2 the wood at a constant speed. Changes in wood resistance are displayed on a graph as changes in amplitude. Areas of pro- longed low resistance indicate decay, cavities, or cracks. Be- cause it requires drilling into the tree, this test is considered minimally invasive. We report the results of using a combination of visual inspec- tion plus single-path stress wave, acoustic tomography, and re- sistance microdrilling tools to detect internal defects in century- old red oak trees. MATERIALS AND METHODS Two red oak (Quercus rubra) trees located at the Capitol Park in Madison, Wisconsin, were evaluated for structural stability in autumn 2005 (Allison 2005). The trees in this study were iden- tified as No. 307 and No. 123. To screen for trunk decay and defects, a visual inspection was conducted looking for anomalies such as fungal conks, cavities, cracks, seams, bulges as well as root-related problems. Further screening for internal trunk defects using single-path stress wave testing was conducted using a Fakopp Microsecond Timer (Fakopp Enterprise, Agfalva, Hungary). The trunks were tested by aligning the two probes on the trunk in a level north– south position for the first test and in a level east–west position for a second test. An electronic caliper was used for accurate measurement of the distance between the probes. It took a single arborist less than 15 min per tree to conduct and record the results of the visual inspection and single-path stress wave screening tests. Next a Picus Sonic Tomograph tool (Argus Electronic Gmbh, Rostock, Germany) was used to conduct acoustic tomograph measurements on the trees. The Picus Sonic Tomograph mea- surement system consisted of 12 sensors, which were evenly placed around the trunk in a horizontal plane during testing. Each sensor was magnetically attached to a pin that was tapped into the bark and sapwood. Acoustic wave transmission data were collected by sequentially tapping each pin using the steel ham- mer. A complete data matrix was obtained through this measure- ment process at each testing location. The tomograph measurement was conducted at one elevation of 100 cm (40 in) aboveground level for tree No. 123 and at three elevations of 10, 100, and 200 cm (4, 40, and 80 in) aboveground level for tree No. 307. At each elevation, the circumference and distances between sensors were measured using a tape measure and an electronic caliper. This information was used as an input for the system software to map the approximate geometric form of the cross sections. Because the cross-sections of the oak trees tested were irregular, the “free geometry” feature of the program was selected to reconstruct the geometry of the cross-sections. On completion of acoustic measurements, a tomogram was con- structed for each cross-section using the Picus Q70 software. Using information provided by the tomographs regarding the acoustic characteristics of each trunk cross-section, resistance microdrilling was conducted using a F400S Resistograph (IML, Inc., Kennesaw, GA). The drilling paths were selected to enter the area of trunk cross-section displayed in the tomograph as possible decay. The goal was to determine if the tomograph display represented an area of hollow, decay or was a crack- induced acoustic shadow in an area of solid wood. The trees were felled and a 10 to 15 cm (4 to 6 in) thick disk was cut from each elevation. All the disks were then transported to the USDA Forest Products Laboratory in Madison, Wiscon- ©2008 International Society of Arboriculture Wang and Allison: Decay Detection in Red Oak Trees sin, for physical examination. A digital picture of the cross- section was also taken for each disk. RESULTS For red oak No. 307, the visual inspection revealed trunk seams, Ganoderma applanatum fungal conks, and lack of root flare. The single-path stress wave tests revealed stress wave transmission times ranging from 1640 to 3248 s/m (500 to 990 s/ft). These values were significantly higher than the anticipated transmis- sion times for intact oak of 621 to 724 s/m (189 to 221 s/ft) (Wang et al. 2004). The tomographs indicated a large area of defect at all three elevations. The Resistograph drilling revealed that some areas displayed in the tomographs as potential decay or cavity were actually sound wood and that large cracks were creating an acoustic shadow display in the tomograph. After being cut down, tree No. 307 was found to have heart- wood decay at all three elevations. Laboratory examination con- firmed the presence of white rot decaying fungus. The decay was less severe in the upper cross-section (200 cm [80 in] elevation), but it increased in size as the elevation dropped. In addition to decay, major internal cracks were present in the cross-sections of tree No. 307. The combination of extensive decay and large lateral cracks caused the base disk to fall into several pieces during transportation. The photographs of the disks show that lower and middle cross-sections had multiple lateral cracks and the upper cross-section had one large lateral crack. Figure 1 shows the comparisons of acoustic tomographs and photographs of the cross-sections at three elevations for red oak No. 307. The dark-colored zones (brown if it is in color print) in the tomograms represent solid wood, and the light-colored zones represent potentially decayed wood (if displayed in color, the tomograms use green, violet, and blue to represent increasing degradation by decay). It is clear that the tomographs show a strong correspondence to the images of the disks. Extensive decay and radiating lateral cracks in the lower and middle cross- sections were reflected by large light-color zones in the tomo- graphs. The tomograph of the upper cross-section accurately located the position and orientation of the big lateral crack and heartwood decay. For red oak No. 123, the visual inspection revealed trunk seams and cracks with cankering on the south side with associ- ated Ustulina deusta fungus. The single-path stress wave tests revealed a stress wave transmission time ranging from 1908 to 2320 s/m (581 to 707 s/ft). The tomography revealed a large area of defect. The Resistograph drilling revealed a large snake- shaped crack. Figure 2 shows the comparison of the acoustic tomograph and photograph of the cross-section for red oak No. 123. DISCUSSION The visual inspection in combination with the single-path stress wave testing provided a quick screening to justify the need for more advanced testing. The acoustic tomographs obtained using the Picus Sonic Tomography provided strong evidence of struc- tural defect in both trees. The defect areas identified by the tomographs showed strong correspondence to true physical con- ditions of the cross-sections. Most decay pockets in the cross- sections were well reflected in the tomographs. However, the light-colored potential decay zones shown in the tomographs
January 2008
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