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Good day, ladies andgentlemen. I have been asked today to talk to you about the urban landscape.There are two major areas that I will focus on in my talk: how vegetation can have asignificant effect on urban climate, and how we can better plan our cities using trees to provide a more comfortable environment for us tolive in.
Trees can have a significantimpact on our cities. They can make a city, as a whole, a bit less windy or abit more windy, if that's what you want. They can make it a bit cooler if it'sa hot summer day in an Australian city, or they can make it a bit more humid ifit's a dry inland city. On the local scale -- that is, in particular areaswithin the city --trees can make the local area more shady, cooler, more humid and muchless windy. In fact, trees and planting of various kinds can be used to makecity streets actually less dangerous in particular areas.
How do trees do all that, you ask?
Well, the main differencebetween a tree and a building is a tree has got an internal mechanismto keep the temperature regulated. It evaporates water through its leaves and that means that thetemperature of the leaves is never very far from our own body temperature. Thetemperature of a building surface on a hot sunny day can easily be twentydegrees more than our temperature. Trees, on the other hand, remain cooler than buildingsbecause they sweat.This means that they can humidify the air and cool it-a property which can beexploited to improve the local climate.
Trees can also help break the force ofwinds.The reason that high buildings make it windier at ground level is that, as thewind goes higher and higher, it goes faster and faster. When the wind hits thebuilding, it has to go somewhere. Some of it goes over the top and some goesaround the sides of the building, forcing those high-level winds down to groundlevel. That doesn't happen when you have trees. Trees filler the wind andconsiderably reduce it, preventing those very large strong gusts that you sooften find around tall buildings.
Another problem in built-upareas is that traffic noise is intensified by tall buildings. By planting a belt oftrees at the side of the road, you can make things a little quieter, but much of the vehiclenoise still goes through the trees. Trees can also help reduce the amount of noise inthe surroundings,although the effect is not as large as people like to think. Low- frequencynoise, in particular, just goes through the trees as though they aren't there.
Although treescan significantly improve the local climate, they do however take up a lot ofspace.There are root systemsto consider and branches blocking windows and so on. It may therefore be difficult to fit treesinto the local landscape. There is not a great deal you can do if you havewhat we call a street canyon --a whole set of high-rises enclosed in a narrowstreet. Trees need water to grow. They also need some sunlight to grow and youneed room to put them. if you have the chance of knocking buildings down andreplacing them, then suddenly you can start looking at different ways to designthe streets.
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