Arboriculture & Urban Forestry 36(6): November 2010 Arboriculture & Urban Forestry 2010. 36(6): 243-252 243 Quantifying Wood Decay in Sydney Bluegum (Eucalyptus saligna) Trees Denise Johnstone, Michael Tausz, Gregory Moore, and Marc Nicolas Abstract. The evaluation of decayed wood in a tree is essential for tree risk assessment in arboriculture. It is not feasible in practice 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 of decay in the wood of a whole tree is the focus of this study. The volume of wood decay in Eucalyptus saligna trees was estimated post harvest by measuring the whole tree wood density of 36 trees. Linear, logarithmic, and multiple regression statistical models were used to exam- ine whether whole tree wood density data could be correlated with three ways of directly measuring wood decay in a single cross-section, two pri- or to tree felling and one post felling method. The decay estimation techniques were measured in cross-section—a Picus® Sonic Tomograph sys- tem, an IML-Resi system, and a visual method. In this study the Resi System for estimating wood decay showed a correlation with whole tree wood density in the entire tree whereas the Picus system and the visual method did not show any statistical correlation with whole tree wood density. Key Words. Eucalypts; Hazard Trees; Picus® Sonic Tomograph; Resi; Resistograph; Tree Failure; Tree Risk Analysis; Wood Decay. Evaluating the amount of decayed wood in a tree is essential for tree risk assessment. It is not feasible in practice to measure the amount of wood decay in all the wood of every tree in an urban landscape. The extent of decay in Eucalyptus globulus and Eu- calyptus nitens is greater axially than either radially or tangen- tially (Deflorio et al. 2007), suggesting a stem measurement may detect wood decay, if present, in Eucalyptus saligna. Therefore, the capacity of a single measurement taken in cross-section to approximate the volume of decay in the wood of a whole tree at the time of testing is the focus of this study. This study does not attempt to predict the progression of decay within trees over time. Field devices for measuring decay in the cross-sections of trees vary greatly in their operating principles and are often similar to the equipment used for measuring wood density, as decay results in a decrease in wood density or mass (Beall and Wilcox 1987). Two of the most common devices for quantifying wood decay in urban trees are the Argus-Picus Sonic Tomograph, which records the time of transmission of multiple acoustic stress waves through the stem of the tree in a cross-section, and the IML-Resi which is a constant feed drill that records “resistance” on a graph trace. The aim of the investigations described in this paper was to exam- ine whether quantifying the area of decay in cross-section could be extrapolated to estimate the volume of wood decay in Euca- lyptus saligna. Methods using the Resi and the Picus were used to estimate wood decay in a trunk cross-section of the trees be- fore they were felled, and a visual method was used after felling. Electrical conductivity meters such as the Shigometer were not used because interpreting data from these instruments has been shown to be difficult in certain genera – particularly Eucalyptus (Wilkes and Heather 1983). The Fractometer was not chosen be- cause the 5 mm core sample required by the instrument proved too brittle to be assessed successfully in Eucalyptus globulus in a study by Matheny et al. (1999). Currently available devices are similar to the ones used in the experiment, although they may have more sophisticated data recording, software, and data processing. Field instruments quantify decay in small sections only, so the methods of wood decay estimation were compared to an estima- tion of the density of the wood in each tree or the “whole tree wood density” to gauge which method best reflected the amount of wood decay in the entire tree. It must be emphasized that in this study, “whole tree wood density” includes trunk or branch wood sections that may be hollow or very decayed, rather like a pipe, and hence have very low density. Thus, whole tree wood density is a measure of wood decay in the entire tree. Whole tree wood density in this study should not be confused with “basic wood density.” There are five standard ways of describing wood density in the timber industry: oven-dry density, air-dry density, green density, nominal density, and basic density (Walker et al. 1993). All of these density measurements are taken on wood samples that appear completely sound (nondecayed) as were the basic wood density samples in this study. Thus, the mea- surement of basic wood density in this study does not relate di- rectly to the decayed wood in the trees, unlike the measure of whole tree wood density, which includes visibly decayed wood. Wood decay was measured in plantation trees as this enabled a statistically significant number of even aged trees to be mea- sured with relative ease, and to reduce the variation in whole tree wood density data that may be due to the environmental effects on branch growth such as plant–soil–nutrient interactions and cli- mate (Casella and Sinoquet 2003). Open grown or urban trees would not have been suitable to test the effectiveness of single location decay methods for describing the volume of wood decay ©2010 International Society of Arboriculture
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