Arboriculture & Urban Forestry 37(4): July 2011 electrics of two different materials, the larger the radar wave. For example, the dielectric of water is 81 and that of an aver- age soil is approximately 13, producing a “dielectric contrast” of 6.2:1 (81/13), which is large and will cause most of the ra- dar wave energy to be reflected back to the surface antenna. Root detection is possible in principle because of the mois- ture content within the woody root that provides an excellent contrast with the soil matrix. Roots that are dying will have very little or no moisture content, due to fungal attack for instance, and will be either weak or nonreflective targets and, hence, not detectable. In fact, this is an inferential way to determine root health (Burgess et al. 2001; Shigo 2003). Even roots located in high water table soils are detectable (Gormally et al. 2010). Although roots are detectable, it may only be possible to estimate a bulk property of the roots structure such as root density (Butnor et al. 2003). However, to create a true root morphology map that shows not only density but also individ- ual roots that are above and below others, bifurcating, merg- ing (grafting), and other means, require considerably more effort. Offline signal analysis software can be employed us- ing properties of the electromagnetic radar wave to “disentan- gle” the root mass and produce the desired morphology map. The goal was to use an existing set of installed sidewalks in an experimental grove to ground-truth GPR data collections. The study authors intended to locate tree roots with the GPR sys- tem and then count excavated roots in the same soil volume to compare the accuracy of the GPR system with true root loca- tion. If GPR technology was found to be accurate, it would be useful to provide data in support of strategies to preserve tree roots during development. By enhancing the ability to retain trees on development sites, GPR technology may enable indi- viduals to maintain and support tree growth in urbanized areas. MATERIALS AND METHODS In the spring of 2003, 72 dormant, lightly branched, bare root lin- ers of Acer platanoides ‘Emerald Queen’ (Emerald Queen Norway maple) trees were planted in two soil media in a research field in Ithaca, New York, U.S. The 2 cm caliper trees were 1.5–2.0 m tall and were budded onto Acer platanoides seedlings of unknown ori- gin donated by J. Frank Schmidt’s Nursery (Boring, Oregon, U.S.). Six trenches, 24 m long, 2 m wide, and 1 m deep were exca- vated and the trenches lined on all sides but the bottom with heavy 4 mil plastic. Three trenches were backfilled in three 0.25 m lifts of silt loam (70% silt, 23% clay, 7% sand), and compacted to 1.7 Mg m-3 . A base course layer of gravel, 0.25 m deep was laid over the surface of the entire trench and then the trench was paved with 10 cm of concrete leaving 0.5 m × 0.5 m openings in the center of 12 equally spaced 2 m × 2 m squares (Figure 1). One liner was planted in each opening in the pavement. The second set of three trenches of the same dimensions was excavated and backfilled in three 0.25 m lifts with CU-Structural Soil made from 20% of the same silt loam soil and 80% ~2.5 cm limestone gravel by weight. Thirty grams of hydrogel (Gelscape made by Amereq, Inc., New City, NY, U.S.) was added to each 100 kg of gravel to aid in the uniform mixing of the CU-Structural Soil (hereafter referred to as CU-Soil). The structural soil was compacted to 1.97 Mg m-3 , veri- fied by sand-cone replacement method. The same 0.25 m base course of gravel and 10 cm of concrete were added on top of the structural soil. Liners were planted in the concrete openings as 161 was done for the trench with the compacted silt loam soil and the trees were watered as necessary. In subsequent growing seasons, the trees were weeded, but there was no additional watering. In spring 2008, one trench of each treatment was scanned us- ing a TerraSIRch™ GPR system (Geophysical Survey Systems, Inc., Salem, New Hampshire, U.S.). The GPR system consisted of an SIR-3000 (Subsurface Interface Radar) computer control unit, a 900 MHz (Model 3101B) radar antenna, and a modified tricycle jogging cart for automated scanning as shown in Figure 2. The TreeWin™ signal processing software package (TreeRadar, Inc., Silver Spring, Maryland, U.S.) was used to process the GPR radar transects offline and produce 2D “virtual trench” maps showing the location (distance along the scan line and depth) of each de- tected root for each scan line. This customized software package, operating in a semi-automated way with the analyst, permitted root clusters to be “broken up” and individual roots determined. Figure 1. Example layout of sidewalk, GPR paths, and resulting scan sections used in analysis. Trees are surrounded by 8 sam- pling sections (labeled 1-8). GPR scans ran across the sidewalk as a width scan and along the sidewalk length (labeled long GPR scan). Tree openings were 0.25 m2 , and not to scale in the above image. As length and width scans overlapped on corner sections (1, 3, 6, and 8), the two GPR data outputs were averaged in analysis. ©2011 International Society of Arboriculture
July 2011
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