Internal decay and cavities in wood are known to reduce the structural functionality of trees. Such damage may lead to detrimental effects not only on the wood, but also on humans. This is especially the case with old, large trees that are more vulnerable to heavy snow and strong wind. Thus, preventative management (e.g., detecting internal wood defects) is essential. The present study investigated the reliability of noninvasive measurements using sonic tomography (SoT) to detect internal defects in Abies holophylla Maxim. trees and compared the results with measurements using the invasive method of resistance microdrilling (RM). The tomograms were visually compared with tree cross-section images. The results of SoT and RM showed no significant differences, while the explanatory power, as determined by a regression analysis, were considerably high at 67% with a positive correlation between the two methods. In comparison to the cross-section images, the tomograms were found to reflect the size and position of internal decay, although the detected size tended to be larger than the actual decay area. Our findings indicate SoT as a promising noninvasive technique for detecting internal defects in A. holophylla trees.

Zelkova serrata (Thunb.) Makino trees, which account for a large proportion of cultural assets and protected trees and have considerable historical and cultural symbolic meaning in South Korea. To verify the reliability of the nondestructive sonic tomography (SoT) method on Z. serrata trees, this study compared the results of SoT with those obtained with the destructive resistance micro-drilling method. With SoT measurement, defects were found in approximately 75% of the target Z. serrata trees and two of these trees were evaluated as having a high risk because their defective portions accounted for approximately 70% or more of the entire cross section. The independent samples t-test indicated that the two methods did not show a significant difference between measurements. Regression analysis indicated that the measurement values of the two methods showed a positive relationship with a high explanatory power of 76%, thus, verifying the reliability of SoT.

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