50 shades of green: cooling the suburbs

Bendigo Streetscape

How many trees are in your suburb: lots, a few, not enough? What about that suburb over the river, are there more trees there? As summer heatwaves hit, the benefits from shady urban trees grow more and more obvious. But why do some suburbs have more trees and shade than others? Is it a matter of ecology, history, policy, or the people who live there? Do the folks down the road just hate trees too much?

Lots of things can influence urban tree cover, including natural features, past land use, planning processes (such as allotment sizes) and socio-economic factors (e.g. residents’ income or home ownership). Many factors are inter-twined, but some will be more important than others. If you had to guess, which do you think would be most important?

shady street
Street trees shade an inner-suburban street. Photo by Jacqueline Yetzotis, Saving Our Trees.

A few years ago, Gary Luck and colleagues at Charles Sturt University (where I work) did a big study of tree cover in residential areas.

They compared levels of tree cover against natural features (annual rainfall, length of the growing season, elevation, topography and soil fertility) and socio-economic factors: population and housing density, income, education, home ownership and the proportion of new immigrants from overseas. They ignored the capitals (Melbourne and Canberra) and sampled nine, large regional cities across Victoria and southern New South Wales, from Mildura to Albury, Traralgon and Warrnambool.

They did something else pretty cool. They examined how the people and trees in each city changed over 15-20 years, to see if patterns of tree cover now were more strongly related to socio-economic factors in the past rather than the present.

Socio-economic factors can be hard to disentangle as they often inter-related. For example, people with high levels of education often have higher incomes and are more likely to own their own home than people with fewer qualifications. This makes it hard to separate the effects of education, income and home ownership. Nevertheless, the most important factors – out of all those listed above – were the density of housing, education levels and the number of recent immigrants to Australia in a suburb. The direction of these trends may surprise you.

suburban pool
Cooling off in the shade-free suburban yard. Original photo: Canberra Times.

Tree cover was low in suburbs with the highest housing density. That makes sense, as you can’t fit many big trees in a small block. Suburbs with ‘medium’ housing density (about nine houses per hectare) had the most trees. Tree cover was lower in the biggest blocks, which had larger lawns. These labels – high, medium and low housing density – describe the variation across regional cities, and many new housing estates have high density housing, and few trees, by these standards.

Curiously, home ownership wasn’t closely associated with overall tree cover, but suburbs with high and low levels of home ownership did contain different types of plants:

Home ownership … was positively correlated with … nectar-rich plants and trees, and negatively correlated with percent cover of impervious surfaces [concrete]. This may reflect a greater orientation toward elaborate and native gardens with home ownership. The same relationships were recorded with income … so this pattern may reflect an interaction between disposable income and home ownership, whereby home owners have the motivation to invest in garden maintenance and those with greater disposable income have the capacity to do so.(Luck et al. 2009, p. 614)

What about education and immigration? The study found there were more shady trees in suburbs with more new immigrants and where residents had more educational qualifications. The authors of the paper expected that suburbs with more migrants would have fewer trees but their findings blew this suggestion out of the water. Why? Well, we just don’t know (but stay tuned for next week’s blog for more tantalizing suggestions).

new suburb
No room for trees in high density suburbs. Photo source: Herald Sun News.

The result that I think is most cool is – the factors that were best associated with tree cover in 2006 were the socio-economic factors from 10-15 years earlier. In one analysis, tree cover in 2006 was best explained by the proportion of new immigrants back in 1991, and levels of education and housing density in 1996. This highlights the lag effect, or inertia, in social and ecological systems. Past social and economic changes affect our cities now, and social and economic changes now will influence our cities for decades to come.

Trees don’t read resident’s CVs or income statements. Unlike many residents, trees don’t care about race or immigration. How do we explain why suburbs with more migrants and higher education levels have more trees and shade?

NASA urban heat map
Trees cool the eastern suburbs in this urban heat map. Source: NASA Urban Heat Islands.

Big surveys like this are important because they show us general, widespread relationships, not local idiosyncrasies. The patterns are indisputable; even if they don’t match our own local experience. But all surveys – no matter how big or small – show us patterns, not causes. We have to infer the causes from other information. In science-speak, we can never infer causation from a correlation. Every correlation might have a multitude of causes.

To illustrate, the strong relationships between social factors and tree cover could indicate (Option A) that social factors influence tree cover – ‘let’s go buy some trees, we’ve got enough money’ – or (Option B) that tree cover influences social factors – ‘I want to buy a house with a lovely garden no matter how much it costs’ – or (Option C) that trees and people are both influenced by other, unrelated factors; ‘we’re not living near the cleared industrial area, it’s too smelly’.

All of these processes interact. People with enough money to buy a house in an attractive shady suburb may choose to plant more trees in the future, and will certainly have enough money to do so. Indeed, the study found that the relationships between tree cover and socio-economic factors grew stronger over time, which suggests that suburbs grow more and more different as they get older. In the author’s words:

Intuitively, this makes sense as vegetation cover may be more similar across neighborhoods at the time of neighborhood establishment … and diverges as neighborhoods develop, influenced by the socio-economic profile of residents.(Luck et al. 2009, p. 615)

Urban trees are a cheap, effective way to lower temperatures, improve human health and reduce the urban heat island effect. When the next heatwave hits, spare a thought for everyone (human and non-human) in the leafless suburb down the road. Under global warming, 50 shades of leafy greens beats 50 shades of grey concrete.

Further reading

Luck, G.W., Smallbone, L.T. & O’Brien, R. (2009). Socio-economics and vegetation change in urban ecosystems: patterns in space and time. Ecosystems 12(4), 604-620 [Full paper available for free from this link].

Professor Gary Luck kindly fact-checked this week’s post. This is the first in a series of posts on urban trees; the next post widens the canvas to examine street trees across eastern Australia. The wonderful web site, The Conversation, has a number of short articles on the importance of urban trees, including:

Related posts

21 thoughts

  1. Thanks for reminding me why I abandoned city and suburban living for the fire-prone countryside – those treeless macmansions make me shudder. However the correlations and variability of tree cover and socio-economics are interesting and hopeful. Jamie Kirkpatrick at UTAS is also interested in gardens and street trees, including the effects of people’s attitudes to them. He notes that trees in cities are mostly killed by tree lovers, who just happen to hate particular types of trees that they replace when moving house. He also has a relevant article on the Conversation at http://theconversation.com/sustainable-cities-need-trees-not-freeways-382
    It seems the value of a tree goes beyond the personal taste of landowners.

    1. Hello Gwenda, thanks for writing in and for the link to The Conversation article by Jamie Kirkpatrick, I hadn’t read that one, it’s great. I’m hoping to write another couple of articles on urban trees, and I’ll feature one of Jamie’s papers in the next blog, so stay tuned for more. Best wishes Ian

  2. Those patterns are really interesting, and don’t match my own (22 years long) local experience in a Sydney suburb. I have noticed the loss of a number of significantly sized trees along my usual ‘walk to the station’ route, but it was a shock to see a comparison of images via Google and NearMap of my place and my immediate surrounding streets, when producing documents in relation to a neighbouring DA. So many trees had disappeared (often by stealth I’m afraid) in just a 5-10 year period. I have taken to photographing nearby houses with any significant trees whenever I see a ‘For Sale’ sign go up. I once deliberately went to a neighbouring open house which had the most significant (truly magnificent) tree in the landscape; visible from a great distance away, and planted in a good location on the property. I listened to one after another prospective buyer quiz the real estate agent about the chances of removing the tree. Interestingly and thankfully, without exception he said “you’d have buckleys”. And thankfully it is still there, having survived 2 new owners so far.

    I wonder if there is something about regional cities compared to big cities like Sydney, Melbourne etc. It would be good to see similar work sampling these bigger cities. What factors perhaps attract migrants to these regional areas, instead of, or away from the big cities. And are there factors relating to country/culture of origin, as well as socio-economic factors also at play. I look forward to the suggestions in next week’s blog.

    1. Hi Deb, thanks for your comment, its great to hear of an example where a wonderful big tree was spared. One possible reason for the discrepancy between the data and your observations may be that the two are focusing on different things. The study by Gary Luck and co looked at the cover of all trees, rather than the number of big old trees. If a big old tree was cut down, for example, and was replaced by a few smaller trees, which collectively had more cover than the original, then this would show as an increase in cover, and the loss of the big tree wouldn’t be noticed. I’ll write more about the capital cities in the next blog, so stay posted.

      1. Ian, I think I am seeing both – the loss of significant, stand out trees, and smaller ones; ie overall cover. And it seems to be so often replacement with paving/concrete, and shrubs or smaller plants, seldom what could be classed as trees (although perhaps they haven’t yet grown significantly to be noticeable yet). This is particularly pronounced with cases of ‘knock down and re-builds’ which is an increasing trend in some areas (eg middle ring suburbs, such as my own) as people seek new and bigger houses on old blocks – often to be occupied by smaller families than the previous owner’s generation. The patterns in capital cities should be interesting.

  3. Great post, Ian – city trees are so important! Look forward to further installments and will send this round to students and colleagues as there’s a lot of interest in urban ecology within my department. By coincidence I posted a brief piece on city trees in Brazil when I was there back in November: http://jeffollerton.wordpress.com/2013/11/10/a-city-without-trees-is-like-a-bird-without-feathers-brazil-diary-2/

    In it I posed a question to which I’ve still not found an answer: Do trees in temperate cities also insulate against cold in the winter? Have you seen anything written about this? One can imagine evergreen species, at least, providing some insulation for heat radiating from roads and buildings at night, as well as reducing wind chill. But is there a significant effect?

    1. Hello Jeff, thanks for writing in.I’m afraid that I haven’t read anything about whether trees help keep cities warm in winter. All of the studies I have heard of have focused on reducing summer heat. The abundance of deciduous trees in many cold cities would presumably limit the number of areas where the question could be tested. I love the title of your blog – “A city without tree is like a bird without feathers”. Best wishes Ian

  4. Thankyou Ian for a very important article. I am heavily involved in trying to save Daly Nature Reserve in Gisborne, Victoria from a development by the Macedon Ranges Shire Council. This article, and others are priceless for the help they give. Thankyou

  5. Hi Ian, Great to see the profile of the private urban forest being raised. For your follow up article, you might be interested in the work of Jamie Kirkpatrick, Aidan Davison, Anna Wilson, Dave Kendal (and perhaps even myself). Just wanted to flag here that I’ve sent you an email with more info. Keep up the great work, Lil.

    1. Hi Lil, thanks for the great suggestions. I’ve got a pile of great papers by these and other authors on my desk. The next blog will focus on a recent paper by Jamie, Aidan and others. I’m not sure of the order of the posts after that, but will try to cover all of the people you’ve suggested. Thanks again, best wishes Ian

  6. Ian again a good topic. Two comments: Prof Jamie Kirkpatrick in Tasmania has been doing, with his students, some interesting and related work on class/wealth/location/gardens/trees for some years, see his beautiful small book: “Ecologies of Paradise – Explaining the Garden Next Door” (2006) self published obtainable from him. Second in Singapore, where I visit for family reasons regularly, they have now developed a sensational tree valuation system which includes each tree’s carbon sequestration potential, sure a small scale but important, see this work on the Singapore National Parks Board web site; and finally would love to share a photo of a superb car park in France totally covered using pleached Plane Trees, aesthetically and environmentally superb, happy to share the pic if you tell me how.

    1. Hi Max, thanks again for writing. I’ve read lots of Jamie’s papers on gardens but hadn’t heard of his book, I shall try to chase it up. I’ll feature one of his group’s studies in the next post. The car park photo sounds fascinating. I shall email you separately. Perhaps I can include it in an upcoming blog? Best wishes Ian

      1. I have sent the photo of the French car park by email to you.

  7. Thanks for writing on the very important topic of urban forests, especially in the Australian context. Research in urban ecology is not yet informing planning policies with regard to retention of urban trees and canopy preservation. In some established areas of Sydney, we are losing huge numbers of private trees to building extensions, plot subdivision and backyard granny flats. Furthermore, the combination of McMansions and small plot sizes in developing suburbs means that trees are being “designed out” of these suburbs, with no space for trees in the future, which is likely to be hotter in any case. It would be tragic to end up with our only suburban trees being street trees, constrained as they often are by infrastructure, both above and below ground. Why isn’t a tree for wildlife and its cooling shade as important as a place to park a car (or three), a place to dry clothes outdoors, or a veggie patch?

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