Arboriculture & Urban Forestry 39(2): March 2013 creasing cultural importance of gardens as bearers of cultural conformity (see Timms 2006) and their increasing importance as sources of privacy and as sites in which to express individu- ality (Daniels and Kirkpatrick 2006a; Kirkpatrick et al. 2009). The intensifying of a risk avoidance and public liability culture over the past twenty years in Australia has reduced the attractiveness of large (especially brittle) trees to residents, not least because of increased maintenance costs. Planting guides suggest that cypresses and eucalypts unpredictably drop large limbs, while paperbarks and wattles are also prone to collapse at a relatively young age. Additionally, the impact of the Victorian bushfires in the summer of 2009 and the subsequent amendments to the planning scheme as a result may have influenced home- owner perception of fire risk from trees (Kirkpatrick et al. 2012). Perhaps the most significant ways in which trees bear cultural meaning in Australia is in relation to questions of place and be- longing (Holmes et al. 2008), questions that run deep in colonial societies (Kirkpatrick 2006). The latter part of the twentieth cen- tury saw a surge of expression of nativism in urban gardens in Australia (Timms 2006; Head and Muir 2007). Some gardeners restricted their tree palette to natives, or local indigenous spe- cies, while others used natives in combination with exotic trees (Daniels and Kirkpatrick 2006a; Kirkpatrick et al. 2007; Kendal et al. 2012). This shift is reflected in the data, particularly in the prevalence of bottlebrushes (Callistemon spp.), native trees that flower profusely, do not grow too tall, and rapidly form dense, obscuring foliage (see Appendix). Habitat gardening is an emerg- ing private garden ideology directed toward the succor of native birds, reptiles, frogs, and mammals (Grant 2003). Large trees provide better and more habitat for many native species than an equivalent area of smaller trees, shrubs, or lawn (Stagoll et al. 2012). Although native animal species use many exotic trees for food and shelter (Low 2003; Daniels and Kirkpatrick 2006b), habitat gardeners tend also to be nativists of the local variety. However, habitat gardeners are too rare in suburbia to have much influence, judging by the very small incidence of com- plex native gardens in Hobart suburbs (Kirkpatrick et al. 2007). Changing environmental values have also seen increased uptake of chemical-free, productive gardening in Austra- lian cities, sometimes under the banners of organic garden- ing or permaculture (Timms 2006). This change has recently seen the longstanding habit of planting fruit trees in Australian backyards extended to front gardens (Kirkpatrick et al. 2007). Several biophysical factors are likely to be implicated in the observed tree species composition. The droughts and heat waves of the 1990s and 2000s resulted in morbidity and mor- tality of a wide variety of trees (Kendal 2011). For example, personal observations on the part of the authors suggest that there was widespread mortality of Camellia spp. and Betu- la spp. in Melbourne during the February 2009 heat wave, and the city’s dominant CBD tree, Platanus, suffered greatly (City of Melbourne 2011). Pests and pathogens have also led to the loss and reduced planting of some species (e.g., Ulmus spp. in Melbourne, Acacia iteaphylla in Hobart). The recent- ly introduced myrtle rust may threaten a large proportion of the garden trees of Melbourne, particularly natives. Pine and cypress trees may have become unpopular because they pro- vide excellent breeding habitat for the native brush-tailed and ring-tailed possums, which in turn feed on suburban fruit trees, exotic ornamentals (e.g., roses and camellias), and vegetable 79 crops (Kirkpatrick 2006). These biophysical factors highlight a vulnerability that comes with relying on a few species of trees. The trend toward smaller urban trees is also a reflection of the changing physical structure of the urban environment. There has been a marked increase in the residential density of Australian cities over recent decades (Beeton et al. 2006). This closer settlement has been achieved with the subdivision of existing suburban plots and the building of new houses on smaller lots. Coupled with a significant increase in the size of Australian homes, these changes have seen the extent of pri- vate garden space reduced considerably (Hall 2010). Com- bined with heightened interest in risk avoidance, this trend toward smaller gardens militates against the planting of tall trees. The recent uptake of rooftop solar power systems in suburbs and the growing trend for vegetable gardening are likely to further contribute to the planting of smaller trees. The trend toward shorter stays in individual residences, whether renting or owning, is also likely to contribute to a shift away from slower growing, larger, and long-lived tree species. Given the eight-year mean turnover of house resi- dency (ABS 2009), planting a tree that takes decades to reach a substantial height requires selflessness and foresight. Planning and Management Implications Changes in the species composition of the private tree estate reported here have important implications for management efforts to optimize the social and environmental benefits afford- ed by the urban forest. The finding that large trees are likely to be less common in private gardens than in the past is especially significant as large trees are widely regarded to deliver a dis- proportionate share of urban forest benefits (Moore 2009). For example, large trees tend to become independent of watering by humans as their roots gain access to groundwater, provide vital urban habitat for many native animal species, and may figure more prominently than small trees in social processes of place- making (Wolf 2005), as they are highly visible in the landscape and are experienced as shared characteristics of a local place. Moreover, the challenging conditions and space constraints experienced by street trees mean that unless new and novel opportunities to plant large trees in public spaces (such as in the roadway) are afforded, the population of large trees will continue to decline. The canopy cover targets that are commonly used to indicate urban forest benefits by municipal tree managers will be considerably harder to deliver with a smaller private tree stock. An important management context for such canopy cover tar- gets is the potential for anthropogenic climate change to exac- erbate urban heat island effects (e.g., City of Melbourne 2011), thereby posing risks to public health and well-being, built infra- structure, and urban biodiversity, amongst others. In this context it needs to be noted that the large deciduous trees in group 3, if properly situated in relation to adjacent housing, are known to moderate extremes of temperature and humidity and thereby lower domestic energy consumption. The replacement of these trees by the smaller species in group 4, along with the potential heating effects of increases in the built fabric, raises the pros- pect of positive feedback between increased domestic energy consumption, global climate change, and local urban heat island effects (Hamin and Gurran 2009; Kendal 2011). Additionally, carbon storage by large trees is up to 1000 times greater than ©2013 International Society of Arboriculture
March 2013
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