©2023 International Society of Arboriculture Arboriculture & Urban Forestry 49(1): January 2023 17 street trees are planted for environmental, aesthetic, and statutory reasons, but when it comes to selection, species characteristics, site factors, costs, and man- agement and maintenance issues drive the process. The availability of species, their pest and disease resistance, the appropriateness of the species for nearby ecosystems, and their biogeographical fea- tures are also significant factors in selection (Behrens 2011; Conway and Vander Vecht 2015; Moore 2021b). Many commonly planted urban trees have a wide phenotypic plasticity for stress tolerance, growth in restrictive urban space, and limb breakage and sever- ance (Sæbø et al. 2003; Kendal et al. 2012). There has been considerable research into the eco- nomic and environmental benefits provided by street trees as part of the urban forest, which demonstrates that these benefits exceed the costs associated with managing the trees (McPherson 2003; Nowak et al. 2010; Soares et al. 2011; Moore 2022). Generally, the larger a tree and the longer it lives, the greater the value of the carbon sequestered and its other environ- mental services (McPherson 2003; Moore 2022). Many street trees die (often 20% to 40%) within the first 2 to 5 years post-planting, but others die after about 3 decades, just when they reach a size that makes them valuable assets (Roman et al. 2015; Hil- bert et al. 2019). Eucalyptus leucoxylon street trees are a significant component of the urban forest of the city of greater Melbourne, providing shade, seques- tering carbon, reducing the urban heat island effect (UHI), and mitigating the effects of climate change. They need to be managed to maximise both their per- formance and life spans. Whether they are excurrent or decurrent as adult trees, young trees tend to be excurrent. Later, decur- rent species develop a canopy where the lateral branches outgrow the main leader or trunk and form a more rounded, spreading canopy (Harris et al. 2004). In selecting street trees, the usual aim is for trees that are tall and possess a straight trunk with a dense, spread- ing canopy above head height. The trees also need to be tolerant of environmental stresses, pests and dis- eases, urban pollutants, and regular pruning (Ryder and Moore 2013). They need to be neat and tidy so that soil surfaces and infrastructure are unaltered by root systems, and the shedding of flowers, fruits, foli- age, and bark is unproblematic. Tree performance is assessed by how the tree grows within the landscape over time and is often 1850 to 1870 preferring evergreen trees with fast growth rates. Exotic conifers were the most popular, but a number of native species were also used, most notably the blue gum (Eucalyptus globulus Labill). The use of E. globulus, however, was short-lived, as many became unsightly by the late 1880s (Spencer 1986). The second phase of planting was from 1870 to 1910, using European species such as oaks, pop- lars, elms, and plane trees. Melbourne’s spectacular avenues of tall elms, with their wide, overarching canopies unravaged by Dutch elm disease, were planted at this time (Spencer 1986). From 1910, the rapid development of suburbs con- taining narrow streets within native bushland pre- sented more challenges, and eucalypts, Jacaranda, Prunus, Pittosporum, Tristania, and Lophostemon spe- cies were planted (Spencer 1986). The use of euca- lypts Melaleuca and Callistemon increased after the Second World War, when Australian native species became popular due to their ability to maintain native ecosystems within peri-urban zones and attract native fauna. This led to experimentation in the use of local indigenous species and the use of trees such as Euca- lyptus maculata, E. nicholli, E. linearis, E. leucoxylon, E. sideroxylon, and Corymbia citriodora (Spencer 1986). Interrogating the tree inventory data held by the 31 local municipal councils making up the city of greater Melbourne identified nearly a million trees from 1,127 different taxa, with the proportion of Australian species being 60%. Seventy percent of the most fre- quently planted species were native (Frank et al. 2006). There was greater taxonomic diversity among exotic species than native species, despite their lower pro- portion. Evergreen (71%) and broadleaf taxa (92%) were the most commonly planted trees, and the most popular native species were from the Acacia, Cal- listemon, Eucalyptus, Melaleuca, and Lophostemon genera, which comprised 43% of all trees (Frank et al. 2006). The genus with the highest number of taxa and the largest number of individuals overall was Eucalyptus, and the most common species was yel- low gum, E. leucoxylon, with over 20,000 individuals making it the seventh most common of the 1,127 taxa identified (Frank et al. 2006). Despite its common occurrence, the performance of E. leucoxylon in streets can be quite variable (Beardsell et al. 1993). Much is expected from urban street trees (Núñez-Florez et al. 2019). In an Australian context,
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