94 Burcham et al.: Detecting Mechanically Induced Internal Voids in Syzygium grande bark surface temperature, (2) determine the influence of mechani- cal void size and position on bark temperature, and (3) evaluate the use of an IR camera in the detection of mechanical voids un- der equatorial climatic conditions where temperature variations are small (typically between 24°C to 34°C) throughout the year. MATERIALS AND METHODS One Syzygium grande (Myrtaceae) was randomly selected from a row of 26 individual trees planted in the roadside verge along Jurong Town Hall Road in Singapore (1°19’53.72”N, 103°44’24.55”E). The selected tree (41.2 cm DBH) was in healthy condition without internal decay or defects. The stem of the selected tree was harvested into one, two meter segment on March 18, 2010, and transported to a research laboratory. The stem segment was further sectioned into four equal 45 cm long sections. The diameter of each section decreased with greater height from the trunk base, measuring 44 cm at 0.25 m, 41.5 cm at 0.75 m, 40.8 cm at 1.25 m, and 37.5 cm at 1.75 m. Relatively greater diameter at the trunk base was caused by trunk taper and buttress root formations, the former contributing to even circum- ferential expansion and the latter causing circumferential undula- tions. The average bark thickness, measured on all of the exposed ends of the cut sections, was 1.2 cm. An identical mechanical void was created in three sections and denoted as Void A. The cylindrical void was enlarged subsequently to produce Void B and Void C (Table 1). The mechanical voids were oriented longi- tudinally 1 cm interior to the vascular cambium on the east stem aspect (Figure 1). One stem section without a void served as the control for the study. The control section and the three sections with Void A were then exposed to direct sunlight for four hours at midday. The stems were oriented with the east stem aspect facing the 90° azimuth, and the cut surface proximal to the root col- lar was placed downward. The stems were subsequently moved to a covered, shaded location for evaluation with an IR camera. Infrared images were collected with a Thermoteknix VisIR 640 IR camera (Cambridge, UK) calibrated using ambient tem- perature and emissivity values prior to evaluation. Two IR im- ages were captured for each stem section at a distance of one meter. One image was collected by directly focusing at the trunk surface concealing the underlying mechanical void (east aspect) and one additional image was captured of the stem surface direct- ly opposite (west aspect). The depth of the voids, when viewed from these evaluation aspects, varied among treatments (Figure 1). The east aspect received greater amounts of direct solar ir- radiance from primary exposure during the diurnal solar cycle. Table 1. Mechanical void dimension, volume, and stem cross-sectional area in cut sections of Syzygium grande. Treatment Control Void A Void B Void C Dimension (cm) 0 × 0 × 0 5.1 × 5.1 × 16 10.2 × 5.1 × 16 15.3 × 5.1 × 16 Volume (cm3 ) 0 327 745 1159 Cross-sectional area (cm2 ) 0 20 46 72 The images were captured regularly at 30-minute intervals for 150 minutes. Atmospheric conditions were recorded hourly during the four-hour time period of stem exposure and subsequent evalu- ation period, including ambient temperature (°C), relative humid- ity (%), and solar irradiance (W/m2 ). The experiment was repeated with Void B and, subsequently, Void C, on separate days. In total, the experiment contained three factors at different levels (void size4 × aspect2 × time6 ). Each treatment combination was performed in triplicate. At the study’s conclusion, all stem sections were dis- sected with a chainsaw to verify the absence of internal defects. The sections were bisected longitudinally, cut into cross sections at 10 cm intervals, and visually inspected for internal defects. All thermal images were processed using the TherMonitor® Reporter System (Thermoteknix 2001) after field data collection. An initial visual comparison of the IR images in time-series was performed to identify and examine surface temperature anoma- lies qualitatively. In qualitative visual examination, relatively dissimilar regions of temperatures in the images were compared Figure 1. Mechanical voids were introduced incrementally in a radial direction towards the pith at a constant depth of 16 cm be- fore exposure. Following exposure, infrared images of the stem surface were collected from two perspectives, diametrically op- posed, in line with the radius occupied by the void and corre- sponding with the stem surfaces exposed to the east and west cardinal directions. ©2011 International Society of Arboriculture Figure 2. A rectangular transect was imposed onto each infrared image to demarcate the boundary of temperature values extract- ed for quantitative analysis. The transect was positioned on the bark surface immediately above the underlying, concealed void on both aspects.
May 2011
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