©2023 International Society of Arboriculture 34 where the smooth bark was exposed, adding to the uniqueness and aesthetics of each tree. Flower colour does not breed true in E. leucoxylon (Nicolle 2006), but most trees (67.3% [202]) had flowers that were a variant of red or pink, which are the popularly preferred colours for this species as a street tree. Eucalyptus leucoxylon is known to flower throughout the year, and flowering has been reported in every month (Hall 1972; Brooker and Kleinig 2006; Nicolle 2006). In this research, the flowering period of E. leucoxylon ranged over 6 months, with most flowering occurring in the period from August to October. The trees provide a significant source of pollen, nectar, and seed, which attract large numbers of insects in the warmer months, including bees, and significant numbers of native honey-eaters and seed-eating parrots, such as rainbow lorikeets and sulphur-crested cockatoos (Hall 1972; Nicolle 2006). They are an important food source, particularly over winter. The large numbers of trees growing along streets provide valuable wildlife corridors that link other larger green spaces within the city of greater Melbourne. CONCLUSION This research assessed 300 E. leucoxylon street trees growing across the city of greater Melbourne for their performance against arboricultural criteria relating to canopy structure and density; straightness of the trunk; health; flower colour; and root systems. The results confirmed its suitability as a street tree and revealed that most specimens exhibited good habit, vigour, and health (assessed on canopy, trunk and branch condition, production of exudates, and presence of fungal fruiting bodies). The trees had traits such as live crown ratio, height, flower colour, and capacity to cope with pruning that are considered arboricultur- ally appropriate for a street tree. It seems likely that the species has the potential to be a successful street tree not only in Melbourne, Australia, but in other parts of the world where it has been grown success- fully in forestry plantations under warmer and drier conditions. The dense canopies and high live crown ratios of E. leucoxylon street trees provide shade that can reduce the urban heat island (UHI) effect. As so many trees have been planted across the city of greater Mel- bourne, this could be important, as the city’s tempera- tures are predicted to rise significantly under climate directly below or close to power lines, with over 60% of these trees interfering with cables. This did not include trees that were still young and which may cause future problems. Inappropriate tree planting will increase maintenance, which is already high, with over 65% of the observed trees having been pruned, which can increase the chance of insect and fungal attack (Fazio and Krumpe 1999; Montagu et al. 2003). The form and habit of E. leucoxylon and its capac- ity to cope with regular pruning means that it is suit- able for planting where power lines exist. The rapid growth of epicormic shoots in response to pruning for line clearance also means that shade and habitat value are quickly restored, as is carbon sequestration in the canopy. There is also an opportunity for selecting lower growing E. leucoxylon varieties from known seed sources. However, given the large number of trees planted across the city of greater Melbourne, it could also be argued that undergrounding of power lines with the consequent lower tree maintenance costs could be cost-effective. Generally, trees were healthy, but some were decayed and exuding resin mainly associated with damage to the trunk or the removal of limbs. Epicor- mic shoots were present in response to damage or pruning. Typically, eucalypts respond to fire damage, branch losses, and pruning by producing fast growing epicormic shoots, and E. leucoxylon possesses large numbers of epicormic buds on its trunk and branches (Moore 2021a). Many of the larger limbs removed were at a height that was intended to improve visibil- ity and sight lines for vehicles and pedestrians. The canopy of E. leucoxylon was usually distributed over a number of substantial lower-order branches, which contributed to the round to broadly round can- opy that occurred in most trees (75%). The canopy and radial branches were evenly distributed in this multi-branched habit, and the canopy typically occu- pied at least half the height of the tree. There were very few trees that had dominant leaders (22.6%) and even fewer trees that had a single leader (6.4%). An important characteristic of street trees is the form of the trunk. The grand majority of trees (32.3% [97]) had an upright trunk without a lean, but the remainder were leaning to some degree. This lean, combined with any twisting and kinking of the trunk, exposed the patterning of the different mottled bark colours typical of this species. This was especially prominent Moore and Chandler: The Potential of Yellow Gum as an Urban Street Tree
January 2023
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