Arboriculture & Urban Forestry 42(2): March 2016 Arboriculture & Urban Forestry 2016. 42(2): 111–119 111 Preliminary Evaluation of Electrical Resistance Tomography for Imaging Palm Trunks Monica L. Elliott, Timothy K. Broschat, and Lothar Göcke Abstract. In a preliminary study, electrical resistance tomography (ERT) was used to obtain trunk images of Syagrus romanzoffiana, a common ornamental palm grown in southern Florida, U.S. Seven palms, four healthy and three diseased, were evaluated in the middle of the dry season, with four located in an irrigated site and three in a non-irrigated site. Two healthy and three diseased palms were felled and cross sections obtained to examine their internal structure and compare to the tomograms obtained. ERT was effective in illustrating the relative electrical resistance value of healthy palms, as the tomograms obtained for palms situated in the irrigated site (low electrical resistivity) were distinctly different from the tomograms of palms situated in the non-irrigated site (high electrical resis- tivity). ERT was also effective in visualizing internal palm trunk areas affected by the fungal pathogens Ganoderma zonatum, a wood decay pathogen, and Fusarium oxysporum f. sp. palmarum, a vascular wilt pathogen, as these areas had a low electrical resistivity. Key Words. Electrical Resistance Tomography; Electrical Resistivity; Florida; Fusarium oxysporum f. sp. palmarum; Ganoderma zona- tum; Palms; Syagrus romanzoffiana; Wood Decay. Palms are important landscape elements in tropi- cal, subtropical, and Mediterranean climates, and cold-hardy palms are oſten used in the landscape in southern temperate climates. As monocotyledonous trees, palms differ significantly from dicotyledon- ous and coniferous trees. Palms have no vascular cambium and, therefore, no sapwood, heartwood, or phloem rings. Instead, a palm trunk is com- posed of thousands of vascular bundles, with each bundle containing xylem vessels, phloem sieve tube cells, and fibers. The fibers are the primary tissues that provide mechanical support to the trunk as the amount of cellulose and lignin deposition increases over time. The lowest portion of the palm trunk is the oldest portion and, therefore, has the greatest amount of cellulose and lignin. While vascular bun- dles are dispersed throughout the palm trunk, their distribution within the trunk is species specific, with some palm species having more vascular bundles concentrated near the outside of the trunk and oth- ers having the bundles evenly dispersed throughout the trunk (Tomlinson 1990; Tomlinson et al. 2011). As with dicotyledonous and coniferous trees, non-destructive methods that examine the internal structure of the palm trunk are highly desirable, especially to confirm internal trunk decay or dam- age due to pathogens or insects. One such method is electrical resistance tomography (ERT). As it applies to trees, this method uses pulsed electric current (DC) to examine tree properties that affect the resistivity of the wood. The primary properties are water content, cell structure, and ion concen- tration, which are properties that change if decay in the tree is detected. The resulting measurements are displayed on a two-dimensional map, showing the apparent electrical resistance of the wood, called an electrical resistance tomogram. In general, areas of low resistance to electrical current correspond to areas with high water content or high ion concentra- tion, whereas areas of high resistance correspond to areas of low water content or low ion concentration (al Hagrey 2006; al Hagrey 2007). While the use of electrical resistance to evaluate wood decay (Tat- tar et al. 1972) or stem water potential (Nadler and Tyree 2008) is not new, using electrical resistance to develop a tomogram is relatively new. ERT has been used to evaluate trunk decay or the sapwood–heart- wood interface in dicotyledonous and coniferous ©2016 International Society of Arboriculture
March 2016
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