Ecological Sustainability and Urban Green Space
and Green Structure Case Studies Ringkøbing
Impacts of sustainable Housing Helsinki -
Espoo -SU Political
Urban Density and Green Structure Case Studies
Social Impacts of sustainable Housing
Helsinki - Espoo -SU
Norway - N
Jakub Szacki, Institute of Physical Planning and Municipal Economy, Warszawa
This paper presents some general information on the functioning and designation of green networks in cities. It appears to be a very challenging task, definitely more difficult that in the case of networks in the natural or semi-natural environment. This is for several reasons. First, there is a need to reconcile the requirements of nature protection with the needs of city inhabitants. Secondly, cities differ from one another (in terms of their size, pollution, density of buildings, degree of degradation of natural elements, land use, etc.) more than, say, one forest from another. It is possible to work out some general rules for network designation, but when it comes to details and specific solutions it is very difficult, if at all possible, to generalise. In addition, urban areas are very much mosaic systems where quite natural ecosystems are close to dense buildings and degraded industrial areas; thus, identification of individual units that may constitute a network is more difficult than in the case of more homogenous landscapes. Finally, green networks in cities have various and sometimes contradictory functions.
Recently, conservationists have realised that nature conservation in protected areas such as national parks and nature reserves is not sufficient to maintain a high level of biodiversity and proper functioning of natural systems. It was articulated both in the Rio Declaration and the Convention on Biodiversity, as well as in other significant international documents. It has led to several concepts of ecological networks such as ECONET, PEEN, Natura 2000 and Emerald at national and international levels, some of them being already implemented in several countries. The listed networks cover substantial parts of individual countries and comprise not only natural habitats but also considerably transformed areas, including rural and urban areas (where necessary and possible).
Cities are often viewed as areas void of natural values and consequently unworthy of any conservation measures. Several studies carried out in Poland as well as in other countries have shown that this is rarely the case. For instance, in the centre of Warsaw many species of birds were recorded both while migrating and while breeding, including birds of prey and Charadriidae. In London over 1800 species of plants and 200 of birds were recorded (Sukkop, Werner 1982). Some rare species of mammals spend some part of their life histories more often in cities than outside them, as do numerous species of bats. It shows that there are good reasons for nature conservation in cities. That was recognised in some countries, as the American statement of policy of the American Society of Landscape Architects (Lyle 1987 after Adams, Dove 1989) shows: 'providing suitable conditions for plant and animal communities is a goal for every landscape everywhere, including urban, suburban and rural landscapes and a goal that should be seriously pursued at every level of environmental planning and design'.
Green networks in cities are needed on two scales. first, there should not be any kind of barrier within ecological networks at regional or national levels. That is especially important as links within ecological networks are very often ensured by riparian corridors and cities are usually located on river banks. So, it is the case that urban areas frequently disturb functions of large scale natural corridors along river valleys. Secondly, green networks are needed on a local scale to ensure the proper functioning of nature and to improve living conditions of inhabitants.
Generally, the main aim of core areas and corridors constituting green networks in cities is to guarantee the appropriate functioning of nature in urban areas and simultaneously to produce appropriate living conditions for humans. More specifically, they should shape the natural environment through the improvement of biotic, climatic and hydrological conditions.
Specific aspects of the urban microclimate, considerable pollution, overheating and overdrying of air negatively affect human health, plant and animal populations and other elements of the natural environment. As regards climate, a network should be based on extensive areas of air regeneration usually located in suburbia and areas enabling relatively free inflow of fresh air into a city and outflow of polluted air from it. Therefore, the improvement of climatic conditions is one of the important functions that should - and could - be fulfilled by urban networks of green areas.
Frequently, the gradual lowering of the ground water table may be observed in cities. It is a highly disadvantageous phenomenon, as it leads to the disappearance of numerous water bodies and streams and excessive drying of soil. The main reason is the disturbance in natural water circulation, mainly due to covering the surface with impermeable material (asphalt, concrete), increased runoff of precipitation water (sewage system, river regulation) and excessive water intake (depression cones). For these reasons green networks should produce an appropriate humidity and water regime both for humans and other biota.
Besides the disadvantageous effects of physical and chemical conditions in cities, another important factor playing a part in the functioning of urban nature is the fragmentation of natural areas. This may be considered unimportant from the point of view of human needs, as it does not directly influence living conditions. Nonetheless it is a serious threat to the existence of sustainable networks of green areas. The effects of fragmentation have been evidenced in many empirical studies and theoretical models. The most important of these include: increased probability of extinction of various species as a result of insufficient population size (due to stochastic processes); difficulties in re-colonisation of once extinct populations; a decline in the rate of migration, which forms a necessary element in the life history of many species; genetic impoverishment due to reduced gene flow among local populations, etc.
Fragmentation of natural areas is an important problem everywhere, including in tropical forests, but it is especially so in cities. Green areas are usually small, so the effects of fragmentation are strong and the degree of isolation is high due to the existence of roads, buildings and other infrastructure hostile to living organisms. In this case the role of a green network is to ensure links between individual green areas and where possible to change an archipelago of green islands into an extensive system that may be used by various species. A special role should be assigned to ecological corridors that are described by Soule and Gilpin ( 1991 ) as "bandages for a wounded landscape".
d. Other functions
Existing networks of green areas should also play the part of recreational grounds for city dwellers. Many surveys of public opinion show that contact with nature is in much demand. In Poland it is also demonstrated by the large number of summer houses in the countryside which are now owned by a considerable proportion of inhabitants of large cities. In addition, elements of green networks are valuable as educational tools and urban nature. The aesthetic role of green areas should also be noted.
In all landscapes, including urban ones, there are habitats capable of having a positive influence on neighbouring areas and there are those demanding such influences. Therefore, a network of green areas in cities should constitute a consistent system composed of units that are resistant to human pressure and capable of influencing other parts of a city in terms of microclimate, hydrology and biological relationships. That is the basis for delimiting the network, or the Urban Natural System - UNS, as it is called in Poland (Szulczewska, Kaftan 1996). This title shows that ecological networks are intended as real systems and not just left over green areas unsuitable for investment. Thus, the UNS should be a part of a city (consisting of appropriately selected landscape units), that has not only an environmental role (mainly climatic, hydrological and biological) but other functions (e.g. housing, recreational, aesthetic), consisting of core areas and corridors connected to one another and also connected to regional and national ecological networks with natural processes. Only then can it constitute a functional and stable system which does not require constant human activity to survive.
a Core areas
Since a network of green areas in a city needs to meet the requirements of inhabitants, prescribing the limits of such a network cannot follow the methods applied in natural landscapes. In the case of the latter, the main criteria for demarcating core areas are a degree of naturalness and richness of species, whereas in cities other factors become more important, such as the amount of oxygen produced, the capacity to absorb pollution and noise, and aesthetic values.
Core areas should be those areas which fulfil the role of shaping climatic, hydrological and biotic functions in adjoining terrain. Typically they consist of rich and balanced landscape units that are resistant to external influences. The main criterion of their quality is their extent of influence on neighbouring areas (outside the network).
As regards climatic conditions, special attention should be paid to the character of the biologically active surface (the ratio of artificial surface to that covered by vegetation and the spatial distribution of vegetated surface, supply of oxygen and other substances, the influence on humidity and horizontal movements of air masses (coarseness of surface and location in relation to dominating directions of winds), providing conditions for vertical movements of air and capacity for absorbing pollutants. So the more extensive the green area, the more significant is its role as a climatic factor (greater biomass production means higher production of oxygen). Water bodies or areas of high humidity due to a high level of ground water are important in areas of dense development, as the air is dry there. Urban landscape mosaics increase vertical air movements; this is of special importance in cities where horizontal movements are limited due to physical barriers. Usually river valleys produce advantageous conditions for horizontal air flow, although the occurrence of dense vegetation (that is advantageous for biotic and hydrological reasons) may reduce air flow.
The following factors are important for hydrological reasons: capacity of water retention, capacity of recharging the ground water system, the connection between ground water and surface water, the direction of water flow (ground and surface), seasonal changes in the ground water table, water pollution. So the most important role should be assigned to river valleys and valleys of periodic runoff and areas of high retention (e.g. peat areas), areas of permeable ground (due to recharging the ground water system) and forested areas, due to the retention and protection of surface pollution (and simultaneously ground water), against pollution.
When it comes to biotic conditions the most eminent features are richness of species (especially the proportion of native species, but also the presence of species tolerating pollution), the complexity of the ecosystem that usually leads to greater stability, the amount of biomass and its capacity for restoration (it also affects oxygen production), the vertical and horizontal structure of vegetation (it is related to the number of niches and consequently richness of species), the consistency of vegetation with habitat features), and the proportion of natural and semi-natural areas to areas of development. Certainly, a core area does not necessarily need to meet all the requirements, for example, it may improve climatic and hydrological conditions in adjoining areas considerably, but may be of modest significance for biota.
It does not necessarily mean that core areas have to be natural or even semi-natural. For instance, all woods, meadows, reeds and peat habitats may be important as biological components of a green network in urban areas, irrespective of their ecological value and acreage. Very often even much degraded areas may be the important for the survival of species. In Silesia, a heavily polluted part of Poland, several species of rare orchids were found in the close vicinity of non-ferrous smelter. All woods and forests, parks, gardens accompanying the few single family houses and cemeteries may constitute parts of green networks in cities. Their role depends on their size and quality but also on their relation to the areas that are to be influenced by them. Even a very rich ecosystem has no or little influence over adjoining areas which are very different from them. For instance, wetland habitats will be of little importance for biota in city parks on mineral soils, due to a completely different pool of potential species.
"Corridors" has become a very fashionable term, but (or perhaps because of it), this description remains somewhat vague. There are several definitions, but I believe that the most succinct is to regard a corridor in terms of its functions. Thus, a corridor would be any part of a landscape that links two or more habitat patches (core areas) and which facilitates any movement of organisms between them, no matter what its shape. While core areas in urban areas have various functions, the main function of corridors, as the term suggests, is to facilitate movement (corridors also have the roles assigned to core areas, but they are not specific to them). Thus, they should be designated to counteract threats resulting from fragmentation and isolation. In cities the restoration of sufficiently large habitats is often not possible due to necessary urban and industrial management; the only other possible solution would be to ensure links between individual habitat fragments. That would ensure access to a sufficiently large area of suitable habitats, access to breeding grounds and overwintering habitats as well as opportunities to breed and re-colonise formerly inhabited patches.
The main function of a corridor is to enable links between individual habitat patches. That may seem trivial, but when it comes to defining those links, the question is no longer so trivial. One has to remember that physical continuity is neither a necessary nor sufficient condition of a corridor. In fact, most existing corridors are non-continuous structures. However, they may play their role as pathways facilitating movement. A linear habitat which may seem to be a corridor may not actually be one, because it may be unsuitable for organisms of a species of interest to move between core areas. On the other hand the role of corridor may be played by a non-continuous habitat (for example, so called stepping-stones being places of shelter, rest or forage for migratory species, such as birds). For that reason some landscape ecologists introduced two separate terms: connectivity and connectedness. The first one describes actual functional linkages between habitats and the second physical, structural links. As we are more interested in the effects of corridors as actually existing routes of dispersal, migration and gene flow, more stress should be put on how well a particular piece of land fulfils several functions, rather than the structural features of a corridor. In other words, the connecting function is more important than the actual structure, but we may only anticipate function on the basis of those structures. In particular we have to attempt to develop corridors which would provide avenues along which:
animals can travel, migrate and meet mates
The demarcation of corridors is based on complex criteria: abiotic factors (for example, hydrographic and soil conditions, landscape use, naturalness), biological diversity and general similarity of corridors to core areas which are to be connected (for example, meadows in a river valley would not be a functional corridor connecting an arid forest on a high plain and a city park on the other side of the valley) and specific requirements of the species which we want to protect (if any). In the case of the latter, the presence of a given species is an important criterion of the suitability of that corridor, as it shows the existence of appropriate conditions for the species. If it is not present in some parts of a corridor it is important to learn whether the absence of a species is due to existing barriers or the unsuitability of corridor habitats, as it is often possible to reduce the negative effects of existing barriers, for example, by building flyovers or underpasses under roads and railways, and re-constructing the elements of corridors where needed. Usually in urban areas the demarcation of continuous corridors is possible only along a river valleys. No other habitats are in a suitable condition and there is little chance of restoration or constructing corridors in cities. However, since a corridor is not necessarily a continuous structure, we should search for some landscape configurations or mosaics in urban areas, which can have the functional role of a corridor rather than that of continuous linear habitats.
Demarcation of urban green areas may be a very exciting intellectual exercise, but when it is not implemented on the ground it is a useless. Unfortunately, there are several serious problems that make the implementation of properly functioning networks of green areas in Polish cities questionable. Firstly, there is the problem of ownership. This is still a problem even several years after the collapse of communism, as more and more former owners demand their properties back and nobody knows what the ultimate outcome will be. At the same time administrative reform is to be carried out and for the moment people are uncertain about future land ownership and about authorities within cities. Secondly, there is a problem of low environmental awareness by the public and, even worse, by the authorities, who are not able to take environmental requirements seriously. In addition, there is the problem of funds. Local authorities are very anxious to earn money, so they are very reluctant to undertake any decisions that are not directly linked with immediate profits. Finally, the designation and demarcation of the system of green areas requires expert knowledge from various professional backgrounds and organising such an interdisciplinary team is very difficult. As a result, in spite of a sound theoretical background and a long tradition of environmentally friendly town planning, we are still far from achieving the goal of sustainable networks of green areas in Polish cities.
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Sukopp H, P. Werner 1982. Nature in cities. Council of Europe, Strasbourg.
Soule M.E. and Gilpin M.E. 1991. The theory of wildlife corridor capability. In: Nature Conservation 2. The role of corridors. [Eds.: D.A. Saunders and R.J Hobbs]. Surrey Baetty & Sons, Chipping Norton. pp: 3-8.
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