Trees in towns bring with them both benefits and costs. Whilst many of the costs are well known to managers of urban trees, who watch the budgets and answer the phone to disgruntled residents, the benefits can be seen as nebulous and difficult to quantify or justify. Never the less, a considerable and expanding body of research exists on the benefits that urban trees bring. Here we attempt to summarise some of the benefits of urban trees.
A study by the University of Washington established a number of benefits in terms of consumer experiences of business districts with trees (Wolf, 1998(a), Wolf, 1999 and Wolf, 2003). Consumers reported consistently higher ratings for a number of categories related to their perception of business districts with trees. They reported a willingness to pay more for parking in landscaped car parks and on average reported a willingness to pay about 11% more for goods in a landscaped business district than a non landscaped district, with this figure being as high as 50% for convenience goods.
Both the business community and consumers were found to favour business districts with good landscaping (Wolf, 1998(b)).
The quality of landscaping along approach routes to business districts has also been found to positively influence consumer perceptions (Wolf, 2000).
The attractiveness of an environment is an important factor in attracting inward investment. Both consumers and businesses have been found to favour districts with high tree cover and the increase in retail prices that can be commanded in well landscaped areas can reasonably be assumed to be a positive benefit in attracting businesses to the district.
Several studies in the USA have analysed the effect of tree cover on the price of residential house sales, finding that values 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 translation of these data is not possible. Never the less, 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 street were said to be in more demand and to sell faster.
The conventional wisdom has been that trees and other vegetation have a negative impact on crime because they provide cover for criminals and reduce opportunities for casual surveillance.
Research in a particularly deprived area of inner city Chicago has suggested that this is in fact not the case and that appropriate vegetation cover can lead to reduced crime rates (Kuo and Sullivan, 2001(a)). The study dealt largely with mown grass and high canopy trees, which do not provide cover in the same way as, for example, shrub planting. It looked at an area with relatively homogenous architecture and a relatively homogenous population but with differing levels of vegetation. Areas with higher vegetation cover were found to have lower rates of crime, as measured by reports to the police.
Two mechanisms are suggested by which crime rates might be reduced by trees. The first is through an increase in surveillance, essentially because public open space with trees tends to be used much more than space without trees. The second mechanism relates particularly to violent crime and relates to evidence that vegetation has a mitigating effect on mental fatigue, itself often a precursor of outbursts of anger and violence (Kuo and Sullivan, 2001(b)).
Other social benefits
A wealth of research has been undertaken by the Human-Environment Research Laboratory at the University of Illinois and has identified numerous beneficial effects that trees have on society. A good summary of these is a paper by Frances E. Kuo, “The Role of Arboriculture in a Healthy Social Ecology”, which is attached to the PDF version of this document (Kuo, 2003).
Many of these benefits relate to encouraging people out of their homes and into public open space, where they react more with others and build stronger social relationships. An additional benefit of interest 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 experience 23% less time off sick than those who can not see any nature. Desk workers who can see nature also report greater job satisfaction (reported by Wolf, 1998(d)), whilst hospital patients with views of trees have been found to recover significantly faster than those who can not see any natural features.
Research undertaken in the West Midlands by Lancaster University (Hewitt et al, undated) has established that trees can remove a number of pollutants from the atmosphere, including ozone, nitrogen dioxide and particles. The news is not all good though. Trees also produce volatile organic compounds, VOCs, which in combination with some man made pollutants can lead to an increase in ozone, particulates and other pollutants.
Different species of tree have different net effects on air quality. Willows, poplars and oaks can potentially worsen air quality during hot weather, whilst ash, alder and birch have amongst 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.
It is well known that trees, in common with all vegetation, absorb carbon dioxide (one of the principal greenhouse gases) and release oxygen during the process of photosynthesis. The carbon absorbed by trees in this process is stored in the wood.
Whilst this most well known of benefits is real it seems it is often overstated. The study by Lancaster University of trees in the West Midlands estimated that the total amount of carbon stored in trees within the conurbation represents the equivalent of about three weeks worth of CO2 emissions. Never the less, trees do have an important role to play in reducing the effects of greenhouse gases, not only through carbon sequestration but perhaps more importantly through the effects that careful planting can have on fuel use.
Careful tree planting can reduce the amount of fuel used on both heating and cooling buildings. A considerable amount of research has been undertaken to quantify this in the United States, but little such research has been undertaken in the UK. Clearly differences in climate mean that figures here can not be directly related to any part of the USA.
Trees provide shelter and reduce windspeed, thus reducing heat loss from buildings during winter. They also provide shade in the summer, whilst the evapo-transpiration of water from the leaf surface has a general cooling effect on surrounding air. This can significantly reduce the need for air conditioning during hot weather.
Trees and other vegetation can play an important role in attenuating noise through reflecting and absorbing sound energy. One estimate suggests that 7db noise reduction is achieved for every 33m of forest (Coder, 1996) whilst other reported 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 through interception of pollution. Perhaps the most important effect in Britain at present, given the trend for increasing winter flooding, is the reduction in ground water run-off. 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 birds and other wildlife. Trees that bear berries are also a direct source of food for many bird species.
In an urban setting, linear corridors of habitat are among the most important, connecting otherwise isolated areas to each other and out to the rural surroundings. Trees and other vegetation along highways, waterways and railways are particularly important to wildlife in the respect.
Trees can help improve road safety in a number of ways.
Trees lining streets give the impression of narrowing the street and encourage slower driving.
The stress reduction effects of trees (Wolf 1998(d), Kuo and Sullivan 2001(b)) are likely to have the effect of reducing road rage and improving the attention of drivers.
Trees along streets also provide a buffer between pedestrians and vehicular traffic.
Managers of both trees and highways are well aware of the detrimental effects that trees can have on the surface of footways and carriageways through direct damage by roots. Less well known is the fact that the shade cast be trees can significantly increase the life of road surfaces by reducing the temperatures which the surface reaches during hot weather.
Papers marked * are included in the appendices of the PDF version of this document.
*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 Behavior 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 Behavior 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 Behavior 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