Weatherwatch

Paul Brown
The Guardian, Monday 20 April 2009

Local authorities and homeowners cut trees down far too readily in urban areas. This is partly because they fear branches might fall on someone and also that insurance companies say tree roots can cause subsidence if they are too close to buildings.
However, the benefits of urban trees far outweigh disadvantages, particularly as climate change makes urban areas too hot for comfort in the summer. Increasing tree cover by 10% in an urban area reduces the summer temperature between 3C and 4C. Green corridors of trees and grass create oases of cool air.
Trees are also vital in reducing urban flash flooding. More intense rainfall is leading some authorities to consider new systems of storm drains although trees can be more effective. They absorb around 30% of rain through their leaf system before it hits the ground and about the same again through the roots. More than half the rain is transpired back into the air so avoiding the need for extra drains.
Trees have direct health benefits by soaking up pollution. They turn carbon dioxide back into oxygen along busy streets and deal with a large part of the deadliest form of pollution, the tiny particulates from vehicle exhausts. These kill thousands of people in UK cities each year, and trigger asthma attacks. In tree-lined streets levels of pollution are far lower because the particulates stick to leaves and twigs. When it rains the particulates are washed harmlessly away.