I was listening to the replay of By Design on Wednesday - the host of the program Alan Saunders, was interviewing Dr Peter Fisher about the idea of 'Urban Forests'.

Fisher is a physicist and climate change consultant, and on the show he argued for the value of trees in urban areas as a way of making our cities more ecologically sustainable. If you'd like to, you can listen to the whole program here.

Saunders and Fisher discussed the fact that during the last few dry summers, local councils have allowed large trees in urban areas throughout Australia to die, and Fisher estimated that up to 15% of large trees have died in some areas.

Handling 'green assets' (as our trees are known) in a water crisis presents some real dilemmas for councils. And I do understand the reasoning behind the application of strict water restrictions in public parks and gardens: It shows local government acting as a role model - and also makes those of us who live in the city and suburbs aware of something of the real scale of the water problem. It also saves a significant amount of water. But I also think that there is an argument for protecting a few green public areas in each part of the city and suburbs for people to enjoy; people who have little enough exposure to nature already.

And it's a very high price to pay if we lose our avenues or stands of established trees permanently. Disturbingly, Fisher believes that we will not be able to grow really big tree's again in our urban areas because of ongoing drought conditions and water restrictions and so, he argues, we need to protect the ones we have.




Fisher called himself a proponent of "old fashioned shade," he points out the role mature trees play in our urban areas:

• Trees work to reduce the impact of greenhouse gases by carbon sequestration
• By shading houses, footpaths and roads, Trees reduce the 'heat island affect' caused by heat from the sun being absorbed by hard surfaces and later released as radiated heat. This extra heat can make our cites 3-4 degrees warmer than surrounding areas.
  
• By absorbing water in their root system, trees reduce the problem of overloaded storm water systems in heavy rains, and so reduce the likelihood of flooding.
• Each shade tree over a house saves 30kwh per year in air conditioning, (Fisher argues that this power reduction then saves water used in the powerplant - so keeping the tree's alive actually saves water).
• Trees filter and absorb toxins and particles from vehicular traffic and industry which cause asthma, respiratory illness and other serious health issues.
  

Fisher and Saunders went on to discuss the problem I looked into in an earlier post about weighing up the benefits of deciduous trees (mainly exotic) versus those of evergreen indigenous/native trees. They focused on the problem of shade from trees reducing the effectiveness of solar panels. All trees will reduce the effectiveness of solar panels in summer, but trees which lose their leaves in winter allow higher levels of productivity from solar panels than trees which keep their leaves all year round (like most native species).

This argument plays out differently depending on which state or rather, which climate you're in. Here in Melbourne, most of our energy use, and therefore carbon emissions, is from heating in winter - so maybe we're better off with deciduous exotics (although I hate to admit that - because personally I'm a big fan of indigenous planting). Fisher is from Queensland, and common sense suggest that most of their energy use would be cooling - so it might make sense to plant indigenous trees which can also support birdlife and local urban ecosystems. Especially if summer shade from trees significantly reduces energy use anyway.

Of course, it's important to remember than here in Melbourne, even deciduous trees can cause problems in terms of reducing carbon emissions. A deciduous tree planted to shade your living area, will significantly reduce the solar gain (warmth from the sun) inside your house during spring and autumn, which means you're more likely to need to turn on the heater. Operable shade systems which allow you to keep sun out in summer and let it in when it gets cooler are more effective. I'm not sure how the carbon impact of extra energy use weighs up against the carbon stored by the tree. And I certainly don't know which is the best solution once you take in to account the other benefits of trees, like cleaner air, better soil, bird life... and so on.

The other possible conflict with trees v's solar panels is that it's difficult to map tree growth, or predict it. If you've just installed $12 000 PV array, you can usually do something about your next door neighbour putting up an extra floor and blocking out your sun, but it's harder to prevent the Corymbia maculata they planted from shooting up 5 meters in as many years and causing the same problem.

Image of tree in bag from New Stiletto