Arboriculture & Urban Forestry 48(2): March 2022 health do not necessarily have problems with struc- tural stability. Therefore, if we consider that tree health and structural stability are closely related, this sug- gests that healthy trees can produce compensatory growth to reduce the damage caused by decay, break- age, and injury. Therefore, in this study, a visual tree assessment (VTA) of the trees was conducted from the following 2 viewpoints, as suggested by Mattheck and Breloer (1996): 1. The biological point of view, including the growth of the tree; the growth of branches and leaves; the presence of pests and diseases; the presence of fungal parasites on branches and roots, decay, and cavities; the presence, size, and location of wounds; or the presence of bulges on the trunk of the tree. 2. The mechanical point of view, including (1) an assessment of the possible breakage of the trunk, such as swelling, tumors, holes, decay, wounds, tilting, and cracks, and the endogenous bark of a certain part of the trunk, and (2) an assessment of whether the tree will fall, such as the condi- tion of the root surroundings, root tension, root injury and decay, and soil conditions. After completing the collection of various visual evi- dence, the health of the tree was evaluated to determine if further testing was merited. The assessment of decay usually utilizes instru- ments owing to the excellent concealment of inner decay. There are 3 conventional instruments that can acquire the situation of inner decay and related infor- mation: destructive test (DT), micro-destructive test (MDT), and non-destructive test (NDT)(Goh et al. 2018; Coelho-Duarte et al. 2021). DT can be estab- lished using a drilling machine, endoscopy, or moni- tor drill top. The principle of DT is to drill one or multiple holes to observe the inner decay of a tree, but it will wound the trees. In addition, it can only observe the monitoring points and may ignore other types of decay. This situation restrains the precision of DT, and secondary infections can happen to the objective (Johnstone et al. 2007). MDT is also a type of destruc- tive test, but the hazards are much lower than those of DT owing to its small- or medium-sized probe. NDT utilizes sound, light, electricity, magnetism, or other media, which would not destroy the tree but indi- rectly detect the inner decay. Although the precision is higher and the range of detection is larger than that of other methods, NDT is more expensive to use than 51 other methods. In recent years, its cost has been decreasing and it has replaced DT for use in detecting the decay in living trees (Ponneth et al. 2014). Additional ways of inspecting trees, considering the portability of the instrument and operational safety, include acoustic-based stress wave or ultrasonic inspec- tion tools such as the FAKOPP Microsecond Timer, the ArborSonic 3D Acoustic Tomograph, or the CBS- CBT Sylvatest TRIO (ultrasonic timer), which are more commonly used. These tools are used to record the distance and time between the transmittance and receiving ends by actively or passively emitting a specific frequency of sound waves, which are con- verted into values for the speed of sound, the speed of sound of the decay rate, the chordal to radial speed of sound penetration time, and the radial to chordal speed of sound ratio, among others, to assess whether the cross section of the tree is decayed. There are many NDT instruments and products with different practi- cability, convenience, safety, and economy, among others. The most commonly used methods to assess wood decay include stress waves and ultrasonic tech- niques, which have already been accepted by tree health managers and practitioners. The range of use is also very broad (Johnstone et al. 2010; Ponneth et al. 2014). Stress waves and ultrasonic techniques both use acoustical instruments to measure the velocity of energy in a test wood. The main difference between these methods is that the ultrasonic waves are gener- ated from a known frequency pulse, but the stress waves are not. In theory, the wave velocity in the wood is much faster than it is in the air, and it depends on the type of wave and the elasticity and density of the materials. Thus, the inner decay of a tree will change the velocity in different inner conditions (Wang 2013; Li et al. 2014). We have tested instru- ments that utilize both methods. In recent years, a diagnostic method and instrument called “PONTA” (Suyama et al. 2010; Suyama et al. 2013) was invented in Japan, which utilizes the stress waves of a knock on the tree. After a crosswise knock, using fast Fourier transform (FFT), the stress waves are recorded and the frequency distribution is analyzed. The high- est peak of frequency is analyzed and compared with the diameter of other healthy trees to detect the abnor- mal phenomena. This NDT can detect the damage to a tree through a simple and time-saving method (Yamada et al. 2019). ©2022 International Society of Arboriculture
March 2022
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