Are trees good for you?

Of course they are.  I firmly believe that.  Obviously, so do advocates of urban forests. Oh, yes, I am one of those.

Advocacy for increased attention to tree scale planting in cities, for the establishment of viable urban forests, is at least a generation old.  And even though rarely adopted as a coherent policy by cities during that time, there has nevertheless been a distinct trend for the public and private domains to acquire significantly more green fuzz.

Paddington, Sydney 2012

It is quite enlightening to compare old aerial photographs of 'inner ring' suburbs of a city like Sydney, with recent images.  It becomes clear how little greenery featured in these places when they were a dense matrix of unregulated industry and predominantly worker housing, strung together by tramways and above ground electricity and telephone wiring.  In contrast, gentrification also seems to bring a commitment to transforming bare streets to treed, and small gardens to intensely planted havens.  In particular, not so many people these days seem to share the former suburban phobias about gutters blocked by leaf litter, so they don't maintain personal vendettas against trees taller than three meters.

Paddington, Sydney, 1947

But lately there also seems to be a more concerted push towards urban forests, driven by a loose conception that greening the city will help do away with the so-called 'urban heat island effect'.

The latest such initiative I have come across is from Melbourne, Australia, already one of the worlds most livable cities.  The article is on Sourceable.net, and may very well have some of the usual baggage of journalistic oversimplification, but I suspect that the point I am about to tease apart is not one of them.

Thermal image of inner Melbourne

As almost always, the key image in the argument for the putative benefits of a green canopy is an aerial view as a 'false colour' thermal photograph.  The image supporting the Melbourne press releases is the prettily coloured one I reproduce in this post.  What you are meant to see is that all the parklands show up as reassuringly blue-green cool, in contrast to the warm yellow-reds of all the concrete and asphalt of buildings and roads.
 
But now look more carefully.  Look for all the coolest surfaces, the ones in the most intense blues.  I am not really familiar with Melbourne, but I can tell by the shapes that the coolest surfaces are in fact the metal roofs of the sporting venues, and indeed of the large industrial and warehouse sheds.  Now, I don't know too much more about the total heat balance over time, of these lightweight roofs as opposed to the grass and canopies of the areas of vegetation.  But I am sure my reading of the pretty picture is not going to be popular with the people who would like it to be self-evident, that literally greening the city will by itself make the urban heat islands go away.

Most readers of my blog would be familiar with my obsession with looking more carefully at data and imagery, particularly when looking for evidence of technical performance.  Well, this is one more example.  In fact, US research suggests that simply extending green surfaces by way of street planting and vegetated roofs makes far less impact on the city's average albedo than we would hope.

Which is not to say that a lot of the other benefits of trees in cities are not worth it for their own sakes.  I'd rather a shaded street sidewalk to walk along than to bake in the merciless sun of a Melbourne summer, I'd rather the leaves overhead cooling and humidifying by transpiration the microclimate under them.  But to capture that correct depiction of what happens, you need to show thermal images of the street at pedestrian level.  It would be just as compelling as the misinformation of the thoughtlessly chosen and misleading aerial.

Read the Sourceable.net article here.

To see another similar aerial thermal image, with exactly the same issues, see the ones of Sydney, here.  But if the reader is interested, this page also links to other microclimate monitoring by the City of Sydney.  If really, truly interested, take the trouble to go to the UNSW link and download the full report of the pilot project
Micro-Urban-ClimaticThermal Emissions: in a Medium-Density Residential Precinct.  Don't be put off by the poorly written report itself, and spend your time instead on the comprehensive literature and methodology review in Appendix 1.

Micro-Urban-Climatic Thermal Emissions: in a Medium-Density Residential Precinct - See more at: http://www.be.unsw.edu.au/sustainability-and-climate-change-adaptation/projects/thermal-impact-designed-environment-urban-heat#sthash.Y6relWBT.dpuf

Thermal Impact of the Designed Environment on the Urban Heat Island - See more at: http://www.be.unsw.edu.au/sustainability-and-climate-change-adaptation/projects/thermal-impact-designed-environment-urban-heat#sthash.Y6relWBT.dpuf