38 Burcham et al.: Infrared Measurements of Mechanically Induced Voids day (Appendix). It is possible that the planting depths used for each species may have contributed to the temperature differences between D. fragrans and S. grande stems containing the 3.8 cm void (Figure 2). The greater amount of soil surrounding the deeply planted D. fragrans stem may have acted as a buffer against tem- perature increases at the stem surface by cooling the air within the void, resulting in a relatively cool stem surface temperature above the 3.8 cm void for this species. Although the extension of the largest D. fragrans void into the soil facilitated its detection using the IR camera in this experiment, this unintentional artifact of ex- perimental design is not likely to be encountered in the landscape. In contrast to the mean temperatures, the standard deviation and skewness of the surface temperature distributions within the rectangular transects were, in some cases, significantly different from that exhibited by the irrigated controls (Plant 2). Howev- er, there was an inconsistent relationship between the absolute value and sorted position of these statistics among the experi- mental treatments. For example, the standard deviation of sur- face temperature distributions within the transect measured on D. fragrans Plants 5, 6, and 7 was significantly lower than the irrigated control (Plant 2), while the standard deviation measured on S. grande Plant 8 was significantly higher than the irrigated control. These statistics failed to draw any meaningful connec- tions between the treatments and values obtained, and irregular changes in the magnitude and direction of both the temperature standard deviation and skewness among all treatments suggest they may not be analytically reliable when applied in the field. Linear trending analyses of stem surface temperatures largely supported the visual image evaluation process. Plants with the 3.8 cm void displayed a sharp deviation in the linear tempera- ture trend near the position of the artificially induced void, and the magnitude of these localized deviations was significantly greater than those measured at the same position on the irri- gated control (Plant 2). Although linear temperature deviations on both D. fragrans and S. grande measured 0.3°C below their respective averages, the linear temperature trend for S. grande visibly portrayed a comparably small and less distinct deviation around the 3.8 cm void when viewed in the coordinate plane. The deviation exhibited by S. grande remained noticeably dif- ferent from the irrigated control, but the temperature gradient around the void position exhibited a shallower slope compared to the relatively steep gradient exhibited at the same position on D. fragrans. This disparity may be caused by the different stem geometry or bark thickness between these two species, with these features being substantially more pronounced in S. grande, or the different planting depths used for the two species. The IR images were especially easy to interpret visually using the irrigated control temperature distribution as a refer- ence case with which to compare other stems containing inter- nal defects. In the field, the availability of a physically compa- rable reference specimen without internal defects would also be an important consideration when applying this technique. A control specimen would provide the comparative basis for evaluating temperature anomalies on the test subject, their ab- sence from a control specimen, subjected to similar atmospheric conditions and solar radiation being a condition for positively identifying an internal defect. Overall, the qualitative image evaluation and linear trending analysis adequately identified the temperature anomaly associated with the largest internal void, while the temperatures extracted from rectangular tran- ©2013 International Society of Arboriculture sects were, in some cases, contradictory to the conclusions facilitated by the other methods. In field applications, it may be adequate to rely exclusively on a visual image evaluation and lin- ear trending analysis to identify similar temperature anomalies. The basal gradient of progressively cooler temperatures on irrigated stems provided corroborative evidence of the effect of sap flow on surface temperature distributions (Wul- lschleger et al. 1998; Bauerle et al. 2002; Burcham et al. 2012). This natural gradient is caused by the actively conducting xylem fluids gradually warming from root to aboveground tem- peratures within the stem, and this has been reported in other species as being especially pronounced near the soil surface in relatively open or low-density forest areas (Köstner et al. 1998; Do and Rocheteau 2002). This temperature gradient will be common in relatively open urban landscapes, and its pres- ence should not be erroneously attributed to internal defects. Overall, these results are largely in agreement with earlier reports that stem temperature anomalies are limited to stems with internal defects occupying at least 76% cross-sectional area (Burcham et al. 2012). However, these findings extend current knowledge by demonstrating diurnal variation in the relation- ship between internal stem condition and surface temperature. Based on this variation, the observed temperature anomalies were likely caused by the reduced stem wall thickness made appar- ent only after the stems were passively heated by the sun. The change in stem geometry for those with the 3.8 cm void induced a measurable change in the stem’s specific heat capacity, a ma- terial property predicting the change in temperature exhibited by a particular substance following a heat transfer (Joel 1996). Theoretically, specific heat capacity is described by an equa- tion ( ) where the ratio of heat transfer per unit mass (J/kg) and the resulting temperature change (K) is described in units of J/kg K (Joel 1996). This property is mostly influenced by the temperature and moisture content of wood and is largely unaf- fected by density or species (Forest Products Laboratory 2010). Whereas the stem’s specific heat capacity can be described by that of its wood in the absence of defects, a composite specific heat capacity must be considered for those in this experiment with a reduced stem wall thickness and air-filled voids. The two species exhibited measureable temperature changes in the eve- ning when the defect exceeded 76% cross-sectional area, and the relatively low stem wall thickness in these specimens may have resulted in a measurable cooling effect by the air-filled void. Although the wood anatomy of D. fragrans and S. grande are markedly different from one another, these two experiments allowed similar conclusions of this diagnostic technique’s prac- tical value. Still, the mechanically induced voids in this study can only be considered physically analogous to fungal-induced cavitations, and they likely did not accurately represent the complex physical, chemical, or biological conditions within a wood decay lesion (Adaskaveg et al. 1990; Deflorio et al. 2008). Similarly, the characteristic elements of a host defensive response, including cellular polyphenolic deposits, lignification, and suberization, were likely not manifest at the void periphery within the experimental timeframe (Pearce and Woodward 1986; Pearce 1996; Deflorio et al. 2009). The comparative physical complexity of wood decay lesions in mature, urban trees pres- ent a challenge to adapting the technique for practical risk as- sessments. Ultimately, it will be crucially important to further evaluate this technique using landscape trees with severe wood
January 2013
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