Urban forests and trees provide both tangible and less tangible benefits important for a good quality of life. The consumable products include fuelwood, food, fodder, and poles. They improve air, water and land resources, provide habitats for wildlife, control erosion, protect watersheds for urban water supply and can be an outlet for safe disposal of urban wastes. Additional benefits to society, including it low-income citizens are significant and relate to improvement of health, recreation, environmental education, aesthetics, and enhancement of landscape.
Depending on urban forest management objectives, the focus is quite different in wealthier cities and poorer settlements. Multiple purpose urban forests are required for both rich and poor cities. Examples of multi-functional parks can demonstrate how many benefits can be combined in urban improvement projects working with the poor (Table A1). Appropriate urban forestry has to focus on those benefits, which are desired for local value first.
Table A1. Major benefits of the urban forest in conventional and development-oriented forestry
Multipurpose Urban Development Forestry = Urban Trees for Local Values | |
Conventional urban forestry focuses on amenity value in "developed countries" |
Development forestry focuses on economic benefits, employment and support of agriculture in low income cities, mostly in "developing countries" |
· reduces noise |
· provides food |
· reduces air pollution |
· provides fuel |
· reduces climatic extremes |
· provides fodder |
· cools cities and planet |
· provides fencing material |
· conserves energy |
· provides timber |
· provides beauty and shade |
· provides medicine, oil |
· improves water quality |
· provides raw material, fibre |
· controls water runoff |
· increases cash/subsistence income |
· provides habitat for wildlife |
· provides employment |
· increases recreation value |
· improves gardening conditions |
· increases health/well being |
· plus all benefits of conventional forestry |
Source: modified from Kuchelmeister 1991
The majority of the people in the region are poor and have an urgent need for necessities for a reasonable quality of life: adequate food, shelter, potable water and jobs. Urban forests can provide a significant portion of these needs. Urban forests can be set aside for food production, timber for shelter and fodder for livestock. Urban green space also provides recreation and employment opportunities. Diverse basic human needs can be satisfied with products from trees and shrubs; food and fuel are among the most pressing needs in developing countries. Tree products, if sold, provide direct cash benefits; if used within the household they provide indirect cash benefits by freeing cash income for other uses. Trees themselves can improve existing savings/investments, secure tenure or increase property value.
In many smaller urban centres in Asia and Africa, between 50 and 90% of domestic energy supply comes from biomass fuel (WRI 1996). Poor people use small twigs and leaves for firewood. Most fuelwood is bought from peri-urban areas or beyond (Carter 1993) but it seems that a considerable proportion is collected within the city (Kuchelmeister 1991). Aware of this demand, landless and marginal farmers have resorted to collecting fuelwood, especially in open-access forest. As a result forests around urban centres are being degraded.25 Poor people often have to spend a large proportion of their meagre income on fuelwood and thus have no choice but to over-exploit any trees within reach. Urban forests can alleviate this pressure (Kuchelmeister & Braatz 1993).
Recognising that deforestation in and around arid urban areas is closely related to fuel (energy) requirements, it must be assumed that fuelwood plantations adjacent to or in close proximity to population centres can make an important contribution to meeting urban demands. However, such urban related plantations (usually of exotic species) with few notable exceptions, such as Ethiopia, have not improved fuelwood supply. Often, instead of fuelwood more lucrative poles were produced or poor people did not have sufficient income to buy fuelwood. In order that this paradox can be resolved, less formal "softer" methods involving a mixture of agriculture, agroforestry and plantations (established and managed by small holders) or natural forest management should be adopted (Cline-Cole 1991, Leach & Mearns 1989).
Fuelwood related activities such as urban energy surveys and peri-urban fuelwood production are fields of action in which forestry projects in the region have been engaged for a long time in activities.
Building with organic materials in urban areas is less common than in the countryside. Nevertheless, organic materials are still very widely used in the urban areas of the poorest countries. In Bangladesh 53% of walls and 40% of roofs of urban houses are made of bamboo or straw. Availability of an adequate supply of organic materials of the right quality for building is a problem for a significant and growing number of households. The problem is most severe in arid areas and in the vicinity of cities (Wells 1995).
Building materials like poles, branches, leaves for thatching, shade trees for human livestock and crops; windbreaks and shelterbelts for protection of settlements against sand and wind; and living fences to protect and screen living sites are other appreciated values of urban trees. Progress has been made in incorporating timber harvesting and related forest products with intensive outdoor recreation activities in urban forest (Kuchelmeister 1991).
In the low-income cities of many countries urban green space provides grazing for livestock belonging to urban residents. Trees are an important source of animal fodder, particularly during dry seasons. In some countries like Pakistan the need for fodder is so great that even amenity trees are lopped (Carter 1993).
Many urban trees suitable for resource-poor settlements can provide food, particularly fruit, but also edible leaves, shoots and even flowers. Urban tree crops have been overlooked in nutrition surveys, but they can significantly contribute to food security in poor areas (UNDP 1996, Smit 1997). Often low-care wild edible plants are excellent candidates for use as ornamental roadside plantings (Kuchelmeister 1993). In Queensland, Australia, a park has been turned into an 'edible' public park to provide fruit, herbs, flowers and vegetables to anyone walking by. Local residents and schools carry out any necessary work and maintenance. Research for the Himalayan region showed that wild fruit trees and other multi-purpose trees are excellent candidates for urban forestry because they are both ornamentals and bear fruit and other valuable products (Parmar 1989).
Food from agroforestry gardens in the Pacific region (Thaman 1987) and elsewhere is significant.
Urban areas tend to be much warmer than the surrounding countryside. Urban vegetation can moderate the heat island effect of urban areas by (i) direct effect on human comfort and (ii) effect on the energy budget of urban buildings, where air conditioning is used. Effects can be either significant or negligible, depending on the size, spacing and design of the urban forests.
Trees modify climate in three ways: by acting as a windbreak, by providing shade and through evapotranspiration. It is reported that tree shade can reduce the average air temperature in buildings by as much as 5°C (Akbari et al 1992). Studies in Malaysia showed that under trees air temperature could be 4°C lower than in exposed spaces (Yap 199). For Nangjing, China, it is claimed that the average summer temperature has decreased from 32.2° to 29.4° between 1949 and 1991 by the cooling effect of the trees extensively planted during this period (CARTER 1993). Scientists in Beijing calculated that with every 10% increase of green space the temperature decreases by 1°C. Since the heat island intensity of Beijing has been calculated at 4 to 5 °C, it may be possible (at least in theory) to control the heat island by increasing of green space by over 50% (Profous NN).
The urban poor appreciate trees for the shelter they provide in lieu of permanent structures; many small business people use the shade of street trees. In Calcutta, avenue trees provide space to a large number of poor people who cannot afford to pay the high rent needed to rent space in the city market (Malhotra & Kumar 1987).
While air pollution in many cities in the more developed countries in the region has dropped over the years, air pollution level has been rising in other cities. Planting vegetation to reduce air pollution is increasingly utilised as an effective approach (IDB 1997). An increasing number of urban forestry projects address pollution control, e.g. Kuala Lumpur (Abedullah 1990), Bangkok, Manila, Hong Kong, and Seoul. As an example from outside the region, Stuttgart, an industrial city in south-west Germany, used the "smog-busting" ability of trees to literally change its urban climate. The arrangement of buildings in the city centre blocks the wind, allowing air pollution to build up. Changes in overall city planning were instituted; some buildings were removed and tree cover was re-established, creating corridors that significantly improved the city's air quality (Whiston Spirn 1984).
Box A1. Multipurpose values of avenue trees in Calcutta
|
A survey of avenue trees growing along a 4.6-km stretch of a trunk road in Calcutta was undertaken in 1996. Out of 400 trees, 142 were associated with some kind of human activities. Since the total number of mature trees was 138, more than half (64%) were in use. A number of trees were associated with more than one activity. Religious activities occurred in the form of places of worship like temples. Open or covered structures. Public utilities include places for parking cycle rickshas and carts, bus stop shelters, rest and recreation, advertisements and notice boards, clothes drying, etc. Economic activities relate to selling of food, repair of cycles, barbershops, teashops, etc. The most heavily used trees are large Banyans (Ficus benganensis). The most important shade trees are fast growing native evergreens that provide shade throughout the year. Other trees withstand storms with a high probability of survival, while still others are fruit bearing, or nesting habitat for birds. The avenue trees provide space to a large number of poor who cannot afford to pay the exorbitant rates to rent a place to operate in the organised city market. The study showed a direct link between high human activities and a high density of trees. Local peoples protect and care for the trees. If people worshipped a particular tree, its chance of survival was almost 100%. Source: Malhotra & Kumar 1987 |
Urban trees interact with the atmosphere and surrounding urban surfaces. It has been suggested that they affect air quality in the following ways. (i) conversion of carbon dioxide to oxygen through photosynthesis;(ii) Trees intercept particulate pollutants (dust, ash, pollen and smoke) and absorb toxic gases such as ozone, sulphur dioxide, and nitrogen dioxide, thus removing them from the atmosphere. (iii) Trees emit various volatile organic compounds, such as isoprene and monoterpenes, that can contribute to ozone formation in cities. (iv) By transpiring water and shading surfaces, trees lower local air temperatures. This cooling can reduce emission of volatile organic compounds from both biogenic and anthropogenic sources, and can alter ozone-forming processes, thereby reducing local ozone levels. In the absence of the cooling effect of trees, high temperatures would tend to contribute to the formation of photochemical smog, since as much as one-third of smog is directly related to the heat island effect (greater warmth compared to rural environment). (v) by reducing building temperature extremes in both summer and winter, trees can reduce energy use in buildings and consequently reduce pollution emissions from power-generating facilities.
Box A2. Air quality improvement: Results from the Chicago Urban Forest Climate Project
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Results from a research project indicate that trees (11-percent tree cover) in the city of Chicago, Illinois (USA), removed 591 metric tons (t) of air pollution in 1991. The greatest removal was of particulate matter less than 10 microns (212 t), followed by ozone (191 t), nitrogen dioxide (89 t), sulphur dioxide (84 t), and carbon monoxide (15 t). Average hourly improvement in air quality (in-leaf season) due to pollution removal from trees in the Chicago area was less than 0.4%. Maximum hourly improvement was estimated at 1.3% for sulphur dioxide, through local improvements in air quality can reach 5 to 10% or greater in areas of high tree cover (Nowak 1995). |
Conversely, it must be stressed that air pollutants injure trees by damaging their foliage and impairing the process of photosynthesis. Research on trees which are relatively resistant to pollutants has been conducted in China (Kuchelmeister 1991), in India, Republic of Korea, India (Khan et al 1989), Malaysia and Thailand (Kaitpraneet et al 1978), to name a few.
By lowering air temperature and shading buildings, trees can reduce the use of energy for air conditioning in the summer, and through blocking winter winds, they can reduce the consumption of energy for heating (Mcpherson & Rowntree 1993).
In the tropics and subtropics the shade of a tree at the southern side of a house can reduce the absorption of temperature of a building of 50 watt/m2 to one fifth. A tree with a 10-m wide canopy saves 50 m2 energy. 250,000 trees can save as much energy as is produced by a modern nuclear plant of 12,000 megawatts. A tree can be a natural air conditioner. The evaporation from a single large tree can produce the cooling effect of 10 room-sized air conditioners operating 20 hours a day.
Energy saving through tree planting around houses ranges from 10 to 50% for cooling and from 4 to 22% for heating.
Trees and related urban vegetation can significantly contribute to improving the air quality by cooling and cleaning the air. Energy conserving landscaping by strategically planting trees can maintain comfort without air conditioning and thus needs to be systematically incorporated in housing projects in resource-poor settlements. Since urban trees reduce the need to burn fossil energy, they are a more important investment for green house mitigation than rural trees.
Use of renewable energy sources at a local level conserves fossil fuels and the land or waters from which they are recovered, reduces the transport of fuels, reduces need for the building of transport routes (power lines, roads and rails), and reduces processing (refineries, power stations, etc.). While the role of urban forests in carbon sequestration is limited compared to forests in rural areas, urban trees do play a role in controlling global climate change through indirectly decreasing the use of fossil fuels for building heating and cooling.
Carbon dioxide is a most important component of air pollution and smog, and is a principal contributor to the greenhouse effect. Any action that lowers fuel consumption also lowers the amount of carbon injected into the atmosphere and global warming is thereby slowed down. Tree type, planting location and its intensity, and climate variations are just a few of the factors that determine the extent of saving (Akbari et al 1992).
The major carbon impact of urban trees is through energy conservation. Since this value is directly related to canopy cover, it follows that average tree size, health and age are critical factors in determining the urban forest's ability to offset carbon emissions. Investment in tree care is thereby critical to maximising the urban forest's effect on global warming (Kuchelmeister/Braatz 1993). When added to the ability of urban trees to store carbon, the energy saving potential (from lower air condition requirements) of an urban tree is up to fifteen times higher than the benefit of a rural tree (Moll & Young 1992).
Nonetheless, energy conservation through strategic planting of trees is very seldom deliberately included in urban housing projects in low-income settlements.26 Research on landscape planning for energy conservation including tree planting has been conducted in South Korea (Kim 1986).
Noise often reaches unhealthy levels in large cities. Typically, noise from cars, trains and planes can exceed 100 decibels, twice the level at which noise becomes troublesome. The health risk is high, as shown by research in developed countries. Poor people living close to heavy industry, commercial and traffic corridors often get exposed to the highest levels of noise particularly since all too often the building materials used in low-income settlements do not insulate residents from noise pollution.
Trees and vegetation can help to abate noise through transferring sound to other objects, altering the direction of sound, bouncing the sound back to its source, bending sound waves around an object, and mixing unwanted sound with more pleasing sounds. Trees and other vegetation in conjunction with land forms reduce highway noise by 6-15 decibels while trees in combination with solid barriers reduce noise by 5-8 decibels; by comparison, a typical masonry wall sound barrier reduces noise levels by 15 decibels. Especially advantageous to humans is the fact that plants absorb more high frequency noise than low, since higher frequency noises are most distressing to people (Miller 1997).
Deterioration of watersheds in and around cities has alarming consequences in terms of insufficient quantity and quality of water for urban dwellers. Trees and other vegetation can help in protection of urban water supply, wastewater treatment systems and storm water management.
Water supply: Protection of the suburban and rural areas that serve as the source of cities' water supplies is one of the more traditional fields of action of urban forestry, as witnessed in the case of Hong Kong, or Nepal (Braatz 1983). There is still much scope for integrating forestry with other water resource initiatives.27
Storm water control: High rainfall areas are subject to flooding along streams and rivers. Floods cause considerable damage in the region. Since many informal settlements are located in flood prone areas, they are the most hit and often the least assisted after flooding. As more forested areas are replaced by pavement, less storm water is infiltrated into the ground and runoff volume increases.
Trees can thus be purposefully used to help achieve the objectives of storm water management at optimal costs, which are to prevent the loss of life, to reduce property damage by runoff of severe storms; to prevent land and watercourse erosion, to protect water resources from pollution, to preserve natural watercourses and their ecosystems and to achieve objectives. The traditional engineering approach accepts higher flow rates and reinforces the watercourse with concrete to prevent damage by flooding. This is effective, but expensive and has no ecological value (Parks Department 1994).
Integrated storm water and pollution control may involve water collection through the following urban greening techniques: (i) using wetland and parks as important components of a city's flood control system; (ii) designing roofs and pavements to distribute water onto grassed/vegetated areas or sumps or bioswales. They also recharge aquifers and reduce runoff, flash flooding and pollution of the natural drainage system. Further they also provide or augment water supply from natural aquifers.
Parks can be designed for flood control. Interference with other park uses (such as recreation), occurs only in the short periods when the parks' wetland and flood plains are actually flooded. Tree species can be selected which can withstand water for up to a week or more. In Tokyo urban forest management systems have changed to multi-storied forests from single-storied to improve water conservation (Yuji 95).
It may be noted that storm water runoff has become the major source of pollutants entering many urban waterways and is a major cause of reduction in the quality of waters in rivers and the coastal environment; runoff can also pollute groundwater, in some cases irreversibly (for an example from Australia, see Box 15). The quantity of storm water that runs off of Australian cities each year is about equal to the amount consumed from domestic storage. Therefore there is a great potential for expanded collection, storage and re-use of storm water for non-drinking purposes (French & Sharpe 1976).
With steep terrain and where there is little vegetation and harsh seasonal rains, landslides can be common and can be a constant threat to people's lives and homes. Trees and forests can through water (run-off) management contribute to achieving the best soil erosion control.
Urban forestry is an opportunity to enhance water services in an integrated and system-wide manner. Most poor cities in the region face significant wastewater treatment challenges and could integrate stabilization ponds into park systems and could reuse wastewater for urban forestry. Tree planting can also offer a beneficial use for solid waste landfill sites.
Solid waste and land reclamation: Wealthy cities are vast producers of solid waste, the disposal of which has become a serious problem. Per capita solid waste generation is still low in cities such as Calcutta, India but as their per capita income increases, the quantity of solid waste is likely to grow. Urban greening offers some solutions, such as composting. Recycling of waste from urban forest can play a large role in solid waste management, especially in cities in developing countries, and should be encouraged not only to reduce the need to dispose of vast amounts of waste but also to secure new raw materials from extraction for re-use.
Unused and degraded land and landfill sites can be reclaimed through afforestation (Wong 1995). It is a practice applied in Hong Kong (Chan et al 1996). The idea of greening parks on terminated landfills, i.e. where controlled disposal of waste material to land is exercised seems to have become popular (IDB 1997).
Waste water: In the developing world, it is estimated that more than 90% of sewage is discharged directly into rivers, lakes, and coastal waters without treatment of any kind. Disposal of domestic wastewater remains a problem in wealthier regions, although by no means as severe as in poor areas (WRI 1996).
Irrigated tree plantations can be a safe and productive means of wastewater disposal; not surprisingly, this has been a tradition in many arid zones like Egypt and Iran (Braatz 1993). Reused waste city water not only recharges aquifers but also reduces the demand exerted on scarce water reserves. The practice of at least partially treating wastewater in stabilization ponds integrated into park systems and other green areas must be considered as an economic and ecological alternative to conventional urban wastewater treatment. Recycling wastewater into green areas is more economical than finding ways to dispose of it somewhere else (IDB 1997). A study in the city of Battambang (the second largest city in Cambodia) confirmed that engineered ponds and wetlands are a less costly alternative to conventional waste treatment.
The major disadvantage of treated wastewater is the large land requirement for treatment. Making the open space economically attractive through multiple use can counteract this. The practice of treated sewage fisheries along with garbage and sewage farming in peri-urban wetlands play a significant role in waste recycling and urban sanitation in Calcutta. When using wastewater to irrigate edible crops the potential health risks must be evaluated (IDB 1997, FAO 1989). A slum upgrading in Durban, South Africa demonstrated that wastewater treatment and recreation are compatible and are accepted (IUCN 1994, ICLEI NN)). In order for the treatment plant to be self-sustaining, revenue must be generated through fish production, user fees and other means.
Conservation of biodiversity and especially wildlife does not come to mind as a general function of city trees and forests; these roles are predominantly served by rural forest and woodland ecosystems rather than urban forests. Nevertheless, given that increased diversity is now considered to be paramount in the field of nature conservation, the urban forest is expected to play its part, even if limited. Thus, although generally highly valued in urban areas, wildlife receives relatively little consideration in day-to-day urban forest management (Grey 1996). Biodiversity is increasingly stressed in urban forestry management in the region, e.g. in Malaysia, where selected forest species in urban area may serve as a form of ex-situ conservation (Yap 1995). Kuala Lumpur is the only city in the region with primary forest in its centre (Ariffin 1989).
Although urban forests may contain less biological diversity than rural woodlands, the animals that occur in the urban forests are still numerous (Moll & Young 1992). For instance, a study in Jakarta found that birds in an urban environment tend to have low species-richness but high density (Indrawan & Wirakusumah 1995.). Quite often botanical gardens, located in the vicinity of urban centres have a richness of biodiversity (Katzir 1996).
Older gardens and parks, not to mention churchyards, often contain noticeably rich biodiversity; these are the main habitats of urban plants and animals. Older, well-established installations attract, for instance, birds and mammals, the natural habitat of which is the forest (Nilsson & Rundup 1997).
On a larger scale urban forests can create or restore biological diversity that will reconnect a city to its surrounding bioregion. Suburban wetlands can be some of the most productive natural ecosystems and can provide important habitat for fauna. Incorporating green areas through networks will improve biological conservation and biodiversity; e.g. greenbelts and greenways (linear parks) can serve as biological corridors (Groome 1990, IDB 1997).
Research indicates that vegetation and nature reinforce spontaneous attention by people, allowing sensory apparatus to relax and infusing viewers with fresh energy. Visits to green areas bring relaxation and sharpen concentration, since people only need to use their spontaneous attention. Also, fresh air and sunlight are essential for diurnal and annual rhythms.
There are some reports that hospitalised patients recovered faster when they had a view through a window, allowing them to see trees (Ulrich 1984) and that periods spent outdoors in parks have an actual medicinal value for patients and residents of hospitals, old people's homes and homes for the sick. People became happier, slept better, needed less medication, were less restless and far more talkative (Ulrich et al. 1991).
Certainly, improving air quality through planting vegetation has passive impact on health with such obvious benefits as decreased incidence of respiratory illnesses. Shade trees reduce ultraviolet light exposure, thereby lowering the risks of harmful health effects such as skin cancer and cataracts. For that reason urban forests are increasingly recognised as a component to mitigate ground-level ozone and to reduce air pollution (IDB 1997).
Urban forestry can provide jobs for the poor as both skilled and unskilled labourers. Tree planting and especially urban agroforestry systems can be labour-intensive and provide both initial start up jobs as well as more permanent employment in tree care (IDB 1997).
In wealthier countries the arboricultural industry is a significant business, often using heavy machinery like cranes for tree pruning. There is also considerable income in growing and selling flowers and ornamental plant seedlings. In less prosperous cities urban forest management is labour intensive. There are also opportunities for all kinds of formal and informal enterprises related to recreation.
Urban forests provide many educational opportunities. A number of cities in the region have botanical gardens, zoos, natural trails and even visitor information centres that can inform people about flora and fauna. In Singapore a remnant rainforest park at Bukit Tenara and other parks have been established (Carter 1993).
Education opportunities for urban residents are rare opportunities to learn about nature through first-hand experience. For example, the Mahim Nature Park in Mumbai was a treeless garbage dump, with sprawling slums to one side and a polluted creek to the other. Today it provides a rare oasis of green and an important educational resource, not just for the urban poor, but for school children and college students throughout Mumbai. At the heart of the Park, a garden with over 105 species of ayurvedic plants is used to teach traditional medicine (Pye-Smith 96).
The urban poor normally have few affordable options for recreation and thus place a high value on green areas. Lower income residents tend to frequent city parks more than wealthier citizens do because they lack the financial constraints and leisure time to reach distant recreation sites. For instance, in Bangkok on Sundays and holidays 10,000 people visit Lumphini Park, most of them from low-income families in nearby residential areas (Pleumaron 1988). Green space for the children of low-income families is very important in Bangkok (Chalermchai 1980) and elsewhere in the region. In Malaysia recreation areas that are developed and managed by government agencies have mainly satisfied the outdoor recreation needs of the urban lower income groups; commercial outdoor recreation areas have mainly catered for the middle and higher income groups (Wan & Wan 1993).
To be useful to low-income people, forests and green areas must be within an affordable travelling distance and have the amenities which people prefer (IDB 1997).
While not considered as important as meeting basic needs, the aesthetics of green areas can also be very meaningful for urban dwellers. Vegetation reduces sun glare and reflection, complements architectural features and lessens the harshness of dense buildings (Miller 1997). Garden cities with enough greenery to be aesthetically appealing are attractive to residents and investors alike. The beautification of Singapore and Kuala Lumpur was one of the factors that have attracted significant foreign investors who assist rapid economic growth in those cities (Braatz 1993, Ariffin 1989).
Through landscape enhancement, urban forests and trees add to the property values in the vicinity. In the USA, in one city the average house with five trees in its front yard was sold for up to 4.5% more than houses without trees. In Connecticut, urban trees account for 6% of total property value (Moll 1992).
25 Examples about India (Bowonder et al 1987), Nepal (Poudyal 1996, Thapa & Weber 1994), Malaysia (Hadikusamah & Karyono 1991). Pakistan (FAO 1993), Philippines (Cruz et al 1991, Remedio & Bensel 1992), Thailand (Polthanee et al 1991), Vietnam (Bailey 1990) and other countries in the Region are well documented.
26 A good example is a slum-upgrading project in Florida (Parker 1997).
27 E.g. water supply catchment should be an important input to the deliberations on water resources of the UN Commission on Sustainable Development during its 6th session in April 1998 (Michaelsen 1997).
