A study by the University of Washington found that people visiting business districts were prepared to pay more for parking in landscaped car parks and, on average, 11% more for goods in a landscaped business district than a non-landscaped district. This figure was as high as 50% for convenience goods (Wolf, 1998(a), Wolf, 1999 and Wolf, 2003).
Both the business community and consumers were found to prefer business districts with good landscaping (Wolf, 1998(b)).
The quality of landscaping on approach routes to business districts positively influences visitor perceptions of an area (Wolf, 2000).
Both visitors and businesses have been found to favour districts with high tree cover and this increases inward investment to an area. The increase in retail prices that can be asked for in well landscaped areas can be assumed to attract businesses to the district.
Several studies in the USA have looked at the effect of tree cover on the price of residential house sales. They found that the value of properties in tree lined areas may be up to 6% greater than in similar areas without trees (Wolf, 1998 (c)).
The market in the UK is different and a direct comparison of the data is not possible. However, an informal telephone survey of estate agents in the Warwick area suggests that tree cover has a positive effect on saleability, if not directly on price. Properties on tree lined streets were said to be in higher demand and sell faster.
Traditionally, it has been thought that trees and other vegetation have a negative impact on crime because they provide cover for criminals and reduce opportunities for casual surveillance.
However, research in an area of inner city Chicago has suggested that this not the case. It found that appropriate vegetation cover can lead to reduced crime rates (Kuo and Sullivan, 2001(a)). The study looked at areas with similar architecture and population statistics but differing levels of mown grass and high canopy trees. Areas with higher vegetation cover were found to have lower rates of crime.
How do trees reduce crime? Public open space with trees tends to be used more than space without trees and this increases casual surveillance. There is also evidence that vegetation reduces mental fatigue, which is often a precursor of outbursts of anger and violence (Kuo and Sullivan, 2001(b)).
Other social benefits
Research has been undertaken by the Human-Environment Research Laboratory at the University of Illinois which has identified numerous beneficial effects of trees to society. A good summary of these is 'The Role of Arboriculture in a Healthy Social Ecology' (Kuo, 2003).
Trees encourage people out of their homes and into public open space. Outside, people interact more with others and build stronger social relationships. An additional benefit is the positive effect that contact with nature can have on children with Attention Deficit Disorder (ADD) (Taylor, Kuo, Sullivan, 2001).
Dr Rachel Kaplan has found that desk workers who can see nature from their desks take 23% less time off sick than those who cannot see any nature. Desk workers who can see nature report greater job satisfaction (Wolf, 1998(d)). Hospital patients with views of trees have been found to recover significantly faster than those who can not see any natural features.
Research by Lancaster University (Hewitt et al, undated) has found that trees can remove a number of pollutants from the atmosphere, including ozone, nitrogen dioxide and particulates. However, trees also produce volatile organic compounds (VOCs), which can lead to an increase in ozone, particulates and other pollutants when combined with man-made pollution.
Different species of tree have different net effects on air quality. Willows, poplars and oaks can worsen air quality during hot weather, whilst ash, alder and birch have the greatest beneficial effects.
The study estimates that doubling the number of trees in the West Midlands would reduce excess deaths due to particulate pollution by up to 140 per year.
Trees absorb carbon dioxide (one of the main greenhouse gases) and release oxygen during photosynthesis. The carbon absorbed by trees is stored in the wood.
A study by Lancaster University estimated that the total amount of carbon stored in trees within the West Midlands represents only three weeks worth of carbon dioxide emissions. However, trees still have an important role to play in reducing the effects of greenhouse gases through carbon sequestration.
Careful tree planting can reduce the amount of fuel used to heat and cool buildings. Lots of research has been done in the USA, but little research has taken place in the UK.
Trees provide shelter and reduce wind speed, thus reducing heat loss from buildings during winter. They provide shade in the summer and evapotranspiration of water from the leaves has a cooling effect on the surrounding air. This can significantly reduce the need for air conditioning during hot weather.
Trees and other vegetation can play an important role in reducing noise. One estimate suggests that a 7db noise reduction is achieved for every 33m of forest (Coder, 1996) whilst other field tests show apparent loudness reduced by 50% by wide belts of trees and soft ground (Dwyer et al, 1992).
Trees have a number of hydrological effects. These include reducing erosion and improving water quality by intercepting pollution. They also reduce ground water run-off, which helps reduce flooding. One study has estimated that for every 5% increase in tree cover area, run-off is reduced by 2% (Coder, 1996).
Trees are an important wildlife habitat. They provide nesting sites for birds and support a wide range of insects that are an important food source for wildlife. Trees that produce berries are a direct source of food for many bird species.
In an urban setting, linear corridors of habitat are important. They connect otherwise isolated areas to each other and allow wildlife to move around. Trees and other vegetation along highways, waterways and railways are particularly important linear corridors.
Trees can help improve road safety in a number of ways:
Tree lined streets make it feel like the street is narrower and encourage slower driving
They reduce stress (Wolf 1998(d), Kuo and Sullivan 2001(b)) and are likely to reducing road rage and improve the attention of drivers
Street trees provide a buffer between pedestrians and road vehicles
Trees can have negative effects on the surface of footways and carriageways through direct root damage. However, the shade cast by trees can significantly increase the life of the road by reducing the temperature that the surface reaches during hot weather.
Coder, KD, 1996, Identified Benefits of Community Trees and Forests, University of Georgia Cooperative Extension Service - Forest Resources Publication FOR96-39
Dwyer, JF, McPherson, EG, Schroeder, HW and Rowntree, R, 1992, Assessing the Benefits and Costs of the Urban Forest, [in] Journal of Arboriculture 18(5), pp 227 - 234.
Hewitt, N, Stewart, H, Donovan, R and MacKenzie, R, undated. Trees and Sustainable Urban Air Quality, Research summary from Lancaster University at http://www.es.lancs.ac.uk/people/cnh/docs/UrbanTrees.htm
Kuo, FE and Sullivan, WC, 2001(a), Environment and Crime in the Inner City. Does Vegetation Reduce Crime [in] Environment and Behaviour 33(3), pp 343 - 367
Kuo, FE and Sullivan, WC, 2001(b), Aggression and Violence in the Inner City - Effects of Environment via Mental Fatigue, [in] Environment and Behaviour 33(4), pp 543 - 571
Kuo, FE, 2003, The role of Arboriculture in a Healthy Social Ecology [in] Journal of Arboriculture 29(3), pp148 - 155
Nowak, DJ, undated, The Effects of Urban Trees on Air Quality, USDA Forest Service, Syracuse, NY
Taylor, AF, Kuo, FE, Sullivan, WC, 2001, COPING WITH ADD - The Surprising Connection to Green Play Settings [in] Environment and Behaviour 33(1), pp 54 - 77
Wolf, K, 1998(a), Trees in Business Districts - Positive Effects on Consumer Behaviour, University of Washington College of Forest Resources, Factsheet #30.
Wolf, K, 1998(b), Trees in Business Districts - Comparing Values of Consumers and Business, University of Washington College of Forest Resources, Factsheet #31.
Wolf, K, 1998(c), Urban Forest Values: Economic Benefits of Trees in Cities, University of Washington College of Forest Resources, Factsheet #29.
Wolf, K, 1998(d), Urban Nature Benefits: Psycho-Social Dimensions of People and Plants, University of Washington College of Forest Resources, Factsheet #1.
Wolf, K, 1999, Grow for the Gold, [in] TreeLink 14, Washington State Department of Natural Resources
Wolf, K, 2000, Community Image - Roadside Settings and Public Perceptions, University of Washington College of Forest Resources, Factsheet #32.
Wolf, K, 2003, Public Response to the Urban Forest in Inner-City Business Districts, [in] Journal of Arboriculture 29(3) pp 117 - 126