As stated in the definition of urban forestry adopted in section one, trees growing in urban areas bestow a great variety of benefits and problems. While this section will concentrate mainly on the benefits that trees provide, many are not gained without some cost. For example, street trees providing shade and ornamentation often, especially if their location was not carefully planned at planting, raise and crack the pavement with their roots. The very use of a tree for one purpose by a certain individual may be seen as a problem by another; the lopping of a tree for fodder will alter its shape and render it, in the eyes of some, far less attractive in ornamental terms.
Little use or value is generally attached to the raw products of urban trees in developed countries; even fruit trees are rarely planted in public places, although they still have an important place in urban gardens. Although perhaps over emphasizing the case, Webb (pers. comm.) comments,
“One would be hard put to find a group of plants more unproductive or more useless to man than those currently used in urban landscape design. It is almost as though a useful tree or shrub in a landscape scheme was somehow shameful, yet some of the most beautiful trees and shrubs which we can grow yield edible fruits and nuts.”
He adds (quoting Mollison, 1984) that Adelaide is one of the few cities which supports useful public plantings, in their case olives, which are gathered by people who use the oil and preserve the fruit. Why trees producing edible fruit may not be favoured in urban plantings is discussed in section six.
In contrast to urban trees in the developed world, those in developing countries provide a variety of products that are used by urban dwellers to meet both subsistence and income generation needs. The importance and potential of trees in this regard probably varies widely from country to country, and even settlement to settlement. This section discusses what urban tree products are known to be used by local people in different parts of the Third World.
Fuelwood
The primary source of energy in many towns and cities of the Third World is wood fuel, either as wood or charcoal. Among poorer people it is also common for small twigs and leaves to be burned for fuel. Most wood fuel is brought in from peri-urban areas or beyond, although at least a small proportion (particularly with regard to gathered twigs and leaves) is obtained within the city area.
Fuelwood from within the immediate area of towns and cities
It is likely that among the very poor in Third World cities, particularly those living in self-help
housing areas or on the streets, urban greenspaces are an important source of gathered
fuel (both in the form of tree twigs and leaves, and as dung from grazing animals). In
pointing this out, Leach (1987) also notes that the inefficient open fires in which such fuel
is burned are the only option for people living on the streets or in squats. Not only can they
not afford the regular purchase of fuels such as paraffin or gas; there is no point in them
saving for a costly appliance that they have no secure place in which to keep. Pollard
(1977:294) recorded among his impressions of urban forestry in China that,
“…we observed that dead limbs and litter [from trees] are almost instantly recovered - to the detriment of soil condition sometimes - by local peasants for fuel.”
Fuelwood from peri-urban forests, plantations and “parklands”
The role and potential of peri-urban plantations and forests in supplying urban fuelwood
needs received considerable attention in the 1980s. With the realization of the heavy reliance
on wood fuel of most people in Third World cities, and the perception of a growing gap
between demand and supply, great efforts were made, with the assistance of bilateral and
multilateral organizations, to establish fuelwood plantations on the periphery of cities to
provide for future needs. The FAO itself funded a number of ‘green-belt’ projects to this
effect in Africa, on the outskirts of Ouagadougou (Burkina Faso) and Kinshasa (Zaire),
N'Djamena (Chad), Nouakchott (Mauritania), Maputo (Mozambique), among other places.
Such plantations have not always met with great success, for reasons well summarized by Cline-Cole (1990) and Shepherd (1990), which may be outlined only briefly here. Usually, peri-urban fuelwood lots used fast-growing exotic species (notably Eucalyptus spp.) in a “package” design of,
“tried and tested species and proven technology, which give the most favourable economic rates of return on investment. ‘Package’ plantation designs which specify probable species choices, recommended cultural practices, etc., for given sites have for this reason become tremendously attractive.” (Cline-Cole, 1990:328–329).
However, the “package” often was applied in a blanket fashion, regardless of site conditions, and productivity was not always as high as predicted. More importantly, the introduction of the external technology failed to take into account local knowledge, preferences and uses. Traditional fuel use patterns were not investigated, resulting in wrong assumptions being made. Cline-Cole notes that the entire financial viability of many peri-urban fuelwood lots is dubious, for the following reasons:
The recommendation of both Cline-Cole and Shepherd (opp. cit.) is that where possible, more emphasis and support should be given to the appropriate management of indigenous woodland in peri-urban areas, as well as trees cultivated in “parklands”, here used to describe trees scattered on land used for grazing and agriculture.
Having said this, peri-urban fuelwood plantations will be appropriate and necessary, at least in the short term, in some circumstances (as Cline-Cole and Shepherd themselves acknowledge). Around some Third World cities, natural forests may have become so badly degraded that the only management option is to replant with species that can survive and grow on very poor sites. An immediate need for fuelwood and timber may also sometimes dictate the choice of fast-growing species; what is important is that this should be determined with local people, not simply assumed. The peri-urban plantations around Addis Ababa, described in box 4.1, provide an example of how Eucalyptus plantings may come represent a very important source of fuelwood for an urban population.
A further important point to note with regard to the supply of woodfuel to towns and cities from peri-urban areas is that its collection is often an important source of income generation to urban dwellers (for example, Kamara, 1984; Haile, 1991). This is well-illustrated by the case of Addis Ababa with regard to some of the poorest city residents who earn very little for their labour. Supplying wood fuel to cities may also provide large profits to city businessmen, although in some cases, at least, this is more true of organised transport from substantial distances away than from areas on the city fringe (Cline-Cole et al. 1987).
Food
Some of the trees most commonly reported to be grown by Third World urban dwellers are those which provide food - particularly fruits, but also edible leaves, shoots and even flowers. As in cities of the developed world, food producing trees are not widely grown in public places, although Webb (pers. comm.) reports that in Beijing persimmon and walnut trees are grown in parks, and in Singapore the Housing Authority has a policy of growing fruit trees in housing areas for the benefit of elderly people.
| Box 4.1. | Fuelwood from the peri-urban plantations of Addis Ababa |
| Urbanization began in Ethiopia around the turn of the 19th century, during the reign of King Menelik II (1890 – 1914). With the cities' need for wood fuel and construction timber, deforestation in their hinterland became an increasing problem, and initially forced the relocation of Menelik's capital several times. When he based himself close to the present location of Addis Ababa, a system of controlled harvesting of the nearby natural forests was introduced. However, demand soon out-stripped supply as the population of Addis Ababa grew, and Menelik responded by introducing Eucalyptus plantations (mainly E. globulus) around the city. He ordered the distribution of 100 seedlings to each resident, who was required to plant, tend and utilise them. It is reported that within six or seven years, people had begun to sell Eucalyptus for fuelwood, and that large landlords took over the management and control of plantations. Private ownership of Eucalyptus plantations continued until the 1974 revolution in Ethiopia, after which the State took over forest ownership. By this time, some 20,000 hectares of plantations had been established by private growers around Addis Ababa. These came under the control of state agencies, urban dwellers associations and peasant associations, and were rapidly exploited in an unplanned manner. Estimates based on satellite imagery indicate that in the three year period 1973–1976, the peri-urban plantations of Addis Ababa decreased by 33%. | |
| Addis Ababa is now a city of over 1.8 million people (figures will have fluctuated during the civil war). Furthermore, since 1989 the administrative area of the city has included rural areas in its immediate environs, with a total population of some 2.3 million in 1991). There continues to be a heavy demand for fuelwood and construction timber, the bulk of the former, at least, being supplied from forests and plantations within a 100 km radius of the city. In recent years, the Government of Ethiopia has taken steps to improve wood supplies to Addis Ababa through programmes to upgrade the Eucalyptus plantations, in collaboration with international donor and lending organizations such as the World Bank and the African Development Fund. Draft forest policy aims to re-introduce private forest ownership, among other ownership categories. | |
| A large number of fuelwood carriers and their families depend upon this work for their livelihood. Most of these carriers are women and children; their estimated number is well over 10,000. Collectively, they supply Addis Ababa with about one-third of its fuelwood requirements. Many of the carriers are young (more than 40% are under 20 years old), and poorly educated (almost 75% are illiterate). They are often recent migrants to the city, and the majority (60%) are the head of their household. Working an average 10-hour day, they spend some 7 hours of this in fuelwood gathering and carrying, walking an average 30 km round trip and carrying a load of some 30 kg. The work is clearly arduous, and often brings them into conflict with State officials responsible for forest control and management, notably forest guards. The latter see the women as illegal destroyers of the forest, who are conducting their activities purely for immediate financial gain without any thought for forest conservation. Yet investigations showed that the vast majority of the women were, | |
“amazingly conscious about the value of the forest. A large number of them indicated the fact that trees are not only useful as a source of income, fuelwood and construction material but essential in protecting the soil from erosion, giving the land its green beauty and attracting rain.” (Haile, 1991:19). | |
| It is not ignorance, a lack of education, or a desire for quick and easy profits that has led the women into fuelwood carrying; it is poverty and a lack of any alternative. Efforts are now under way to facilitate other means of income generation. | |
| Source: Ferkete Haile (1991) ‘Women Fuelwood Carriers in Addis Ababa and the Peri-Urban Forest’. | |
As noted previously, urban agriculture is an important source of food and income for many residents of Third World towns and cities, and it is in such situations that food-producing trees are often found combined with other food crops in agroforestry systems. An example is provided by the Pacific island urban gardens (box 4.2), while Smit (1992:12) has noted that,
“In Panama, agricultural shantytowns produce forest and vegetable crops just across the bridge over the Panama Canal from downtown Panama City.”
Fruit tree cultivation by urban dwellers is widely reported in Asia, Africa and Latin America both as an indigenous activity and one which has been encouraged in tree planting programmes (Niñez, 1985; Wade, 1987; Lipkis and Lipkis, 1990; Honghai, 1992).
A food and income-generating activity which may be complementary to tree cultivation is bee-keeping. According to Webb (pers. comm.), some extremely productive hives are now found in towns and cities in developed countries, where bees gather nectar from lime (Tilia spp.) and sycamore (Acer pseudoplatanus) trees. The Mazingira Institute study of six Kenyan cities (Lee-Smith et al. 1987) found that 48 households in their sample (3% of the total) kept bees. Numbers varied between cities; in the towns of Kitui and Isiolo, where bee-keeping is traditional, 10% and 9% respectively of the households questioned were bee-keepers. While in some cultures bee-keeping is viewed as dangerous, and would be especially unwelcome in densely populated areas, there may be considerable under-exploited potential in urban bee-keeping in some developing countries, particularly in peri-urban locations, that could be enhanced by the cultivation of appropriate tree species.
Although associated with rural areas, bushmeat is also sometimes harvested from peri-urban areas. Certainly agricultural plots on the edge of urban settlements are inhabited by a number of animals which (while they are agricultural pests) are prized as bushmeat; examples include “cutting grass” (Thryonomys swinderianus) and brush-tail porcupines (Atherurus africanus). This is a subject which may be worthy of further investigation, not only in West Africa. Undoubtedly some peri-urban forests, at least, already provide some animal protein to local residents, and possibly could be managed to produce more.
Fodder
Livestock raising is a common practice in many towns and cities of some Third World countries, but as Smit (1992:10–11) has commented,
“There has been little research and reporting on this common practice as distinct from rural livestock raising, except for poultry..... This farming system is widely distributed among large producers and small producers and between high-income and low-income farmers.”
He went on to elaborate slightly,
“The small producer typically will have a small herd of goats or a few pigs, and some agreements about access to grazing land and/or access to market or restaurant waste. The backyard rabbit hutch and chicken coop is the common denominator.”
As an example of the extent of urban livestock rearing, the Mazingira Institute study of six Kenyan cities (Lee-Smith et al. 1987) found that 17% of all households kept livestock with them in the urban area where they live. It also estimated that some 1.4 million head of livestock were kept in all towns in Kenya at the time of the survey.
| Box 4.2. | Urban agroforestry for food security and improved nutrition: Pacific island homegardens |
| Homegardens are a ubiquitous feature of urban areas in the Pacific islands. Within one garden many types of tree, shrub, climbing, ground and root crops are cultivated, diversity being further increased by the use of numerous varieties of a particular crop. Homegardens can play a vital role in feeding and maintaining the nutritional status of many urban dwellers, as well as providing other benefits. Many trees provide important food crops; indeed, on low-lying islands such as the atolls of Kiribati, the main staples are virtually all tree crops (coconut, bread-fruit, Musa clones, pandanus, and Ficus tinctoria). Other important food trees grown in homegardens are papaya, citrus species, avocado, guava, Annona, Syzygium and Terminalia spp., Spondias dulcis, and Pometia pinnata. | |
| In recent years there has been a widespread shift in the dietary habits of Pacific urban dwellers away from the consumption of traditional foodstuffs and towards imported foods of inferior nutritional value. This has become a serious health problem, which the consumption of fruit and vegetables from homegardens can do much to alleviate. A number of studies have shown that urban dwellers with homegardens are better nourished than those without. In Honiara (Solomon Islands), people without homegardens were found to have a lower intake of iron and vitamins A and C. In Kiribati campaigns have been held to promote homegardening and the consumption of both traditional foods and the edible leaves of local trees such as Morinda citifolia, Pisonia grandis and Polyscias spp. The latter do not form a usual part of the local diet, but are rich in iron and vitamin A. As a result of the campaign, anaemia and vitamin A deficiency reportedly decreased. | |
| Urban homegardens may also contribute to food security, through the sale of food and non-food produce. This cash income may be particularly important in times of inflation, and to families near subsistence level, when it provides an insurance against increasing food prices. However, there are signs that the younger generation of Pacific island urban dwellers are less interested in cultivating homegardens than their parents and grand-parents. There thus appears to be a need to encourage and publicize the value of urban agroforestry at both government and community level. | |
| Source: Thaman, R R (1987) ‘The Pacific Islands and Beyond’. | |
Grazing for livestock
In many developing countries urban greenspaces and peri-urban forests provide grazing to
livestock belonging to urban residents, and may have scope for more productive
management. To quote again from the Mazingira Institute study, over 60% of all households
in the study who were keeping urban livestock allowed their animals to graze freely, the
percentage being slightly higher in the wet season (66%) compared with the dry season
(62%). Allowing animals to fend for themselves was common in all towns except Nairobi,
where most livestock were stall-fed. It was further observed that,
“Some urban farmers tend to lock up their animals so as to avoid quarrels with their neighbours, and the destruction of flowers, trees and other plants. The pressures to do this are obviously greater the more densely the town is developed.” (Lee-Smith et al. 1987:210).
Tree foliage as cut fodder
Trees are an important source of animal fodder in many developing countries, particularly
during dry seasons when little ground vegetation is available and crop residues are running
low. A wide variety of species are utilized, and in some cultures, as in Nepal, it is
considered important to provide a range of fodders to maintain animal health (Carter, 1992a).
In some places, the need for fodder is so great that amenity trees are even lopped, as
illustrated by a Pakistani writer:
“The greatest damage to arboriculture in this country is caused by cattle, goats, camel and lopping by graziers” (Sheikh, 1976:211).
The extent to which urban trees are currently used for fodder is impossible to evaluate, and probably varies greatly in different parts of the world.
A distinction should be drawn between fodder for livestock being brought into cities for sale, in which case their herders are on the move and ready to lop whatever fodder they see in passing, and fodder for livestock living permanently within towns and cities. For the latter, their keepers must have regular and reliable access to fodder. The Mazingira Institute study (Lee-Smith et al. 1987) found that the major feed used was grass, followed by garbage, with crop residues playing a minor feed component. “Other types of fodder” were used by 29% of households in the dry season, and 27% in the wet season; these were said to be “mainly leaves, vines and twigs from bushes”. Tree fodder could well have been among this. During the dry season, 31% of households purchased animal feed, an example being napier grass purchased from farmers at the urban fringe. Various urban garden schemes have reported encouraging the use of fast-growing tree species such as Leucaena leucocephala, which produce fodder, among other benefits (Siki, 1984; Wade, 1987). The susceptibility of L. leucocephala to a psyllid pest has proved disastrous in some areas, and perhaps serves as a warning that attempts to improve fodder availability for urban livestock would do well to begin with trying to improve the supply of locally used fodders, building on local knowledge.
In a number of urban settlements in Asia, sericulture is practised and a large demand for fresh mulberry leaves (Morus spp.) must be met. This is reported, for example, in China by Honghai (1992) and in the old city of Delhi by Smit (1992). Where the leaves are brought from in the latter case is not made clear, but it would seem that in peri-urban areas of some Asian cities the cultivation of mulberry bushes is already a thriving income-generating activity.
Timber and poles
Urban settlements consume vast amounts of timber for the construction of buildings and furniture. At first thought, the current potential for supplying such needs from within and immediately around towns and cities would appear to be slim. However, there are some examples of this taking place, at least in peri-urban areas. Smit (1992:4) reported that in metropolitan Baltimore, Paulownia is cultivated for export to Japan as veneer timber. Webb (pers. comm.) has also noted that in Britain urban foresters are actively examining the possibility of using the various exotic species grown in urban areas as a source of craft and specialist timber once they have to be felled. In much of Europe, peri-urban forests used primarily for recreation are also managed for limited timber production. As far as construction materials are concerned, the current use and potential of peri-urban forests and plantations in developing countries probably consists mostly of supplying poles rather than larger timber. In theory, these can be produced on a rotational basis from fast-growing tree species such as Eucalyptus spp or even bamboo, although, in practice, local species preferences and site conditions may dictate other choices.
Trees growing within the urban metropolis may be put to use as timber on occasions. For example, Pollard (1977) reported that street trees in Peking (Beijing) provided material for temporary shelters after catastrophic earthquakes which occurred in 1976. It is likely that the urban poor seeking to build shelters in urban areas throughout the Third World resort to using the timber and branches of urban trees whenever they can.
Spices, fibre, medicines and other non-timber products
As with fruit trees, trees which bear spices or have uses for medicine, fibre and other purposes are rarely cultivated in public places. Indeed, Denne (pers. comm.) reports a concern among some Asian and African forestry students that some amenity trees are harmed through their bark being removed by local people for medicinal use. This has been observed with species of Melaleuca in Sri Lankan towns (Carter, pers. comm.); the bark is reportedly an important ingredient of ayurvedic medicine, although the genus (originating as is does from Australia) is not indigenous.
It is in urban homegardens of the Third World that trees valued for spices, fibre, mushroom cultivation, perfume, handicrafts, dyes, etc. are quite commonly grown. Thaman (1987), for example, reports medicinal plants to be a “critical economic and cultural resource” in the Pacific Islands. Of the 93 medicinal plant species found in native urban gardens in Fiji, Tonga, Kiribati and Nauru, 55% were trees and another ten were woody shrubs. Sacred or perfumed plants were also widely cultivated for income generation as well as household use, their flowers, leaves, fruits and bark being sold for use by the tourist industry. Smit (1992:12), although he gave no examples, noted that medicinal crops are traditional in many small urban farms, and that he found mushrooms being cultivated in urban forests.
Trees may also provide materials other than poles and timber for building shelters. The leaves of palms, for example, are commonly used as roofing by many poor urban dwellers.
Environmental problems in some Third World cities and towns are so great that tree cultivation might be dismissed as a trivial consideration. Space and land availability are certainly likely to be limiting factors in many circumstances, as indicated in section three. This section concentrates on the potential environmental benefits to be gained from trees, space permitting, and examines to what extent they may be already performing such functions in urban settlements of developing countries.
Landscape enhancement
As already noted, there is a long history of tree cultivation in urban settlements for the purpose of enhancing their visual character. Apart from adding variety and richness to urban landscapes with their different foliage and blossoms, heights, colours and shapes, trees can enhance the living environment by reducing glare and reflection. They can also add to the coherence of an urban landscape, complementing architectural features. As noted by Webb (pers. comm.), urban forestry and the concept of a “green city” can be a source of civic pride, and used to attract investment into an area. Well-known examples include Kuala Lumpur and Singapore, as well as the garden festival programme in Britain, used to reclaim derelict areas in a number of cities. The potential role of businesses in supporting urban forestry because of the economic benefits to be gained from an attractive urban environment are discussed further in section seven.
The choice of species in regard to landscape improvement (as well as other factors) is discussed further in section six. Beyond what is chosen for planting on technical grounds, cultural considerations may also be involved. Furthermore, in certain countries certain species are invariably allowed to survive if they regenerate naturally, even in an urban environment. Thus, for example, the tree species under which Lord Buddha gained enlightenment, Ficus religiosa, is commonly allowed to grow wherever it comes up in Sri Lankan towns and in other countries where Buddhism is strong. Jim (1991:145) comments that,
“Trees constitute an important ingredient of the cultural landscape of human settlements…. Amenity trees in a given city can.…be interpreted as an interplay between nature and culture.”
Trees in an urban environment help to establish a sense of cultural identity. Thus Asians who have settled in East Africa or in the Pacific Islands (such as Fiji) commonly raise species such as mangos, jackfruit, drumstick trees (Moringa oleifera), and curry leaf (Murraya koenigii) (Kaudia, pers. comm. and Thaman, 1987). The limited number of species planted as amenity trees by Europeans in their overseas colonies is possibly also in part a reflection of the need for familiarity. Even today, the traveller who has visited Nairobi and seen the mauve blossoms of Jacaranda mimosifolia often has a sense of comfort in visiting say, Delhi and seeing the same sight. In neither region is the tree indigenous.
In a somewhat similar sense, trees in urban landscape may serve as important territorial markers (Denne, pers. comm.). This is implicit in the fact that tree names are often incorporated into place names. On a smaller scale, trees may be planted by individuals to indicate ownership of a plot of land; indeed, in some countries, planting a tree may represent a claim to ownership (Fortmann and Riddell, 1985) - a matter that may take on particular significance in squatter settlements.
Educational value
Urban parks, and particularly botanical gardens with their wide collection of trees and other plants, have a huge educational potential. While this tends to have been exploited to a greater extent in developed countries, it is by no means restricted to them. As noted by Webb (pers. comm.), the remnant rainforest park at Bukit Temara, Singapore is used extensively as an educational resource for students. Similarly, in Hong Kong the Urban Services Department has established tree trails in some of its parks. In the Botanic Gardens of Limbe in SW Cameroon, school parties are regular visitors, and there are plans to establish nature trails through the garden as an education tool (Carter, 1992b).
Recreation
Urban parks and peri-urban forests are an important recreational facility in developing as well as developed countries. Thus on public holidays, numerous urban dwellers from Kathmandu throng the nearby Queen's Forest and the botanical gardens of Godavari. In Mexico City, the Chapultepec forest, among other parks and gardens, serves a similar function. Examples could be quoted from all over the world. At a smaller level, even a small patch of ground supporting a few trees can have great recreational value to children as a playground.
A sense of well-being
There is some evidence to justify the sense of well-being which, at least for some people, is engendered by a tree-filled landscape. According to Ulrich (1990), people derive quantifiable benefit from the passive experience of viewing trees, the positive effects being both psychological and physiological. Concerning the former, he commented that,
“Compared to urban scenes lacking vegetation, views containing trees and other vegetation elicit preference or liking and can have positive influences on a range of other important feelings having a central role in psychological wellbeing.…many scenes dominated by trees foster [psychological] restoration because they elicit positive feelings; reduce negatively toned emotions such as fear, anger, and sadness; effectively hold interest; and, accordingly, might block or reduce stressful thoughts.” (Ulrich, 1990:29).
Research by others has indicated similar results (Hull and Harvey, 1989). With regard to physiological benefits, a much-quoted study by Ulrich found that,
“Twenty-three surgical patients assigned to rooms with windows looking out on a natural scene had shorter postoperative hospital stays, received fewer negative evaluation comments in nurses' notes, and took fewer potent analgesics than 23 matched patients in similar rooms with windows facing a brick wall.” (Ulrich, 1984:420)
Although more research is needed to establish this, it may make sound economic sense for hospitals to ensure tree-filled, landscaped surroundings for their patients to view.
It could be argued that for many urban dwellers of developing countries, issues such as landscape, amenity and a sense of well-being are an irrelevance. That the urban poor have pressing needs cannot be disputed, but it should not be assumed that these entirely erase from them any aesthetic appreciation of their surroundings. Where there is no space for trees in high-density residential areas, parks and open spaces even some distance away from home may be valued for occasional recreational visits by poor as well as more affluent urban dwellers. Street trees may also have an important recreational role as the site of tea shops and a gathering spot for poorer people to sit and chat (see box 4.4). Furthermore, a number of researchers in self-help housing areas/squatter communities have recorded the planting of trees for ornamental purposes (or at least the planting of trees which are valued for this as well as other products). Examples range from Trinidad, where pink and yellow poui trees (Tabebuia spp.) are planted by squatters around their homes on the outskirts of Port-of-Spain (Bass, pers. comm.) to the Pacific islands, where ornamental species are commonly planted closest to the home in backyard urban agroforestry plots (Thaman, 1987).
A habitat for wildlife
The diversity of habitat provided for wildlife by urban forestry is valued in many developed countries, and particularly by conservation groups. The extent to which this is viewed as important varies greatly in different cultures and in different groups of the same society (wildlife conservation concerns being often a prerogative of the middle classes). In India, there is a considerable conservation lobby, among whom urban parks and gardens are valued as a wildlife habitat; for example, Shyam Sunder (1985) mentions the importance of cultivating Ficus spp. in Bangalore parks to provide food for birds. Interest in urban forests as a wildlife habitat might focus on the potential bushmeat to be gained (as discussed in section 4.1 above).
Climatic modification
The effect of urban trees on the local micro-climate has been a subject of considerable investigation in the last twenty years or so in developed countries. As a result, it is clear that trees can have a significant and quantifiable effect on the immediate local climate, although general claims such as a overall reduction in air temperatures and increase in rainfall have not been proved. This would probably be disputed by many Chinese urban foresters, who claim to have markedly altered the climate of some cities through widespread tree planting (see box 4.3 on Nanjing). Miller (1988) divides potential climatic modification into two main categories; direct effect on human comfort, and effect on the energy budget of urban buildings.
| Box 4.3. | Tree planting for micro-climatic amelioration: Nanjing, China |
| With a current estimated population of about 1.5 million, the highly industrialized city of Nanjing is known as one of the five “furnace cities” of the Yangtse Valley. Since 1949, some 34 million trees are reported to have been planted in and around the city with the specific objectives of reducing summer temperatures and generally regulating the local climate; purifying the air; and beautifying the environment. It is claimed that a drop in the average summer temperature from 32.2°C to 29.4°C over the period 1949 to 1981 is directly attributable to the cooling effect of trees planted during this time. Over the 32 years, some 23 trees per city inhabitant were planted. Tree plantings have included block afforestation of degraded hillsides, windbreaks, triple rows of trees along railways, and the lining of street sides. | |
| Source: Beatty (1985) ‘Planning guidelines for urban forest management’, quoting Bartenstein (1981). | |
Human comfort
Perhaps the most important contribution of trees to human comfort in hot countries is shade,
both directly and indirectly (by covering surfaces that reflect heat). They also provide
protection from heavy rain, and for the urban poor are a commonly used shelter, both at
night for sleep and during the day.
| Box 4.4. | Human activities associated with avenue trees on the Barrackpore Trunk Road, Calcutta, India. |
A census of avenue trees growing along a 4.6 km stretch of the Barrackpore Trunk Road was
conducted in March 1986. Out of the 400 trees, 142 were associated with some kind of human
activity, most of these (88) being more mature trees of over 5 years in age. Since the total
number of mature trees was 138, more than half (64%) were in use. The researchers classified
activities into three broad categories, notably
| |
| The researchers further commented that although they had no quantitative data, they had observed similar patterns of street tree utilization in other Indian cities such as Patna, Ranchi, Bhagalpur, Pune, Ahmednagar, Kolhapur, Delhi, and Rajkot. | |
| Source: Malhotra K.C. and Vijayakumar M. (nd) ‘Man-tree relationship in an urban setting: A socio-ecological study’. | |
As a study conducted on a truck road in Calcutta illustrates (see box 4.2), street trees in Third World towns and cities are often used by small businesses for the shelter that they provide to the trader and client alike. This commented that,
“In short, the avenue trees provide space to a large number of poor people who can not afford to pay the exorbitant rents and have a place to operate in the organized city markets. These persons sell and/or provide services at a much cheaper rate compared to their counterparts who operate in the organized markets. Thus, these people cater to the needs of the weaker section of the city dwellers.” (Malhotra and Vijayakumar, undated:283–284).
If carefully planned, trees may also be used to improve human comfort in urban areas by directing air movements. A line of trees, for example, may serve to obstruct, guide, or deflect a current of air, while a tree canopy of only medium or light density can serve to filter the current in its passage. Some observers have claimed that trees increase humidity and precipitation, but according to Miller (1988), trees do not have a measurable impact on relative humidity in the urban environment.
Energy budget of buildings
There is considerable interest in the potential ameliorating effects of surrounding vegetation
to reduce the costs of the winter heating and summer cooling of buildings. Miller (1988:53)
notes that,
“..vegetation can significantly affect building heating budgets. Windbreaks have been found to reduce home heating costs by 4 to 22 percent, depending on site windiness and how airtight the structure is. On the other hand, vegetation that shades a home in winter can increase heating costs.”
In most developing countries, the cooling effect of trees is likely to be more pertinent to urban dwellers. Deering (1954) found that in non-air-conditioned homes in California, interior temperatures were as much as 20°F (about 9°C) cooler in summer months. Studies have shown that in fact the costs of air-conditioning a building can be reduced by up to 50 – 60%, depending on the location of the building and the trees around it (Miller, 1988). The energy budget of buildings is certainly a factor considered by landscape architects in many countries. In a temperate, Northern climate it would be usual, for example, to plant deciduous shade trees on the east and west side of buildings, so that their shade provides summer cooling and their autumn leaf shed allows some solar radiation in winter (Sand, 1993).
Air quality
Air pollution is not exclusively a problem of developed countries; in urban areas of many newly industrialized and developing countries also suffer from serious levels of air pollution. Indeed,
“In many Third World cities, the concentrations and mixes of air pollutants are already high enough to cause illness in more susceptible individuals and premature death among the elderly, especially those with respiratory problems.” (Hardoy et al. 1992:76).
The same authors provide various examples of air pollution, including the fact that in 1991, the urban core of Shanghai recorded an annual average concentration of sulphur dioxide that was more than twice the WHO recommended average, with peak concentrations during the winter. In São Paulo, suspended particulates pose a severe problem, as do carbon dioxide and ozone levels (both of which routinely exceed air quality standards). Available evidence (which in many places is scant) indicate that air pollution is growing worse. One example is provided by Mexico City, where the average level of particulate suspension in the atmosphere has risen from 65 mg/m3 in 1974 to 400 mg/m3 in 1990. Over the same time span, atmospheric sulphur dioxide levels rose from 60 mg/m3 to 120 mg/m3 (Chacalo and Pineau, 1991). Air pollution may be compounded by local conditions, notably air inversions (warm air lying over cold air) which trap polluted air over cities or towns for prolonged periods. Examples of this phenomena include Mexico City and Kathmandu, Nepal.
Controlling urban air pollution involves political and social commitment, and the addressing of often complex scientific, economic and ecological factors. Tree planting alone is unlikely to have a significant effect in cleansing the atmosphere of pollutants. However, trees and other vegetation may contribute to this goal if used in conjunction with other measures. Particular success has been reported in Chinese cities, as illustrated in box 4.5.
| Box 4.5 | The greening of a factory: Capital Iron and Steel Corporation, Beijing City |
| The Capital Iron and Steel Corporation is now considered to be “just like a garden”. Over the 12 years prior to 1991, the Corporation has planted 3,390,000 trees, such as white poplar, paulownia, Chinese little-leaf box, pine and bamboo. It has also planted out an area of 904,000 m2 with grass and 8,590,000 flowers. The walls of the tall buildings inside the factory grounds are covered with climbing plants, the vertical green area reaching 46,500 m2. The Corporation has tried hard to develop production and protect the environment, combining economic efficiency with social responsibility. In 1990 the output of steel increased 2.37 times over that in 1979. The amount of smoke and dirt emitted dropped by 50%. The sky above the plant is now reportedly always clear and bright and the air is clean, providing favourable working and living conditions for the factory employees as well as improving the quality of the environment throughout the region. | |
| Source: Honghai (1992) ‘Urban agriculture as food supply and environmental protection subsystems in China’ (quoting Wu, 1991). | |
The potential role of trees in ameliorating atmospheric pollution is illustrated in box 4.6, which is drawn from information in Miller (1988), quoting Smith and Dochinger (1976) and Smith (1978). Almost all research on this subject has been conducted in North America and other developed countries where climatic conditions, the combination and level of pollutants, and the tree species planted are generally different to those in developing countries. The limited studies conducted elsewhere have not gone beyond basic investigations. Trees do have the potential to make a marked improvement on air quality by absorbing carbon dioxide and other pollutants, filtering airborne particulates and transpiring water. However, if trees are to be planted for this purpose in Third World cities, the initiative should be backed by further research.
Key general points to be noted regarding the potential role of trees in ameliorating atmospheric pollution include the following.
The importance of the soil
Soil plays an important role in reducing atmospheric pollutants. It is a sink for pollutants
filtered by the vegetation above, as well as through direct deposition. The soil may also
neutralize some pollutants through chemical reactions. In this respect, forests in and around
urban settlements have greater potential for atmospheric cleansing than avenue and other spot
plantings, where much of the soil is covered by a layer of concrete or tarmac.
Damage to trees and vegetation caused by pollutants
Some species are far more susceptible to air pollution in general than others, and some have
a low tolerance to particular substances. (This in itself may be exploited as an indicator of
pollution levels, as is reported to occur in China. Pollard (1977:295) noted that the
yellowing leaves of Prunus davidiana were used for this purpose around a Chinese factory
that he visited, although he does not state to which particular substances the tree reacts.) If
trees are to be used in the reduction of atmospheric pollution, it is clearly desirable to select
species that will thrive in such conditions. In North America and Europe much is already
known about the susceptibility of different tree species to different pollutants; furthermore,
among commonly used species, particular provenances are often recognised as having high
or low tolerance. An example is provided by a study of five cultivars of the honey locust
(Gleditsia triacanthos var inermis), which found that resistance to ozone damage ranged from
high (the cultivar “Majestic”) to very low (the cultivar “Imperial”) (Miller, 1988, quoting
Smith and Brennan, 1984). This information may be put to use in the planning of urban tree
planting in temperate parts of the developing world. For example, the tolerance to pollution
and other urban environmental stresses of Platanus acerifolia, a tree widely planted in
European cities, is well recognised in China, where it is widely cultivated in warm temperate
and to a lesser extent in sub-tropical cities (Jim, 1991).
Research on the air pollution tolerance of tree species suitable for cultivation in tropical cities has been scant in comparison with temperate regions. Through a process of trial and error those which are highly susceptible to urban stresses have probably been eliminated. However, for tolerance to present day, increasingly polluted urban environments, systematic screening has been generally lacking and more information is reportedly required (Jim, 1990a; Chacalo and Pineau, 1991).
Generalizations regarding the effectiveness of vegetation in decreasing atmospheric
pollution
A number of broad generalizations have been made regarding the theoretical “ideal” for plant
associations that will reduce air pollution. These are listed in box 4.7. Although based on
research in North America, most of these theoretical points are applicable worldwide
(excepting number 2, based on widespread leaf drop in winter).
| Box 4.6. | Effects of trees and associated vegetation on air pollutants | ||
| Component | Key Examples | Levels Reduced by Trees | Comments |
Particulates Solids (organic, inorganic) Liquids (aerosols) | various | yes | Removed from plants by sedimentation |
| (under the influence of gravity); impaction (under the influence of precipitation). Wind velocity is reduced by plants, resulting in heavier particles settling out. As air currents divide to pass around plants, particulates impact on their surfaces. | |||
Gases Primary (released directly) | |||
| Inorganic | Nitrogen oxides | yes | Removed by foliar uptake; some nitrogen utilization by plants. |
| Sulphur dioxide | yes, but very limited | Amount removed by woody plants reportedly negligible when compared to atmospheric concentrations. Many species encounter foliar damage. | |
| Carbon monoxide | yes, but limited | Some reduction by woody vegetation reported. | |
| Chlorine and Fluorine | yes, particularly chlorine | Many plant species are susceptible to damage by halogens. | |
| Hydrogen sulphide | no | (No reported impact). | |
| Ammonia | yes | Absorbed and utilized by plants (for nitrogen) to a certain extent. | |
| Organic | Hydrocarbons, aldehydes, mercaptans | no | (No reported impact) |
Secondary (synthesized in the atmosphere) | Ozone | yes | Removed at a rapid rate, but foliar damage occurs to many species. |
| PAN (Perozyacetyl nitrate) | yes, but limited | Low levels reduced by plants, but foliar damage occurs at higher concentrations. | |
| Box 4.7. | Theoretical considerations governing the choice of trees and plant associations for reducing atmospheric pollution |
| 1. Removal of airborne particulates. Preference should be given to species with high ratios of leaf circumference to area and surface to volume, and with surface roughness. | |
| 2. Particulate removal during winter (in temperate regions) Suitable species are conifers, and deciduous species with a dense branch and twig structure. | |
| 3. Removal of gaseous pollutants Tree species with a high tolerance to urban environments are most suitable, as they are likely to have the highest metabolic rate, and as a consequence, stomatal opening. A high resistance to drought is particularly important. | |
| 4. Greenbelts for a significant impact on urban air quality Relatively large areas are required for any significant potential reduction in atmospheric pollution; the minimum width indicated is some 150 m, although this will vary according to local conditions. | |
5. Urban forest and greenbelt density and structure
| |
| 6. Minimising sudden failure Mixed plantings of evergreen and deciduous species minimise the danger of a sudden loss of function due to one species succumbing to high environmental, entomological or pathological stress. | |
| Source: Miller (1988) ‘Urban Forestry Planning and Managing Urban Greenspaces’ (quoting both Smith, 1978, and DeSanto et al. 1976) | |
Noise reduction
Noise in urban environments is often excessive and discomforting, particularly when it occurs at high frequencies (short wavelengths). Although generally poorly documented, it is certainly a problem in Third World cities, and probably particularly to the poorest inhabitants since they are forced to live in the least desirable locations (Hardoy et al. 1992). As a possibly extreme example, in Mexico City it is reported that noise levels intermittently reach 100 dB(A),1 and are permanently at a level of some 75 dB(A) in areas in the vicinity of the major urban highways and the airport (Chacalo and Pineau, 1991:50). As the same authors point out, this is worrying when compared against the report that loss of hearing can be caused after prolonged exposure (of more than eight hours) to noise levels of 85 – 90 dB(A).
As with air pollution, the cultivation of trees cannot solve the problem of noise, but may help to reduce it to possibly more acceptable levels, especially if combined with other measures aimed at controlling noise emissions. For example, Cook (1978) found that trees and other vegetation in conjunction with landforms reduced highway noise by 6 – 15 dB, while trees in combination with solid barriers reduced noise by 5 – 8 dB. Noise pollution is reduced by trees through five mechanisms, notably sound absorption, deflection, reflection, refraction and masking. With regard to absorption, plants generally absorb high frequencies at a greater rate than low frequencies, meaning that they selectively remove the frequencies most distressing to human ears. The effectiveness of noise deflection, reflection and refraction depends on the configuration in which trees are planted. Masking involves the substitution of desirable sounds for undesirable ones. Vegetation generates sound such as the rustling of leaves in the breeze, and humans tend to selectively filter out undesirable city noise in preference for more natural sound (Miller, 1988, quoting Robinette, 1972).
Erosion control
Many Third World cities experience serious soil and coastal erosion, especially in cases of recent expansion where they have grown far beyond their original site. Steep, erosion prone slopes are a particularly common site of self-help housing. Their danger is fully recognised; indeed, settlement is usually in contravention of local laws. According to Douglas (1983), many countries have had legislation to prevent urban development on excessively steep slopes for many years. As an example, he cites the Brazilian Forest Law of 1959 which (although not applying only to urban locations),
“made construction work above a specified level illegal in order to preserve the stabilizing influence of forests and to prevent construction across springs.” (Douglas, 1983:125).
Yet for example in Rio de Janerio, three million people, making up two thirds of all favela (self-help housing) dwellers, live on steep slopes surrounding the city, and mudslides after the rainy season are a constant threat to peoples' lives and homes.
Whether trees can help control such soil movements will depend to a certain extent on the nature of the slope and local conditions. The subject is one that has been much researched; readers are referred to Coppin and Richards (1989) for more details. It should certainly not be assumed that tree cover always reduces or controls soil erosion. Raindrops falling from a tree canopy may easily reach terminal velocity before they hit the ground, and, since droplets coalesce on the leaves they may well be larger, and thus more damaging, than raindrops falling from the sky unimpeded. It is the understorey cover that is important in this regard (Clarke, pers. comm. see References section).
| Box 4.8. | Trees for erosion control and other purposes: Working with the dislocados of Nampula City, Mozambique |
| The capital of Nampula Province, Nampula city is the third largest city in Mozambique with a population conservatively estimated at over 200,000 people. Numbers in fact fluctuate considerably, according to the local security situation. Mozambique's civil war has caused large numbers of refugees - dislocados - to flock into the cities, establishing homes in bairros, peri-urban squatter settlements. At least 80% of Nampula's current population lives in the bairros. Located on steeply sloping lands around the original hill-top city, the bairros have no proper drainage system, and are prone to soil and gully erosion, to the extent that houses and productive land are washed away in heavy rains. All registered dislocados are entitled to small plots of land for agricultural production (machambas), but these are often located a long way from the city and are not serviced by public transport. There is an increasing fuelwood problem in the area, and a reduction in tree cover around the city. | |
| Since 1987, the Irish charity CONCERN has been working with local people to develop soil conservation and sustainable land use techniques in the bairros, through a project which includes horticulture (fruit trees), agroforestry, “forest” tree planting and the construction of non-erodible drains and check-dams. Overall objectives include improving farming practices, reducing rural deforestation and urban soil erosion, and promoting environmentally sustainable land use practices. In achieving these aims, CONCERN tries to work with and strengthen the capacity of the government bodies involved, to ensure institutional sustainability once funding is withdrawn. An early part of CONCERN's work was to support the establishment of an Environmental Division of the Nampula City Council, known as GAMA. GAMA has now become an official structure and since 1991 has received its own funding. | |
| Much of the “forest” tree planting programme originally focused on Leucaena leucocephala, as a species that would both provide fuelwood and serve a useful role in soil conservation and amelioration. Some 200,000 seedlings were raised in 1990/91, of which the majority were Leucaena (the remainder being Cassia siamea and Casuarina spp.). About half of the seedlings were distributed free of charge in the bairros and to farmers interested in setting up alley cropping trials; the rest were distributed to other projects. In response to local demand for a greater variety of “forest” tree species, seedlings that have been raised subsequently include Acacia albida, Albizia lebbek, Eucalyptus camaldulensis, E. citriodora, Gliricidia sepium, Melia azedarach, Moringa olifera and Prosopis juliflora. Demand for fruit and “forest” tree seedlings has increased as a result of an awareness raising campaign funded by CONCERN. This has included radio coverage, and the performance of a forestry play. Despite the positive response, it is uncertain whether the campaign will influence the behaviour of all bairro residents. Wilful damage to trees, and simple neglect or failure to protect saplings from browsing animals, are still problems. | |
| Source: CONCERN internal documents. | |
Whatever the situation, trees will only be fully effective in soil conservation if used in conjunction with complementary measures of both a physical and institutional nature. Although institutional aspects are discussed specifically in section seven, some of them are indicated in the example of interventions aimed at erosion control in Nampula City, Mozambique which is given in box 4.8.
Watershed management: peri-urban forests as catchment cover for urban water supplies
An issue related to the use of trees and forests in controlling soil erosion is that of their use as a watershed catchment cover. In many parts of the world, strict control is exerted over urban water supply catchment areas, which are normally kept under tree cover. In some cases external assistance has been sought in this matter. For example, the FAO has supported watershed management schemes for the urban water supplies of Kathmandu, Nepal; Freetown, Sierra Leone; and Tegucigalpa, Honduras. As shown by the case of Kathmandu (box 4.9), the management of peri-urban water catchment areas for the benefit of urban water consumers can engender a clash of interests. These need to be handled with sensitivity, acknowledging the different needs of both urban and local peri-urban residents.
Tree cultivation using urban wastes
The way in which urban wastes are viewed may be starting to change; in the words of Smit and Nasr (1992:143),
“A vision of metropolitan areas is evolving from primarily open loop systems with one-way flows of resources (in) and wastes (out) to primarily closed loop systems where the definition of wastes and resources becomes blurred.”
Urban wastes may be broadly categorised into wastewater and solid waste. The former is clearly a resource of particular potential value in arid and semi-arid areas, where nutrient-rich wastewater can be an important input for agriculture or forestry. Since a possible hazard is the presence of pathogens and vectors, it can be argued that it is actually more suitable to grow non-food forest crops than agricultural ones for human consumption. Smit and Nasr (opp. cit.) also consider that the use of wastewater in urban areas of the Third World has greater potential than in industrialized countries, since their wastes contain less chemicals and toxic materials. The potential will certainly vary between countries, and in some places its practice will involve overcoming a variety of cultural and socio-political obstacles. The raising of trees using wastewater irrigation is already practised in Lima, Peru; Cairo, Egypt and in Jordan among other places.
| Box 4.9. | The Shivapuri Watershed Management and Fuelwood Plantation Project, Nepal |
| One of the main sources of drinking water for the inhabitants of Kathmandu (Nepal's capital city) is the 144 km2 Shivapuri watershed. During the 1970s, it was perceived that the quality and quantity of water derived from this area was declining due to a loss of tree cover, over-grazing, and cultivation on steep slopes. His Majesty's Government of Nepal decided in 1975 that the bulk of the area (112 km2) should be protected as a watershed and wildlife reserve. A boundary wall and road was constructed around the reserve, and the majority of scattered settlements within it removed (leaving only two villages). Hunting, the grazing of livestock, and fuelwood and fodder collection within the reserve were prohibited, and army guards posted at check posts along the wall to enforce these rules. | |
| Residents requested to move off the land were compensated for the value of their lands and homes, but there was deep resentment at the loss of access to the forest. Many families particularly resented the ban on fuelwood collection; previously they had gained a significant part of their income from the sale of fuelwood to the Kathmandu market. Eventually, in 1985, the government initiated a programme to address the needs of the Shivapuri people. The Shivapuri Watershed Management and Fuelwood Plantation Project was established with assistance from the FAO and Norwegian Trust Fund. Income-generating activities were introduced and improved, including fruit tree and vegetable cultivation; mushroom production; sericulture; and private plantations to produce fuelwood, fodder and timber. Improved stoves (chulos) were also distributed, to minimise local fuelwood consumption, and wage labour offered for Project works. Some, although not all, of these initiatives are considered to have been a success, and those which have are claimed to have benefitted only certain individuals-generally the more wealthy and educated residents. According to one critique, | |
“Efforts at controlling forest degradation simply cannot succeed without the commitment of the locals. Ad hoc measures, such as the introduction of improved chulos, vegetable cultivation and tree plantation, do not address the fundamental insecurities that farmers in and around Shivapuri face every day.” | |
| The challenge for the future management of the Shivapuri watershed seems clear, although some practicalities remain unresolved. | |
| Source: Rankin and Joshi (1992) ‘Speaking for Shivapuri's Rim Dwellers’. | |
Solid waste may be divided into organic and inorganic, the use of the former (as compost) being already quite widespread in urban agriculture. Of particular note in the current context is its common use in the cultivation of fruit trees and tree seedlings. In China, the utilization of solid wastes is particularly well developed in virtually ‘closed system’ urban gardens (Honghai, 1992). Another aspect of organic wastes in cities is those acquired through the maintenance of parks and street trees. In a number of towns and cities in the developed world, tree prunings are chipped and used as mulch, while leaf material (including grass mowings) is composted. Some municipal authorities pride themselves on being virtually self-sufficient in this regard. Similar systems may be possible in cities such as Hong Kong and Singapore, but in many developing countries it is difficult to envisage. Tree and grass trimmings have a greater alternative value as fuel or fodder (see section 4.1).
According to Smit (1992:12),
“There is a huge untapped possibility of combining urban forests and urban wastes to increase biodiversity, provide recreational open space, and produce food, fuel and construction material in many countries.”
This may be true, but major organizational changes will be needed first.
A number of the potential problems of trees in urban areas are discussed further in section six, with regard to species selection. However, a brief review is of use here, both of the problems and possible means of avoiding them.
Cost
Urban forestry initiatives conducted on a scale beyond small homegardens can cost a large amount of money to implement. This is particularly the case if instant results are wanted in amenity plantings, so large saplings are planted which require intensive after care in the first year or so of establishment. Maintenance costs, in particular irrigation, can be very high in such situations. Poorly run tree planting campaigns can also prove to be very costly, if mortalities are high as a result of inadequate or misdirected support (see section seven for comment on such campaigns). There are numerous ways in which costs can be minimised and benefits maximised through appropriate technology and careful planning (see sections five and seven respectively), but arrangement for regular maintenance is crucial.
Threats to human safety
Poorly planted or inappropriate tree species can serve as a hazard to urban inhabitants, either directly (through falling branches or the falling over of the entire tree) or indirectly. The former may be particularly likely in countries where typhoons or hurricanes are regularly experienced. It is possible that they are also of increasing occurrence in former colonies where colonial tree plantings are now over-mature and in need of replacement (Denne, pers. comm). Whereas in many developed countries there is provision for ensuring the removal or treatment of dangerous trees, this may not exist, or fail to be implemented in some developing countries. The general result is that there is probably more cause for genuine concern about the safety of trees in cities of developing rather than developed countries. Onganga (1992) comments that in Kenya, for example, problems with “trees blocking highways and falling on roofs of houses are common in urban areas.” Careful planting and choice of species, regular maintenance and a clear line of responsibility for dealing with dangerous trees would help to increase human safety.
Indirect threats to human safety caused by trees include branches catching in overhead power lines, tree canopies obscuring vision and thus causing accidents, and trees serving as a screen for assailants. The former two can be generally avoided by a careful choice of species and regular maintenance - as, to a certain extent, can the latter. In the case of urban parks, in particular, the discouragement of would-be attackers requires additional measures. In Limbe, SW Cameroon, the Botanical Gardens were generally avoided by local residents for fear of assailants until a ‘clear-up’ campaign was initiated in the late 1980s. This entailed clearing of undergrowth, the maintenance of fences and the employment of watchers - as a result of which the incidence of attacks has greatly decreased. Fear of assailants hiding behind trees is quite often expressed by residents, particularly women, of European towns and cities (Hill, pers. comm.). Such possibilities can be reduced by ensuring that paths are bordered by ground vegetation that tapers upwards into bushes and then into trees only some distance from the path. In areas where urban dwellers are likely to be more vulnerable to attack, the choice of tree species should be restricted to those that will never grow large enough to provide adequate cover for an assailant.
Structural damage
The roots of street trees often cause the cracking of roads and pavements and sometimes water pipes. Urban trees can also cause structural damage to buildings, both at foundation level due to their roots, and through the falling of whole trees or branches. As with human safety, such problems can be minimised by careful species choice and maintenance (Biddle, 1987).
Vandalism and browsing
Damage may be inflicted on trees simply out of intent to destroy; out of casual disregard; as a consequence of harvesting tree products; and by browsing livestock. While many foresters and arboriculturalists would classify all these as vandalism, there are clear differences. Only deliberate and casual vandalism are generally a problem in the developed world, whereas all four occur in Third World cities. Apart from any other considerations, this probably renders them a more difficult environment in which to raise trees.
The most important issue in combatting all forms of human and animal-induced tree damage is gaining local people's support for and active involvement in tree cultivation, a matter discussed in section five. Apart from this, urban amenity plantings can be planned to minimise the likelihood of vandalism. Trees planted within cultivated ground tend to be less susceptible to deliberate or casual damage than ones surrounded by tarmac or concrete, as are ones planted in groups compared with lone trees (Hill, pers. comm.). This is substantiated by observations of street trees in Bangalore, India. Here it was noted that,
“The extent of mutilation is clearly inversely proportional to the extent of tree cover in a locality. The fewer the trees, the more insidious the process of destruction…. We have either very little, or almost total mutilation in any locality…. It seems as if there is a psychological threshold involved: once people get over the inhibitions and into the habit of hacking trees, they go and hack every one of them.” (Gadgil and Parthasarathy, 1977:65).
The position of tree stakes can also influence vandalism; trees with stakes that reach to breast height are more likely to be snapped off at this point than ones which have lower, less obvious stakes.
Levels of deliberate vandalism to urban trees probably vary in different parts of the Third World, but are reported in a variety of places (one example being in the CONCERN Mozambique project, described in box 4.6). The following observation is salutary.
“It is common in Kenya, during funerals of important people or when a home team wins a prestigious cup, for people to cut trees and carry branches as a sign of sorrow or victory. One day's riot can leave an entire park stripped of thousands of trees.” (Onganga, 1992:219).
The damage of trees in the process of harvesting their products is primarily an issue revolving around the purpose of urban trees in different places, discussed in section six (species choice). Damage from livestock may be minimized by the choice of non-fodder species, and the erection of individual tree guards around vulnerable plants (notably roadside saplings), as discussed in some depth by Shyam Sunder, 1985).
Unorganised waste disposal
Rather than being a means of recycling urban waste, urban forests may be used as dumping grounds in manner that is wholly deleterious to the environment. As one writer comments,
“Urban forests are considered by many people as the most ideal place to dump industrial waste. This is a major problem which is not easy to solve in Kenya because it involves very rich and influential people. Waste from tires, bottles, and other industrial by-products quite often covers several acres that otherwise could be used for tree planting. These waste products have also become a health hazard to the urban dwellers” (Onganga, 1992:219).
Access to solar energy
In developing as well as developed countries which receive significant solar radiation, solar power is an increasingly utilized energy source. While trees may be valued for their cooling shade, if this reduces solar radiation falling onto a solar panel, it may be viewed as a nuisance. In many States of the USA, there are now laws regarding access to solar energy which effectively require trees blocking solar radiation to be pruned or removed (Miller, 1988). Although such legal difficulties are unlikely to affect urban tree growers in developing countries, the issue of access to solar energy is one which may be of increasing future importance.
To conclude this section, urban trees may be seen to provide a wide range of benefits to Third World urban dwellers, especially if their cultivation is linked with complementary activities designed to improve urban living conditions. Certainly the full potential of urban forestry will only be met if solid socio-political and institutional support is forthcoming. Similarly, the minimalization of many of the potential problems posed by trees in cities requires action at the socio-political and institutional level. Such matters are discussed further in sections five and seven.
