208 Lin et al.: Inspection and Evaluation of Decay Damage Using Nondestructive Techniques velocities of Japanese cedar, Norfolk island pine (Araucaria heteophylla), and hoop pine trees are 1,354, 1,129–1,296, and 1,154–1,164 m/sec, respectively. If detected values of nondestructive evaluation are lower than these reference values, the wood quality of the trunk brings up ques- tions that could require further investigation. In this study, lower transverse stress wave velocities (map grids) were observed inside of the decay-damaged trees. Severe wood decay defects have been reported when the stress wave veloc- ity reduced to 70% of the characteristic values of sound wood (Bethge et al. 1996). In this study, the average V value in the undamaged trees were 1,642 m/sec with the threshold at 1,148 m/sec (1,642 × 0.7 m/sec). Moreover, the minimum V values of the tomogram in the undamaged trees were 1354 m/sec. Therefore, the minimum V values (1,148–1,354 m/sec) can be considered as the threshold values for diagnosis by stress wave velocity tomogram. Furthermore, the range of demarcation between decay-damaged and undamaged wood occurred at an approximate transversal stress wave velocity of 1,148–1,354 m/ sec. The reduction in V is indicative of serious damage, the location and extent of which can be seen in the map grids. The decay-damaged trees had lower average and individual stress wave velocities compared with the undamaged trees. Some studies have reported that an acoustic tomogram cannot precisely evaluate the extent and location of decay or the type of defect (Gil- bert and Smiley 2004; Wang et al. 2007; Wang et al. 2009; Li et al. 2011). For example, an acousti- cal tomogram underestimates the internal decay and overestimates that in the periphery of the trunk. Therefore, to make better assessments of internal conditions and decay of trees, other more effective methods (e.g., visual drawings of the increment core, drilling resistance, and use of a fractometer) should also be adopted in combi- nation to enhance the accuracy of information. In-depth tree assessments are warranted when a tree poses a high degree of risk to public safety and exhibits defects that cannot be fully evalu- ated by visual inspection (Pokorny 1992). How- ever, micro-destructive methods can destroy the compartmentalization zone and break the existing barrier zone within the tree, allow- ©2016 International Society of Arboriculture ing decay to spread into healthy wood. There- fore, when using decay detection devices, the number of drill holes or sensor sites for col- lecting the required critical field data should be kept to a minimum (Wang et al. 2007). A larger thickness of the peripheral region and a higher ratio of peripheral wood toward the trunk base have significant implications for the tree structure and safety (sound and health). When Japanese cedar trees have trunk decay, deterioration, or hazardous defects, the residual wall thickness (shell) and wood qual- ity have been found to be marginally suffi- cient. Most experts (Pokorny 1992; Matheny and Clark 1994; Harris et al. 2004; Hayes 2007) agree that a 30-35 ratio of percent sound wood in the remaining wall is the threshold beyond which some action should be taken. The acoustic velocity values tended to increase with increasing diameter in this study (Figure 4). In this experiment, transversal V was detected by eight fixed probes along the circum- ference on the trunks. Moreover, the average crushing strength values of the larger diameter class were only slight lower than those of the smaller diameter class (Table 3) in the undam- aged tree group. The properties (quality) and thickness (residual wall) of the peripheral wood in a tree is very important for the tree’s struc- tural safety and hazard evaluation. Generally, larger diameter trees with larger crowns need greater support, while the trunks of smaller diameter trees with smaller crowns need to withstand smaller forces. The deadweight and crown volume of the larger diameter class was larger than that of the smaller diameter class. The most important and dangerous load on trees is undoubtedly that created by wind, which can introduce bending stresses near the periph- ery of the stem (Mattheck and Breloer 1994). Previous research has indicated that the maxi- mum V values of lean Norfolk island pine and lean hoop pine trees are greater than those of normal non-leaning trees (Lin et al. 2015; Lin et al. 2016). The leaning of a tree could result in reaction wood or larger gravity effects in the trunk of the tree. However, the V values of the cross section are influenced by the distribution of the cell structure, reaction wood, gravity, own
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