250 Johnstone et al.: Quantifying Wood Decay in Sydney Bluegum (Eucalyptus saligna) Schwarze 2008). Most researchers agree the Picus Sonic Tomo- graph is less likely to contribute to further wood decay than a Resi drill, as nails are inserted only a few millimeters into the xylem with the former (Gilbert and Smiley 2004), rather than holes being drilled directly through the xylem with the drill. However, it is clear from this and other studies that the Picus Sonic Tomograph does have difficulty with accurate decay esti- mation, such as when the origin of the decay is in the sapwood (Deflorio et al. 2008; Wang et al. 2009), when there are cracks and cavities present (Schwarze and Heuser 2006; Wang and Al- lison 2008; Wang et al. 2009), the location of the wood decay is not necessarily centrally located in a cross-section (Schwarze 2008), and as in this study where the trees are small (from 142 to 318 mm in diameter at 1.3 m in height). Even when wood decay is centrally located, Wang et al. (2009) reported the Picus can underestimate the amount of wood decay. As in this study, Wang et al. (2009) reported that the Picus cannot reliably detect small amounts of sapwood decay and insect holes in mostly sound wood. The percentage wood moisture content measured at 1.5 m showed no statistical relationship with the Picus, Resi, or visual wood decay estimation methods in this study. This is a posi- tive result for these methods, as it is unlikely that wood mois- ture content is causing unexplained variation in the wood decay estimation data. Moisture content did affect the Resistograph and the Decay Detecting Drill resistance values in some stud- ies (Seaby 1991; Rinn et al. 1996; Lin et al. 2003), but mois- ture content did not affect average drill resistance in a previous study by Johnstone (2005). It is very probable the Resi sys- tem removes the variable of moisture content, if present, from the drill resistance raw data. Though the velocity of sound in wood is affected by moisture content (Mishiro 1996), the Picus system was not affected by moisture content in this study. The unexplained residual variation when each wood decay es- timation method is compared to whole tree wood density—the best proxy for “true” wood decay values—is quite high for both the Resi and Picus systems. It is greater than 70% in all cases, even when the statistical relationships are significant. The possi- ble error in the Picus and Resi systems is probably high, as for ex- ample the “linear distance” measurements used in both methods may be affected by variable bark thickness between trees, which in turn affects the cross-sectional (wood) area of the trees at 0.3 m. The most successful method, the resi system, is a more subjec- tive method than the picus system, because interpreting graphs is subjective, particularly ascertaining a lack of growth incre- ments, but also the yield in the graph can be obscured by “noise” from residual resistance evident on the graph (Johnstone 2005). Tree risk assessment cannot be achieved without evaluat- ing the amount of decayed wood in a tree, but the exact loca- tion and amount of decay is difficult to assess. It is not practi- cal to measure the amount of wood decay in all the wood of every tree. Therefore, the capacity of a single measurement taken in cross-section to approximate the volume decay in the wood of a whole tree was the focus of this study. Overall the Resi system appears to be the most suitable method for esti- mating the volume of wood decay in Eucalyptus saligna trees. Acknowledgments. We would like to acknowledge the support of the De- partment of Sustainability and Environment, Victoria, Australia, for the use of their plantation trees for this research. We would also like to thank our anonymous reviewers for their time and consideration of this paper. LITERATURE CITED Anonymous, 2004. Picus Sonic Tomograph Manual. Rostock, Germany, Argus electronic gmbh. 52 pp. Beall, F., and W. Wilcox. 1987. Relationship of acoustic emission during radial compression to mass loss from decay. Forest Products Journal 37:38–42. Casella, E., and H. Sinoquet. 2003. A method for describing canopy ar- chitecture of coppice poplar with allometric relationships. Tree Phys- iology 23:1153–1170. Costello, L., and S. Quarles. 1999. Detection of wood decay in blue gum and elm: an evaluation of the IML-Resistograph and the portable drill. Journal of Arboriculture 25:311–317. Deflorio, G., K.M. Barry, C. Johnson, and C.L. Mohammed. 2007. The influence of wound location on decay extent in plantation-grown Eu- calyptus globulus and Eucalyptus nitens. Forest Ecology and Man- agement 242:353–362. Deflorio, G., S. Fink, and F.W.M.R. Schwarze. 2008. Detection of incipi- ent decay in tree stems with sonic tomography after wounding and fungal infection. Wood Science Technology 42:117–132. Evans, R., G. Gartside, and G. Downes. 1995. Present and prospective use of Silviscan - an instrument for the rapid measurement of wood properties. Research Report. Melbourne, The CRC for Hardwood Fibre & Paper Science 47. 4 pp. Gilbert, E., and E. Smiley. 2004. Picus sonic tomography for the quan- tification of decay in white oak (Quercus alba) and hickory (Carya spp.). Journal of Arboriculture 30:277–281. Harris, R.W., J.R. Clark, and N.P. Matheny. 2004. Arboriculture. Inte- grated Management of Landscape Trees, Shrubs, and Vines (4th Edi- tion). New Jersey, Prentice Hall. 578 pp. Helliwell, D.R. 2007. A short note on effects of boring holes in trees. Arboricultural Journal 30:245–248. Isik, F., and B. Li. 2003. Rapid assessment of wood density of live trees using the IML-Resistograph for selection in tree improvement pro- grams. Canadian Journal of Forest Research 33:2426–2435. Johnstone, D.M. 2005. The field assessment of wood decay and wood density in eucalypts. Master of Forest Science Thesis. University of Melbourne, Vic. 221 pp. Johnstone, D.M., P.K Ades, G.M. Moore, and I.W. Smith. 2007. Predict- ing wood decay in eucalypts using an expert system and the IML- Resistograph drill. Arboriculture & Urban Forestry 33:76–82. Kersten, W., and F.W.M.R. Schwarze. 2005. Development of decay in the sapwood of trees wounded by the use of decay-detecting devises. Arboricultural Journal 28:165–181. Lenth, R.V. 2001. Some Practical Guidelines for Effective Sample Size Determination, The American Statistician 55:187–193. Lenth, R.V. 2006. Java Applets for Power and Sample Size [Comput- er software]. Retrieved Sept. 26, 2009. Lin, C.-J., S.-Y. Wang, F.-C. Lin, and M.-C. Chiu. 2003. Effect of mois- ture content on drill resistance value in Taiwania plantation wood. Wood and Fiber Science 35:234–248. ©2010 International Society of Arboriculture
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