Introduction

 Green spaces are increasingly recognized as an important component of the urban environment. According to a recent report by the United Nations Department of Economic and Social Affairs (DESA) entitled "Prospects for Global Urbanization", the world's population will increase by 2 billion people over the next 30 years, from 7.7 billion today to 9.7 billion in 2050, two-thirds of this population will live in an environment. urban. This will have a greater impact on two continents, namely the African continent and the Asian continent, due to the fact that its population will be three times larger. We are talking about Africa's urban population increasing from 414 million to 1.2 billion, and Asia's urban population from 1.9 billion to 3.3 billion by 2050. 

The rapid development and growth of these urban areas puts increasing pressure on the environment, so nature in the city is of great interest to managers, politicians, scientists and the public.

Vegetation in the urban environment provides answers to some development questions. Therefore, it is strategically important for local actors to have operational mapping of vegetation throughout the urban area, for the management and control of green spaces, which today is impossible without the use of remote sensing data.

In the age of technological advancement, this new module enables AI-powered mapping based on deep learning image recognition algorithms to digitally map large areas quickly at a lower cost. This contrasts with traditional digital mapping, which required a huge amount of work and a private army of cartographers to map even a single city. It was a tedious and expensive job.

The purpose of this chapter is to present the characteristics of urban vegetation and the functions and services it provides from an ecological, human and economic point of view.

  To achieve this goal, the following tasks were identified:

1. In-depth study of the technology and methods of remote sensing of the Earth (ERS);

2. Training in working with the QGIS geographic information system;

3. Studying the architecture of the QGIS GIS and gaining skills in working with the basic set of its functions;

4. Acquisition of skills in working with the Mapflow module (from Geoalert);

  1. Vegetation in urban areas: features and characteristics

 1.1. Features of the urban environment 

In contrast to the natural, semi-natural and agricultural environment, the city (urban environment) is a highly anthropogenic space that combines various activities and functions and includes a wide variety of objects.

The urban environment is a highly fragmented environment that forms a real mosaic. The fragmentation of the urban environment is concretized, in particular, by obstacles that may be impassable for fauna (buildings, roads, walls, fences.....).

The city is also a highly artificial space, dominated by mineral surfaces in the form of built-up areas, roads, train stations, factories, etc. Vegetation is often present in residual form.

The highly artificial and fragmented nature of the city has important implications, especially for climate and biodiversity (see 3rd paragraph of the author). Indeed, strong artificiality leads to the fact that the temperature in cities is higher than in rural areas, usually by 2-3 degrees.

 1.2. Forms of urban vegetation: diversity and modes of representation

 1.2.1. Wide variety of forms

 Vegetation in the urban environment has a large number of different shapes and configurations: tree lines along roads, flower beds on roundabouts, individual private gardens, public gardens or large urban parks, usually located on the outskirts of cities. In addition to the natural vegetation present in the city, there is also vegetation on roofs and facades.

The diversity of the observed forms is due to the great diversity of species found in cities, as well as the way they are managed: they can be spontaneous or cultivated.

Thus, the term "green space" itself reveals a wide variety of meanings depending on its composition, use and context.

Thus, urban vegetation is, by definition, a complex object that must be understood from a physical, spatial, biotic and social point of view, and therefore in an interdisciplinary way.

To date, there is no nomenclature that allows us to simultaneously consider the physical, spatial, biotic and social components of urban vegetation. Therefore, five main types of typologies can be distinguished:

 

-Botanical or phytosociological typologies that propose nomenclatures based on genera/families/species, habitats, or ecological functions;

-Structural typologies, which are based on more physical and geographical approaches to the study of the shape and organization of vegetation. These analyses are being developed widely in the space science community

-Regulatory typologies based on the legal or land status of vegetation and, therefore, determining its urban use (public, private, ....).

-Typologies based on management methods. They are developed by managers and services of green spaces in order to ensure the subsequent and optimal management of these spaces;

-Typologies of use and functioning. This type of approach is mainly based on the type of use by citizens

 

Within the framework of this research work on the development of operational mapping of vegetation in urban areas, structural typologies and typologies on management methods are of particular interest.

  Structural typologies

Structural typologies usually describe urban vegetation in two components: horizontal and vertical.

Vegetation, like any object inscribed in the territory, is characterized by the method of planting in accordance with the horizontal component: this method is either point (individual tree or shrub), or linear (flower beds, alignment of trees, ....), or superficial (flower beds, herbaceous areas, ...).

However, the horizontal component of urban vegetation includes not only the ground part: it also includes the vegetation canopy and, in particular, the canopy surface. Thus, the same element of vegetation can be represented differently depending on the horizontal component. For example, a tree may be represented by a dot to indicate its location or a surface to indicate the area of its canopy.

 

Vegetation can also be classified according to its vertical component, based on the different strata that compose it. Thus, four main strata can be distinguished (Fig 1).

1- The moss layer, composed mainly of mosses and lichens, rarely exceeds a few centimeters.

2- Herbaceous layer, consisting of a large floristic variety, with a maximum height of 1.5 m.

3- Shrub layer, including shrubs and shrubs from 2 m to 7 m.

4- A shrub tier that includes species growing above 8 meters.

 

In urban areas, managers use these different layers, individually (roadside tree lines) or in combination (garden beds), to structure the landscape.

 

Figure 1. Ways of representing urban vegetation

 

The diversity of forms of urban vegetation raises the question of the ways in which it is cartographically represented, which by their nature are a simulation and, therefore, a simplification of the observed reality.

 Modes of spatial representation of vegetation are largely derived from the sciences of geographic information systems (GIS). Indeed, each object, depending on its characteristics, can be integrated into the GIS according to two modes of representation, vector mode and raster mode.

 In vector mode, each of the three graphic primitives used (bridge, line, polygon) represents a plant element, the attribute table contains information characterizing it (coordinates, size, planting date, etc.). This information often has high spatial and thematic accuracy and is necessary for the management of urban vegetation.

 If the representation in vector mode is most often used by managers, it nevertheless has certain limitations. Indeed, it is difficult to represent vegetation objects with the help of a single graphic primitive. For example, it seems simplified to represent a tree only by a point or surface, taking into account the descriptors associated with it in databases (crown height and span, view, type of control, and so on).

Therefore, it is necessary to multiply representations of the same plant object in order to identify all these characteristics.

 In raster mode, vegetation can be represented in two scales: pixel, while vegetation is analyzed according to its spectral and/or textural characteristics;

- subject, resulting from the segmentation of the image into homogeneous zones according to its spectral, textural, shape and situational characteristics in relation to neighboring objects.

 The vegetation-by-object approach has allowed managers to start using information derived from remote sensing data, with managers more accustomed to dealing with information in the form of objects rather than in the form of pixels. Of these two modes, raster mode will be used in the configuration of our project.

 2. Functions and services provided by urban vegetation

Vegetation in the urban environment has been studied in terms of ecosystem functions and services for more than thirty years.

At the environmental level, green spaces in the city are multifunctional, that is, they perform a complex of environmental functions. These functions can be studied through the composition, structure of vegetation or through various processes, such as, for example, productivity.

 The positive services or benefits associated with urban vegetation can be divided into three dimensions of sustainable development: environment, people (health and well-being, social connections) and economy (increasing the value of biofuels, increasing the value of plant products, urban agriculture and the attractiveness of the territory) figure 2.

Figure2: The Three Pillars of Sustainable Development

 

In contrast to services associated with the concept of benefit, services are defined as negative effects produced by natural and semi-natural ecosystems.

 These goods and services have a positive or negative impact on human well-being (regulatory services, ethics, recreational, etc.). They can be assessed using environmental, sociological, and economic approaches.

The functions and services of the impact of vegetation on the urban environment are described in detail according to these three dimensions: environment, people and economy.

 2.1 For the environment

•   Klimat

Vegetation plays an important role in regulating the climate at a subtle level in the urban environment. Indeed, the city, by virtue of its mineral character and special morphology (strong verticality, ....), provokes the creation of an urban "heat island", which is all the more pronounced the denser the city. This heat island is spatially expressed in higher temperatures in the city center than in the countryside.

Vegetation is an element of the landscape matrix that, through various physical processes, makes it possible to attenuate this heat island effect:

- Evaporative breathing, which refreshes the air by evaporating the water present in the soil and plants. (Figure 1.3a)

- Sweating, which is the release of water by the plant;

- Shading effects.

 Vegetation, intercepting solar radiation, limits the heating of surfaces. This mechanism mainly depends on the air volume of the vegetation and the interception capacity of the canopy. (Fig. 4 (a-b))

                                        a)                                                                                           b)

Figure 3 (a, b): Physical processes of radiation and thermal regulation of vegetation

These different processes make it possible to reduce the temperature of plant spaces by 1-50⁰C compared to non-plant spaces.

 Urban vegetation, depending on its organization and morphology, can also perform protective functions (windbreaks, shelters, etc. ....), identifying an obstacle to air flwo.

On a more global scale, vegetation makes it possible to reduce the greenhouse effect through the process of photosynthesis, in which the leaves of chlorophyll plants absorb CO2.

Urban vegetation, and in particular woody and shrub-type vegetation, is actively involved in the absorption of carbon from the atmosphere in these woody tissues. The rate of sequestration is difficult to accurately estimate, but scientific evidence suggests a significant influence of vegetation on carbon dioxide accumulation (Yang et al.2005; Cox,2012: Wang et Lin ,2012). 

They show that the amount of CO2 absorbed by woody vegetation depends on the species in question, the age of the trees, their height, diameter or even density of plantations. It also depends on the quality of the air and light (variable depending on the time of day, the season) and the available water. This work also shows the large role of urban forests in this process.

However, it should also be noted that plants also emit CO2 when they respire or, for example, when vegetation is exported in the form of slices or their decomposition.

 

• Air quality

 Vegetation in urban areas also improves air quality. This impact on air quality is direct, through stomatal absorption of pollutants, and indirect, through the influence of vegetation on climate regulation, which reduces energy consumption and therefore GHG (greenhouse gas) emissions.

Thus, many studies have been conducted on the ability of vegetation to capture various types of pollutants such as nitrogen dioxide, ozone, coarse and fine particles.

It is important to note that the regulatory effects of vegetation on air quality are more or less contrasting depending on the type of vegetation in question (deciduous or evergreen, shrubby or herbaceous), as well as the time of day or season.

  

• Water flow and soil protection

 Among the various effects of vegetation on water and soil, water retention by vegetation through its root system can be noted, which makes it possible to limit surface runoff and, consequently, overload of drainage systems.

This is not a direct influence, but an indirect one. vegetation to store water that will be released by evaporation or evapotranspiration.

The effect of vegetation on water circulation in a city depends on many factors, such as rainfall intensity or vegetation type. Parks and gardens, green roofs, vacant lots, and woodlands are permeable surfaces that can temporarily trap water, slow flow, or absorb it by infiltration.

 In addition, the presence of vegetation cover in the city protects undeveloped soils (non-artificial soils) from erosion and compaction caused by exposure to precipitation and runoff.

 

•  Biodiversity

 Biodiversity, which is the diversity of ecosystems, genes of species and their relationships, is necessary to maintain the basic functions, structure and processes of the urban ecosystem.

It is end-to-end for the various services provided, as it intervenes at the level of environmental regulation, supply and cultural services.

The quantity and quality of green spaces in a city determine the level of biodiversity in it.

On a more global scale (city scale), only the development of a "green network" (i.e. a green grid with more or less continuity, including vegetation of buildings and private spaces) will allow the preservation of ordinary biodiversity (native species) to the city center and can play a role in the regional green network.

The diversity of species and habitats is related to the characteristics of the environment (building density, degree of isolation).

Studies have shown that species diversity decreases with urbanization. Urbanization also favors certain species, more versatile or exotic species to the detriment of undesirable species.

  

2.2 For the individual and society

•   Health

 The impact of vegetation on human health in urban areas is mostly indirect. Vegetation acts as a lever for one or more environmental factors (reduction of urban heat islands, reduction of pollutants) that lead to the improvement or deterioration of human habitats.

Vegetation influences several components of health, which is defined here in the sense of the WHO (World Health Organization): "Health is a state of physical, mental and social well-being, and not simply the absence of disease or infirmity."

 

• Effects on physical health

Several studies have shown a link between the importance of vegetation and the ability of some patients to recover (Tzoulas et al 2007).

Thus, we observe a shorter recovery time in patients with a view of a green space than in patients with a view of a very mineral space.

Green spaces also have an indirect therapeutic effect for people with respiratory diseases or sensitive people for whom air pollution is a health risk factor (Tzoulas et al 2007). 

However, some types of plants are harmful to people who are allergic to pollen grand and can cause, for example, asthma attacks, skins dieases.

As another example of an indirect service provided by urban vegetation, the proximity of urban green spaces facilitates access to physical activity (walking, running, cycling, etc.), which leads to a reduction in obesity and its associated risks (according to a 2008 Potwark survey).

Such proximity also leads to a significant increase in soft mobility (cycling and walking), which improves the quality of the environment by reducing emissions of pollutants associated with modes of transport. In addition, the use of green spaces reduces the likelihood of developing certain pathologies, such as cardiovascular diseases, diabetes and certain types of cancer.

 

•  Influence on the comfort of the "mental" environment

Green spaces reduce "anxiety, stress, irritability or difficulty concentrating, improve workplace well-being and overall human performance" (Birds 2007.2004; Brown,2007; Cuny et al2015).

Green spaces also promote positive emotions, allowing people to "let go of themselves" and emotionally discharge from the stress in which they live.

 

• Impact on the comfort of the environment (ecological)

 Many studies have established a link between noise reduction and vegetation. Noise reduction depends on a variety of variables. Noise reduction correlated with foliage development.

The study found that a semi-open vegetation barrier can reduce noise propagation by almost 50%, and that, contrary to popular belief, dense vegetation is not more effective than less dense vegetation.

Thus, they show that the noise reduction is proportional to the density of the plant, but only up to 5 m of the thickness of the "hedge".

In addition, noise reduction is no longer essential. It is important to emphasize that, despite empirical measurements of noise reduction in different vegetation contexts, psychological aspects are involved in the perception of noise by urban dwellers.

So, the type of barrier (type of vegetation and aesthetic value), allowing or not allowing to interact with the interfering element, enhances or, conversely, reduces the perception of noise by a person.

In addition, vegetation acts as a filter for climatic conditions, be it temperature (cooling), precipitation or wind.

 

• Social connection

By all accounts, green spaces seem to facilitate communication and encounters (Long et Tonini 2012.). The landscape context, in which vegetation predominates, forces the user to "instinctively" open up to the other. Green spaces are also the places where recreational practices are most frequent, and the types of green spaces (urban parks, golf courses, lots, and communal gardens) provide varying degrees of satisfaction or enjoyment.

 

 2.3 For the economy (In the interest of the economy)

In general, the proximity of green spaces or urban forest increases the price of land (Crompton, 2001). In addition, the view of the green area or the pond is also a value for the property.

On a more global scale, the presence of green spaces a priori attracts tourists. Even if it is true that the users of parks and gardens are mainly residents of the city, and with the exception of some notable green spaces that motivate visiting the city, all the vegetation contributes to the image of the city, to its atmosphere, to its aesthetics, which attracts tourists.

From an economic point of view, the conversion of plant waste into energy today represents a significant contribution to the economy, since we are talking about firewood and grass residues. Plant waste can also be used as mulching or coating elements in landscaping.

Urban plants are also used for food production.

 Conclusions

The updating of cartographic documents relating to the vegetation of the city of Brazzaville is primarily the responsibility of environmental engineers, topographic geologists and remote sensing engineers. At present, these disciplines give very satisfactory results, and combining them will only be useful for city leaders in decision-making. 

Thus, creating operational mapping with geo-referencing and accuracy is a very technical task, and the research carried out shows that thanks to their services, we can better navigate the management of development projects.