Arboriculture & Urban Forestry 37(3): May 2011 of these thermodynamic processes could provide an explanation for the temperature reductions associated with external defects. Quantitative analysis exposed uneven representation of surface temperature by the three evaluated statistics. Mean temperature and mean normalized temperature val- ues did not show a significant change in temperature com- pared to the control, and these statistics belied any relation- ship between the internal defects and surface temperatures. The values provided by these two statistics gave a measure- ment of central tendency for the temperature range exhibited. Mean relative temperatures calculated from the first two treat- ments, contrastingly, showed that the mechanical voids had a sig- nificant (P < 0.05) effect on surface temperature. This statistic represents surface temperatures independent of the absolute tem- perature values, and it could prove particularly useful while making comparisons among images captured on various dates and times. The mean relative temperatures recorded were higher com- pared to the control stem sections and this is a divergence from earlier reported findings of comparatively lower surface tempera- tures above structural discontinuities (Catena 2003). The dis- crepancy in results may be caused by the absence of active water movement within the stem sections obtained for the study. Water movement through the vascular system could alter heat energy along a living tree stem, but the effects of this process on the axial surface temperature gradient are not well understood (Potter and Andresen 2002). The largest mechanical void, Void C, oc- cupying 1.9% of total volume, did not exhibit mean relative tem- perature values significantly different compared to the control. A significant change in mean relative temperature was also discovered on the two separate evaluation aspects above Void B, but this temperature difference was observed only once while comparing the evaluation aspects independently. There was no evidence indicating that the internal position of the artificial de- fect, segregated within two evaluation aspects in this experiment, had any effect on surface temperature distributions compared to the control. Still, this basic comparison does not sufficiently dem- onstrate that internal voids located immediately beneath the trunk surface have a similar effect on surface temperature compared to those located near the center of the stem. In order to circumvent uncertainty related to irregular internal positions, a circumferential average of exhibited temperatures may increase the technique’s sensitivity and subsequent ability to identify internal anomalies. This circumferential surface temperature average may comprise sufficient values so as to preclude any consideration of void depth. This method may facilitate more straightforward identification of internal voids, but it is unclear how this would affect any method- ological sensitivity in measuring the amount of degraded tissue. Further development of image collection, data processing, and quantitative analysis methodologies will be required to achieve a diagnostic technique with reliable consistency. The selective dis- covery of a relationship between internal mechanical voids and surface temperature is a small positive affirmation of the tech- nique’s foundation, and further development of the analytical methods may provide the substantial improvement required for arboricultural implementation. However, it is not possible to di- rectly translate results obtained in this study to field applications of the technique. The typical features of partially decayed wood, including changes in cationic concentrations and gradual decom- position of cellulose and lignin, were not replicated by the intro- duced mechanical voids (Jellison et al. 1997; Schwarze et al. 1999). 97 Additionally, the mechanical voids did not accurately replicate the features typical of reaction zone boundaries, including suber- ized tissue and phenolic infusions (Pearce and Woodward 1986). CONCLUSIONS In this study, the IR camera diagnostic technique was evaluated using tree sections containing mechanically induced voids. After field image collection, the raw temperature data was extracted for analysis with minimal post-processing, and the mechanical voids had an inconsistent effect on surface temperature distributions. The mean relative temperatures showed that the two smaller voids (Void A and Void B) had a significant effect but the largest void (Void C) did not have a significant effect. However, the re- sults of this study should be interpreted with reference to the void sizes evaluated. The sizes created in this study were small, oc- cupying between 1.5% and 5.4% total cross-sectional area of the stem, and a fungal induced cavity of this size will not frequently substantiate great concern during tree risk assessments. The mean relative temperature represented the data comparably better than mean normalized temperature for this purpose, and this statis- tic should be preferentially evaluated in similar applications. In order to further corroborate the findings of this study, future research is needed to clarify the effect of fungal induced cavi- ties on surface temperature distributions and any diurnal varia- tion in this relationship. The effects of active fungal decay, wood anatomy changes, metabolite status, and translocation through the vascular system on surface temperature should be system- atically evaluated to characterize the precise limitations of this technique. Moreover, further analytical methods referencing the relative spatial extent of the temperature values should also be as- sessed to assist in isolating specific areas of concern on the trunk. Acknowledgments. The authors would like to thank Ms. Phyllis Ng Qianyu and Ms. Jackeline Tay Jit Ling for their hard work and persistence creating artificial voids. This project was made possible through funding provided by the National Parks Board of Singapore. LITERATURE CITED Bellett-Travers, M., and S. Morris. 2010. The relationship between sur- face temperature and radial wood thickness of twelve trees harvested in Nottinghamshire. Arboricultural Journal 33:15–26. Bethge, K., C. Mattheck, and E. Hunger. 1996. Equipment for detec- tion and evaluation of incipient decay in trees. Arboricultural Journal 20:13–37. Catena, A. 2003. Thermography reveals hidden tree decay. Arboricultural Journal 27:27–42. Catena, A., and G. Catena. 2000. Ricerche per la conservazione e la ges- tione del verde urbano a Roma: l’esempio di Villa Sciarra. Thesis. Catena, A., and G. Catena. 2008. Overview of thermal imaging for tree assessment. Arboricultural Journal 30:259–270. Catena, G., L. Palla, and M. Catalano. 1990. Thermal infrared detection of cavities in trees. European Journal of Forest Pathology 20:201–210. Dwyer, J.F., E.G. McPherson, H.W. Schroeder, and R.A. Rowntree. 1992. Assessing the benefits and costs of the urban forest. Journal of Arboriculture 18:227–234. Hunt, J.F., H. Gu, and P.K. Lebow. 2008. Theoretical thermal conductiv- ity equation for uniform density wood cells. Wood and Fiber Science 40(2):167–180. ©2011 International Society of Arboriculture
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