04:51

Chapter 4

Part I

4.1

Introduction

 

 

 

 Trees and forests are, because of seasonal changes and their size, shape, and color, the
most prominent elements of urban nature. Their benefits and uses range from intangible
psychological and aesthetic benefits to amelioration of urban climate and mitigation
of air pollution. Historically the main benefits of urban trees and forests relate
to health, aesthetic and recreational benefits in industrialized cities. Moreover, green
areas have provided people with subsistence by providing food, fodder, fuel, wood and
timber for construction (see Chap. 2).
Today, woodland, woods and trees are important to people especially through symbolizing
personal, local, community and cultural meanings. They provide aesthetic
enjoyment and create a pleasant environment for different outdoor activities. Woodland
can provide an experience of nature in the middle of urban life. In particular, old
woodland with big trees may provide urban people with the opportunity to recover
from daily stress, revive memories and regain confidence. There is also an important
educational value of urban forests. Contact with trees, in particular for children, can
help people learn about nature and natural processes in an otherwise artificial environment.
Urban trees and woodland also contribute to an attractive green townscape and
thus communicate the image of a positive, nature-oriented city. Indirectly, urban trees
and forests can promote tourism and enhance economic development. At the local level
trees contribute to the quality of housing and working environments and their benefits
are reflected in property values. The same urban woodland areas and trees may have
multiple benefits that reinforce each other. Recreational woodland, for example, also
reduces wind speed and traffic noise as well as improves the landscape in a nearby
residential area. To a certain extent the distinction between different categories of
benefits is artificial. However benefits have their own special features and therefore
can be presented separately (Table 4.1).
While these benefits of urban woodland, other tree stands and individual trees are
not new they are still insufficiently recognised in urban planning and development
processes (see Chap. 5). There is need to provide more knowledge on the role of urban
woodland and trees in improvement of the environment and relate this to their social
functions such as fostering mental and physical health.
This chapter aims to give insight into the current state of knowledge about benefits
and uses of urban forests and trees in Europe. This is a difficult enterprise due to the
complexity of the European continent. Urban forest research is largely national or even
Benefits and Uses of Urban Forests and Trees
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82 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
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local, and results are often only disseminated in the national language (Forrest et al.
1999). Moreover, the benefits of woodland and trees can differ widely between European
cities and towns due their different environmental and socio-cultural background.
The recreational and aesthetic benefits are traditionally important especially in the
Nordic countries, whereas the protective and climatic uses of vegetation are more
emphasized elsewhere in Europe. Furthermore, while the use of trees to shelter from
strong winds is an important issue in the north-western part of the continent, shading
is a more important concern in hot climates, for example in the Mediterranean. In
practice, management of the urban forest is a challenging task not only because of
harsh growing conditions but also because of various, often conflicting, demands and
goals. Therefore, this chapter will also address geographical and socio-cultural differences
in benefits and uses between European regions.
4.2
Social and Aesthetic Benefits of Urban Forests and Trees
4.2.1
Urban Woodland and Parks As a Recreational Resource
One of the generally acknowledged functions of in particular urban woodland and
parks is the provision of recreational opportunities. Urban green-space recreation was
a genuine phenomenon of the mid-European bourgeoisie culture of the early
19th century. In earlier times, royal and aristocratic parks as well as urban woodland
were used as deer parks and hunting grounds to display the splendour of court life.
Tree alleys, promenades, malls and the king’s way represented the power of the political
system (Poëte 1913; Chap. 2). As well in countries with a long democratic tradition
such as Switzerland, the role of trees, parks and alleys has been remarkable. In Calvinist
Geneva, for instance, there was literally a tree cult from the 16th century onwards
and spring was officially announced when the buds of a particular tree appeared and
were seen by a state employee (Silva 1996).
Table 4.1. Benefits and uses of urban forests and trees (adapted from Tyrväinen 1999)
Chapter 4 · Benefits and Uses of Urban Forests and Trees 83
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The French Revolution put an end to many aristocratic privileges in all spheres of
social life and citizens gained free access to parks and forests across the countries.
During the Napoleonic wars and in the era of political restoration, the lifestyle characterized
by bourgeois values was spread all over mainland Europe. This also paved the
way for outdoor recreational use that had so far been unknown. Industrialization led
to a massive transfer of labor from the agricultural sector to the newly established
centers of industry and mass production. Insufficient hygiene, poor housing conditions
and long working hours were major threats to millions of people who had either
no access to, time for or interest in green-space recreation (see also Chap. 2).
Only in the late 19th century and in the first half of the 20th living conditions of the
urban working class improved. A sports and outdoor movement emerged that used
urban green space for recreation. Leisure time, being once a luxury good of the upper
class, became more common among other social classes. The formal separation of a
person’s life time into working hours and leisure time made recreation an explicit
social demand. The provision of green space in and around cities became a representation
of middle class values. Its design and function became an attribute of urban
culture itself and were spread all over the globe. In the post-industrial era of the late
20th century, parks with a postmodern design emerged in large central European cities
like Zurich. This has been a remarkable trend as the municipal area of Zurich has a
green-space cover of no less than 43%, even with real estate prices higher than the
average in central Europe.
Today, outdoor recreation is a type of activity many people participate in, all across
Europe. Participation in the most common recreational activity, walking, stands at about
81% in Finland (Pouta and Sievänen 2001) and 74% in The Netherlands (Statistics
Netherlands 1997). Many of these recreationists have considered natural environments
more attractive as activity settings than built-up areas. Among natural areas, forests are
considered one of the more attractive types of nature. In Italy, 96% of the population
participates in recreation activities involving the forest (Scrinzi et al. 1995). In Denmark,
this proportion is about 91% (Jensen 1999). There are, however, large regional differences
in the supply of forests in and around cities. For example, in Finland forests cover
about 86% of the land areas and they are also the prevailing type of urban green area,
whereas in The Netherlands forests cover only 10% for the total land area. If we look
Fig. 4.1.
Recreationists in the Helsinki
urban forest (photo: City of Helsinki,
Environmental Centre)
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at the amount of forest per capita, the differences become even larger: 51 000 m2 in
Finland versus about 220 m2 in The Netherlands (Sievänen et al. 2000). The attractiveness
of forests as a recreational environment is also evident from the distance that
people are willing to travel to visit a forest. According to Scrinzi et al. (1995), Italians
travel about 32 km (single distance) to a forest visit. This is about the same distance
that residents in the western part of The Netherlands – the most urbanized and “forest-
poor” part – travel to their most often visited forest site (De Vries 2000).
Accurate information on the actual level and type of recreational use of forests is
still relatively scarce for most countries. In Italy a first national study was carried out
in 1995 (Scrinzi et al. 1995). For the German-speaking countries a review study including
articles from more than 60 periodicals in forest sciences gives a good overview for
the period between 1960 and 1995 (Schmithüsen et al. 1997). Moreover, an overview of
the recreational use of forest in the Nordic countries has been provided by Jensen (1995).
Participation frequencies derived from interviews or mail surveys may not always
coincide with figures obtained through observations of actual forest visits; in retrospect
respondents tend to exaggerate the number of visits they have made to forests
(Jensen 1999). In Denmark, the average annual number of forest visits is somewhat less
than 40 times before correction, and about 13 times after correction. In Italy the average
frequency of visits is only four times a year, however, the average duration of a visit
is almost four hours. Finland scores much higher with an estimate of between 72 and
110 visits per year. The duration of a visit is usually from half to one hour (Tyrväinen
1999). It is unclear to what extent this high frequency is due to the abundant supply of
woodland in Finland or caused by different measurement methods. Therefore, more
systematic research and international comparisons are needed.
In urban forests walking tends to be the most common recreational activity. Other
common activities are cycling, jogging, picnicking as well as picking berries and
mushrooms (Fig. 1). However, there exist clear differences between European countries.
Cycling within forests is not that common in Italy. Picking berries and mushrooms
is relatively infrequent in Dutch and Danish forests, while cross-country skiing
in winter is very common in Finland, Sweden and Norway. These differences are related
to the recreation possibilities that the nearest forests in one’s environment offer,
in combination with the forests’ proximity. Using a forest environment for daily physical
exercise takes place only if such an environment is available nearby (Tyrväinen
2001; De Vries and Goossen 2002).
Experiences that are sought after are predominantly enjoying the natural scenery,
and peace and quietness. On a scale from wilderness to developed natural areas, forested
areas tend to be located closer to the developed side, although still less developed
than urban parks. This is partly a consequence of proximity to a large concentration
of inhabitants. If open to the public, recreational use tends to be rather intensive.
The Dutch State Forest Service suggests approximately 1 000 visits ha–1 yr–1 to be
common for this type of forests. This implies that there are likely to be other people
present during one’s visit. Although this is not likely to contribute to experiencing
quietness, forests have a relatively large ‘social capacity’ per hectare, i.e. because of the
trees there can be many people present without the area feeling crowded. This makes
forests a relatively efficient type of resource for nature-based recreation, compared to
for instance agricultural areas. The perception of crowdedness obviously also depends
Chapter 4 · Benefits and Uses of Urban Forests and Trees 85
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on visitor expectations. Although many urban forests are unlikely to be selected as a
destination for the opportunities they offer with regards solitude, during some days
and time points they might actually provide this experience. However, people’s recreational
motives vary and different user interests often lead to conflicts. For example,
those who want to go for a walk in a quiet and natural environment may feel disturbed
by others, who pursue hobbies such as horseback riding and mountain biking
(e.g., Seeland et al. 2002).
A rapidly growing segment of the population in many European countries consists
of ethnic minorities. Often very little is known on their desires and use of urban green
space. Language problems have frequently prohibited their participation in surveys,
unless special measures are taken. In the few studies that are available, Dutch ethnic
minorities (predominantly people from Turkey, Morocco, Suriname and The Netherlands
Antilles) appear to be more focused on recreation in urban green areas than in
the countryside (e.g., Jókövi 2000). The social aspect of recreation, being together with
family and friends, seems to be more important to them than to the indigenous population.
The common Dutch activity of bicycling is less popular among the people from
these ethnic minorities. However, the composition of this segment is rapidly changing,
due to the large influx of asylum seekers originating from different countries. It is even
less clear what the needs and desires of these new groups will be regarding urban
greenery and outdoor recreation.
From social demands regarding the type and amount of forests it seems to be only
a small step to demands based on ecological motives such as conservation and
biodiversity. Most visitors appreciate the idea of the naturalness of an urban forest, and
the importance of ecological management has increased during the past decade
(Tyrväinen et al. 2003). However, the relation between the ecological and the social
function is not a simple one. On the one hand, appreciating nature may lead to increased
support for ecological goals, but on the other hand, recreational usage may
endanger fragile ecosystems. To many people, however, rare animals and plants are not
especially important in selecting a destination area. Some people will not even pay
attention to or recognise them during the visit. Environmental information and education,
however, can increase the awareness of residents and help them appreciate urban
flora and fauna. Furthermore, people like to have easy access to the forest, whereas
ecologists prefer to minimize disturbance. For urban forests the primacy of the social
function is essential. By offering people ample and high quality recreation opportunities
nearby, they will be less inclined to visit ecologically fragile environments located
further away. However, even for urban forests with a predominantly social function,
some ecological preconditions have to be taken into account, to provide a sustainable
recreation environment.
4.2.2
Health Benefits of Urban Forests and Trees
Urban forests and trees contribute to a better quality of living environment in cities,
for example by improving air quality and consequently the health of urban residents.
The leaves of trees can take up many pollutants, e.g. ozone, nitric acid vapor, nitrogen
dioxide, ammonia, sulfur dioxide and particles (aerosols and dust). Some of these pollut86
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ants can cause serious health problems. Trees also provide valuable shading from the sun.
An individual tree can provide a Sun Protection Factor (SPF) of 6 to 10, which means a
level of exposure to ultraviolet radiation of one sixth to one-tenth of full sun (NUFU 1999).
There are also other ways in which urban forests may improve public health. By
offering an attractive environment for recreational activities, urban forests may seduce
people with a sedentary life style to become more active during their leisure time.
Activities such as recreational walking and cycling already have a positive effect on
one’s health. It has indeed been shown that more green space within the living environment
leads to people visiting natural environments more often (e.g., Grahn and Stigsdotter
2003). However, a higher number of visits to green areas does not necessarily
mean that these people are more physically active. For example, people living in a less
green environment may still walk often, but do so more frequently in a built-up area.
Nearby urban forests and parks are especially important for elderly and young people
who are restricted in their capacity to move. The most active users of neighborhood
forests are probably children. There are also programs that try to stimulate people to
become physically active within the local natural environment, for example in the United
Kingdom (Ashcroft 2002). When they do go for a walk, a lack of nearby nature-based
opportunities tends to increase the number of people using a car and subsequently
leads to driving longer distances to visit an attractive natural area (De Vries 2000). The
key factor for active use is easy access to the areas, preferably within walking distance
from home. In a survey study in Salo, Finland half of the respondents noted that the
main reason for not using urban recreation areas was the distance (Tyrväinen 2001).
An important positive effect of natural scenery on health is its stress reducing effect.
Research similar to original studies in the United States (Ulrich et al. 1991) has led to
similar results in Sweden (e.g., Hartig et al. 1996). Just visually experiencing a natural
setting reduces stress. Stress relief, as measured through muscle tension, blood pressure
and electrical brain activity, can be demonstrated within some minutes of exposure
to a green environment (Ulrich et al. 1991). Moreover, viewing or visiting natural
environments (compared to built urban environments without natural elements) after
stressful or mentally fatigued situations, produces greater physiological changes toward
relaxation and faster recovery of attention-demanding cognitive performances
(Parsons et al. 1998). Research has shown that even quite ordinary urban green areas
have a stress-reducing influence in everyday life. In Sweden, Grahn and Stigsdotter
(2003) demonstrated that the more often one visits green areas the less often one reports
sickness from stress.
It is unclear to what extent the mechanism behind this restorative effect is evolutionary
in character and/or cognitively mediated. As a consequence, also very little is
known about how to design and maintain urban green spaces in such a way as to optimize
their health benefits. A high aesthetic quality may not be required for a stress
reducing effect, but might be helpful to attract people to the green area. One precondition,
however, is quite generally thought to be important for restorative effects: safety.
The (assumed) presence of dangerous others will diminish positive health effects. As
mentioned before, common motives for visiting forests are experiencing solitude, peace
and quietness. These qualities may also be conducive to the stress-reducing effect.
However, crime statistics, for example in the United Kingdom, show that physical attacks
are rare in woods, and that such concerns are often based on perceptions rather
Chapter 4 · Benefits and Uses of Urban Forests and Trees 87
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than reality. One of the key factors for security is visibility, which requires active management
of the understorey, and giving the impression that the area is controlled
(Tyrväinen et al. 2003).
Another possible mechanism relating nature to health is that of social interaction
and cohesion. While European research in this topic is still scarce, several studies conducted
in Chicago, USA suggest that green space, especially trees, may help to facilitate
(positive) social interaction with neighborhood members (Kweon et al. 1998). This is
suggested to reduce feelings of social isolation, which is a risk factor related to depression.
Although it is still unclear what are the most relevant mechanisms behind the
health effects, recent Dutch research has shown that the relationship between the amount
of green space in the living environment and self-reported health is positive, even after
controlling for relevant socio-demographic and socio-economic characteristics (De
Vries et al. 2003).
4.2.3
Social Potential and Trends in Urban Forest and Tree Benefits and Uses
Nowadays, different sections of urban society tend to share more collective values
regarding sound management of the environment, including the importance of green
space for the well-being of growing urbanized societies. Ongoing social change and
increased pressure on the different types of green spaces is a challenge for traditional
concepts of maintenance. Conflicts and maintenance problems have developed during
recent decades due to a lack of information about the social needs and expectations of
various user groups. Due to this limited knowledge, urban greening projects are often
designed according to architectural and aesthetic standards which have little reference
to the local population with its specific needs.
The demographic development in the service-oriented societies of central and other
parts of Europe shows a trend towards further urbanization, a remarkable increase in
the number of elderly people and groups with special demands for a certain social infrastructure
(e.g., disabled people, asylum seekers, unemployed people, drop-outs, and so
forth), a decreasing tolerance to car traffic, and a desire for close-to-nature recreation
in or near cities. With regard to these trends, the need for detailed information on
urban woodland, parks and trees on public and private land will only increase. The key
issues related to the future benefits of urban forests and trees include what is demanded
and perceived by whom from urban green spaces, and to what extent and how green
spaces could be preserved in and around cities during modernization of cities.
Beautification of the city with gardens and parks for the sake of an image of
splendour and generosity was an important aspect of greening cities throughout the
feudal and bourgeoisie eras. To have one’s recreational needs served within the living
environment was a privilege of only few and closely connected with the location of
housing quarters. With an increase of urban population and particularly the middle
class, entertainment, sport and recreation went along with a daily or weekly visit to
urban green space to counterbalance stress and compensate for the lack of private
home gardens. Large private parks in the core cities were often opened up for public
use and thus a democratization of green space private property became widespread
all over central Europe.
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Today, event-culture is provided where attractive entertainment is expected; and
this applies to the media as well as to open-air events in public green spaces. What
common access meant to the middle and lower classes of an emerging urban society
in the 18th and 19th century has become a rising public demand for fun-parks and entertainment
facilities at the beginning of the 21st century. Apart from dense networks
of paths and other recreational infrastructure, there are often special attractions in
urban forests such as zoos, amusement parks and platforms for open-air concerts. Green
space with related amenities and social and cultural services to make it more attractive
seems to be the demand of today and probably even more for tomorrow.
Although a close-to-nature living environment seems to be a desire to many people,
at the same time cities and towns have become more compact. Migration studies (e.g.,
Willaert 1999) point out a steady flight from Flemish cities with relatively low amount
of green areas, especially since the late 1980s. Also in sparsely populated countries such
as Finland, nature and peaceful environment attract people from urban areas to more
rural surroundings. Compact city policies and ‘infill’ in existing housing areas has
resulted in an increasing demand for land within city limits and demands to build on
land allocated to green spaces. This means decreasing amount of green spaces within
the easy daily access for residents as well as increased use pressures on the remaining
green areas, which often leads to overuse, congestion and the depletion of nature.
In general, as lifestyles in Europe have become more urban, the demands for urban
woodland and trees become more diverse. Although urban forests are places for social
contacts and bringing people together, at the same time many users are looking for
solitude and peace and quiet. Moreover, awareness of the importance of ecology and
preserving urban biodiversity is increasing among the residents. Compact city policies,
however, provide less green areas resulting in decreased possibilities to maintain
natural vegetation in urban areas. In addition, parallel to traditional ways to use urban
nature, more adventurous and active forms of recreation have increased including
mountain biking, skateboarding, survival games and paintball. In this respect, the social
carrying capacity of urban open green areas depends on the type of use.
In conclusion, public green spaces have multifunctional purposes such as those mentioned
above practically all over Europe. There is an increasing need to define and promote
the socially integrative potential of woodland, parks and trees and to integrate people
with specific needs and demands, deriving from their social status age, gender and ethnic
background (German-Chiari and Seeland 2004). Due to an increase in the multicultural
set-up of urban populations in the wake of European political integration and the influx
of non-European immigrants and asylum seekers, and the increment of the number of
singles among the urban population because of the fragmentation of families, socializing
on the occasion of urban outdoor events (e.g. open-air concerts, summer festival weeks
etc.) gains momentum. To meet people outside their homes and working places has always
been a major purpose of urban green spaces. Be it urban woodland, parks or trees,
there tends to be fewer differences and more commonalties in globalizing societies. Public
green space offers a great opportunity for all sections of an urban society to meet in an
arena that can be designed and used in a participatory way to benefit all. Public, open
green space matters the more where informal social conventions increase. This trend of
social inclusion among the younger urban generations is perhaps a counter-current to
more and more cyber-based forms of communication and access to reality.
Chapter 4 · Benefits and Uses of Urban Forests and Trees 89
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4.2.4
Architectural and Aesthetic Benefits
Architectural benefits deal with the use of vegetation in urban planning and development
(see also Chap. 6). The main purpose of trees and forests is to improve and to
restore constructed townscapes. Vegetation is used in defining open space and inte-
Fig. 4.2a,b. Seasonal variation in urban forests (photos: E. Oksanen, Metla)
a
b
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grating the buildings to the surrounding environment. According to Robinette (1972)
plants form walls, canopies or floors of varying heights and densities; these are architectural
characteristics. Landscape variation is created through different colors, textures,
forms and densities of plants. Urban trees can direct vision, break up large spaces,
and define space. They can be used to frame scenes and to provide foreground and
backgrounds for landscape features.
Aesthetic benefits relate to people experiencing different colors, structure, forms
and densities of woody vegetation (Fig. 4.2). Much of the aesthetic experience is subjective
in nature and has impacts on people’s mental and emotional state (e.g., Kaplan
and Kaplan 1989). Even a single tree carefully placed can make an important contribution
to the aesthetic quality of the location. A great deal of the consumption of amenities
occurs indoors through a window or from a car or bicycle. Visual variation is often
stressed as being a key factor for aesthetic experiences (e.g., Axelsson-Lindgren 1995).
In landscape research there are many different research paradigms dealing with
aesthetic values including psychophysical, cognitive (psychological), experiential (phenomenological)
and expert approaches (Zube et al. 1982; Daniel and Vining 1983; Lothian
1999). These different research approaches produce different type of information for
design and management of urban forests. The psychophysical and expert approaches
provide information more easily applicable for practical purposes than other approaches.
Psychophysical research has tried, first and foremost, to analyze and rank
the preferences of people related to various types of urban forest environments (Daniel
and Vining 1983; see also Karjalainen and Tyrväinen 2002). The cognitive approach
(Kaplan and Kaplan 1989) has provided a framework for preferences and their links to
cognitive aspects of the environment. The most common concepts derived from this
knowledge base applied in practical planning guidelines in urban woodland have been
diversity, scale, visual accessibility, stewardship, naturalness-continuity and coherence
(Ode and Fry 2002).
In preference research aesthetic values are thought to be linked to the evaluation
context as well as respondents’ characteristics such as education, recreational activity,
nature relationship, age and gender. Preference studies mainly from North America
have shown that attitudes towards the wooded environments differ between children,
teens and adults (Kaplan and Kaplan 1989). Youths appreciate the wild, dense, and hidden
forest more than cultivated and open forest. Moreover, adults and children appear to
value open-forest landscape more than dense forest. For children, structurally diverse
natural places have been stressed as being more inspiring and imaginative, even compared
to a well-organized playground (Kaplan and Kaplan 1989; Grahn 1997).
The visual quality of urban forests and people’s preference can be examined through
various approaches. Verbal information has been shown to have an effect on people’s
acceptance of various management actions in a recreational forest area (Jensen 1999).
The main part of aesthetic perception occurs through the sense of sight and therefore,
visualization of landscapes is a central part of forest landscape perception and preference
research. Today digital image editing (Fig. 4.3) and in the future virtual landscape
simulators offer the most sophisticated means of visualization for landscape research
(Karjalainen and Tyrväinen 2002).
People’s within-forest landscape preferences correlate strongly with the characteristics
of the forest stand. People prefer stands of tall trees, but the preferred tree speChapter
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Fig. 4.3a,b. Pair of slides produced by photo manipulation illustrating different management options
(photos: E. Oksanen, Metla)
cies relate to the specific geographical region in question. In Finland, for example, the
most appreciated species in woodland are Scots pine (Pinus sylvestris) and silver birch
(Betula pendula). Furthermore, in urban woodland the within-forest visibility should
be relatively good. In general, old and mature forest stands are preferred over young
a
b
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and small trees, but small trees, if they form the lower canopy layer of a two-storey
stand, are considered to improve the aesthetic value of the stand. Variation is greatly
appreciated, not only due to mixtures with other types of trees, but also combination
of trees with fields, meadows and, in particular, water elements (Schmithüsen et al.
1997). In particular, forest edges, which are many in urban settings, are essential for
human aesthetic experience and visual perception. A well-designed edge consists of
mixture of bush and tree species, which have not only aesthetic but also ecological
importance (Lucas 1991).
Moreover, the aesthetic valuations may partially change over time and are influenced
by trends, cultural system and knowledge. Studies on forest management preferences
show that regeneration, in particular clear-felling, is visually the least acceptable
management practice (Ribe 1989). Thinning treatments affect the stand’s scenic beauty
less than regenerative cuttings. Moreover, the length of time since the treatment has
been found to correlate positively with the stands’ visual quality (Silvennoinen et al. 2002).
An increasingly important question is whether people find managed forests more attractive
than unmanaged ones. Previous forest preference studies conducted mainly during
the 1980s suggest that residents prefer managed forests if traces of human activity are
not visible. Although both types of results exist, many studies suggest that areas that are
thought to be in a natural condition are perceived to be more beautiful than if traces of
human activity are visible (e.g., Axelsson-Lindgren 1995). Furthermore, logging residues,
dead snags and decayed wood left in the forests are not appreciated. Today, the importance
of ecology has increased and values related to woodland are more conflicting than
earlier (e.g., Seeland et al. 2002). On the one hand, management is needed because of
security aspects and aesthetic reasons, while on the other hand there is an increasing
demand for unmanaged areas based on ecological arguments.
In practice, preferences for urban forest management may be rather different among
user groups. In a study in Helsinki, Finland, the majority of residents preferred managed
forests probably because of security and cultural reasons (Tyrväinen et al. 2003).
The most disliked stands were unmanaged forest vistas where young coppice limited
sight and accessibility. In general, residents also disliked dead or decayed trees left in
the forest. However, younger, higher educated residents and active urban forest users
preferred more ecologically-oriented management compared to older, less-educated
residents and less active users. Also, housing type was connected to the preferences of
urban forests. Residents living in one-family or terraced houses preferred to have
managed forests more than residents living in blocks of flats (Tyrväinen et al. 2003).
Forest landscape preferences have a strong cultural dimension. Nordic residents have
a relatively abundant supply of woodland within their living environment, but in many
densely populated countries in central Europe the daily contact with natural woodland
is less frequent. In a study conducted in Redditch, United Kingdom (Coles and Bussey
2000), open structure woods were found to be preferred over woods with a dense canopy
cover, in particular because of security concerns but also as open woodland offers a
more varied environment. Interviews revealed that escape from urban life and activities
was considered the most important motive to visit a wood, in order to seek a sense
of tranquillity. Whether the woodland was a plantation or a natural woodland did not
appear to matter. Woodland visitors described “natural” mostly as a contrast to the
Chapter 4 · Benefits and Uses of Urban Forests and Trees 93
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urban setting, and every sign of urban intrusion reduced the pleasure of experiencing
nature. Rubbish, signs of vandalism, and management were seen as particularly negative
impacts.
Physical design parameters for urban woodland were also investigated by Coles and
Bussey (2000) in Redditch. A size of 2 ha was identified as the smallest wood that people
wish to visit regularly. Small woods could be attractive when linked together by footpaths.
Shape became particularly important in small woods of less than 5 ha in size.
Blocks of woodland that allow circular walks were preferred over narrow belts.
4.3
Climatic, Engineering and Ecological Benefits
The urban forest can play a major role in improving urban environmental conditions
and safeguarding biodiversity. Environmental benefits do not relate solely to areas of
woodland, however; smaller groups, avenues and isolated trees can equally improve
environmental conditions in urban areas.
4.3.1
Air Quality
Improving air quality has been an imperative of environmental policies throughout
the 20th century. Installation of filtering devices in power plants, the switch to less
polluting fuels and other technical measures have achieved significant improvements
in this respect. Yet air quality remains a major concern. While the concentration of air
pollutants such as sulfur dioxide has been successfully reduced in urban areas, other
– mainly car induced – pollutants such as nitrogen oxides, ozone and volatile organic
compounds are of increasing concern. Climate change is likely to add further to these
air quality problems because rising air temperatures and higher levels of radiation can
lead, for instance, to higher concentrations of ozone in the air. Particular attention also
needs to be paid to ultraviolet radiation, which can cause skin cancer, in the southern
parts of Europe.
A particular challenge lies in the fact that the sources of air pollutants are often
diffuse. In this context, the role of urban green space and in particular that of the urban
forest in removing air pollutants in urban areas has become of interest. Green spaces
and trees are widespread in urban areas and thus could provide an effective means to
improve air conditions locally and provide shelter from ultraviolet radiation. A number
of studies in the United States have shown the potential of the urban forest for
improving air quality (e.g., McPherson 1994; Nowak et al. 2002).
In Europe, evidence is still limited but results of previous studies clearly show that
trees have an important role to play in removing air pollutants in urban areas. A woodland
in Nottingham was estimated to reduce concentrations of sulfur dioxide and nitrogen
oxides in the air by 4–5% (Freer-Smith and Broadmeadow 1996). More important,
however, is the function of trees to capture dust. Evergreen tree species, and in
particular conifers, filter more dust than deciduous species, but conifers are also more
sensitive to damage caused by air pollutants (Däßler 1991; Beckett et al. 1998).
94 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
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Single trees filter less dust than groups or rows of trees. Trees capture air pollutants
most efficiently when they are planted close to the source of emissions. Woodland canopies
are more effective than other vegetation types at trapping particle pollutants because
of their greater surface roughness (Manning and Feder 1980). Studies in North American
cities have estimated the overall removal of air pollutants by trees (McPherson 1994).
While there can be little doubt that the urban forest has a largely beneficial effect on
air quality, the emission of volatile organic compounds (VOC; Beckett et al. 1998) as
precursors of ozone has recently gained attention. This may be an issue in hot climates
with intensive solar radiation such as are experienced in Mediterranean cities. Also,
the production of pollen from tree species such as birch needs to be carefully considered
when tree species are selected for urban plantings, because of its allergenic effect.
A recent large-scale study in the West Midland region of England estimated the overall
removal rates of air pollutants by the urban forest (Stewart et al. 2001). The study was
based on a sample of over 30 000 trees. An air quality model was developed to assess the
potential of the urban forest to remove air pollutants such as ozone, nitrogen oxides and
carbon monoxide and also to estimate potential emissions of volatile organic compounds.
At the time of writing, results had yet to be published in their entirety. However, the data
that are available suggest that planting additional trees on land theoretically available for
this purpose could lead to a significant reduction in concentrations of airborne particles
in the West Midlands once these trees have matured. Planting of all available land could
achieve a reduction of up to 25% of small airborne particles.
4.3.2
Urban Climates
Thermal imagery has been widely used to assess thermal conditions and develop climate
strategies on a city level (e.g., Nachbarschaftsverband Stuttgart 1992). On these images,
urban woodland and trees are consistently among the coolest surfaces during hot summer
days. On these hot summer days, air temperatures within large parks can be 2–3 °C
lower than in the surrounding built-up areas. A significant climatic function can only be
expected when park size exceeds one hectare, while a size of 10 ha is required to lower air
temperatures by 1 °C (Kuttler 1993). The results from extensive studies in Göteborg, Sweden
clearly show the climatic impact of green space (Eliasson 2000). Air temperatures
were significantly lower inside parks as compared to the surrounding built areas, but
temperatures were also reduced in a transition zone outside the parks. However, even
large parks lower the air temperatures in adjacent built-up areas only to a distance of
approximately 200–400 m on the windward side on days with low wind speed when the
urban heat island effect is strongest. Therefore, it is important to protect the vegetation
within urban land uses and to create a dense network of publicly accessible green spaces.
An ideal urban climate would offer a great range of different microclimatic conditions
within walking distance (= 150 m; Mayer 1990), while avoiding climatic extremes.
Trees are the most effective vegetation element for reducing overheating in urban areas.
Figure 4.4 shows the mean surface temperatures for urban surfaces in Munich (Pauleit
and Duhme 2000a,b, see also Chap. 3). Open space types, and in particular those with a
high percentage cover of trees and water surfaces, were the coolest areas in the city. An
increase of tree canopy cover by 10% reduced surface temperatures on average by 1.4 °C
Chapter 4 · Benefits and Uses of Urban Forests and Trees 95
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Fig. 4.4. Mean surface temperatures in urban morphology units and cover of trees ands shrubs for the example of Munich, Germany (adapted from Pauleit and Duhme 2000)
96 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
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during daytime on a hot summer day. Low density residential areas were characterized by
a cover of trees and shrubs greater than 20%. These areas were significantly cooler during
hot summer days than densely built-up housing and commercial areas.
Temperature reduction by trees is mainly caused by two factors: direct shading and
evapotranspirational cooling (Oke 1989). On a hot summer day, for instance, a significant
decrease of air temperatures by more than 2 °C could be observed during daytime
under trees on a car park as compared with sun exposed sites in front of south facing
walls (Brahe 1974). Airflow, on the other hand, can be significantly reduced through
tree plantings. This reduced airflow can decrease energy demand of adjacent buildings
for heating and air conditioning but it can also have negative impacts, as air pollutants
may concentrate under the tree canopy, and sultriness may increase in hot-humid climates
(Givoni 1991). Therefore, plantings schemes are required that reduce overheating
but maintain good ventilation.
An example of climate conscious planning is the redevelopment of the former Munich
airport as a new mixed neighborhood (Burkhardt and Duhme 1996, Fig. 4.5). Approximately
one third of the neighborhood was dedicated to green space. A big park of 200 ha
size in east-west direction fulfills important climatic functions as a corridor enhancing
ventilation of the inner city and the neighborhood (Fig. 4.5, level 1). Green wedges within
the built-up area will allow country breezes from the south to penetrate and thus improve
air quality as well as reduce air temperatures on hot summer days (Fig. 4.5, level 2). For
access streets in east–west direction, it was suggested to plant trees in front of the north
facing fronts of the houses (Fig. 4.5, level 3). This would allow good air exchange due
to circulation driven by small-scale temperature differences. Planting trees on the north
facing side would also avoid strong shading of south facing windows. However, in hot
climates such as the Mediterranean, plantings should be preferably made on the southfacing
front to avoid excessive temperature loads on the buildings while temperature
differences with the north side of the street would still allow for small scale circulation.
Fig. 4.5. Principles for climatic planning on the levels of residential area, block and street in the new
neighborhood of Riem, Munich (source: Burkhardt and Duhme 1996)
Chapter 4 · Benefits and Uses of Urban Forests and Trees 97
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Deciduous trees with open crowns such as black locust (Robinia pseudoacacia), honey
locust (Gleditsia triacanthos) or Japanese pagoda tree (Sophora japonica) would be
particularly well suited for this purpose.
Climatic modeling is becoming increasingly available to assess the bioclimatic conditions
on urban places and in street canyons (e.g., Matzarakis 2002; Maye and Matzarakis
2003), yet its application in climate planning is still limited (Eliasson 2000). These
models assess the effects of trees on air temperature and thermal comfort of pedestrians,
and thus provide tools for climate planning from a small scale to the city level.
The role of trees to improve climatic conditions in cities and towns differs across
Europe. Overheating of inner cities during summertime is in particular an issue in the
countries in the south, south-east and to some extent also in central Europe. In urban
areas in the north and north-west provision of shelter from cold winds will be of greater
importance, even under climate change scenarios.
4.3.3
Hydrology
Hydrographs show how urbanization increases the peak flow intensity and quantity
during rainfall events. Urban forests and trees can reduce surface runoff and thus alleviate
the strain from the urban sewage system and dampen peak flows of streams.
The main ways that individual trees reduce runoff are by
the interception of precipitation, which is stored and/or evaporated from the tree
(Xiao et al. 2000),
the increase of rainwater infiltration into the open soil under the canopy,
an increase of water storage capacity of soils through evapotranspiration,
the reduced impact of raindrops and consequently less soil erosion and pollutant
wash-off.
Protection of riparian woodland can be of particular importance for surface water
quality. The hydrological function of urban woodland and trees is increasingly stressed
as protection of drinking water resources. For example, in Denmark new woodland
areas established close to cities consider this function as a primary one next to recreational
benefits (Jensen 1995).
Open spaces covered by trees and other pervious surfaces enable infiltration of
rainwater and thus recharge the groundwater in an otherwise sealed urban area. However,
no models are currently available to assess these effects quantitatively on the level
of cities, neighborhoods or single sites in European cities and towns.
The role of vegetation in reducing surface runoff was estimated using a standard
curve method in 11 residential areas for the Merseyside conurbation, England (Whitford
et al. 2001). The results show a clear inverse relation between green-space provision
and rainwater runoff. A simple approach based on empirical coefficients derived from
several studies was used to estimate the hydrological impact of green spaces in Munich
(Pauleit and Duhme 2000a). Different runoff and infiltration coefficients were assigned
to land cover types. This exercise provided only very rough estimates and would need
further refinement and verification. However, the results show clearly how well-greened
98 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
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urban areas and particularly woodland reduce surface runoff and increase rainwater
infiltration compared to built-up areas, as can be seen from Fig. 4.6. In a next step, the
amount of pervious surface required to infiltrate the surface runoff completely within
the land use unit was calculated. On this basis, the need for green areas in general and
urban forests in particular for a more sustainable urban hydrology was quantified.
4.3.4
Energy Demand and Carbon Sequestration
While some North American research has studied the reduction of energy demand
through shading of houses in summertime and shelter in winter (e.g., McPherson 1994),
no comparable studies have been performed in a European context. Energy studies
have concentrated mostly on the built environment. The overall direct carbon sequestration
by urban trees accounts for less than one percent of carbon emissions from
urban areas (McPherson 1994). Still, more important will be the indirect effects of trees
in reducing energy demand of buildings. It has been estimated that, for Sacramento
County, California, the urban forest of approximately 6 million trees sequesters 238 000 t
of CO2 per year. The urban forest further reduces energy consumption indirectly as
Fig. 4.6.
Runoff curves in urbanized
catchment areas (source:
Emschergenossenschaft/
Lippeverband, 1979, in: SRU
1987, p 299)
Chapter 4 · Benefits and Uses of Urban Forests and Trees 99
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carbon dioxide emissions from the local power plant are avoided by an estimated
75 600 t CO2 yr; this is because the presence of trees leads to reduced demands for
household heating and air conditioning (McPherson 1998).
In Liverpool, United Kingdom, carbon sequestration was estimated for four residential
areas with a different provision of vegetation (Whitford et al. 2001). With up
to 0.13 t ha–1 yr–1, well-treed areas sequestered more than double the amount of carbon
than those areas with a poor provision of green spaces. Even more striking were
the differences in carbon storage between residential areas. Carbon storage could be
as high as 17 t ha–1 in a residential area with a mature, dense stock of trees, whereas
it was below 1 t ha–1 in an area with an overall low provision of green space and almost
no trees.
Moreover, the use of wood as a biofuel could substitute for fossil fuels. Modern biofuel
and power plants would allow for a wider use of wood without unacceptable impacts
on air quality. This could be an interesting alternative in particular for smaller settlements
where sufficient land is available for growing energy crops such as poplar plantations
or coppice woods.
4.3.5
Biodiversity
The biodiversity of urban woodland in European cities and towns is relatively well
investigated (Gilbert 1989; Sukopp and Wittig 1993). The main factors that influence
biodiversity of woodland are:
Woodland origin and naturalness, with a gradient of species richness from relics of
primary woodland, over secondary, naturally developing woodland to recently established
and managed woodland. Even after several centuries, secondary woodland
could be still distinguished from primary woodland by a lack of species of low
colonizing ability (Peterken 1974).
Size: large forests offer more different habitat types as well as habitats for species
with larger area requirements. In a study in The Netherlands, only woodland areas
of at least 10 ha in size harbored interior woodland birds (Van Dorp and Opdam
1987), but much larger woodland areas would be required to accommodate more
demanding wildlife (>>100 ha).
Intensity of interventions through management and use: Intense recreational activities
may have a negative impact in particular on breeding birds and other disturbance
sensitive species (Van der Zande 1984). Monoculture woodland mainly
planted and managed to produce timber is less biodiverse than naturalistic plantations.
Moreover, single old trees in parks can be an important habitat for birds, bats and
invertebrates. Habitat surveys and floristic and faunistic studies have shown the importance
of tree cover in urban land uses such as residential areas for biodiversity. Tree
crowns can provide habitat for birds and invertebrates in otherwise intensively managed
and used gardens. Density of tree cover, overall extent of stands of trees and age
of trees are especially important factors influencing biodiversity. An overall tree cover
100 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
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of at least 20% was proposed as a target for urban forestry planning for residential
areas on this basis for the City of Munich (Duhme and Pauleit 1992). In fact, the
biodiversity in urban areas is in part high because of human influence and due to many
exotic species. This richness of species in urban nature could be used more for educational
purposes, i.e. to show the residents that city nature has its own special features
and diverse values (see Chap. 8).
While the importance of urban areas for biodiversity is increasingly recognised, there
is a significant lack of guiding models for biodiversity planning on the city and neighborhood
level. One approach is the linking of individual sites, through wildlife corridors or
green corridors into a “green network” of wildlife sites and open spaces (Barker 1997).
Whilst research findings are calling into question assumptions about the functioning of
ecological connectivity within urban environments (Dawson 1994), there is little doubt
that both people and wildlife benefit from connections to the overall landscape. Urban
woodland is a major component of green networks. This requires strict protection of
existing natural woodland that cannot be recreated. Furthermore, although in North-
Europe woodland areas preserved from natural forest vegetation are common they are
often intensively managed. For example, in Finland there is an increasing demand to leave
unmanaged areas also in urban forests to create habitats for old growth forest species in
the long run. In practice, the more the urban forests become fragmented in a city structure
the more difficult it will be to reach the ecological objectives.
Furthermore, increasing attention needs to be placed on successional woodland on
post-industrial land. These areas offer significant opportunities for the creation of urban
woodland (Perry and Handley 2000), providing valuable habitats and creating a new
landscape character. Therefore, the model of compact cities and the redevelopment of
brownfield sites (i.e. abandoned former industrial sites) needs to be carefully balanced
against their value for recreation, biodiversity and landscape character. Landscape
ecology also stresses the importance of patch shape and boundaries (e.g. hard or soft,
straight or curved) for biodiversity. An overview of landscape ecological principles for
the design of woodland is provided by Bell (1999). Finally, the Munich study shows
how targets for urban forests within urban land uses can be developed based on habitat
surveys (Pauleit and Duhme 2000).
4.4
Economic Benefits of Urban Forests and Trees
4.4.1
Economic Values of Urban Forests
In environmental economics a specific taxonomy of values related to natural resources
has been developed, although definitions of these values seem to be somewhat unclear
or overlapping (Turner et al. 1994). The values attached to public environmental goods
are usually classified as use values and non-use values. Use values are divided further
into consumptive and non-consumptive use values, while non-use values are often
divided into option, quasi-option, bequest and existence values.
The consumptive use values of forests include values of market-priced products such
as timber, game, berries and mushrooms. Timber is traditionally seen as the most
Chapter 4 · Benefits and Uses of Urban Forests and Trees 101
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important market-priced product of forests in many rural areas across Europe. However,
the values of timber production and of picking mushrooms and berries in urban
forests are lower than in rural areas. This is because the environmental conditions for
growth are limited due to pollution, fragmentation of forests and trampling effects. In
addition, the net revenues from timber are usually fairly small if any, because management
of areas is relatively expensive due to small-scale management practices. Moreover,
the market price of berries and mushrooms reflects their true value only partially,
as the recreational value of their picking is excluded. The value of game is also unimportant
as hunting is often not allowed in urban areas.
In fact, the main values of urban and peri-urban forests have no market-price. These
values are termed as non-consumptive use values and include benefits derived for
example from a pleasant landscape, clean air, peace and quiet, as well as recreational
activities (Fig. 4.7, Tyrväinen 1999). This category also includes benefits such as reduced
wind velocity, balanced microclimate, shading, and erosion control, the economic
value of which may be determined through for example reduced costs of heating or
cooling or alternative costs of environmental control.
The non-use values (option, bequest, existence) may be less important in
urban than in natural forests but still worth taking into consideration. The possible
motives for these values are altruism, heritage or existence, but theoretically the
different value categories have not been consistently defined. Option value is defined
as individual willingness to pay (WTP) for ensuring the future availability of a
particular amenity. These values can be attached for example to well-known public
parks such as Central Park in New York or Bois de Boulogne in Paris, which can be
expected to have importance for people other than residents of the city. A related
form of value is bequest value, a willingness to pay to preserve the urban forest as
a resource, not for the current valuators, but for a potential future use by their descendants
(Turner et al. 1994). Many parks have, among other values, this type of
cultural and historical importance. Moreover, the existence value is derived from the
knowledge that the resource continues to exist and it is often connected to extinction
of species. There are species, for instance, that have found suitable habitats only in
urban environment.
Fig. 4.7. The total economic value of urban forests (Tyrväinen 1999, adapted from Turner et al. 1994)
102 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
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4.4.2
Quantifying Amenity Benefits in Monetary Terms
The economic value of urban forest can be estimated in different ways. Traditional
methods include opportunity costs, estimation of maintenance costs and of the production
value of forest. These methods are based on valuing market-priced goods and
therefore their use in urban forests is limited (Tyrväinen 1999). The quantification of
amenity values of urban forests is complicated, because these are not sold and bought
through markets. The urban forest resource can be viewed as a public good, but not a
pure one. Usually municipalities are in charge of providing the services, excluding urban
trees on private property, and in principle everyone has a possibility of consuming, for
example, the pleasant wooded landscape. However, households also have an option of
paying for the environment as a joint product with a job or a house by, for example,
choosing a house close to a public park.
The methods used in estimating non-priced benefits of forests include the contingent
valuation method (CVM), the hedonic pricing method (HPM), and the travel cost
method (TCM). Furthermore, approaches such as tree pricing and environmental benefit
valuation have been applied in assessing urban forest benefits (Tyrväinen 1999). The
methods have different abilities to capture different benefits (Table 4.2). For example,
hedonic pricing mainly captures recreational and aesthetic benefits of green areas,
whereas environmental benefit valuation focuses on air quality and the energy saving
function of trees.
In the first approach, CVM, hypothetical markets for the environmental goods
are created, i.e. the respondents are asked what they are willing to pay (WTP) for
the preservation/establishment or improvement of urban forests. The researcher
can then estimate the monetary value of the asset by calculating the average WTP
of respondents and multiplying this by the total number of consumers (Mitchell
and Carson 1989). In Germany, Elsasser (1996) applied the CV method for valuing
the recreational use of two large urban fringe forests in the Hamburg region. The
Table 4.2. Methods of estimating the amenity value of urban forests in monetary terms (Tyrväinen 1999)
Chapter 4 · Benefits and Uses of Urban Forests and Trees 103
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data set was exceptionally large (3 500 respondents). The mean WTP/year for the
use of the forests was around Õ42 person–1 yr–1. In Finland, Tyrväinen (2001) asked
people’s willingness to pay (WTP) for forested recreation areas in two study towns,
Joensuu and Salo. More than two-thirds of the respondents were willing to pay for
use of the recreation areas. Good location and active management raised the average
WTP. The average use value per visitor in different recreation areas ranged from
Õ5.2–12.7 per month.
In the second approach, hedonic pricing, the value of the urban forest amenities
for people is observed through housing market transactions. If a household
wishes to enjoy a view onto a forest or a park or to have easy access to wooded
recreation areas, it will buy this type of property and pay a premium for it. A hedonic
price model can be computed from data concerning the prices and different
features of properties (Palmquist 1991). The main advantage of the method
is that it relies on actual market data rather than hypothetical valuations. In Finland,
Tyrväinen and Miettinen (2000) demonstrated that a one kilometer increase in
the distance to the nearest urban forest area led to an average 6% decrease in the
market price of the dwelling (Fig. 4.8). Furthermore, dwellings with a view onto
forests were on average 5% more expensive than dwellings with otherwise similar
characteristics. In The Netherlands, Luttik (2000) found that a pleasant view alone
leads to a considerable increase in house price (6–12%), particularly if a house overlooks
water or open space. Proximity of public parks, however, yielded ambiguous
results; only when water was a distinctive feature in the park could a premium on the
house be demonstrated.
However, it is worth noting that proper maintenance of urban forests is essential to
sustain the flow of green benefits. A deteriorated park, for example, may become a
negative externality within a neighborhood, and may also prompt decision-makers to
consider it for non-park development options. Moreover, social congestion caused by
a heavy use of public parks may cause negative externalities to adjacent houses and
may even decrease their prices.
The basic idea of the travel cost method is to estimate the demand for recreational
benefits using the costs of travel, which are used as a proxy for price. The method is
problematic in urban settings because there are usually no or only small costs involved
in traveling to the site (e.g., Tyrväinen and Väänänen 1998). However, the method is
useful in a setting where large urban forests within city limits are scarce and people
have to travel further to reach the areas.
Tree pricing assumes that the tree value is based on several factors such as size,
expected age, aesthetic value, location, form and other special features (Table 4.3).
The method is based on a cost depreciation approach, and it is widely used in
many large European cities due to its practicality. Determining prices for single trees
has been necessary for estimating the compensation of injuries to the landowner
caused, for example, by vehicles or construction. This is, however, based to some
extent on subjective judgment. Tree pricing has typically been used for single or
groups of trees, but it is not suitable for forest areas. Moreover, it does not explicitly
account for environmental services such as shading and adsorption of pollutants that
trees provide.
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Valuing the environmental benefits of trees aims at quantifying the impact of trees
on urban climate (shading, evapotranspiration and air flow modification). These benefits
can be valued through the alternative costs of environmental control, such as
people’s WTP for air pollution control and noise abatement, or directly through, for
example, energy savings in heating or cooling (McPherson 1994). Nowak (1994) found
that a 5–10% improvement in localized air quality was possible in areas with relatively
high tree cover. In 1991 the researcher estimated the value of pollution removal to be
US$1 million for trees in Chicago. Furthermore, the potential of trees to reduce the
residential heating and cooling energy was investigated. An increase in tree cover of
10% (approximately three trees per building) could reduce heating and cooling energy
by 5–10%. Although the approach is suitable for valuing all green areas, many countries
have insufficient quantitative information on the impacts of trees on urban climate.
Table 4.3. Example of a tree pricing formula used in Danish cities (Randrup et al. 2003)
Chapter 4 · Benefits and Uses of Urban Forests and Trees 105
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4.4.3
Cost–Benefits Analysis in Urban Forestry
Frequently, the amenity benefits of urban forests are not well enough articulated in
land use decisions and green-space policy-making. In many cities there are increased
pressures to convert urban forests to other use, as well as financial problems related to
management of the areas. If the socio-economic value of ecological factors can be
demonstrated, for example through a premium on house price, this strengthens the
position of existing green areas in the policy decision process. Today land-use planning
procedures and greening projects do not include any systematic quantitative assessment
of the benefits of green areas (Tyrväinen 1999). Decision-makers compare
economic factors like contribution to the tax base and employment or the value added
to the local economy against the value of environmental factors. By expressing the latter
in monetary terms they become comparable to the former. This will put more weight
on environmental factors in the decision making process, although by no means all
environmental values can be put into monetary terms (Luttik 2000).
Economic estimates of benefits of urban forests and trees are useful in decisions
regarding town planning, urban forest policy and budget allocation. The application
possibilities include an assessment of lost benefits due to ‘densification’ or ‘infill’ of
town structure and reduced tree canopy as well as economic gains of establishing new
green areas. At the municipal level it is necessary to assess whether the provision of
recreation services is in balance with the demand. The analysis also provides information
for assessing cost efficiency and alternative management structures. The use of
economic analysis in decision-making has different levels. At a minimum level it might
stimulate public and policy-makers’ awareness of potential values. At the project level,
it could influence or identify decisions through cost–benefit analysis.
Frequently, the costs of urban forest management are poorly documented in cities
and towns. A study in Munich, Germany, addressed the question of how to optimize
management costs of the urban green structure. An economic approach was used to
structure the quality of urban forests in relation to necessary maintenance to get this
quality. Information on resources and average time spend on certain management tasks
in different areas was gathered. An optimization model was applied to plan the use of
management resources (Steidle-Schwahn 2002).
The total value of a green area depends on its location, size, quality, use intensity
and the amount of available substitute areas. In general, the scarcer the resource becomes,
the higher the value per hectare. Moreover, the value of single trees in a city is not the
same, even if trees are identical and in the same location. The law of decreasing marginal
benefits applies to aesthetic and recreational values of trees: the first tree in an open
area makes the biggest difference after which every additional tree counts for less.
So far, research seeking to determine the amenity benefits in monetary terms has been
limited in Europe, although the level of activity and the results have more or less passed
the demonstration stage. There are cultural differences in attitudes towards the urban
forests and the supply of the green areas varies. Therefore, results are not easily transferable.
More research is needed to create feasible models for assessing the benefits in practice.
By monetary valuation methods it is possible, however, to create local or regional
estimates for the economic value of green areas.
106 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
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4.4.4
Tools for Decision-Making
Valuing Benefits of Urban Woodland
Property Value Models
The next example shows how a property value model can be used to calculate the value
of an individual urban forest park indirectly. The idea behind this procedure is (1) that
the total value of the forest/park views is determined and (2) that attention is given to
the effect of the distance to the forest park. Here the calculation is illustrated by using
the property value model (Tyrväinen and Miettinen 2000) presented in the previous
section (Fig. 4.8). In this example it is assumed that size of the forest park is one hectare
and it is circular. Given the average yard size of 400 m2, the number of dwellings
next to the park is 35. The total value of these apartments is approximately Õ2.24 million,
and the value of the forest view is 4.9% of the total value of dwellings, which is Õ110 000.
Here, the average size of an apartment is 90 m2 and the average unit price was at the
time of investigation Õ710 per m2.
Moreover, according to the semi-logarithmic model an increase in distance of one
kilometer reduced the average price of a dwelling by 5.9% (Fig. 4.8). Here the price
effect is calculated only up to 600 m, because it is the case of a small park. The total
value of apartments within 600 m from the park is Õ76.35 million. The aggregate value
of the park reflected to dwelling prices is Õ3.73 million and can be calculated geometrically
using the price model. When the value of the view is added to this figure, the total
value of the park is Õ3.84 million. If conversion of the park into other use is considered,
Fig. 4.8. Effect of distance to the nearest forested area on apartment price (1 FIM = Õ0.168) (Tyrväinen
and Miettinen 2000)
Chapter 4 · Benefits and Uses of Urban Forests and Trees 107
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this value should be compared to increased costs when providing the building sites
somewhere else. A more detailed calculation example is presented in Tyrväinen and
Miettinen (2000).
In this example it was assumed that the park was the only one influencing the apartment
prices within 600 m. If the construction intensity is higher (i.e. there would be
mostly blocks of flats), the estimated value of the park would also be considerably higher,
approximately Õ11–15 million. Here it should be stressed that the social and ecological
carrying capacity of the green area are limited. After a certain limit, the high use of the
park becomes also a nuisance and may decrease property values.
It should also be noted that hedonic models express only the benefits associated
with housing. Excluded are recreational benefits by others than residents in the area.
The residents may not be aware of other benefits such as rainwater and erosion control.
Therefore, the approach capitalizes only a portion of urban forest benefits to nearby
properties and the method gives just a minimum value for the areas.
Use of Economic Estimates from CV Studies
The next examples demonstrate the use of economic estimates derived form CV studies.
The cases deal with urban fringe woodland and therefore it is relevant to compare
the amenity value of the areas with timber production benefits. In this analysis information
on forest stock (inventory data) and timber values is needed. On the cost-side
information on annual maintenance costs of vegetation and recreation facilities is
required. Moreover, the costs should include opportunity cost of land, i.e. reduction in
timber values due to management adjustments such as prolonged rotation period and
small-scale management units required by the recreational use. In the Nordic countries,
usually the loss of timber production value in recreation areas compared to commercial
forests is between 10–30% (Tyrväinen 2001). On the benefits side, the aggregate
amenity benefits can be estimated by calculating the average WTP of users and
multiplying this by the total number of consumers.
Tyrväinen and Väänänen (1998) estimated the present timber value of the study area
(46 ha) in Joensuu, Finland to be Õ0.19 million and the residents’ total WTP for keeping
it in recreational use was Õ1.38 million (1997) using a 5% interest rate. On this basis
the amenity value of urban forests was 7.3 times higher than the value of the area in
timber production. It has to be kept in mind, however, that the recreational use of the
area does not completely exclude timber production, but rather decreases the received
economic benefits and vice versa. Furthermore, Tyrväinen (2001) also showed in a
comparative study that amenity benefits of various recreation areas in two study towns
were clearly higher than their current management costs. The estimated aggregate recreation
value was 7 to 26 times higher than the total costs of providing recreation services
in the area depending on their characteristics, management and use intensities.
Moreover, the infill of existing housing areas may lead to a considerable loss of
amenity benefits of urban forests experienced by the local residents. In the study of
Tyrväinen and Väänänen (1998) people were willing to pay most for the green space
where the development scenario meant condensing the housing area. The aggregate
total WTP for preserving this urban forest was Õ1.38 million. It exceeds the value of
similar size, unimproved land area by Õ120 000. This sum can be used to cover the
increased costs of infrastructure when building elsewhere, for example, at the urban
108 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
Part I
fringe. This example suggests that infill of present housing areas is not always worthwhile
from the point of view of society, if the losses of green-space benefits are taken
into account.
Valuing Benefits of Street and Park Trees
Recently, some large research projects, most of these undertaken in the United States,
have studied the benefits of the urban forest, for instance to improve urban climates
and abate air pollution (McPherson 1994). American Forests, a non-profit citizens’
conservation organization, has developed a geographical information system, which
allows rapid assessment of the benefits of trees on urban green space (CITYgreen 2003).
This kind of information system informs planning and decision-making on the role
trees and woodland play in the urban environment.
CITYgreen is a GIS-based software product, which has been used to analyze the
local ecosystem and to calculate the benefits from urban forestry in Garland, Texas,
United States. Ten sites were selected to represent a broad spectrum of land use and
land cover, including single family residential, commercial and industrial sites. The
main problem in the town was storm-water management. The computer software
package calculated the effects of urban tree cover and impervious surfaces on forest
health, air quality, carbon sequestration, energy use and storm-water runoff. An important
characteristic of the software is the ability to put dollar values on all these
effects. The software can calculate the financial benefits from tree cover (compared
with a zero tree cover situation).
Fig. 4.9. Conceptual approach to modeling benefits or street and park trees (McPherson and Simpson 2002)
Chapter 4 · Benefits and Uses of Urban Forests and Trees 109
Part I
A study by McPherson and Simpson (2002) analyzed the benefit–cost ratios of street
and park trees in two cities, Modesto and Santa Monica in California, United States.
The approach considered functions such as energy savings, air quality improvement,
storm-water runoff reduction and atmospheric CO2 reductions and aesthetic benefits
(Fig. 4.9). The estimated benefits of trees were 1.85 to 1.52 times higher than the actual
costs. The aesthetic and other benefits accounted for a large part, 50–80% of total
benefits, while pruning accounted for half of the maintenance costs.
4.5
Conclusions
Nowadays, urban woodland and parks in or in the vicinity of large cities serve as areas
for recreation and entertainment, as well as space for biodiversity to compensate for
the built parts of the city. It is therefore important to emphasize the multifunctional
use of trees, green spaces, parks and woodland and draw the attention of city dwellers
towards the maintenance of biodiversity, of plant succession and the dynamics of lowcost
ruderal places. This may allow them to develop a more conscious attitude towards
nature in their immediate surroundings. In this context, more research will have to be
done on how wilderness is perceived and accepted or demanded by urban dwellers. So
called ‘loose-to-fit’ green spaces, where people can participate to design public green
space according to their own taste and recreational preferences will have to be given
more attention. Increasing maintenance costs of intensively managed parks with flower
beds and urban woodland with a neat infrastructure will favor a trend towards lowcost
greens.
Most of the research on the recreational function of urban green space has not yet
been translated into generally accepted practical guidelines and criteria, or into other
types of policy and management instruments. Partly this is due to the normative content
that is almost inherent to such guidelines and criteria: they fall within the domain
of the policy-maker as much as within that of the researcher. Suitable guidelines and
criteria depend on the goals that have been set. What is an adequate local supply of
outdoor recreational opportunities measured both quantitatively and qualitatively,
however, is still unclear. Although a start has been made (see e.g., Van Herzele and
Wiedemann 2003), more work remains to be done before empirically founded policy
making and planning with respect to urban greening is put into practice.
In the future, a comprehensive picture about the perception and acceptance of trees
should be obtained. Today, there is considerable loss of trees in urban areas due to
pollution, vandalism, traffic damage, use of de-icing salt, storm events, lack of adequate
space to grow and over-aging. Substantial research on these topics has been done representing
a need and strength of the technical and green-space management sciences.
Economical valuation studies have brought about many valuable insights into the benefits
and costs of urban green spaces and their management at the end of the 20th century
that was previously not available.
There is also an increasing need to define and promote the socially integrative potential
of woodland, parks and trees and to integrate people with specific needs and demands.
Parks and woodland areas at the peri-urban belt of large agglomerations are important
social meeting places for elderly people, youth, ethnic minorities of different cultural
110 Liisa Tyrväinen · Stephan Pauleit · Klaus Seeland · Sjerp de Vries
Part I
background and disabled citizens, to mention only a few. The role of cities as focal points
of cultural life has been supplemented by providing their inhabitants with green spaces
designed close to nature. Multicultural residential areas showing a preference for certain
tree or other plant species and outdoor amenities are particularly important.
The results from various research studies stress the importance of the quality of
the near-home environment and encourage the maintenance and establishment of
new green areas near homes. Economic valuation studies indicate that on social
grounds urban forests are a cost-effective concept. Although there is increasing evidence
also of the environmental benefits of the urban forest, most of this evidence is
gained indirectly, for instance more generally investigating the urban climate and the
role of green space therein. There are few studies in Europe on the environmental
performance of forests, and the different advantages and disadvantages of spatial forest
patterns and types of tree plantings. The information available is mostly limited to a
few urban areas; there is a significant lack of comparative information of cities across
Europe. There can be little doubt that the environmental functions and benefits of the
urban forest will vary widely between cities in southern, central and northern Europe,
depending on regional climates, the specific structure of the urban forest (Chap. 3)
as well as local needs. Yet, until now these differences have not been explored. As a
consequence, there is a lack of guiding principles for urban forest planning specific
to the different regions of Europe.
The particular challenge for the future will be to establish links between the environmental,
social, economic and aesthetic functions of the urban forest, for instance to
assess how mitigation of heat island effects and the improvement of air quality are
related to human health. This agenda has only recently become addressed (NUFU 1999,
2002) but there is still a significant lack of information. Collecting this information
will support the development of urban forest policies on a strategic level and the setting
of clear targets for provision of trees and woodland in urban areas.
In many countries innovative means to raise public awareness and also funding for
management and establishment of green areas are needed. Concretizing the amenity
benefits of urban forests and trees through various types of research contributes to
raising the decision-makers awareness of the consequences land use alternatives and
compact city policies. Moreover, this will produce more information on the benefits
that people actually receive from various types of green areas for practical planning.
The amount and quality of urban forest is, in the end, a political question and a matter
of whose interests are to prevail in decision-making. Given many residents’ high appreciation
of urban forest benefits, it is worth to fully account for urban forest benefits
and elaborate more detailed criteria for green-space development in European cities.
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Planning and Design
of Urban Forests and Trees
Chapter 5 Urban Forest Policy and Planning
Chapter 6 Design of Urban Forests
Chapter 7 The Role of Partnerships in Urban Forestry
Chapter 8 Involving People in Urban Forestry –
A Discussion of Participatory Practices
throughout Europe
Part II