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Geographers have described the internal structure of cities using various models, based on ecological principles (Burgess's theory), economic principles (Hoyt's theory), and a combination of factors (Bradford and Kent, 1977). Many models have used the city of Chicago as their example, and their fundamental limitation in this regard is that they are largely based on the growth patterns of cities in the North (Drakakis-Smith 1987). They are not particularly helpful in explaining the structure and growth of towns and cities in developing countries, where, as the preceding section has indicated, a range of specific historical, economic and other influences have played a part in shaping current settlements. Nevertheless, Third World towns and cities are characterized by different zones, within which the current extent and potential for urban forestry vary. This section considers three ways in which this zonation may be perceived; in biogeographical terms, in terms of land ownership, and in a simple spatial model.

3.1. Biogeographical zonation

The biogeographer's view of an urban settlement is of an environment which has been heavily modified by man to the extent that some living organisms are eliminated while others invade, colonize and multiply. Different associations of organisms occupy different intra-urban ecosystems, which fit together in “a mosaic of paved or roofed surfaces and vegetated surfaces” (Douglas, 1983:128). The same author adds that the vegetated surfaces,

“…vary from carefully tended gardens to derelict wasteland and patches of mature trees and shrubs. This vegetal diversity is related in part to the nature of the soil, much urban soil being fill, or rubble, or compacted soil on which it is difficult for many native plants to become established. The vegetal diversity also reflects the deliberate or inadvertent introduction of exotic species and modification of the plant nutrient status of soils by fertilizers, compost, dumping of wastes or pollution. The nature of intra-urban ecosystems is thus partly a function of land management, varying in time and space according to changes in land ownership and land use.” (Douglas, 1983:128).

Thus urban trees viewed in this perspective form a part of certain intra-urban ecosystems. By their very nature, such ecosystems are in a state of change; nevertheless, Douglas (opp. cit.) offers a table of major types of urban land cover and biogeographic conditions, which is reproduced (with a slight modification) in box 3.1. Unfortunately, it has been drawn up with Western European, North American and Australian cities particularly in mind. Drawing on, but modifying Douglas's work, a possible zonation for urban forestry potential in developing countries is provided in box 3.2. The extent to which urban forestry is already being practised in these different zones in Third World towns and cities obviously varies within and between countries, and depends not only on the determinants outlined, but also on current social and political circumstances.

Box 3.1.Types of urban land cover and biogeographic conditions
Nature of land coverType of locationNature of biotic zone (after Dorney 1979)Trend over time
Paved, roofed, densely urban complexes devoid of open areas of vegetation and water bodiesCity central business districts. Large shopping centres and car parks. Some industrial zonesCliff/organic detritusBirds and insects develop an increasing number of niches. Micro-organisms colonize
Suburban mosaic of houses, roads, gardens and mature treesOlder, spacious inner suburbsOld urban savannaAlready diverse and mature, future changes depend on management
Corridor zones of wild plantsRailway, canal, power line and some arterial road reservationsGrassland/ weed complexIncreasing biotic diversity unless interference occurs
Landscaped parks and open spacesCity parks, recreation grounds, golf coursesMowed grasslandCareful management keeps biotic diversity low
Derelict land construction sitesAbandoned industrial sites, old waste tipsAbiotic/weed complexTemporary opportunities for plant colonization removed when development is completed
New suburbs, devoid of mature trees and high grassland: cultivated garden ratioOuter suburban areas, new housing estates, some modern industrial estates and office building complexesNew urban savannaBiotic diversity increases as gardens become established. Rate of increase depends on occupants' attitudes
Grassland on reclaimed soil with streets, car parks and buildings but few or no mature treesInner city redevelopment areas with large open spaces, modern flats or row houses, some new industrial estatesNew urban savanna/ mowed grasslandManagement tends to preserve low biotic diversity
Small woodland and rural areas within the cityPatches of woodland left as quasi-natural areas, urban commons or heathsRemnant ecosystem/ natural islandsOld woodland diversity maintained if human interference is not increased
Water bodiesLakes, rivers and reservoirsLake-stream/ aquatic complexesStatus dependent on management
Wetlands and much modified water bodiesMarshes, sewage farms and gravel pitsDerelict/weedy grassland and aquatic complexIf neglected biotic diversity increases. Likely to be much disturbed
Source: modified from Douglas (1983:128) ‘The Urban Environment’

Box 3.2.A biogeographic zonation of urban forestry potential in developing countries
Nature of land coverType of locationType of urban forestry
Paved, roofed, densely urban complexes devoid of open areas of vegetation and water bodies.City central business districts; market areas and possibly shopping centres; some industrial zonesAvenue trees on main streets; as lines screening markets or factories.
High density, low income housingProbably limited.
Suburban mosaic of houses, roads, and gardens.Older, spacious high income inner suburbs; often former expatriate residential area from colonial timesMature avenue trees. In gardens, trees primarily for ornamental purposes; perhaps some food trees.
Medium density housing with small back-yards.Probably younger avenue trees. In gardens, some trees for ornament and food.
New suburbs, devoid of mature treesOuter suburban areas, new housing estates; some shops and office complexes.Possibly some young trees already planted; potential for increased tree planting.
Corridor zones of wild plantsRailway, canal, power line and some arterial roadsidesTrees lining communication links; possible environmental and production benefits.
Landscaped open spacesCity parks and gardens, racecourses and sports groundsTrees primarily for environmental enhancement; in parks and gardens, could be managed more for production?
Derelict land and construction sitesLand used illicitly for agriculturePossible cultivation of fast growing trees for fencing, fuel, fodder. Fruit trees if land usufruct perceived to be secure.
Land adopted by the local communityPotential for community tree planting if usufruct perceived to be secure.
Steeply sloping, erosion prone land on urban fringeRecently established self-help housingFew trees; possibly some planted as boundary markers.
Well established self-help housingTrees planted for production and ornament in homegardens.
Protected vegetated slopesTree cover managed for watershed protection, recreation and (possibly) limited production.
Small woodlands within the cityPatches of sacred forest, woodland parksMature indigenous trees, generally left intact.
Tree covered areas on the city outskirts (other than on steeply sloping land)Degraded natural forest or plantations (often Eucalyptus spp.)Indigenous and exotic species used for fuel, timber, grazing land.
Water bodiesRivers, canals, lakes and fish pondsNone within the water body itself.
Modified water bodiesSwamps, coastal marshes inhabited by fishing communitiesMangrove swamps on coasts; use of fast-growing trees to dry out marshes?

3.2. Ownership of, and access to, land and tree resources

Another way of conceptualizing the potential for urban forestry in different areas of Third World towns and cities is to classify land in terms of ownership. Whether actual (in legally defined terms) or perceived (by common local consent), land ownership is an important aspect in determining the potential for tree cultivation in a particular location.1 A related issue is who has access to this land, and to the use of the trees on it.

1 Differences may exist between local peoples' perceptions based on customary law, and ownership as legally defined by the State. An example is provided by Nigeria, where the implementation of a Land Use Act, introduced in the late 1970s to facilitate urban planning, has met with many difficulties (Famoriyo, 1984).

Tree planting on land privately owned or occupied

It is well-known, mainly from rural studies, that people are usually unwilling to plant and tend trees on land over which they have no security of tenure. This is reinforced in a number of countries by the fact that tree planting is a symbol of land ownership, and is therefore not permitted by landlords. Yet in many urban areas of the Third World illegal occupation of land is a norm; indeed, in many cities, “the most conspicuous political action of the urban masses is the illegal occupation of land” (Gilbert and Gugler, 1992:223). In such circumstances, legal security of tenure may be less important in dictating people's behaviour than perceived security of occupation, often expressed in terms of illegal land prices.

Tree planting on company land

Much potential for tree cultivation may exist on land owned by companies, both as a means of creating a successful corporate image (offices surrounded by landscaped greenspace generally create a more favourable impression than mere tarmac and concrete), and as a means of screening industrial sites. Trees may also be used to rehabilitate spoil heaps and other former industrial sites; in South Africa, for example, techniques for this have been well developed in gold mining areas (Poynton, 1976). In many of the larger cities of developing and newly industrialized countries large national and multi-national companies own substantial blocks of land. The extent to which tree cultivation possibilities have been fully utilized on such land is uncertain.

Box 3.3.Ownership of land in urban settlements
categorysub-categoriespeople's accessurban forestry potential
private landindividual owner occupiedclosed; usually strongly controlled by ownerhigh; trees planted for ornamental and material benefits
tenanted (short period - perhaps one year)closed; fairly controlled (by owner and tenant)low
leased (often for a period of many years)closed; controlled (by owner and lessee)medium; possibly some trees cultivated for material benefits
illegally occupiedvariable, as is the control (by occupant and owner) over land uselow, unless tenure is perceived de facto as fairly secure.
corporate (company) landmulti-national company (premises in worldwide locations)closed; usually strongly controlled (by company)high; trees around premises planted for environmental enhancement
national company (premises in various parts of the country)closed; probably strongly controlled (by company)variable, but could be tree planting for environmental enhancement
small local business (single premises)closed; control (by business owner) probably variesvariable, as above.
“public” landState - departmental control (eg. irrigation, highways, forestry, etc.)often open; control (by department) variesprobably high; trees planted for environmental enhancement
urban/municipal city counciloften open; control (by council) variesprobably high; trees mainly planted for environmental enhancement
community land, owned collectively under customary law, or donated for the use of local peopleusually controlled by common property arrangementsmay be high; trees may be planted for a variety of purposes
church or temple land school landusually controlled by common property arrangementsmedium; possibilities for tree planting for environmental, educational and material purposes
illegally occupied (may fall under any of the above categories, but most likely State or council land)variable, as is the control (by occupant and owner) over land uselow, unless tenure is perceived de facto as fairly secure.

Tree planting on ‘public’ land

Most trees on ‘public’ land are generally planted and/or maintained for environmental purposes in Third World cities, although there may also be some material production, and potential for more. Depending on the body controlling land management, different tree species and spatial arrangements are likely to be chosen according to different specific objectives. It may also be noted that access to trees growing on ‘public’ land by local people may be open, or controlled to a variable extent. This too may influence decisions about what and how tree plantings are implemented, as indicated by examples given later in this document.

Access to land and tree usufruct

A wide range of possibilities exist in urban areas of the Third World with regard to peoples' access to land, and usufruct of trees growing on it. Situations vary from open access and poor control over usufruct to privately owned and strong control over usufruct. Many trees in urban areas are effectively open access resources, used by all. This is particularly true of those on ‘public’ land such as roadsides and derelict plots. Vandalism or excessive harvesting may be a particular problem in such circumstances. Trees planted through community initiatives may be managed more as common property resources, particularly if located on land strongly associated with that community such as around a school or a plot donated for the purpose. In some circumstances, specific usufruct rights may be defined and allocated, for example the right to harvest fruit from trees growing in parks.

A number of key ownership categories and sub-categories are given in box 3.3, which also indicates the likely extent of peoples' access to trees growing on such land, and its urban forestry potential. The range of land ownership sub-categories occurring in a given city or country will vary, as will the proportion of the total urban land area that each occupies. The extent to which such information is readily available in developing countries is uncertain.

3.3. A simple spatial model

A more simple model of tree growing zones in urban areas of developing countries is provided by Smit (1992), although he used it to describe urban farming systems. The model divides urban settlements into four broad spatial categories, as follows:

Into these categories can be fitted the more detailed zones outlined in boxes 3.2 and 3.3.

This section has indicated where trees may be found growing in Third World towns and cities, and where there may be unexploited potential for their cultivation. It remains a fact that in most urban environments, space is precious, and subject to many competing land use pressures. Much of the challenge of urban forestry lies in making optimal use of the limited area available for urban trees.

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