36 Grabosky et al.: Observed Symmetry and Force of Plantanus × acerifolia (Ait.) Willd. Roots morphology in size, branch occurrence, or cross-sectional shape. As a root grows, it must displace the surrounding soil. While root growth pressure measurement is an important aspect for study, the ability to exceed the surrounding soil’s nonconfined shear strength is required to displace soil to accommodate radial growth. As such, the soil environment is of practical importance in considering urban tree root growth patterning. The pattern of perennial growth in a woody root is possibly an indication of displacement opportunity and likely stress distribution growth response. Relationships between mechanical advantage and root cross-sectional shape in shallow horizontal roots close to the root-shoot transition are well developed in some forest conifers (Eis 1974; Coutts 1983; Coutts et al. 1999). Eccentric root growth can reflect impediments to displacement in the soil by stones or structures. There is commonly accepted field wisdom in root de- cay investigation for an expectation of an upward offset in cam- bial growth. Such growth offset would be consistent with both lesser confining resistance (unit soil weight above versus displac- ing downward against all subtending soil solids) and loading in- duced growth adaption from wind-sway in i-beam and t-beam root descriptions (Coutts et al. 1999; Weber and Mattheck 2005). Discussions of tensile loading on buttress roots and the de- velopment of root plate architecture (Fayle 1968; Vogel 1996; Coutts 1983; Coutts 1987; Gartner 1997; Nielson 2009) sug- gest an upward growth adaptation. However, at some distance, root form shifts to a different configuration, wherein roots adopt a rope-like morphology in secondary growth (Fayle 1968; Eis 1974; Wilson 1975; Coutts et al. 1999). What seems to be lack- ing is data on radial growth direction from the pith as distance from the trunk increases through and beyond the structural root plate of an urban tree. A root could be concentric or even have downward growth offset, but display a net lift in soil position if the resistance to displacement downward was high compared to a lesser energy use requirement for an upward soil displacement. Smiley (2008) presented data from an experiment where London Plane trees Plantanus × acerifolia (Ait.) Willd. were grown next to various pavement protection systems for 10 years. One treatment used a double layer of extruded rigid polysty- rene foam board (Foamular 150, Owens-Corning) to separate the root zone from the pavement wearing surface. At the con- clusion of the experiment, the pavement surface was removed and roots were counted and measured under the foam layer. It was discovered that a limited number of roots had grown be- tween the two layers of foam. Upward pavement surface dis- placement was measured in the study and was reported to be 1.5 mm mean lift in the foam protection treatment (Smi- ley 2008). The presumption is that root expansion resulted in corresponding deformation or displacement of the foam. The study presented a novel system to begin definition of a root element to model growth impacts in a pavement section fi- nite element model. The roots were between two homogenous foam layers of equal likelihood for displacement. The foam is able to be defined and the root-caused deformation can be rep- licated in the laboratory with standard engineering test methods. Root growth pattern upward versus downward, or horizontally versus vertically in cross-sectional view, could verify any need for growth directional offset in a computerized root for first it- eration definition of the computerized root growth model. While the comparison of root caused foam deformation and laboratory simulated testing cannot define the maximum force generated by the roots, it can certainly establish a pos- sible minimal force. This information can then be applied to an FEM for further modeling of root-pavement conflicts. FEM has been initiated with empirical testing of a mechani- cal simulation of a root in compacted sand (Grabosky 2009). The purpose of the current study was to use the roots grown between foam layers to: document gross root growth pattern, that is, to determine if there is an upward or down- ward growth offset in lateral root growth; to document the nature of the foam failure caused by radial root growth; and to replicate the foam deformation to estimate the minimum forces generated by radial tree root growth. In a study like this, having a large number of roots to examine would be highly desirable. However, since it was the intent of the origi- nal research project to exclude roots from between the foam boards, the opportunity to have well-developed, ten-year-old roots grown in this foam board environment could not be dis- carded. The information derived is only a starting point for the study of tree root growth morphology and forces. As such, the data presented are to inform computer model root simulation characteristics, and not to be considered generally representative of a root inventory or general species behavior for growth under pavement. The data does provide a first, al- beit limited, data set for tree roots growing under pavement. MATERIALS AND METHODS Root sections from trees in the foam treatment set were opportu- nistically harvested, photographed for growth direction, labeled with associated foam pieces, and shipped in boxed sections for analysis at Rutgers Urban & Community Forestry Labs. Two ma- jor roots each from two trees as treatment replicates (labeled tree A and B for this paper), were available for analysis. The foam- pavement system was located 50 cm from the center of the tree trunk, which had grown from 4 cm measured at 0.15 m elevation to a final 8.7 and 7.1 cm diameter at 1.37 m elevation (trees A and B respectively), in the ten-year duration of the study. Pavement surface lift from trees A and B were 2.41 and 1.78 mm, respec- tively, as measured against elevation benchmarks (Smiley 2008). Smaller roots were not measured for sectioning and measurement as the study authors were interested in the larger foam deforma- tions in this limited set of root observations. The foam sections were labeled upper and lower and taped into position around the excised root section for shipping, thus determining and preserving the direction of the root (up versus down). The plane between the foam sections was considered the horizontal measurement plane. Roots were measured in transverse section every 5 cm. The bark inclusive radius distance from root perimeter to the pith in the vertical and horizontal directions were measured to the nearest millimeter. Mean radius for each section was used to develop whole-root sample means. Upward growth radius from pith was subtracted from downward growth radius to provide an estimator for vertical growth offset for each section, with zero representing the pith centered within the root. Data reported here divided this estimator by its section vertical diameter and were compiled and reported as percentages. Percentage data were transformed by taking the arcsine of the square root prior to analysis. Similarly an estimator for horizontal root growth ©2011 International Society of Arboriculture
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