R.J. Olembo and P. de Rham
R.J. Olembo is Deputy Director of the Department of Environmental Management, United Nations Environment Programme (UNEP), Nairobi; P. de Rham is a former Senior Programme Officer in the Environmental Management Service, UNEP and at present works as a Swiss-based environmental consultant
Municipal intervention, where it exists, is often limited to some street planning; it practically never provides for future green space... New Third World urban areas are thus commonly treeless.
· There is consensus in the view that the quality of urban life depends largely on the amount and quality of green space within it or close to it. This green space, when it exists, is usually occupied by trees which, apart from their aesthetic value, are increasingly considered to play a vital role in the protection of the urban environment. Given the ever accelerating rate of urbanization today all over the world, it is therefore to be expected that urban forestry will become a very important branch of forest science.
In 1974, it was estimated that 34 percent of the world's population lived in urban areas; by 1984 this had increased to 41 percent; and it is expected to reach 48 percent by the year 2000.
The increase of urban dwellers is much greater in developing than in developed countries. The growth rate of urban population in developing countries was 3.7 percent annually in the period 1976-80 whereas in developed countries it stood at only 1.3 percent in the period 1974-84, and is expected to decrease further to only about 1 percent by the year 2000. In Belgium, for example, only 2 percent of the country's population lived in cities in the year 1800; by 1950 the urban population had reached 20 percent; and probably more than 60 percent will live in cities by the year 2000 (Impens and Stenbock-Fermor, 1976).
The high growth rate of urban population in developing countries is easily explained by high birth rates and by large-scale migration from rural to urban areas, a phenomenon often exacerbated by misfortunes such as drought and local wars.
For our purpose, however, it is not only the number of people that matters but also the total surface area affected by urbanization. If surface area, and not merely population, is taken into consideration, it can be said that the process of urbanization is still expanding rapidly in highly developed countries as well as in developing ones.
In developed countries, one commonly observes a concentration of population in attractive areas, such as seacoasts, rather than an influx to the established cities. The populations of older cities have decreased as people have left the congested centres for the more congenial suburbs. This evolution, largely brought about by the automobile revolution, has given rise to a host of new urbanization problems, both in the old centres, which often fall into economic and social decay, and in the sometimes huge new urban and suburban areas.
"Street forestry" can and has undoubtedly been a very efficient tool when used in conjunction with other measures to rehabilitate old city centres. However, because of the great amount of land involved, it is in the new suburbs that the greatest challenges for urban forestry are to be found.
The main characteristic these newly urbanized areas share is a relatively low density of inhabitants. This density is, of course, much higher than that in the remaining rural areas, which not so uncommonly see their populations decrease. Although density in newly urbanized areas has a tendency to increase with time, it is still in most cases much lower than that in the old city centres or even in their older suburbs.
The negative effects of this suburban sprawl on the environment are numerous and severe. Usually the best bottom agricultural land is the first to be urbanized, with a concomitant loss of agricultural production, which then is pushed back to less suitable areas. Natural and semi-natural ecosystems such as forest are threatened by pollution and by greater pressure from human use and misuse. Water quality and management are altered. Perhaps the worst impact is the increasing quantity of biologically productive soil which is buried under concrete and asphalt as houses, factories, supermarkets, roads, etc. take over the land. In this way the low-density urbanization taking place in developed countries is extremely wasteful of natural resources and land.
If one considers the tree cover situation, however, the picture is not always so dismal. Forests, which usually belong to the state or its subunits, are very often better protected than agricultural land. They are then incorporated within the urban tissue and, even if modified in the process, commonly subsist. Many large European cities are surrounded by extensive forests which create at least a partial green belt: Paris provides a good example. In the United States, part of the original forest cover is often maintained in low-density residential areas, where landscape architects like to "design with nature" (McHarg, 1969).
More surprisingly, in many areas occupied previously by low tree-density agriculture and where the resultant poor soils have been subjected to arid or semi-arid conditions, the number of trees and bushes can actually increase significantly with a residential development. Suburban areas around some Swiss cities such as Geneva clearly demonstrate this trend. As a consequence of such an environmental change, suburban gardens have today the highest densities of passerine birds to be found in Switzerland. While this may be of debatable importance, it does indicate that a diverse and biologically productive ecosystem has replaced the previous rural ecosystem.
Singapore has perhaps the best record, but other cities in Southeast Asia and Latin America could also be mentioned.
Constant pressure to gather fuelwood thus makes the task of would-be urban foresters practically impossible in many cities.
Unfortunately, such a pleasant picture, although not unheard of, is seldom to be found in Third World countries. Here, as stated above, urbanization affects more people than in rich countries because of high birth rates and immigration from the countryside. As most newcomers to Third World cities belong to the poorest classes of society and are unemployed, they can afford only the minimum plot necessary to set up their dwelling - if they can afford any kind of dwelling at all. Vast expanses of productive land are thus lost to urbanization, given the sheer number of people involved as well as the most common type of housing adopted. The limited capacity of the kind of simple, one-floor structures most commonly erected is often the most important factor controlling the number of households that can crowd into an area. These settlements are often spontaneous, sometimes illegal. Municipal intervention, where it exists, is often limited to some street planning; it practically never provides for future green space. With a lack of space and the unsuitability of the environment, new Third World urban areas are thus commonly treeless, even when they have taken over previously forested areas.
The lack of trees and green space is only one aspect of the often appalling environmental picture in many urban areas. Quite understandably, tree planting cannot always be expected to receive a high priority when corrective measures are eventually undertaken.
Greater publicity has been given to the dramatic problem of providing fuelwood or other cheap forms of energy to the urban areas of developing countries than to urban forestry in general. Satisfying wood-fuel demand presents, at the same time, both a formidable obstacle and a real challenge to the development of urban forestry. Coupled with overgrazing and environmentally harmful agricultural practices, fuelwood collection has created extensive decertified areas around many urban settlements, mainly in arid and semi-arid regions. Severe environmental degradation, however, is also observed under wetter conditions where the human population is high. As a result of vegetative destruction, drinking-water resources disappear and the urban communities are exposed to the full force of sand and dust winds in the dry season and floods in the rainy season. Extensive urban forestry programmes could help to alleviate such conditions.
Fortunately, some Third World cities have done well in maintaining or even enhancing their green space. Among them Singapore has perhaps the best record, but other cities in Southeast Asia and Latin America could also be mentioned. One cannot fail to notice, however, that this seems possible only when the standard of living and education has risen significantly. Many colonial towns, previously well-endowed with trees and greenery, have deteriorated in this respect as standards of living have declined.
FORESTED PARK IN YOKOHAMA a luxury or a necessity?
When comparing the situations in rich and poor countries, one must be aware that intermediate cases are numerous, and that what has been said is valid only in broad statistical terms. Some depressed urban areas found in so-called developed countries can have much in common, including a rapidly increasing population, with Third World urban settlements. Conversely, rich residential quarters of Third World cities can be comparable or even superior to those in developed countries in terms of urban forestry. Such a juxtaposition of conditions is common in the larger Latin American cities, where the most blatant exterior signs of socio-economic inequality exist.
TREE-LINED STREET IN AFRICA shade, fuelwood, fodder and hod
In spite of the fuelwood crisis and land hunger, a few Third World cities have been able to maintain more-or-less extensive forest tracts close to their borders. Very often this has been done to protect important watersheds. A good example is the Banco Forest on the outskirts of the West African city of Abidjan. Although today practically surrounded by urban development and partially degraded, it is the largest forest block of this type (moist tropical rain forest on sedimentary sands) still remaining in Côte d'Ivoire. The well-known forest of Tijuca, now a national park which crowns the heights of Rio de Janeiro, is an even more striking case. This forest had largely disappeared at the time the area was first protected some 60 years ago. Today the natural vegetation has to a large extent reverted to its original state; this fact serves, first of all, to show that such a reversal is in fact possible. Moreover, apart from its great scenic beauty, the forest - as no one would disagree - fulfils an Indispensable role in providing environmental protection for the lower parts of the city.
Trees in urban environments
Few solid scientific data seem available on the measurable positive effects of trees on the urban environment. This probably results in part from the attitude of the public, which, especially in developed countries, regards trees and forests as unquestionably having such positive effects and therefore as being very desirable assets for the community. The large parks of London have been called the ''lungs" of the city for a long time without anyone really knowing much about how this beneficial "breathing" function works. Therefore the incentive to measure such beliefs has not been very great. The public's energy has instead - and probably rightly - concentrated on how to save the trees. The very strict laws protecting trees in many Western cities indicate that those efforts have often been successful.
There is little doubt also that it is the aesthetic and recreational values of trees and forests which are best perceived by the public, especially the urban public. The more alienated from nature human beings become and the less directly their living depends upon biological processes, the more they wish to re-establish some links with a previous environment. Trees, because of their size, shape, colour, seasonal changes and importance in the landscape, are the natural living element most visible and therefore the most cared for. Green is the colour most restful for our eyes, presumably because we evolved in a predominantly green environment. Trees bring back a certain harmony to the urban environment and therefore play a very important social role in easing urban tensions.
In addition, however, trees do have proven beneficial physical effects. First, let us consider the city environment as it differs from the countryside environment. Cities are characterized by a predominance of stone, concrete, asphalt and metal, all materials which accumulate and radiate heat and which have a high reflective power for light and sound. Metabolic and economic activities heating, cooling, transport, heavy industry, etc. - produce great amounts of heat. The air becomes filled with carbon dioxide (CO2), carbon monoxide (CO), sulphur dioxide (SO2), many other pollutants and dusts. As a result the climate of large cities, especially in badly ventilated areas, differs considerably from that of their surroundings. The median temperature is higher, the air is drier and the sunlight is often partially blocked by haze or even fog induced by emissions. Rainfall, however, can be higher and tends to be more torrential.
Trees have proven beneficial effects in that they reduce dust, fix some toxic substances, reduce high temperatures and increase relative humidity. In Frankfurt, the Federal Republic of Germany, green belts, even having widths of only 50 to 100 m, have had an important effect of reducing the temperature by 3.5° C through evapotranspiration and by increasing the relative humidity by 5 percent compared with that in the city centre (Bernatzky, 1969).
TREE-LINED AVENUE IN BEIJING...
Airborne dust will settle and remain trapped on leaves. Because of the large total surface area occupied by their leaves, trees act as dust filters. Comparative experiments on the filtering effects of different canopies have been made by forest experts. Another important feature is the reduction of wind speed in avenues and parks, which can enhance the air-filtering effect of vegetation. In addition to carbon-dioxide fixation by photosynthesis, plants are able to absorb some atmospheric pollutants through their stomata. In this way they contribute to purifying the air in cities. Of course, there is a limit to the concentration of pollutants a plant can absorb and still function properly. Noise reduction by trees, although widely believed in, is difficult to prove. It is possible that trees, rather than directly blocking sound emissions, reduce the vibration, reflectibility and resonance of sounds produced in the immediate environment. In addition, vegetation - and especially trees - can be used in urban areas as excellent indicators of the quality of the environment since they react by integrating external factors, both positive and negative.
Even if the balance is largely positive, trees in urban areas may also present some inconveniences. However popular trees are, individual citizens often object to a tree that blocks me light reaching their house or apartment. Leaves have to be swept and collected if they are not to create problems on streets and pavements. Large parks and urban forests offer shelter to criminal activities and must be strictly controlled if they, are to serve their recreational purposes. Because of these reasons and many others, the maintenance of a healthy and useful tree population in an urban area costs more than is generally realized.
CAR-FILLED STREET IN HONG KONG both now have urban forestry programmes
The above has been said mainly with large Western cities in mind. However, most of it would apply equally well or even better to tropical cities, where heat, pollutants, dust and noise levels are often even much higher. But, as previously mentioned, the gap between rich and poor countries is also widening in the field of urban forestry.
In the less densely populated suburban areas, it is in great part a result of the abundance of trees that the environment is felt to be more congenial to man. Every effort should be made for this to remain so, otherwise the move out of old city centres would be totally in vain. The French humorist Alphonse Allais had already perceived the problem well when he advised that in order to escape from the hardships of life in the cities, the latter should be built in the countryside.
Problems and opportunities for urban forestry
Urban forestry, in its broadest sense, must face up to its task if the immediate environment of what will soon be half of humanity is not to be irretrievably degraded. Even today the rehabilitation of the extensive slums found in most large urban areas of the Third World is one of the most formidable obstacles to overcome if sustained socioeconomic development is to be obtained. That urban management services, among them forestry, cannot fulfil this task alone is certainly true. The chief difficulty is to give, through employment opportunities, an economic basis to the life of the urban multitudes. However, development and environmental actions must proceed together. It will be difficult to achieve any social and economic progress within an unhealthy and depressing environment. This must have been understood by the authorities in cities such as Singapore, where dynamic economic growth exists together with an impressive level of green space enhancement.
The living environments of plants, especially long-living trees, in urban areas are extremely diverse. They present many difficulties for the urban forester. In high-density urban areas trees will often have only a few cubic metres in which to develop their root systems whereas conditions found in urban forests and suburban areas, although more closely related to rural conditions, still present very special problems.
Urban soils can be deficient in many ways. They commonly lack water and oxygen or are contaminated by methane gas (from leaking pipes), hydrocarbons, heavy metals and other toxic substances. Soil compaction and loss of fertility through trampling and lack of organic matter renewal are other common problems which can be encountered even at some distance from the city centres.
Similarly, microclimatic and pollution situations can vary. Faster-than-normal growth rates of trees in urban areas have been recorded, and are attributable to high levels of carbon dioxide, but any positive effect is usually neutralized by the influence of dust and poisons such as carbon monoxide, oxides of nitrogen, heavy metals, bromides and chlorides, to cite only a few. Resistance to toxic substances greatly varies from one species to another. It is interesting to note that among the most pollution-resistant species suitable for a temperate climate is to be found the Ginkgo (Ginkgo biloba L.), an archaic deciduous conifer originating in China, which would probably be extinct had it not been for human intervention. As such, it is a good example of the unexpected potentialities that a tree species can have.
URBAN TREES IN CAPE VERDE an essential part of planning
Recreation activities, for which forests near urban centres are increasingly being used, can also bring many new problems. Soil compaction is one, but the vegetation itself can also suffer, especially during its formative stage. Regeneration is commonly more difficult in forests situated close to urban areas. A more insidious form of degradation may be brought about by the invasion of exotic species. Disturbed ecosystems are more easily invaded by new species which often adapt better to unfavourable conditions. Up to now, landscapers and gardeners have felt free to use whatever species best suited their purpose. Evergreen or fast-growing species have often been preferred to the local deciduous or slower-growing trees and bushes. This was of little importance when urban and suburban areas occupied only a small proportion of the land. But with generalized "suburbanization", this could result in the drastic reduction or even elimination of certain tree species together with their associated flora and fauna. In suburban areas, it should be made a policy to use native species to the extent possible.
Urbanization is especially prevalent within certain geographic areas such as seacoasts. If preventive measures are not taken, coasts all around the world may well be urbanized in the not too distant future. This poses a well-recognized threat for many ecosystems such as certain types of forests and wetlands which occur only in coastal areas. A good example of this is the destruction of marshy areas through the extension of Rio de Janeiro. In eastern Brazil, species of orchids and primates have disappeared or are highly endangered as a consequence of the destruction of local forests, in great part a result of urbanization. Large-scale destruction of mangroves in southern Florida in the United States through the dredge-and-fill technique used to develop new residential areas was stopped only when it was clearly shown that mangroves were an irreplaceable habitat for the juvenile stages of important commercial species such as shrimps.
Although forests and trees are not normally the best means to produce large quantities of food, they can provide many other resources in great demand in urban areas, the most important ones being water and wood, especially fuelwood.
Contrary to common belief, a forested water catchment will normally produce less water than an unforested one of the same size receiving the same amount of rainfall (cloud forests can behave differently). This is a consequence of the high evapotranspiration rate of trees, which reinject an important part of the available water back into the atmosphere. This should be taken into account when the maximum water yield is sought. However, water coming from a forested watershed is clean and is released by the porous soil over a long period. The advantage of having a constant flow of clean water is usually preferred to having to deal with irregular, larger volumes of dirty water that are always difficult to treat and to store.
Remarkable results can be obtained by protecting watersheds. The 5000 ha Reserva Estadual de Cantareire, established some ten years ago with the assistance of an FAO project, provides 4.0 percent of the drinking water of São Paulo. Apart from providing water, the reserve is also used for recreation and for the conservation and management of a diverse fauna, among which are different species of venomous snakes whose venom is used for the large-scale preparation of antiophidic serum. That such a relatively small area can provide such significant resources for one of the world's largest cities is a success well worth noting and one that should be replicated elsewhere whenever possible.
Wood-fuel (firewood and charcoal) collection is one of the main causes of deforestation around many Third World cities. Because transport costs are high, the woody vegetation closest to the urban areas is collected first, then afterward collection proceeds at greater and greater distances, following the main access roads. As exploitation is in most places completely uncontrolled, it tends to continue until the last tree has disappeared. No provision is normally made for regeneration. Worse, any regeneration which might occur will be removed immediately either by the local inhabitants, who have no other wood source, or by the city poor if they can get to it. Cases of rapid destruction, normally nocturnal, of city parks have been reported. Constant pressure to gather fuelwood thus makes the task of would-be urban foresters practically impossible in many cities.
Large urban concentrations are a new phenomenon in most of the tropics, especially in Africa where the wood-fuel problem is at its worst. The new urban dwellers still meet their domestic energy needs largely in the same way they did when living in rural areas. The resulting exploitation of trees, which was supportable when it was geographically dispersed, has become devastating for the environment with concentration. Chidumayo (1983) has shown how even in a country like Zambia, with a large territory and a small population, urbanization can lead locally to desertification.
Because of tradition and low economic revenue, the difficulties of switching to new energy sources such as electricity or kerosene, even when they are available, are immense.
For some urban areas, such as Cotonou, Benin, for instance, rough calculations have been made to evaluate the size of reforestation programmes which would be needed to meet the present demand. It was found out that present efforts should be multiplied by ten, 20 or more times. Many doubt that this will ever be possible. Apart from the lack of economic resources, land in sufficient quantity to grow the trees needed is not available close to urban areas. The solution, if any, will probably come from the rational management of all available forests in the concerned countries-plus a partial conversion to other energy sources.
Some urban forestry techniques could help at least on a local scale. The successful use of sewage water to grow urban forests in Peru, under desert conditions, could be one of them. In Lima vast expanses of eucalyptus are grown in this way. In a smaller area, close to the northwestern city of Piura, the very useful local tree algarrobo (Prosopis sp.) is thriving with extremely fast growth. The water used here, although somewhat purified in cesspools, would not have the quality required to grow crops or vegetables. This method seems highly applicable for arid areas where the natural growth of rainfed trees is extremely slow.
Most urban experts agree that although it appears very difficult, everything possible should be done to prevent the larger Third World cities from reaching the frightening population levels projected for the next few decades. One possible solution would be the creation of new satellite towns, self-sufficient in services, whose population would be limited to a fixed and largely self-governable number. In such a scheme, the use of trees and urban forests would be absolutely essential, both to protect the environment of the new towns and keep them within their assigned limits. Such success could be achieved only if the towns were kept well separated. Main communication roads should also avoid the towns and preferably pass through forested areas to diminish pollution and noise. Secondary ramifications would link the general road network to the towns, so as to be used only by the people living or having business in one particular town.
Unfortunately, under present circumstances, such plans for ecologically sound new towns seem rather utopian. Large financial resources would be needed, as would an enormous political will. It is always very difficult to oppose or even channel the "natural" spreading, or sprawl, of existing cities.
More realistically, urban forestry will have to work within the existing situation, however unsatisfactory it may be. As previously seen, the difficulties are enormous, but past experience shows that progress can be made if all levels of the population feel concerned and if urban forestation is not forced upon the poor areas from outside. As in most development endeavours, the active participation of the concerned population is the key to success. In this case, ideally, the trees should be planted and looked after by the people themselves.
The outlook for urban forestry is very different if one considers what is happening in the rich developed world compared with the poor Third World countries. In the short or medium term, the future of urban forestry may even seem bright in developed countries, where low-density urbanization is taking place and a growing awareness by the public of environmental issues should at least give good employment opportunities for those who specialize in this field. However, the world is finite and tremendous population growth in the South is bound to affect the non-affluent regions of the North sooner or later. Therefore greater efforts should be made to implement urban forestry programmes in the rapidly urbanizing Third World if we do not wish to see an irremediable degradation of the quality of life for the majority of the world's population.
The implementation of urban forestry programmes in developing countries is a very difficult proposition. The urban forester deals mainly with organisms - trees - which, apart from a few fast-growing species of limited use, normally take up to 40 years or more to become useful, be it for timber, recreation or environmental purposes. This is an extremely long time for countries where the basic requirements in food and energy are commonly not assured for the next few months - if they are being met at all. However, trees and forest remain indispensable to provide for certain resources such as water and wood-fuel; above all, they are one of the only ways to conserve some diversity and stability in our overstrained environment.
BERNATZKY, A. 1969, Zur Praxis der Begründung von Schutzpflanzungen. Proc. 1st. European Congr., Influence of Air Pollution on Plants and Animals. Wageningen.
CHIDUMAYO, E.N. 1983, Urbanization and deforestation in Zambia. In Desertification Control Bulletin (UNEP). November-December.
IMPENS. R. & STENBOCK-FERMOR, K. 1976, Role and impact of green spaces in urban planning. In The environment of human settlements. Vol. 2. Proc. Conf. Held in Brussels, April 1976. Oxford, Pergamon Press.
MCHARG. I. 1969, Design with nature. Published for the American Museum of Natural History. Garden City, NY, The Natural History Press.