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?
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.
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).
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?
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.
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:
- Trees are a city’s air conditioners, so why are we pulling them out?
- Spending wisely now will make heatwaves less costly later
- Our cities need more trees and water, not less, to stay liveable
- Won’t the real Shady City please stand up?
- Why do we plant and remove urban trees?
- The natural secrets of urban well being