12155-015-9607-0