228 DBH values, the odds ratio of protrusion could be computed and compared among possible tree species to be selected (Table 4). An acceptable odds ratio would imply a species’ suitability for roadside greening. The required open soil area could then be calculated with species-specific regression equations (Table 5). A database archiving the key dimensions of a mature specimen of the selected tree species would be neces- sary. The known values related to tree species as well as planned pavement configurations could be substi- tuted into the equation. Normally, protruding roots and/or flares would stick out from the tree stem located in the tree pit centre. Therefore, in order to avoid pavement damage and tree-health decline, the halved length and width of the open soil area must exceed the predicted magnitude of protrusion. If exceeded, an expansion in the open soil area would be required. For existing trees showing signs of potential protru- sion, tree-pit enlargement could be justified by regres- sion outputs generated in the manner of this study. In some studies, substrate amendments such as a gravel base layer or structural cell and soil texture modifications were shown to buffer root diameter growth and direct roots downward (Smiley 2008; Rahardjo et al. 2016; Giuliani et al. 2017; Ow and Ghosh 2017; Johnson et al. 2019; Lucke and Bee- cham 2019). In the case of tree pits on pavement, the possibility of using such measures may be restricted. The required load-bearing capacity may be offered by intentional compaction of backfilled, native, fine-soil particles without additional structures. Eventually, the chief strategy must be to reserve sufficient open soil area for healthy tree growth. The enormous amount of information in Tables 4 and 5 may be overwhelming. The complexity posed a huge contrast to the simple allometric equations pre- sented in past studies (e.g., Hilbert et al. 2020). In fact, a reduced set of significant predictors could be conve- niently distilled using hierarchical and stepwise regression. However, in this non-interventional tree survey, experimental control was severely limited. Total control of planting environment was impracti- cal. Hence, with the ability to exercise statistical con- trol, simultaneous regression was preferred (Keith 2019). Nevertheless, future studies could be con- ducted in the format of controlled experiments. Limitations and Future Studies Pavement damages may occur well before protruding roots and/or flares touch the edge of paving materials. ©2022 International Society of Arboriculture Lee: Prediction of Pavement Damages Associated with Urban Trees In fact, during the tree survey, spalling and cracks were observed even when flares and roots were yet to be reckoned as protruding. Techniques such as ground- penetrating radar, of course, could facilitate the detec- tion of root architecture which is invisible at the surface. Root position of urban trees, mostly up to 0.6 m deep, would be detectable within the reach of ground- penetrating radar in urban applications (Jim 2003; Grabosky and Bassuk 2016; Altdorff et al. 2019). Although technically possible, the time and financial costs might render these technologies ineffective, if not impractical, for a district-wide sampling as in the present research. The propensity of protrusion to cre- ate damage merits another lengthy examination. When further analyses are published, the understand- ing of protruding roots and/or flares as an indicator of pavement damages can be improved. Physiological compromises related to the tree roots were not examined in this study. It is acknowledged that deep burying, girdling roots, and root pruning may hinder initial tree establishment, taper develop- ment, and reduce the long-term survival of urban trees (Arnold et al. 2005; Arnold et al. 2007; Blunt 2008; Day and Harris 2008; Day et al. 2009; Gilman and Grabosky 2011; Benson et al. 2019a, 2019b). While root-related defects had been noted during the tree survey, the present research was purposed to analyse the dendrometric and habitat measurements with atten- tion to the protruding parts of the trees. A future research direction would be to expand the allometric analysis in this study to tree defects and disorders, not just in the root-soil system, but also on other parts of trees. CONCLUSION In this study, protruding roots and flares, in tandem with other dendrometric and habitat factors, were measured. Emphasis was placed on trees growing in tree pits along pavements built with bricks or con- crete. Various statistical comparisons and regression models were carried out to quantify the relationships between pavements and trees. Among 14 species, 1,100 trees showed complex patterns and divergence in the distributions of key dimensions and habitat conditions. Based on linear regression, allometric relationships between DBH and TFD were estab- lished. DBH was a significant predictor of TFD. But DBH-TFD relationships were weakened if only trees with protruding roots and/or flares were included in the prediction models. Still, with the purpose of avoiding pavement damage, more conservative TFD estimates
July 2022
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