246 Moore and Ryder: Ground-Penetrating Radar to Locate Tree Roots 2000) and commonly extending to 2–3 times the dripline or 1–2 times the height of the tree (Perry 1982; Pirone et al. 1988; Schnelle et al. 1989; Hitch- mough 1994). Perry (1982) suggests that roots may extend up to seven times the diameter of the drip line. There are exceptions, with Eucalyptus marginata (Jarrah) having roots that penetrate the soil to 20 m (Shigo 1991), while Kozlowski (1971) provided evi- dence of a number of species that have demonstrated deeper rooting, oſten in response to dry conditions. Tree roots are opportunistic and tend to grow where soil conditions, such as low bulk density and supplies of oxygen, moisture, and nutrients, are best (Perry 1982). At the point of attachment to the trunk, tree roots are usually few and large (Pirone 1988). These large roots tend to taper rapidly and branch into rope-like strands that can extend for many meters (Perry 1982). When roots encounter favorable conditions, it is common for them to branch many times, creating a fan-like structure to exploit the favorable soil conditions and reserves (Coder 1998). Ground-penetrating radar (GPR) has been in used for other applications and in other disciplines, such as archaeological investigations, bridge deck analysis, detection of landmines, pipe and cable detection, and planetary exploration for about forty years (Daniels 2004; Gibson and George 2004). Over the past decade, the technology has been applied to mapping tree roots (Hruskra et al. 1999; Cermák et al. 2000; Butnor et al. 2001; But- nor et al. 2003; Barton and Montagu 2004; Guo et al. 2013). Barton and Montagu (2004) attempted to determine the diameters of roots buried in sand with some success, and Hirano et al. (2012), using GPR, found that their system estimated 68% of the excavated root biomass. Methods are improving, however, and Wu et al. (2014) used GPR to recon- struct three-dimensional coarse root structures with an accuracy of 83%, based on measurements of different size classes of roots from a previously excavated shrub root system. Mapping of general root architecture (Wu et al. 2014) with GPR has proved accurate, as have estimates of root biomass (Cui et al. 2013) and fresh root biomass is more accurately determined than dry root mass (But- nor et al. 2003; Wu et al. 2014). Bassuk et al. (2011) also used GPR to locate tree roots under pavement. GPR is an electromagnetic technique that can be used to detect physical changes in the medium ©2015 International Society of Arboriculture through which the GPR signals are transmitted. The signals respond to the relative dielectric per- mittivity, which is a general measurement of how well electromagnetic radiation passes through a medium such as soil (Guo et al. 2013). GPR sys- tems use signal processing both during and aſter scanning for roots, usually to improve the signal to noise ratio, but such processing is usually done in a way that the arboricultural field user of GPR is unaware of its application (Wielopolski et al. 2000; Butnor et al. 2003; Guo et al. 2013). Raw radargrams are processed so that they clean up and adjust an image so that it can be more readily classified and interpreted for root detection (Guo et al. 2013). Some soil characteristics, such as texture, bulk density, and water content, can affect the soil’s dielectric properties, and so impact upon the use of GPR by making it difficult to contrast roots from the medium in which they occur or by increasing signal reflection (van Dam et al. 2005; Guo et al. 2013; Isaac and Anglaaere 2013). Many GPR studies have been conducted under controlled circumstances where soil conditions have been optimized for GPR use (Cui et al 2013; Guo et al 2013). The best quality of GPR root detection is achieved in well-drained soils under dry conditions (van Dam et al. 2005; Zhu et al. 2011; Guo et al. 2013) and in electrically resis- tive soils, such as sands (Guo et al. 2013). Soils with less than 30% volumetric water content give the best contrast between roots and the surrounding soil (Cui et al. 2013). Consequently, soils with a higher sand content are superior to clays for GPR detection of roots, especially as the depth of the roots increases (Guo et al. 2013; Isaac and Anglaaere 2013). The use of GPR as a non-invasive, non-destructive, and efficient method of locating and mapping roots has advantages that go beyond reducing the unnec- essary removal of, or significant root damage to, valuable urban trees (Guo et al. 2013). It allows the scanning of large root systems quickly and economi- cally, does not disturb the soil, allows repeated mea- surements, and can detect roots under hard surfaces (Bassuk et al. 2011). However, when GPR-generated root maps were compared with actual field obser- vation of the root systems aſter excavation, there were variations, especially in vertical views that were used to construct a three-dimensional image of the root system (Guo et al. 2013). In many field situations, a planar map of the root system would be
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