*3.2. Avenidas—the Trees as a Line*

The Avenidas is one of the richest urban and architectural environments of Lisbon, not so much by the representativeness of its buildings, but mainly due to an eclectic assortment of types and styles formed since the late nineteenth century by the Avenidas plan. Designed by Ressano Garcia, the plan led the development of the city to the north, using an elementary urban system composed of an orthogonal design matrix similar to what happened in other European cities of the nineteenth century. It clearly appears in response to the emergence of a new bourgeois class claiming a habitable, spacious and airy city with green spaces.

The plan is based on a juxtaposed structure of three different urban layouts organized around three central axis sequentially articulated along the valleys by squares. In addition, the proposal structure includes the integration of a set of pre-existing elements: some rural plots, buildings and, above all, a primary structure of roads connecting the city with the small rural nucleus beyond Lisbon [17].

The plan composition encompassed two opposite approaches when considering, on the one hand, the detail given to the design of public space and, on the other hand, significant uncertainty about private space by the absence of architectural regulations for buildings. These guiding principles, or the lack of them, have remained until the present day, forming an urban unit where public space remains practically untouched and the built fabric lives in constant transformation. In a built fabric formed mainly by housing typologies of different configurations, a new trend appeared from the 1970s onward to integrate services and commercial buildings into the main axes of the Avenidas. This process reinforces a tendency to concentrate human activities in the squares that articulate them.

In the genesis of the plan for Avenidas, Ressano Garcia and António Maria Avellar had the intention of planting different tree species in each avenue—*Platanus x hybrida* for Avenida da República, *Jacaranda mimosifolia* for Avenida 5 de Outubro, *Celtis australis* for Avenida Duque d'Ávila, *Tipuana tipu* for Avenida Conde Valbom, *Populus tremula* for Avenida Fontes Pereira de Melo, amongst others (Figure 6, right; Figure 7). This characteristic, that is still evident today, gives a singular character to each street, "the trees created the initial identity of the street" [13].

Plantations in this urban structure are mostly linear, emphasizing the rectilinear tracings of the different avenues (Figure 6, left). These linear structures, formed by the orderly distribution of trees along an axis, constitute alignments that develop parallel to the building that forms the street. Alignments allow the delimitation, separation and framing of elements and spaces that form the street cross-section, depending on their position. Usually, the alignments we find in the city of Lisbon are formed by two parallel lines of trees that are positioned on the sides of the street, separating the road from the pedestrian. This principle is mainly verified in the streets created between the late nineteenth and early twentieth centuries, yet we often find variations of streets with central alignments.

The case of Avenidas is a good example where the presence of these two types of alignments—lateral and central—is recurrent and balanced, such as the Alexandre Herculano and Filipe Folque streets (Figure 8). These two examples also allow the demonstration of different ways of forming these alignments, evidencing the existence of single lines and lines that can be multiplied within the same street space (Figure 9), usually composed of trees of the same species. Exceptionally, these alignments derive from tree line aggregations of different species, such as the case of Avenida da Liberdade (Figure 8), highlighting the structuring role of this street space in the context in which it is inserted. This example highlights the importance that alignments can have in the definition of a hierarchical urban structure, such as the case of Avenidas.

**Figure 6.** The regularity of tree alignments in the urban structure of Avenidas (left). The different tree species on each avenue (right).

**Figure 7.** The diversity of formal relations between different tree species and the street space in Avenidas.

**Figure 8.** Three types of tree alignments in Avenidas (left to right): complex alignment—Avenida da Liberdade; simple and lateral alignment—Rua Alexandre Herculano; multiplied and central alignment—Rua Filipe Folque.

**Figure 9.** Two types of tree alignment in Avenidas: multiplied alignment—Avenida da Liberdade (above); simple alignment—Avenida Duque de Loulé (below). Photographic source: Formaurbis LAB.
