32 Burcham et al.: Infrared Measurements of Mechanically Induced Voids wood decay lesions, cavitations, or cracks, may initiate chang- es to the wood’s thermal properties (i.e., thermal conductivity, heat capacity, thermal diffusivity) that produce visibly differ- ent surface temperature distributions under certain heating and atmospheric conditions. These thermal properties are mostly governed by wood density, moisture content, extractives, grain direction, structural discontinuities, and cellular microfibril angle; and some defects may, in certain cases, significantly alter these characteristics (Forest Products Laboratory 2010). The association of decayed wood with tree failures has resulted in this defect being of particular concern for practicing arborists, and the close association of relatively cool stem surface temperatures with internal decay was reported as the basis of this diagnostic technique (Catena et al. 1990; Schwarze et al. 2000). However, initial reports largely employed qualitative analytical methods to interpret the images for surface temperature anomalies indicat- ing the presence of internal defects, and significant unrealized development potential remains in designing the analytical tech- niques used to interpret the images (Catena and Catena 2008). In recent studies, the relationship between a stem’s internal condition and its surface temperature has been systematically evaluated (Bellett-Travers and Morris 2010; Burcham et al. 2011; Burcham et al. 2012). In particular, one study demon- strated a relationship between these two features exclusively in stems containing internal defects exceeding 76% stem cross- sectional area (Burcham et al. 2012). These results suggest a relatively low workable resolution for the device that omits defects below this minimum threshold value, and these find- ings are critical in defining the conditions under which the IR camera technique may practically be deployed for use in the field. In a separate study, the internal condition of landscape Casuarina equisetifolia specimens did not explain the variabil- ity (r2 = 0.001–0.022) in surface temperature measurements captured with an IR camera when internal defects occupied up to 21.8% stem cross-sectional area (Burcham et al. 2012). These results are indirectly corroborative by failing to detect defects not exceeding the minimum threshold value previously determined. Still, the major conclusions about the technique’s performance were derived primarily from experiments con- ducted with Dracaena fragrans, a monocot, under controlled environmental conditions, and it is critical that the veracity of these results be confirmed with eudicot species in an uncon- trolled, outdoor environment. The anatomy of monocots dif- fers substantially from that of eudicots, the class of flowering plants containing most hardwood tree species planted in cities. In this case, the important difference is the distribution of tis- sue systems within the stems, with eudicots having a continu- ous cylinder of vascular tissue enclosing some ground tissue (i.e., pith) and surrounded by more (i.e., cortex), and monocots having vascular bundles scattered throughout the ground tis- sue (Evert 2006). Therefore, two experiments were designed using a monocot and eudicot species, Dracaena fragrans (L.) Ker Gawl. (Agavaceae) and Syzygium grande (Wight) Wight ex Walp. (Myrtaceae), respectively, chosen for their frequent use in Singapore. The objectives of the study were to evaluate, under controlled conditions in a nursery: 1) the effect of mechanically induced internal voids on the stem surface tem- perature distributions of monocot and eudicot species using an infrared camera, and 2) diurnal variation in this relationship affect- ed by ambient atmospheric conditions in the equatorial tropics. ©2013 International Society of Arboriculture MATERIALS AND METHODS In this study, two similar experiments were conducted using different plant species. In the first, seven D. fragrans speci- mens were selected from a single nursery source using three predefined selection criteria, including a 5 cm stem diameter (measured at 10 cm height), an absence of stem defects, such as mechanical damage, cracks, or detached bark, and a single straight apical leader. The relative physiological maturity of the plants was held constant by selecting individuals from the same nursery crop rotation. In the second, eight S. grande were selected from a single nursery source using the same criteria. Subsequently, the commercial growing media was removed from the root systems of individual plants to transfer them into a larger container size. During this process, longitudinal voids were introduced mechanically into selected stems using wood auger drill bits. The voids were introduced axially from the bot- tom of the stem, made accessible by the bare root system, and centered along the stem midpoint (pith in S. grande). Root dam- age was mostly avoided with D. fragrans as the adventitious roots generated during vegetative propagation were arranged at the stem periphery, but some damage was inevitably caused to the wood structural roots in secondary growth that obscured the bottom of the S. grande stem (Figure 1). The voids occupied between 25%–76% of stem cross-sectional area and extended 5–35 cm above the root flare. In each experiment, the voids were introduced into all specimens except for two (i.e., Plants 1 and 2), which were preserved as control treatments (Figure 2; Table 1). Figure 1. Commercial growing media was removed from the root systems of Dracaena fragrans (a, b) and Syzygium grande (c, d) to access the bottom of the stem, and longitudinal voids were subsequently introduced using wood auger drill bits. Inadvertent root damage was avoided insofar as possible during the void introduction process.
January 2013
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