Tourism brochures would have us believe that almost every Australian city is a ‘city of gardens’. Since tourism marketeers are just real estate agents on uppers, it’s reasonable to ask sceptically, ‘which cities do have the most green space?’ or more simply, ‘which cities have the most trees?’
Ecologists and geographers love to search for spatial patterns, and might re-frame these questions in a different way: does the density of trees vary across cities in a predictable way? To what extent is urban tree density influenced by climate or latitude (from north to south) or socio-economic factors?
If we counted trees in six big cities in eastern Australia – from tropical Townsville in the north, through Brisbane, Sydney, Adelaide, Melbourne, to Hobart in the south – would we see a north-south trend? Would tropical cities have fewer or more trees than the cooler southern cities? Alternatively – if you prefer to punt for your home town – would any of these cities have more trees than the others, regardless of where it is? To paraphrase the song, which city is ‘the real City Shady’?
Fortunately, we can ignore the brochures and address the issue factually, based on a study by Jamie Kirkpatrick and colleagues at the University of Tasmania. Like the Victorian study in my last blog, the researchers counted trees in selected suburbs in each city, and compared the numbers against a range of socio-economic factors. They ignored city parks and counted trees in front and back yards and along city streets.
Who wins the Shady City Award?
The answer (as always) depends on where we look and what we count. When we peek over the fence and count how houses do and don’t have a tree in the yard, the answer is – surprisingly – a dead heat. Most yards in each city – from 70% to 85% – contain a tree, and this percentage doesn’t differ significantly across the six cities.
But some yards have more trees than others. The average density of garden trees ranges from about 60 trees per hectare in Adelaide and Hobart to over 100 trees per hectare in Melbourne and Brisbane. By this measure, Melbourne and Brisbane win the Green Yard Trophy while Adelaide and Hobart share the wooden spoon.
Garden styles change from north to south. Eucalypts, conifers and deciduous trees are uncommon in northern gardens, and most trees in Townsville and Brisbane are other evergreen trees (e.g. sub-tropical trees and palms). Sydney gardens have the most conifers and few deciduous trees, and the three southern cities contain a mix of tree types.
Who wins the Shady Street Award? Unfortunately, it’s not Hobart. Hobart bottoms out again, and just 18% of houses in Hobart have a tree on the nature strip. That’s a pretty bare street-scape. The winners are … Melbourne (once again) and Adelaide. In both cities, 54% of the surveyed nature strips contained a street tree.
On both counts – the density of garden trees and the number of nature strips with trees – my old hometown Melbourne is, without a doubt, ‘the real City Shady’. All the others are just imitating. And Hobart? Well I guess the people of Hobart get a lot of good views.
The unshaded suburbs
My last blog showed that – in cities in regional Victoria – tree cover was highest in suburbs that had more educated residents, more migrants and more middle-sized blocks.
The Kirkpatrick study found similar patterns in the capital cities. More house blocks contained trees in suburbs that were older, had bigger blocks and smaller houses, more employment and more residents who were born overseas. [The density of trees in yards showed a more complex pattern, because many expensive rental properties in inner suburbs also contain lots of trees].
Thus, in capital and regional cities we generally find more trees in older suburbs, on larger suburban blocks, where education, employment and/or income are high; and where there are many recent migrants.
Surprisingly, street trees follow much the same pattern; even though street trees are planted by local government rather than residents. More houses have a tree on the nature strip in older suburbs and suburbs with more employed residents; and there are slightly more street trees outside each property where residents earn more money. Why? The authors suggest:
The fact that the number of street trees in 2006 was greater where median household income was higher could result from a reluctance of the poorer LGAs [Local Government Areas] to spend on a perceived luxury, but … may also reflect pressure from ratepayers to plant street trees in richer suburbs and pressure not to plant trees, or to remove them, in poorer suburbs. (Kirkpatrick et al. 2011, p. 250).
These findings … are consistent with the idea that unequal power relationships between the community and local government can lead to increased public investment in public landscapes in more advantaged areas (Kendal et al. 2012, p. 262).
Both studies counted trees in the ground, not planting rates or budgets. It would be fascinating to compare tree planting, survival and turnover rates across socio-economic gradients in a future study. An American study, for example, found that street trees were more likely to die in areas of high unemployment.
Is it getting better?
Our cities need more trees, but are we getting them? The strongest – and perhaps most unexpected – pattern in the Kirkpatrick study is the big increase in trees in all six cities over the past half a century. The authors counted trees on air photos from 1961 and 2006. Over the 45-year period, when averaged across all six cities:
- the proportion of houses with a tree in the yard increased from 57% to 78%,
- the density of trees in residential gardens increased from 26 trees/ha to 85 trees/ha,
- and the proportion of houses with a street tree increased from 22% to 41%.
That’s a fantastic change; although there’s lots of room for improvement. The trend may be hard to continue in the future, especially on new sub-divisions with small blocks and big houses. But we can easily plant more street trees.
The capacity for planners and land managers to increase the number of trees in their cities is greatest on streets, with 60% of our sampled houses having no tree in front of their block, while only 20% lack a tree in their garden. If it is assumed that the 20% of householders who do not have a tree in their garden would react negatively to attempts to plant trees on their streets, there is still a capacity to double the number of houses with street trees (Kirkpatrick et al. 2011, p. 251).
I can’t imagine Slim Shady would ever say it, but everybody ‘please stand up, please stand up, please stand up’ for our street trees. So the shady marketeers of the future can truthfully claim every single city as the ‘the real City Shady’.
Dr Dave Kendal and Professor Jamie Kirkpatrick kindly fact-checked this week’s post. The wonderful image of the Brisbane swimming pool is taken from cuttsblog. This is the second in a series of posts on urban trees. In the next post: why do residents plant and remove urban trees?
Kendal, D., Williams, N.S.G. & Williams, K.J.H. (2012) Drivers of diversity and tree cover in gardens, parks and streetscapes in an Australian city. Urban Forestry & Urban Greening 11(3), 257-265.
Kirkpatrick, J.B., Daniels, G.D. & Davison, A. (2011) Temporal and spatial variation in garden and street trees in six eastern Australian cities. Landscape and Urban Planning 101(3), 244-252.
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- 50 shades of green: cooling the suburbs
- Why do we plant and remove urban trees?
- The natural secrets of urban well being