©2023 International Society of Arboriculture 18 and (3) access must be available to measure the can- opy spread in a north-south and east-west direction. Data were sought on the age of the trees, but the infor- mation was not available, and so DBH was used as the basis for selecting trees for assessment. In gen- eral, assessed trees were considered to be older than 10 years. Sufficient trees were identified to allow 300 specimens growing across the city to be assessed with the following information and data gathered. Location, Site, and Soil Tree position and a detailed description of the tree’s location on the street were recorded (Table 1). The street tree environment included road type, soil profile, and compaction and physical constraints that might affect a tree, such as the presence of utility lines and compe- tition from groundcover vegetation and from other trees. Site soil-surface and paving, as well as any root damage to landscape infrastructure, were also noted. Soil compaction was measured by pushing a 10-mm-diameter, 1-m-long steel spike into the ground within 1 m of the base of the tree trunk, which is a common field assessment technique used by arborists in Australia. Pressure was stopped when resistance was met. This was done at least 3 times, to ensure there was no obstruction to penetration. If measure- ments differed within the 3 tests, the process was repeated until there were 3 consistent measurements. When pushing the spike into the ground, the resis- tance of the soil to its insertion was recorded on a 5-point descriptive scale: very soft, soft, medium, firm, and impenetrable. In many cases, street trees were growing directly under electrical or telecommunication cables, and so the incidence of trees growing under or near service lines and whether tree canopies interfered with cables were recorded (Table 1). The position of street trees in relation to other trees and infrastructure, such as buildings and utility poles, and whether trees made contact by foliage or smaller or larger branches were recorded (Table 1). Growth Habit and Phenotype Data were collected to provide a phenotypic descrip- tion of E. leucoxylon trees growing across the city of greater Melbourne (Table 2). Height was measured from soil surface to the highest living canopy point using a Silva® clinometer/heightmeter (Silva, Sandy, UT, USA), and DBH at 1.3 m was recorded. Canopy characterised by measures such as growth rate, height increment, canopy spread and density, and measure- ments of diameter increase. Other components of per- formance include crown condition and integrity, and tree health, which includes structural integrity and expected longevity (Harris et al. 2004; Day et al. 2009; Day et al. 2010; Gilman et al. 2010; Leers et al. 2018; Núñez-Florez et al. 2019). Consequently, this research was undertaken with 2 primary objectives. The first was to assess the characteristics and perfor- mance of E. leucoxylon currently growing across the city of greater Melbourne against arboricultural crite- ria that would provide a measure of their performance and suitability as street trees. The second objective was to use the data obtained to identify variability occurring between individuals and to establish crite- ria which could assist in selecting superior trees with an appealing urban phenotype for future propagation and planting across the city. MATERIALS AND METHODS All 31 municipalities within the city of greater Mel- bourne were contacted to inquire as to the details of the locations of E. leucoxylon plantings within their region. Extensive lists were provided by only 2 of these city councils, but responses from others mentioned a few selected streets with specimens. Initial assess- ments commenced in Frankston, but sampling single trees in different streets involved too much travel time to be practical, so sampling was based on choos- ing streets at random where at least 3 to 5 E. leucoxy- lon trees could be assessed (so that a total of 300 trees [n = 300] could be assessed). Suburbs were visited, and locations where there were sufficient trees were identified so that trees could then be selected ran- domly for assessment. Trees were sampled from a wide range of locations across the city, and the large numbers provided an indication of species perfor- mance within the city. Field work was undertaken between the months of April and October (Australian fall, winter, and spring), and it should be noted that under Australian conditions, evergreen species (such as eucalypts) can grow year-round but may have win- ter and/or summer dormancies depending on the tem- peratures in winter and the aridity of summer. In selecting trees for assessment, the following cri- teria were applied: (1) trees must have a diameter at breast height (DBH at 1.3 m) of at least 10 cm; (2) there must be clear access to the base of the trunk; Moore and Chandler: The Potential of Yellow Gum as an Urban Street Tree
January 2023
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