Trees a 'low-cost' solution to air pollution and biodiversity loss in cities
Ecologist2nd July, 2010
Native woods and trees in urban areas, including gardens, provide haven for wildlife, reduce air pollution, surface run-off and flooding. Reversing the declining numbers of native trees and woods in cities would provide numerous benefits at 'relatively little cost', says a report from the Woodland Trust.
As well as access to green space, the report, 'Greening the Concrete Jungle', says trees provide a wide range of free ecosystem services including reducing the risk of surface water flooding and improving air quality that could save millions in flood defence and healthcare costs.
The UK has one of the world’s highest rates of childhood asthma, around 15 per cent, particularly amongst lower socio-economic groups in urban areas. Research shows asthma rates among children aged four to five falls by a quarter for every additional 343 trees per square km, as they help keep the air clean and breathable and reduce ambient temperature.
Trees are also able to reduce the pressure on the drainage system during flooding. The University of Manchester has shown that increasing tree cover in urban areas by 10 per cent reduces surface water run-off by almost 6 per cent.
A major international study published earlier this year, The Economics of Ecosystems and Biodiversity (TEEB), put the global value of services provided by forests and biodiveristy at between £1.2-2.8 trillion a year.
Decline in native woodland
Despite these 'invisible' benefits, the report says urban tree cover is actually deteriorating in many areas, with concerns over tree safety and insurance claims as well as development. Many places have seen a decline in older trees with large spreading crowns, replaced with smaller, more manageable alternatives. Smaller crowned trees have less capacity to intercept rain.
Fewer than 10 per cent of city dwellers have access to local woodland within 500m of their homes. Evidence suggests proximity to a wood encourages physical exercise, whilst a woodland walk lowers the heart rate and mental stress.
The Woodland Trust said socially disadvantaged groups were the most likely to lose out with around two-thirds of urban trees in private or less accessible public grounds. It is campaigning to plant 20 million native trees annually accross the UK for the next 50 years.
'Towns and cities also tend to put into sharp relief some of the key problems we are facing as a society - physical and mental health problems, childhood obesity and asthma, differences between rich and poor, air pollution, soaring summer temperatures, flash flooding, energy conservation, diminishing wildlife - so they are a good place to start when trying to illustrate just where green space can deliver significant improvements for relatively little cost,' said report author Mike Townsend.