Trees in towns and cities bring both benefits and costs. While many of the costs are well known, the benefits can be more difficult to measure. But lots of research has been carried out on the benefits trees bring, which include:
A study by the University of Washington showed the benefits of trees in business districts. People were found to be willing 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 and the quality of landscaping along approach routes to business districts has also been found to have a positive influence on consumer perceptions. (Wolf (c))
The attractiveness of an environment is an important factor in encouraging inward investment. Both consumers and businesses have been found to favour districts with trees and the increase in retail prices in well landscaped areas can help to attract businesses.
Several studies in the USA have looked at 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)).
It is often thought that trees and other plants can provide cover for criminals and increase crime, but in a survey in Chicago in the USA, areas with higher vegetation cover were found to have lower rates of crime.
Crime rates could be reduced by trees for two reasons: they encourage people to use and enjoy open spaces and they help to reduce mental fatigue, which can lead to outbursts of anger and violence (Kuo and Sullivan, 2001(b)).
A lot of research has been undertaken by the Human-Environment Research Laboratory at the University of Illinois and has identified many benefits that trees have on society.
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. Another benefit is the positive effect that contact with nature can have on children with Attention Deficit Disorder (ADD) (Taylor, Kuo, Sullivan, 2001).
Workers who can see nature from their desks take 23% less time off sick than those who can't and report greater job satisfaction. And hospital patients with views of trees have been found to recover quicker than those who can't.
Research in the West Midlands by Lancaster University (Hewitt et al, undated) has shown that trees can reduce pollution - although in some cases they can produce organic compounds that can add to pollution.
Different species of tree have different effects on air quality. Willows, poplars and oaks can potentially worsen air quality during hot weather, whilst ash, alder and birch have some of the greatest benefits.
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. There are differing views over the amount of carbon involved, but trees do have an important role to play in reducing the effects of greenhouse gases.
Careful tree planting can reduce the amount of fuel used on both heating and cooling buildings. Trees provide shelter and reduce windspeed, thus reducing heat loss from buildings during winter. They also provide shade in the summer, and the way water evaporates from leaves 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 cutting noise pollution by reflecting and absorbing sound energy.
Trees have a number of water-related effects. These include reducing erosion and improving water quality by absorbing pollution. Perhaps the most important effect in Britain at present is reducing groundwater run-off and the threat of flooding.
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 towns and cities it is important separate green spaces are linked to each other and the countryside. Trees and other vegetation along highways, waterways and railways are particularly important to wildlife in this way.
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 traffic.
It is well-known that tree roots can damage roads and footpaths, but less well known is the fact that the shade cast by trees can greatly 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.