276 Miesbauer et al.: Impact of Planting Depth on Fraxinus pennsylvanica ‘Patmore’ Picea sitchensis (Bong.) Carr. trees with a rootable depth greater than 80 cm than those with less than 80 cm. Simi- larly, utilizing finite element modelling of root systems, Dupuy et al. (2005) found that heart root systems and tap root systems had a higher resistance to overturning than herringbone and plate root systems. Furthermore, Peltola et al. (2006) found that root depth was positively correlated with maximum resistive bending moment. Unfortunately, urban soils (as currently constructed) may not support root growth and development, especially deep into the soil pro- file (Watson et al. 2014). Ow et al. (2010) investigated if planting trees in topsoil or structural soils with stone:soil ratios of 80:20 and 50:50 affected bending moment, but results were inconclusive. However, they did report that root plate size was positively correlated with bending moment, which is similar to our findings. Increasing the depth of favorable soil for root growth through processes such as soil profile rebuilding (Layman et al. 2016) might prove to be an effective management strategy for improv- ing tree stability, especially in locations where lateral root growth is impeded. Future research should consider inves- tigating soil improvements on tree stability. In an early iteration of our analysis, we assessed the impact of planting depth (i.e., depth to topmost root only) on bending moment and found no statistical significance. Though we eventually adopted the analysis reported above (which takes into consideration the architecture of the whole root system), these early results were similar to the findings of Gilman and Grabosky (2011), who found that planting depth had no impact on bending stress of Q. vir- giniana trees planted in sandy soils. Similarly, Gilman and Wiese (2012) found no difference in stability between Q. virginiana ‘Highrise’ trees planted deeply in containers during nursery production and those with the root flare at grade throughout production. More recently, 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 bending stress) than those planted at grade. Every tree in this study had several large structural roots growing horizontally as well as vertically, well beyond 60.1 cm deep (Figure 2). Given the favorable growing conditions for roots deep within the soil profile at our research site, it is not surpris- ing that planting depth (a somewhat coarse metric given the complexities of root systems) was not a significant pre- dictor of bending moment. It is important to note the limitations of this research. The root volume that was measured only included those roots contained within the 244 cm diameter root ball. In reality, root systems of the trees in this study extended well beyond the width of the hydraulic tree spade used to exca- vate the root systems. That said, utilizing 3D models to measure total volume of all structural roots within the root ©2019 International Society of Arboriculture ball gives a similar, but more complete, picture than mea- suring root cross-sectional area of the main structural roots. Another limitation is that although fine root (< 1 cm diam- eter) dry mass was measured, we did not consider the over- all impact of fine roots on stability in our model of bending moment. All fine roots for each tree were combined in the field and weighed as one measure, preventing us from sec- tioning them as we were able to for the larger root volumes. Despite the above-mentioned limitations, we feel the methods outlined in this paper have the potential to advance tree biomechanics research. Once our whole root system models were generated, we were able to measure them several different ways to address questions that occurred during analysis. For example, while our interest in the presence of defects among the treatments (Table 1) was spurred by observations made in the field, logistical con- siderations made it impossible to investigate further onsite. Having a digital copy of our trees preserved this data and allowed us to go back and tally root defects and other con- ditions like adventitious rooting (Table 1). CONCLUSION Deep planting continues to be a complex issue that varies in its impact on tree health and stability. In our nine-year study, neither tree height nor trunk diameter were affected by rooting depth. Bending moment varied based on the volume of roots (opposite the pulling direction) located both at the soil surface (0 to 10 cm) and deeper in the ground (i.e., 40.1 to 50 cm and greater than 60.1 cm). Con- necting this back to our original research questions, the depth of roots (as measured by root volume) was impacted by our original planting treatments. Given that many urban trees are initially planted too deep, creating environments that are favorable for roots to grow deeply in urban soils may increase rooting stability beyond initial establishment if the species, nursery production method, and planting conditions are such that health is not impacted or the root system is not predisposed to significant root defects. ACKNOWLEDGEMENTS The authors thank the Wisconsin Arborist Association for partially funding this research effort. A special thanks also goes out to Gary Watson, Angela Hewitt, Robert Miller, Jeremy Shafer, and William VanderWeit for their efforts on the project. LITERATURE CITED Arnold, M.A., G.V. McDonald, D.L. Bryan, G.C. Denny, W.T. Watson, and L. Lombardini. 2007. Below-grade planting adversely affects survival and growth of tree species from
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