Arboriculture & Urban Forestry 39(2): March 2013 of individuals in a class declines as tree size become larger), it is assumed that the species constantly produces seedlings most of which suffer mortality at various stages before reach- ing maturity. This inferred continuous recruitment contrasts with the episodic recruitment that is implied by a lack on individuals in the small size classes. If, as in the cases of Eucalyptus spp. and Callitris rhomboidea at Paradise Gorge in Tasmania, Australia (Harris and Kirkpatrick 1991), one species (Eucalyptus) has individuals only in the larger size classes, and the other (Callitris) demonstrates a reverse-J curve, it is reasonable to deduce that in the continued absence of disturbance suited to the regeneration of Eucalyptus, the stand will be dominated by Callitris. By extension, in the urban forest a reverse-J curve could indicate that the species has enjoyed continuing popularity among private residents, while a gap in the smaller size classes could indicate that the species is no longer planted. The causes of natality and mor- tality in urban trees are likely to be very different to those in natural forests, and so researchers might expect other pat- terns of size class distribution than the two described here. In the present paper, size class analysis was applied to contrib- ute to an understanding of the role of private residents in shaping the dynamics of the urban forest in ten suburbs in Melbourne and Hobart, two southeastern Australian cities in temperate climates with populations of 4 million (Melbourne) and 220,000 (Hobart) people. After assessing the suitability of the method, researchers deduce the major compositional changes that are taking place. The authors then speculate on the reasons for these changes and discuss their ecological, social, and functional implications. METHODS Data Collection Five suburbs each in the cities of Melbourne (37°47’S, 144°58’E) and Hobart (42°52’S, 144°58’E) were chosen to represent a wide range of socioeconomic and settle- ment characteristics (Table 1). There is considerable vari- ation in household income and educational status in the sample, the extremes being Albert Park and Malvern at the high end and Risdon Vale at the low end (Table 1). The inner suburbs of Fairfield, Albert Park, and North Hobart largely consist of Victorian era conjoined or ter- % Aus born % renting Albert Parkz Croydonz Claytonz Malvernz Sandy Bayy North Hobarty Lenah Valleyy South Hobarty Risdon Valey z Melbourne suburbs. y 39 42 25 54 27 33 50 22 36 27 75 race houses, typically 100–150 m2 with small front gar- dens and larger back gardens. Block size is small com- pared to the outer suburbs, many of which had substantial native vegetation cover as recently as 1961 (Table 1). Outside the Victorian era suburbs, the typical residence is a single-story, detached dwelling, typically 150–200 m2 , sitting on the middle of its allotment. Front and back gardens are typically substantial. Ten blocks with- in each suburb were selected randomly as this number was enough to typify the suburb. An average suburban block in Melbourne and Hobart contains 40 dwellings. Tree species were defined as woody species that attain a height of 8 m or greater on maturity fol- lowing the most widely used Australian structural classification (Specht 1972). In some cases, it was not- possible to consistently distinguish between related spe- cies when viewing them from public spaces. These spe- cies were aggregated into groups of species with similar appearance for analysis. In most cases, this aggregation took place at the generic or family levels. In each block, all individuals of all tree species or species groups visible from streets or laneways were recorded in one of the fol- lowing height classes: <1.5 m, 1.6–3 m, 3.1–5 m, 5.1–8 m, and >8 m (hereafter referred to as: <1.5 m, 1.5–3 m, 3–5 m, 5–8 m, and >8 m). The height classes were selected to enable easy estimation by reference to adjacent houses and fences and to incorporate two commonly used lower height limits for trees, 5 m and 8 m. The rationale for recording individuals in height classes was that within a taxon, tall- er individuals are likely to be older than shorter individu- als. Thus, a predominance of tall individuals over shorter ones indicates that a tree taxon was planted more so in the past than the present, and a predominance of shorter over taller individuals indicates that a tree taxon was planted more in recent years than in earlier years. These infer- ences can be used to predict the general nature of com- positional change in the urban forest, once differences in growth rate and mature tree height are taken into account. Each tree was noted to be from one of front gardens, back gardens, streets, or other public land. In the case of back gardens, it was only possible to consis- tently record individuals >8 m tall. Therefore any vis- ible, shorter individuals were not used in backyard analyses. Table 1. Characteristics of suburbs (Bureau of Statistics 2008), in Melbourne (VIC) and Hobart (TAS), Australia. Suburb Median household % residents weekly income (AUD$) Fairfieldz 71 74 80 33 75 73 81 86 82 89 1046 1619 934 736 1720 1082 947 1062 963 637 33 41 18 30 40 37 31 24 35 3 Mean block tertiary educated size 2006 (m2 487 268 953 666 573 819 473 729 1464 * ) 100 94 27 67 94 72 94 24 69 * % blocks with % blocks bush house in 1961 in 1961 0 0 20 0 0 6 0 35 18 * Hobart suburbs. Note: Asterisk (*) indicates data were not available for Risdon Vale, as it was collected via aerial photographs for each of the other suburbs by Kirkpatrick et al. (2011). ©2013 International Society of Arboriculture
March 2013
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