Arboriculture & Urban Forestry 32(4): July 2006 161 nooka (Tristaniopsis laurina [Smith] Peter G. Wilson and Waterhouse) because these taxa have good characteristics for street tree planting and are present in sufficient numbers to allow suitable selections to be made from extant populations. Of the exotic trees, various ornamental Prunus cultivars make up the largest group with 9% of the total tree popula- tion. These are mostly forms of cherry plum (Prunus cera- sifera Ehrh). Planes are also a popular street tree as a result of their urban tolerance. They are a common avenue tree in Melbourne CBD and many inner city streets. Oaks (Quercus), ash (Fraxinus), and elms (Ulmus) are also common. Elms (Ulmus) in Australia have not been exposed to Dutch elm disease and consequently they form significant, world- renowned mature avenues leading into the city of Melbourne (Spencer et al. 1991). Biodiversity Maintaining a diverse urban forest has significant ecologic implications as well as potentially minimizing dramatic loss of trees as a result of pest and disease outbreaks associated with monoculture systems (Bassuk 1990). As urban consoli- dation continues, the streetscape can become important open space for the conservation, enhancement, and sustainability of Australia’s genetic resources. For example, the Maryland Department of Natural Re- sources has developed a methodology for assessing biodiver- sity in existing street tree populations (Galvin 1999). That methodology sets target levels for taxon diversity within a street tree population using the guidelines suggested by San- tamour (1990) that there should be no more than 30% of any one family, 20% of any one genus, or 10% of one species in an urban tree population. In this study, Myrtaceae comprises 52% of the total plants. This exceeds the 10:20:30 rule. However, Myrtaceae does have the greatest diversity of genera at 8%. Myrtaceae is one of the most characteristic Australian families, providing the dominant taxa in most Australian vegetation communities except for treeless areas, rainforests, and those communities of arid and semiarid regions dominated instead by Acacia (Mimosaceae) (ANBG 2004). The high percentage of this family in streets could be seen as being analogous to natural Australian plant communities, especially in the southern part of the continent, and may not be a point for concern. As an example of this pattern, metropolitan Melbourne covers land that contains a number of different native vegetation types, including a range of forests and woodland/savannahs. The overstorey of these treed landscapes, with only very minor exceptions, was entirely composed of Eucalyptus spp. (Savio 2001). Eucalyptus is the most common genus with 13% of total trees which fits within the 30:20:10 rule. The most common species, Queensland brush box, at approximately 7% of the population, also fits within the 30:20:10 philosophy. The survey contains 76 taxa that are indigenous to the Greater Metropolitan Melbourne area (SGAP 1991). These taxa, which include yellow gum, red ironbark, red box (Eu- calyptus polyanthemos Schauer), narrow-leaved peppermint (E. radiata Sieber ex DC.), manna gum (E. viminalis Labill.), blackwood (Acacia melanoxylon R. Br.), and lightwood (A. implexa Benth.), accounted for 41,853 individuals (approxi- mately 4.5% of the surveyed population). The use of indig- enous vegetation in urban settings has become one of the foci of moves toward more sustainable horticulture (Dunnett and Clayden 2000). Restoring landscapes to the point of long- term sustainability is also a primary objective of the Victorian state government’s native vegetation framework (Department of Sustainability and Environment 2002). Finding species with indigenous distribution in this survey appears encourag- ing, but many of these taxa have wide distributions and the records did not indicate the exact provenance of any of the trees. It is highly probable that many of them were propa- gated from seed sourced outside of Melbourne. The use of locally indigenous selections of street tree species could be increased. This would increase diversity of these populations but, by fragmenting the street tree population further, would make other activities such as genetic improvement more ex- pensive to implement. CONCLUSIONS Street tree population data were collected from each of the 23 municipalities of Greater Melbourne that had conducted sur- veys. This data were combined into one superset of data for analysis. A total of 922,353 trees were captured in this su- perset of street tree inventory data comprising approximately 1127 taxa. Australian native taxa made up the majority of the trees. Wattles (Acacia spp.), bottlebrush (Callistemon spp.), gum trees (Eucalyptus spp.), paperbarks (Melaleuca spp.), and Queensland brush box comprised 394,730 speci- mens, or 43% of all plants. Of the exotic taxa, Prunus spp. were the most common with 86,227 specimens, or 9% of the total plants. The family Myrtaceae is the most represented comprising 52% of the total plants, with family member Queensland brush box being the most common taxon. The genus Euca- lyptus had the largest number of taxa and also the largest percentage of individuals. This exceeds the diversity guide- lines suggested by Santamour (1990); however, this figure could be considered analogous to natural plant communities found in southeastern Australia. Considering the importance of Australian native tree taxa within the street landscapes of Melbourne, it is interesting to note that only a small percentage of total trees (4.5%) used in streets could be indigenous to metropolitan Melbourne and that very little genetic improvement or other selection re- search has been undertaken on native plant trees used in streets. There is potential both for increased use of indigenous ©2006 International Society of Arboriculture
July 2006
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