Arboriculture & Urban Forestry 46(5): September 2020 Arboriculture & Urban Forestry 2020. 46(5):321–332 URBAN FORESTRY ARBORICULTURE Scientific Journal of the International Society of Arboriculture & Strain Patterns Across the Root-Stem Transition Zone in Urban Trees By Kenneth E. Beezley, Gregory A. Dahle, Jason Miesbauer, and David DeVallance Abstract. Trees are subjected to mechanical loading during their life span or face premature mortality. The strain resulting from loads inter- cepted by the canopy and transferred throughout the tree is of significant importance, not only for the survival of the tree, but for the safety and well-being of the human population found in close proximity. To test the function of tree orientation to an applied load, static load tests were conducted on 15 mature pin oak trees (Quercus palustris Muenchh.). We applied the static load tests to tilt the trees 0.1° from natural position. We used a digital image correlation system to map strain in the leeward, windward, and tangential roots in the root-stem transition zone. Results indicate that mean maximum strain magnitudes are similar in the leeward and windward orientations and lower on the tangential orientation. The leeward orientation experienced compressive strain, the windward orientation experienced tensile strain, and the tangential orientation had both tensile and compressive strain. This information provides the arboricultural and plant science sectors with a better understanding of how loading force moves through trees and will further enhance tree risk assessment and root zone management protocols. Keywords. Bending Moment; Digital Image Correlation; Static Load Test; Strain; Tree Stability. 321 INTRODUCTION Trees have the ability to manage strain (ε) induced from an array of naturally occurring environmental and man-made stresses placed upon both above- ground parts (leaves, branches, woody tissue) and below ground in the structural root support system. Neild and Wood (1999) describe the two main princi- ples of plant growth as the economy of nutrients for light competition and the reaction of the plant to load- ing stress by the forming of reactive tissue material in areas where strain is greatest. The response of trees to external loading events, of which wind is the single most abiotic destructive mechanism (Niklas 1992; James et al. 2006), is important to understanding tree stability, especially during formal tree risk assess- ments. Trees are prone to failure as a result of ele- vated and sustaining winds (James et al. 2014) as well as short, sporadic wind gusts (Coder 2008). In many cases, however, these failures may not be instanta- neous (Hale et al. 2010). Research has shown that trees manage stress and the resulting ε across various parts of their architecture through mass damping (James 2003; Moore 2014), shedding of woody parts (James et al. 2006), bending of stems and branches (Brudi and van Wassenaer 2002; Dahle and Grabosky 2010), stem deflection and tilt (Neild and Wood 1999; James et al. 2013a), root-plate tilt (Jonsson et al. 2006; James et al. 2013b), root anchorage (Dupuy et al. 2005), soil-root interactions (Genet et al. 2005), and adaptive growth (Niklas 1992; James 2003). However, there is limited knowledge of how loading force, stress, and resulting ε is transferred from the main stem into the root zone (Beezley et al. 2020). With the recent increase of catastrophic weather (wind or ice) events in the USA, Europe, and Australia (Moore and Maguire 2004; Yang et al. 2014), liability concerns arising from the retention and preservation of large amenity trees is of increasing concern. As more managed trees are found in populated areas, arborists and urban foresters are faced with the task of determining whether a tree should be removed to limit the risks associated with failure (Dahle et al. 2014). Predisposing factors can increase the likeli- hood of tree failure during wind events even under ©2020 International Society of Arboriculture
September 2020
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