**2. Urban Morphology: From City to Urban Greenery**

Since the mid-twentieth century, city forms have taken a central role in the understanding of the dynamics and complexity of urban fabric.

Camillo Sitte [1] and Joseph Stübben [2] were the first urbanists to contribute towards the systematic study of the city form, later known as the discipline of urban morphology. Between the Great Wars, geographers from the German and French schools, and the Italian architecture school, also began to work on this discipline, with a particular following in France [3].

These practitioners (namely, Muratori, Caniggia, and Maffei) call themselves typologists as they reveal the urban physical and spatial structure based on a detailed classification by the types of elements that compose it: "they study the pieces or cells—buildings and open spaces contained within the framework of a discrete piece of land in single ownership or use—that generate and change the cityscape" [4].

The typo-morphological analysis aims to individualize each element, recognizing and systematizing its characteristics, its differences, and its relationship with the urban context, historical period, and the society that originated it [5]. Yet, as argued by Aymonimo [6], the identification of the type and its corresponding typology should not be solely understood as a methodical act of classification, but also as an important design tool. Not only does it decode the city's physical and spatial structure, but it also systematizes the inherent processes of consolidation and design [7]. It is, therefore, an important tool to consolidate ideas and concepts.

Lynch [8] and Cullen [9] also read and perceived the city and its built environment from the decomposition of its morphological elements, such as streets, squares, buildings and topography, amongst others [8,9]. They compared, connected, and related each morphological element of urban fabric in each context, and with each other, to contribute to the explanation of the city. Through this method of urban fabric decomposition, the relationship between the different elements that form the city through history is more easily interpreted [10].

The morphological analysis is a classic reading tool of the city that simplifies what is naturally complex. These procedures, in their different approaches, allow the understanding of the whole through the reading of each element, their structural relations and interaction over time.

When considering the specific identification and characterization of the different types of vegetation forms within the city, two references must be highlighted: "L'arboriculture urbaine" [11], a work by Laurent Mailliet and Corinne Bourgery, and "L'urbanisme vegetal" [12], authored by Caroline Stefulesco. Both references contribute to initial identification of the most important principles of the composition of urban greenery. In fact, the former highlights "the many ways to compose vegetation and combine its effects" [11] (p. 67):

(1) Regular or Random: Regular plantations usually emphasize rectilinear tracings and other geometric figures inspired by classical architectonic compositions. Random plantations introduce an irregularity into the city that is intended to be natural, yet they require significant know-how.

(2) Volume: Vegetation, even leafless, constitutes volumes comparable to architectural structures. Vegetation monuments allow tree compositions of exceptional dimensions.

(3) Dome: Foliage may constitute domes that define interior spaces, sometimes with strong architectonical features. They form a converting that tempers the excesses of heat or luminosity.

(4) Border: When vegetation borders set the boundaries of space. They consolidate and explicit urban textures. In the proximity scale, they create privileged places.

(5) Staging: When vegetation arrangements emphasize and value sights and landscapes as well as buildings and monuments, framing them on a regular or irregular basis.

(6) Apparatus: When vegetation amplifies the staging by overflowing the composition beyond the venue, building, or monument;

(7) Accompaniment: When vegetation planted in the domains contiguous to the public space participates in an important way in the collective landscape. Often, vegetation overflow completely covers the walkways.

(8) Landmark: When a tree or vegetation complex is notable for its size, architecture, flowering or foliage, and contributes to orientation and location.

(9) Covering: When climbing or suspended vegetation covers buildings, especially when the necessary conditions for its growth have been integrated into the architectural proposal [11].

This systematization is relevant as a principle for the understanding of the multiple roles and forms that vegetation can have in the construction of urban space. While this previous reference combines the formal vegetation aspects of element appearance with other aspects of specific positioning and urban space definition, the work developed by Sérgio Proença [13] on the streets of the city of Lisbon offers a more simplified view, solely focused on trees. By separately addressing the different aspects that determine the relationship between tree planting and street space—the conformation of its layout, cross-section, and partition—and the associated typological synthesis, Proença identified four types of aggregation and distribution of urban greenery:

(1) Unique elements: Trees that are placed sporadically and in an isolated way in the city, reinforcing the uniqueness of a given context.

(2) Simple alignments: The most common form of street layout partitioning, by distributing trees of the same type in single or multiplied lines and a lateral or central position to the street space.

(3) Complex alignments: Alignments defined by different tree species.

(4) Sets of assemblages: The spaces of a more informal nature, composed by the aggregation of trees that are arranged organically, forming a green mass.

The analysis of both references serves the work that is presented here, not only in the understanding of the formal potentialities of vegetation forms within the city but also in the specificity of trees as an element of street spatial composition and in its systematization in particular types that explain the phenomenon.

Bearing in mind that Lisbon (Portugal) is the chosen case study, two additional references are explored: "The Tree in Portugal" [14] and "Trees in the City" [15]. These works offer better knowledge about the types of vegetation formations in the Portuguese context and their circumstances in Lisbon's urban system.
