124 Johnstone et al.: The Measurement of Wood Decay in Landscape Trees Fractometer. When intact, 5 mm cores were obtained from Victo- rian blue gum (Matheny et al. 1999); they broke when the lever arm was placed against the sample and no results could be recorded. Bethge et al. (1996) observed that the Fractometer distinguished between types of decay. Brown rot leads to very small fracture angle whilst advanced white rot results in much larger angles. Timber in which lignin is degraded may appear to resist bending but does not have the stiffness of less decayed wood. On the other hand, timber with cellulose degradation will be low in elastic strength but stiff. Timber that is not decayed will be very stiff but also very strong. The Fractometer II determines the longitudinal compres- sion failure strength as well as the radial fracture bending strength (Fractometer I measures the latter only) (Bethge et al. 1996). Fractometer III measures all of the above, the tangential bending fracture strength, and the radial and tangential shear strength. The developers maintain however that the only nec- essary Fractometer for successful field diagnosis is the Fracto- meter I, as excessive fiber loading is most common in the ra- dial direction. Methods that require core sampling are some of the most invasive of the decay detecting devices, causing decay in yellow birch (Betula lutea Michx.) basswood (Tilia ameri- cana L.) and sugar maple (Acer sacchrum Marsh.) (Lorenz 1944). However, core sampling did not contribute to tree mor- tality over a 12-year period in white fir (Abies concolor), which is said to decay rapidly after mechanical wounding, and red fir (Abies magnifica) (van Mantgem and Stephenson 2004). COMPUTERIZED TOMOGRAPHY Computerized tomography can employ acoustic rays, electri- cal resistance, and thermal or radar techniques (Nicolotti et al. 2003). For electrical resistance and acoustic measurements, sen- sors are usually placed around a tree (from 8–16 but occasion- ally more), and multiple measurements are gained by sending a signal from one sensor to the others (Nicolotti et al. 2003; Gil- bert and Smiley 2004; Bucur 2006b). In radar or thermal imag- ing techniques, the signal is delivered and allowed to bounce off internal, and in the case of thermal imaging, external structures (Bucur 2003; Nicolotti et al. 2003; Catena and Catena 2008). These instruments produce cross-sectional “pictures” of the stem, via a computer programmed with complex conversion al- gorithms. X-rays, microwave technology, nuclear magnetic reso- nance (NMR) and neutron imaging for decay detection are all possible, but are currently very expensive and usually used for more sophisticated scanning of wood properties (Bucur 2003). Thermal imaging with an infrared camera scans for wood defects but cannot accurately quantify the amount of wood decay (Catena and Catena 2008). Images are species spe- cific. Thermography cannot assess residual wall thicknesses (Catena 2003). Thermal imaging has the advantage of be- ing noninvasive. It can detect wood decay in large tree roots or the root collar (Catena 2003; Catena and Catena 2008). Georadar devices are usually used to locate tree roots (Ouis 2003; Hagrey 2007). Images are generated via the re- flection of electromagnetic waves (Nicolotti et al. 2003). Georadar techniques were successful in detecting wood de- cay in the study by Nicolotti et al. (2003), but required con- siderable processing of the data. Georadar is noninvasive. Nicolotti et al. (2003) assessed results from electrical resis- tance and ultrasonic tomography and georadar. They reported ©2010 International Society of Arboriculture good results with electric tomography but the number of repli- cates was two. Electrical tomography was deemed promising by Hagrey (2007), but results were qualitative rather than quantita- tive. Problems with the Shigometer in eucalypts may be simi- lar to electrical tomography, as the raw data is the same (elec- trical resistance). However the electric tomography described by Nicolotti et al. (2003) is less invasive than the other electri- cal resistance devices, because the electrodes are only driven into a depth of 10 mm rather than placed in a predrilled hole. The PUNDIT (Portable Ultrasonic Nondestructive Digi- tal Indicating Tester) uses ultrasonic tomography. The operat- ing frequency of the PUNDIT is 33 kHz and it is possible with the 16 sensors to obtain 120 travel time measurements for each trunk cross-section (Nicolotti et al. 2003; Socco et al. 2004). Signal processing for the data collected in the study by Nico- lotti et al. (2003) was carried out with Migratom software. Two samples used in the study were London plane (Platanus hybr- ida Brot.), and were decayed rather than hollow in the center, with strength losses of between 22.7% and 53.6% (Nicolotti et al. 2003). Moisture content was higher in the decayed zones than in the surrounding sound wood. The ultrasonic transducers were used with a coupling gel placed directly on the bark, but without removing a bark plug. The ultrasonic signal can be pro- cessed by a cathode ray oscilloscope to further manipulate and control the data supplied by the PUNDIT (Socco et al. 2004). The Picus Sonic Tomograph uses sonic tomography. Raw data is the time of transmission of the sound of a hammer tap on one sensor to each other sensor, 8–12 for each stem cross-section (Gilbert and Smiley 2004). The Picus is self- calibrating in that the fastest acoustic transmission time rela- tive to distance is deemed “sound” wood (Rabe et al. 2004; Schwarze 2008). A sound wave produced manually is called a stress wave (Wade 1975; Bulliet and Falk 1985; Mattheck and Bethge 1993). A disadvantage of the Picus over the PUNDIT is the Picus does not deliver a sound pulse of known frequen- cy, which can lead to inaccuracies in recording the speed of propagation time (Nicolotti et al. 2003). It is possible to de- liver sonic waves at predictable and repeatable frequencies (Schubert et al. 2009) but this is not the mode of operation of the Picus. The conversion algorithm for both stress and ultra- sound waves is complex because the propagation of sound is not always in a straight line (Bucur 2003; Maurer et al 2006). Gilbert and Smiley (2004) evaluated the Picus for location and extent of decay. Decay was defined as both an absence of wood and wood that could be deflected with finger pressure. There was a high correlation between the amount of decay de- tected by the Picus and the extent of decay assessed visually, fol- lowing felling (r2 = 0.90) (Gilbert and Smiley 2004). The Picus slightly underestimated decay in most cases, with an average discrepancy of 6%. The stem cross-sections exhibited decay not detected by the Picus in 9% of the readings. The range for error was from minus 3% to minus 20%. Tree diameter ranged from 250 mm to 490 mm (Gilbert and Smiley 2004). Similar results were obtained by Rabe et al. (2004). Decay in the sapwood was not accurately assessed (Deflorio et al. 2008), and was depen- dent on the host/pathogen combination. The precise location of the decay was also found to be less accurately represented by the Picus in some studies (Rabe et al. 2004; Wang and Allison 2008). The Picus is minimally invasive, as 2 mm nails are insert- ed a few millimeters into the xylem (Gilbert and Smiley 2004).
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