Arboriculture & Urban Forestry 45(6): November 2019 Harris et al. (2016) reported that Acer rubrum L. ‘Franksred’ planted with the root flare 30 cm deep were more resistant to uprooting (i.e., had greater bending moment and bend- ing stress) than those planted at grade. Peltola et al. (2006) observed that increased depth of the root plate was posi- tively correlated with maximum resistive bending moment. In contrast, Papesch et al. (1997) found that diameter of the root plate had a significant effect on resistance to overturn- ing while root plate depth did not. Research into the impact of root architecture on tree sta- bility is limited, as root systems are complex and their architecture is difficult to quantify. One of the measures commonly employed in addressing this challenge is the root cross-sectional area (Coutts et al. 1999; Gilman and Grabosky 2011; Gilman et al. 2013) at different depths, directions, and distances from the trunk. Utilizing this approach, Gilman and Grabosky (2011) found bending stress to be positively correlated with root cross-sectional area 20 to 30 cm and 40 to 50 cm below the soil surface for Q. virginiana trees six years after being planted into sandy soil. Despite its demonstrated utility, cross-sectional area only provides a snapshot of root architecture and distribu- tion at the point of measurement. What lies beyond this measurement point (either towards the trunk or outward beyond the measurements) is unknown. In contrast, mea- surements of root volume or surface area in different loca- tions within the root ball can offer a more complete measure of root architecture. Submerging roots in water to observe displacement (Harrington et al. 1994) is one poten- tial means of measuring the former metric, though this becomes difficult when one works with larger root sys- tems. However, calculating the amount of root surface area in contact with the surrounding soils is challenging (if not impossible) with traditional assessment methods. Noting these limitations, several programs have been developed to create three-dimensional models and esti- mates of root volumes. Examples include, AMAPmod, FSPM, GROGRA, and SimRoot (Danjon and Reubens 2008). Beyond these purpose-built programs, other pro- grams exist which may be useful for digitally recording tree root systems for architectural characterization and analysis. Derived from remote sensing technologies, numerous programs exist which allow users to generate 3D models from data derived via laser scanning (i.e., LiDAR) or structure from motion (SfM) photogrammetry. Of these two means of collecting spatial data, SfM photo- grammetry requires relatively basic equipment (a digital camera) making it more accessible for arboriculture researchers (Morgenroth and Gomez 2014). In their proof of concept paper, Morgenroth and Gomez (2014) found that aboveground tree height and stem diam- eter could be obtained using SfM photogrammetry with minimal error (RMSE < 4%). While somewhat less accurate, they were also able to obtain estimates of aboveground 271 volume from digital models derived from SfM photogram- metry. Volume estimates for the main stem had an associ- ated RMSE of 12%, while estimates of branch volume had a higher RMSE of 47.5% (Miller et al. 2015). In using SfM photogrammetry to model diameter at breast height (dbh), Liang et al. (2014) obtained a similar, though smaller RMSE value (6.6%). The accuracy of the latter work was deemed to be on par with results typical for terrestrial laser scanning (Liang et al. 2014). Following these findings, Koeser et al. (2016) utilized SfM photogrammetry to model volume and sur- face area for individual root segments and whole root sys- tems. In comparing their computer model-derived volumes to volumes derived via water displacement, they observed an RMSE of 12.2%, with the SfM photogrammetry-based measurements having a positive bias of 5.3% (Koeser et al. 2016). Building on these past works, the purpose of this study was to: 1) determine the effects of planting depth on above- and belowground tree growth; 2) use root architecture measures derived from SfM photogrammetry to assess the impact of root depth and distribution on resistance to uprooting; and 3) determine what impact planting depth had on root architecture. This study serves as a continua- tion of past efforts to demonstrate the potential of SfM photogrammetry in tree root system modeling and serves as preliminary evidence for the utility of the method in tree biomechanics research. MATERIALS AND METHODS In April 2004, a block of 60 three-year-old Patmore green ash (Fraxinus pennsylvanica ‘Patmore’) bare root liners were planted at the University of Illinois Agricultural Experiment Station in Urbana, IL (USDA Hardiness Zone 5b)( Jarecki et al. 2005). The predominant soil type on the site was a Drummer silty clay loam with a 0 to 2% slope (USDA Natural Resources Conservation Service 2013). Trees were 2 to 2.75 m (7 to 10 ft) in height. At planting, trees were assigned one of three planting depth treatments: 1) deep-planted trees were planted with the graft union 15 cm (6 in) below the soil surface; 2) moderately deep- planted trees were planted with the graft union at the soil surface; and 3) properly planted trees were planted with the root flare at the soil surface. Trees were planted in a com- pletely randomized design with 20 replicates per planting depth treatment. Trees were planted at a spacing of 3.7 m (12 ft) within the rows, and rows were spaced 4.6 m (15 ft) apart. A subset of 32 trees (i.e., 11 deep planted, 11 moder- ately deep planted, and 10 properly planted) were selected for growth comparisons and stability testing. To avoid edge effects created by trees growing on the predominantly windward (west) or leeward (east) sides of the block, only trees from interior rows were selected for final assessment. ©2019 International Society of Arboriculture
November 2019
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