The study of the architecture of urban trees is important for the management of urban forests to optimize their ecological and socioeconomic services. Trees have a fractal-like architecture which is disrupted by competition for light. Therefore, studying the architecture of open-grown urban trees should provide a better understanding of the inherent fractal-like character of trees. Terrestrial Laser Scanning (TLS) technology provides detailed data of tree architecture. The main scope of this dissertation was to model the fractal-structural complexity of urban trees based on different fractal analysis methods in relation to their physiological and functional traits. In the second chapter of the dissertation, a variant of the "two-surface" method was used to estimate the fractal dimension of thousands of urban tree crowns from a publicly-available dataset across the USA. It was found that urban trees reduced their crown fractal dimension to reduce water loss through transpiration in hotter cities depending on the level of urbanization at smaller spatial scales. The functional group and the life-history traits of the studied urban trees significantly affected their crown fractal dimension in response to their growing environment. In the third chapter, forty-five trees of different deciduous species (Gleditsia triacanthos L., Quercus macrocarpa Michx., Metasequoia glyptostroboides Hu & W.C. Cheng) were laser scanned in leaf-on and -off conditions on the Michigan State University campus to study the role of leaves in the fractal-structural complexity of urban trees using the "box-dimension" (Db) metric. It was found that the presence of leaves significantly increased the Db metric of all study trees, and the contribution of leaves decreased as branch network complexity increased. The leaf-on laser point clouds of the study trees were also virtually defoliated with a leaf-removal algorithm. It was found that the algorithmic leaf-removal caused biased estimates of the Db of the G. triacanthos and M. glyptostroboides trees. In the fourth chapter, the leaf-off laser point clouds of fifty-six urban trees of the aforementioned species were used to generate quantitative structural models (QSMs) to quantify their woody surface area (WSA) allometry. It was found that the variation in the above-ground WSA of the study trees related to their fractal dimension quantified with the Db metric and the distribution of "path" lengths from the tree base to every branch tip. It was also found that the urban trees allocated the largest portion of their WSA to their branches, which varied with branch order, branch-base diameter, and branch-base height. This study also showed a positive relationship between the WSA and the crown surface area of the urban trees. The fifth chapter included laser point clouds of thirty-one trees of deciduous and evergreen species that were sampled on the Michigan State University campus and the Harvard Forest in Petersham, MA, USA to model their above-ground woody biomass. QSMs were generated to estimate the total tree volume and component volumes of the study trees. Biomass estimates were produced by multiplying the TLS-based volumes with measurements of tree basic density from sample disks from stems and branches obtained after destructively sampling the trees, and also with published basic density values at species level. The leaves of the trees that were scanned in leaf-on condition were artificially removed before QSM generation. It was found that TLS technology can be used to produce reliable total and component biomass estimates of trees. The biomass estimates quality can be affected by the growing environment, the leaf condition of the laser-scanned trees and the basic density values that are used.
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