There are several types of green spaces in the contemporary city and all of them have their own role in the urban context. These different types of greenery are designed in different urban scales with several functions in the city. Green spaces elements (i.e., trees, small gardens, green parks or green surfaces on buildings) are all identifiable components in the city structure, making part of the composition of the urban fabric, contributing to the organization and definition of spaces. In this sense, it can be said that the priority given to green spaces has not changed much in city’s policy. The provision and the localization of green spaces have particular importance in the configuration of the city due to their influence in the quality of life of citizens in many ways. The integration of plants affects positively the urban environment, because vegetation is a climate moderator, helping to reduce the imperviousness of soil and contributing to storm water management, improving the air quality through the production of oxygen, absorption of pollution like C02 and heavy metals, and trapping dust particles.
On the one hand, green spaces can have a positive effect on urban environmental conditions and consequently in the health of citizens. When applied in a significant scale they can be beneficial to the thermal comfort of cities, providing shade, promoting the evapotranspiration, inducing a cooling effect and contributing to the mitigation of urban heat island effect. On the other hand, they have a social and emotional importance in the citizen’s quality of life and well-being. ln fact, the presence of nature in cities encourages the use of outdoor spaces, increasing the interaction and social integration among citizens.
Given that the main part of the world population is living in cities, it can be said that the green spaces have a strategic importance in the task of rethinking contemporary urbanity
Importance of greenery in the cities
Green spaces as landmarks in the urban history Vegetation was always present in the construction of cities and in some cases, green spaces were landmarks in the urban history. Greenery has been used for many centuries in urban context and integrated with buildings. One of the most famous example is the Hanging Garden of Babylon ordered by Nebuchadnezzar in the 7th and 8th centuries B.C. This garden was characterized by terrace construction with trees of many species of fruits on the top of small hills, with waterfalls and irrigation systems maintained by slave labour. It was built as part of the ornamental roof used by the ancient civilizations of the Tigris and Euphrates river valleys in Mesopotamia, creating the perception of being suspended, being visible from a vast distance from the palace walls.
In the ancient Greek and Roman empires, plants such as olive trees and grapevines were cultivated
purposely so as to climb buildings walls.
With the dawning of industrial city and a new concept of spatial planning practise, green spaces (including parks, gardens, tree-lined avenues or curtains of trees) began to be used as landmarks in urban context, as parts of the city make-up and of urban design proposals. A notable example is the model of the garden city proposed by Ebenezer Howard at the end of the 19th century. The British cities of Letchworth (1904), Hampstead (1906) and Welwyn (1921) were the first garden cities built on the outskirts of London. They were part of an idea of spatial planning emerging at the time in the United Kingdom, which had the aim of integration cities and green elements. Thus, green elements started to be used in order to define the design of public spaces by ample parks and gardens, wide avenues as a type of boulevard or in addition to private plots of dwellings. A dense curtain of trees separating the industrial zone from the urban fabric, was essential to guarantee the city’s autonomy and productivity.
Such as in other countries which were not in the first line of the spatial planning innovations in the late 19th century, in Portugal, there are no real examples of garden cities. However, there are planned garden suburbs, especially in the decades from 1940 to 1960, built during an urban expansion process as new residential areas with large green spaces and a wide range of public facilities.
Urban density and sustainability
As part of a process of increasing urban density, cities began to unveil a new organizational model based on expansion and dispersion of even more distant suburbs. This process, based on the accessibility of the population, moving every day from residence to workplace, the commuters, required efficient network infrastructures and transport systems. Thus, urban expansion resulted in increasing levels of air and noise pollution, energy consumption, lack of amenities, unhealthy housing, high maintenance costs of infrastructures or absence of green spaces. In recent years, the containment of cities has been called for instead of their expansion. The authors indicate the urgency to create cross-functional policies in order to support the concentration of activities, discouraging the extension of infrastructure, which would reduce the need for mobility and transport, and consequently resize urban perimeters. That’s why is vital to redefine green areas to turn them into being more liveable and sustainable.
Nowadays, urban areas occupy around 3 per cent of the earth’s entire land. At the same time, the growth of urban population is a universal tendency in modern societies. For the first time in history, half of the world’s population is living urban spaces, which “is expected to be 69 per cent urban in 2050”. In this scenario, urban sustainability as a concept has been discussed intensively since the 1990s. In addition to traditional environmental dimension there are also the economic and the cultural dimensions to consider. On the environmental side, urban sustainability requires that the rate of consumption of renewable resources (including water and energy) does not exceed the replacement rate, and that the consumption of non-renewable resources does not exceed the capacity to develop renewable resources.
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