**3. Lisbon's Urban Form: Alfama, Avenidas, and Alvalade**

In Lisbon (Figure 1), the heterogeneous nature of urban space is the result of formal characteristics that are representative of the intersection of different eras and peoples, contextualized by its privileged situation in the territory. Lisbon has always profited from its strategic positioning, essentially given by the navigability and natural harbour conditions of the Tagus Estuary, which promoted human presence and the establishment of open ports for all. The Atlantic history of the city and the landscape from which it emerged, combining the Tagus Estuary with an irregular topography dominated by a set of hills and valleys, determined the medieval occupation of Lisbon. This occupation developed from a dual need to accommodate the urban fabric to the topography whilst benefiting from a safe and flourishing port. Its history is marked by a succession of earthquakes such as the one in 1755 that forever transformed its image, creating a new identity, defining new boundaries, and further propelling the emergence of new neighbourhoods. Like other nineteenth-century European cities, avenues ripped open, expanding the city northward and along the valleys. The city's growth would later cover all directions, driven by the twentieth-century demographic boom, which justified the emergence of new developments.

In Lisbon, the tension between diversity and unity is evident due the existence of urban fabrics that has undergone long processes of sedimentation (Alfama), urban fabrics that result mainly from an idea of urban production based on the conception of public space (Avenidas), others designed as an integral unit (Alvalade), operations which reject the classic elements of urban composition such as the street and block (Olivais), or more recent urban fabrics representing the recovery of these elements in city urban composition (Parque das Nações). All of these emerged between different development phases of the city and among different natural land morphologies such as hills, valleys and the riverfront (Figure 2). The combination of urban fabrics in the city not only translates into its formal complexity, but also into the dialogue between its public form and the presence of trees (Figure 3).

**Figure 1.** Lisbon's urban morphology. Identification of the three homogeneous areas analyzed: (1) Alfama, (2) Avenidas and (3) Alvalade.

**Figure 2.** Morphological table of the three homogeneous areas analyzed: Alfama, Avenidas and Alvalade. Photographic source: Filipe Jorge [16].

*Land* **2020**, *9*, 18

**Figure 3.** The diversity of formal relations between trees and street spaces in Lisbon.

In Lisbon, the presence of trees is significant and varied but was largely confined to the private courtyards within the urban blocks until the nineteenth century. At this time, the presence of arboreal elements in the public structure was meticulous, punctuating spaces of a more singular nature, as can still be observed in Alfama.

It was not until the mid-nineteenth century that trees began to play a truly essential role in the qualification and beautification of the city, especially in exceptional public spaces such as belvederes, squares or gardens, but also in streets of larger sections. This idea was supported by hygienist principles that favoured the healthiness of urban space, envisioned as broad, airy and with the presence of vegetation. This tendency quickly became a rule for both new and pre-existing spaces, in a logic that also became a hierarchy of the urban structure of the city [13].

The present form of the city clearly expresses this evolutionary process that we will here aim to synthesize by focusing on the analysis of different areas of Lisbon. The three chosen areas to be explored—Alfama, Avenidas and Alvalade—are sufficiently distinct in their form and context, allowing a comparative interpretation of the differences between their main morphological elements. Because of their historical and/or landscape context, each homogeneous area now presents a distinct physical structure with diverse urban functions and human activities. Furthermore, it is possible to recognize how different vegetation is composed and how the presence of vegetation influences the acknowledgement of the particular urban morphology of each area and of the city as a whole.
