City thresholds . The role of urban green infrastructures in Madrid

This paper presents an exploration of the thresholds of the city, embodying the concept of Urban Green Infrastructure. In particular, it is a journey through the urban fringe of Madrid, where these green infrastructures, due to their form and history, achieve the sense of urban threshold and act as identity generators of the city. We examine the concept of peri-urban landscape in relation to nowadays challenges of sustainable development, as well as the benefits of Urban Green Infrastructures in the contour of the city. We then take a brief tour though the peripheral landscape of the city of Madrid, where we analyse metropolitan parks and historical green areas that comply its proximity image. After identifying the green infrastructures acting as thresholds in the city of Madrid, we focus on the south-east diagonal of the capital in order to reaffirm its importance in the construction of the image and identity of the city. We defend the importance of Urban Green Infrastructure to and from the city, suggesting the necessity of a supra-municipal planning tool to take change of the peri-urban landscape, usually perceived as subsidiary, to deem the proximity visions of the city as relevant for its design.

1 Beatriz Fernández Águeda, "Del límite de la urbanización al límite de lo urbano: la construcción territorial del Gran París y el Gran Madrid (1910Madrid ( -1939 Landscape and the Contour of the city Throughout history, the image of the city contour has been linked to the image of its infrastructure. Even though green spaces have always been present, to a greater or lesser extent, in the design of the growth and edge of the city, in recent years we are paying special attention to the concept of Urban Green Infrastructure (UGI). UGI is defined by its connectivity, multifunctionality and accessibility and can alter and enrich the peri-urban space.
Within contemporary processes of metropolitan growth many spaces remain unresolved, generating voids and geographical and sociological disconnections which condition the character of the city's edge and alter the traditional transition between city and countryside. The complex morphology of the contemporary city, in contrast to delusive administrative borders, reveals the idea that the urban limit is not defined by its urbanization but by its area of influence, 1 meaning Green Infrastructure located in the periphery acquires a valuable role in understanding the identity of the city's contour, from the point of view of the landscape, and not only by the limits of artificial land.
This article proposes a morphological analysis of the contour of the city of Madrid based on the Green Infrastructure that encircles it, enhancing the dialogue between nature and the city, especially from its impact on perceptive aspects. 2 We emphasize the importance of the metropolitan parks built in recent decades, specifically on the southeast diagonal of the capital, when characterizing the entrance to the city, which generates threshold spaces. Spaces that make more amiable areas more initially neglected in their design, when compared to the equivalent areas in the northwest of the city.

The landscape of the Contemporary City: The Peri-urban environment and the challenges of Sustainable Development
After the industrial revolution and with the rise of the bourgeoisie in the 19th century, a new model of city emerged, "the contemporary city" 3 which, among other aspects, reconsidered its relationship with nature. The city was defined by its urban centre and by the concentration of workers and consumers. After the political, economic and social changes of the 20th and 21st centuries, the importance of both the centre and the peri-urban space was established. 4 It is this diffuse, transitional space that we refer to what we refer to with the concept 'peri-urban environment'. This interstitial space between urban and rural, with a fragmented and hybrid character, is often forgotten and lacks an overview when it comes to planning the city. 5 The European Landscape Convention defines the contemporary concept of landscape, 6 highlighting both natural and cultural heritage as the basis of its identity. It also defines the meaning of "landscape management", as actions from a perspective in line with Sustainable Development, as well as the "types of landscape" among which is the urban landscape. 7 Thus, with the increase of population, climate change and the various challenges raised by the New Urban Agenda, 8 the United Nations is promoting the implementation of a series of sustainable development objectives within the framework of Horizon 2030. By 2050, it is expected that more than 70% of the world's population will live in cities. 9 Therefore, Goal 11 10 refers to cities and communities to preserve their natural and cultural heritage. One of the measures is the provision of universally accessible and inclusive green and public spaces in cities, considering the economic, social and ecological links between urban and 11 Cfr. Eva J. Rodríguez Romero (dir), Paisajes de aproximación a la ciudad de Madrid (Madrid: Conarquitectura Ediciones, 2018 peri-urban space and areas of influence, which leads us to link it directly to Urban Green Infrastructures.

Green Infrastructure in the edges of the city
As well as, on an architectural scale, patios or galleries act as a filter between the interior and exterior of a building, at an urban scale we find other transitional elements. Proximity Landscapes 11 of the city are those perceived when the city is a point of destination or departure. Hence, the city contour and its threshold spaces become a fundamental factor when defining its entrance and exit character. In this context, the Urban Green Infrastructures (UGI) stand out as "buffer areas" for the effects of urbanization, favouring sustainable development, as they provide territorial cohesion. 12 Historically, the infrastructure network of a city, understood as the set of elements, equipment or services needed for its proper functioning, has been linked to the edge of the city. 13 Typically, the landscape around a city was defined by its productive or rural appearance, yet since the late 19th century some projects have understood the latent role of green areas in changing the dynamics of the city on its urban edge. 14 Some recent policies, like the 'Green Belts' in England, executed metropolitan green belts to control growth, for example, the Metropolitan Green Belt in London (1935). In Spain, the 'Turia Garden' in Valencia (1986), the Green Belt in Vitoria- Gasteiz (1993) or the Green Belt in Zaragoza (2008)  Natural spaces that comprise Green Infrastructure act as a response to this growing fragmentation of the landscape, 17 linking urban and natural spaces. Connectivity between ecosystems and citizens is, therefore, one of the main characteristics of these spaces, along with multifunctionality or capacity to house different activities, and accessibility from different points. The advantage of Green Infrastructure in the peri-urban space is its influence from and towards the city, as well as its enormous capacity to improve formal and perceptive aspects of that often forgotten space.
Thus, interstitial spaces found on the urban edge play a fundamental role and can be incorporated into the existing UGI.
UGI benefits range from environmental 18 (they promote the principles of ecology, increase biodiversity, mitigate the effects of climate change, etc.), to economic (they help to reduce costs, attract tourism, encourage entrepreneurship, etc.) and to social factors (they improve physical and mental health, allow contact with the community, improve the sense of place, etc.) In short, they are areas with a distinct identity that directly affect people's quality of life and their perception of the city.
In Spain, UGI have been designed independently and as complementary tools compared to the overall strategies of "grey infrastructure". Although many cities already have Green Zone Strategic Plans, they do not always involve planning around existing structures, and therefore do not fully achieve their potential as a networked system. 19 Although there are different types of UGI according to their scale and form, this article focuses on the analysis of urban green systems, which affect the urban contour in its role of visually improving the experience when approaching a city.

Journey through Madrid's peri-urban landscape and the character of its green spaces
Madrid's urban development has historically been linked to the natural environment where the city was located and to the mainly rural landscape surrounding the capital.
The powerful geomorphological support of its location ( where one recognizes that one has entered the city and left the countryside behind; urban thresholds. Among different places which may act as urban thresholds, the role played by urban green infrastructure stands out, especially given its power to produce visual connectivity between the environment and the city, as well as its ability to improve in various aspects the materialisation of urban borders and influence the perceived image of the city as we approach it. Consequently, the large transport infrastructures such as radial and ring roads, railways or Barajas airport, due to their large extension over the territory, are elements which also define the character of the urban fringe, in dialogue with UGI. However, while the latter visually connect the peri-urban landscape with the urban one, the transport infrastructures fragment the territory and focus the access experience on a determined point.  One of the first projects to understand green infrastructure as a system, the 'Suburbs Development Plan' by Nuñez Granés, proposed a green belt around the city, providing service and connecting the workers' centres on the outskirts of the city. However, it was never carried out. Another project on the edge of the city, following hygienist theories, was 'Lineal City' by Arturo Soria.  (Madrid: CSIC, 2019). The International Urban Planning Competition of 1929 promoted the expansion of the city to the north, and the industrial area to the south, proposing the conservation of "the charm of the city in this area" in the southwest and west, with the construction of promenades with plantations and the conservation of the views towards the valley, advocating a system of free spaces "with a certain law". This exploration of the limit was present in the project of Zuazo-Jansen, finalists of said competition. It focused on the dialogue between satellite populations, the centre, and the possibilities of growth of the city, trying to order the urban areas that had emerged spontaneously on the edge of the city.
24 The limitations and deviations from the Plan are set out in the review of the Plan carried out by the City Council's Urban Planning and Housing Area in 2012. It proposed actions that materialized in the eastern zone of the city: Valdebebas Park, Airport City, reforestation of the southeast zone of the municipality, areas of the Manzanares Linear Park and the Regional Park in the lower courses of the Manzanares and Jarama rivers and the Gavia Park.
25 Ayuntamiento de Madrid, 'Plan de infraestructura verde y biodiversidad. Resumen ejecutivo del diagnóstico de situación del plan estratégico' (Madrid, 2018). In densified cities like Madrid, the need for UGIs for development is essential for building cities in accordance with the postulates of sustainable development, as they provide economic, social and environmental benefits. Furthermore, the location of this network of metropolitan parks in Madrid's peri-urban environment enables the lookouts establish connections of identity with the city from their views, bringing it closer to neighbourhoods that would otherwise be isolated, thus fostering their sense of belonging.
However, despite the opportunities this network of green infrastructure offers, there is a need to improve aspects of design, accessibility and connectivity. For example, new developments like 'Valdebebas Park' need more time to grow their trees in order to favour a more intensified use of the park, given its large scale. An increase in UGIs along the Cycling Green Belt would densify the network and provide support to a greater number of neighbourhoods, currently degraded, by creating a real network of interconnected green infrastructure, where unresolved interstitial spaces become areas of opportunity.
There is a need of an overview of the city's periphery when tackling Madrid's urban planning, considering the city's threshold spaces and its image in a holistic way, starting, for example, with a supra-municipal body. In this context, the peri-urban environment acts as a space of opportunity for future global developments, which [Fig. 4]. The industrial-productive landscape type, with the corresponding system of green spaces and their role as viewpoints towards the city. Source: prepared by the authors. must be planned and properly addressed, as they are the welcoming landscapes of the city.
The daily use of these spaces by neighbours becomes especially relevant when assessing their role in the city. Therefore, we need an in-depth study of the perception of these places to provide new data on the impact they have on people's quality of life. New lines of research would address the potential for comparing UGIs with "grey infrastructures".
In short, the urban planning of the contemporary city and its peri-urban space should benefit from UGI for designing a strategic consistent network, especially in the most degraded areas, to offset the existing land fragmentation. Thus, instead of generating limits that act as barriers, the thresholds will be transitional spaces that dialogue with nature and with the recognisable image of the city as we approach it.