Arboriculture & Urban Forestry 41(5): September 2015 249 Figure 3. Prepared stepped trenches for Experiments 1 and 2 at 200 mm, 400 mm, and 800 mm depth. its range. The trenches were offset from each other to ensure that the roots could not overlap between the different depths. The trench was refilled and GPR scans were conducted within seven days of the roots being placed in the ground to reduce the risk of desiccation. Aſter the roots were put in place and buried, the soil surface was made level and the ground marked to show a line perpen- dicular to the buried roots along which to scan. The site was scanned by wheeling the cart across the ground, with the GPR device emitting a radar pulse every 5 mm. All three replicates of both exper- iments were scanned in one pass over the ground and this was repeated three times to determine the consistency of GPR root location. For each pass, the depth setting was altered to determine whether the effect of this setting affected the accuracy of the GPR in locating the roots. Although it was not expected that there would be any significant effect, the experiment tested this aspect of GPR use. For the 200 mm trench and the 400 mm trench, the GPR was set to scan at three different depths. Pass one was set to scan to the maximum depth of 1000 mm. Therefore, on the 200 mm deep trench, the roots should show up in the top 20% of the display. Pass two was set to scan to a maximum depth of 700 mm, and pass three was set to scan to a depth of 500 mm. For the 800 mm trench, shallower scans could not be used due to the depth of the trench, and reducing the depth of the scan would have been restrictive due to the depth of placement of roots. Therefore, pass one and two were set to 1000 mm and pass three was set to 900 mm. In total, nine scans were taken across the three trenches. The final experiment scanned the root system of a Pistacia chinensis, one of a row of trees growing in the Burnley field station, which was approxi- mately three meters tall, with a DBH of 80 mm. The roots were scanned using linear scans on all four sides of the tree, creating a small grid (Fig- ure 4). Scans of the root system were taken at distances of 1000 mm, 1500 mm and 2500 mm from the trunk. Twelve scans were taken around the tree, each being 5 m long (60 m in total). In this experiment, both root location and num- bers were of interest as in contrast to the first two experiments, neither the number of roots nor their locations were known prior to scanning. The lines along which the GPR scanned were then excavated to test the accuracy of the GPR. All grass was removed from around the tree using a Bobcat skid steer loader and the scanned lines were trenched using an AirSpade tool (Series 2000 150/90, Air-Spade Technology, Verona, Pennsylva- nia) (Figure 5) so that there was no grass present during GPR data collection. The AirSpade was coupled to a 180 cfm Ingersoll Rand compressor which delivered air from the nozzle at mach 2 (approx. 2380 km hr-1 ). The benefit of using an air excavation tool was that the soil was blasted away while leaving the roots intact (Nadyezd- hina and Cermàk 2003). Following excavation, all roots over 5 mm in diameter, measured with cali- pers, were counted along each trench. The roots were put into two categories, less than 10 mm and greater than 10 mm. Although root sizes of 10 mm diameter and above were used in the first two experiments, the smaller root size class was collected in the field experiment in case the data were needed for interpretation of the root scans. The roots that crossed each trench were counted and compared to the GPR results. Roots were counted in three squares, but not at the corners due to the possibility of overlap and double counting. The GPR scan profiles were processed through TreeWin, a specialist soſtware package for use with the Tree Radar designed to increase the sig- nal to noise ratio (Guo et al. 2013). The processing includes algorithms that reduce noise, clutter, and signal amplitude losses. Following collection and visual examination of the data, scans at 200 mm and 400 mm depth were subjected to a two-tailed paired-sample t-test to assess significance differ- ence (α = 0.05) between the predicted and known horizontal and vertical root locations along the trenches. At 800 mm, the presence of a large num- ©2015 International Society of Arboriculture
September 2015
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