78 Pearce et al.: Using Size Class Distributions of Species for the Private Urban Forest and Perkins 2005). The high proportion of large trees on pri- vate land in the current sample emphasizes the importance of understanding the dynamics of the private tree estate. Size Class Analysis as a tool to Deduce Urban Tree Dynamics Urban trees in private gardens are less limited by soils and climate than in natural systems, as gardeners can and do provide supplementary water and nutrients as well as, in some cases, sheltering of young trees from common causes of mortality, such as severe frost and drying winds. This nur- turing could be expected to result in lower mortality in age classes, resulting in much flatter reverse-J curves than in a natural forest, as could the lesser competition for space. Given their nurturing, if garden trees are left to grow as they will, height is a good surrogate for age. However, height is subject to modification by the gardener, who may remove the tops of trees for various reasons, and many of whom use tree taxa for hedging and topiary, although both phenomena are uncommon. Taxa pruned for these purposes include Cupres- sus macrocarpa, Photinia glabra, and Camellia species. In the cases of the self-seeding taxa, such as Cotoneaster spp., Pit- tosporum undulatum, and Prunus spp., their voluntary nature and their tendency to be a nuisance in adjacent bushland can lead to an early demise (Zagorski et al. 2004). This trunca- tion of their potential lifespan may concentrate their incidence in the classes of lesser stature. The recent tendency to plant advanced tree stock also complicates interpretation of the data set. The fact that there was no relationship between growth rate and height class distribution removes one possible dif- ficulty in the interpretation of the results. Small propor- tions of individuals in the smaller height classes can there- fore be most reasonably interpreted as a recent failure to plant potentially tall trees rather than the result of any par- ticularly rapid transition from seedling to 8 m tall tree. The Changes The size class distribution data strongly suggested that the private urban tree estate will change in its species composi- tion over the coming decades. The study authors are confident that these changes are real because there are many species of trees that lack, or almost lack, representation in the smallest height class, and many others that lack, or almost lack, repre- sentation in the largest height class. It is inferred that the for- mer species are not being planted at a sufficient rate to main- tain their present density as mature trees, and that the latter will become more prominent in the tallest class in the future. A prime example of species in the former class is the slow- growing Liquidambar styraciflua (Figure 2). The general pat- tern is a downward trend for the potentially tall exotic trees, such as the deciduous trees, Larix spp., Quercus spp., Ulmus spp., Fraxinus spp., and Liquidambar styraciflua, and the ever- green Picea spp., and Pinus spp. (Figure 2). The native rainfor- est tree Grevillea robusta is also not prominent in the smaller height classes. Tall forest trees have given way to glossy-leaved small trees, such as Magnolia grandiflora and Michelia spe- cies. Although some of these species are widely used as hedg- ing plants, most are also planted as specimen trees, and some, such as Michelia, were not observed by us in hedges. ©2013 International Society of Arboriculture None of the four groups of trees possessed a reverse-J curve, on average. Although trees in group 4 came closest, their greatest density is in the second smallest class rather than the smallest (Figure 1). The patterns in all four groups are typical of the pat- terns shown in episodic recruitment in natural forests (Figure 1). Explaining Private Tree Preferences Researchers postulate the following possible explanatory factors for the observed shifts in private tree species composition: the vagaries of fashion consequent on the increased influence of gar- den-related industries and popular media on private tree choices; shifts in cultural meanings invested in urban trees ranging from issues of belonging to perceptions of risk; biophysical drivers, such as drought and diseases; and changes to housing trends and urban form, leading to reduced provision of private garden space. In the 1950s, the ornamental trees for front yard display that were most favored by homeowners and stocked by nurseries in Melbourne were Liquidambar sp. and Betula sp., while the Cupressaceae family and Photinia robusta were used for tall hedg- es (Kirkpatrick 2006). Today glossy-leaved evergreen trees, pref- erably with spectacular or interesting flowers or fruits (e.g., Acme- na spp. Magnolia grandiflora, Michelia doltsopa) or plants with unusual foliage color (e.g., Pittosporum tenuifolium) are being used as accents. This shift is likely to have been influenced by the increas- ing scale of garden-related economic consumption over the past fifty years allied to the growth of private garden industries, chiefly nursery, maintenance, landscaping, and arboricultural businesses. In the second half of 2009, the Australian garden nurs- ery and garden maintenance industries alone accounted for a turnover of AUD 3.8 billion (NGIA/HA 2010). The increased economic importance of garden consumption has been accompanied by increased investment in tree-related adver- tising and significant growth in gardening content in popular media, such as television programs and magazines (NGIA/ HA 2010). While responding to resident tree species prefer- ences, this market activity explicitly seeks to influence con- sumer demand and, in particular, to promote novelty and changing fashion as mechanisms of sustaining the consump- tion of tree ‘products’ in the face of the longevity of many trees. This activity raises the prospect that many healthy trees may be removed at a frequency dictated by the vagaries of fashion (Kirkpatrick et al. 2012). It is interesting to note that currently the most highly fashionable trees, those in group 4, are among those that demand the greatest amounts of wa- ter, suggesting that such fashion is not necessarily attuned to environmental conditions. This finding is indeed remarkable in the Melbourne context, as the city experienced prolonged drought through most of the first decade of this century, lead- ing to water restrictions, increased water prices and govern- ment education programs aimed at promoting ‘water-wise gardening.’ Interestingly, the desire for water efficiency in plants is conflicted by a converse preference for broad leaved, dense canopy trees, which, due to their shade and high levels of evapotranspiration, cool cities (City of Melbourne 2011). In cities and elsewhere, trees are carriers of cultural mean- ing (Jones and Cloke 2002). Thus, changes in tree preferences are likely to reflect underlying cultural shifts. The finding that recent tree plantings (group 4) are likely to provide better pri- vacy than earlier, tall-tree plantings is in keeping with the de-
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