122 Johnstone et al.: The Measurement of Wood Decay in Landscape Trees to any great depth in a tree is more difficult than using a constant feed drill that is able to gather data as it drills. As with the Shigom- eter, results must be dependent on the tightness of fit of the elec- trode (Seaby 1991). The advantage of the Plant Impedance Ratio Meter is in the measurement of the vitality of tissues near the sur- face (with double-needle pins, rather than a long probe) rather than in decay detection in the deeper tissues of the tree. Tree vascular tissues must be wounded to the depth of desired decay detection. The four-point resistivity (RISE) method passes a current through an object with one pair of electrodes, while measuring the voltage difference with another pair of electrodes (Larsson et al. 2004). The constant current is passed vertically though the stem rather than horizontally. The resistivity must be measured against other trees of similar water content and species and at a similar tempera- ture and humidity and must be normalized for stem cross-sectional area. It cannot be used to assess the volume or location of the decay. CONSTANT FEED DRILLS Constant feed drills are simple drills that control and measure the rate of feed of a drill bit, and map the results either elec- tronically or onto a graph. The most common devices for tree inspection are the Resistograph (also known as the IML-Resis- tograph or the IML-Resi) and the Sibert DDD 200. A simple mechanical drill has also been used as a decay-detecting device, where the drill operator senses the changes in drill resistance and evaluates the wood shavings (Costello and Quarles 1999). The Resistograph is a portable, constant feed drill that records on a strip chart or electronic data recorder the drilling resistance as the bit penetrates the tree at a constant drive (Mattheck et al. 1997). The amplitude of the graph trace indicates resistance. De- cay in the path of the drill is represented by a fall in drilling resis- tance. A shaft covers the drill bit to prevent the operator forcing the drill faster than the constant drive. The drill bit tip (needle) is 3 mm wide (Bethge et al. 1996). The penetration speed may be set on the device and the depth of drive is 300–1500 mm, so it has the potential to be used on quite large diameter trees (Rinn et al. 1996). A study compared the Resistograph to drilling with a por- table drill in Victorian blue gum (Eucalyptus globulus) and golden elm (Ulmus glabra) (Costello and Quarles 1999). Wood density levels below a critical level (less than 500 kg m-3 Victorian blue gum and golden elm (less than 400 kg m-3 ) in ), were considered decayed. The depth to the point of decay was grouped into three categories 0–5 cm, 6–10 cm, and 11–15 cm. For Victorian blue gum, 85.5% of Resistograph and 73% of the portable drill results were in a 0–5 cm deviation from accura- cy category. For golden elm, 100% of Resistograph and 81% of the portable drill method were in the 0-5 cm deviation from accuracy category. This is a very accurate result for the Resis- tograph and a moderately accurate result for the portable drill. The IML-Resi was tested in conjunction with an “expert sys- tem” to assess the accuracy of decay detection in Victorian blue gum (Eucalyptus globulus subsp. pseudoglobulus) (Johnstone et al. 2007). The compartmentalization of decay in trees (or CODIT) model (Shigo 1979) was combined with raw data from the IML- Resi to predict the cross-sectional area of decay. A statistically significant relationship was established between the predicted to- tal area of decay in a wood section and the actual area of decay. Using a linear regression analysis of variance, 76% of the variation in the readings could be explained by the predicted area of decay. ©2010 International Society of Arboriculture Correlations between wood density charts and the Resis- tograph measurements have been established for air seasoned wood samples in six tree species: European silver fir (Abies alba), European larch (Larix decidua), Norway spruce (Picea abies), Swiss pine (Pinus cembra), bigleaf linden (Tilia platyphyllos), and a poplar (Populus sp.) (Rinn et al. 1996). A progeny trial Isik and Li (2003) found a weak to moderate relationship be- tween wood density and the amplitude of drilling resistance in loblolly pine (Pinus taeda L.). A strong correlation between aver- age wood density and resistance was also found in Victorian blue gum (Johnstone 2005). The sensitivity of the Resistograph (or the IML-Resi) to wood properties meant that drill resistance was affected by the moisture content of the wood (Rinn et al. 1996; Lin et al. 2003), but not in Victorian blue gum (Johnstone 2005). An English prototype for a constant feed drill was devel- oped, the “Decay Detecting Drill” (DDD), with a 2 mm flared drill bit tip (Seaby 1991). The number of revolutions per cm of penetration was plotted against wood density (kg m-3 ) to test the device’s effectiveness. According to Seaby (1991), there was a direct relationship between wood density and resistance except when there was variable moisture content in the sample. Rota- tional drag did not affect the results but longitudinal drag reduced drill bit tip pressure by approximately 10% cm-1 . This meant that the drill produced results that indicated wood density increased with the depth of penetration, even when this was not the case. Seaby (1991) speculated that better drill bit tip design might improve drag. The DDD 200 is faster than the Resistograph but provides less data per millimeter (Nicolotti and Miglietta 1998). Both the Resistograph and another device, the Densitomat-400, have a probe design that cuts a hole wider than the drill shaft, which may reduce resistance problems. Probes of constant feed drills are flexible, causing inconstancies in the data if the probe rubs against the side of the hole, and may cause deviations in the drilling path (Nicolotti and Miglietta 1998; Dolwin 1999). Other possible causes of drill bit drag may be the sharpness of the drill bit and the heating of the drill bit. Heating as a cause of drill bit drag however is very unlikely as most drill bits are steel, and the coefficient of area of expansion for steel is 24 x 10-6 for every 1°C (Giancoli 2005). Thus, even if a drill bit heated to 500°C, which is very unlikely, only a 1% increase in the cross- sectional area would occur. Moore (1999) found that drill bit drag (or friction) was severe enough to prevent the detection of decay by the Resistograph, a result inconsistent with others (Costello and Quarles 1999; Nicolotti et al. 2003; Johnstone et al. 2007). Kersten and Schwarze (2005) found that the IML-Resistograph provides a substrate for decay fungi as shavings from drilling re- tained in the hole, and Toussaint et al. (2004) found decay increased along drill-needle paths in Tilia sp. (Linden). As a result of Resis- tograph drilling, Helliwell (2007) found both 6 mm and 10 mm drills resulted in wood staining and discoloration two years after the holes had been drilled. Weber and Mattheck (2006) argued that constant feed drills did not result in long-term decay. They claimed that negative short-term wood decay was counteracted by the suc- cessful formation of compartmentalization reaction zones (Shigo 1979), after a longer period of 8–10 years (Weber and Mattheck 2006). No decay extended past the compartmentalization bar- rier zone (Shigo 1979), in any of the trees they tested, and there was no decay in trees with no preexisting decay prior to drilling.
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