The vital role of urban trees in helping to lower temperatures in cities is described in a new publication by scientists from Forest Research, the research agency of the Forestry Commission.
Built up urban environments can be as much as 9°C warmer than the surrounding area with buildings, road traffic and human activities all contributing to the ‘urban heat island’ effect. Vegetation, especially trees, has a crucial role to play in lowering temperatures by providing shade, reflecting sunlight and by evaporation. Strategic selection and placement of trees in cities can cool the air by between 2°C and 8°C which could reduce heat-related stress and premature human deaths during heatwaves.
Under the changing climate predicted for the UK there are likely to be an increasing number of incidences where the average daytime temperature is 30°C or above and the night time temperature exceeds 15°C for two consecutive days – the Department of Health threshold value for temperatures that could significantly affect health. While the South East, London, the East and West Midlands are projected to be the most vulnerable, excess deaths due to heat are forecast to increase in Scotland and Wales. The risks are greatest in large metropolitan areas such as London, Manchester and Birmingham.
Dr Kieron Doick and Tony Hutchings from Forest Research have undertaken an extensive survey of research on the cooling effects of urban trees. In this review for the Forestry Commission, “Air Temperature regulation by urban trees and green infrastructure” they examine how the type of tree, its planting location, together with the mix of vegetation and paved surfaces in greenspace all contribute to countering the effects of urban heat islands.
“Around 20% of London is under the canopy of trees and about 38% of its land area is ‘greenspace’. While we do not know what proportion of London’s population is cooled by these trees and greenspaces, greenspaces are recognised to lower local ambient air temperatures by 2°C to 8°C. If we assume that London’s trees and wider green infrastructure are preventing an additional 2°C of additional urban heat island warming; then the current provision of greenspaces in London may be saving 16 to 22 lives a day during spells of hot weather. Thus it is very appropriate that, in Climate Change Week, we recognise the important role that trees have to play in preserving and improving our quality of life as the climate changes and temperatures increase”.
The report recommends that planners and developers can help to combat UHI by greening the urban environment giving priority planting to trees with large canopies. Greenspaces should be a minimum of 0.5 ha so that they have a significant effect on cooling beyond their boundaries.