The benefits of urban trees

Economic benefits

Consumer behaviour

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).

Inward investment

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.

Property values

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.

Social benefits

Crime reduction

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.

Environmental benefits

Pollution interception

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.

Carbon sequestration

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.

Fuel use

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.

Noise reduction

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).

Hydrology

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).

Wildlife benefits

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.