Arboriculture & Urban Forestry 41(5): September 2015 many more roots were observed than were predicted, probably because multiple small roots (<10 mm) showed as one root (Butnor 2001; Butnor 2003). The GPR instrument used in this research operated with a 900 MHz antenna with 100% bandwidth, which allowed scanning within the frequency range 450–1350 MHz with a claimed minimum detectable root diameter of 10–15 mm. Although this should have been sufficient to define the roots that were buried, the use of a higher fre- quency antenna would have been beneficial and the device can be coupled to a 1500 MHz antenna that would resolve smaller roots but only to depths of 500 mm. Wielopolski et al. (2000) used a 1500 MHz antenna to a depth of 700 mm and under optimum conditions resolved a 2.5 mm twig. The GPR was not recalibrated using the Autocal function every time the depth settings were changed. Parameters such as operating frequency, gain, bandwidth, relative permittivity, resolution, depth, antenna choice, and sampling intervals all interact with each other and are important in determining the best setup for the desired application (Daniels 2004; Gibson and George 2004). It might be wise operat- ing procedure to recalibrate the machine before each pass, but this has to be balanced against the time con- straints that apply in the field arboriculture appli- cation. If soil conditions are relatively consistent, re-calibration aſter each pass is unnecessary as long as one calibration for signal velocity has been made. GPR is complex technology with wide-ranging application and its use in arboriculture is rela- tively novel. The instrument used in this research is among the first generation of instruments with signal processing designed for root scanning use; future improvements can be expected as opera- tors become better trained and experienced in its use for different species growing in different soil types and under varying landscape condi- tions (Wu et al. 2014). The ramifications of miss- ing roots could result in an underestimation of the extent of the structural roots of a tree, which could see a sound and safe tree removed unnecessarily, while the reverse could apply for false positives. GPR accurately detected larger roots and particu- larly those at depths of less than 400 mm under dif- ficult field conditions. Under these challenging and uncontrolled conditions, where there is the potential for signal interference from disturbed soils and from 257 the sides of trenches, the GPR accurately located roots of 10 mm, 20 mm, and 40–50 mm diameter to a depth of at least 400 mm. This would be sufficient for many urban soils where soil disturbance during development leads to shallow spreading root sys- tems. Development of an effective method of non- invasively mapping tree roots provides a significant advance in the management of tree roots systems and their interaction with urban infrastructure. Acknowledgments. This research was part of a Bachelor of Applied Science Horticulture (Honours) project undertaken by C.M. Ryder at the University of Melbourne, Burnley Cam- pus. Use of the field station and the support of technical staff is gratefully acknowledged. Mr. R. Knott, R&T Tree Services is thanked for allowing the use of the Tree Radar instrument, as is Mr. D. Gunter, arborist with R&T Tree Services for help in oper- ating the Tree Radar and providing the technical support. Dr. A. Mucciardi, president, Tree Radar Inc. is thanked for allowing the independent testing of Tree Radar in Australia and giving technical advice on GPR. Mr. D. Hammersley provided labor in adverse conditions. Dr. Peter Ades, Melbourne School of Land and Environment, The University of Melbourne, is thanked for assisting in data analysis. Ms. E. Moore, linguist, and Ms. R. Ryder are thanked for their critical reading of the manuscript and their helpful suggestions. LITERATURE CITED Akinnifesi, F.K., B.T. Kang, and D.O. Ladipo. 1999. Structural root form and fine root distribution of some woody species evalu- ated for agroforestry systems. Agroforestry Systems 42:121–138. Barton, C.V.M., and K.D. Montagu. 2004. Detection of tree roots and determination of root diameters by ground-penetrating radar under optimal conditions. Tree Physiology 24:1323–1331. Bassuk, N., J. Grabosky, A. Mucciardi, and G Raffel. 2011. Ground- penetrating radar accurately locates tree roots in two soil media under pavement. Journal of Arboriculture 37:160–166. Butnor, J.R., J.A. Doolittle, K.H. Johnsen, L. Samuelson, T. Stokes, and L. Kress. 2003. Utility of ground-penetrating radar as a root biomass survey tool in forest systems. Soil Science Society of America Journal 67:1607–1615. Butnor, J.R., J.A. Doolittle, L. Kress, S. Cohen, and K.H. Johnsen. 2001. Use of ground-penetrating radar to study tree roots in the southeastern United States. Tree Physiology 21:1269–1278. Bureau of Meteorology. 2013. Daily Temperature and Rainfall Data, Melbourne Airport, 1971–2013. Australian Government. Cermák, J., J. Hruška, M. Martinková, and A. Prax. 2000. Urban tree root systems and their survival near houses analyzed using ground-penetrating radar and sap flow techniques. Plant and Soil 219:103–116. Coder, K.D. 1998. Root growth control: Managing perceptions and realities, pp. 51–81. In: D. Neely and G.W. Watson (Eds.). The Landscape Below Ground II, paper presented to Second Inter- national Workshop on Tree Root Development in Urban Soils. International Society of Arboriculture, Champaign, Illinois, U.S. Conyers, L.B., and C.M. Cameron. 1998. Ground-penetrating radar techniques and three-dimensional computer mapping in the American southwest. Journal of Field Archaeology 25:417–430. ©2015 International Society of Arboriculture
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