Arboriculture & Urban Forestry 39(1): January 2013 Arboriculture & Urban Forestry 2013. 39(1): 31–41 31 Infrared Camera Measurements Reveal Diurnal Variation in the Effect of Mechanically Induced Internal Voids on Stem Temperatures of Small Trees Passively Heated by the Sun Daniel C. Burcham, Eng-Choon Leong, Yok-King Fong, and Puay-Yok Tan Abstract. The relationship between stem temperature measured by an infrared (IR) camera and a tree’s internal condition was proposed as a basis for di- agnosing potentially hazardous defects in the landscape. Before practical application, this diagnostic technique needs clarification to guide expectations of its resolution and precision. In this study, cylindrical voids of varying size were created in 5 cm diameter stems of Dracaena fragrans and Syzygium grande by mechanically removing tissue from specimens in an outdoor nursery, and the surface temperature of these plants was measured twice daily at 0830h and 1830h using an IR camera. The collected IR images were analyzed using a concurrent mixed methods approach with qualitative image evaluation and quanti- tative temperature analysis where the temperature distributions of stems containing internal voids were compared to those without defects. For both species, there was no difference in stem temperatures, as observed in IR images, among any experimental plants at 0830h, but an anomalous temperature reduction was exclusively apparent at 1830h near the 3.8 cm void, occupying 76% stem cross-sectional area. There was also a larger decrease in the linear temperature trend (0.34°C–0.51°C) near this void margin compared with other treatments, although this was more pronounced in the monocot species than eudicot. The remaining treatments did not exhibit stem temperatures visibly different from the control. Under experimental conditions, the technique identified relatively large internal defects, but the reduced heat capacity of stems containing such defects is only apparent in the evening after being passively heated by the sun. Key Words. Diagnostic Device; Dracaena fragrans; Infrared Camera; Internal Defect; Singapore; Syzygium grande, Temperature; Thermal. The application of diagnostic devices during tree risk assess- ments is common among practicing arborists who need detailed information about a tree’s internal condition. The devices measure material property changes in the stem associ- ated with degraded wood to assess the presence, and in some cases, severity of internal defects. The devices often employ physical measurement techniques to evaluate internal defects, including penetration resistance, acoustic stress wave transmis- sion, and electrical resistivity (Nicolotti and Miglietta 1998; Larsson et al. 2004; Deflorio et al. 2008; Wang and Allison 2008). Ultimately, arborists consider the output of these devices during formal tree risk assessments when judging the overall risk presented by a tree in its surrounding landscape. These assessments are informed by three separate considerations, including the probability that a tree will fail within a given time period, the likelihood this failure will result in damage or harm, and the severity of these outcomes, often measured in financial terms (Matheny and Clark 1994; Ellison 2005). Of these considerations, the first is often the most difficult to resolve accurately. A subjective assessment prescribing the likelihood that a tree will fail is often imprecise and ambigu- ous, and accurate formulas describing strength loss caused by internal decay are currently not available (Kane and Ryan 2003; Kane and Ryan 2004; Norris 2008). The availability of accurate and easy-to-use tree diagnostic devices, in par- ticular, is essential to support this decision-making process. Over the past fifty years, forestry and lumber profession- als have developed many of the techniques for identifying and assessing internal defects (Beall and Wilcox 1987; Brashaw et al. 2009), and the arboriculture profession has adopted several of these for practical use (Matheny and Clark 2009; Johnstone et al. 2010; Leong et al. 2012). However, the great size and longev- ity of many tree species and complex plant–microbe interactions, coupled with limited scientific resources, has constrained the extent of systematic testing imposed on these devices. Arbori- culture research addressing this topic has frequently relied on a relatively small sample of trees and inconsistent methodological approaches to determine a device’s measurement resolution (i.e., detection capability) and precision. As a result, the conclusions drawn from these studies may inhibit their straightforward appli- cation to an urban forest (Matheny and Clark 2009). Additional research focusing on diverse tree species, pathogens, and habitats is needed to help clarify the capabilities and constraints of the devices. Recently, the capability of an infrared (IR) camera to be employed as a tree diagnostic device has been the focus of sev- eral studies. Its proposal as a rapid, general, and accurate tree diagnostic device suggests its utility in managing the time requirements and providing detailed information for tree risk assessments (Catena and Catena 2008). The proposed tech- nique evaluates stem surface temperature measurements cap- tured by an IR camera for thermal property changes associated with degraded wood. The presence of structural defects, such as ©2013 International Society of Arboriculture
January 2013
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