218 for pavement may allow some degree of soil moisture replenishment, whereas the surface runoff on concrete surfaces would be lost to the urban drainage system. Therefore, characterising the habitat conditions by the dimensions and type of pavement is vital. Without appropriate care, pavement may interfere with root growth. In the literature, belowground root expansion was detected and predicted with various approaches, such as statistical modelling (Johnson et al. 2019), numerical analysis (Giuliani et al. 2017; Li and Guo 2017), mechanical analysis (Grabosky and Gucunski 2019), and ground-penetrating radar (Krainyu- kov and Lyaksa 2016; Altdorff et al. 2019). But, in frontline operations, a site manager often needs to take care of a large number of trees. Simple and rep- licable methods could raise the efficiency of tree inspection. For researchers, with a considerably large sample generated from tree survey, reliable allome- tric relationships could be drawn via statistical means. Flares stemming from a trunk tapering near the ground can be quantified by trunk flare diameter (TFD), which is predictable by diameter at breast height (DBH). In order to avoid pavement damage, minimal open soil surface area could be determined according to predicted TFD values (North et al. 2015; Hilbert et al. 2020). Hilbert et al. (2020) also regressed the occurrence of pavement damage on dendrometric and habitat variables. However, in their model, the classi- fication of possible damages was lacking. The central tenet of prediction models was the higher likelihood of damage with larger TFD. Yet confounding factors (such as the geometrical shape of trunk flare) added complexities to the damage prevention through TFD prediction. Holding TFD constant, flares reaching lat- erally towards a corner of a tree pit, instead of a side, have more room for extension before inflicting dam- age. A more-direct indicator is desperately needed. Emphasis should be placed on flares and roots which are longer and larger, providing higher concern over their potential to inflict damage. Therefore, a more direct, quantitative variable could be utilised in order to characterise the potential conflict between pave- ment and trees. A narrow selection of tree species from a few gen- era were covered in previous studies on pavement damage in relation to trees. Some examples are Acer, Fraxinus, Gleditsia, Koelreuteria, Melaleuca, Plata- nus, Populus, Pyrus, Quercus, and Zelkova (D’Am- ato et al. 2002; Blunt 2008; Smiley 2008; Gilman and ©2022 International Society of Arboriculture Lee: Prediction of Pavement Damages Associated with Urban Trees Grabosky 2011; North et al. 2015; Grabosky and Bassuk 2016; Johnson et al. 2019; Lucke and Bee- cham 2019). A wider range of tree species for urban greening should be examined. Growth behaviour of trees is species-specific, so the allometric models should be as well (Semenzato et al. 2011; Marziliano et al. 2013; Oldfield et al. 2015; Benson et al. 2019b). Non-conspecific allometric equations should be care- fully and never indiscriminately applied. Previously, no attempts at allometric modelling were conducted on samples entirely consisting of trees which were in possible conflict with pavement. Therefore, more urban tree species could serve as potential samples for the present research. In Hong Kong, pavement damages have been observed around trees whose roots or flares have reached or breached the edge of the open soil surface of a tree pit. Such roots or flares are visually detect- able by inspecting the interface between open soil and pavement material. The specific terms, namely, protruding roots or protruding flares, are defined in the Materials and Methods section. The aims of this research are to (1) establish allometric relationships between DBH and TFD of common urban tree spe- cies and (2) identify factors affecting the presence and magnitude of protruding roots and flares. Practi- cal recommendations for urban greening and infra- structure management are distilled from the findings. MATERIALS AND METHODS Study Location This research was conducted in Hong Kong, China (22.3° N, 114.2° E). Approximately 24.9% of the 1,100 km2 land area was classified as built-up land (Planning Department 2020). The population size of 7.34 million are packed into limited-development areas due to the hilly terrain (Census and Statistics Department 2018). The resulting high building den- sity leaves very little space for street-level greening. Nevertheless, from 2010 to 2020, approximately 544,300 trees were planted in urban areas (Greening, Landscape & Tree Management Section Develop- ment Bureau 2021). Even in paved areas, trees are planted for landscaping purposes. Transport land use occupies 46 km2 of land surface, where tree pits dot the pavements along many roads. With a rationale of preventing pavement damage around tree pits, this empirical study focused on trees growing in tree pits in Chai Wan, Eastern District,
July 2022
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