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Cities are complex entities, with their own rules and dynamics of growth, behaviour and evolution, which result from interactions between biophysical and socioeconomic forces (Alberti et al., 2003). They have their own structure and functions. While the urban structure comprises its elements, its functions are based in the variety of services that cities provide (Pickett and Cadenasso, 2008). Its physical structure is composed by four main elements: buildings, build infrastructures, technical infrastructures and green infrastructures (Sandström et al., 2007), being complemented by the society component. These elements form a complex mosaic of patches, in a matrix of infrastructures, organizations and social institutions (Alberti et al., 2003).

The Urban Green Structure (UGS) is a component of the urban system. Like other systems, it has a function and a structure, which result from interactions between its elements and with their surroundings (Flores et al., 1998). According to the Portuguese Law-Decree 380/99 (which establishes the legal basis of the spatial planning instruments), the UGS consist of areas, values and networks which are fundamental for the urban system equilibrium.

The UGS contributes to an healthy environment for residents and users of the city (McPherson, 1992), as well as an entire set of ecological, economic and social benefits which allow the liveability, quality of life and sustainability of urban areas. It is an opportunity of spatial planning based in ecological, constructed, cultural and mobility elements, aiming to obtain a sustainable urban landscape.

Despite its crucial value, these areas have received little interest in the spatial planning policy-making, due to the lack of understanding of its importance, potential and fragility, particularly in the urban systems. In Portugal, this planning element has been considered mandatory since 1999. However, it was only applied to a restrict group of municipalities (Quintas and Curado, 2009), which proves the need of a strategy to promote the UGS in the planning process.

In order to understand the functional dynamics of landscape, the ecological and social processes should be considered, being essential in the holistic comprehension of the territory and its planning (Turner et al., 2001; McHarg, 1969). The ecologysociety relation is important for the analysis of the UGS, and “(…) since ecological networks consist of both ecological and human components, the interaction between nature and culture is a priority to consider in both nature conservation and sustainable development” (Jongman and Pungetti, 2004:5).

The Urban Green Structure is a system, composed by elements and fulfilling a vast diversity of functions. It can be analyzed as a landscape structure, using landscape metrics concepts, such as patch, matrix, edge and corridor (Forman and Gordon, 1981), which allow an evaluation not only of its ecological character, but also of its social value. “Assessment of ecological network viability can be undertaken by analyzing the inherent characteristics of the landscape elements, the interrelationships between landscape elements and external factors affecting the functioning of the ecological network” (Cook, 2002:270).

The UGS is organized in patches (with a core and an edge) and corridors. The patch characteristics are responsible for the correct performance of its functions. Thus, the patches should have qualities and have easy access, allowing natural processes and social activities to occur. The attractiveness of one patch is dependent on their access and their qualities (Figure 1).

In a patch core, indicators such as size, heterogeneity, diversity, typologies and dynamics of elements can be evaluated, both in social and ecological level. These factors, that determinate the quality of the patch, influence the movement to that patch and also the willing to stay. The edge effects take into account the relationship between patches and their surroundings, which influence the local character. They should allow a good ecological connectivity, and also attractiveness and access to people. “The availability of accessible and attractive green spaces is an integral part of urban quality of life” (Van Herzele and Wiedemann, 2003:109).

The corridors (and stepping stones) allow the continuity and the different connections in landscape, with the flux of matter, energy and organisms, but also regarding the transport systems and movement of people.

The analysis and evaluation of these components of the UGS will increase the knowledge of its importance in the urban system, the assessment of its state and provide basis for strategies of landscape planning and management in urban areas.



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