Overstory #229 - Urban tree benefits
More than 80 per cent of us live in urban areas, and many more work or spend a substantial part of our lives in and around towns and cities. They are important for us socially and culturally, they are the places where we live and work, raise families, socialise and relax, from which we draw identity and pride. The quality of urban areas is of great importance.
Good architecture and design are clearly essential, but of equal importance is the quality of the green space. Public parks and gardens, the landscaping around buildings, street trees and highway verges, the wilder corners along river banks and canals, on railway sidings and industrial sites, and our own gardens.
Trees are often the dominant features of green space; their stature and beauty make them the defining elements of urban spaces. They cast shade in the heat of summer, provide shelter from the rain and wind, help to keep the air clean and breathable, support wildlife, and add value to the culture and economy of our towns and cities.
Urban trees, woods and health
Trees and woods are vital to health and wellbeing. There is a strong relationship between the quality of urban green space and people’s health and wellbeing (1, 2, 3). Increasing tree cover mitigates some of the effects of a warming climate, reduces the impacts of poor air quality, and increases the opportunities for people to adopt a healthy lifestyle.
Urban heat island effect
Increasing tree cover in urban areas can help mitigate the ‘urban heat island effect’. This occurs as the buildings, concrete and other hard surfaces such as roads act as giant storage heaters, absorbing heat during the day and releasing it at night. The resultant effects can be dramatic; on some days there is a difference of as much as 10oC between central London and its surrounding suburbs (4). Projections for our changing climate suggest this problem will get markedly worse.
Higher temperatures increase ground level ozone exacerbating the symptoms of chronic respiratory conditions. In addition prolonged high temperature can bring on cardiovascular or respiratory failure or dehydration, particularly amongst the elderly, very young or chronically ill (5). In the 2003 summer heat wave over 2,000 people died in Britain alone and more than 35,000 died across Europe as a result of the heat.
Green space, and trees in particular, provide both direct shade and reduce the temperature through the cooling effect of evaporation from the soil and plant leaves. One mature tree transpires up to 450 litres of moisture a day – equivalent to five room-sized air conditioners left on for 19 hours (6).
Research at the University of Manchester using computer modelling has shown how increasing urban green space can mitigate urban heat island effect. Without any increase in green space, by 2050 the temperature in Manchester is projected to rise by 3oC. However if the amount of green space increases by just 10 per cent this could potentially eliminate the effects of climate change on increasing surface temperatures. However, reducing tree cover by the same percentage could lead to an increase of 8.2oC under some scenarios (7).
Reducing air temperature is only part of the picture. Radiant heat – direct sunlight – is often more important in terms of people’s comfort, and carries a health risk when it results in sunburn. Children’s skin is more sensitive to UV damage and the amount of sun exposure during childhood is thought to increase the risk of developing skin cancer in adult life. Shading is particularly important in school grounds and where children play. Providing direct shade using trees in playgrounds reduces the risks from UV radiation (8).
Trees and woodland improve air quality (9) by adsorbing pollutants such as sulphur dioxide and ozone, intercepting harmful particulates from smoke, and dust and of course release oxygen through photosynthesis. This helps to alleviate the problems caused by chronic respiratory disease.
Each year, 24,000 people in the UK die prematurely from air pollution (10). Research by the British Lung Foundation suggests that one in every seven people in the UK is affected by lung disease, almost 8 million people (11). The UK also has one of the world’s highest rates of childhood asthma, with about 15 per cent of children affected and a higher prevalence in lower socio economic groups in urban areas (12). Columbia University researchers found asthma rates among children aged four and five fell by a quarter for every additional 343 trees per square kilometre (13).
Trees will have a proportionately greater effect in urban areas, where they are close to sources of pollution and nearer to people who might be affected. Street trees in particular, close to sources of pollution, can intercept particles from traffic and other emissions (14). It is important to remember that despite the significant benefits of trees on air quality, some people do suffer allergies to tree pollen, particularly in the early spring (15).
Green space and healthy lifestyles
Proximity of green space to people's homes increases the likelihood of the residents choosing walking over other forms of transport (16,17,18). With nearly a quarter of both men and women in the UK classed as obese, the Government is looking at the role of trees, woods and other green space in encouraging physical activity.
Over a third of people are on incapacity benefits because of mental health problems or muscular or skeletal disorders – both of which can respond to tailored physical activity programmes. If just one per cent of people on incapacity benefit could be helped back into the workplace through active lifestyles, it would save the country £67 million a year (19).
The Campaign for Greener Healthcare and the initiative to establish an NHS Forest (20) illustrate a growing consensus amongst health professionals of the importance of trees to people's health and wellbeing. With plans to plant a tree for every one of the 1.3 million NHS employees, the campaign endorses the role of trees in air quality, improved health outcomes and reducing negative environmental impact.
With 80 per cent of people living in urban areas, but fewer than 10 per cent having access to local woodland within 500m of their home (21), it is vital that the Government sets targets for new woodland that will meet the need near where people live.
There is evidence that trees not only provide physical benefits but can also be important to mental health.
Trees and woods can have a restorative and therapeutic effect on the mind (22). Studies have looked at the beneficial effects of natural surroundings on children with Attention Deficit Hyperactivity Disorder (23). Trees have been found to enhance mood, improve self esteem and lower blood pressure. The quality of natural features and trees in the city helps reduce mental fatigue and stress (24), and has important benefits for child development (25).
Research in the Netherlands and Japan indicated that people were more likely to walk or cycle to work if the streets were lined with trees and feel better and live longer as a result (26).
It is hoped that in coming years there will be an increasing emphasis towards long-term disease prevention through adopting healthy lifestyles. Around £110 billion is spent each year in the UK on healthcare, equal to 8.5 per cent of all income. It has been estimated that if every household in England had good access to quality green space it could save around £2.1 billion (27).
Trees and flooding
An increase in hard surfaces in urban areas, unable to absorb rainfall, often means drains are overwhelmed and water quickly collects on the surface rushing down streets and over paving. Following the 2007 flooding, paving over of gardens was identified as having a major impact on drainage of surface water in urban areas.
Around two-thirds of the 2007 flooding was a result of surface water, with 3.8 million homes in England susceptible (28, 29). The insurance cost of the 2007 floods was thought to have been around £3 billion (30), but the Environment Agency expects the regular annual cost of damage to property alone to be in excess of £1 billion. When the cost of further disruption, damage to infrastructure and loss of business is added this increases to £2.5 billion and could rise to £4 billion by 2035 (31).
Interception of rainfall by trees in urban areas can be critical in reducing the pressure on the drainage system32 and lowering the risk of surface water flooding. Slowing the flow increases the possibility of infiltration and the ability of drains to take away excess water.
The world is losing biodiversity at an accelerating rate, due largely to a combination of habitat loss and climate change. Aside from any intrinsic value, biodiversity is important for helping to maintain the stability of natural systems and in the supply of a range of ‘ecosystem services’. These include flood attenuation, pollination of crop plants, soil conservation and climate regulation. Native woods and trees in urban areas, including gardens can be vital to a wide range of wildlife, providing food, shelter and places to breed.
As well as remnant pockets of woodland and more natural space, urban areas have parks, private gardens and planted shrubberies which can support a large number of invertebrate and bird species, especially in the suburbs. These include uncommon species, including for example juniper fauna which has adapted to garden junipers (39).
An important characteristic of urban areas is their mosaic of habitats. Industrial sites such as demolition sites, disused railway lands or unused industrial land can be rich in species. Later stages of succession through to woodland contain many uncommon invertebrates with flies, bees and wasps, including some parasitic species and sawflies.
Trees in urban areas support a wealth of wildlife, from the common, such as robins, blackbirds and tits, to bats and bees, many of which are in decline. Native tree species are particularly important in supporting wildlife – native willows for instance may support over 450 species, many of which are insects that provide food for birds (40).
Civic amenity and economic benefits
The beauty of towns and cities arises from a mix of good architecture and design, and the landscape of public spaces. There is strong evidence that improving green infrastructure and the urban environment helps promote inward investment by creating a more attractive environment for businesses and their staff (41).
Trees are a vital element in providing structure and texture to green infrastructure, and yet this has been eroded in many places. Maintaining what we have, ensuring future generations of trees to replace those that are being lost, and imaginative creation of more places rich in trees is central to making towns and cities places people want to live in, visit and do business in.
Trees are multi-purpose tools for urban adaptation and design. Any measures which undermine current levels of tree cover are likely to be damaging to adaptation, whereas well planned and well maintained urban tree cover can greatly increase the adaptive capacity and resilience of the city.
1 Sadler, J.P., Bates, A.J. & Hale, J. (in press) Bringing cities alive: the importance of urban greenspaces for people and biodiversity. Urban Ecology (ed K. J. Gaston). CUP
2 Maas, J., Verheij, R.A., Groenewegen, P.P., de Vries, S. & Spreeuwenberg, P. (2006) Greenspace, urbanity and health: how strong is the relation? Journal of Epidemiological Community Health, 60, 587-592.
3. Maas, J., Verheij, R. A., Spreeuwenberg, P. & Groenewegen, P. P. (2008). Physical activity as a possible mechanism behind the relationship between greenspace and health: a multilevel analysis. BMC Public Health, 8, 206.
4 BBC web site, Urban Heat Islands. Available at: http://www.bbc.co.uk/weather/features/understanding/urban_heat_islands.shtml [accessed 9th June 2010]
5 Shaoni Bhattacharya (2003) European heatwave caused 35,000 deaths, New Scientist online, 10th October 2003, Available at: http://www.newscientist.com/article/dn4259-european-heatwave-caused-35000-deaths.html [accessed 9th June 2010]
6 Nicholas-Lord, D. (2003) Green cities and why we need them, New Economics Foundation, London, p.13. Available at: http://www.urbanwildlife.org.uk/assets/userfiles/000074.pdf [accessed 9th June 2010]
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8 Heisler, G.M., Grant, R.H., 2000. Ultraviolet Radiation, Human Health, and the Urban Forest. U.S. Department of Agriculture, Newtown Square, PA, General Technical Report pp. 35.
9 Nowak, D., Crane, D. & Stevens, J. (2006) Air pollution removal by urban trees and shrubs in the United States, Urban Forestry Urban Greening, pp. 115-23.
10 HM Government (1998). UK Environmental Accounts. HMSO, London
11 British Lung Foundation. Facts about respiratory disease. Available at: http://www.lunguk.org/media-and-campaigning/media-centre/lung-statsand-facts/factsaboutrespiratorydisease.htm [accessed 9th June 2010]
12 Townshend, J., Hails, S. & McKean, M. (2007) Diagnosis of asthma in children, British Medical Journal, 28; 335(7612), pp. 198-202.
13 Lovasi, G., Quinn, J., Neckerman, K., Perzanowski, M. & Rundle, A. (2008) Children living in areas with more street trees have lower prevalence of asthma. Journal of Epidemiology & Community Health, 62(7), pp. 647-649.
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19 Department of Health press release, 13th August 2009, downloaded at: http://www.dh.gov.uk/en/News/Recentstories/DH_104254
20 The Campaign for Greener Healthcare, downloaded at: http://www.greenerhealthcare.org/nhs-forest
21 The Woodland Trust (2004) Space for People. Available at: http://www.treeforall.org.uk/AboutTreeForAll/WhyTreeForAll/Science/spaceforpeople.htm [accessed 9th June 2010]
22 Hartig, T., Evans G.W., Jamner L.D., Davis D.S., and Gärling T. (2003). Tracking restoration in natural and urban field settings. Journal of Environmental Psychology 23, 109-123.
23 Taylor, AF et al (2001) ‘Coping with ADD, The Surprising Connection to Green Play Setting’, Environment and Behaviour, Vol. 33, January 2001, pp 54-77
24 Ulrich, R.S., Simons, R.F., Losito, B.D., Fiorito, E., Miles, M.A. and Zelson, M. (1991). Stress recovery during exposure to natural and urban environments. Journal of Environmental Psychology 11: 201-230
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26 Van den Berg, A.E., Koole S.L., and van der Wulp N.Y. (2003). Environmental preferences and restoration: (how) are they related? Journal of Environmental Psychology 23, 135-146
27 Natural England (2009) Our Natural Health Service - the role of the natural environment in healthy lives. Page 8. Available at; http://www.naturalengland.org.uk/Images/nhsmanifesto_tcm6-12022.pdf [accessed 18th May 2010]
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30 Newratings, 24th June 2007, UK floods likely to cost £3bn in insurance. Available at: http://www.newratings.com/en/main/company_headline.m?&id=1577047 [accessed 9th June 2010]
31 Environment Agency, ‘New reports highlight GBP20Bn investment over 25 years is needed to protect England from flooding’ downloaded 29th July 2009 at: http://www.environmentagency.gov.uk/news/108705.aspx [accessed 9th June 2010]
32 Gill, S (2009) ‘The Essential role of trees – adapting cities to climate change by managing high temperatures and reducing pressure on drainage systems’, in proceeding Trees and Urban Climate Adaptation: a social agenda for liveable cities, 19th November 2009
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40 Trees for Cities. Trees and urban biodiversity. Available at: http://www.treesforcities.org/page.php?id=490 [accessed 10th June 2010]
41 The economic benefits of Green Infrastructure: Developing key tests for evaluating the benefits of Green Infrastructure Natural Economy Northwest, Commissioned from ECOTEC by The Mersey Forest on behalf of Natural Economy Northwest
This article was excerpted with the kind permission of the publisher from:
Townsend, M. 2010. More Trees More Good - Urban Trees, Greening the Concrete Jungle - Policy Brief Holmgren Design Services. Woodland Trust.
Publisher contact info:
The Woodland Trust | Autumn Park | Grantham | Lincolnshire | NG31 6LL
About the Author
Mike Townsend, MBE is conservation policy team leader at Woodland Trust, the UK’s leading woodland conservation charity. It has 300,000 members and supporters. The Trust has three key aims: i) to enable the creation of more native woods and places rich in trees ii) to protect native woods, trees and their wildlife for the future iii) to inspire everyone to enjoy and value woods and trees. Established in 1972, the Woodland Trust now has over 1,000 sites in its care covering approximately 20,000 hectares (50,000 acres). Access to its sites is free.
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