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Economic Benefits
Consumer behaviour
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).
Inward investment
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.
Property values
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.
Social Benefits
Crime reduction
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.
Environmental Benefits
Pollution interception
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.
Carbon sequestration
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.
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.
Noise reduction
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).
Hydrology
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).
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 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.
Other Benefits
Road safety
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.
Road surfaces
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.
Bibliography
*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
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