Overstory #142 - Urban Trees and Forests
A canopy of benefits
Urban trees and forests provide both tangible and less tangible benefits important for a good quality of life. The consumable products include fuelwood, food, fodder, and poles. Trees 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 its 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 (see table below). Appropriate urban forestry has to focus on those benefits which are desired for local value first.
TABLE. Major benefits of the urban forest in conventional and development-oriented forestry
Conventional urban forestry focuses on amenity value in "developed countries"
- reduces noise
- reduces air pollution
- reduces climatic extremes
- cools cities and planet
- conserves energy
- provides beauty and shade
- improves water quality
- controls water runoff
- provides habitat for wildlife
- increases recreation value
- increases health/well being
Development forestry focuses on economic benefits, employment and support of agriculture in low income cities, mostly in "developing countries"
- provides food
- provides fuel
- provides fodder
- provides fencing material
- provides timber
- provides medicine, oil
- provides raw material, fibre
- increases cash/subsistence income
- provides employment
- improves gardening conditions
- plus all benefits of conventional forestry
Source: modified from Kuchelmeister 1991
Meeting basic needs
The majority of the people in the world 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 and increase property value.
In many smaller urban centres in Asia and Africa, between 50 and 90% of domestic energy supplies come 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. 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.
Timber and poles
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 forests (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 1995). 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).
Air quality improvement
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. 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).
Energy savings, global warming and carbon dioxide reduction
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.
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.
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).
Water use, reuse and conservation
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.
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.
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.
Natural conservation - wildlife habitat and biodiversity
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).
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.
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.
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).
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 1996).
Recreation and Aesthetic
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 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).
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).
Abdullah, Z.H. 1990. Urban forestry in Malaysia: Current and future. IUFRO 19th World Congress, Montreal, August, 1990. Proceedings Division 1. Hull, Quebec, Forestry Canada, IUFRO Organising Committee. Vol. 2:228-235.
Akbari, H. Davis, S. Dorsano, S. Huang, J. & Winnett, S. (eds.) 1992. Cooling our communities, a guidebook on tree planting and light-coloured surfacing, Laurence Berkley Laboratory Report LBL-31587, Washington D.C.
Ariffin, I. 1989. Nature conservation in urban areas. Urban Forestry Bulletin, FRI; Malaysia Vol.3, No.1, April 89:5-6.
Braatz, S. 1993. Urban forestry in developing countries: status and issues, Proceedings of the Sixth National Urban Forestry Conference, Washington D.C.
Carter, J. 1993. The potential of urban forestry in developing countries, a concept paper, FAO, Rome.
Cline-Cole, R.A. 1990. The urban fuel plantation in tropical Africa: A case for re-evaluation. Land Use Policy, Vol.7, No. 4:323-335.
Grey, G.W. 1996. The urban forest, comprehensive management, New York.
IDB (Inter-American Development Bank) 1997. Good practices for urban greening, Social Programme and Sustainable Development Department, Environment division, Env-109, Washington D.C.
Indrawan, M. & Wirakusumah 1995. Jakarta urban forest as bird habitat: a conservation view, Tiger Paper Vol. 22, no 1 Jan.-March 1995:29-32.
Katzir, R. 1996. Agroecological aspects of the periurban process. Market gardening, farm associations, and good provision in urban and periurban Africa. The Green Beach Hotel, Netanya, June 23-28, 1996, Bar-Ilan University, Ramat-Gan, Israel.
Kuchelmeister, G. 1991. Peri-urban multipurpose forestry in development cooperation: experience, deficits, and recommendations, funded by the Commission of the European Communities, Contract Article B946/90-19, Illertissen (unpublished).
Kuchelmeister, G./BRAATZ, S. 1993. Urban forestry revisited. Unasylva 173, Vol. 44:13-18.
Leach, R. & Mearns, R. 1989. Beyond the woodfuel crisis: People, land and trees in Africa. London.
Mcpherson, E.G. & Rowntree, R. 1993. Energy Conservation Potential of Urban Tree Planting. Journal of Arboriculture, 19(6):321-331.
Miller 1997. Urban forestry: Planning and management of green space. Upper Saddle River, New Jersey.
Moll, G. and Young, S. 1992. Growing greener cities, Los Angeles.
Parmar, C. 1989. Wild fruits of the sub-Himalayan region of India. Agroforestry Today Jan.-Mar. 90, Vol.2, No.1.
Pleumarom, Nitichan 1988. Soziale, ökonomische und ästhetische Aspekte der Freiraumplanung in Bangkok. Urban et Region 47/1988. Kasseler Schriften zur Geographie und Planung, Berlin.
Profous, G. V. NN. Report: the structure and functions of the urban forest in Beijing, US Forestry Service (mimeo) The People's Republic of China, New York.
Pye-Smith, CH. 1996. Building green islands in Bombay. People & the Planet 1996, Volume 8.4, November 1996.
Smit, J. 1997. The Urban Agricultural Network, Coordinator, pers. communication.
Thaman, R. 1987. Urban agroforestry. The Pacific Islands and Beyond. in Unasylva, Vol.39, No.155, pp. 2-13.
UNDP 1996. Urban Agriculture. Food, jobs and sustainable cities, New York.
Wan Sabru & Wan Mansor 1993. Forest for outdoor recreation in Malaysia, Proceedings of the Asean Forestry Congress, Manila, 10-15 Oct. 1993.
Webb, R. 1998. Urban and peri-urban forestry in South-East Asia: A comparative study of Hong Kong, Singapore and Kuala Lumpur. Paper prepared for FAO, Rome, draft.
Wells, J. 1995. Population, settlements and the environment: the provision of organic materials for shelter, Habitat International Vol. 19, No 1, 73-90.
Whiston Spirn, A. 1984. The granite garden, Urban nature and human design, Basic Books, Harper Collins Publishers, NN.
WRI (World Resource Institute) 1996. World Resources 1996. Washington D.C.
Yap, S.K. 1995. Urban planning and biodiversity conservation in Malaysia, Proceedings of the 7 th National Urban Forest Conference. Washington, D.C. American Forests.
This article is excerpted with the gracious permission of the author from:
Kuchelmeister, G. 1998. Asia-Pacific Forestry Sector Outlook Study: Urban Forestry in the Asia-Pacific Region - Situation and Prospects. TREE CITY, Illertissen, Germany.
This study has been published as Working Paper No: APFSOS/WP/44 by FAO, Forestry Policy and Planning Division, Rome and Regional Office for Asia and the Pacific, Bangkok at: http://www.fao.org/docrep/003/x1577e/X1577E00.htm
About the authors
Guido Kuchelmeister is an independent consultant specializing in agroforestry and urban forestry issues. With more than 20 years experience, he has worked internationally with many organizations including FAO (Food and Agriculture Organization); GTZ (German Agency for Technical Cooperation) and ADB (Asian Development Bank), KfW (Kreditanstalt fuer Wiedeaufbau) and World Bank. He is Coordinator of the TREE CITY Initiative in Illertissen, Germany. Web site: http://www.kuchelmeister-consult.de .
Related editions to The Overstory
- The Overstory #126--Trees for Urban Planting
- The Overstory #113--Forest Biodiversity
- The Overstory #106--Hidden Bounty of the Urban Forest
- The Overstory #99--Grey Water for Trees and Landscape
- The Overstory #87--Urban Forestry
- The Overstory #72--Microenvironments (Part 1)
- The Overstory #64--Tropical Homegardens
- The Overstory #60--Trees as Noise Buffers
- The Overstory #46--Human Health and Agroecosystems