Arboriculture & Urban Forestry 36(6): November 2010 249 Figure 7. (a) The percentage of decay using the visual method versus the whole tree wood density in kg/m3 lyptus saligna trees. (b) The percentage of decay using the visual method versus the whole tree wood density in kg/m3 . Includes all 36 Euca- . These data exclude smaller trees; trees #17, #19, #24, #25, #31, and #34 are excluded. Therefore, 30 Eucalyptus saligna trees are included in this data set. ables. There was no significant statistical relationship be- tween whole tree wood density and the visual method data, even when only larger trees were analyzed [N = 30, P (whole tree) = 0.9174, P = (basic density) = 0.3032, r2 = 0.0501]. DISCUSSION In this study, the estimation of wood decay given by the Resi system showed a statistical correlation to the volume of wood decay in Eucalyptus saligna, whereas the Picus system and the visual method of wood decay estimation did not show a statistical correlation. The accuracy of the Resi system is sup- ported by Costello and Quarles (1999), who showed the Re- sistograph had a very low deviation from accuracy in assess- ing wood decay in Victorian bluegum (Eucalypus globulus). There was a very small variation in the estimated volume of decay using the visual method, from only approximately 0.03% (tree #19), to 13.53% (tree #26), with 94% (all but two trees) estimated as having less than 4% wood decay. The small variation in data would have contributed to the lack of statisti- cal relationships between the visual method and whole tree wood density. The decay in a wood section must be advanced or at least at an intermediate level in order to be detected with this method. To be advanced, the wood becomes fibrous and the wood structure is altered or nonexistent (Harris et al. 2004). Intermediate means the decay is clearly recognizable and there is a change in wood structure but remains intact. The other two stages—early decay, where there are slight changes in wood color, texture, and brittleness; and the incipient stage of decay, where there is a thinning of xylem cell walls and wood may be discolored—are not able to be detected by the visual method. Basic wood density measured at 1.5 m also showed a correla- tion with the percentage of decay as calculated using the Resi system. This is not surprising as decay results in a decrease in wood density or mass (Beall and Wilcox 1987). Much of the wood strength in decaying wood is believed to be lost at incipient decay level, with up to 50% by around 1% mass loss (Beall and Wilcox 1987). Five to ten percent mass loss can only be detected by light microscopy, and is not yet visible (Beall and Wilcox 1987). Slight changes in wood density may not be detected by the visual meth- od if only at the early or incipient level, but may be measureable using the Resi system described in this paper. The IML-Resi and the Resistograph showed clear correlations between basic density and raw drill resistance values in many studies that did not apply the Resi system described here (Rinn et al. 1996; Isik and Li 2003; Lin et al. 2003; Johnstone 2005). However applying the Resi sys- tem described here locates pockets of lower drill resistance that indicate putative decay (Figure 3b), not overall reduced drill re- sistance compared to other trees. The basic wood density mea- surement, as a sample taken across the whole of each tree at 1.5 m may be of reduced density due to incipient decay in the samples. Whole tree wood density and basic density showed a very significant linear and logarithmic relationship. This is not sur- prising as whole tree dry density is strongly influenced by the density of the sound wood, as well as the decayed wood in a tree, particularly if the estimated volumes of decay are rela- tively low, as in this study [Picus system (excluding tree #24) between 0% and 23.5%, Resi system between 0% and 23.19%, visual method between 0.03% and 13.53%]. Separating a mea- sure of decay and a measure of wood density is difficult and may require sophisticated measurements of the exact density across a wood section, such as SilviScanII, which was beyond the scope of this study (Evans et al. 1995). Multiple regres- sion analysis was performed comparing each decay estimation method as a dependent variable and whole tree wood density and basic density as partial regression coefficients, in an at- tempt to remove the interplay between these two variables. The Resi system still showed a significant correlation with the whole tree wood density coefficient within the multiple regression. The Resi system and the Picus system are both nondestruc- tive methods of wood decay assessment, however there is con- siderable debate over whether the Resi drilling causes further wood decay in trees with some researchers claiming the damage is negligible (Weber and Mattheck 2006), while others imply it may be significant (Kersten and Schwarze 2005; Helliwell 2007; ©2010 International Society of Arboriculture
November 2010
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