ವ ತಿನ್ನು

ಮಾಡುವ ಕೆಲಸಗಳ

ವಿಜ್ಞಾ ನ ಮ್ಣ್ಣಣ

ಮಿಥುನ್

Keywords: substrate, leaf area index, climate, meteorological conditions, retention, detention, thermal insulation, energy demand, ecosystem services

Green roofs are expected to contribute to the mitigation of multiple environmental issues that affect urban areas. Owing to their composition, organization, and external factors, the performances of green roofs have been demonstrated to be overall positive but strongly variable. Our work first aims at proposing consensual definitions and a frame adapted to these biotic-abiotic systems. It also aims at shedding light on the qualitative relations between various internal properties and external factors of green roofs on their hydrological and thermal performances. One hundred relevant study papers were filtered from 395 papers as per our defined search criteria based on originality and precision. The expectations were to be capable of hierarchizing factors and properties that would influence the performances of green roofs. The main findings highlighted that most factors and properties have a positive influence on the performances of green roofs, showing there are many existing levers to enhance the green roof performances and tackle some of the main urban environmental issues. However, even if previous research has already explored various relations, in the final filtered consideration of 6 performances and 30 factors and properties, there was a possibility of 180 combined factor-propertyperformance relations studies overall. Out of these possibilities, only 82 have been studied at least once, leaving the other 98 relations (54%) unexplored. Considering that these lists were far from exhaustive, a huge potential in determining green roof performances remains unearthed. In this regard, various proposals have been made regarding: (i) identification of levers to enhance the performances of green roofs; (ii) filling the gaps: the exploration of the unstudied relations; (iii) promotion of deeper and innovative experimental approaches for research on green roof performances; and (iv) the shift from mono to transdisciplinary research about green roofs.

List of

List of figures. The densification of urban areas tends to strongly modify the living environment. Impervious surfaces (e.g. roofs, paved driveways, roads, and paved parking lots) occupy a predominant place in these spaces highlight the danger of waterproofing cities (Lazzarin et al, 2005;[START_REF] Vergriete | Analyse préliminaire de l'aménagement du site du garage municipal de Ville de Laval[END_REF], a phenomenon that promotes the increase of surface runoff. Since the first cities were built at the end of the Neolithic period, the phenomenon of urbanization has been intensifying over time. But it was after the industrial revolution, with the rural exodus, that the process really took off. Currently, 4 billion people live in urban areas. If current trends continue, it is projected that 7 billion people would be living in cities by 2050 [START_REF] Ritchie | Urbanization. Our world in data[END_REF]).

List of tables.

This urban growth raises complex issues and questions the sustainability and functioning of urban models through issues such as the urban heat island effect, increasing energy demands, soil sealing (IPCC 2022) and loss of biodiversity.

These problems lead us to look at and develop different solutions. The concept of ecological engineering, based on action by and/or for living things, as well as Nature-based Solutions (NBS)

appear to be relevant approaches to meet the challenges of urban environments. Ecological engineering was defined in 1998 by William Mitsch as "the design of sustainable ecosystems that integrate human society and its natural environment for the benefit of both" (Mitsch et al,1998). NBS are closely related to the concept of ecological engineering but encompasses all "Actions to protect, sustainably manage, and restore natural or modified ecosystems that respond to societal challenges in an effective and adaptive manner, while providing human well-being and biodiversity benefits."

(Cohen-Shacham et al, 2016).

Among the techniques allowing introduction of nature into the city, green roofs hold a great potential.

Considering various concepts, green roofs have been immensely appreciated and investigated in many cities around the world as a tool to contribute to solve the urban environmental problems. Integrated into the building, they do not consume any additional space and can even be implemented on existing buildings, which is likely to encourage their massive diffusion. The momentum of democratization of these concepts and their application has notably led to the development of green roofs in cities. Their development has been favoured by certain public policies (e.g. green, and blue networks, article 12 of the Grenelle 2 environmental law) [START_REF] Chauveau | Les perspectives du développement des toitures végétalisées en France : construction d'un marché et évolutions des pratiques[END_REF].

This popularity is mainly defined by their ability to respond to these new urban challenges by providing so-called "ecosystem services" (Table 1.1). The latter are defined as the goods and services that humans can obtain from ecosystems directly or indirectly to ensure their well-being. Urban soils provide many ecosystem services essential to urban environments [START_REF] Blanchart | Vers une prise en compte des potentialités des sols dans la planification territoriale et l'urbanisme opérationnel[END_REF] and green roofs, specifically, provide several services from which humans can benefit (Table 1.1).

They are classified according to three main categories of services: regulation, goods and services and cultural. Green roofs in the urban area provide ecosystem services that meet the above-mentioned challenges: reduce energy consumption through thermal insulation of buildings, mitigate the urban heat island effect through evapotranspiration, reduce stormwater runoff by retaining a portion of precipitation in the substrate, sequester carbon dioxide and bind pollutants in the biomass, improve the exterior aesthetics of the built environment, potentially provide recreational benefits, create wildlife and floral habitat, or reduce noise in buildings (Oberndorfer et al, 2007;Dunnett and Kingsbury, 2008;[START_REF] Dusza | Toitures végétalisées et services écosystémiques: favoriser la multifonctionnalité via les interactions sols-plantes et la diversité végétale 209[END_REF]). Considering that science was behind the schedule on the evaluation of such performances, this thesis aims to assess, evaluate and measure few prominent ones among such claimed performances and more importantly their evolution and interaction with green roof system over time and analyse their correlation and sustainability. This can be useful for the concerned city planners and developers in design and monitoring of green roofs for desired ecosystem services.

Some of these services have been known to humans for hundreds of years and plants on roofs have been mentioned: from the mythical gardens of Babylon to the insulating roofs of Scandinavian villages [START_REF] Dunnett | Planting green roofs and living walls[END_REF]. However, although green roofs have been used or talked about by many civilizations for a long time, the current technologies for green roofs are very recent! It was in the 1970s and 80s, in a context of emerging international environmental concerns, that the current concept of green roofs was created. After the Brundtland report in 1987, which introduced the pillars of sustainable development for the first time, Germany launched an active environmental policy (Oberndorfer et al, 2007). This would allow, among other things, the emergence of modern green roofs. In France, it was not until the early 2000s that the first "professional regulations for the design and construction of green roofs and terraces" were established by the "Association française des toitures et façades végétales". Also, the vast majority of green roofs in France, and even in the world, are recent systems, installed less than 20 years ago. This implies that we have very little experience of their durability.

After major technological developments, various scientific communities (hydrology, climate, ecology, etc.) have recently taken an interest in this field of study in order to evaluate its performance, in particular by quantifying certain ecosystem services such as the regulation of heat and water flows or urban cooling. It has to be noted that most of these research works have been done considering that the green roof substrates are inert, and their properties remain constant over time. This raises a curious but an important question,

Are green roofs really affected by ageing?

The answer to this is found through various chapters of this PhD, First, the state of the art, which highlighted the high need for nature based solutions to mitigate the environmental issues that cities are facing and introduce green roof as a solution; then the metaanalysis chapter, where it was demonstrated that green roof can be more or less performant to answer to those issues; such performances levels rely on the one hand on factors that are hardly manageable, and properties that are easily manageable, but that appear to evolve over time because of ageing.

In the next step, there was an experimental study of the effect of mono-factor on properties under controlled conditions to apprehend the kinetic and intensity of such evolution on an individual scale.

Finally, there was the in situ study of the effect of multiple factors on properties in real situation to understand how green roof evolves in real scale.

Through this there was a discussion of the importance of various internal properties (such as composition of substrates) and the influence of external factors on them; and thus, how this evolution of properties directly has their potential influence on performances.

2 Literature review

History and evolution of green roofs

The voluntary use of plants on roofs is an ancient concept. If these green roofs historically have had a protection role primarily, it was since the second half of the 19th century and development of flat roof concept that they started to appear as a new space to be invested in major Western cities.

It was not until the 1970s-1980s that the current concept of green roofs started to bloom. This was characterized by the emergence of international environmental concerns. In 1972, the report "The limits to growth", commissioned by the Club of Rome at the Massachusetts Institute of Technology (Donella H. [START_REF] Meadows | The Limits to growth; a report for the Club of Rome's project on the predicament of mankind[END_REF], demonstrated for the first time that Humanity is using the planet's finite resources faster than they are produced. This report marked the emergence of reflection on the fragility of the planet in the decades that followed (Oudot and Chaillan 2010).

Description of the current green roof technics

Currently, green roofs are defined as roofs where plants and substrate are placed on the roof. Plants as overload and leakage issues were crucial not just to the integrity of the building but mainly the comfort and safety of the people living or working there.

Biotic and abiotic components of green roofs

Green roof is a multi-layered structure, several layers are necessary to be installed before any vegetation is possible. This includes at least a waterproofing membrane, and other layers such as insulation, drainage or water retention, for one of which an anti-root membrane can also be added, depending on the builders and their necessity (Figure 2.1). The first layer consists of the insulation and waterproofing the support roof. Then, it is topped by a draining layer that facilitates the storage and drainage of water. This is covered with a filter that prevents the drive of the upper elements and an organic and mineral substrate. Finally, the plants are put in place (Gendreau et al, 2007).

Each of them has a particular purpose: a waterproof layer, a layer protecting the building from root infiltration, a drainage and filtration layer often made of gravel or thermoformed plastic. Above comes the geotextile to prevent the escape of particles from the substrate during rainy periods. The geotextile will host the substrate and the vegetation. Other layers can be added or removed depending on the builders' approach (Vijayaraghavan 2016). There are three types of green roofs which are classified according to the thickness of the substrate layer according to the French professional rules. Extensive roofs are characterized by a substrate depth of between 4 and ~20 cm, semi-intensive roofs by a depth of ~12 to 25 cm and intensive roofs by a depth greater than 30 cm (sometimes 15 cm). (Figure 2.2) They each have different uses, extensive roofs will, for example, be a source of aesthetics mainly while intensive and semi-intensive roofs will be a way to reconnect with nature in the city, by gardening or even growing vegetables.

Drainage layer and geotextile

The drainage layer creates a drainage space that directs rainwater to roof drains and prevents vegetation deterioration by keeping the roots dry.

The function of the geotextile is to retain the fine particles of the substrate. It has also as a function of preventing clogging of the drainage layer.

Substrate

The properties of the substrate layer of these green roofs determine the communities of organisms present. Here "substrate properties" refer to all the bio-physicochemical parameters intrinsic to the substrate such as pH, granulometry, density of microorganisms, etc. Substrate properties associated with the organisms present and the structure of the vegetated roof (e.g., slope, depth of substrate) condition the ecosystem services provided by the latter.

The substrate is the major component within a green roof system, and the majority of green roof benefits can be attributed to its physical properties. (De-Ville, 2017).

The organic part of the substrates aims to provide nutrients necessary for plant development [START_REF] Thuring | Green roof plant responses to different substrate types and depths under various drought conditions[END_REF].

Specificity of green roof substrates

Soils are composed of mineral and organic matter, water, air and living organisms. Other than this some can also be considered as transportic soils [START_REF] Rossiter | Classification of urban and industrial soils in the world reference base for soil resources (5 pp)[END_REF]. The properties of substrates parent materials (significant organic fraction -peat, bark or compost: coarse texture -pozzolona, crushed brick or expanded clay). In addition to that, imply specific characteristics (strong microporosity) and original and diverse properties in terms of hydrophobicity and water availability (Séré, 2010 Cannavo and[START_REF] Cannavo | Peat particle size effects on spatial root distribution, and changes on hydraulic and aeration properties[END_REF].

The green roof substrates can very well belong to this category considering

• Its ability to evolve under the actions of external factors and parameters.

• The human action involved in selection from the diverse origin of its parent materials and the way they were mixed together and then implemented on a roof over a geotextile and a drainage layer.

Vegetation

Modern green roof systems must be created with the aim of minimizing the overload for targeted buildings. The substrates must not only be lightweight, but also shallow. These conditions, combined with high solar exposures, potentially strong wind and limited water resource in dry weather [START_REF] Cao | Wind tunnel investigation of wind loads on rooftop model modules for green roofing systems[END_REF], create unfavourable growing conditions for most plant species.

Hence, the choice of the plant is sensitive as many properties are expected, in particular (Nigel Dunnet,

2008):

• Ability to cover a ground

• Frost resistance • Drought resistance
• Capacity for self-regeneration and growth.

The species of the genus Sedum, derived from the family Crassulaceae, i.e. succulent plants, have made it possible to meet these expectations.

Sedums are widely used mainly because they can grow in inhospitable environments. This type of vegetation generally grows on rocky soils in the regions in the northern hemisphere (Europe, North America). It also develops in areas used to dryness due to its ability to close its stomata during the day. Exchanges are made mainly at night to reduce water losses by evapotranspiration. In addition, most of the sedums can store a large amount of water in their leaves [START_REF] Sayed | Crassulacean acid metabolism 1975-2000, a check list[END_REF].

The proliferation and success of sedums in green roofs is also attributed to their ability to capture CO2 present in the atmosphere [START_REF] Durhman | Effect of watering regimen on chlorophyll fluorescence and growth of selected green roof plant taxa[END_REF]. However, the ability of sedums to contribute directly to stormwater management has yet to be demonstrated (Lundholm and Marlin, 2006). The combination of Sedum and artificial substrate constitutes most roofs vegetated in France and around the world.

Green roof thermal performances

Thermal protection and energy efficiency of buildings

Thermal protection of buildings and mainly the reduction of heating in summer and the consequent reduction in air conditioning requirements are the most important factors in the services most widely studied in the literature on green roofs (Dusza, 2017.) [START_REF] Dominique | Thermal Behavior of Green Roof in Reunion Island: Contribution Towards a Net Zero Building[END_REF] showed that an extensive roof composed of a 10 cm substrate and of Sedum vegetation in Saint-Pierre (Reunion Island) could reduce the temperature of a building from 5 to 7 °C. [START_REF] Kokogiannakis | The role of Green Roofs on Reducing Heating and Cooling Loads: A Database across Chinese Climates[END_REF] have modelled different conditions for green roofs in Beijing (China) and showed a 7°C reduction in ambient air in a building in summer for a soil of 70 cm and herbaceous vegetation. (Jaffal et al, 2012) have observed a 2°C reduction due to a Sedum green roof (depth unknown) within a building in La Rochelle (France).

This type of study to measure temperatures remain rare and are difficult to generalize, since factors such as insulation, building layout can have a significant influence on the final thermal and energy gain [START_REF] Dusza | Toitures végétalisées et services écosystémiques: favoriser la multifonctionnalité via les interactions sols-plantes et la diversité végétale 209[END_REF]. For example [START_REF] Synnefa | Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions[END_REF] studied the effect of a green roof (no information on substrate and vegetation) in Athens (Greece) on the air conditioning needs of the building according to the layout of the rooms. The reductions for the entire building fluctuated between 6 and 49%, and between 12 and 87% for the top floor.

Finally, the energy efficiency induced by a green roof can be very variable depending on climatic conditions [START_REF] Dusza | Toitures végétalisées et services écosystémiques: favoriser la multifonctionnalité via les interactions sols-plantes et la diversité végétale 209[END_REF]. [START_REF] Saiz | Comparative Life Cycle Assessment of Standard and Green Roofs[END_REF]) evaluated the impact of a roof composed of a substrate of 9 cm and succulent plants on the energy demand of a building in Madrid (Spain). If during heat peak the reduction was 25%, it decreased to 6% over whole hot season and 1% over the year.

A more common approach is to compare the heat fluxes exchanged with the roof level in the presence and absence of a green roof system. Several authors consider that the interest of roofs green is not in the new building sector, but in the renovation sector [START_REF] Kolokotsa | Green and cool roofs' urban heat island mitigation potential in European climates for office buildings under free floating conditions[END_REF]) (Castleton et al, 2010). Castleton et al noted that as over half of the existing UK building stock was built before any roof insulation was required, it is the older buildings that will benefit most from green roofs. [START_REF] Oke | City size and the urban heat island[END_REF] reports that a city of more than 1 million inhabitants can have an average temperature 1 to 3°C higher than the surroundings while in calm and clear night conditions, this difference can reach

Combating the effects of urban heat island

12°C

Studies such as [START_REF] Fioretti | Green roof energy and water related performance in the Mediterranean climate[END_REF] show the reduction potential of the urban heat island (85% reduction in roof surface temperature). However, the effect that partial greening of cities would have on the urban heat is more difficult to estimate. [START_REF] Santamouris | Cooling the cities -A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments[END_REF]) in a literature review, compared the potential of green roofs to the "cool roofs". Cool roofs, or reflective roofs, are roofs with high albedo and generally white, whose objective is to reflect solar radiation and avoid daytime storage of heat in cities. [START_REF] Santamouris | Cooling the cities -A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments[END_REF] considered that the potential of green roofs to reduce the roof surface temperature was similar, or better, to that of cool roofs.

Influence of substrate on thermal performance

As described above, the ecosystem services of water and thermal regulation provided within the urban area are water retention, temporal shifting of urban runoff peaks [START_REF] Dunnett | Planting green roofs and living walls[END_REF], thermal insulation of buildings and creation of urban cooling islands. The properties of the substrates vary the hydric and thermal performances.

The substrate layer acts primarily as a thermal insulator, with the characteristics more or less favourable to limit the heat penetration. (DelBarrio, 1998.) using a modelling approach, concluded that porosity favoured thermal protection. Similarly, Lin and Lin (2011), by studying the thermal resistance of several 10 cm deep substrates in Kaohsiung (Taiwan), showed that the most porous substrate (incinerated sludge) achieved a 75.3% reduction in heat fluxes, compared to 63.9% for the least porous substrate (sand).

Sailor et al noted that the addition of organic matter reduced thermal conductivity by 0.05 W.m -1 .K -1 .

(Dusza, 2017.) noted that to their knowledge is the only author to have studied the links between organic matter and insulation.

Influence of vegetation on thermal performance

The role of vegetation has been studied very little in the context of thermal regulation services.

Vegetation is considered in thermal models of green roofs mainly via their LAI (leaf area index), which represents the density of the cover [START_REF] Theodosiou | Summer period analysis of the performance of a planted roof as a passive cooling technique[END_REF], and percentage of vegetation cover. However, weather conditions make it difficult to generalize, particularly because that plant cover is intrinsically linked to stress and plant survival [START_REF] Dusza | Toitures végétalisées et services écosystémiques: favoriser la multifonctionnalité via les interactions sols-plantes et la diversité végétale 209[END_REF]. Dusza also noted that the choice of plant species has the potential to possess a very strong influence on thermal services.

Hydrological performances

Water dynamics in a green roof

To reduce the volume of runoff water and reduce the flow of rainwater into the sewer system, the use of green roofs is proving to be an interesting solution. The green roof is a structure increasingly integrated into urban rainwater management. Studies being conducted are focused on the quantities of water retained by green roof, particularly during rainy events (Mentens et al, 2006). [START_REF] Kok | EVALUATING THE PERFORMANCE OF GREEN ROOF FOR STORMWATER MANAGEMENT OPTIONS FOR GREEN TECHNOLOGY[END_REF] shows that the risk of runoff increases with waterproofing of urban surfaces (study carried out in Montreal, Canada). It shows that the roofs represent 30% of the waterproofed surfaces in the city.

Green roofs play to some extent the same role as above-ground plant biomass, which intercepts rainwater before it reaches the ground. The vegetation that develops there also allows faster evaporation (location on the roof, which promotes exchange with the atmosphere), thanks to the accumulated water as observed and reported by (Vergriete et al, 2008)

Rainwater detention and retention

Stormwater and stormwater runoff management is part of the services most frequently mentioned by local and regional authorities when they invest in green roofs. Climate change phenomenon now poses a risk of an increase in frequency and intensity of rainfall events, providing a context of increasing flooding (IPCC, 2022).

The ability of a green roof to limit rainwater runoff is related to two similar functions: detention and These cumulative values highlight a positive effect of green roofs but do not consider the response of a green roof to the frequency or to intensity of rainy events. However, the reduction of runoff is more essential during heavy rainfall, when the risk of pipe saturation is high.

Influence of substrate on rainwater detention and retention

The properties of the substrates vary the hydrological performances. Indeed, a study published in 2016 by Bouzouidja et al, suggested a decrease in the hydraulic performance of a green roof following a change in the physical organization of its substrate (e.g., porosity, soil structure). Others have also highlighted this phenomenon: (Graceson et al, 2013;[START_REF] Ding | Using the double-exponential water retention equation to determine how soil pore-size distribution is linked to soil texture[END_REF]. More fertile substrates (e.g., containing more organic matter nitrogen/carbon, substrate composition) allow for more plant biomass [START_REF] Kanechi | Performance of herbaceous Evolvulus pilosus on urban green roof in relation to substrate and irrigation[END_REF].

Substrate composition

The role of substrate composition on rainwater runoff has received little attention, but is mainly valid through its retention capacity. ( (Mentens et al, 2006) in their meta-analysis mentioned above found a positive correlation between soil depth and water retention. Logically, the storage capacity of green roofs is proportional to the thickness of the substrate. Indeed, for an average substrate thickness of 100mm (extensive green roofs), green roofs retain up to 52% of rainwater while a thickness of 150mm allows retention up to 75%.

Influence of vegetation on rainwater detention and retention

Vegetation can influence the limitation of runoff through two functions. Firstly, it can directly limit the amount of water that touches the substrate during rainy event. It can then participate in the evapotranspiration of water out of rainy events and thus regenerate the substrate's ability to retain water. In fact, it is the initial soil moisture (Bouzouidja et al, 2016.) the rain event which will be decisive [START_REF] Dusza | Toitures végétalisées et services écosystémiques: favoriser la multifonctionnalité via les interactions sols-plantes et la diversité végétale 209[END_REF].

(Nagase and Dunnett, 2012) tested the effect of twelve species of plants representing three distinct groups: Sedum, Graminoid and Non-Herbaceous Herbs grasses planted on 10 cm of substrate in Sheffield (United Kingdom). They showed that the plant species characterized by higher height, larger diameter and larger root biomass were the most effective in limiting runoff rainwater. In contrast, the Sedum species, which combined small size, small diameter and low root biomass, resulted in water retention lower than the bare substrate.

Thermal and hydrological performances partly rely on the properties of substrate (i.e. depth, composition) and it seems to be possible to clarify these relations. These properties evolve during the early pedogenesis of green roofs which induce evolution of the performances.

Thus, the nature of the substrate, as well as the weather conditions, seem to strongly influence the nature and consequences of this evolution. However, it should be noted that we have not identified any studies entirely dedicated to the evaluation, under controlled or real conditions, of the link between changes in properties and performance.

Pedogenesis of green roof

The main components of green roof substrates such as pozzolana or peat and man-made products such as bricks and compost; all of which are defined as 'artefacts' (IUSS 2014). green roofs substrates are strongly influenced by the origin of parent materials of very contrasting origins and characteristics that are deliberately combined (Bouzouidja et al, 2018). Thus, like most Technosols, they are then reactive media that can also be subject to often early and rapid pedogenesis Hence, considering the nature and composition of the green roof substrates, the environmental factors can modify the properties of the substrates of these green roofs. Few articles on the effect of these factors on Technosols are available in the literature, but there are a number of articles on various natural soils.

Rainfall events can produce crumbling effects, mechanical disruption of aggregates and compaction.

Macroporosity, pore size distribution and pore shape also varied. These processes occur on the surface horizon (0-3cm) but also on the layer immediately below (3-6cm) (Rousseva et al, 2002). Rain events can also leach nutrients from green roofs such as phosphorus or nitrogen (Akter et al, 2020). (Le Bissonais et al, 1989) found that for rainfall events of < 2 mm, degradation of soil aggregates occurs without particle displacement, but a rainfall event of 7 mm begins to dislodge microaggregates and cause them to enter the soil inter-aggregate spaces.

Moreover, the position of green roofs on top of buildings gives them the characteristics of a very special ecological niche, subject to constraints of remarkable nature and intensity (Sutton et al, 2012).

Indeed, the substrate of a green roof is disconnected from the surrounding natural soils and is therefore not connected to the natural regulation networks of the soil parameters. The dispersal capacities of soil fauna species do not all allow them to reach these niches and contribute to biogeochemical cycles within the green roofs. The soil, separated from some of its major components that provide essential soil functions for its structure, is gradually changing. Green roofs are also particularly exposed to the climatic agents mentioned above (e.g. rain, wind, solar radiation), which are themselves disturbed by the urban environment. The thermal inertia of buildings induces a temperature regime significantly different from natural conditions. Moreover, the establishment of a high to medium density of specific vegetation dominated by sedum species, and its control by weed control, creates specific conditions.

Most of the factors controlling pedogenesis (i.e. parent materials, climate, vegetation) are combined to favour a rapid evolution of their bio-physical-chemical properties during the ageing of green roofs.

These modifications over time, i.e. this early pedogenesis, is likely to result in a loss of performance and therefore of the services rendered by the green roofs. However, current results on the evolution of the properties of green roofs are controversial and often contradictory, reflecting a lack of understanding of the processes involved. For example, a decrease in organic matter in the substrates over time was noted by Emilsson et al, in 2007, while other authors have described an enrichment in both organic carbon and total nitrogen (Schrader and Boening, 2006). The water retention capacity of evolved substrates increased compared to a new substrate in the subjects studied by (Getter et al, 2007), while (Mentens et al, 2006) showed no influence of age on its hydraulic behaviour.

As mentioned before, the very nature of the components of green roof substrates resulting from the mixing of very different materials of natural (e.g. peat, pozzolan) or artificial (e.g. compost, bricks) origin gives them a potentially high reactivity Technosols -more particularly classified as "Histi-spolic

Objective

We have thus seen that several studies mentioned above have highlighted the contribution of green roofs in various ecosystem services such as reducing rainwater runoff, thermal insulation of the building and reduction of the urban heat island. Most of these studies conducted to date have attempted to characterize the effect of the thickness or nature of the substrate and vegetation type on these performances, neglecting the temporal dynamics of green roof evolution.

Beyond that, additional results suggest that these changes in properties lead to variations in the performance levels of green roofs. These phenomena are complex because the various studies contradict each other, as shown on the one hand by the drop in performance by (Bouzouidja et al, 2018) and on the other hand by the improvement modelled by (De-Ville et al, 2017) This could mainly be due to the fact that they were working with substrates of different compositions and origins and hence evolving differently.

In this context, the main objective of the thesis will be to describe the influence of different external factors (e.g., temperature regime, precipitation, biological activity) on the biophysical-chemical properties (e.g., poral architecture, organic matter concentrations, grain size, water properties) and the consequences in terms of green roof performances (e.g., water retention capacity, thermal insulation, physico-chemical fertility for plants). (Figure 2.4) shows how green roof evolution is an amalgamation of both ageing of materials and pedogenesis of its natural components, modifying its internal properties and process through external factor action thus potentially impacting its desired performance. Based on the analysis of the abovementioned sources it is agreeable that the originality of this thesis will lie in the fact that we consider green roof as a living, interactive and evolving media unlike the general approach of previous research which have mainly divided them into layers / material components and studied their individual effects.

Originality of the thesis

Throughout the course of the PhD, we shall try to cross/merge these two approaches between materials and living media to know the similarities, their interactions, as well as inter-relations between these two approaches. The idea is to be inspired by both approaches and reach a crossroad point between ageing --which is the term dedicated to evolution of inert materials and pedogenesis --which is the term that describes evolution of living media.

Scientific challenges

Through various stages of exploration including an in-depth literature review, an extensive metaanalysis, controlled condition experiments and in situ aged samples characterization, this PhD aims to

provide a relevant answer to the above raised questions. The work was focused on the scale of the green roof substrate sample (a few hundred grams) to the green roof plot (a few dozen m²).

• The first scientific challenge was to correlate the level of influence of external factors with the evolution of the properties. For this, experimental protocols under controlled conditions to reproduce and accelerate meteorological and biological factors observed in real conditions can be determined.

• The second challenge was to describe the evolution kinetics of the properties and performance of green roof substrates in the years following their implementation. It was then necessary to isolate the potential impact of each factor on the biophysical and chemical properties by implementing ageing tests under controlled conditions. So, in a second step, representative scenarios integrating all the factors were determined and implemented in microcosm to obtain aged samples of green roof substrate. Finally, explicit performance indicator measures were implemented. The properties and performance of green roof in real conditions, were used to ensure the validity of the experimental results obtained.

In the end, the thesis will provide new knowledge on the early pedogenesis of green roof substrates Each of these stages are presented as separate chapters [START_REF] Foley | Global Consequences of Land Use[END_REF]4,5). The final Chapter 6 concludes this part of the research work by summarising the findings, evaluating the work based on the objectives it started on and thus suggesting modifications/enhancements in the methods used and also paving a path for the future studies to be conducted to continue and improve the research in this domain. The constant expansion in terms of space and density of urban areas has numerous consequences on city infrastructure dynamics and surrounding environment mainly through impervious surfaces (such as roofs, paved driveways, roads, and paved parking lots) occupying a predominant place in these spaces (Lazzarin et al, 2005;[START_REF] Vergriete | Analyse préliminaire de l'aménagement du site du garage municipal de Ville de Laval[END_REF]. They replace the vegetated, pervious surfaces and create the danger of waterproofing cities, a phenomenon that promotes increase of surface runoff. Furthermore, fewer vegetated areas in turn cause a decrease in canopy interception and transpiration within the city, leading to increased temperature and decreased air humidity, leading to environmental issues, such as urban heat island and massive consumption and loss of energy [START_REF] Foley | Global Consequences of Land Use[END_REF].

Chapter 3

Since the 1950s and mainly in the past decade, to address such concerns, new forms of nature-based solutions for urban areas have been emerging. However, many of those techniques require availability of land space, which is usually not available in densely built downtown urban areas (Czemiel Berndtsson, 2010). In every city, the availability of roof area is very likely. Hence, among the techniques allowing introduction of nature into the city, green roofs hold a significant potential. Integrated into the building, they do not consume any additional space and can even be implemented on existing buildings (a traditional green roof can exert a load of up to 250 kg per m 2 ; however, the widely used sedum-based ones are 6 times lighter on an average and are suitable for most buildings; specific guidelines can be found in (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau, 2013), [START_REF] Breuning | Introduction to the FLL Guidelines for the Planning, Construction and Maintenance of Green Roofing[END_REF])), which is likely to encourage their massive diffusion. This transformation of the roofs with soil and vegetation is widely believed among the research community to contribute to counter the urban environmental problems mentioned above (Berardi et al, 2014).

After major technological developments, various scientific communities (e.g., hydrology, climate, ecology) have quite recently taken an interest in this field of study in order to evaluate its performances, in particular by characterizing some of them that are sometimes approached as ecosystem services, such as the regulation of heat and water flows or urban cooling. By studying green roofs in the UK, (Castleton et al, 2010) demonstrated their contribution to the insulation of buildings, with significant influence of the substrate properties but also of the actual thermal insulation systems. (Jaffal et al, 2012), (Battista et al, 2016) showed a moderate contribution to the reduction in annual energy demands as a function of the seasons. Oppositely, under tropical climate, (Rakotondramiarana et al, 2015) found insignificant contribution of green roofs to insulation and energy savings. In Sweden, (Villarreal and Bengtsson, 2005) demonstrated a positive impact of green roofs on the reduction of runoff water as a function of the depth of substrate and its moisture prior to the rain event but failed to highlight a correlation with the slope. Under various conditions, (Fassman and Simcock, 2012), (Stovin et al, 2012) measured a large range of water retention performance as a function of the substrates properties but also of the season and meteorological conditions. This last aspect and the consequent substrate's moisture were also strongly highlighted by (Sims et al, 2016) in three distinct climate regions. The capacity of green roofs to delay rainwater runoff has been less studied, but evidences of the potential contribution of green roofs has been noticed (Ryad Bouzouidja et al, 2018a; De-Ville et al, 2018).

Even if authors such as (Jelinkova et al, 2016),(McPhillips, 2018) have studied the temporal evolution of green roofs, it has to be noted that most other research have been conducted considering that green roofs are inert or abiotic systems and that their properties remain constant over time. However, not only the very nature of the components of their substrates but also the functions provided by green roofs require to consider them as soils and more specifically Isolatic Technosols, which makes them highly reactive and submitted to an early pedogenesis (R. Bouzouidja et al, 2018). As a result, a significant evolution of the porous architecture of substrates over time is noted, modifying their internal properties [START_REF] Kutílek | The structural porosity in soil hydraulic functions -a review[END_REF]. Beyond that, additional results suggest that these changes in such properties lead to variations in the performance levels of green roofs (Ryad Bouzouidja et al, 2018a; De-Ville et al, 2017).

Our paper aims to describe the influence of factors and properties of green roofs by extracting such factor-property-performance relations (defined in Section 2.1) from the previously conducted studies. Indeed, some researchers conducted various in-situ or controlled-conditions experiments or modelling approaches in order to study performances-at a certain time, some on ageing, some on the influence of the properties of materials. With such a diverse literature, meta-analysis can serve as a useful tool to parse through both main findings until now to moderate and evaluate what works best for a particular scenario and the ways to improve that approach for future studies. It can also serve as a go-to bibliography for further relevant studies. A fruitful review has already been conducted that showed the wide diversity of performances of green roofs across the world (Suszanowicz and Więcek, 2019). Our expected additional contribution is to qualify the factors and properties that would influence the performances of green roofs in order to: (i) highlight the most relevant levers that could be used to enhance their contribution to urban environmental issues and (ii) suggest future directions for research on green roofs.

Materials and Methods

Definitions and Frame of the Analysis

Performances of a green roof are the direct and indirect contributions and benefits to human wellbeing (Schade et al, 2021; Zhang et al, 2021; Zheng et al, 2021); more specifically, it describes the ability of a green roof to provide ecosystem services (Zheng et al, 2021). Some authors also mentioned them as "operational environmental cost-benefits" (Hashemi et al, 2015). In this work, we focused our attention on hydrological and thermal performances (Table 3.1). Such performances are supposed to be under the influence of factors and properties.

Retention is defined as the capacity of a green roof to limit the quantity of drained water as a function of the incoming water (rainfall) (R. Bouzouidja et al, 2018;[START_REF] Czemiel Berndtsson | Green roof performance towards management of runoff water quantity and quality: A review[END_REF] In other terms, it is "the amount of stormwater that will not become runoff [of] the roof"(Andenaes et al, 2018). The water is both retained into the substrate (due to capillary forces) or the drainage layer (depending on its configuration) and is later evaporated or uptake and transpired by vegetation. Detention is the capacity of a green roof to delay the peak of discharge of drained water as a function of the start of the rainfall thanks to its flow into the substrate and the drainage layer [20,24,28,29].

Insulation is the ability of the roof to "reduce the heat flux through a building envelope since the growing medium acts as an insulating layer" (Schade et al, 2021). Energy demand of a building is the amount of energy used for cooling and heating (Schade et al, 2021). Reduction of surface temperature is the ability of green roof to reduce the roof surface temperature, thereby helping in the reduction of energy needs of the building. Inner building temperature is the building's interior temperature, which can be dependent on the efficiency of green roof to prevent solar radiation from heating interior spaces of buildings (Schade et al, 2021).

From an historical soil science perspective, we chose to very clearly distinguish the characteristics from the environment of green roofs designated as factors and the characteristics of the green roof system designated as properties according to the seminal work from [START_REF] Jenny | Factors of Soil Formation: A System of Quantitative Pedology[END_REF].

Factor is an external characteristic of the environment of the green roof that may influence and affect its performances (Hashemi et al, 2015). They are related to weather conditions (e.g., air temperature, wind conditions, humidity); to the characteristics of rain events, such as its intensity, frequency, and duration; or related to building characteristics (Czemiel Berndtsson, 2010). A total of 25 factors were initially considered and progressively restricted to 12 (Table 3.2). The restriction was done after going through the available research articles and analysis the relevancy of the factors and availability of research material for the particular factor.

Property is here specifically defined as an internal characteristic of the green roof that may affect its performances. The properties can be related to the number of layers; the global, physical, and chemical characteristics of the substrate (e.g., depth, composition, porosity); and of the vegetation (type, density) [4]. It has to be noted that certain authors named these properties as "structural factors" (Gong et al, 2020;Liu et al, 2021Liu et al, , 2019)). A total of 33 properties were initially considered and progressively restricted to 18 (Table 3.3), similar to the methodology considered in restricting factors.

Tables depicting initial factors and properties considered before finalization are mentioned in Table 1 of Appendix A The term "performance relation" is used in order to express the way the studied factors and properties influence the different hydrological and thermal performances of the green roof. FPR refers to factor-performance relation, such as effect of rain intensity on retention. PPR refers to property-performance relation, such as effect of substrate depth on retention. FPPR refers to combined factorproperty-performance relations. Each influence of a factor on a performance studied by an author is considered as one FPR; similarly, each influence of a property on a performance studied by an author is considered as one PPR. Hence, a same study and a single experiment had the possibility to result in multiple FPR and PPR.

Influence indices (positive-P, negative-N, neutral-O) are the ways the factor/properties influence the performances. A positive influence would mean that the factor/property increases the performance provided by the green roof system. For example, an increase of the substrate depth was demonstrated to increase the retention capacity (Liu et al, 2019); hence, the property "substrate depth" will be considered by these authors as positive on the performance "retention". On the contrary, an intense rainfall can reduce the retention capacity of a green roof (Getter et al, 2007;Ladani et al, 2019); hence, the factor "rainfall intensity" will be considered by these authors to have a negative influence on the performance "retention". Determination of the influence indices was done by evaluating the details of the experimental observations of all studied papers and the way the authors expressed the influence to be positive/negative/neutral on any performance. Different papers could find different influence indexes for the same FPPR depending on what aspect of the performance the studied author concentrated on.

The green roof system can therefore be seen as a complex system that could be described by its "properties", considering on the one hand the nature and physical, chemical, and thermal properties of its abiotic components (i.e., substrate and drainage layers) and on the other hand its biotic components (i.e., planted or seeded and spontaneous vegetation, spontaneous fauna, and microbiota) (Figure 3.1). As in all biotic/abiotic systems, complex interactions happen. First, the external system-here described as "factors"-induces an ageing effect that results in the evolution of "properties" over time (e.g., rain may induce leaching of fine particles; cold temperature may alter the vegetation development). Moreover, interactions between abiotic and biotic components may also induce evolution of "properties" (e.g., plant litter may increase the organic matter content in the substrate; decrease of the substrate physico-chemical fertility can decrease the biomass production).

Figure 3.1 Green roof system's interactions between its global characteristics, internal properties, and external factors.

Eventually, it can be said that such inter-relations and interactions between all "factors" and "properties" control the level of performances that could be submitted to changes over time. Though the system is complex, our approach sheds light on the potential of simplifying each FPPR and hence understanding the system evolution and performances better (Figure 3.1).

Search Strategy

To find the articles that studied these FPPR, the Web of Science database was the chosen source of the data from the three initially considered sources (Web of Science, ScienceDirect, and Google scholar). The keywords were selected based on their popularity of utilisation among the authors in this field by trying out various possible and relevant words related to hydrological and thermal performances. The search was conducted using the following terms: [TS = (green roof* AND hydrological performance), TS = (green roof* AND hydric performance), TS = (green roof* AND thermal performance), TS = (green roof* AND thermic performance)].

The reference list of identified papers was initially manually checked for their relevancy based on their title and keywords and were filtered suiting the purpose of this paper (hydrological and thermal performances). After the relevant articles were selected (number of selected papers, N = 395), their abstracts and type of study were analysed, and any review / non-experimental based articles were eliminated (N = 187).

The remaining studies (N = 208) were selected if they met the following criteria:

The study conducted has at least one experiment/modelling approach where the authors studied at least one factor/property and tested the effect of this factor/property by comparing different values of it and directly mentioning the influence of factor/property on the hydrological/thermal performance of green roof and their influence index. Studies were excluded if they lacked any relevant information, either missing or ambiguous, as per the understanding of the author. Any other publication lacking primary data and/or explicit method descriptions were also excluded.

As a result, 100 articles were considered for the rest of this work. These are mentioned in Table A2 of Appendix B

Data Extraction

Data extracted from the eligible studies were the FPR and PPR and their influence indices. Other than this, the following information was also noted: (i) first author's name, (ii) first author's expertise (based on the theme of the published journal), (iii) journal aims and scope, (iv) year of publication, (v) study location, (vi) study design, (vii) climatic condition, (viii) type of study, and (ix) keywords.

Results

Characteristics of the Included Studies

Eligible studies were published between 1997 and 2020, with the biggest chunk of the studies in the past decade (80%) (Figure 3.2).

Majority of the studies were sourced from four countries, including the USA (12); the UK (11); France (9), whose research has been consistent throughout the past decade; and China (11), which has had a boom in green roof research in the last three years. Other than these, to a lesser extent, studies have also been conducted in Italy, Sweden, Spain, Belgium, the Netherlands, Madagascar, Norway, Brazil, Turkey, and Canada during the past decade.

While countries from Europe and North America have been contributing steadily throughout the past two decades, countries mainly from sub-tropical climate regions and Asian origin, such as China, Malaysia, Japan, Australia, Singapore, Israel, and Iran, have contributed significantly during the last half of the decade, thereby almost tripling the overall research conducted in this area. Before this subtropical influx, most of the studies were from the temperate maritime and continental regions.

Almost half of the studies (47%) were conducted using modelling approach (mostly since 2017), whereas 34% were in-situ experiments, and 19% were lab experiments (Figure 2). Until 2011, there was frequent but little research done as per our relevance. Since 2011, there has been a gradual increase, especially in modelling studies.

Though it is difficult to pinpoint the first author's expertise of the articles, it can be said that most of them are of hydrology background, followed by energy engineering, environment, soil, and horticulture, among other similar fields.

Total Number of External Factors and Internal Properties (Considered vs Studied)

Out of the 25 factors initially considered, based on the criteria explained in the materials and methods section, 12 factors were studied at least by a single article. Similarly, compared to external factors, more internal properties (33) were initially considered, and more (18) properties were actually studied by the authors. Properties related to the set-up of the green roof system (e.g., composition, substrate depth) and vegetation were comparatively well studied, whereas chemical properties, such as nitrogen content or chemical conductivity, were ignored by almost all the authors considered.

Influence of External Factors

Empty spaces are prominent (70.8%) in the FPR heat map (Figure 3.3), showing limited cases have been considered by the articles and that not all FPR are studied. In fact, overall studies conducted are also less than half that of the PPR. The same percentage of FPR (29.2%) for both the hydrological performances (7 out of 24 FPR studied) and thermal performances (14 out of 48) are studied. However, among the studied ones, thermal performances are studied better, with an average of three studies per FPR. Reduction of surface temperature is the best-studied thermal performance, with 15 FPR studies. Among the hydrological performances, retention is well studied, with 19 FPR. Detention and insulation performances are almost ignored. It can also be noted that there is not even a single specific factor that has been studied well for both hydrological and thermal performances. Factors related to rain have been decently studied for retention performance, , with seven studies. The studied factors are also considered to a much lesser extent, with only six FPR studied more than four times overall. On an average, each nonempty FPR is studied by 2.4 publications.

Figure 3.3 Heatmap depicting external factors influence on performances (FPR).

Only two factors (solar radiation and height of the building) have been studied for their influence on at least three performances. The average here is two.

Influence of Internal Properties

Even if empty cells are still found in the PPR Heatmap (Figure 3.4), they are much less prominent compared to FPR (42%), suggesting that they have been better studied.

Figure 3.4 Heatmap depicting internal properties influence on performances (PPR).

The performances are more uniformly studied, with at least five properties being studied for all the performances. Among the hydrological performances, retention and detention are equally well studied. Thermal performances have been less studied, with the reduction of surface temperature being the best studied. Relating to substrates, substrate nature and physical properties, in particular composition and depth, have been studied the most by the authors overall considering all the performances, with it being mentioned 26 times and 38 times, respectively. On an individual scale, substrate depth's influence on the retention performance is the most studied PPR, with authors studying it 17 times.

Relating to vegetation, two properties (percentage of vegetation and leaf area index) have been studied for their influence on at least four performances. The average here is two. Among the ones studied, percentage of vegetation has been studied the most by the authors overall with respect to performances, with it being studied for 40 PPR, making it the most studied property. On an individual relation scale, leaf area index's influence on the reduction of surface temperature and percentage of vegetation's influence on retention are the most studied PPR, with authors studying it 11 times and 13 times, respectively. In a rather contrasting manner, while substrate and vegetation properties are vastly studied for all the performances, others have had a great decline, with them being studied for just one or two performances. On an average, each nonempty PPR is studied by 2-3 authors.

By considering six performances and a final total of 30 factors and properties, there was a possibility of 180 FPPR studies overall. As presented previously, out of these possibilities, only 82 have been studied at least once, leaving the other 98 relations (54.4%) unexplored.

Factor-Performance Relations Influence

It can be seen that the studied factors have a major negative influence on the performances, especially the hydrological performances, owing to the rainfall prominently (e.g., the highest is the rainfall intensity; the worst is the retention performance of the green roof) (Figure 3.5). In fact, 90 % of the hydrological FPR have negative influence. With regard to thermal influence, the positive influences are more noticeable, with 45 % and 55 % being negative. Only one FPR was found to have a neutral influence. Building-temperature-related FPR has the most positive influence, with 55%.

Figure 3.5 Influence index of FPR on respective hydrological and thermal performances, showing largely negative influence.

In general, it can be seen that most studies are in accordance with each other on the type of influence. In one case (building slope), there is a difference in opinion among the studies with respect to their influence in the hydrological-retention performance study.

Property-Performance-Relations Influence

Contrary to the influence of factors, the studied properties have an almost completely positive influence (95%). Results are very consistent for all performances regardless of whether they are hydrological or thermal or regardless of whether they are very well studied (such as retention) or less (such as insulation). (Figure 3.6).

Figure 3.6 Influence index of PPR on respective hydrological and thermal performances, showing largely positive influence.

The difference between various authors in type of influence indices in the PPR studies is higher than that of FPR, with 5 out of 18 PPR having differences in their type of influence. Here, along with four cases of retention (diversity, percentage of vegetation, composition, and water-holding capacity), there is also one case of insulation (layering) where difference arises.

Percentage of vegetation, the highest studied PPR, has only one disagreement (in retention). Leaf area index, porosity, and substrate depth, the other well-studied PPR, have 100 percent homogenous observations, each being studied for all the performances and having positive influence on all of them. Diversity of vegetal species is the most diverse FPR with respect to influences, with studies showing it can influence retention performance positively, negatively, and neutrally, too.

Overall, with only five FPR studies among all the studies not being in accordance with others, it can be said that the type of influence is highly homogenous.

Discussion

Identification of Levers to Enhance the Performances of Green Roof

Our main findings highlighted that most properties have a positive influence on the performances of green roofs, showing there are many existing levers to enhance green roof performances and tackle some of the main issues already mentioned, i.e., flooding and urban heat island.

A few of these could easily be controlled by manufacturers and have been largely mentioned already, such as increasing the depth of the substrate, which could help increasing the retention and detention of water alongside contributing to insulation and consequently to the decrease of energy demand and inner building temperature in hot periods. Substrate composition and layering of various materials are undoubtedly another key parameter to control the level of performances provided by green roofs (Baryła et al, 2018; Eksi and Rowe, 2019). However, the relations between them and performances remain complex to establish, considering that it is highly dependent on many internal properties (nature and proportion of the substrate's parent materials) (Yio et al, 2013) and external factors, such as the climatic conditions (Mentens et al, 2006;Stovin et al, 2015). Further research that could help in predicting the contribution of various components and their association on green roof performances would be highly needed. Vegetation also appears as a valuable lever to increase performances in relation with their physiology (i.e., transpiration, shadowing, biodiversity habitat). Authors such as (Ouellet and Charron, 2013) have already mentioned the importance of choosing local and native plants and of maintaining an optimal vegetation development through a possible input of fertilizers (Suszanowicz and Więcek, 2019).

Some other properties have a significant impact on performances but are more difficult to handle, as they are not directly manageable. For example, the higher the porosity, the higher the retention and detention of water (Getter et al, 2007;Graceson et al, 2013) and the better the insulation (DelBarrio, n.d.). Similar thoughts could be made on bulk density or water-holding capacity. However, such physical characteristics rely not only on the properties of each constituent of the substrate but also on the result of their formulation to design the substrates. In addition, a change over time in the chemical (lower pH, higher organic carbon, and total nitrogen levels) and physical (settlement) characteristics of different green roof substrates was identified by (Schrader and Böning, 2006) without the consequences of these changes on hydraulic properties being measured. As a consequence, further research on the physical, hydrological, and thermal properties of substrate would be required in order to enhance the way they control the behaviour of green roofs (Lin and Lin, 2011; Liu et al, 2019). More globally, the relations between substrates fertility (i.e., concentration in nutrients, organic matter content, water available for plants) are also strongly correlated with all characteristics related to vegetation. As a consequence, an agronomical approach of green roofs appears promising to describe the interrelations of such properties.

Finally, factors are, by definition, absolutely unmanageable, as they are dependent on either climatic, meteorological, and building properties or the ecological continuum (Johannessen et al, 2018;Stovin et al, 2015). Based on that, a deep work on the adaptation of the properties of a green roof to its environment should be promoted (Viola et al, 2017).

Filling the Gaps: The Exploration of the Unstudied Relations

Previous experimental and modelling research has explored various relations between factors or properties and hydrological or thermal performances of green roofs. However, more than half of the potential relations we identified-considering that our lists were far from exhaustive-remains unexplored. Even if they do not have all the same relevancy, many outlooks emerged from such a review analysis. (González-Méndez and Chávez-García, 2020), in their study, highlighted the need and complexity of green roof designing due to unmanageable factors and time-and location-specific constraints. As more and more FPPR are explored, the complexity of green roof becomes more manageable with regards to designing for optimal performance. This is mainly possible, as through this approach instead of the whole system redesign, specific factors and properties can be controlled and monitored. Most research through the years, such as (Mentens et al, 2006;Sandoval et al, 2015), have also highlighted the need for specific approaches and expansion of the green roof horizon. But it seems that due to design and implementation constraints, only the tried and tested approaches, factors, and properties are preferred.

Concerning the impacts of the studied factors, it appears that the ones that are related to hydrology (i.e., rain characteristics) are dominantly separated from the ones related to heat fluxes. Indeed, the combined effects of thermal and hydrological behaviour of the green roof on its performances remain largely unknown. For example, the effect of dry conditions prior to a rain event are known to have an impact on both water and heat fluxes into the substrate as well as, more specifically, the impacts of the temperature or the wind speed on the plant transpiration (Mentens et al, 2006). At the scale of the whole building, the consequences of its various characteristics are complex to capture with experiments and mainly rely on modelling approaches, which require specific skills (Cascone et al, 2019).

The study of green roof properties appears complex, as all the mentioned characteristics are not only correlated but also evolving over time. The very nature of artefacts, dominant components of green roof substrates, are well known to be extremely reactive to environments, which can result in intense and rapid changes in their properties (Séré et al, 2010). Apart from its own effects, vegetation could lead to an evolution of Technosols' physico-chemical characteristics (Cheik et al, 2019), and so could the soil fauna and even microorganisms. Various studies, such as (Ryad Bouzouidja et al, 2018a; De-Ville et al, 2017), already suggested that the temporal changes in properties lead to variations in the hydraulic performance levels of green roofs. Again, relations between various chemical properties of green roof substrates (e.g., relative abundancy of organic and mineral contents, availability of nutrients) and their dynamic over time that control the development of vegetation appear as a major gap in the study of the green roof performances. As an explicit example, fine particles' eluviation from substrates surface have been monitored on green roofs, which may lead to physical constraints for plant growth (R.

Bouzouidja et al, 2018).

There is a need for studies that consider both approaches and reach a crossroad point between ageing-which is the term dedicated to evolution of inert materials-and pedogenesis-which is the term that describes evolution of living media-to know the similarities, their interaction, as well as inter-relation between these two approaches.

Promotion of Deeper and Innovative Experimental Approaches for Research on Green Roof Performances

Everybody seems to agree and appreciate that green roofs are beneficial, but as seen in this article, scientifically speaking, there is very less validation with regard to it yet. There are many studies about green roofs that were not adequately designed on purpose to study the dedicated effect of factors or properties, owing to the fact that they either considered the green roof as a whole system, such as (Oberndorfer et al, 2007), or limitations in specific approach development, such as (Zhang et al, 2019). There is a need to develop specific approaches relevant and customized to the green roof scale and behaviour to explore the green roof potential to the fullest extent possible. In their review, (Czemiel Berndtsson, 2010) clearly echoed the same sentiment: that there is a need for more research into green roof performance in an urban environment, as the differences measured by few existing studies between the early years' performance of green roofs and the later years indicate a need for long-term monitoring of green roofs. (Andenaes et al, 2018) also highlighted there is scarcely any literature on the operation of full-scale, building-implemented green roofs, and no articles were found on the building technical performance of aged green roofs. That is not only a risk factor that fails to evaluate long-term green roof performance but also an utter disregard to the potential improvements in efficiency of green roof performances by not exploiting it.

In-situ experiments are very rich in terms of data that could be monitored and harvested under real conditions (Schindler et al, 2019;Soulis et al, 2017). Therefore, it is consistently prominent in the past decade considering its longevity. On the downside, they need long-term observation and adapted monitoring devices to be implemented at the beginning, and hence, even if large-scale data are available, their exploitation can be time consuming, tedious, and impractical. Therefore, the definition of some shared protocols and knowledge about good practices is expected. Then, implementing longterm observation sites under various situations would also serve as a reference to all the concerned studies. However, it is found that the vast majority of in-situ green roof research conducted has been on smaller green roof test beds or isolated components (Andenaes et al, 2018). This is an interesting observation because, as this paper highlights, pedogenesis and evolution of green roof components have a great potential to modify the performance over time. However, the test beds are usually discarded/not maintained after the experiments, hence denying the possibility of temporal evolution analysis.

Comparatively, lab-scale experiments are promising [16,17,20] as they, under controlled conditions, could reproduce in a very specific way some variations of factors and properties. However, they might not be multipurpose and very specific to certain properties or factors. Unfortunately, this is the least-utilized method for now considering the difficulties in designing them. Hence, there is a need to design and create such lab prototypes to understand the influence of each relation and also in order to lighten the long-term experiments, too.

Considering various implementation and monitoring complexities in the other two approaches, the modelling approach has been the go-to suggestion by most researchers and reviewers in the past three decades, showing its potential (Mentens et al, 2006;Sandoval et al, 2015). However, the existing modelling approaches are not perfectly suited to the evaluation of green roof performances, as they are more focused on the thermal and hydrological processes (i.e., transfer and fluxes). (Quezada-García et al, 2020) noted that currently, there are many mathematical models in green roofs, but as each one of them has their own notation, it complicates the study of this field. Even if modelling approaches have the liberty to simplify systems, a more realistic and green-roof-specific consideration can be adapted. For example, modelling approaches do usually consider 100 percent vegetation on green roofs, which is rarely the case owing to maintenance issues and easy foreign matter growth. (Sandoval et al, 2015) also highlighted the need for a green-roof-specific approach because of the complex interactions between the hydraulic and thermal processes that occur in these systems.

From Mono to Transdisciplinary Research about Green Roofs

Throughout the study, it was noted that all the research was made by the authors who had specific interests and studied mostly that specific aspect. Their interest would be either about hydrological performances or thermal performances, rarely considering both performance groups together and not specifically related to green roofs (Piro et al, 2018). A broader, transversal, and complex study could lead to a more reliable and precise estimation and assessment of FPPR.

The expertise and interests of the published authors have also confined them to more prominent properties, such as the substrate composition and the vegetation. They have been demonstrated to have direct influence on the performances, and hence, most of the studies concentrated on them, as they were easier to characterize, ignoring the other less common but possibly impactful study possibilities. Despite the study of many different substrates with various composition, almost no research has yet been conducted to establish the clear relations between physical properties (e.g., bulk density, particle size of the components) or chemical characteristics (e.g., organic matter or nitrogen contents) and performances (Conn et al, 2020;Mitchell et al, 2018).

Other disciplines could help in understanding the various properties and factors that influence the behaviour of green roofs, such as ecology, biology, soil science, agronomy, and architecture (Semeraro et al, 2021). In this regard, (Suszanowicz and Więcek, 2019) suggested a research on finding the coefficients of the impact of green roofs to universalize the effect of green roofs on performances, as the results presented by different authors (most often based on a single or limited number of case studies) differ significantly from each other, owing to change in the climate, geography, study concerned, etc.

Conclusions

This paper first contributed to the definition of a framework likely to be considered to describe the intrinsic complexity of green roofs as biotic-abiotic systems-at the crossroad between architecture and horticulture-and its relation with external factors, notably meteorological conditions, that contributed to multiple ecosystem services, here focused as hydrological and thermal performances. As part of the drastically increasing scientific interest for green roofs, our meta-analytic review highlighted many researches dedicated to the impact of green roof properties and factors on such performances. Even if these properties-performance relations are seemingly more important, the comparative ignorance of more than half of the other possible factors-properties relations also holds important implications considering their potential in defining green roof performances. It could also be seen that the studied factors have a major negative influence on the performances (68%), whereas the studied properties have a major positive influence (95%), which highlights that even if the green roof is a complex system, there exists different levers to be considered for performances enhancement. The trend of more and more researchers opting for modelling approaches in the past decade also paves way for further studies with promotion of deeper and innovative experimental approaches for research on green roof performances, especially in real site conditions to understand the performance changes in a realistic way. Even if this can be a tedious job considering the difficulties in design, possible inter-relations, and time consumption, it is the need of the hour considering their possible impact in improving green roof performances and easing the design process. The first step regarding this could be developing clear protocols for particular factors-properties-performance studies to simulate real site conditions. This can ideally be suitable to be able to implement in multiple climatic conditions by modifying certain parameters based on the regional characteristics. Along with this, there is a necessity to develop longterm observations for better understanding and control of the evolutionary and early paedogenic performance changes.

Factors or Actors?

A controlled condition accelerated ageing experimental evaluation of influence of individual external factors on the green roof pedogenesis.

Abstract

Purpose:

The objective of this study was to improve knowledge on the evolution of certain major bio-physicochemical properties of green roof substrates over time, under the influence of three main factors (rain, temperature, vegetation) through experimental study under controlled conditions. The experiment was conducted in mesocosms by monitoring the properties of the substrates over time.

Methods:

Three substrates with different compositions representative of the actual diversity of green roofs were tested in controlled conditions with artificial ageing of the substrates applied over a period of 2 months. Three main evolutionary factors were experimentally reproduced in order to accelerate their early pedogenesis: rain simulation, freeze-thaw simulation, vegetation simulation. For each substratefactor association, 3 replicates were created in order to have statistically accurate results. In addition, 3 harvesting times with sacrifice of boxes to perform the measurements. A total of 108 mesocosms were created (3 substrates x 4 modalities x 3 replicates x 3 time periods). The substrate properties (bulk density, WHC, organic content, porosity, granulometry) were measured over different time periods.

Results:

The study revealed traces of early pedogenesis. Substrates, albeit in a different scale, seems to undergo a marked evolution, It appears that the composition and nature of the substrate is an essential parameter of this evolution as the evolution pattern of finer P substrate is different to that of coarser I and X. Depending on the substrate, Vegetation stimulates or maintains microbiology, rain and frost modifies granulometry through fractionation and leaching processes. There was also change in the poral structure thus modifying the retention performance. In fact, all the significant changes (>25 %) were observed in volumetric water content of the water retention curve (WRC Retention performance showed considerable decline for all substrates compared to the reference samples. Other than that, the changes in the performance of other studied properties seem more due to the temporal evolution rather than factorial based where most changes occur at T2 (up to T4 in some cases) and tend to stabilise after.

The results obtained and also further analysis on previous research also showed a contradiction between in situ and non in situ studies in results obtained in property evolution emphasising the need to improve non in situ studies to be better in sync with the real conditions and behaviour.

Introduction

State of the art

The substrate of green roofs can be described as "Technosol" according to the World Soil Classification (IUSS 2014). This soil type is characterized by soil properties and pedogenesis that are dominated by technical origin., as evidenced by the substantial presence of artefacts, impermeable constructed geomembrane or technical hard rock (Rossiter 2004;[START_REF] Lehmann | Technosols and other proposals on urban soils for the WRB [World Reference Base for Soil Resources[END_REF]). Indeed, green roofs' substrates are composed of a mixture of extracted and transported materials of natural origin such as pozzolan or peat and man-made products such as bricks and compost; all of which are defined as 'artefacts' (IUSS 2014). Green roofs can specifically be classified as Isolatic Technosol (Drainic, Folic and Transportic) (Bouzouidja et al, 2018). Like many other Technosols, they are a reactive media due to their high porosity, the heterogeneity of their parent materials that were deliberately combined and their exposition to environmental factors (i.e. biological activity, meteorological parameters). As a consequence, they are subject to a potential early and rapid pedogenesis (Séré et Property is here specifically defined as an internal characteristic of the green roof that may affect its performances. Here, we call "substrate properties" all the bio-physicochemical parameters intrinsic to the substrate such as pH, granulometry, density of microorganisms, etc. It has clearly been demonstrated that different values of a property induced different level of performances (e.g. the deeper is the substrate, the more efficient is the green roof to retain water) [START_REF] Hanumesh | A roof of greenery, but a sky of unexplored relations-Meta-analysis of factors and properties that affect green roof hydrological and thermal performances[END_REF]. Furthermore, the evolution of green roof properties, during early pedogenesis, is also likely to influence the green roof performances over time. A study published in 2016 by Bouzouidja et al, suggested a decrease in the hydrological performance of a green roof following a change in the physical organization of its substrate (e.g., porosity, soil structure).

Subsequently, vegetation has many effects that all act on properties which eventually influence hydrological and thermal performances such as: albedo effect, evapotranspiration rate [START_REF] Blanusa | Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer better 'cooling service[END_REF] and even water retention rate (Dunnett et al, 2008[START_REF] Lundholm | Plant species and functional group combinations affect green roof ecosystem functions[END_REF], Nagase and Dunnett 2012) Vegetation, although an integral part of the green roof and thus a property by definition, will be considered in this study as a factor modifying the properties of the soil. Indeed, as this study focuses on the substrate and everything that can alter, modify, or change it, making it more interesting to consider it as a factor because during its growth, many of its actions can greatly modify the properties of substrates., particularly the microbial biomass and organic carbon content [START_REF] Bürgmann | Effects of model root exudates on structure and activity of a soil diazotroph community[END_REF] The environmental factors (other than vegetation) tested that could potentially change the properties of the substrates are rainfall events, temperature variations and especially freeze/thaw cycles of the substrate. Few articles on the effect of these factors on Technosols are available in the literature (mentioned in chapter 3), but there are several articles on various natural soils. Considering the very limited number of research on the relations factors have on properties on the green roof, the frame of analysis has been enlarged to all types of soils Freeze-thaw cycles can, for example, increase nitrogen mineralization in natural soils (DeLuca et al, 1992). In controlled laboratory incubations, freeze-thaw cycles decreased the stability of soil aggregates, particularly at high soil moisture, and these effects accumulated over successive cycles [START_REF] Oztas | Effect of freezing and thawing processes on soil aggregate stability[END_REF]. It has also been shown that the consequences of freeze/thaw cycles are more severe for macro-aggregates than for micro-aggregates [START_REF] Six | A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics[END_REF]. Other studies have shown that erosion is accentuated by freeze/thaw cycles and when humidity is high [START_REF] Chow | Seasonal distribution of runoff and soil loss under four tillage treatments in the upper St. John River valley New Brunswick, Canada[END_REF][START_REF] Ferrick | Quantifying the effect of a freeze-thaw cycle on soil erosion: laboratory experiments[END_REF].

Zhang et al, (2012) found that the changes in vegetation type may have largely influenced carbon accumulation in the soil. This process could seriously influence soil organic matter content and soil bulk density (Wang et al 2013) and thus eventually the retention performance. Vegetation, and in particular root exudates, can lead to organic carbon loss phenomena due to strong microbial development [START_REF] Keiluweit | Mineral protection of soil carbon counteracted by root exudates[END_REF] but also increase the stability of aggregates (Traoré et al, 2008). Root exudates allow, for the plant, to regulate the composition of the microbial and fungal community [START_REF] Broeckling | Root exudates regulate soil fungal community composition and diversity[END_REF]. It was described by Bürgmann et al, in 2005 that artificial exudates and simple sugars induced rapid nitrogen fixation and the development and activation of multiple strategies in different microbial species.

Rainfall events can produce crumbling effects, mechanical disruption of aggregates and compaction. Macroporosity, pore size distribution and pore shape also varied. These processes occur on the surface horizon (0-3cm) but also on the layer immediately below (3-6cm) (Rousseva et al, 2002). These poral restructuring can have an impact on retention performance. Rain events can also leach nutrients from green roofs such as phosphorus or nitrogen (Akter et al, 2020). Le Bissonais et al, (1989) found that for rainfall events of < 2 mm, degradation of soil aggregates occurs without particle displacement, but a rainfall event of 7 mm begins to dislodge microaggregates and cause them to enter the soil interaggregate spaces.

The analysis of the scientific literature revealed that factors controlling pedogenesis (i.e. parent materials, climate, vegetation) are combined to favour a rapid evolution of their bio-physical-chemical properties during the ageing of green roofs. However, current results on the evolution of the properties of green roofs are controversial and often contradictory, reflecting a lack of understanding of the processes involved. For example, a decrease in organic matter in the substrates over time was noted by Emilsson et al, in 2007, while other authors have described an enrichment in both organic carbon and total nitrogen (Schrader and Boening, 2006). The water retention capacity of evolved substrates increased compared to a new substrate in the subjects studied by Getter et al, 2007, while Mentens et al, (2006) showed no influence of age on its hydraulic behaviour.

Objective

The influence of different factors on the processes such as the characterization of the intensity and kinetics of these variations remains largely unknown for green roofs.

The objective of this study was therefore to improve knowledge on the influence of three main factors on the evolution of the properties of various representative green roof substrates over time, under controlled conditions. The substrates (Figure 4.2 and table 4.1) are chosen considering their varied properties and granulometry as the literature review and meta-analysis both highlighted that substrate type and components are one of the most influential parameters of green roof pedogenesis owing to their different potential pedogenesis)

The research work was thus carried out around the following problematic:

What are the properties of the substrates modified over time by these factors and to what extent? (Figure 4.2)

Hypothesis

On the basis of the literature review, several hypotheses are put forward: H1: Depending on the nature and composition of the substrates, the properties will evolve differently. H2: The vegetative cycle will improve microbiological activities and will induce changes in organic carbon and total nitrogen concentrations. H3: Rain events will cause vertical transfer or leaching of particles. H4: Successive freeze/thaw cycles will modify the particle size of organic and mineral constituents, especially the coarser ones.

The purpose of this study was to reproduce certain factors considered as influential (i.e. rain, vegetation and freeze/thaw alternation), under controlled conditions, on mesocosms of vegetated roofs by following the properties of the substrates over time. Artificial ageing of the substrates is applied over a period of 2 months. The intention behind accelerated ageing was to mimic the real time green roof evolution of multiple years within a few weeks in the lab scale, so that this protocol would help future researchers too to understand green roof evolution through less tedious experiments.

Materials and methods

Substrates studied

Three types of substrates were tested here, all from the same supplier (SOPREMA®): Sopraflor P, Sopraflor I and Sopraflor X The substrates have different compositions and properties (Figure 4.3, Table 4.1, Appendix ii, Appendix iii) with substrate I and X being similar compared to P. The intention was to select substrates of contrasting composition and granulometry (P vs I and X) so that their evolution patterns can be compared, but also substrates of similar composition but in different percentages (I and X) to understand their evolution and impact ability.

Substrate P is composed of brown peat humus, aged bark, sand, compost, calcite lime, gypsum, magnesium oxide and fertilizer.

Substrates

Description of mesocosms and numbers

The mesocosms reproducing the green roofs consist of a plastic box (175x366x100mm), a piece of geotextile (325x439x4 mm) covering the entire walls and bottom of each box. The substrate is brought without compaction to a height of 8 cm; the initial masses vary according to the type of substrate (2.1 kg for the P and I substrates and 2.3 kg for the X substrate). Each mesocosm is then identified with a unique label.

Three substrates as well as four factor modalities were tested: rain simulation, freeze-thaw simulation, vegetation simulation and a reference modality. For each substrate-factor association, 3 replicates were created in order to have statistically accurate results. In addition, 3 harvesting times with sacrifice of boxes to perform the measurements, increased the number of mesocosms. A total of 108 mesocosms were created (3 substrates x 4 modalities x 3 replicas x 3 times).

The mesocosms for the reference, vegetation and rain models were placed on shelves in a room that is hermetically sealed from solar radiation and where the temperature is stable and monitored by a sensor. The mesocosms for the freeze/thaw modality were placed in a thermostatically controlled chamber. A ventilation system was present to promote evaporation and limit the development of moulds due to overly stagnant air.

Substrate P Substrate I Substrate X

Applied treatments and factor simulation

All mesocosms were therefore maintained at the same content throughout the experiment. For each of the 108 mesocosms, the water masses were calculated implying that the green roof was at a constant matrix potential corresponding to 80% of the moisture at pF 2 (pressure of 0.1 bar) (including geotextile and substrate). Every 2 or 3 days, water was added to maintain this objective. This was done by adding the necessary water under the geotextile which would then spread by capillary action, except for the rain factor test. This technique did ensure there was no creation of any erosion effect, leaching, etc.

Reference (27 mesocosms)

The reference samples, to understand the evolution on natural conditions, were not subject to any additional individual factors, they were let at natural temperature, water supply was capillary action and there was no vegetation simulation.

Rain simulation (27 mesocosms)

The quantity of water necessary to maintain the water content was supplied here, but not by capillary action as in the other experiments. It was done through a system simulating rain (Figure 4.4) through a peristaltic pump. Rain was simulated every Monday and Friday in order not to saturate the mesocosms with water. The rain episodes lasted 30 seconds in total, 15 seconds on each half of the devices, and were equivalent to a flow rate of 885 L.m-².h -1 .

Freeze-thaw simulation (27 mesocosms)

Mesocosms for testing abrupt temperature changes were placed in a climate chamber. The temperature cycle applied was as follows: -10 °C for 2 days and then 20 °C for 2 days.

Vegetation simulation (27 mesocosms)

During the vegetation simulation, two processes were studied. First, at the beginning of soil ageing, artificial root exudates were added to the mesocosms with the irrigation water. The artificial exudates solution was made at a rate of 100 mg COD.kg After one month of experimentation, 70 g of sedums were added to each mesocosm to mimic a litter deposit. The sedums were collected from the experimental green roofs of the Cerema. They were previously crushed in order to mimic the action of detritivorous organisms not present in the mesocosms and thus force the degradation process.

Sacrifice time

Before initially adding the substrate to the mesocosms, they were homogenised through manual mixing and then evenly spread out in a large and high chamber and air dried for 48 hours. After two weeks (t+2 weeks), the 9 mesocosms intended for each modality were sacrificed (36 in total) to carry out the measurements of our 9 parameters. This was similarly repeated, after one month (t+4 weeks) and two months (t+8 weeks) (Figure 4.5) Where ρa and ρr are respectively bulk and solid density (kg m -3 ).

The soil water retention characteristics, i.e. volumetric water content at various matrix potential, were determined on a pressure plate apparatus at saturation, (Pf 0), -10 kPa (pF 2, that was considered to be moisture at field capacity: 𝜃 𝐹𝐶 ), -33 kPa (pF 2.5) [START_REF] Bruand | Backscattered electron scanning images of soil porosity for analyzing soil compaction around roots[END_REF]. Drainage and Macroporosity (D+M), Mesoporosity and Microporosity (m+μp) were estimated according to (Peverill et al, 1999) (Eq.4.2 and Eq. 4.3):

𝐷 + 𝑀 = 𝛿 -𝜃 𝐹𝐶 (Eq.4.2) 𝑚 + 𝜇𝑝 = 𝜃 𝐹𝐶 (Eq.4.

3)

The granulometry of the soil is measured by sieving the entirety of each layer of soil sampled. The sieving is carried out on dried samples (40°C for 24 hours) thanks to 2 mm and 5 mm sieves, thus giving access to the percentage of particles < 2mm, between 2 and 5mm and finally > 5mm. It is important to specify that during this measurement each of the aggregates is forced to pass through the meshes thanks to a pressure and a circular movement breaking the less solid ones.

Fluorescein di-acetate (FDA) is a molecule that can be hydrolyzed by many microbial enzymes including esterases, proteases and lipases. FDA hydrolysis is therefore a generalist enzyme activity, with a much broader spectrum of action than other specific enzyme activities and is thus widely accepted as an accurate and simple method for measuring the overall microbial activity of soils. The term "metabolic activity indicator" or "microbial activity index" (MAI) is often used because FDA hydrolase correlates well with microbial biomass and is a good indicator of organic matter supply (Adam and Duncan 2001).

It is measured by determining its reaction product which is fluorescent. After 1h45 of incubation with agitation, the filtrate is recovered. Absorbance measurements are performed with a spectrophotometer at 490nm and compared to a standard range.

This protocol allows a measurement of the carbon and nitrogen microbial biomass of a soil. The notion of microbial biomass covers all the micro-organisms in the soil (bacteria, fungi, etc.). It is a "biocidal" method, consisting in measuring the carbon (or nitrogen) contained in the living beings of the soil by fumigation-extraction using chloroform vapors. The difference in soluble organic carbon or nitrogen between fumigated and non-fumigated samples gives the amount of "extractable" carbon or nitrogen of microbial origin. This amount is directly proportional to biomass [START_REF] Wu | Measurement of soil microbial biomass C by fumigation-extraction-an automated procedure[END_REF].

After incubation of our fumigated samples (10g) for 24 hours at 30°C in a vacuum desiccator with chloroform vapors, a K2S04 solution was added to all samples. The samples are then stirred for 45 min and filtered. The filtrates are analyzed with a liquid phase dissolved organic carbon and nitrogen analyzer (TOC). The carbonaceous microbial biomass (BM-C) is then calculated as the difference between the soluble organic carbon of the fumigated samples (CF) and that of the non-fumigated samples (CNF).

In order to measure the water pH of a soil (NF ISO 10390) the test samples (10g of unground soil) are added to 25mL of water and then shaken for 1h. The samples are then left to rest for at least 1h and at most 3h. The pH readings are taken with a pH meter after the soil particles are resuspended and homogenized.

The organic carbon and the total nitrogen of each layer is realized by dry combustion (NF ISO 10694 and 13878). The soils to be analyzed are previously finely ground (< 250 μm). Tin capsules are created containing a precise mass of soil to be analyzed by combustion. The capsules for organic carbon analysis are enclosed in silver capsules and decarbonated with a hydrochloric acid solution (i.e. remove the carbon in mineral form: carbonates). i.e., all the results of each time period were normalised with that of the reference sample at T0. On a global view, major chunk of the blocks (60.8 %) is in the ratio of 0.8-1.2, indicating that in most cases there is no drastic difference compared to the initial reference sample. In fact, 100 % of properties related to porosity, density, pH and granulometry stay within this ratio, irrespective of the substrates, factors, and time periods. All the significant changes (>25 %) are observed in volumetric water content of the water retention curve (WRC) where the ratio goes as low as 0.2 (Ө = 0.3 at T8 for the vegetation sample of substrate I). Majority (72 %) of the values stay between the ratio 0.4-0.6 here.

Results

Global evolution visualisation

Considering the time periods, T4 seems to undergo most changes (more blocks outside 0.8-1.2 ratio range) compared to the other two time periods. There is a trend indicating gradual changes deviating away from initial value at T2 and T4 and then slight inclination towards the initial values again at T8. This is especially noticeable in WRC.

Considering the substrates, the evolution behaviour is different for different substrates depending on the property considered. For instance, substrate I has more pronounced evolution for WRC, followed by X and P respectively. Whereas, in contrast, substrate P evolution is more pronounced in properties related to pH, density and porosity, followed by X and I respectively.

Considering the factors, the influence seems to be uniformly dispersed depending on the property. However, it can be noted that the factors influence substrates I and X in a similar way compared to P. %) is in the ratio of 0.8-1.2, indicating that in most cases there is no drastic difference compared to the reference sample of that particular time period. In fact, again, 100 % of properties related to porosity, density and granulometry stay within this ratio, irrespective of the substrates, factors, and time periods. All the considerable changes (>25 %) are observed in water retention curve (WRC). However, this time, wherever the changes are noticed, the influence is not just negative (reduction) like in figure 4.6. The ratio goes as low as 0.5 (Ө = 0.3 at T8 for substrate X submitted to precipitation) but also as high as 1.4 (Ө = 0.3 at T4 for substrate P submitted to freeze/thaw and Ө = Өsat at T8 substrate I).

Considering the time periods, T8 seems to undergo most changes (more blocks outside 0.8-1.2 ratio range) compared to the other two time periods which is mostly positive in WRC and mostly negative for other properties.

Considering the substrates, the evolution behaviour is different for different substrates depending on the property considered. But Substrate I and X have comparatively similar evolution, different to P.

Considering the factors, the influence seems to be uniformly dispersed depending on the property. However, it can be noted that vegetation has higher influence for substrate I and precipitation for substrate X, compared to others. No significant evolution over time is observed. However, for density, it can be noted that, albeit small, all 3 substrates evolve differently, especially after time period T2, which can be noted in the bulk density graph. While the evolution of reference samples is different for different substrates with no trends, it can be seen that for all the substrates and all the considered factors, there is a gradual increase in drainage porosity and decrease in micro+meso porosity at time periods T2 and T4. However, no such trend was observed at T8. Hence, drainage porosity is highest at T4 for all the factors and substrates, and then tends to decrease at T8.

Density and porosity evolution

There are no such clear trends found for Macro-porosity. for the rain and freeze/thaw factors, after an initial decrease at T2, Macro porosity tends to remain constant without considerable evolution for all the substrates at T4 and T8 time periods. However, this is not true for the precipitation factor (whose behaviour is erratic at T8)

Water retention evolution

.11 represents evolution of WRC of different factors and reference for the substrates P, I and X considering the average of all time periods. While the evolution pattern for each of the substrate is different, it can be noted that there is not a significant difference in their values and tend to be similar, especially for the substrates P and X. Samples related to the factor freeze/thaw has higher MWHC compared to the reference samples in all the substrates. . For the reference samples, evolution pattern for substrate P is different compared to that of I and X. Substrates I and X have a significant decrease at T2 (>30 %) and lesser but considerable decrease again T4. However, the behaviour at T8 is different as it tends to increase slightly towards initial values for substrates P and X. There is a similar evolution pattern to that of the reference samples when the factors' average is considered. However, the tendency to lean towards the initial values at T8 is more prominent here compared to the reference samples. This is evident for substrate P where the WRC is higher at T8 than at T2 and T4.

Evolution of microbiological properties

At T4, the microbial biomass (SMB) of substrate P (Figure 4.14 1A) is significantly higher than the initial value and also is higher than the reference sample throughout the time periods. Whereas for substrate I (Figure 4.14 1B), the reference sample is significantly higher at T2 until T4 compared to the beginning of the experiment. No difference is detected statistically between the vegetation and reference samples at the beginning of the vegetation simulation and at the end. However, an increasing trend is visible for the SMB of substrates P and I as well as for the FDA of substrate P. For substrate X (Figure 4.141C). only the SMB measurement at T4 for the vegetation sample is different from the beginning of the experiment as well as from the reference.

The microbial activity index (MAI) of the P substrate reference (Figure 4.14 2A) remained stable over time. Furthermore, no significant difference was observed between the reference and vegetation samples, although the vegetation sample remained higher than the reference sample. For substrate I (Figure 4.14 2B), the microbial activity index is different only between measurements at T2 and T4 for the vegetation sample. No significant difference is shown for substrate X (Figure 4.14 2C) between the reference and the vegetation sample. However, the vegetation sample tends to be above the reference sample for both parameters from T4. Moreover, a significant increase exists from T4 for the vegetation sample.

Evolution of pH under the effect of different factors

Globally, all the factors induced an evolution of pH (Figure 4.15) for each of the substrates over time. Indeed, the pH significantly decreased for substrate P compared to the beginning of the experiment and increased respectively for substrates I and X. However, no significant difference is observed within each substrate between samples. Compared to the other samples of substrate I, an increase (7.9) in the pH of sample I under the influence of factor F can be noted. Another tendency to increase can be observed on sample V for substrates P and X (respectively 6.8 and 7.

3) compared to their other samples.

Evolution of particle size

No significant changes over time in the fine fraction are highlighted for the P substrate (Figure 4.16 A). However, clear trends appear. An enrichment in fine fraction is visible between 2.5 and 5 cm depth for the freeze/thaw sample after 8 weeks compared to the control at t+4 weeks. There is a tendency for an increase in the fine fraction at the surface depth was observed for this PP sample at T8. In addition, a slight tendency of fine fraction depletion was also observed at T4 for the PP sample, but this was not confirmed at T8.

For substrate I a significant enrichment in fine particles (< 2 mm) of the surface (0-2.5 cm) and depth (5-8 cm) horizon is observed for the freeze/thaw sample at the end of the experiment compared to the beginning of the experiment. No other trends could be statistically supported, although the sample I under the influence of factor rain shows an enrichment of fine particles in the depth horizon (5-8 cm) at the end of the experiment.

For substrate X only an enrichment in fine fraction (< 2 mm) of the lower layer (5-8 cm) of the freeze/thaw sample compared to the upper layer of the rain sample could be shown by the statistics.

A trend towards enrichment of the lower layer of the rain sample at the end of the experiment is visible but without any statistical support.

Organic carbon and Nitrogen concentration evolution

Little significant variation was observed in organic carbon concentrations (Figure 4.17 A). A decreasing trend in content over time was observed for substrate P from 10.3% at T0 down to 8.1% as well as for substrate X. In all cases, the addition of synthetic exudates and sedum litter in modality V partially or totally compensated for the decreases in organic carbon concentrations.

No significant variation in total nitrogen concentrations was observed among the modalities studied (Figure 4.17 B). Similar to organic carbon, a slight decreasing trend in this concentration is visible for the substrates P and X, but not significantly. In a similar way, the concentrations of the V samples are significantly higher than the other samples (R, P and F) and very close to the initial substrates but without any statistical support.

After statistical analyses of organic carbon and total nitrogen concentrations in the surface horizons (results not shown), no loss or enrichment could be demonstrated. However, for the rainfall sample of substrate P, the average organic carbon and total nitrogen were lower than the other samples. The average organic carbon for the precipitation sample of substrates I and X also tends to be lower than the content at the beginning of the experiment. This last observation is not true for nitrogen.

Discussion

It was presumed that depending on the composition and nature of each of the substrates, the properties would evolve differently. Indeed, this holds true, overall, with each substrate responding and evolving to different factors in their way owing to its constituents. Nevertheless, they seem to be in line with one of the main characteristics of young Technosols: undergoing early pedogenesis, i.e. rapid and intense under the effect of such environmental factors as rain, temperature and biological activity induced by vegetation (Bouzouidja et al, 2016).

The storm within: Evolution of reference substrate samples

The reference samples of each of the substrates underwent a pH evolution: an acidification for substrate P and an alkalinization for substrates I and X. The latter two substrates have very similar compositions and properties. As water was added, basic elements could be released by solubilization or desorption. Moreover, a bio-alteration or alteration of minerals by exudates [START_REF] Keiluweit | Mineral protection of soil carbon counteracted by root exudates[END_REF] could also be carried out by soil microorganisms releasing bases or acids depending on the initial composition of each of the substrates and the microorganism communities present. Such difference was not assessed here but could lead to further investigation.

The nitrogen content of the different substrates varied little over time, although a slight decreasing trend could be observed for the P and X substrates (which were the best supplied). A decrease in organic carbon could also be observed in a more notable manner. This is probably explained by the mineralization of organic constituents associated with a potential volatilization of nitrogen, but which could not be measured during this course. Again, these results can be compared with the non in situ experiments from [START_REF] Emilsson | Effect of using conventional and controlled release fertiliser on nutrient runoff from various vegetated roof systems[END_REF] and Bouzouidja et al 2018 who also described such a decrease in organic matter over time. However, they are also contradictory with the in situ studies of those of Schrader and Boening (2006) who described an enrichment in organic carbon and total nitrogen.

Comparing evolution of water retention curves of reference samples for different time periods for the substrates P, I and X. with that of average of all factors being similar seems to show that effect of time is more relevant than effect of factors (the evolution of intrinsic biotic vector involving complex processes which are yet to be explored is more prominent than the effect of external factors). And also, substrate P, with a much finer granulometry and richer organic matter, behaves very differently from the other two substrates I and X justifies the prominence of the substrate composition's impact.

For the reference sample of substrate P, no evolution of microbiological properties is highlighted statistically. This stagnation or even trend decrease of microbial activity and biomass in substrate P can be explained by very high microbial indicators at the beginning of the experiment compared to substrates I and X. The nutrients contained in substrate P seem to be poorly available, thus limiting the development of microorganisms. The second hypothesis justifying this stagnation or decrease in microbiological properties may be the production of a metabolic waste or compound that inhibits or blocks the multiplication of microorganisms. Indeed, there are three phases of development of populations of microorganisms in culture: exponential growth phase, stagnation phase and decline phase [START_REF] Monod | The growth of bacterial cultures[END_REF]. On the contrary, for substrates I and X, a significant increase or trend in these properties can be noticed. This increase may be due to a strong oxygen supply during the setting up of the experimental devices, stimulating certain aerobic populations of microorganisms, which gradually fades away over time. On the other hand, pH variations making certain elements more or less bioavailable can also explain these evolutions.

Factors or actors? Effect of evolutionary factors

Effect of freeze thaw on WHC

It can be seen that freeze/thaw has the highest MWHC among all the factors, irrespective of the substrate, showing that the alternate freeze/thaw process indeed might breakdown the coarser aggregates into finer particles and thus also modifying their poral architecture.

It is possible that water infiltrating the pores of coarse particles, by becoming solid, increases its volume and thus produces a fractionation of the particles (Getter et al 2007).

Freeze/Thaw samples also have the highest percentage of macro and micro+meso porosities overall. These two may directly influence the retention performance. However, increase in macro porosity could also mean a dent in detention performance as bigger pores facilitate quicker movement of water out of the substrate.

Also the water holding capacity tends to be similar as the applied pressure increases as most of the difference in poral architecture lies in drainage porosity and macro porosity and not in micro porosity.

Freeze/thaw also had a tendency for substrate P and a significant effect on substrate I and X. Indeed, an enrichment of fine particles on the three depth horizons could be observed. This phenomenon can be explained by a fractionation of coarse particles (> 5 mm) which enrich the fine fraction under the effect of freezing/thawing. This is confirmed by a reduction in the fraction (2 < x < 5 mm). Detailed information regarding this is found in [START_REF] Maeder | Etude du vieillissement des toitures végétalisés: performances & perspectives de durabilité[END_REF]. These results are also reported in the literature before. [START_REF] Six | A history of research on the link between (micro)aggregates, soil biota, and soil organic matter dynamics[END_REF] observed destabilization of macroaggregates and Ferrick and Gatto (2005) observed erosion under the effect of freeze/thaw cycles.

Effect of vegetation on microbiology

The vegetation factor seems to change the microbial properties associated with the substrates. Indeed, as mentioned in the previous section, vegetation, through microorganism's activation may induce some modification of pH, Corg and N. The substrate P does not seem to evolve alone over time. However, the vegetation factor seems to allow the microbial biomass to increase significantly over time compared to the control and this after the addition of sedum litter. The addition of exudates and litter therefore apparently supported the development of the microbial community within substrate P. The same phenomenon occurred within substrate X for the microbial biomass. Some microorganisms in the substrates thus seem to use these resources to grow. On the other hand, for the BMC of substrate I, a non-significant increase compared to the control can be observed. These increases, whether significant or trendy, fade over time except for the microbial biomass of substrate P and the activity of substrate X. This phenomenon may be due to a cessation of litter supply (i.e. cessation of nutrient supply) or a metabolic waste produced during exponential growth, which then inhibits the development of microorganisms [START_REF] Monod | The growth of bacterial cultures[END_REF]. The level reached at 4 weeks in the BMC of substrate P and the activity of substrate X suggests that the populations of microorganisms find enough nutrients to perpetuate their growth or activity. Speculatively, this could be explained by cometabolism [START_REF] Keiluweit | Mineral protection of soil carbon counteracted by root exudates[END_REF] or bio-stimulation [START_REF] Hazen | Biostimulation (No. LBNL-1691E[END_REF] phenomena allowing, even after nutrient supply is stopped, to continue to make nutrients available. The study by [START_REF] Bürgmann | Effects of model root exudates on structure and activity of a soil diazotroph community[END_REF] suggested the development and activation of different strategies in multiple microbial species. This supports the hypothesis of the establishment of co-metabolisms.

Effect of rain and freeze/thaw on grain size

Contrary to what was expected, no significant change in fine fraction with depth was observed for substrate P despite trends in fine fraction in the depth horizon for the rainfall modality at the end of the experiment. The expected leaching of fine particles from the upper horizon under the effect of rain was thus observed in a tendency at t+4 weeks but was not confirmed at t+8 weeks. For substrates I and X, the expected rain-induced leaching of fine particles from the upper horizon, as suggested by [START_REF] Bissonnais | Soil characteristics and aggregate stability[END_REF], was also difficult to observe, although vague trends emerged (surface depletion at t+4 weeks and between 2.5 and 5 cm depth at t+8 weeks). We are not ruling out methodological bias here, but another hypothesis is also admissible: the trapping of particles in the geotextile. The visual observation of fine particles located in the underlying geotextile contributes to attest the phenomena of fine particle transfer. Nevertheless, this phenomenon could not be quantified during the course.

The above results are again different from those in situ results of De-Ville et al, (2015) who described, on two different aged substrates, an increase of finer particles while the lower part showed little change. This increase was attributed to atmospheric particle deposition and weathering processes which explains the large difference between the two results and the impact of real site condition interactions and inter relations. Indeed, our experimentation took place indoors with pure water spraying with minimal external Influence.

Effects of factors on organic carbon and total nitrogen

All experimental conditions led to a more or less marked mineralization of the organic matter contained in the substrates. This result is consistent with the microbial activity measurements which show the presence of microorganisms. The most significant decrease concerns substrate P which was initially the richest. This more intense phenomenon of carbon and nitrogen leaching in the most fertile substrates is also highlighted in many studies (Vijayaraghavan, Joshi and Balasubramanian 2012; Beecham and Razzaghmanesh 2015; Emilsson et al, 2007; Clark and Zheng 2014). To a lesser extent, this decline also affected substrate X. The 'vegetation' modality also showed that this mineralization could be partially compensated by root exudates and litter from characteristic extensive roof vegetation. It should be noted that, as for organic carbon, root exudation and litterfall tend to compensate for, or even increase, the nitrogen concentration over time.

The specific observation of the organic carbon concentrations of the surface horizons does not bring more relief to the results obtained over the entire thickness, except for the "vegetation" modality which is significantly more enriched. This seems to indicate that the mineralization and integration processes of organic matter are active throughout the thickness (8 cm) of the substrates studied.

On the other hand, the specific study of surface horizons for the T, P and F modalities, with higher N concentrations than at depth, makes it difficult to identify the active processes responsible for the trend decline in N at the scale of the substrates on their full depth. Nevertheless, the positive contribution of the vegetation is more evident.

Ageing: accelerated or overrated?

As it can be seen, irrespective of the substrate, most changes for WHC occur at T2 compared to the other time periods. This is also true for microbiological properties. This could serve as an important testimony to the conducted accelerated ageing because it not just is in accordance with the typical Technosol evolution of high initial pedogenesis and being more stable after, but also it has to be noted that even if their results were contradictory to each other due to their approach, many authors made similar observations. Simon de Ville (2018) through in situ experiments and Bouzouidja et al (2018) through simulations noted drastic changes occurred in the initial 2-3 years of the green roof implementation and less evident from there on. Our accelerated ageing process, especially rain and freeze/thaw were designed to try to simulate years of ageing in weeks which has made positive beginnings with comparable trends in evolution to that of in situ samples. This of course needs more tests and validations.

This could also mean that the intrinsic biotic vector which seemed to have higher influence than the factors is valid for a certain time / biological cycle period and then stabilises after from where on external factors could have a more prominent influence. i.e. within the first years, the pedogenesis trajectories of green roof substrates are mostly dominated by the nature and composition of their parent materials. Then, after a while, the factors could take the lead

Limitations of methodological approaches

Several limitations are to be highlighted in this experiment. Indeed, the coarse granulometry of our substrates forced us to adapt the protocols. The adaptation of our protocols consisted of increasing by five the number of test samples to obtain more representative samples. However, no surface exchange ratio was taken into account for the measurement of pH for example.

When designing the protocol, it was decided not to drill the mesocosms for free drainage of excess water. However, for the rainfall sample, the water tended to saturate the mesocosms. The effect of this is evidently noted in the erratic T8 time period of retention curves for all substrates. We therefore do not rule out the existence of a methodological bias that would have caused a re-distribution of leached fine particles towards the surface. Moreover, this saturation phenomenon could also have limited the vertical downward transfer of particles.

The design of the experiments could finally explain the numerous non-significant trends in properties. The time between measurements may have been too long, thus not giving access to a fine enough scale of the evolution of the properties and making the interpretation and identification of the phenomena more difficult. Although accelerated ageing process gave similar overall final results to non in situ experiments, it could further be improved to know and evaluate these evolution scales for better understanding of green roof relations.

The Retention contradiction or the in situ-situation?

The global view noted regarding retention is that the evolution of WHC of our study where it decreases over time. Indeed, we noticed a decrease of micro+mesoporosities. While this can be a clue to the decrease of the retention performance, but absolutely not a demonstration for many reasons and most of all:

Retention performance is multifactorial; even if meso+microporosities are of utmost importance, the other porosities are also significant as well as the development of vegetation.

Due to analysis bias, it was not possible to accurately distinguish meso and microporosity. However, their contributions to retention performance are different (notably mesoporosity that provide available water for plant); Hence, further analysis is required to evaluate the retention performance.

However, a difference in retention performance evolution may already be noted in the literature based on the type of study where the considered experimental studies conducted in situ (observed in meta-analysis study of chapter 3), there was an increase over time.

Getter et al 2007, mentioned retention may be increased by roof age owing to changes in porosity. Speak et al 2013, noted Organic matter content in the aged green roof substrate was relatively high, leading to an increase in the retention capacity of the roof. Simon De Ville, in his thesis (2017) mentions that the main conclusion from the analysis of retention with roof age was that there was generally evidence of an increase in retention performance (MWHC) between year 2 and year 3, but there would be little significant change after this point.

However, as mentioned before, it has to be noted that those experiments were on real site and cumulative factors whereas our study was in controlled conditions, individually analysing the evolution based on the influence of each individual factor while keeping other parameters constant.

The differences in results due to non in situ approach was also noted in 2 relevant studies who approached to resolve similar questions through non in situ experimentations.

Bouzouidja et al (2018), conducted simulations considering the same initial water content but with the hydraulic parameters previously adjusted for each substrate with the lab experiment data and his results unequivocally suggested that the water retention performances of green roof substrate decreased overtime in the initial years. Another modelling study of long-term green roof retention performance by Stovin et al (2013) also noted retention decreases significantly for high return period events.

This contradiction of results of retention performance evolution based on the type of study and the source (increase in situ and decrease in controlled/ simulated) once again reiterates the complex system full of nuances that the green roof is with multiple effective interactions within and outside its system and the challenges associated to reproduce them at a lab scale.

Proposal for further experiments

To validate or invalidate the hypotheses presented in this discussion and the observations made during the study, several complementary experiments could then be proposed.

As it was seen during the study and also in the literature that there is a contradiction between the studies made in situ and non in situ. In order to complete and compliment these experiments under controlled conditions, measurements on green roof substrates that have evolved under real conditions would not only make it possible to identify certain trends in the evolution of properties, but also help in finding ways to improve non in situ studies to be able to mimic and predict in situ evolution.

Additional microbiological measurements could be taken to refine the cometabolism and bio stimulation hypotheses. It would then be interesting to identify and monitor the evolution of microbial populations and communities during ageing by adding artificial exudates and litter.

In order to compensate for the possible loss of representativeness of our test samples, due to a too large granulometry, additional analyses on each type of coarse elements (pozzolana, pumice, rock, organic elements) could be carried out. This would help explain the large variability observed in the measurement replicas and provide a better understanding of certain mechanisms.

To confirm the leaching while removing the bias created by the retention of excess water, an additional experiment, where it would still reproduce the rainfall, but with the tanks pierced to ensure free drainage of the excess water could be conducted. In addition, an analysis of the geotextile could quantify the quantity of fine particles trapped during the leaching process.

Conclusion

The study of the effect of the ageing of green roof substrates under controlled conditions has revealed traces of early pedogenesis. Substrates, albeit in a different scale, seems to undergo a marked evolution, which was expected for such young Technosols. This evolution, prominent in the initial time periods and showed that the designed accelerated ageing process was successful in leading to an evolution even if questions remain on the realism of such protocols.

It also appears that the composition and nature of the substrate is an essential parameter of this evolution. This justifies the interest of individual study of substrates of different composition. The evolution was also more time based (especially at T2) rather than being factor based, emphasising the assertion that the pedogenesis trajectories of green roof substrates are mostly dominated by the nature and composition of their parent materials in the initial years.

The results obtained and also further analysis on previous research also showed a contradiction between in situ and non in situ studies in results obtained in possible performance evolution (subject to further analysis) emphasising the need to improve non in situ studies to be better comparable with the real conditions and behaviour.

The tests of the different factors reveal several significant or trend effects. Among these effects, vegetation on soil microbiology plays a stimulating role or simply allows the microbiological properties of the substrates to be maintained. On the other hand, rain and frost tend to change the granulometry according to fractionation and leaching processes. However, these processes remain to be confirmed by further experiments and measurements.

The main limitations of the experiment to be highlighted are a small scale of monitoring and the high granulometry of our substrates making it difficult to make the test samples representative.

The monitoring of the controls nevertheless revealed significant changes in the properties of the substrates: the poral architecture was modified, the pH decreased or increased depending on the substrate, and slight variations in soil microbiology and organic carbon and nitrogen concentrations were measured. This shows that the vegetated roof also evolves according to an internal nature and composition of the substrates involving complex processes that have yet to be explored.

In situ Situation

A real site green roof substrate property evolution-evaluation over 3-and 10-year periods.

Abstract

Purpose:

While most of the meteorological factors such as air temperature, humidity, wind speed can be simulated individually in the lab scale, simulating them all at once can be tedious and mostly impossible. So real life data exploitation would be the most practical source to analyse their inter relations

Methods:

This work was based on two in situ experimental GR plots settled in Tomblaine (north-east of France, 48°40'N 6°13'E, under temperate climate) and in Saint-Mars du Désert (west of France, 47°19'N 1°28'W, under oceanic climate). The sampling has been performed in 2020. The strategy was based on the observation of the substrates vertical profiles from previous observation (Bouzouidja et al, 2016). We decided to split them into two sub-horizons: 0-5 cm (or 2-7 cm) and 5-10 cm (or 7-12 cm), depending on the substrate depth. Each depth was replicated three times. Each time, undisturbed soil cores were sampled. Bulk density (ρa) has been measured thanks to core-sampling. The solid density (ρr) was measured with a helium pycnometer (Quantachrome UltraPyc 1200) based on NF P18-554. Total porosity (δ) was then calculated. The soil water retention characteristics, i.e. volumetric water content at various matrix potential, were determined on a pressure plate apparatus at -10 kPa (pF 2, that was considered to be moisture at field capacity: 𝜃 𝐹𝐶 ), -33 kPa (pF 2.5), -500 kPa (pF 3.7) and -1500 kPa (pF 4.2).

Results:

Study revealed that the evolution is mainly driven by the age where the younger substrates (3 years) could be seen undergoing a rapid pedogenesis compared to the older substrates (10 years). Effect of depth was more drastic but settled in older substrates while they were haphazard but evolving in younger substrates. This strengthened the argument that evolution is more time-based emphasising the assertion of intrinsic biotic vectors of the green roof in the initial stages. The results obtained and also further analysis on previous research also showed some contradiction between in situ and non in situ studies in results obtained in evolution emphasising the need to improve non in situ studies to be better in sync with the real conditions and behaviour and to try to simulate the combination of factors like in real conditions.

Introduction

As a living and evolving media, a green roof is influenced by its environment through multiple interactions of factors, individually and also cumulatively. While most of the individual meteorological factors such as air temperature, humidity, wind speed can be simulated individually at the lab scale, simulating them all at once can be tedious and mostly impossible under controlled conditions. So real life in situ experimentations and data exploitation would be the most practical source to analyse their inter relations and cumulative effects.

Also, depending on the age of the substrates considered and if the composition is similar, there can be a comparative analysis regarding the possible inter relations and interactions between the factors and also those type of influence of external factors that cannot always be easily simulated in the laboratory. For example, the raindrops of high-intensity rainfalls tend to have more energy when they strike the soil surface. Compared with low-intensity rainfalls, they are more effective in sealing the exposed soil surface and hampering infiltration [START_REF] Scott | Soil physics: Agricultural and environmental applications[END_REF]. Also, unlike in laboratory conditions, the rainfall in real life is seldom uniform varying its degree of downpour, also affected by windspeed etc. So, comparing the results of both real life and controlled experimental data would benefit to know their level of comparability.

In-situ experiments are very rich in terms of data that could be monitored and harvested under real conditions (Schindler et al, 2019;Soulis et al, 2017). However, they also need long-term observation and adapted monitoring devices to be implemented at the beginning. Hence, even if large-scale data are available, their exploitation can be time consuming, tedious, and impractical and explaining why such studies are limited. It is found that the vast majority of in-situ green roof research conducted has been on smaller green roof test beds or isolated components (Andenaes et al, 2018). This is an interesting observation because, as Hanumesh et al, (2021) meta-analysis highlighted, pedogenesis and evolution of green roof components have a great potential to modify the performance over time. However, the fact that the test beds are discarded/not maintained after the experiments, would deny the possibility of temporal evolution analysis because a very important criteria to evaluate green roof performance is the length of time that the study is conducted. Most plant evaluation studies of green roof taxa have only been conducted for one or two years [START_REF] Dvorak | Green roof vegetation for North American ecoregions: a literature review[END_REF]. Hence, in-situ measurements of green roofs, maintained for a certain period in a real site (and not on test beds) with a possibility of long-term analysis are the need of the hour.

Among the properties, substrate components and depth are known to be the most prominent drivers of green roof pedogenesis. Hanumesh et al, (2021) noted that substrate material was the most important factor influencing the runoff retention in green roofs, followed by substrate depth, roof slope, and vegetation.

It was seen in the meta-analysis that substrate depth has a significant impact on the green roof hydrological and thermal performances [START_REF] Hanumesh | A roof of greenery, but a sky of unexplored relations-Meta-analysis of factors and properties that affect green roof hydrological and thermal performances[END_REF]. Mentens et al, (2006), too in their meta-analysis mentioned above found a positive correlation between soil depth and water retention.

Other Studies such as Vandegrift, (2019) mentioned the chances of vegetation development and survival are higher with higher substrate depths. Substrate depth was also directly related to water holding capacity and acts as a buffer for temperature extremes, as shallow substrates are more subject to fluctuations to them (Eksi et al, 2017). Reyes et al, (2016) with their experimental analysis on 3 different depths (5 cm, 10 cm, 20 cm) showed that substrate depth controls the amplitude in substrate temperature. The deeper 10-cm and 20-cm depth modules showed a significant damping effect on substrate temperature, also provided a stable and suitable substrate water content while the 5-cm depth green roofs were more affected by the atmospheric demand.

However, most of these studies compare different green roofs of different substrate depths. Sampling at different depths of the same green roof substrate and analysing them would give new perspectives about substrate internal evolution, especially with regards to compaction, organic content and poral architecture.

All these properties can also have an impact on each other, the extent depending on the microclimate and the meteorological factors. For example, Reyes et al, (2016) noted extensive green roofs (less than 10 cm of growing medium) are unlikely to be recommended in arid or semiarid climates due to the fact that they experienced daily thermal amplitudes beyond the recommended value for plant development.

Also, regarding vegetation and their type, substrate depth along with roof age were found to be the predominant factors influencing plant communities as deeper substrates supported meadow plants while shallower roofs were limited to sedum and mosses [START_REF] Gabrych | Substrate depth and roof age strongly affect plant abundances on sedum-moss and meadow green roofs in Helsinki, Finland[END_REF]. Substrate depth has been shown to strongly influence plant survival and long-term plant communities [START_REF] Rowe | Long-term rooftop plant communities[END_REF]. A test of 15 herbaceous perennials and grasses over a six-year time period showed that the greatest survival, diversity, size, and flowering performance occurred at a substrate depth of 20 cm relative to 10 cm (Dunnett et al, 2008). Even Sedum, the drought tolerant succulent species is influenced by substrate depth. (Rowe et al, 2012[START_REF] Thuring | Green roof plant responses to different substrate types and depths under various drought conditions[END_REF][START_REF] Emilsson | Comparison of establishment methods for extensive green roofs in southern Sweden[END_REF][START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF]. The role of substrate composition on rainwater runoff has received little attention, and when it has, it is mainly for its influence retention capacity. [START_REF] Getter | Carbon Sequestration Potential of Extensive Green Roofs[END_REF]. This substrate composition, even if altered by a bit can impact the green roof pedogenesis, as shown in studies such as Liberalesso et al, (2021) where it was seen that adding carbonized rice husk (CRH) improved substrate' physicochemical properties.

Hence, considering all these factors, a 10-year-old and a 3-year-old fully fledged green roof sites which are not test beds, were selected for this study, to help understand the long-term evolution of the green roofs. At least three different substrate and vegetation combinations were sampled from each site for diversity, and they were sampled at two different depths to understand how the same green roof substrates evolve under same external factors at different depths.

On the basis of the initial literature review and controlled condition experiments, several hypotheses are put forward:

H1: Depending on the nature and composition of the substrates, the properties will evolve differently.

H2: The kinetic of green roof substrate's evolution is decreasing over time and therefore maximum during the first years

The purpose of this study is to evaluate the overall evolution of the green roof system over time in real conditions and compare them with the results obtained through reproducing certain factors considered as influential (i.e., rain, vegetation and freeze/thaw alternation), under controlled conditions, on mesocosms of vegetated roofs by following the properties of the substrates over time.

The accuracy and relevancy of Artificial ageing of the substrates which was applied over a period of 2 months can thus be discussed.

Materials and methods

Experimental sites

This work was based on two in situ experimental GR plots settled in Tomblaine (north-east of France, 48°40'N 6°13'E, under temperate climate) and in Saint-Mars du Désert (west of France, 47°19'N 1°28'W, under oceanic climate).

In Tomblaine (TBLN), the green roof platform, built in July 2011, is placed above an approximately 6 m height flat roof building. The overall GR surface area is 600 m². For the purpose of our study, we focused our attention on: i) the Stock40 plot (CS) which describes an extensive green roof (extensive substrate + sedums), that has a 40 mm thick honeycomb structure (Nidaroof@60, NIDAPLAST) with a double function of drainage and water storage (surface: 28 m 2 ); ii) the Coco plot (CC) which describes an alternative extensive green roof (extensive substrate with addition of coco fibres + sedum; iii) the semi-intensive plot (CI) (semi-intensive substrate + sedums); iv) the Vosges plot (CV) that describes an alternative semi-intensive green-roof (semi-intensive substrate + lawn vegetation from the Vosges mountains . More details about this experimental site could be found in (Bouzouidja et al, 2016) and (Schwager, 2014).

In Saint-Mars du Désert (SMDD), an experiment was launched in 2017 in order to study the vegetation growth on various GR extensive substrates: a classical (PRC), a second one made with the same substrate + addition of concrete (PRB), a third one made with the same substrate + addition of expanded glass (PRV). All substrates were placed, under real climatic conditions, in boxes (0.7 m x 0.7 m, 0.35 m deep). Three replicates for all treatment were installed.

Green roof substrates and vegetation

Three types of vegetation have been studied depending on the treatments:

• Extensive lawn is composed of Allium schoenoprasum, Sedum album, Sedum spurium, Sedum larynx, Sedum reflexum, Sedum sexangular, Poa alpine, Potentilla erecta;

• Sedums is composed of Sedum album, Sedum reflexumlarix, Sedum reflexum germanium, Sedum sexangulare, Sedum floriferum;

• Grass is composed of Lolium perenne, Festuca pratensis, Trifolium repens.

Five types of substrates have been studied depending on the treatments:

• Semi-intensive is composed of pozzolana, expanded clay, recycled terracotta, coal slag, compost and bark (supplier: Végétoit);

• Extensive (type 1) is composed of pozzolana (60% of 3 -6 mm particles and 20% of 7 -15 mm particles) and organic parts (10 % of peat dust and 10% of maritime pine bark) (supplier: Faliénor);

• Extensive (type 2) is composed of pozzolana (60% of 3 -6 mm particles and 20% of 7 -15 mm particles) and 20% of coco fibers (supplier: Faliénor);

• Extensive (type 3) is composed of 70% of mineral parts (pozzolana, perlite and sand) and 30% of organic parts (peat and compost) (supplier: La Florentaise);

• Extensive (type 3) + concrete has the same basis as the previous one + 20% of crushed concrete (supplier: La Florentaise);

• Extensive (type 3) + expanded glass has the same basis as the previous one + 20% of expanded glass (supplier: La Florentaise). No significant evolution over depth is observed. However, it can be noted that the depth wise evolution of TBLN is different to SMDD for bulk density (increase in TBLN on an average of 2 percent and decrease in SMDD for PRB and PRV by 2 percent) and total porosity (decrease in TBLN and increase in SMDD). This increase is notable in substrates PRB and PRV where there is a 5 percent augmentation.

Coming to the poral architecture, it can be seen that, irrespective of the substrates, there is an increase in drainage+microporosity at D10 compared to D5 by 2-5 percent. This means that the difference in behaviour of total porosity evolution between TBLN and SMDD substrates is due to mesoporosity+microporosity which decreases at D10 for all TBLN substrates and increases at D10 for all SMDD substrates. While the evolution pattern for each of the substrate is different, it can be noted that the reduction in volumetric water content as the matrix potential increases tends to occur at a higher rate for SMDD substrates compared to TBLN for both the depths D5 and D10 (More than 5 percent decrease for SMDD and around 2 percent decrease for TBLN on an average). This phenomenon is even more pronounced at D10 than D5. These can also be noticed in There are no significant difference percentage wise where there is a slight increase (2%) of fine particles for PRC substrate and slight decrease (4%) of fine particles for PRB and PRV substrates. However, there is a significant decrease (40%) in the weight of fine particles for the substrate PRB at D10 (not shown here).

Water retention evolution

Discussion

Impact of substrate type

For the younger substrates, there is a 30% decrease in bulk density for substrates PRB and PRV compared to PRC. This is due to the fact that even if the base of all 3 substrates was the same, there was either an additional 20% of crushed concrete (for PRB) or expanded glass (for PRV). They, create more pore spaces and consequently decrease the bulk density. These additions have a significant impact in increasing the retention capacity (15% increase for PRB and 11% increase for PRV at D5 and even further increase of 20% for PRB and 15% for PRV) compared to PRC. While the increase at both depths can be attributed to addition of respective porous coarse materials, the increase in WHC at D10 can also be attributed to the fact they have a significant impact in the poral architecture too, probably due to their crushed pattern and possible fractionation and settlement of particles over time.

There is a reduction in drainage porosity (where the water stored can be lost due to gravity) and a significant increase in meso+microporosities (the smaller pores which hold the watertight and impact the retention capacity directly) due to possible fractionation of particles. This increase in microporosity is almost double in the case of PRB, explaining why it has the best WHC among all.

For the TBLN, it is seen addition of coco fibres create the same impact. It is seen that CC has 10% additional retention capacity compared to the other two substrates. This is again attributed to the changes in the poral architecture where the microporosity of CC is the highest among all the TBLN substrate samples irrespective of the depth.

Impact of substrate age

Comparing the kinetics depending on the age is difficult, considering all the initial virgin substrate data weren't available, especially for the SMDD substrates, and also the fact that the two sites are situated in different micro climate and subject to different vegetation. In the WHC, it can be seen that, irrespective of the depth, SMDD substrates, (3-year-old) undergo more rapid changes compared to TBLN substrates (10 years old) for the rising matrix potential. This seems to be in accordance with the literature review of Technosol behavioural characteristic of early rapid pedogenesis and also the controlled condition experimentations (chapter 4) that most of the changes occur rapidly in the initial years of the green roof largely due to the internal substrate evolution action and then it tends towards stability.

Impact of Substrate depth

It can be seen that irrespective of the substrates, the drainage porosity increases as the depth increases. Drainage porosity is known to vary with depth and time (Bear, 1972;Butler Jr. et al, 2007). Also, mature substrate exhibits greater values for porosity, free airspace (macropores). Increased free air space, resulting from factors such as channels formed by decaying roots or burrowing insects, may increase preferential macropore flow through the substrate (thereby also resulting in quicker initial runoff) (Getter et al, 2007).

For the substrates of SMBB the porosity increases at D10 for all the substrates, showing that there is a possibility of fragmentation of particles and restructuring of the substrates in the initial years of its pedogenesis. This is untrue for TBLN substrates as the porosity remains the same or rather decreases slightly at D10, indicating that over the 10-year duration, the substrate might have undergone a lot of compaction and hence having less scope for differences. Overall pore space and changes in pore size may occur over time as a result of settling or as a result of changes in organic matter content (Getter et al, 2007). The root action is thought to stabilise substrate organic content levels over the years (De ville et al, 2018)

Depth wise, while the substrates of SMDD undergo different changes, without a common pattern, according to the substrate type's evolution showing that intrinsic biotic vector isn't settled yet and also prone to rapid changes, the TBLN substrates all have a massive decrease at D10 comparatively and have a similar evolution trend with the increasing matrix potential, showing that the substrates intrinsic biotic vector seems stable and settled.

Our study shows decline in WHC for the deeper D10 and is mostly opposite to the results in literature review. But it is important to note that they would have considered the whole substrate depth and compared different beds of different depths, whereas in our case, it was depth based sections of the same bed, in each case. Our study case shows that there is varied property evolution at different depths of a green roof, especially over time, mostly due to compaction and settlement. Also, all of those studies where deeper substrates were related with higher runoff retention, were specific cases in which they exhibited higher initial moisture contents which resulted in reduction of the expected runoff retention. Other meteorological factors such as antecedent dry period, seasonal variations play a huge role.

Scope for further studies

Though some of these were envisaged for our work, the time constraints did not permit us to conduct them. Nevertheless, these further studies will help to better understand the in situ green roof evolution. Our meta-analysis and also the controlled conditions experiments showed that vegetation has a huge influence, mainly through enrichment of organic content addition, root growth, addition of finer particles, impacts the retention performance among others. Hence, considering the fact that TBLN and SMDD had different vegetation, it would be beneficial to study their impact to know how they affect the green roof performance overall. Even if there are no exact same substrates with different vegetation, there are substrates with the same base and same vegetation (at SMBB) and hence the impact of vegetation on the organic content or root development and their eventual effects on green roof performance can be studied.

Also, it has been stressed that green roof evolution is a complex process where substrates undergo internal pedogenesis while also being influenced by external factors. Same substrates being installed in different microclimates and monitoring them in the long run would greatly help to quantify these.

Conclusion

The study of the effect of the ageing of green roof substrates under real conditions has revealed that while the composition and nature of the substrate is an essential parameter of this evolution, it is mainly and primarily driven by the age where the younger substrates (3 years) could be seen undergoing a rapid pedogenesis compared to the older substrates (10 years). This strengthens the argument that evolution is more time-based emphasising the assertion of intrinsic biotic vectors of the green roof in the initial stages.

Depth wise, there was an increase in fine particles for all the younger substrates at D10 showing there is fragmentation and settling of substrates going on. Also, for other properties, while there are definite differences, is no proper evolution pattern at D10 compared to D5 for the younger substrates showing they are prone to rapid changes in the initial stages whereas for the older substrates, there exists a pattern and also a considerable change in properties such as WHC showing the substrate's internal evolution seems stable and settled over time.

The results obtained and also further analysis on previous research also showed a contradiction between in situ and non in situ studies in results obtained in evolution emphasising the need to improve non in situ studies to be better in sync with the real conditions and behaviour and to try to simulate the combination of factors like in real conditions.

6 General discussion and conclusion The thesis initially originated to have more understanding of green roof evolution considering it as a living media as majority of the state of the art considered them otherwise. While meta-analysis chapter demonstrated that green roof can be more or less performant to answer to those issues; it also showed that such performances levels rely on the one hand on factors that are hardly manageable, and properties that are easily manageable, but that appear to evolve over time because of ageing. Hence, this thesis had the objective to describe impact of such external factors influences the biophysical-chemical properties and the consequences in terms of green roof performances. Such a goal was conducted by trying to reach a crossroad approach between ageing -a term that describe the evolution of the green roof as an inert system as seen through the eye of a building materials scientist -and pedogenesis -a term that describe the evolution of the green roof a combination of virgin green roof biotic and abiotic system, as seen through the eye of a soil scientist -. The aim was to apprehend the green roof system by simplifying it in order to describe the complexity of its evolution.

Discussion

This approach, through three stages, namely meta-analysis, controlled experiments and in situ experiments have given answers and have also raised new questions, some similar, some different to each other, depending on the stage, in understanding the green roof interactions and inter-relations. While they have been discussed in detail in their respective chapters, they are summarised in this chapter. While doing that, the questions this PhD originated from and the answers this PhD found to them are also mentioned.

Do external factors influence the properties and performance?

It was seen that, there was an affirmation, both in the meta-analysis and experiments, that the factors certainly do have an influence, whether positive or negative, on the properties and eventually the performance of the green roof.

In the meta-analysis, their influence on performances could be quantified with 90% of the studied hydrological factor-performance relations having negative influence and 55% being negative influence on the thermal performance.

While in the experiments, the influence of factors on performances were not directly studied, key properties that influence the performances of green roof such as the porosity (and the poral architecture) or organic carbon changed in various degrees under the action of external factors.

These findings highlighted that most factors and properties have a direct and considerable influence on the performances of green roofs, while their level of controllability is variable, it still shows that overall, there are many existing levers to enhance the green roof performances and tackle some of the main issues of the urban environment.

Also, it was seen in the meta-analysis that more than half of the considered (after initial filtration) factor-property-performance relation influences haven't been explored yet; it provides a great path for further exploration and understanding of green roof evolution.

Is age just a number?

While age is just a number, in the case of green roof evolution, less is more. i.e. considering the average lifespan of green roof to be 25 years, irrespective of the substrate/factors applied/microclimate, most changes occur in the lesser initial years of 0-3 in real site conditions. This was noted not just in the SMDD (3 years) substrates in our study but also comparing the evolution pattern of properties of TBLN (10 years) substrates with the results of some of the properties measured during the thesis of Schwager (2014) on the same substrates when they were 3 years old, It could be seen that majority of the changes occur in the initial 3-year time period in a substrate specific way and tends to slow down/stabilise with a predictable, similar trend after. At a different timescale, (considering it was accelerated ageing in the lab scale), Similar observations were made in the controlled condition experiments where majority of the changes happened at T2 time periods (2-4 weeks out of 8 weeks monitored).

Our suggestion is therefore that the kinetic of green roof substrates ageing could be described by a logarithmic curve: fast and intense at the beginning and slower later on. This could be due to that while Factors have a constant influence on the performances throughout but, during the initial years, they also have an influence on the properties leading to pedogenesis. This along with the said internal evolution of the substrates depending on their composition accelerates the kinetics of ageing in the initial years.

Does substrate type matter?

While the answer to this was a solid yes already from the initial literature review, our research opened up new horizons, especially about its pace and duration of evolution (as mentioned in the section above).

During the initial active pedogenesis period, the substrates undergo varied changes, thanks to their specific internal evolution due to, properties such as vegetation, composition or granulometry.

This shows that, depending on the substrate types, there are varied degrees of interactions and inter relations within the substrate components and outside the substrate, especially at a rapid scale in the initial stages, thus modifying the properties and eventually the performances.

Considering Figure 6.2, which shows the available coarse aggregate data for all the substrates considered in this thesis at their respective initial and aged periods, it can be seen that all of them evolve in a different way over time. Only substrates which don't have a considerable evolutional change are the ones which are either composed of mostly fine aggregates (substrate P) or mostly coarse aggregates (LECA). This seems to show that the initial substrate composition does matter for granulometric evolution.

Also, considering the substrates I, X, Coco (CC) and EXT (extensive) which have similar percentage of coarse aggregates initially, it can be seen that all of them evolve differently. I and X (evolved in controlled conditions) see a similar decrease of 5 percent in coarse particles whereas coco and extensive (evolved in real site conditions) are rather opposite, with a significant increase of more than 20 percent. In fact, the extent of increase among the two is also different. Hence, this shows that the composition of the substrates plays a huge role in the pedogenic evolution.

Finally, it can be noticed that all the substrates evolving under lab scale undergo evolution different to that of in situ substrates. A slight decrease in coarse aggregation is found for controlled condition reference sample substrates (This effect is more noticeable under the action of individual external factors such as rain and freeze/thaw, as seen in chapter 4) showing there is possible natural fragmentation of particles over time, which can be accelerated by external factors. Whereas the increase in coarse particles could be due to various external factors such as the loss of fine particles due to wind and erosion, atmospheric/animal deposits of coarse particles.

This also once again reiterates the difficulties of mimicking real site conditions in a lab scale.

Does substrate depth matter?

Substrate depth, when considered as a whole has a huge and mostly positive influence over time on the green roof performance over time as seen in the Meta-analysis and literature review.

However, in the experimentations when depth of the same substrate bed is considered at different sections, It was seen they behave differently, especially in the initial stage of pedogenesis and then it tends to settle down. This can be noticed in Figure 6.3 which indicates the evolution pattern of the granulometry of the controlled condition substrates for the time periods T4 and T8. For a particular Fraction percentage, same colours at different depth indicate either decrease (green) or increase (red) as the depth increases. Seven homogenous set of blocks like these at T8 compared to Four at T4 shows that the substrates get well settled over time.

The substrate I, has this homogenous pattern already at T4 for all the fractions, whereas the substrates P and X have it only for 2 fractions even after a higher time period T8, showing the pace and type of evolution largely depends on the substrate composition and internal characteristics.

Also, depth wise, the value of section D5, the middle one, also had the values closest to the mean of all depths considered for a particular substrate, for all the fractions. Granulometric evolution pattern over time of studied control condition substrates at various depth

Internal pedogenesis vs external influence

While Factors definitely do have an influence as seen both in the in situ and controlled condition experiments, it should be noted that most of the changes occur in the initial stages of controlled conditions (up to T2) and in situ (up to 3 years) experiments irrespective of the type of substrates. While it is undeniable that these factors have an additional influence on this initial pedogenesis depending on the type of substrate and their interactions with them varying their extent of evolution, it is also true that the factors and their intensity remained the same for the controlled conditions and will also remain comparably same on an yearly average (barring some natural calamities in rare cases and sites) for the in situ measurements throughout the duration of the study.

Considering these, it can be said that within the first years, the pedogenesis trajectories of green roof substrates are mostly dominated by the nature and composition of their parent materials. Then, after a while, the factors could take the lead. The factors, continue to have a constant influence on the performances throughout but, during the initial years, they also have a significant influence on the properties leading to accelerated pedogenesis during that time.

This, if validated through further tests and studies, could be very beneficial not just to understand the green roof evolution better, but also to save time, manpower and costs required to maintain and monitor the green roofs as they could be cut down after the initial 3-year period as most of the changes would have already occurred hence monitoring it to the same extent of initial 3 years would not be necessary.

This information would help the researchers in encouraging to monitor green roof rigorously in the initial years of the green roof genesis to have a clearer idea about the intricacies of how each substrate, based on its type and composition reacts to its own intrinsic biotic vector and also to the external factors influencing it.

This, after correlating with the performances, would also help the designers and city planners to predict and estimate more accurately the long-term role of the green roofs to provide the services they are expected to.

Future considerations

While considerations pertaining to respective studies have been mentioned explicitly in their respective chapters, few additional global considerations are mentioned here.

Evaluation of Hydrological and Thermal performances

While the initial idea was to analyse both the hydrological and thermal performances, it was done only in the meta-analysis, and none of the factors influence on the performance could directly be studied. While some key properties, like the water holding capacity through retention curves gave an idea about the retention performance, It would be interesting to directly study and evaluate them and their behavioural pattern as the Meta analysis showing that the external factors affect thermal performances more positively than the hydrological performances.

We have thus seen that several studies mentioned above have highlighted the contribution of green roofs in various ecosystem services such as reducing rainwater runoff, thermal insulation of the building and reduction of the urban heat island. Most of these studies conducted to date have attempted to characterize the effect of the thickness or nature of the substrate and vegetation type on these performances, neglecting the temporal dynamics of green roof evolution.

Data analysis

To properly understand the pedogenic evolution of green roof performance and also how the external environmental factors affect them, it is necessary to compare and evaluate the timely data of green roof performances with the local meteorological data such as the rainfall duration and intensity, wind speed or even temperature as it is well documented in the literature review that the microclimate greatly influences the green roof performance, especially in the initial time period. This data would help to design, improve and be precise for the non in situ studies in successfully reproducing the real time evolution.

General conclusion

Nature-based solution like green roof can hold a prominent role in answering to urban environmental needs through its performances. However, these performances are subject to evolution and hence changes considering the composition of green-roofs and their ability to interact with external factors. One of its main components, the substrate of the green roof, is characterised by intrinsic properties that condition their hydrological and thermal performances. Due to the position of the green roof on the top of the building and its nature as a Technosol, it is disconnected from natural regulation networks. Moreover, its exposure to climatic agents (e.g. rain, temperature, wind, solar radiation) is very high. It therefore evolves more rapidly and this phenomenon is called early pedogenesis. The aim of the study was to quantify the evolution of the substrate properties over time under the effect of factors (rain, vegetation, freezing/thawing).

This was done through three stages:

• An extensive meta-analysis which showed that while majority of factor-propertyperformance relations were unexplored, the studied factors had a major negative influence on the performances (68%), whereas the studied properties had a major positive influence (95%), which highlights that even if the green roof is a complex system, there exists different levers to be considered for performances enhancement.

• Controlled condition experiments where green roof mesocosms, of 3 substrates, were aged for 2 months under controlled conditions associated with a simulation of environmental factors. Analyses of substrate properties were performed at t0, t+2, t+4, t+8 weeks. The results showed evidence of early pedogenesis, especially for the finer substrate. This supports the fact that substrate composition is an essential parameter and that individual characterization and study of them are therefore necessary. Monitoring of the reference samples revealed: a decrease or increase in pH depending on the substrate, small variations in microbiology and in Corg and total N concentrations. There was also change in the poral structure thus modifying the Water holding capacity. This proves that the substrate specific characteristic evolution intervenes in the evolution of the green roof. While the external factors also had influenced the substrates (Vegetation stimulates or maintains microbiology, freeze/thaw modifies granulometry through fractionation and leaching processes etc) their individual effects were not as prominent as time.

• In situ experiments where substrates from two different sites of age 3 years and 10 years with different compositions were studied revealed that the evolution is mainly driven by the age where the younger substrates (3 years) could be seen undergoing a rapid pedogenesis compared to the older substrates (10 years). This strengthened the argument that evolution is more time-based emphasising the assertion of intrinsic biotic vectors of the green roof in the initial stages. The results obtained and also further analysis on previous research also showed a contradiction between in situ and non in situ studies in results obtained in evolution emphasising the need to improve non in situ studies to be better in sync with the real conditions and behaviour and to try to simulate the combination of factors like in real conditions.

Considering these, it can be said that I. Within the first years, the pedogenesis trajectories of green roof substrates are mostly dominated by the nature and composition of their parent materials. Then, after a while, the factors could take the lead showing that there are levers, both in time scale and component scale, to evaluate, monitor and enhance green performances over time. The performance thus is expected to continuously evolve over time.

This, after further validation, could be very beneficial not just to understand the kinetics of green roof evolution better, would help us to also to save time, manpower and costs required to maintain and monitor the green roofs as they could be cut down after the initial 3-year period as most of the changes would have already occurred and its evolution could be predicted from there on. This would also mean while there are many criteria, components, factors, levers involved in the definition of green roof evolution, it can be considered as it is primarily time driven as most of the changes occur in the initial 0-3 years of the green roof genesis.

II.

Green roof evolution is result of not just a multilevel interaction and inter relations of green roof substrates with external factors but also the internal pedogenesis of the substrate components. While on an individual scale, factors definitely have an impact on the green roof evolution, considering green roof as a whole, in the initial years, the impact of internal substrate specific pedogenesis overshadows that of the factors. But over time, as they tend to stabilise, the effect of factors seems more pronounced. Knowing that there are many such existing levers to enhance the green roof performances and tackle some of the main issues of the urban environment, the fact that more than half of these influences haven't been explored provides a great path for further exploration and understanding of green roof evolution.

III.

The designed accelerated ageing design and approach, subject to further improvements, has a promising debut in being able to mimic the technosol character of rapid initial pedogenesis and being able to be in sync with the results and evolution patterns of non in situ experiments.

IV.

While modelling and lab scale experiments tend to be preferred recently for their comparative ease of design and give a great insight on individual factor property relations, however, its efficiency to mimic the real time in situ needs considerable improvement. In fact, some of the studied properties and performances gave a contradictory evolution response depending on whether the studied methods were in situ or non in situ methods.

As a final step, in situ aged property evolution measurements from 7 substrates originating from two different sites of different ages and 3 different vegetation, revealed that the evolution is mainly driven by the age where the younger substrates (3 years) could be seen undergoing a rapid pedogenesis compared to the older substrates (10 years) whose evolution seem to have comparatively settled.

Considering the results overall, it can be said that within the first years, the pedogenesis trajectories of green roof substrates are mostly dominated by the nature and composition of their parent materials. Then, after a while, the factors could take the lead.

Résumé (French) (Long)

Un toit végétalisé, mais un ciel de relations inexplorées -Méta-analyse des facteurs et des propriétés qui affectent les performances hydrologiques et thermiques des toitures végétalisées

Dans un premier temps, une méta-analyse a été menée sur 100 articles répondant à nos critères, dont les principaux résultats ont mis en évidence que la plupart des facteurs et propriétés ont une influence significative sur les performances des toitures végétalisées, montrant qu'il existe de nombreux leviers pour améliorer leurs performances et résoudre certains des principaux problèmes environnementaux urbains. Cette analyse a contribué à la définition d'un cadre conceptuel pour décrire la complexité intrinsèque des toitures végétalisées en tant que systèmes biotiques-abiotiques -à la croisée de l'architecture et de l'horticulture -et sa relation avec les facteurs externes, notamment les conditions météorologiques, qui ont contribué à de multiples services écosystémiques, ici axés sur les performances hydrologiques et thermiques. Dans le cadre de l'intérêt scientifique fortement accru pour les toitures végétalisées, notre revue méta-analytique a mis en évidence de nombreuses recherches consacrées à l'impact des propriétés et des facteurs des toitures végétalisées sur ces performances. Même si ces relations propriétés-performance sont apparemment plus importantes, l'ignorance comparative de plus de la moitié des autres relations possibles facteurs-propriétés a également des implications importantes compte tenu de leur potentiel dans la définition des performances des toitures végétalisées. On peut également constater que les facteurs étudiés ont une influence négative majeure sur les performances (68%), alors que les propriétés étudiées ont une influence positive majeure (95%). Le fait que de plus en plus de chercheurs aient opté pour des approches de modélisation au cours de la dernière décennie, combinées ou complémentaires d'approches expérimentales plus approfondies et innovantes pour la recherche sur les performances des toitures végétalisées, en particulier dans des conditions réelles de terrain, ouvre également la voie à d'autres études pour comprendre les changements de performances. Bien que cela puisse être un travail laborieux compte tenu des difficultés liées à la conception, aux éventuelles interrelations et au temps, l'approche expérimentale est un besoin urgent compte tenu de leur possible impact sur l'amélioration des performances des toitures végétalisées et la simplification du processus de conception. La première étape à cet égard pourrait être de développer des protocoles clairs pour des études particulières des liens facteurs-propriétés-performance pour reproduire les conditions réelles de terrain en se rapprochant des spécificités régionales. Parallèlement, il est nécessaire de développer des observations à long terme pour mieux comprendre et contrôler les changements évolutifs et précoces des performances au cours de la pédogenèse.

Évaluation expérimentale du vieillissement accéléré en conditions contrôlées de l'influence des facteurs externes individuels sur la pédogenèse des toitures végétalisées.

La deuxième étape a consisté à mener des expériences contrôlées à l'échelle du laboratoire afin de comprendre l'influence des facteurs individuels sur les propriétés du substrat.

Objectif : L'objectif de cette étude était d'améliorer les connaissances sur l'évolution de certaines propriétés bio-physico-chimiques majeures des substrats de toiture végétalisée au cours du temps, sous l'influence de trois facteurs principaux (pluie, température, végétation) par le biais d'une étude expérimentale en conditions contrôlées. L'expérience a été menée en mésocosmes en suivant les propriétés des substrats au cours du temps.

Méthodes : Trois substrats avec différentes compositions représentatives de la diversité réelle des toitures végétalisées ont été testés dans des conditions contrôlées avec un vieillissement artificiel des substrats appliqué sur une période de 2 mois. Trois principaux facteurs d'évolution ont été reproduits expérimentalement afin d'accélérer leur pédogenèse précoce : simulation de pluie, simulation de geldégel, simulation de végétation. Pour chaque association substrat-facteur, 3 réplicas ont été créés afin d'avoir des résultats statistiquement précis. De plus, 3 temps de récolte avec sacrifice des boîtes pour effectuer les mesures. Au total, 108 mésocosmes ont été créés (3 substrats x 4 modalités x 3 réplicas x 3 périodes). Les propriétés du substrat (densité apparente, WHC, contenu organique, porosité, granulométrie) ont été mesurées sur différentes périodes de temps.

Résultats : L'étude a révélé des traces de pédogenèse précoce. Les substrats, bien qu'à une échelle différente, semblent subir une évolution marquée. Il semblerait que la composition et la nature du substrat est un paramètre essentiel de cette évolution car le schéma d'évolution des substrats P plus fins est différent de celui des I et X plus grossiers. Selon le substrat, la végétation stimule ou entretient la microbiologie, la pluie et le gel modifient la granulométrie par des processus de fractionnement et de lixiviation. On observe également un changement de la structure porale modifiant ainsi les performances de rétention. En effet, tous les changements significatifs (>25%) ont été observés au niveau de la courbe de rétention d'eau (WRC). La performance de rétention a montré une baisse considérable pour tous les substrats par rapport aux échantillons initiaux de référence. En dehors de cela, les changements dans la performance des autres propriétés étudiées semblent plus dus à l'évolution temporelle plutôt qu'aux facteurs, où la plupart des changements se produisent à T2 (jusqu'à T4 dans certains cas) et ont tendance à se stabiliser ensuite.

Les résultats obtenus, ainsi qu'une analyse plus poussée des recherches précédentes, ont également montré une contradiction entre des études in situ et en conditions contrôlées dans les résultats obtenus au niveau de l'évolution des propriétés.

Evaluation de l'évolution des propriétés du substrats des toitures végétalisées en site réel sur des périodes de 3 et 10 ans.

L'étape finale a consisté à mener des expériences sur site réel pour comprendre l'évolution des propriétés du substrat dans des conditions réelles sous l'action de facteurs externes au fil du temps.

Objectif : Alors que la plupart des facteurs météorologiques tels que la température de l'air, l'humidité, la vitesse du vent peuvent être simulés individuellement à l'échelle du laboratoire, les simuler tous en même temps peut être fastidieux et pratiquement impossible. L'exploitation de données réelles serait donc la source la plus pratique pour analyser leurs relations mutuelles.

Méthodes : Ce travail s'est basé sur deux parcelles expérimentales in situ de GR installées à Tomblaine (nord-est de la France, 48°40'N 6°13'E, sous climat tempéré) et à Saint-Mars du Désert (ouest de la France, 47°19'N 1°28'W, sous climat océanique). L'échantillonnage a été réalisé en 2020. La stratégie a été basée sur l'observation des profils verticaux des substrats à partir d'une observation précédente (Bouzouidja et al, 2016). Nous avons décidé de les diviser en deux sous-horizons : 0-5 cm (ou 2-7 cm) et 5-10 cm (ou 7-12 cm), selon la profondeur du substrat. Chaque profondeur a été répliquée trois fois. A chaque fois, des carottes de sol non perturbées ont été échantillonnées. La densité apparente (ρa) a été mesurée grâce à l'échantillonnage par carottage. La densité solide (ρr) a été mesurée avec un pycnomètre à hélium (Quantachrome UltraPyc 1200) basé sur la norme NF P18-554. La porosité totale (δ) a ensuite été calculée. Les caractéristiques de rétention d'eau du sol, c'est-à-dire la teneur en eau volumétrique à différents potentiels matriciels, ont été déterminées sur un appareil à plaques de pression à -10 kPa (pF 2, qui a été considéré comme l'humidité à la capacité au champ : θ_FC), -33 kPa (pF 2,5), -500 kPa (pF 3,7) et -1500 kPa (pF 4,2).

Résultats : L'étude a révélé que l'évolution est principalement déterminée par l'âge, les substrats les plus jeunes (3 ans) subissant une pédogenèse rapide par rapport aux substrats plus anciens (10 ans). L'effet de la profondeur était plus drastique mais fixe dans les substrats plus anciens alors qu'il était aléatoire mais évolutif dans les substrats plus jeunes. Cela a renforcé l'argument selon lequel l'évolution est plus basée sur le temps, soulignant l'affirmation des vecteurs biotiques intrinsèques de la toiture végétalisée dans les étapes initiales. Les résultats obtenus, ainsi qu'une analyse plus approfondie des recherches précédentes, ont également montré une certaine contradiction entre les études in situ et non in situ dans les résultats obtenus au niveau de l'évolution, mettant en évidence la nécessité d'améliorer les études non in situ pour qu'elles soient plus en phase avec les conditions et le comportement réels, et pour essayer de simuler la combinaison de facteurs comme dans les conditions réelles.

Ainsi, en considérant ces trois étapes, on peut dire que dans les premières années, les trajectoires de pédogenèse des substrats de toiture végétalisée sont principalement dominées par la nature et la composition de leurs matériaux originels. Puis, après un certain temps, les facteurs peuvent prendre le dessus, montrant qu'il existe des leviers, à la fois à l'échelle temporelle et à l'échelle des composants, pour évaluer, surveiller et améliorer les performances des toitures végétalisées au fil du temps.

Discussion et conclusion générales

Les principaux résultats de la thèse peuvent être résumés comme suit : I. Nos principaux résultats ont mis en évidence que la plupart des facteurs et des propriétés ont une influence directe et considérablement positive sur les performances des toitures végétalisées. Bien que les possibilités de les contrôler soient variables, cela montre tout de même qu'il existe globalement de nombreux leviers pour améliorer les performances des toitures végétalisées et s'attaquer à certains des principaux problèmes de l'environnement urbain. Le fait que plus de la moitié de ces influences n'ont pas été explorées offre une grande voie pour une exploration et une compréhension plus poussée de l'évolution des toitures végétalisées.

II.

Bien qu'il existe de nombreux critères, composants, facteurs et leviers impliqués dans la définition de l'évolution des toitures végétalisées, on peut considérer qu'elle est principalement liée au temps, car la plupart des changements se produisent au cours des 0 à 3 premières années de la genèse de la toiture végétalisée.

III.

L'évolution des toitures végétalisées est le résultat non seulement d'une interaction et d'une interrelation à plusieurs niveaux entre les substrats des toitures végétalisées et les facteurs externes, mais aussi de la pédogenèse interne des composants du substrat. Alors qu'à l'échelle individuelle, les facteurs ont certainement un impact sur l'évolution de la toiture végétalisée, si l'on la toiture végétalisée dans son ensemble, au cours des premières années, l'impact de la pédogenèse intrinsèque au substrat éclipse celui des facteurs. En revanche, au fil du temps, dès lors qu'ils tendent à se stabiliser, l'effet des facteurs semble plus prononcé.

IV.

La conception et l'approche du vieillissement accéléré, sous réserve d'améliorations ultérieures, ont des perspectives prometteuses dans la mesure où elles sont capables d'imiter le caractère Technosol de la pédogenèse initiale rapide et d'être en phase avec les résultats et les modèles d'évolution des expériences non in situ.

V.

Bien que la modélisation et les expériences à l'échelle du laboratoire ont tendance à être préférées récemment en raison de leur facilité comparative de conception et donnent un bon aperçu des relations individuelles des propriétés et facteurs, leur efficacité à imiter le temps réel in situ doit être considérablement améliorée. En effet, les performances étudiées comme la rétention, ont donné une réponse d'évolution contradictoire selon que les méthodes étudiées étaient des méthodes in situ ou non in situ.

Figure 2 . 1 : 18 Figure 2 . 2 : 19 Figure 2 . 3 : 26 Figure 2 . 4 : 29 Figure 3 . 1 37 Figure 3 . 2 39 Figure 3 . 3 40 Figure 3 . 4 40 Figure 3 . 5 41 Figure 3 . 6 42 Figure 4 . 1 : 60 Figure 4 . 2 : 63 Figure 4 . 3 : 65 Figure 4 . 4 : 66 Figure 4 . 5 : 67 Figure 4 . 6 : 71 Figure 4 . 7 : 72 Figure 4 . 8 : 73 Figure 4 . 9 : 74 Figure 4 . 10 :

 2118221923262429313732393340344035413642416042634365446645674671477248734974410 Figure 2.1: Standard stratigraphy of an extensive green roof: 1) building support and insulation; 2) sealing complex; 3) drainage layer; 4) geo-membrane filter; 5) substrate and 6) vegetation, Source: Green Roof service LLC. ............................................................................................................................................ 18 Figure 2.2: Summary of green roof types based on IGRA classification (IGRA, 2008). From Fernandez et al 2013 .............................................................................................................................................................. 19 Figure 2.3: schematic diagram explaining technosol pedogenesis (Huoet et al, 2015) ................................... 26 Figure 2.4 : Schematic diagram representing originality of the Thesis ............................................................ 29 Figure 3.1 Green roof system's interactions between its global characteristics, internal properties, and external factors. .................................................................................................................................... 37 Figure 3.2 Yearly evolution of type of studies (modelling, in-situ experiments, and lab experiments) conducted. .............................................................................................................................................................. 39 Figure 3.3 Heatmap depicting external factors influence on performances (FPR). ......................................... 40 Figure 3.4 Heatmap depicting internal properties influence on performances (PPR). .................................... 40 Figure 3.5 Influence index of FPR on respective hydrological and thermal performances, showing largely negative influence. ................................................................................................................................ 41 Figure 3.6 Influence index of PPR on respective hydrological and thermal performances, showing largely positive influence. ................................................................................................................................. 42 Figure 4.1: Graphical abstract Heatmap depicting property evolution of all factors for each substrate compared to initial reference sample ..................................................................................................................... 60 Figure 4.2: Sketch depicting the problematic of this study ............................................................................. 63 Figure 4.3: pictorial representation of the substrates studied with the piles based on the granulometry (< 2 mm; 2< < 5 mm organic, 2< < 5 mm mineral, > 5 mm organic, > 5 mm mineral) .................................... 65 Figure 4.4: Photo depicting the in house designed rain simulating system with a possibility to set the intensity of precipitation (right) ........................................................................................................................... 66 Figure 4.5: Schematic representation of mesocosms sacrificed for each substrate per modality over time ... 67 Figure 4.6: Heatmap depicting property evolution compared to initial reference sample for different time periods (T2, T4, T8) for the substrates P, I, X for all the three factors (P, V, F) ....................................... 71 Figure 4.7: Heatmap depicting property/performance evolution compared to reference sample of the corresponding week for different time periods (T2, T4, T8) for the substrates P, I, X for all the three factors (P, V, F) ...................................................................................................................................... 72 Figure 4.8: Evolution of bulk density (A) solid density (B) and porosity (C) over the experimental time period for the substrates P, I and X. Vertical lines represent standard deviation. ............................................. 73 Figure 4.9 : Evolution of bulk density for different factors for the substrates P, I and X for reference sample (brown), rain (blue), vegetation (green) and freeze/thaw (red). The color codes for each substrate are depicted in the graph background. ........................................................................................................ 74

Figure 4 . 11 :

 411 Figure 4.11: Evolution of WRC of different factors for the substrates P, I and X for reference sample (dark red), rain (blue), vegetation (green) and freeze/thaw (red). The color codes for each substrate are depicted in the graph background. ..........................................................................................................................

Figure 4 . 12 :

 412 Figure 4.12: Comparison of Evolution of WRC of reference samples for different time periods for the substrates P, I and X. (1) with Evolution of WRC for different time periods for the substrates P, I and X considering Average of all factors (2) .......................................................................................................................

Figure 4 . 13 :

 413 Figure 4.13: Evolution of WRC for different time periods for the prrecipitation factor for substrates P, I and X. ..............................................................................................................................................................

Figure 4 . 14 :

 414 Figure 4.14: Evolution of microbiological parameters of the three substrates under the effect of vegetation. The color codes for each substrate is depicted in the graph background. ..............................................

Figure 4 . 15 :

 415 Figure 4.15: Evolution of the pH between the different substrates (P, I and X) subjected to the different factors. R0: pH of Substrates before ageing, R: Evolutionary reference without any factors applied, P: precipitation, F: Freeze/thaw and V: Vegetation. ..................................................................................

Figure 4 . 16 :

 416 Figure 4.16: Evolution of the fine fraction (< 2mm) within substrate P for different depths (A) and different factors (B) ..............................................................................................................................................

Figure 4 . 17 :

 417 Figure 4.17: Evolution of carbon (A) and Nitrogen (B) concentration of substrates for considered factors ....

Figure 5 . 1 :

 51 Figure 5.1: Graphical abstract Heatmap depicting property evolution of all the considered substrates at the depth of 5-10 cm (or 7-12 cm) compared to top layer of 0-5 cm (or 2-7 cm). .........................................

Figure 5 . 2 :

 52 Figure 5.2: Heatmap depicting property evolution of all the considered substrates at the depth of 5-10 cm (or 7-12 cm) compared to top layer of 0-5 cm (or 2-7 cm). ..........................................................................

Figure 5 . 3

 53 Figure 5.3 Bulk density evolution for the considered substrates depth wise (left) and average of depths (right) for TBLN (green) and SMDD (blue) substrates. ......................................................................................

Figure 5 . 4 :

 54 Figure 5.4: Poral architecture evolution for the considered substrates depth wise (left) and average of the depths (right) for TBLN (green) and SMDD (blue) substrates. ................................................................

Figure 5 . 5 :

 55 Figure 5.5: Evolution of volumetric content in the WRC of different studied substrates at D5 (A) and D10 (B) ..............................................................................................................................................................

Figure 5 . 6 :

 56 Figure 5.6: (A) (average of both depths) and (B) (average of each substrate site at D5 and D10) which represents the evolution of volumetric content in the WRC of different studied substrates .................

Figure 5 . 7 :

 57 Figure 5.7: Sieve analysis results of SMDD substrates by percentage at D5 and D10. ..................................

Figure 6 . 1 :

 61 Figure 6.1: Summary of green roof pedogenesis over time considering the internal substrate evolution and also the influence of external factors. .................................................................................................

Figure 6 . 2 :Figure 6 . 3 :

 6263 Figure 6.2: Substrate coarse aggregate percentage evolution of all considered substrates (initial and aged) ............................................................................................................................................................ Figure 6.3: Granulometric evolution pattern for the studied substrates of controlled condition experiments for the time periods T4 and T8 at various depths (cm). same color at different depth for a particular fraction indicate either a gradual increase (red) or a gradual decrease (green). ..................................

Table 1 . 1 :Table 3 . 2 Table 3 . 3 Table 3 . 4 Table 5 . 5 : 1 BACKGROUND 1

 113233345511 Ecosystem services provided by green roofs ordered as per it's Class, service and category. Adapted from (Blanchart, 2017.) ......................................................................................................................... Final performances considered ....................................................................................................... Final factors considered. ................................................................................................................. Final properties considered. ........................................................................................................... Characteristics of the studied green roofs under real climatic conditions ..................................... Chapter Background and Thesis structure

  are used to improve the performance and/or aesthetics of the roof[START_REF] Snodgrass | The green roof manual: a professional guide to design, installation, and maintenance[END_REF].The ecosystem services of water and thermal regulation can be measured through the evaluation of these performances. The direct and indirect contributions of a green roof to human well-being are referred to here as "performance" (Zhang et al 2021, Zheng et al 2021, Schade et al 2021); more specifically, they describe the ability of a green roof to perform certain potential and expected functions. The performances are, for example, the water retention capacity of the roof, its capacity to insulate the building or its capacity to contain or attract biodiversity.Green roofs are intended to reintroduce a living component into the city by superimposing themselves on urban structures.Two external constraints have defined the development of green roof technologies. The first one is external which concerns the need to maintain the water tightness of the roof despite the presence of roots, as the fundamental role of a roof is to protect the property and its habitants. The development of anti-root membranes combined with conventional root protections for roofs (bituminous layers in particular) has made it possible to solve this problem. The second constraint in this external category is that of the weight. At a time when the precision of architectural techniques allows to calculate precisely the loads supported floor by floor, little load margin is generally left for roofs, except for snow and technical elements. This led to development appropriate membranes and light substrates,

Figure 2 . 1 :

 21 Figure 2.1: Standard stratigraphy of an extensive green roof: 1) building support and insulation; 2) sealing complex; 3) drainage layer; 4) geo-membrane filter; 5) substrate and 6) vegetation, Source: Green Roof service LLC.

  These three types of green roofs are characterized by different types of intentionally introduced vegetation. For example, the extensive roofs with a shallow substrate, which dries more quickly, have a plant diversity based on the Sedum genus. The species of this genus are shallow rooted on the one hand, and on the other, have acquired a physiology adapted to desiccation and water stress. Intensive roofs can accommodate shrub vegetation, even some trees, as well as all the plants of the herbaceous stratum (Nagase et al 2010).

Figure 2 . 2 :

 22 Figure 2.2: Summary of green roof types based on IGRA classification (IGRA, 2008). From Fernandez et al 2013

  They originate from the transformation and modification of parent materials under the influence of pedogenic factors: climate, parent materials, nature, topography and biological activity (Oldeman et al, 1994). Recently, a new soil class has emerged in the classifications to describe soils whose properties and pedogenesis are dominated by their technical origin: Technosols (IUSS, 2014) otherwise called Anthroposols in the French classification (Baize and Girard, 2008). They contain a significant amount of artefacts (something in the soil recognizably made or strongly altered by humans or extracted from greater depths) or are sealed by technic hard material (hard material created by humans, having properties unlike natural rock) or contain a geomembrane. Green roofs appear as types of Technosols with physical and hydrological properties different from natural soils.

(

  [START_REF] Heidarinejad | Numerical simulation of the dual effect of green roof thermal performance[END_REF]) estimated a 77% reduction in heat flux due to Sedum canopy planted on 10 cm substrate, compared to a concrete roof. Similarly, Compared to a gravel roof,[START_REF] Getter | Seasonal heat flux properties of an extensive green roof in a Midwestern U.S. climate[END_REF]) observed a 167% reduction in summer incoming heat fluxes due to the presence of a 5 cm substrate and a Sedum cover. Even if the measurement methods are not the same, the comparison of the temperature of the surfaces located at the base of vegetation systems and that of bare surfaces gives us an idea of effectiveness of a green roof in reducing heat.[START_REF] Susca | Positive effects of vegetation: Urban heat island and green roofs[END_REF] observed at New York (United States) that a black bituminous membrane (very present on flat roofs) could reach 80°C, compared to 40°C for the same membrane protected by 10 cm of substrate planted by Sedum. While the magnitude of results like these are important, effectiveness of the green roof must be put into perspective in view of the technical characteristics of the roof itself such as insulation.[START_REF] Niachou | Analysis of the green roof thermal properties and investigation of its energy performance[END_REF]) thus sought to highlight the way in which three types of insulation (good, intermediate, bad) influenced the incoming heat flows via a green roof (features not specified) and a bare roof. For a poorly insulated roof, the roof reduced incoming flows by up to 89%. On the other hand, for a well-insulated roof, the reduction was only 7%. (Silva et al 2016)

  (Séré et al, 2010; Huot et al, 2015; Leguédois et al, 2016) (Figure 2.3).

Figure 2 . 3 :

 23 Figure 2.3: schematic diagram explaining technosol pedogenesis (Huoet et al, 2015)

Figure 2 . 4 :

 24 Figure 2.4 : Schematic diagram representing originality of the Thesis

Figure 3 . 2

 32 Figure 3.2 Yearly evolution of type of studies (modelling, in-situ experiments, and lab experiments) conducted.

Figure 4 . 1 :

 41 Figure 4.1: Graphical abstract Heatmap depicting property evolution of all factors for each substrate compared to initial reference sample

Figure 4 . 2 :

 42 Figure 4.2: Sketch depicting the problematic of this study

Figure 4 . 3 :

 43 Figure 4.3: pictorial representation of the substrates studied with the piles based on the granulometry (< 2 mm; 2< < 5 mm organic, 2< < 5 mm mineral, > 5 mm organic, > 5 mm mineral)

Figure 4 . 4 :

 44 Figure 4.4: Photo depicting the in house designed rain simulating system with a possibility to set the intensity of precipitation (right)

Figure 4 . 5 :

 45 Figure 4.5: Schematic representation of mesocosms sacrificed for each substrate per modality over time4.3.5 Substrate characterisation and Analysis

Figure 4 .

 4 Figure 4.6 represents an Heatmap depicting property evolution compared to initial reference sample.i.e., all the results of each time period were normalised with that of the reference sample at T0. On a global view, major chunk of the blocks (60.8 %) is in the ratio of 0.8-1.2, indicating that in most cases there is no drastic difference compared to the initial reference sample. In fact, 100 % of properties related to porosity, density, pH and granulometry stay within this ratio, irrespective of the substrates, factors, and time periods. All the significant changes (>25 %) are observed in volumetric water content of the water retention curve (WRC) where the ratio goes as low as 0.2 (Ө = 0.3 at T8 for the vegetation sample of substrate I). Majority (72 %) of the values stay between the ratio 0.4-0.6 here.

Figure 4 . 6 :Figure 4 .

 464 Figure 4.6: Heatmap depicting property evolution compared to initial reference sample for different time periods (T2, T4, T8) for the substrates P, I, X for all the three factors (P, V, F) Figure 4.7 represents an Heatmap depicting property evolution compared, i.e. normalised, that of the reference samples at that particular time period. On a global view, compared to figure 4.6, even more chunk of the blocks (84%) is in the ratio of 0.8-1.2, indicating that in most cases there is no drastic difference compared to the reference sample of that particular time period. In fact, again, 100 % of properties related to porosity, density and granulometry stay within this ratio, irrespective of the substrates, factors, and time periods. All the considerable changes (>25 %) are observed in water retention curve (WRC). However, this time, wherever the changes are noticed, the influence is not just negative (reduction) like in figure 4.6. The ratio goes as low as 0.5 (Ө = 0.3 at T8 for substrate X submitted to precipitation) but also as high as 1.4 (Ө = 0.3 at T4 for substrate P submitted to freeze/thaw and Ө = Өsat at T8 substrate I).

Figure 4 . 7 :

 47 Figure 4.7: Heatmap depicting property/performance evolution compared to reference sample of the corresponding week for different time periods (T2, T4, T8) for the substrates P, I, X for all the three factors (P, V, F)

Figure 4 .

 4 Figure 4.8 represents evolution of bulk density (a) solid density (b) and porosity (c) over the experimental time period for the substrates P, I and X considering the average of all factors.No significant evolution over time is observed. However, for density, it can be noted that, albeit small, all 3 substrates evolve differently, especially after time period T2, which can be noted in the bulk density graph.

Figure 4 . 8 :

 48 Figure 4.8: Evolution of bulk density (A) solid density (B) and porosity (C) over the experimental time period for the substrates P, I and X. Vertical lines represent standard deviation.

Figure 4 .

 4 Figure 4.9 represents evolution of bulk density of all studied factors and reference for the substrates P, I and X considering the average of all time periods. No significant difference is observed. However, trends can be seen. Precipitation is the factor with highest value for all three substrates. Precipitation is also the only factor whose value is higher than the reference sample evolution for substrates P and X, showing positive influence. For substrate I, like other factors, It remains lesser than that of the reference sample.

Figure 4 . 9 :

 49 Figure 4.9 : Evolution of bulk density for different factors for the substrates P, I and X for reference sample (brown), rain (blue), vegetation (green) and freeze/thaw (red). The color codes for each substrate are depicted in the graph background.

Figure 4 .

 4 Figure 4.10 represents evolution of porosity distribution for different factors and different time periods for the substrates P (A), I (B) and X (C).

Figure 4 . 10 :

 410 Figure 4.10: Evolution of porosity distribution for different factors and different time periods for the substrates P, I and X for reference sample (brown), rain (blue), vegetation (green) and freeze/thaw (red). The color codes for each substrate are depicted in the graph background.

Figure 4 . 11 :

 411 Figure 4.11: Evolution of WRC of different factors for the substrates P, I and X for reference sample (dark red), rain (blue), vegetation (green) and freeze/thaw (red). The color codes for each substrate are depicted in the graph background.

Figure 4 .

 4 Figure 4.12 represents comparison of evolution of WRC of reference samples for different time periods for the substrates P, I and X. (1) with Evolution of WRC for different time periods for the substrates P, I and X considering Average of all factors[START_REF] Vergriete | Analyse préliminaire de l'aménagement du site du garage municipal de Ville de Laval[END_REF]. For the reference samples, evolution pattern for substrate P is different compared to that of I and X. Substrates I and X have a significant decrease at T2 (>30 %) and lesser but considerable decrease again T4. However, the behaviour at T8 is different as it tends to increase slightly towards initial values for substrates P and X. There is a similar evolution pattern to that of the reference samples when the factors' average is considered. However, the tendency to lean towards the initial values at T8 is more prominent here compared to the reference samples. This is evident for substrate P where the WRC is higher at T8 than at T2 and T4.

Figure 4 . 12 :Figure 4 .

 4124 Figure 4.12: Comparison of Evolution of WRC of reference samples for different time periods for the substrates P, I and X. (1)with Evolution of WRC for different time periods for the substrates P, I and X considering Average of all factors[START_REF] Vergriete | Analyse préliminaire de l'aménagement du site du garage municipal de Ville de Laval[END_REF]

Figure 4 . 13 :

 413 Figure 4.13: Evolution of WRC for different time periods for the prrecipitation factor for substrates P, I and X.

Figure 4 . 14 :

 414 Figure 4.14: Evolution of microbiological parameters of the three substrates under the effect of vegetation. The color codes for each substrate is depicted in the graph background.

Figure 4 . 15 :

 415 Figure 4.15: Evolution of the pH between the different substrates (P, I and X) subjected to the different factors. R0: pH of Substrates before ageing, R: Evolutionary reference without any factors applied, P: precipitation, F: Freeze/thaw and V:Vegetation.

Figure 4 . 16 :

 416 Figure 4.16: Evolution of the fine fraction (< 2mm) within substrate P for different depths (A) and different factors (B)

Figure 4 . 17 :

 417 Figure 4.17: Evolution of carbon (A) and Nitrogen (B) concentration of substrates for considered factors

Figure 5 . 1 :

 51 Figure 5.1: Graphical abstract Heatmap depicting property evolution of all the considered substrates at the depth of 5-10 cm (or 7-12 cm) compared to top layer of 0-5 cm (or 2-7 cm).

Figure 5 . 2 :Figure 5 .

 525 Figure 5.2: Heatmap depicting property evolution of all the considered substrates at the depth of 5-10 cm (or 7-12 cm) compared to top layer of 0-5 cm (or 2-7 cm).

Figure 5 . 3

 53 Figure 5.3 Bulk density evolution for the considered substrates depth wise (left) and average of depths (right) for TBLN (green) and SMDD (blue) substrates.

Figure 5 . 4 :

 54 Figure 5.4: Poral architecture evolution for the considered substrates depth wise (left) and average of the depths (right) for TBLN (green) and SMDD (blue) substrates.

Figure 5 .

 5 Figure 5.5 represents evolution of volumetric content in the WRC of different studied substrates at D5(A) and D10 (B) respectively. While the evolution pattern for each of the substrate is different, it can be noted that the reduction in volumetric water content as the matrix potential increases tends to occur at a higher rate for SMDD substrates compared to TBLN for both the depths D5 and D10 (More than 5 percent decrease for SMDD and around 2 percent decrease for TBLN on an average). This phenomenon is even more pronounced at D10 than D5. These can also be noticed in Figure5.6 (A) (average of both depths) and Figure5.6 (B) (average of each substrate site at D5 and D10) which represents the evolution of volumetric content in the WRC of different studied substrates. The depiction in figure5.4(B) shows that TBLN (avg at D10) starts at a lesser volumetric content of 32% at pF2 compared to 38% for SMDD (avg at D10), they both end up having similar volumetric content (30% and 31%) at pF4.2.

Figure 5 .

 5 Figure 5.5 represents evolution of volumetric content in the WRC of different studied substrates at D5(A) and D10 (B) respectively. While the evolution pattern for each of the substrate is different, it can be noted that the reduction in volumetric water content as the matrix potential increases tends to occur at a higher rate for SMDD substrates compared to TBLN for both the depths D5 and D10 (More than 5 percent decrease for SMDD and around 2 percent decrease for TBLN on an average). This phenomenon is even more pronounced at D10 than D5. These can also be noticed in Figure5.6 (A) (average of both depths) and Figure5.6 (B) (average of each substrate site at D5 and D10) which represents the evolution of volumetric content in the WRC of different studied substrates. The depiction in figure5.4(B) shows that TBLN (avg at D10) starts at a lesser volumetric content of 32% at pF2 compared to 38% for SMDD (avg at D10), they both end up having similar volumetric content (30% and 31%) at pF4.2.

Figure 5 . 5 : 5 . 3 . 4 Figure 5 .

 555345 Figure 5.5: Evolution of volumetric content in the WRC of different studied substrates at D5 (A) and D10 (B)

Figure 5 . 7 :

 57 Figure 5.7: Sieve analysis results of SMDD substrates by percentage at D5 and D10.

Figure 6 . 1 :

 61 Figure 6.1: Summary of green roof pedogenesis over time considering the internal substrate evolution and also the influence of external factors.

  ion under t he inf luence of int ernal propert ies and ext ernal f act ors evolut ion in t he init ial 3-4 years Prominent f act or act ion af t er init ial 3-4 years

Figure 6 . 2 :

 62 Figure 6.2: Substrate coarse aggregate percentage evolution of all considered substrates (initial and aged)

Figure 6 . 3 :

 63 Figure 6.3: Granulometric evolution pattern for the studied substrates of controlled condition experiments for the time periods T4 and T8 at various depths (cm). same color at different depth for a particular fraction indicate either a gradual increase (red) or a gradual decrease (green).

  Acronyms.

	ADWP	Antecedent dry weather period
	BD	Bulk density
	BMC	Microbial biomass
	CF	Fumigated organic carbon
	CV, CI, CS, CC	Substrates sampled from Cerema, Tomblaine site.
	CNF	Non fumigated organic carbon
	D5 & D10	Depths of in situ substrate sampling in cm
	F, P, V	Factors freeze/thaw, Precipitation & Vegetation
	FDA	Fluorescein di-acetate
	FPR	Factor performance relations
	FPPR	Factor property performance relations
	GR	Green roof
	I	Substrate Sopraflor I (soprema®)
	LECA	Light expanded clay aggregate
	MAI	Microbial activity index
	MWHC	Maximum water holding capacity
	NBS	Nature based solutions
	P	Substrate Sopraflor P (soprema®)
	PPR	Property performance relations
	PRB, PRC, PRV	Substrates sampled from Saint-Mars du Désert
	SOM	Soil organic matter
	SMDD	Green roof site of Saint-Mars du Désert
	TOC	Total organic content
	TBLN	Green roof site of Cerema, Tomblaine.
	WHC	Water holding capacity
	WOS	Web of science
	WRC	Water retention curve
	X	Substrate Sopraflor X (soprema®)

Table 1 .

 1 1: Ecosystem services provided by green roofs ordered as per it's Class, service and category. Adapted from

		(Blanchart, 2017.)
	Classes	Services	Services category
			O3, CO2
		Air quality	Fine particles
			Acid Rain
	Regulation		Local climate
		Climate
			Interior Climate
		Water purification	Pollutant degradation
		Natural risks	Floods
		Food	Vegetables
		Energy	Solar
		Habitation	Biodiversity
	Goods and services
			Vegetation cover
		Ornamental resources
			Flowers
			Scenic / Contemplation / aesthetics /
	Cultural	Environmental attraction	amenities linked to the presence of
			nature

2.5.3.2 Substrate depth

	Getter et al, 2009) studied the evolution over time of a five-year-old
	extensive roof in Detroit (United States of America United States), composed of a 6 cm sandy-silt
	substrate and planted with mixed vegetation. After five years, the field capacity of the substrate had
	increased from 17.1 to 67.44%, probably through the increase in organic matter (from 2.33 to 4.25%)
	and microporosity (from 41,41 à 81,84 %). In addition, Graceson et al, (2013) studied several tile or
	brick-based substrates and have shown that intra-and inter-particle porosity are two important
	factors to be taken into account to promote water retention

Roof of Greenery, but a Sky of Unexplored Relations-Meta-Analysis of Factors and Properties that Affect Green Roof Hydrological and Thermal Performances

	META ANALYSIS
	A (This part of our work is a published article (https://doi.org/10.3390/su131810017) and is presented
	as it is)

3 Meta-analysis (Published article) A Roof of Greenery, but a Sky of Unexplored Relations-Meta-Analysis of Factors and Properties that Affect Green Roof Hydrological and Thermal Performances 3.1. Introduction

Table 3 .

 3 2 Final performances considered

	Hydrological Performances	Thermal Performances

Table 3 .

 3 3 Final factors considered.

	Factors
		Rain intensity
	Rain characteristics	Rain frequency
		Rain duration
	Atmospheric conditions	Air humidity
		Wind speed
		Average temperature
	Temperature characteristics and heat fluxes	Maximum temperature Solar radiation
		Latent heat
		Building slope
	Building characteristics	Building height
		Nature of covering material

Table 3 .

 3 4 Final properties considered.

	Properties
	Age of the green roof	Date of implementation
	Global substrate characteristics	Substrate composition Substrate depth Horizonation/layering
		Porosity
		Granulometry
	Substrate's physical	Bulk density
	characteristics	Solid density
		Water holding capacity
		Hydraulic conductivity
	Substrate's thermal	Thermal resistance
	characteristics	Solar reflectivity
		Percentage of vegetation
	Vegetation	Nature of vegetation
	characteristics	Leaf area index
		Diversity of plant species
	Biological activity	Soil fauna Soil micro organisms

Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable Conflicts of Interest:

  The authors declare no conflict of interest.

	Appendix A

Table A1 .

 A1 Tables depicting initial factors and properties considered before finalization.

			depth
			surface state
			horizonation/layering
			porosity
	Physical		granulometry bulk density
			solid density
			structure
	Class	Initial Factors Considered	aggregate stability Pore-size distribution Characteristics retention curve
		water-holding capacity intensity wilting point
	Hydric Rain	saturation point frequency hydraulic conductivity
			duration preferential flow
		hydrometeor van Genuchten parameters
			air temperature organic carbon
	Temperature		average nitrogen minimum pH
	Chemical		maximum CEC
			solar radiation available phosphorus
		conductive flux from the building mineralogy
	Heat fluxes Thermic		infrared radiation chemical conductivity latent heat sensible flux thermal conductivity thermal capacity
			storage flux germination capacity
	Biological		evenness microbial diversity
			slope microbial abundancy
		total surface organic matter addition (e.g., faeces deposition)
	Building surface	height of the building organic matter transformation
	Living organisms (vegetation, animals,	architecture burrowing activities
	microorganisms)	roughness (surrounding buildings) leaf area
		nature of the covering material biological diversity
	Wind		orientation speed biological abundancy
	Appendix B	air humidity
			fine particles deposition
			cloudiness
	Atmospheric and weather conditions	pollutants concentration greenhouse gases
		Initial Properties Considered
	Class		Characteristics
			composition
	Nature		manufacturer date of fabrication
		date of implementation

Table A2 ,

 A2 List of considered papers for Meta analysis after filtration[1] A. Graceson, M. Hare, J. Monaghan, and N. Hall, "The water retention capabilities of growing media for green roofs," Ecological Engineering, vol. 61, pp. 328-334, Dec. 2013, doi: 10.1016/j.ecoleng.2013.09.030. [2] W. Liu, Q. Feng, W. Chen, W. Wei, and R. C. Deo, "The influence of structural factors on stormwater runoff retention of extensive green roofs: new evidence from scale-based models and real experiments," Journal of Hydrology, vol. 569, pp. 230-238, Feb. 2019, doi: 10.1016/j.jhydrol.2018.11.066. [3] Liu and Chui, "Evaluation of Green Roof Performance in Mitigating the Impact of Extreme Storms," Water, vol. 11, no. 4, p. 815, Apr. 2019, doi: 10.3390/w11040815.

	Chapter 4
	CONTROLLED CONDITION EXPERIMENTS
	Factors or Actors?
	A controlled condition accelerated ageing experimental
	evaluation of influence of individual external factors on the
	green roof pedogenesis.

  al, 2010; Huot et al, 2015; Leguédois et al, 2016).

  I and X are composed of mineral aggregates, pozzolana, pumice, professional peat, sand and vegetable compost, in different proportions. Their Bio-physicochemical characteristics and pictorial representation are mentioned in table 4.1 and figure 4.3 respectively.

Table 4 .

 4

	Subst rat e	P	I	X
	Characteristic
	Organic carbon (%)	7.73	3.04	2.82
	Total nitrogen (%)	0.33	0.05	0.09
	BMC (mgC.kg-1 dry soil)	670.9	115.3	67.4
	FDA (μg FDA.g-1 sol sec /h)	206.4	119.7	87.1
	Granulo < 2mm (%)	68.8	9.07	9.61
	Granulo 2 < x < 5mm (%)	24.35	19.14	29.67
	Granulo > 5mm (%)	6.86	71.79	60.72
	CEC (mol.kg-1)	21.9	3.1	8.5
	Real density (g.cm-3)	2.52	2.52	2.77
	Apparent density (g.cm-3)	0.59	0.65	0.81
	Field capacity	0.22	0.16	0.15
	Total Porosity (%)	0.77	0.74	0.71

1: Bio-physicochemical characteristics of the substrates studied

  -1 of soil. Its composition was inspired by the article by Baudoin et al, published in 2003 and repeated in the article by Liao et al, published in 2021: 26.7 mmol.L -1 glucose, 26.7 mmol.L -1 fructose, 13.3 mmol.L -1 sucrose, 13 mmol.L -1 citric acid, 26.7 mmol.

	L -1
	lactic acid, 19.8 mmol.L -1 succinic acid, 13.3 mmol.L -1 alanine, 13.3 mmol.L -1 serine, and 7.8 mmol.L -1
	glutamic acid yielding an artificial exudate solution containing 1000 mg.L -1 dissolved organic carbon
	(DOC).

  Also, Average WHC mentioned by existing researchers for extensive green roofs are 45% (DeNardo et al, 2005, Mentens et al, 2006) and 60% (Moran et al, 2003), and cumulative annual retentions of 50% (Stovin et al, 2012) and 60% (VanWoert et al, 2005). Our results regarding these are in accordance with this.

  Introduction et état de l'artLa densification des zones urbaines tend à modifier fortement le cadre de vie. Les surfaces étanches (e.g. bâtiments, toitures, allées pavées, routes et parkings) occupent une place prépondérante dans ces espaces, soulignant le danger de l'imperméabilisation des villes, phénomène qui favorise l'augmentation du ruissellement de surface. Cette croissance urbaine soulève des problématiques complexes et questionne la durabilité et le fonctionnement des modèles urbains à travers des problématiques telles que l'effet d'îlot de chaleur urbain, l'augmentation de la demande énergétique et la perte de biodiversité. Le concept d'ingénierie écologique, basé sur l'action par et/ou pour le vivant, ainsi que les solutions fondées sur la nature (NBS) apparaissent comme des approches pertinentes pour répondre aux défis des environnements urbains.Parmi les techniques permettant d'introduire la nature en ville, les toitures végétalisées présentent un fort potentiel. En tenant compte de divers concepts, les toitures végétalisées ont été immensément appréciées et étudiées dans de nombreuses villes du monde comme un outil permettant de contribuer à la résolution des problèmes environnementaux urbains. Intégrées au bâtiment, elles ne consomment pas d'espace supplémentaire et peuvent même être mises en oeuvre sur des bâtiments existants, ce qui est susceptible d'encourager leur diffusion massive. Cet intérêt se justifie principalement par leur capacité à répondre à ces nouveaux défis urbains en fournissant des services dits "écosystémiques". Ces derniers sont définis comme les biens et services que les humains peuvent obtenir des écosystèmes, directement ou indirectement, pour assurer leur bien-être. Les sols urbains fournissent de nombreux services écosystémiques essentiels aux environnements urbains[START_REF] Blanchart | Vers une prise en compte des potentialités des sols dans la planification territoriale et l'urbanisme opérationnel[END_REF] et les toitures végétalisées, spécifiquement, fournissent plusieurs services dont les humains peuvent bénéficier. Etant donné que la science a déjà pris du retard par rapport à l'évaluation de ces performances, cette thèse vise à analyser, évaluer et mesurer quelques performances éminentes parmi ces performances revendiquées et, plus important encore, évaluer leur évolution et leur interaction avec le système de toiture végétalisée au fil du temps et analyser leur corrélation et leur durabilité. Cela peut être utile pour les urbanistes et les promoteurs en ce qui concerne la conception et le suivi des toitures végétalisées pour les services écosystémiques souhaités.La nature même des composants des substrats des toitures végétalisées résultant du mélange de matériaux très différents d'origine naturelle (ex : tourbe, pouzzolane) ou artificielle (ex : compost, briques) leur confère une réactivité potentiellement élevée. Les Technosols -plus particulièrement classés comme " Histi-spolic Technosol andic " (Ryad Bouzouidja et al, 2018) -sont bien connus pour être des environnements extrêmement réactifs, sous l'effet des changements intenses et rapides de leurs propriétés physico-chimiques(Séré et al, 2010). Par conséquent, on peut s'attendre à une évolution significative de l'architecture poreuse des substrats au cours du temps, ce qui modifierait leurs propriétés hydrauliques[START_REF] Kutílek | The structural porosity in soil hydraulic functions -a review[END_REF].Ce sujet est actuellement rarement abordé dans la littérature scientifique qui considère la toiture végétalisée comme un milieu statique. Ce besoin de recherche est né du fait que la plupart des études pertinentes menées à ce jour ont négligé cette dynamique temporelle de l'évolution des toitures végétalisées et des propriétés du Technosol hautement réactives. Cette thèse croise des approches d'une part basée sur le vieillissement des matériaux inertes et d'autre part sur la pédogenèse qui décrit l'évolution des milieux vivants. Elle vise ensuite à évaluer l'évolution des performances des toitures végétalisées au fil du temps. Plus précisément, notre travail cherche à décrire l'influence des facteurs externes sur les propriétés internes des toitures végétalisées et donc leur conséquence sur leurs performances hydrologiques et thermiques.A travers plusieurs étapes d'exploration, dont une revue approfondie de la littérature, une métaanalyse extensive, des expériences en conditions contrôlées et la caractérisation d'échantillons vieillis in situ, cette thèse visait à apporter une réponse pertinente à cette question. Le travail s'est concentré sur une échelle allant d'un échantillon de substrat de toiture végétalisée (quelques centaines de grammes) à une parcelle de toiture végétalisée (quelques dizaines de m²).

Cerema DTer-Est / LRN, 71 rue de la grande haie, Tomblaine F-54510, France; remy.claverie@cerema.fr

Laboratoire Sols et Environnement, Université de Lorraine-INRA, 2 avenue de la Forêt de

Funding: This work is financially supported by the Centre for Studies and Expertise on Risks, Mobility, Land Planning, and the Environment (Cerema).

Chapter 2

LITERATURE REVIEW

Chapter 5 IN SITU EXPERIMENTS

A real site green roof substrate property evolution evaluation over 3-and 10-year periods.

Sampling

The sampling has been performed on October 16, 2020 on a no rainy, non-windy day for optimal sampling conditions. The strategy was based on the observation of the substrate's vertical profiles from previous observation (Bouzouidja et al, 2016). We decided to split them into two sub-horizons: 0-5 cm (or 2-7 cm) (D5) and 5-10 cm (or 7-12 cm) (D10). Each depth was replicated three times. Each time, undisturbed soil cores of volume 251 cm 3 were sampled through careful extraction using steel cylinders.

Substrate characterization

Bulk density (ρa) has been measured thanks to core-sampling. The solid density (ρr) was measured with a helium pycnometer (Quantachrome UltraPyc 1200) based on NF P18-554. Total porosity (δ) was then calculated after (Eq. 5.1).

𝛿 = 1 -𝜌 𝑎 𝜌 𝑟 (Eq.5.1)

Where ρa and ρr are respectively bulk and solid density (kg m -3 ).

The soil water retention characteristics, i.e. volumetric water content at various matrix potential, were determined on a pressure plate apparatus at -10 kPa (pF 2, that was considered to be moisture at field capacity: 𝜃 𝐹𝐶 ), -33 kPa (pF 2.5), -500 kPa (pF 3.7) and -1500 kPa (pF 4.2) respectively [START_REF] Bruand | Backscattered electron scanning images of soil porosity for analyzing soil compaction around roots[END_REF]. saturation, (Pf 0) could not be measured due to apparatus issues. Drainage + Macroporosity (D+M) and Mesoporosity + Microporosity (m+μp) were estimated according to (Peverill et al, 1999) (Eq.5.2 and Eq. 5.3):

𝐷 + 𝑀 = 𝛿 -𝜃 𝐹𝐶 (Eq.5.2) 𝑚 + 𝜇𝑝 = 𝜃 𝐹𝐶 (Eq.5.3) On a global view, it can be seen that 82% of the blocks are in the ratio of 0.85-1.15 (i.e. with 15% change), indicating that in most cases there is no drastic difference between the two depths. In fact, no block has more than 20% change. All the considerable changes (>10 %) are observed in volumetric water content of the water retention curve (WRC) of the samples collected from TBLN and sieve analysis of the samples collected from SMDD. For the WRC of CI substrate θ500kPa ratio goes as low as 0.84 and for the sieve analysis of the coarse fraction of PRB, it goes as high as 1.2.

Results

Global comparison by depth

Considering the sites of the collected substrate samples, there tends to be a contradiction in evolution. The porosity, solid density increased, and bulk density decreased for all the samples collected from TBLN whereas it is the opposite for the samples collected from SMDD. This effect is also noticeable in the retention curve at θ10kPa but not at other matrix potential.

Considering the properties, 100 % of the properties related to porosity and density undergo less than 10% change for all the substrates. Thus, most considerable changes (of >10%) occurred in the retention related properties of TBLN (where all of them decrease). SMDD samples behave differently at different θ and the substrate type, but none of the changes are >10%.

In the granulometry related properties of SMDD there are 3 out of 6 (50%) blocks with more than 10% change (where all of them increase).

Abstract (English)

The green roof system is a complex system that could be described by its "properties", considering on the nature and physical, chemical, and thermal properties of its abiotic components (i.e., substrate and drainage layers) and on the other hand its biotic components (i.e., vegetation, spontaneous fauna, and microbiota). As in all biotic/abiotic systems, complex interactions happen. First, the external system-here described as "factors"-induces an ageing effect that results in the evolution of "properties" over time (e.g., rain may induce leaching of fine particles; cold temperature may alter the vegetation development). Moreover, interactions between abiotic and biotic components may also induce evolution of "properties" (e.g., plant litter may increase the organic matter content in the substrate; decrease of the substrate physico-chemical fertility can decrease the biomass production).

Eventually, such inter-relations and interactions between all "factors" and "properties" can control the level of performances that could be submitted to changes over time. Though the system is complex, our approach sheds light on the potential of simplifying each Factor property performance relations and hence understanding the system evolution and performances better.

This research need originated from the fact that most of the relevant studies conducted to date have neglected these temporal dynamics of green roof evolution and their high reactive Technosol properties. This PhD aimed to reach a crossroad point between ageing -of inert materials and pedogenesis-that describes evolution of living media and to evaluate the performance evolution of green roof over time.

To evaluate this, first, a meta-analysis was conducted whose main findings highlighted that most factors and properties have a positive influence on the performances of green roofs, showing there are many existing levers to enhance the green roof performances and tackle some of the main urban environmental issues. But, considering that these lists were far from exhaustive, it was noted that a huge potential in determining green roof performances remains unearthed.

Thus, experiments were designed and conducted with the purpose to reproduce certain factors considered as influential (i.e. rain, vegetation and freeze/thaw alternation), under controlled conditions, on mesocosms of 3 different green roof substrates chosen based on composition and granulometry. Their substrate properties were monitored over time through self-designed artificial aging over a period of 2 months to mimic real time evolution.

Our results showed evidence of early pedogenesis especially for the finer substrate. Depending on the substrate: i) vegetation stimulates or maintains microbiology; ii) rain modifies granulometry through leaching processes; iii) frost modifies granulometry through fractionation. There was also change in the poral structure thus modifying the water retention performance. Other than that, the changes in the performance of other studied properties seem more due to the temporal evolution rather than factorial based. It was also noted in monitoring of the reference samples which revealed: a decrease or increase in pH depending on the substrate, small variations in microbiology and in organic carbon and total nitrogen concentrations.

Résumé (French) (Short)

Une toiture végétalisée (TV) est un système complexe qui peut être décrit par ses propriétés, en considérant d'une part les propriétés de ses composants abiotiques (substrat et couche de drainage) et d'autre part ses composants biotiques (végétation, faune spontanée et microbiote). Comme dans tous les systèmes biotiques/abiotiques des interactions complexes se produisent. Tout d'abord, le système externe -introduit ici comme étant des facteurs -induit un effet de vieillissement qui se traduit par l'évolution des propriétés. De plus, les interactions entre les composants abiotiques et biotiques peuvent également induire une évolution des propriétés.

Ces interrelations et interactions entre tous les facteurs et propriétés peuvent contrôler le niveau des performances qui pourraient être soumis à des changements dans le temps. Notre approche met en lumière la possibilité de simplifier les relations entre les performances de chaque facteur et donc de mieux comprendre l'évolution et les performances d'une TV.

Ce besoin de recherche est né du fait que la plupart des études pertinentes menées à ce jour ont négligé cette dynamique temporelle de l'évolution des TV et des propriétés du Technosol hautement réactives. Cette thèse vise à atteindre un point de croisement entre le vieillissement des matériaux inertes et la pédogenèse qui décrit l'évolution des milieux vivants puis à évaluer l'évolution des performances des TV au fil du temps.

Pour ce faire, une méta-analyse a d'abord été menée, dont les principaux résultats ont mis en évidence que la plupart des facteurs et des propriétés ont une influence positive sur les performances des TV, montrant qu'il existe de nombreux leviers pour améliorer leurs performances et résoudre certains des principaux problèmes environnementaux urbains. Mais, considérant que ce recensement était loin d'être exhaustif, il a été noté qu'un énorme potentiel dans la détermination des performances des TV reste à découvrir.

Des expériences ont été conduites afin de reproduire dans des conditions contrôlées certains facteurs considérés comme influents (e.g. pluie, végétation, alternance gel/dégel), sur des mésocosmes de 3 substrats de TV différentes choisis en fonction de leur composition et de leur granulométrie. Les propriétés de leurs substrats ont été suivies en suivant un protocole de vieillissement artificiel sur une période de 2 mois pour simuler l'évolution en temporelle.

Nos résultats ont montré des preuves de pédogenèse précoce, en particulier pour le substrat le plus fin. Selon le substrat : i) la végétation stimule ou maintient la microbiologie ii) la pluie modifie la granulométrie par des processus de lessivage iii) le gel modifie la granulométrie par fractionnement. Il y a également un changement dans la structure porale, ce qui modifie les performances de rétention d'eau. De plus, les changements dans les performances des autres propriétés étudiées semblent plus dus à l'évolution temporelle qu'à une influence des facteurs. Le suivi des échantillons de référence ont révélé une diminution ou une augmentation du pH selon le substrat, de petites variations dans la microbiologie et dans les concentrations de carbone organique et d'azote total.

Enfin, des prélèvements in situ, nous ont permis de mesurer l'évolution des propriétés vieillies de 7 substrats provenant de deux sites d'âges différents et de 3 végétations différentes. Elles ont révélé que l'évolution est principalement déterminée par l'âge ; les substrats les plus jeunes (3 ans) subissant