76 Johnstone et al.: Predicting Wood Decay in Eucalypts Arboriculture & Urban Forestry 2007. 33(2):76–82. Predicting Wood Decay in Eucalypts Using an Expert System and the IML-Resistograph Drill Denise M. Johnstone, Peter K. Ades, Gregory M. Moore, and Ian W. Smith Abstract. The evaluation of decayed wood in a tree trunk is essential for hazard tree assessment in arboriculture. In this study, an IML-Resistograph was highly successful as a field instrument in its ability to locate and predict the presence of decay in the trunk wood of eucalypts. A system for using the IML-Resistograph to estimate the area of decay in the cross-section of a eucalypt was evaluated for field use. The data from the IML-Resistograph F300S can be used as part of an expert system to estimate the area of wood decay in a cross-section of a standing eucalypt. Key Words. Eucalypts; hazard trees; Resistograph; tree failure; tree risk analysis; wood decay. The decay of wood within a tree trunk is often the cause of tree failure. There are many field devices for measuring de- cay in trees and they vary greatly in the principles on which they function. See Table 1 for a summary of devices in re- lation to their portability, invasiveness, reliability, cost, and ease of use. The most accurate of these instruments now generate a tomographic image using radar, sound, thermog- raphy, or electrical conductivity. However, these instruments are expensive and are dependent on the complexity of the algorithm that is used to convert raw data to an image for their accuracy. In this research, an expert system was developed to predict the amount of wood decay in eucalypts using the less costly IML-Resistograph drill. The Resistograph measures me- chanical resistance as a small drill bit moves through wood. Data from the Resistograph are recorded in the form of a graph. There are no units of measurement for the mechanical resistance shown by the drill, but the distance on the graph is the actual distance drilled in centimeters. A disadvantage with the Resistograph is it can provide a substrate for further decay in the wood of trees with preexisting decay because the shavings from drilling are left in the hole by the device (Ker- sten and Schwarze 2005). MATERIALS AND METHODS Experiments were conducted on trees at three different sites. Data were collected from an urban location, a forest, and a plantation site. Preliminary experiments were carried out on a Eucalyptus praecox Maiden (Expt. 1) and four Eucalyptus radiata Sieber ex D.C (Expt. 2). The tree species used in the primary experiment (Expt. 3) was Eucalyptus globulus subsp. ©2007 International Society of Arboriculture pseudoglobulus Naudin ex Maiden. Eucalyptus globulus is widely planted as a specimen tree in countries as diverse as the United States, Palestine, India, Italy, and Brazil (Tyrell 1999). Eucalyptus globulus was therefore an ideal choice for the primary experiment. Data from the Resistograph were compared with the ap- pearance of wood decay in a tree cross-section. Wood decay was defined as wood displaying clear changes in texture, structure, and color, that is, decay from the intermediate stage through to advanced decay (Harris et al. 2004). An absence of wood was also treated as decay. Data from the Resistograph used on the Eucalyptus prae- cox were observed to indicate decay on the graph traces when there was significant “yield” on the graph in relation to the rest of the trace and a lack of “peaks” on the graph associated with growth increments (Figure 1A and B). Data from a Resistograph used on the Eucalyptus radiata were then used to predict where decay was occurring without reference to the cut stem (Figure 2A and B). Subsequent to this, data from a Resistograph used on the Eucalyptus globulus subsp. pseu- doglobulus were used not only to predict decay, but to predict how far decay may extend beyond the data collection points in the entire trunk cross section (Figure 3A and C). To predict decay beyond the immediate point of data col- lection, an “expert system” was devised using the compart- mentalization of decay in trees (or CODIT) model from Shigo (1979) in combination with raw data from the IML- Resistograph. The method is an “expert system” because it relies on an expert knowledge of models of decay in trees. In Shigo’s model, the tree resists the spread of decay within compartment walls 1, 2, 3, and 4 (Shigo 1979). “Wall 4” in Shigo’s model is of particular importance. In the CODIT
March 2007
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