Ou world

It is impossible to start a research without the expertise and consult of a supportive promotor. Thank you Martin, for suggesting the green roof topic to me in the first place. Because you are so committed to urban ecology, green roofs and green walls yourself, you were always able to point at interesting research options and eager to ask for my latest research results. We had very productive discussions after which I was always encouraged to continue. I was also surrounded and supported by a co-promotor of the Mediterranean I stitute of Biodi e sit a d Ma i e E olog IMBE at the I stitut U i e sitai e de Te h ologie d A ig o F a e . Thierry, je vous remercie pour votre aide pour le succès de cette thèse. Vous êtes une personne passionnée par la science et plus spécifiquement, tous les aspects concernant la restauration écologique. Comme pour moi, le concept des toitures végétalisées était relativement nouveau pour vous, mais vous avez été immédiatement convaincu des nombreux avantages de cette technique. La décision pour réaliser une convention de cotutelle internationale de thèse avec mon Université en Belgique a donc été prise rapidement. La première a e, j ai pass e uel ues mois dans votre unité de recherche et ous a ez ie aidé durant cette période (visites de terrain, détermination des espèces, recherche de la litt atu e g ise , etc.). Sans votre aide, la toiture végétalisée su le toit de l IUT au ait pas pu tre réalisée. Pe da t les ou tes isites à l IUT, ous a ez également toujours pris du temps pour discuter de mon sujet, et ous a ez ha ue fois proposé de me trouver un hébergement ou de venir me chercher à la gare.

I thank both my assessors, Bart Muys and Dirk Raes, and the other members of the jury, Nathalie Machon and Nigel Dunnett, who took the time to evaluate my work, gave suggestions, advice and comments which allowed me to further improve this thesis.

Special thanks go to Daniel Pavon, Renaud Jaunatre (UMR CNRS-IRD IMBE, Marseille, France) and Eric Van Beek (Division of Forest, Nature and Landscape, KU Leuven) for assistance in the field and help during plant identification. I am also grateful for the green roof materials, necessary for the experimental setup, offered by the following companies: SOPREMA nv; IBIC bvba; Peltracom nv; Rectivel bv. I d also like to tha k E as us, a helo a d aste stude ts. Cla a, Lau a, Karolien and Frederik, thank you all for helping me out and for staying motivated e e he the pla ts did t g o as e pe ted a d he it as f eezi g outside o the roof. It was nice to share my knowledge and enthusiasm about green roofs with you.

Je tie s aussi à e e ie les do to a ts à l IUT. Adeli e et "ol e, ous étiez toujours p se tes ua d j tais là pou uel ues se ai es. J ai pu loge hez ous, a ge hez ous, o oitu e a e ous… Ce fut un plaisir de vous aider, dans la Crau en cherchant le Saga pedo à cinq heures du matin, ou en préparant les terrines pour tester la banque de graines. De retour, et ceci vaut également pour Jean-F a çois, j ai énormément apprécié votre aide pou l installation de mon toit végétalisé. Je au ais ainsi pas pu réaliser ce travail toute seule.

In an era of urbanization, land use change, habitat loss and fragmentation, biodiversity is under pressure more than ever. This is an important issue as biodiversity is considered the major driver of ecosystem functioning and the provision of ecosystem services, and linked to that, our human well-being. Furthermore, urban areas face many environmental problems, including a risk of flooding due to a decrease in pervious surfaces, and above average temperatures related to building materials with higher heat storage capacity and lower albedo. Climate change is predicted to further enhance those problems, and the demand for sustainable strategies that can mitigate the situation is urgent. Urban green infrastructures, like street trees, parks, gardens, green walls and green roofs (GRs), comprise such a strategy. Offering a wide array of urban ecosystem services, including urban heat island effect reduction, stormwater management, habitat provisioning and aesthetic and amenity value, GRs have become an important tool for improving urban environmental quality. In essence, GRs are roofs with a vegetation layer on top. In this thesis we focus on the extensive GR type (EGRs, substrate depth < 20 cm) as these have a broad applicability and are more durable (i.e. less maintenance, self-sustaining).

Most expertise on EGRs is located in the temperate climate and cities in the Mediterranean are rather unfamiliar with the technique, which is probably due to the elevated temperatures and summer drought in these regions. In these regions, water is limited and climate change will lead to an even more severe water scarcity during summer. An irrigation system could help plant growth and survival, but it is perceived as an unsustainable and in some cases even impossible option. Therefore, more research is needed to select plant species suitable for Mediterranean unirrigated EGRs, and to adapt GR structure elements to meet the requirements of the selected plant species. As climate change will also lead to higher temperatures and erratic precipitation events in the temperate climate, these regions will also benefit from the resulting outcome of such research.

The main goal of this thesis is to elaborate and test a new concept for EGR design, comprising both plant selection and GR structure elements. The thesis includes a fieldwork study in southern France in which potential drought-adapted Mediterranean plant species were inventoried based on the habitat template vi hypothesis. In a GR context, this hypothesis states that natural habitats with similar characteristics as on EGRs should be targeted when searching for suitable plant species. Afterwards, the species list was screened for the most potential EGR plants aki g use of the pla ts fu tio al traits and utilitarian aspects. This screening p o edu e offe ed the s ie tifi asis to o st u t a eas -to-ha dle screening tool for optimal plant choice. As a case study, the tool was demonstrated on the Mediterranean plant list of the vegetation survey. In the following chapter, a subset of suitable Mediterranean plant species was tested for two years on experimental EGRs in both Belgium (temperate climate) and southern France (Mediterranean climate). The experimental setup included different substrate compositions (substrate depth 5 or 10 cm; presence/absence of a water retention layer) and exposition (exposed or sheltered). In a second experimental approach, the effects of different GR substrates on the growth of common GR plants were studied. The plots contained either a mineral substrate or one with additional organic material, and half of the plots were given a commercial arbuscular mycorrhizal inoculum. As the conducted experiments both focused on unirrigated EGRs, and observations pointed at severe plant stress during prolonged dry periods, results could probably be improved if regular irrigation had been provided. Therefore, a literature review was performed in order to highlight current irrigation techniques, possibilities to make the techniques more sustainable, and GR irrigation specifications in different climatic contexts. In a final chapter we looked at the biodiversity aspects of commercial GR systems. If we want to create a more sustainable urban environment, ecosystem services need to be improved. There is a consensus that ecosystem services can be predicted through functional (FD) and phylogenetic diversity (PD) of plant communities. In this chapter, currently available GR systems were analyzed in terms of FD and PD and two methods were proposed to select the species combination with maximal FD and hence ecosystem services value.

Based on the ecological and experimental knowledge obtained throughout the chapters of this thesis, a concept for EGR design in terms of vegetation and structure could be formulated. We conclude that Mediterranean vegetation can definitely inspire us as a source for development and improvement of EGR design, whether this is for the current Mediterranean climate itself or for the temperate climate under predicted climate change. Appropriate vegetation choice is essential, as well as the design in terms of sustainable irrigation techniques, appropriate substrate depth and composition, and water retention possibilities. Furthermore, many opportunities arise for additional GR research that will further contribute to more successful GRs with high biodiversity that are able to ensure and even improve ecosystem functioning and services under climate change.

Résumé vii

Résumé

Au ou s d u e e dite A th opo e et a a t is e pa l u a isatio , le ha ge e t d affe tatio des te es, la pe te d ha itat et la f ag e tatio p o o ue t des p essio s t s i po ta tes su la iodi e sit . Il s agit d u p o l e important car la biodiversité est souvent considérée comme le principal moteur du fonctionnement des écosystèmes et comme une source de services écosystémiques, elle-même corrélée avec notre bien-être. De plus, les zones urbaines font face à de o eu p o l es e i o e e tau , o p is le is ue d i o dation du fait de la diminution des surfaces perméables et de l'augmentation des températures relatives aux matériaux de construction. Ceux-ci étant caractérisés par une capacité de stockage de chaleur plus élevée et un albédo plus faible. Le changement climatique devrait aggraver ces problèmes et augmenter la demande en stratégies durables afin d'atténuer cette situation. Les infrastructures vertes urbaines, comme les arbres, parcs, jardins, murs végétalisés et toits verts sont les composantes de cette stratégie. Offrant un large éventail de services écosystémique comme la réduction de l effet d îlot de haleu u ai , la gestio des eau plu iales, des habitats pour la flore et la faune locale ai si u une certaine valeur esthétique; les toits verts sont devenus un outil important pour améliorer la qualité de l e i o e e t u ai . Ces toits e ts so t esse tielle e t d fi is o e des toits avec une couche de végétation dessus. Dans cette thèse, nous mettrons notamment l a e t su les toits e ts dit "e te sifs" (profondeur du substrat < 20 cm) car ils ont une gamme d'applications plus large et sont plus durables (car autonomes et nécessitant donc moins de maintenance).

La plupart des connaissances acquises sur les toits verts l'ont été en climat tempéré et peu de recherches ont été réalisées en climat Méditerranéen, probablement à cause de l'effet des fortes températures et de la sécheresse estivale. De plus, l'accès à l eau est li it et il le se a e o e plus du fait du ha ge e t li ati ue ui occasion e a u e p u ie d eau e o e plus g a e au ou s de la p iode esti ale. La ise e pla e de s st es d i igatio pou ait alo s aide e fa o isa t la oissa e des plantes et leur survie, mais cet aménagement est souvent perçu comme une option non soutenable, voir impossible dans certains cas. Par conséquent, une augmentation du nombre de recherches menées sur ce sujet est nécessaire afin de sélectionner les espèces végétales les plus adaptées aux toits verts extensifs non irrigués de la région Méditer a e e. Il est gale e t essai e d adapte les éléments structurels des toits extensifs pour mieux répondre aux exigences de ces plantes. Vu que les scénarios de changements climatiques prévoient également une augmentation des températures et des précipitations plus erratiques en climat te p , es gio s fi ie o t do gale e t des sultats d u e telle recherche.

L o je tif p i ipal de ette th se est d la o e et de teste u ou eau o ept pour la réalisation de toits verts extensifs, comprenant notamment la sélection de la végétation et les éléments de structure de ces nouveaux aménagements comme le substrat. La thèse comprend une étude de terrain dans le Sud de la France, dans laquelle des espèces végétales méditerranéennes adaptées à la sécheresse ont été i e to i es su la ase de l h poth se de "l ha itat od le". Da s le o te te des Résumé viii toits e ts, ette h poth se o e u il faut i le les ha itats atu els poss da t des caractéristiques similaires aux toits verts extensifs afin de trouver des espèces végétales les plus appropriées. La liste des espèces a ensuite été filtrée pour identifier les plantes qui possèdent le potentiel le plus fort pour être installées sur les toits verts extensifs. Ce filtre est basé sur l'utilisation des traits fonctionnels des plantes et de leurs aspects utilitaires dans ce contexte. Cette discrimination a permis la production d'une base scientifique pour construire un outil facile à utiliser pour choisir les plantes les plus favorables. A titre d'exemple, ce filtre a été appliqué à la liste des pla tes dite a e es utilis lo s de l tude de la g tatio . Da s le chapitre suivant, un sous-ensemble des espèces méditerranéennes a été testé pendant deux ans sur des toits verts extensifs mis en place en Belgique (climat tempéré) et dans le Sud de la France (climat méditerranéen). Le montage expérimental comprend différentes compositions de substrats (profondeur 5 ou 10 a e ou sa s ou he de te tio d eau et diff e tes e positio s plei soleil ou ombre). Dans une deuxième approche expérimentale, les effets de différents substrats sur la croissance de plantes, appliqués généralement sur des toits verts ont également été étudiés. Les parcelles contenaient alors soit un substrat minéral, soit un substrat avec de la matière organique. La moitié des parcelles ont aussi fait l'objet d'une inoculation d'une souche commerciale de mycorhizes. Ces deux expériences ont été menées en l'absence d'irrigation et les résultats ont montré que les plantes étaient particulièrement stressées durant les périodes de sécheresse prolongée. En conséquence, une recherche bibliographique a été réalisée afin de mieux identifier les te h i ues a tuelles d i igatio utilis es, et les possibilités pour rendre ces techniques plus durables afin de proposer de meilleures techniques d i igatio des toits verts pour des contextes climatiques différents. Dans un dernier chapitre, nous avons enfin examiné les aspects concernant la biodiversité des systèmes de toits verts extensif commerciaux. En effet, si nous voulons créer un environnement urbain plus durable, il faut en améliorer les services écosystémiques. Il y a de plus, un consensus croissant sur le fait que les services écosystémiques rendus pourraient être prévus en mesurant la diversité fonctionnelle (FD) et phylogénétique (PD) des communautés végétales. Des systèmes de toits verts écosystémisés ont donc été analysés en termes de FD et PD et deux méthodes ont alors été proposées pour s le tio e la o i aiso d esp es a e la FD a i ale.

Basé sur les connaissances écologiques et expérimentales obtenues tout au long des chapitres de cette thèse, un nouveau concept pour la mise en place de toits verts extensifs (substrat; végétation; irrigation) pourrait maintenant être proposé. Nous concluons ainsi que la végétation méditerranéenne peut être une source d'i spi atio pou le d eloppe e t et l a lio atio de la o eptio de toits e ts extensifs, que ce soit pour le climat méditerranéen actuel ou pour le climat tempéré sous l'effet futur des changements climatiques prévus. Un choix de plantes appropriées est alors essentiel, ainsi que la conception en termes de techniques d i igatio du a le, de p ofo deu et o positio du su st at et des possi ilit s de te tio de l eau. De plus, il e iste e o e de o euses oies pou la réalisation de recherches supplémentaires qui contribueront à la mise en place de toits verts a e u e iodi e sit plus fo te. Ces toits e ts se o t alo s e esu e d a lio e de manière significative la qualité de vie des personnes dans les zones urbaines. ix

Samenvatting

In een tijdperk van verstedelijking, wijzigingen in landgebruik, habitatverlies en fragmentatie, staat biodiversiteit meer dan ooit onder druk. Dit is een beduidend probleem aangezien biodiversiteit beschouwd wordt als één van de belangrijkste drijfveren voor het functioneren van ecosystemen, het leveren van ecosysteemdiensten, en hieraan gelinkt het menselijk welzijn. Bovendien hebben stedelijke gebieden te maken met veel milieuproblemen, zoals een verhoogd overstromingsrisico door een toename aan verzegelde oppervlakte, en een verhoogde temperatuur gerelateerd aan bouwmaterialen met hogere warmteopslagcapaciteit en verlaagd albedo. De klimaatsverandering zou deze problemen nog versterken, waardoor de vraag dringt naar duurzame strategieën die de situatie kunnen verbeteren. Stedelijke groene infrastructuurelementen, zoals straatbomen, pa ke , tui e , g oe e u e e g oe dake o atte zo st ategie. G oe dake zijn een belangrijk middel geworden om de stedelijke milieukwaliteit te verbeteren wegens hun breed aanbod aan ecosysteemdiensten waaronder reductie van het stedelijk hitte-eiland effect en vermindering van de afwatering. Ze bieden ook habitat voor lokale flora en fauna, esthetische waarde en gebruikswaarde. In essentie zijn groendaken daken met een vegetatielaag op. In dit doctoraat focussen we op groendaken van het extensieve type (substraatdiepte < 20 cm) omdat deze breed toepasbaar en duurzamer zijn (i.e. minder onderhoud, zelfvoorzienend).

De meeste expertise omtrent extensieve groendaken is gesitueerd in het gematigde klimaat, en steden in het Middellands-zeeklimaat zijn nog vrij onbekend met de techniek, onder meer door de verhoogde temperaturen en zomerdroogte in deze egio s. Wate is hie geli itee d e de kli aats e a de i g zal leide tot ee og sterker watertekort in de zomer. Een bevloeiingssysteem zou plantengroei en overleving kunnen helpen, maar dit wordt vaak beschouwd als een niet-duurzame en in sommige gevallen zelfs onmogelijke optie. Daardoor is meer onderzoek nodig om geschikte plantensoorten voor mediterrane onbevloeide extensieve groendaken te selecteren, en om de structuurelementen zoals het substraat aan te passen aan de vereisten van deze geselecteerde planten. Aangezien klimaatsverandering ook zal leiden tot hogere temperaturen en onvoorspelbare neerslagmomenten in het gematigde klimaat, zullen deze gebieden ook voordeel halen uit resultaten van dergelijk onderzoek.

Het hoofddoel van dit doctoraat was het uitwerken en testen van een nieuw concept voor extensief groendak-ontwerp dat zowel plantenselectie als structuurelementen omvatte. Het doctoraat startte met een veldwerkstudie in Zuid-Frankrijk waarin potentiële droogte-aangepaste mediterrane plantensoorten werden geïnventariseerd op basis van de habitat sjabloon hypothese. In een groendakcontext stelt deze hypothese dat natuurlijke habitats met gelijkaardige kenmerken als op extensieve groendaken als inspiratiebron kunnen dienen om geschikte plantensoorten te vinden. Vervolgens werd de soortenlijst gescreend voor de meest potentiële plantensoorten door gebruik te maken van hun functionele kenmerken en utilitaire aspecten. Deze screening procedure leverde de wetenschappelijke basis om een gebruiksvriendelijke methode te ontwikkelen voor optimale plantenkeuze. Als case studie werd de methode gedemonstreerd op de mediterrane plantenlijst uit het x veldonderzoek. In het volgende hoofdstuk werd een deel van de geschikte mediterrane plantensoorten gedurende twee jaar getest op experimentele extensieve groendaken in zowel België (gematigd klimaat) als Zuid-Frankrijk (Middellands-Zeeklimaat).

De proefopstelling omvatte verschillende substraatcomposities (substraatdiepte van 5 of 10 cm en aan-of afwezigheid van een waterretentielaag) en exposities (onbeschut of beschaduwd). In een tweede experiment werden de effecten van verschillende substraten op de groei van algemene groendak-planten bestudeerd. De proefvlakken bevatten ofwel mineraal substraat of substaat met extra organisch materiaal, en aan de helft van de proefvlakken werd een commercieel inoculum met mycorrhiza toegevoegd. Beide experimenten waren onbevloeid, en de observaties leerden ons dat de planten stress ondervonden gedurende langdurige droge perioden. Betere resultaten hadden dus bekomen kunnen werdoen indien een bevloeiingsmechanisme voorzien was. Daarom werd een literatuurrevisie uitgevoerd om de huidige bevloeiingstechnieken te beklemtonen, om nadruk te leggen op mogelijkheden om deze technieken duurzamer te maken, en om specificaties voor bevloeiing van groendaken in verschillende klimaten voor te stellen. In een laatste hoofdstuk keken we naar de biodiversiteitsaspecten van commerciële groendak-systemen. Als we stedelijke omgevingen duurzamer willen maken, moeten ecosysteemdiensten verbeterd worden. Er bestaat een groeiende consensus dat functionele (FD) en fylogenetische (PD) diversiteit van plantengemeenschappen ecosysteemdiensten kunnen voorspellen. De FD en PD van huidige beschikbare groendak-systemen werden geanalyseerd en twee methoden werden voorgesteld om de soortencombinatie met de hoogste FD (en dus ook hoogste ecosysteemdienstwaarde) te bepalen.

Op basis van de ecologische en experimentele kennis verkregen doorheen de hoofdstukken van dit doctoraat kon een concept voor extensief groendak-ontwerp in termen van vegetatie en structuurelementen geformuleerd worden. We besluiten dat mediterrane vegetatie wel degelijk als inspiratiebron kan voor ontwikkeling en verbetering van extensieve groendaken, zowel in het huidige Middellands-Zeeklimaat als in het gematigd klimaat onder klimaatsverandering. Een geschikte vegetatiekeuze is essentieel, maar ook het ontwerp in termen van duurzame bevloeiingstechnieken, aangepaste substraatdiepte en compositie, en mogelijkheden voor waterretentie zijn belangrijk. We duiden eveneens op bijkomende onderzoeksmogelijkheden die zullen bijdragen tot het bekomen van goed functionerende groendaken met hoge biodiversiteit. Deze groendaken zullen in staat zijn om de levenskwaliteit van mensen in stedelijke gebieden aanzienlijk te verbeteren.

xi List of Tables Table 2.1 Differences between four vegetation types (clusters) for all variables and Raunkiaer life forms. Significance of differences was tested using Kruskal Wallis tests. Average values and post hoc test results are listed for every cluster, with significant differences expressed by superscript letters. (*: p < 0.05; **: p < 0.001; NS: not significant) .................................................... Table 2.2 Plant traits related to drought tolerance and auto-sustainability, two key criteria for survival on extensive green roofs. (a) traits and criteria used to exclude unsuitable species before scoring; (b) plant traits used for scoring. Plant traits were derived from the TRY initiative (Kattge et al., 2011) 2.5 Medite a ea spe ies ith good EG' pote tial adjusted " o e % . Calculations were based on 10 key criteria for survival on EGRs (cf. Table 2.2). The presence/absence in the GRPL, the original source (MEDVEG or MEDLIT) and the Raunkiaer life form (abbreviations in Table 2.1) are indicated. For species of MEDVEG which were used in the vegetation analysis, the cluster number is displayed as well ........................................ Table 3.3 The relationship between the primary plant traits (P) and other plant traits. (i) Spearman rank correlations together with their significance. (ii) Kruskal Wallis test results together with their significance. (iii) Pearson Chi² test results of the nominal variables. For abbreviations of plant traits see Table 3.1. *: p < . ; **: p .

; N: u e of alid ases; df: degrees of freedom; NA: not applicable). ................................................... Table 3.4 Decision support tool to select species which are potentially interesting for use on EGRs in Mediterranean climates, based on functional (*) plant traits and utilitarian (+) aspects related to frequently applied EGR species. Three groups of selection criteria are shown: exclusion criteria, primary traits and derived traits. Species can be deleted in the first step using exclusion criteria, or get a weighted positive score for the traits for which they meet the condition indicated in the screening tool. ........................... 52 Table 3.5 Mediterranean species for potential EGR application and their screening s o e "" ased o the s ee i g tool i Ta le . . The olu Ne i di ates Medite a ea spe ies that a e u e tl ot used o EGRs. Raunkiaer life forms (abbreviations in Table 3.1) are given as well... Plant List (2013). The AMF dependency of the species ranges from 1 (no/rare) to 3 (frequent/obligate). [a: [START_REF] Fitter | The Ecological Flora Database[END_REF]b: Pánková et al., 2014;c: Gucwa-Przepióra and Blaszkwoski, 2004;d Piotrowski et al., 2004;[START_REF] Sawilska | Efficiency of Sandy Everlasting (Helichrysum arenarium (L.) Moench) cultivation from in vitro seedlings and achenes[END_REF] [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF] and of alternative substrate materials tested in different GR studies. The WHC indicates the ability of a material, in compacted condition, to hold water, and is generally expressed as the water content of a substance after initial water saturation followed by a 2-hour dripping period [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF] 6.5 Summary of irrigation methods and regimes from studies in which outdoor GR experiments were conducted in different climatic conditions. GR type: extensive (EGR), semi-extensive (SEGR) or intensive (IGR). GR dimensions (length x width) were used to calculate the minimum daily water requirements (I day ) during the growing season and summer. ........... xiv Table 7.1 List of 29 traits used for the FD approach, selected after screening of the literature on ecosystem services of GRs. Every trait is supplemented with the following information: ecosystem properties explaining the importance of the trait for the ecosystem service; ecosystem service category (C: cultural; R: regulating; S: supporting; P: provisioning); brief description, variable type (N: nominal; S: scale; O: ordinal) and missing values which indicate the % of species for which no trait information could be found. [Source references [START_REF] Fitter | The Ecological Flora Database[END_REF][START_REF] Shipley | Structured Interspecific Determinants of Specific Leaf-Area in 34 Species of Herbaceous Angiosperms[END_REF][START_REF] Cornelissen | An experimental comparison of leaf decomposition rates in a wide range of temperate plant species and types[END_REF]Kühn et al., 2004;Couplan, 2008;Kleyer et al., 2008;[START_REF] Oudolf | Planting: A New Perspective[END_REF] FDMax1 and FDMax2, offering an exact and practical species list, respectively. Number of groups corresponds to the average species richness (SR) of every type. Species of FDMax1 are underlined and the superscript numbers rank these species according to decreasing functional dissimilarity. The other species (up to 6, for the complete list we refer to Appendix 7.3) can be used as alternatives. For comparison, both the system FD and the maximized FD of FDMax1 for a system with the average SR are shown. . 30% in 1950(United Nations, 2014). The transition from rural to urban life styles has been marked by huge land use changes (e.g. increase in impervious surfaces), habitat loss and fragmentation [START_REF] Antrop | Landscape change and the urbanization process in Europe[END_REF][START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF][START_REF] Lundholm | Vegetation of Urban Hard Surfaces[END_REF]. Those factors have a severe negative impact on biodiversity in terms of local flora and fauna because many plant and animal species have difficulties to migrate or adapt to the novel conditions associated with urbanization [START_REF] Lundholm | Vegetation of Urban Hard Surfaces[END_REF]. As biodiversity is considered the major driver of ecosystem functioning and the provision of ecosystem services, it also impacts our human well-being (Millennium Ecosystem Assessment, 2005). The demographic pressure in cities has also led to a decrease in green areas and hence pervious surfaces, which in turn reduces the infiltration rate and increases surface runoff during and after rainfall [START_REF] Paul | Streams in the Urban Landscape[END_REF]. As a consequence, the risk of flooding has augmented [START_REF] Kazmierczak | Surface water flooding risk to urban communities: Analysis of vulnerability, hazard and exposure[END_REF], causing both human tragedy and severe economic losses. Between 1998 and 2009, more than three million people have been affected by floods in Europe, leaving an economic damage of approximately 60 billion euro (EEA, 2010). In Western Europe, due to the huge economic costs and the increased flooding risk related to the predicted climate change, stormwater management is believed to be the most important environmental challenge in the 21 st century (IPCC, 2013). Also in the United States, there have been significant floods during the 20 st century, causing a ou d illio U"D e o o i da age pe ea O Co o & Costa, . Moreover, the urban heat island (UHI), the phenomenon that temperatures in city centres are generally 1-3°C higher compared to the surroundings because of building materials with higher heat storage capacity and lower albedo, is receiving considerable scientific attention [START_REF] Saadatian | A review of energy aspects of green roofs[END_REF][START_REF] Li | The effectiveness of cool and green roofs as urban heat island mitigation strategies[END_REF] because it lowers the quality of life in cities. The predicted climate change will further amplify both flooding and urban heat island problems, which is a cause for alarm and a trigger to implement more sustainable strategies in order to mitigate the urban environmental problems (cf. [START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF][START_REF] Semadeni-Davies | The impacts of climate change and urbanization on drainage in Helsingborg, Sweden: Combined sewer system[END_REF][START_REF] Harlan | Climate change and health in cities: impacts of heat and air pollution and potential co-benefits from mitigation and adaptation[END_REF]. A strategy that considers green space as a key to ecosystem service conservation and climate change adaptation is the implementation of a green infrastructure network [START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF]. In an urban context particularly, the concept of green infrastructure comprises all the natural, semi-natural and artificial networks of multifunctional ecological systems in urban areas [START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF]Naumann et al., 2011). Urban trees, parks, gardens, green walls and green roofs (GRs) are elements that fall under this definition. GRs are multifunctional engineered ecosystems that are increasingly popular in urban areas [START_REF] Carter | Establishing Green Roof Infrastructure through Environmental Policy Instruments[END_REF]. These elements are the main topic of this thesis and their significance will be brought to light in the following section.

A brief introduction to GR technology

By definition, a GR uses plants to improve the performance and appearance of a roof [START_REF] Snodgrass | The Green Roof Manual. A Professional Guide to Design, Installation, and maintenance[END_REF]. They are also often denoted as living roofs, vegetated roofs, or ecoroofs when the main goal is biodiversity conservation and little or no maintenance or irrigation is required [START_REF] Spolek | Performance monitoring of three ecoroofs in Portland, Oregon[END_REF].

The first modern GRs emerged in the late 1950s in Germany. The environmental benefits associated with these structures were quickly acknowledged and in 1982, the first guidelines were published by the German Landscape, Research, Development and Construction Society (hereafter called FLL;the last version: FLL, 2008). Together with environmental policies, these guidelines made Germany the leading country in GR implementation and success [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF] and the German GR technology is still considered a standard for GR construction all around the world.

Three main types of GRs can be distinguished based on substrate depth. Intensive GRs (IGR, substrate depth > 20 cm) can be considered as roof gardens, offering amenity value and habitat for a variety of flora and fauna. Evidently, this GR type poses constraints for the load bearing capacity of the building and usually demands a lot of maintenance [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF]. On the contrary, the extensive GR type (EGR) is characterized by a light and inorganic substrate and generally has a depth of 2 to 12 cm [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. It is easier and less expensive to design and build, requires little maintenance and is designed to be self-sustaining in the long term [START_REF] Snodgrass | The Green Roof Manual. A Professional Guide to Design, Installation, and maintenance[END_REF]. Between the IGR and EGR categories, a third category is often considered, namely the semi-extensive or simple intensive GRs (SEGR; substrate depth 12-20 cm;[START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF][START_REF] Theodosiou | Green Roofs in Buildings: Thermal and Environmental Behaviour[END_REF].

In line with the increased proportion of the human population that is living in urban areas, there is a growing awareness of the importance of ecosystem goods and services and biodiversity conservation [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF][START_REF] Gaston | Managing urban ecosystems for goods and services[END_REF]. Within an urbanized setting, the concept of novel ecosystems has gained momentum. These are, by definition, self-organizing systems originating from human interference and with actual altered biotic, abiotic and social components as compared to the prevailing historical ecosystems [START_REF] Hobbs | Novel ecosystems: intervening in the new ecological world order[END_REF]. Urban ecosystems, like GRs, parks and gardens, fall within this definition due to their novel elements but differ from traditional novel ecosystems because management is necessary [START_REF] Lundholm | Mini-Review: Habitat analogues for reconciliation ecology in urban and industrial environments[END_REF][START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. Urban ecosystems are characterized by spatial and temporal heterogeneity, which makes them different from surrounding natural habitats [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF][START_REF] Gaston | Managing urban ecosystems for goods and services[END_REF]. Therefore, it has been suggested that urban ecosystems cannot be considered as a substitution for habitat lost in natural ecosystems. General biodiversity conservation strategies therefore focus on the preservation of remnant (semi-) natural habitats and restriction of urban sprawl to a minimum [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF]. However, this traditional conservation view is too narrow as evidence is growing that urban ecosystems can also significantly contribute to biodiversity conservation [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF]). Greening roofs for example, has gained wide international interest lately [START_REF] Blank | Directions in green roof research: A bibliometric study[END_REF] due to the benefits that they offer, ranging from the building to the city scale and encompassing both ecological and economical aspects. The extent to which GRs can perform all these functions depends on many climatic and design factors like substrate depth, age, inclination and vegetation composition [START_REF] Snodgrass | The Green Roof Manual. A Professional Guide to Design, Installation, and maintenance[END_REF]Saadatian et al., 2014). Here we give an overview of the main GR benefits:



First of all, both vegetation and substrate have an important effect on thermal performance and hence contribute to climate regulation [START_REF] Simmons | Green roofs are not created equal : the hydrologic and thermal performance of six different extensive green roofs and reflective and non-reflective roofs in a sub-tropical climate[END_REF]Lundholm et al., 2010;[START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF]. Buildings with a GR are cooler in summer (roof surface: 30-60°C cooler; interior: up to 20°C cooler) compared to conventional roofs due to evapotranspiration and higher albedo [START_REF] Saadatian | A review of energy aspects of green roofs[END_REF].



Evapotranspiration and higher albedo also explain why GRs are effective in mitigating the UHI effect [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].



The evapotranspirative capacity also contributes to stormwater management [START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF]. GRs manage stormwater quantity, by reducing the runoff volume, reducing the peak discharge and releasing the runoff over a longer time period [START_REF] Czemiel Berndtsson | Green roof performance towards management of runoff water quantity and quality: A review[END_REF][START_REF] Guo | Runoff Reduction Capabilities and Irrigation Requirements of Green Roofs[END_REF]. The runoff quality can be enhanced by GRs as well, although caution must be taken when dissolvable fertilizers are used [START_REF] Czemiel Berndtsson | Green roof performance towards management of runoff water quantity and quality: A review[END_REF] or when pollution deposition is high [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF].



Next to runoff quality, the vegetation is able to improve air quality [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. This can be done through the vegetation directly by for example uptake of gaseous pollutants through stomata or interception of particular matter through leaves. Alternatively, indirect air pollution reduction can be attributed to lower building surface temperatures and hence reductions of photochemical reactions that produce pollutants like ozone. Also because of a reduced need for air-conditioning, there will be fewer emissions from power plants [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF].



Considering biodiversity, GRs may serve as a habitat for local fauna and flora and support the pollination process [START_REF] Brenneisen | Space for Urban Wildlife: Designing Green roofs as Habitats in Switzerland[END_REF][START_REF] Colla | Can green roofs provide habitat for urban bees (Hymenoptera: Apidae)? Cities and the Environment[END_REF][START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF]. They even have the potential to act as stepping stones for many high-mobility arthropod species [START_REF] Braaker | Habitat connectivity shapes urban arthropod communities: the key role of green roofs[END_REF].



The cultural service is determined by the GRs aesthetic and amenity value (Lindemann-Matthias et al., 2010;[START_REF] Dakin | The Professional Design Guide to Green Roofs[END_REF].



Depending on their design, GRs can also be used to grow marketable products [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Whittinghill | The role of green roof technology in urban agriculture[END_REF].



As less daily temperature fluctuations occur on GRs, underlying structures are protected from harmful environmental conditions (e.g. ultraviolet radiation) and the life span of the roof membrane can hence be doubled (average life span 40-55 years ;[START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Saadatian | A review of energy aspects of green roofs[END_REF].  Finally, GRs have been shown to significantly reduce the noise pollution in buildings [START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF][START_REF] Van Renterghem | In-situ measurements of sound propagating over extensive green roofs[END_REF].

It is important to conserve and enhance urban biodiversity as it is related to human well-being and because it is an important indicator of ecosystem health [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF]. Compared to homogenous species-poor habitats, urban ecosystems with high species richness (SR) are more resilient (Bengtsson et al., 2002), more resistant to invasions because of more efficient use of available resources (Loreau et al., 2002), and more productive (Tilman, 1997). Those healthy ecosystems are able to ensure normal ecosystem processes and functions, and are therefore essential for different aspects of human well-being and health [START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF]. Most of the benefits provided by GRs summarized above contribute to different ecosystem functions and services. Together with benefits not directly related to ecosystem functions and services, GRs contribute to both ecosystem and human health (Fig. 1.1; [START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF]. It becomes clear that GRs have potential to conserve urban biodiversity and that conservation strategies should not only focus on remnant natural habitats but also include the range of urban ecosystems [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF]. Such a combined conservation approach can be considered as an adaptation to the ongoing urbanization process and a way to ensure the provision of ecosystem services in urban areas under global change [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF].

All types of GRs have the same general structure, consisting of following components: a waterproofing layer on top of the roof deck and insulation layer; a drainage and root-permeable filter layer; a specialized GR substrate and finally the vegetation layer (Fig. 1.2). Sometimes, an additional water retention layer (WRL) is placed above or under the drainage layer. All the components have to be properly designed and work well together to ensure optimal GR performance [START_REF] Snodgrass | The Green Roof Manual. A Professional Guide to Design, Installation, and maintenance[END_REF]. The vegetation layer is the visual part of the GR system and must generally be aesthetically pleasing and meet the recreational or amenity requirements associated with the GR project. Indeed, people are still rather sceptical towards GRs which are not green all year round. It is not yet publicly accepted that EGRs can be dry and rather unattractive during summer due to the natural dynamics of the vegetation, while still being more interesting in terms of building insulation and stormwater management than traditional bitumen or gravel roofs. Plant selection depends on many factors, of which the most important ones are substrate depth and (micro) climatic conditions (Getter & Rowe, 2008b). The EGR vegetation generally consists of following plant communities (arranged according to increasing substrate depth): moss-Sedum (2-5 cm); Sedum-moss-herbs (5-10 cm); Sedum-grasses-herbs (10-12 cm); grasses-herbs (> 12 cm; FLL, 2008). On the shallowest EGRs, the substrate experiences faster desiccation rates and more temperature fluctuations, which make Sedum and moss communities the only vegetation types suitable for this GR type [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. For SEGRs (12-20 cm), vegetation options comprise grasses-herbs, perennials-shrubs and small bushes. On IGRs (> 20 cm), all types of plant species can be considered, including trees if appropriately accounted for in the design [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF]. Whatever plant community is selected, it is expected that plants should establish and cover the roof quickly and they should be long-living and stressresistant (Getter & Rowe, 2008b;[START_REF] Snodgrass | The Green Roof Manual. A Professional Guide to Design, Installation, and maintenance[END_REF]. Indeed, plants face many harsh conditions on roofs in contrast to habitats on ground level (e.g. limiting substrate depth and water availability, heat, frost and wind) and these stressful growing conditions limit the range of plants that can thrive on GRs in the long term. Fig. 1.3 illustrates how the GR vegetation is influenced by microclimatic variation on a carport. The sheltered and probably wetter part accommodates grasses and bulbs, in contrast to the exposed right part, which is dominated by water-stressed Sedum album. Due to the harsh GR conditions mentioned above, drought tolerance is considered one of the most essential characteristics for GR plant selection (Getter & Rowe, 2008b) and until recently, Sedum spp. were regarded as the most appropriate plantings for EGRs and SEGRs [START_REF] Blanusa | Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer ette ooli g se i e ?[END_REF]. They possess traits like CAM metabolism (i.e. closing their stomata during the day to save water), water storage in succulent leaves, very shallow rooting systems, compact growth form, quick ground covering capacity and easy propagation by fragmentation [START_REF] Durhman | Effect of Substrate Depth on Initial Growth, Coverage, and Survival of 25 Succulent Green Roof Plant Taxa[END_REF]. However, a GR only consisting of Sedum spp. has a rather low biodiversity value and its contribution to GR ecosystem services (e.g. cooling potential; stormwater retention) can be questioned [START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF][START_REF] Blanusa | Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer ette ooli g se i e ?[END_REF]. Some alternatives for Sedum, including grasses, perennials and small shrubs, have been investigated in different countries, but with variable success (e.g. Lundholm et al., 2010;Nagase & Dunnett, 2010;MacIvor & Lundholm, 2011;[START_REF] Blanusa | Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer ette ooli g se i e ?[END_REF].

Cu e tl , a est i ted list of t ied a d t ue pla ts is used G' o pa ies a ou d the world [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF], but a tendency to increase GR biodiversity by including native plant material is noticeable in both scientific and GR designer circles [START_REF] Butler | Native plant enthusiasm reaches new heights: Perceptions, evidence, and the future of green roofs[END_REF]. Including native plant species on a GR is indeed an interesting option, as these plants are assumed to be better adapted to local climatic conditions and because they would require less maintenance [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]. GRs with native plant species could also offer an alternative new artificial habitat for other native flora and fauna whose natural habitats are endangered [START_REF] Butler | Native plant enthusiasm reaches new heights: Perceptions, evidence, and the future of green roofs[END_REF]. For examples of studies and projects with a focus on GRs as natural habitat replacement, we refer to [START_REF] Brenneisen | Space for Urban Wildlife: Designing Green roofs as Habitats in Switzerland[END_REF]. It has been shown that EGRs can offer suitable habitats for multiple and sometimes rare or endangered spider and beetle species [START_REF] Jones | Tecticolous invertebrates: A preliminary investigation of the invertebrate fauna on ecoroofs in urban London[END_REF][START_REF] Kadas | Rare invertebrates colonizing green roofs in London[END_REF][START_REF] Madre | A comparison of 3 types of green roof as habitats for arthropods[END_REF], rare birds like the Black Redstart (Phoenicurus ochruros; [START_REF] Frith | The black redstart in urban Britain; a conservation conundrum[END_REF][START_REF] Gedge | Roofspace: a place for brownfield biodiversity?[END_REF], some ground-nesting birds (Brenneisen, 2003) and even endangered orchid species (Loudolt, 2001).

EGR performance could also be improved by increasing biodiversity in terms of plant characteristics [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF][START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]Lundholm et al., 2010;Nagase & Dunnett, 2010;[START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. As stated earlier, biodiversity is considered the major driver of ecosystem functioning and provision of ecosystem services (e.g. [START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF]. There is also a growing consensus that one should rather improve biodiversity by increasing the range and alue of the spe ies fu tio al t aits i stead of i easi g SR and evenness (e.g. [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF][START_REF] Lavorel | Using plant functional traits to understand the landscape distribution of multiple ecosystem services[END_REF]. By increasing the functional diversity (FD) of a plant community, ecosystem functioning can be improved, through ecological processes like niche complementarity and facilitation [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF]. One could also look at and improve the phylogenetic diversity (PD) of the plant community, as it is considered a holistic diversity measure and accounts for the shared evolutionary history of genetically related species displaying similar traits [START_REF] Cadotte | Using Phylogenetic, Functional and Trait Diversity to Understand patterns of Plant Community Productivity[END_REF][START_REF] Devictor | Spatial mismatch and congruence between taxonomic, phylogenetic and functional diversity: the need for integrative conservation strategies in a changing world[END_REF][START_REF] Flynn | Functional and phylogenetic diversity as predictors of biodiversity-ecosystem function relationships[END_REF][START_REF] Srivastava | Phylogenetic diversity and the functioning of ecosystems[END_REF]. As PD accounts for species evolution, it hence encompasses many additional and often unmeasured traits that may be important to determine ecosystem functioning [START_REF] Cadotte | Using Phylogenetic, Functional and Trait Diversity to Understand patterns of Plant Community Productivity[END_REF][START_REF] Srivastava | Phylogenetic diversity and the functioning of ecosystems[END_REF].

Problem statement

As highlighted in the previous section, GRs offer benefits and are in many aspects preferable to bitumen roofs in urban areas. However, the technique is relatively new and many questions still need to be resolved, especially in the context of expected climatic changes for the coming years. It is expected that the environmental challenges of urbanization will increase with climate change. Worldwide, temperatures are expected to increase by 0.1-0.4°C per decade (IPCC, 2007). By 2050, many large urban agglomerations will experience a minimum temperature increase of 2.5°C, as predicted through the Representative Concentration Pathway (RCP) 8.5 scenario in which unchanged current greenhouse gas emissions are assumed [START_REF] Revi | Urban areas[END_REF]. Both changes in mean temperature and episodes of extreme temperatures will severely affect vegetation in many places on earth. Additionally, higher temperatures will increase the evaporative power of the atmosphere (reference evapotranspiration ET0), leading to higher plant evapotranspiration. Also precipitation patterns are predicted to change, with increases in annual precipitation of 1-2% per decade in Northern Europe. Winters are expected to become wetter (1-4% more precipitation per decade), but projections for summer precipitation are less unequivocal [START_REF] Giorgi | Projections of twenty-first century climate over Europe[END_REF]. In Belgium (North-Western Europe), surface temperatures and winter precipitation are expected to increase while summers are predicted to be drier [START_REF] Baguis | Climate change scenarios for precipitation and potential evapotranspiration over central Belgium[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]. In contrast, Southern Europe will experience a decrease in annual precipitation with 1% per decade, and even decreases of 5% are expected in summer [START_REF] Parry | Assessment of potential effects and adaptions for climate change in Europe: the Europe Acacia Project (a concerted action towards a comprehensive climate impacts and adaptions assessment for the European Union)[END_REF].

Current annual weather patterns for Avignon (France, Mediterranean climate, Southern Europe) and Leuven (Belgium, temperate climate, Western Europe), two locations where this thesis focuses on, are given in Fig. 1.4. As a result of the above mentioned climate change predictions, temperatures in both locations will increase, which will also lead to an increased potential evapotranspiration (PET). Avignon already experiences 3-4 months of summer drought (0.5xPET > precipitation), and this period will extend as the PET increases and summer precipitation decreases. In Leuven, summer drought is currently not applicable, but can become a reality in upcoming decades, suggesting that average future climatic conditions will approach the current Mediterranean situation. In the study of [START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF], vegetation drought stress and runoff for the near future (2050) were assessed for different GR types (grass-herb and Sedum-moss vegetation) under different climate change scenarios. With the projections of certain global climate models, which project substantial precipitation decreases and 10 evapotranspiration increases in summer, relative runoff from grass-herb GRs was expected to decrease by approximately 50% in the future relative to the baseline period and on Sedum-moss GRs by approximately 40%, whereas on bitumen roofs with the same climatic scenarios by only 30%. This observation, representative for extreme events, highlights the benefit of a GR in the future.

Although runoff reduction will become more pronounced on grass-herb than on Sedum-moss GRs, grass-herb GRs proved to be more vulnerable to drought stress, a factor becoming more important under climatic scenarios. In the near future, GR designers will need to consider this trade-off between vulnerability to drought-stress and effectiveness to reduce runoff of different vegetation types. In conclusion, the GR industry will need to look for new plant species which are more adapted to the future climatic conditions.

As plant survival is essential to ensure any GR benefit, the most drought tolerant species should be considered. When looking for suitable GR plant species, it has been suggested to target natural ecosystems with similar characteristics as on GRs, a o ept also des i ed as the ha itat te plate Lu dhol , . "pe ies adapted to high temperatures, shallow mineral and free-draining soils, frequent drought and high wind speeds are supposed to be able to survive on EGR as well. Natural habitats demonstrating these conditions include limestone pavements, cliffs, scree beds and even dry grasslands [START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF]. The concept is very promising and is now increasingly adopted when looking for new potential EGR species [START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF][START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF][START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF][START_REF] Caneva | Ecological approach in selecting extensive Green roof plants: A data-set of Mediterranean plants[END_REF]. Selecting drought-adapted species for EGRs in both Mediterranean and temperate climates in order to prepare the GR industry for the predicted climate change is one problem that will be dealt with in this thesis. Based on the habitat template concept, plant species currently growing in the Mediterranean region may serve as representative, potentially suitable species, as many habitats match EGR conditions, and current average environmental conditions in that region approach the prospected future climatic situation in temperate climates.

Next to adaptation of species selection, the design in terms of GR structure can be adjusted. Increasing the substrate depth in order to retain more water for a longer time and hence facilitate plant survival is an obvious option (Getter & Rowe, 2008b; 'azzagh a esh et al.

, ut ot al a s possi le e ause of the uildi gs physical limitations. A deeper substrate may also increase the establishment of undesired vigorous weeds, which contribute to a more time-consuming maintenance procedure (Nagase et al., 2013). Alternatively, the effect of water retention additives on GR plant performance are now increasingly explored [START_REF] Sutton | Media Modifications for Native Plant Asemblages on Extensive Green Roofs[END_REF][START_REF] Farrell | Water-retention additives increase plant available water in green roof substrates[END_REF].

One could also add organic material (OM) or arbuscular mycorrhizal fungi (AMF) to the GR medium to support plant survival. In general, OM increases the substrates nutrient content and improves soil structure and water retention [START_REF] Friedrich | Principles for selecting the proper components for a green roof growing media[END_REF][START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF][START_REF] Molineux | Characterising alternative recycled waste materials for use as green roof growing media in the U[END_REF]. AMF are symbiotic fungi (Phylum Glomeromycota) that form mutualistic associations with plant roots of most plant species [START_REF] Bonfante | Plants, Mycorrhizal Fungi, and Bacteria: A Network of Interactions[END_REF][START_REF] Willis | The Ecology of Arbuscular Mycorrhizal Fungi[END_REF]. Benefits for the plant include better phosphorus and nitrogen uptake and improved water uptake [START_REF] Rillig | Arbuscular mycorrhizae and terrestrial ecosystem processes[END_REF][START_REF] Veresoglou | Suppression of fungal and nematode plant pathogens through arbuscular mycorrhizal fungi[END_REF]. Both substrate additions could be of importance to increase the performance of EGR (cf. [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF], but although the benefits on plant growth and development are acknowledged in horticulture, there is hardly any evidence of the effects on GRs.

Because plants will experience more drought-stress, it may be expected that irrigation will become necessary on EGRs and SEGRs for species survival in summer [START_REF] Macivor | Decoupling factors affecting plant diversity and cover on extensive green roofs[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]. However, in many cities, water scarcity is a yearly reoccurring problem, especially in regions with frequent droughts like Southern Europe and Australia [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]Ascione et al., 2013), which has led to irrigation restrictions for GRs [START_REF] Rowe | Effect of green roof media depth on Crassulacean plant succession over seven years[END_REF]. One should therefore look for alternative water sources and water conservation practices to make GR irrigation practices more sustainable (Moritani et al., 2013). The knowledge on irrigation practices and specifications for GRs in relation to different climates, is however still limited.

In conclusion, given that in particular the Mediterranean region is a hotspot in terms of plant diversity (Médail & Qu zel, , ep ese ti g o e % of the o ld s flo a [START_REF] Lavergne | Fingerprints of environmental change on the rare mediterranean flora: a 115-year study[END_REF], we believe that there is a considerable amount of potential plant species available in nature that is able to offer solutions for better GR success. These species can be used to design more heterogeneous EGRs in terms of SR and FD, hence improving biodiversity value and associated GR ecosystem services. Heterogeneous GRs are also expected to be more resistant, persistent and resilient to climate change. Furthermore, because many of the potential species originate from natural habitats, the implementation of them on GRs is a step towards better biodiversity conservation in urban areas. Adaptations in GR structure design will alleviate the challenging environmental conditions on GRs and hence help survival of the GR plant species.

Research questions and chapter outline

The general aim of the thesis is aptu ed i the title: Natu e as a te plate fo a e concept of EGRs . The fi st pa t of the title Natu e as a te plate efe s to the development of a detailed procedure for applying the habitat template concept. More specifically, potential EGR plant species (e.g. drought tolerant, shallow rooting system) originating from (semi-)natural Mediterranean habitats will be investigated. Knowledge collected during this part will then be used to elaborate and test a new concept for EGR design, comprising both plant selection and GR structure elements.

The main research questions can be formulated as follows:

1. What kind of vegetation can be found in (semi-)natural Mediterranean areas selected according to the habitat template concept of GRs? To what extent do the plants in the resulting Mediterranean plant list (MEDPL) offer new possibilities for use on EGRs? 2. How can the plants in MEDPL be ranked according to their potential use for EGRs by making use of their plant traits? 3. Next to theoretical evidence, can the potential of some Mediterranean plants for EGRs be confirmed experimentally? Do other factors including climate, substrate depth, WRL and exposition affect the suitability of these plants?

Side questions also answered in this thesis include:

4. Plant performance can also be enhanced by adding OM and AMF to the growing medium. Is this also the case for plants developing under EGR conditions? 5. Based on previous research, plant performance is often believed to be enhanced by regular irrigation. What kinds of irrigation methods are commonly used on GRs? Can irrigation recommendations be made for different climates (including the Mediterranean) and are there sustainable methods that should be promoted? 6. Assuming that FD and PD of plant communities have a positive effect on ecosystem functioning, to what extend do different GR systems available on the market contribute to GR ecosystem functioning? Can FD and hence the ecosystem services of GRs be improved?

The flow chart in Fig. 1.5 gives an overview of the different Chapters that will provide answers to the above mentioned specific research questions. In this introduction part, it became clear that there is a need to adjust GR vegetation and design. This is because of the predicted climate change, which will result in longer dry periods that will pose an additional stress on EGR vegetation, and because EGRs are currently not very common in Mediterranean areas although they would be very beneficial here (Fig. 1.6)

The first research question will be the focus of Chapter 2. Natural habitats in the Mediterranean climate will be studied both in the field and from literature sources. Following the habitat template concept, we specially target habitats characterized by shallow, calcareous soils or pavements. This approach should result in a GR species pool which, given the high diversity of plant species in the Mediterranean region, probably will be an extensive list of Mediterranean plants (MEDPL). This list should be analyzed more thoroughly before eventual testing.

In Chapter 3, research question 2, i.e. how we can make use of the plant traits to find the most potential species for EGR application, will be addressed. This Chapter builds further on the MEDPL from the first chapter, together with a plant list of currently used GR plants (GRPL as the a e al ead o side ed t ied-and-t ue a d hence serve as a reference. To be fully applicable we need a tool for screening potentially interesting species. This can be achieved by using functional plant traits related to drought tolerance and self-regulation (the key factors essential for survival on EGRs under dry conditions). But one might argue that more utilitarian aspects (like flower color, flowering period) are also important as these will determine the species that people like and which are or might be available from nurseries. Based on both functional traits and utilitarian aspects, it should be possible to deliver a screening tool in which species are scored and ranked according to their suitability for use on EGRs in regions with frequently occurring dry periods.

As the screening tool from Chapter 3 offers rather a theoretical evaluation, the performance of some Mediterranean species needs to be tested on outdoor GR experiments under varying conditions. Chapter 4 will describe the approach and results of a two year experimental trial and will also discuss research question 3, i.e. if certain factors affect the suitability of those Mediterranean plants to be used on EGRs. More specifically, a subset of Mediterranean plants will be tested in two locations with different climates (cf. Fig. 1.4): Heverlee (Leuven, Belgium, temperate climate) and Avignon (France, Mediterranean climate). As it is impossible to test all possible GR conditions, three were selected playing an essential role in water balance of GRs (5 cm substrate; 5 cm substrate + WRL; 10 cm substrate + WRL). Also, the effect of two expositions (fully exposed and sheltered) will be tested. The results should also offer a first verification of the reliability of the screening tool from Chapter 3.

The focus of the antecedent chapters was on plant species and plant performance. However, GRs are more than just the vegetation layer. In Chapter 4, the effect of substrate depth was addressed as well. However, the substrate composition in all the plots was identical. In Chapter 5, the effects of addition of AMF and/or OM to the mineral GR substrate on initial development of EGR plants are studied. The plants originate from a commercial EGR seed mix and are installed as seeds. The results will be used to discuss research question 4.

Many scientific papers on both GR plant selection and Mediterranean GRs state that an irrigation system is necessary for plant survival on EGRs in regions with frequent dry periods. In both the experiments conducted in Chapter 4 and 5, an irrigation system was not installed for sustainability purposes. Vegetation hence relied on natural precipitation events to irrigate the experimental plot, but it became clear that this was often not enough to ensure plant survival in dry periods. In the literature, no exact information is available on different irrigation systems for GRs, or what kind of irrigation is advised in different climates. Chapter 6 hence reviews the currently available information on (sustainable) GR irrigation. This will offer a response on research question 5.

Conform the approach in Chapter 3, a matrix of EGR species and relevant functional traits was created in Chapter 7. The species were derived from plant lists of GR systems currently available on the European GR market. Clustering techniques grouped the GR systems in different types according to their species composition. FD analysis of the resulting GR types was performed, as well as a PD analysis as PD is considered a valuable proxy for FD. The results give an answer on research question 6. It was found that there is room for improvement of FD, so in Chapter 7, two methods for maximization of FD will be applied and discussed.

In the conclusion part, Chapter 8, the highlights of all the Chapters will be brought together to answer to the main goal of the thesis, namely to formulate a new concept for EGR design and to propose new axes for further research on this subject. Chapter 2. Mediterranean open habitat vegetation offers great potential for extensive green roof design

Introduction

Urban regions are facing worldwide population and housing demand increases, causing a lot of environmental problems as building space becomes scarcer. New buildings replace green areas and make cities less attractive for living and working. In this respect, the application of green roofs (GRs) or ecoroofs proved that, by transferring vegetation to the top of buildings, major urban problems can be reduced (cf. [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. In terms of sustainability, extensive GRs (EGR) are preferred over the intensive ones. The former offer stormwater control and thermal insulation without requiring substantial irrigation and maintenance. Furthermore, EGRs have a shallow substrate (< 20 cm) which makes them lightweight and suitable for wide application on new constructions and for renovation of old buildings [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. Although GRs are manmade, they can be part of nature restoration and even potentially help counteracting the destruction of (semi)natural habitats if local or regional species are used [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Francis | Urban reconciliation ecology: the potential of living roofs and walls[END_REF]. The multitude of ecological and economic benefits, along with some non-negligible factors like psychological and aesthetic effects, makes GRs an important tool for improving urban environmental quality.

An exponential rise of interest in and implementation of GRs has been observed during the past decades particularly in temperate Europe and North-America [START_REF] Köhler | The green roof tradition in Germany: The example of Berlin[END_REF][START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]. In 2007, GR coverage in Germany increased on average 13.5 million m² per year (cf. [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. In France, EGR surface reached 1 million m² in 2011, and is expected to rise to ca. 1.5 million m² in 2015 [START_REF] Lassalle | L'écho des toitures végétales[END_REF]. Modern GR technology is relatively recent and its origins lie in North-and Central Europe, with Germany as leading country.

Research and experience has led to the development of the German FLL guidelines for the planning, construction and maintenance of GRs [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF], which serve as a model throughout the world. Implementation of these guidelines usually guarantees success in the temperate climate, but can pose problems in other ecoregions [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]. In the Mediterranean area for example, plants often face severe water stress as a cause of frequent elevated temperatures and extended summer drought [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]. When plant species commonly used for GR purposes are applied in the Mediterranean climate, they suffer from the additional stress factors, leading to poor GR performance and therefore discouraging both industry and government to promote this innovative tool [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]. Despite these problems, scientists showed that GR advantages are also pronounced in the Mediterranean climate [START_REF] Fioretti | Green roof energy and water related performance in the Mediterranean climate[END_REF], therefore rendering investigations to overcome the poor GR performance very valuable. The incorporation of local or regional plant species can provide a solution to the problem, as they are more adapted to the Mediterranean climate [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. Evidence is provided that EGRs mimic habitats found in nature, a concept which is des i ed as the ha itat te plate h pothesis [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF]. These natural template habitats comprise mostly rocky environments, free draining dunes, open areas on very shallow substrates, and limestone pavements. The Mediterranean region with its exceptional diversity of plant species [START_REF] Médail | Hot-Spots analysis for conservation of plant biodiversity in the Mediterranean Basin[END_REF]) contains a lot of these habitats, so we hypothesized that it should be possible to find drought-adapted, native plant species that could thrive on EGRs.

In this study, we aimed at:

1. describing and classifying the vegetation of open habitats on very shallow, stony soils and analyzing the variation in species composition in relation to environmental factors. A literature research on old vegetation surveys will further complement the potential plant species in the region. This delivers us a potential plant species pool for further analysis. 2. comparing the resulting species list with a list of currently used GR plant species in NW Europe and 3. highlighting the potential of the Mediterranean vegetation as a source of inspiration for development and improvement of EGR design, by considering some major plant traits.

Material and methods

Vegetation description 2.1.1. Study area

As the Mediterranean region is considered a biodiversity hotspot [START_REF] Médail | Hot-Spots analysis for conservation of plant biodiversity in the Mediterranean Basin[END_REF] with approximately 25000 vascular plant species (ca. 7.8% of the number of species in the world), of which about 13000 are endemic to this region [START_REF] Quézel | Definition of the Mediterranean region and the origin of its flora[END_REF], it was impossible to include the whole Mediterranean basin in this project. Therefore, we focused on the southern part of France, more specifically the departments Hérault, Gard, Bouches-du-Rhône, Vaucluse and Drôme in the regions Languedoc- [START_REF] Blondel | The Mediterranean Region. Biological diversity in space and time[END_REF]. In general, summer drought lasts one to four months (Unesco- FAO, 1969) [START_REF] Filippi | The Dry Gardening Handbook: Plants and Practices for a Changing Climate[END_REF][START_REF] Blondel | The Mediterranean Region. Biological diversity in space and time[END_REF]. The mountainous part in this region is dominated by coniferous forest but more to the Mediterranean sea, the vegetation changes in a mosaic of mixed forest (with amongst others Pinus halepensis and Quercus ilex), matorral such as maquis on acidic soils and garrigue on calcareous soils [START_REF] David | Points de vue sur la garrigue: 30 panoramas du Gard et de l H ault. Les e ologistes de l Euzi e[END_REF]. The main blooming period of the vegetation occurs in spring. During summer drought, the aboveground part of many plants dies off, whereas the growth of evergreen shrubs just takes a halt. Altitudinal limits of the Mediterranean climate and flora and fauna is often situated at about 1000 m [START_REF] Blamey | Mediterranean Wild Flowers[END_REF], although others (e.g. [START_REF] Blondel | The Mediterranean Region. Biological diversity in space and time[END_REF] do not recognize this limit. According to the latter, a good delimitation of the region should coincide with the 100 mm precipitation isohyets. We further describe our study area as (sub) Mediterranean.

Data collection

Given the specific conditions on EGRs, only non-forest vegetation on shallow soils and limestone pavements in the (sub) Mediterranean part of France was considered.

The departments under study contain many stony calcareous habitats with a great diversity of plant species (Appendix 2.1). During the growing season of 2011 (April 22 -June 6), 20 locations in an area of 190 x 130 km were visited (Fig. 2.1; 2.2). Given the fact that local conditions on stony or rocky habitats may change over very short distances [START_REF] Verrier | Contribution à la synsystematique et à la synecologie des pelouses sèches à therophytes d'Europe[END_REF], the vegetation was recorded in small plots of 1 m². Cover was estimated according to the decimal scale of Londo [START_REF] Londo | The decimal scale for releves of permanent quadrats[END_REF]. In total, 253 plots were recorded with 12-20 plots per location. 372 species were found (Mediterranean plant list from vegetation recordings: MEDVEG), with on average 18 species per plot. Some in situ measurements of vegetation-related variables included an estimation of the amount of bare soil (BS) and % cover of lichen and bryophytes (BL). In every plot, a number of environmental variables were recorded as well (Table 2.1). Altitude (A), geographic coordinates (WGS 84 reference system) and aspect were measured with a GPS navigator (Garmin eTrex Vista HCx, Garmin Ltd 2007). The two latter variables, together with slope (angle in degrees measured with a clinometer),

where used to calculate the heat load (HL) of the plot (Eq.1) with the formula presented by [START_REF] Mccune | Equations for potential annual direct incident radiation and heat load[END_REF]. HL = 0.339 + 0.808 x cos (latitude) x cos (slope) -0.196 x sin (latitude) x sin (slope) -0.482 x cos (folded aspect) x sin (slope) (Eq.1)

Folded Aspect = |180 -|aspect -225||
The result of Eq.1 is a dimensionless index of the HL. The variables were all converted to radians prior to incorporation in the formula, and the aspect was folded in order to obtain the most accurate approximation, by shifting the equation from a maximum on south slopes to a maximum on southwest slopes and a minimum on northeast slopes.

A small one-piece drill was used to measure average top soil depth (D) and for soil sampling. In the lab, soil pH (H 2 O) and electric conductivity (EC) were analyzed by means of a portable pH/EC/TDS/Temperature gauge (HI 991301, Hanna Instruments Inc.). This was done by inserting the calibrated electrode into a mixture of 25 ml deionized water and 10 g soil, after two shaking moments of 10 seconds (at start and after 30 minutes). A volumetric determination of organic matter (OM) was determined by the Walkley-Black procedure [START_REF] Walkley | An examination of the Degtjareff method for determining organic carbon in soils: Effect of variations in digestion conditions and of inorganic soil constituents[END_REF]. The Coulter Principle was used for measuring the proportion of sand, silt and clay in the soil sample (LS 13320 Laser Diffraction Particle Size Analyser, Beckman Coulter Inc.).

Climate data were obtained by the FAO program New_LocClim v1.10, which uses interpolation methods to investigate the conditions at any possible location on earth. For our study region, the interpolation was done based on data from eight weather stations: Montpellier, Nimes, Marseille, Salon, Millau, Montélimar, Orange and Mont Ventoux. Average annual precipitation (AAP) and temperature (AAT), potential evapotranspiration (PET), the De Martonne Aridity Index, and dry period (days with precipitation < 0.5 PET) were calculated. The annual thermal sum in Growing Degree Days (GDD; Eq.2) is often used as a substitute for temperature and was calculated with the formula of [START_REF] Meunier | Climate and Picea Mariana seed maturation relationships: a multi-scale perspective[END_REF]:

GDD = ∑ n i=1 [(T i, max + T i, min )/2 -5] (Eq.2)
Where T i, max and T i, min are the maximum and minimum daily temperatures for every i-day, i.e. day with mean temperature above 5°C, with exclusion of days in the dry period, as no growth can occur in these circumstances. Furthermore, the life form of every plant species was determined using the database Baseflor [START_REF] Julve | Baseflor. Index botanique, écologique et chorologique de la Flore de France[END_REF], for calculation of the Raunkiaer life form spectra in every cluster [START_REF] Raunkiaer | The Life-forms of Plants and Statistical Plant Geography[END_REF]. Sörensen indices (Eq.3) were calculated to quantify and compare the similarity in species abundance between the emergent clusters. This is the shared species abundance divided by the total abundance, where C stands for the species in common between two clusters and A and B are the separate cluster species abundances [START_REF] Mccune | Analysis of Ecological Communities[END_REF].

QS = 2C/(A+B) = [ A∩B ] / [ A + B ] (Eq.3)
The plot*variable matrix, supplemented with corresponding cluster numbers and life form categories, was imported in SPSS 20.0 to relate the variables to the emergent clusters by means of Kruskal Wallis tests (KW) and subsequent post-hoc tests.

The results of the hierarchical clustering and correlation analysis allowed an adequate description of the clusters. Assignment to a phytosociological class (i.e. a group of plant communities having the same diagnostic species) was based on a comparative description (Appendix 2.2; cf. [START_REF] Loisel | La egetatio de l tage dite a en dans le sud-est continental Français[END_REF].

Species lists comparison

For a more complete overview of the total SR in the selected habitats in southern France, an additional literature search was conducted resulting in four relevant sources: [START_REF] Molinier | La vegetation de la Crau (Basse Provence)[END_REF][START_REF] Van Den Berghen | Etude sur la végétation des Grands Causses du Massif Central de France[END_REF][START_REF] Loisel | La egetatio de l tage dite a en dans le sud-est continental Français[END_REF][START_REF] Rieux | Etude écologique et phytosociologique du Crassuletum tillaeae Molinier et Tallon 1949[END_REF] (see Appendix 2.2). The latter resulted in an additional list of species (MEDLIT). The literature data were not included in the vegetation analysis part of this study, as they had larger plot areas than the minimal area to detect vegetation homogeneity (about 20 m²) and hence contained internal small scale variation which might bias the results. Combining the two lists (MEDVEG and MEDLIT) yielded a total Mediterranean plant list, abbreviated as MEDPL. The MEDPL contained 633 plant species, from which 372 species originated from MEDVEG and 261 species from MEDLIT.

Next to the MEDPL, a list containing plant species commonly applied or spontaneously occurring on EGRs in NW Europe was compiled (GRPL). Seven key sources were used: five literature sources [START_REF] Bornkamm | Vegetation und Vegetations-entwicklung auf Kiesdächern[END_REF][START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF][START_REF] Köhler | Fassaden-und Dachbegrünung[END_REF][START_REF] Kolb | G ee a d ool oofs u a heat island mitigation potential in European climates for office buildings under free floating conditions[END_REF][START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF] and two species lists from international GR Companies (Optigreen Limited and Zinco Gmbh). The GRPL comprised 484 species (subspecies and varieties were not taken into account). Plant nomenclature followed The Plant List (The Plant List, 2010). The two lists (MEDPL and GRPL) are compared in terms of species similarity (Eq.3) and Raunkiaer life form distribution.

Potential of Mediterranean vegetation

A simple screening procedure was used to highlight the possibilities of Mediterranean vegetation for EGR purposes. A multitude of functional plant traits can be informative in this respect (cf. Cook-Patton & Bauerle, 2012), but as an extensive analysis is out of scope of this study, we focused on general traits associated with the survival on EGRs. For the MEDPL, information on ten plant traits (based on [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF] were derived from the TRY trait base (a global archive of plant traits, cf. Kattge et al., 2011). One of the used traits comprises the plant strategy types of Grime [START_REF] Grime | Plant Strategies, Vegetation Processes, and Ecosystem Properties[END_REF], in which plants are grouped into competitors, ruderals, stress tolerants or a combination of these categories. Only species having data for more than five plant traits were considered. Species were also omitted if they met one or more of the following obvious exclusion criteria (Table 2.2a): phanerophytic life form; plant height exceeding 1 m; root system deeper than 20 cm; no drought and/or stress tolerance. Finally, every remaining plant species gets a score of 1 for each trait where it meets the preferred condition as indicated in Table 2.2b. The total score (on 10) is adjusted to account for missing values, through division of the score by the number of traits for which the plant had data.

Results

Vegetation description

PCA results showed that five axes are significant in explaining the relationships between variables (p-value = 0.001). The first PCA axis (Fig. 2.3) explained 30% of the total variance and can be considered as a measure of the climatic variation in the studied sites. Climate variables (AAP, AAT, aridity, dry and PET) and altitude are strongly correlated, either positively or negatively, indicated in Fig. 2.3 by coincident or opposite vectors. Axis two explained 17% of the variance and was linked with soilrelated variables. According to axis two, the % sand was negatively correlated with silt and clay, and OM and EC were positively correlated. The remaining axes account for 28% of the total variance. pH (H 2 0) and GDD correlated with PCA axis 3, although on Fig. 2.3, they are also partly explained by axis 2. The variables SR, Cover, BS, BL and HL have their highest eigenvector on axis 4. Again, the second highest eigenvalue of these variables can be found on axis 2. Finally, EC and D are expressed by axis 5. The main variation in species composition is hence determined by climate and soil related variables. [START_REF] Cornelissen | An experimental comparison of leaf decomposition rates in a wide range of temperate plant species and types[END_REF]Diaz et al., 2004;Paula et al., 2004;[START_REF] Wright | The worldwide leaf economics spectrum[END_REF]Garnier et al., 2007;Kleyer et al., 2008;Green, 2009 Root depth Deep rooting system (> 20 cm) Paula et al., 2004;[START_REF] Green | USDA PLANTS Compilation, Version 1[END_REF] Tolerance to drought No drought tolerance Green, 2009 Grime strategy No stress tolerance Kühn et al., 2004 (b) Plant traits for scoring step

Preferable condition Reference

Grime strategy Ruderal and/or stress tolerant Kühn et al., 2004 Leaf phenology Evergreen [START_REF] Cornelissen | An experimental comparison of leaf decomposition rates in a wide range of temperate plant species and types[END_REF][START_REF] Wright | The worldwide leaf economics spectrum[END_REF]Cornwell et al., 2008;[START_REF] Kattge | Quantifying photosynthetic capacity and its relationship to leaf nitrogen content for global-scale terrestrial biosphere models[END_REF]Paula et al., 2009 Leaf shape Small, acicular, needlelike leaves Kühn et al., 2004;Paula et al.,2004 Maximum plant longevity Annual or perennial Diaz et al., 2004;Kühn et al., 2004;[START_REF] Moles | Small-seed species produce more seeds per square metre of canopy per year, but not per individual per lifetime[END_REF][START_REF] Green | USDA PLANTS Compilation, Version 1[END_REF]

Metamorphosis for storage

Succulence Kühn et al., 2004 Photosynthetic Pathway (facultative) CAM metabolism [START_REF] Cornelissen | An experimental comparison of leaf decomposition rates in a wide range of temperate plant species and types[END_REF]Diaz et al., 2004;[START_REF] Wright | The worldwide leaf economics spectrum[END_REF]Cornwell et al., 2008;Kattge et Paula et al., 2004;[START_REF] Green | USDA PLANTS Compilation, Version 1[END_REF] Tolerance to drought Medium or high drought tolerance [START_REF] Green | USDA PLANTS Compilation, Version 1[END_REF] Typical plant height Small plants (< 1 m) [START_REF] Cornelissen | An experimental comparison of leaf decomposition rates in a wide range of temperate plant species and types[END_REF]Diaz et al., 2004;Paula et al., 2004;[START_REF] Wright | The worldwide leaf economics spectrum[END_REF]Garnier et al., 2007;Kleyer et al., 2008;[START_REF] Green | USDA PLANTS Compilation, Version 1[END_REF] Hierarchical clustering and ISA revealed an optimal division of the plots in four clusters (Fig. 2.3), as the lowest p-value of Monte Carlo permutation test (p = 0.077) was found for this cluster number. Table 2.3 shows a detailed description of the four clusters. KW tests revealed significant differences between the four clusters for all variables (p-value < 0.05), except for pH. The four clusters also significantly differed in life form distribution of hemicryptophytes and therophytes (or annuals; Table 2.1).

In cluster 3 and 4, hemicryptophytes clearly dominate and annual species are very rare. Species with an annual life form are most pronounced in cluster 2 (Table 2.1). Sörensen indices showed the highest similarity in species abundance between clusters 1-2 (QS = 0.45) and clusters 3-4 (QS = 0.47).

We conclude that the four clusters (Table 2.3) can mainly be distinguished based on geographical and climatic factors. The vegetation in clusters 1 and 2 is typical for thermophilic limestone pavements. These species rich habitats contain a high amount of annuals and a mosaic of different biological types (cf. [START_REF] Loisel | La egetatio de l tage dite a en dans le sud-est continental Français[END_REF]. Clusters 3 and 4 contain vegetation that occurs at higher altitudes, are more mesophilic and are scarce in annuals (cf. [START_REF] Van Den Berghen | Etude sur la végétation des Grands Causses du Massif Central de France[END_REF][START_REF] Loisel | La egetatio de l tage dite a en dans le sud-est continental Français[END_REF].

Species list comparison

Of the 484 species in the GRPL, 19 species were noted in five or more of the seven key literature sources on EGRs in NW Europe, of which eight species were also noted in the MEDPL (Table 2.4). All except two species are hemicryptophytes. The other two are a bulbous geophyte (Allium schoenoprasum) and an annual species (Erodium cicutarium). Seven species in the list (36.8%) belong to the genus Sedum.

The species in the MEDPL showed some overlap with the GRPL (131 species in common), although a Sörensen index of 0.24 indicates a low species similarity. 79% of the species in the MEDPL is currently not applied on EGRs.

Fig. 2.4 indicates that hemicryptophytes are predominantly used on GRs (75.4%). The MEDPL also contained mainly hemicryptophytes (55%). Annuals are not applied frequently (13.8%) on EGRs, although they are particularly abundant in the MEDPL (28.4%), with a much higher percentage occurring in MEDVEG (36.3%) compared to MEDLIT (17.2%).

Potential of Mediterranean vegetation

Taking into account the ten main traits (Table 2.2), the MEDPL was analyzed. From the 633 species in the list, 330 species had to be omitted because only data for 1-4 traits could be found. Another 161 species were omitted for the following reasons: 21 for having a phanerophytic life form, 9 for exceeding 1 m in height, 60 because of a too deep rooting system, 2 because of complete lack of or weak drought tolerance and 69 due to unsuitable Grime plant strategy (i.e. no stress tolerance). The adjusted scores of the remaining 142 species ranged between 37.5 and 100%. Table 2.5 lists the species with potential for EGR appli atio adjusted s o e % . I ge e al, Mediterranean species (53%) are currently not used for EGR purposes in NW Europe. These species are promising and it would be interesting to further test them on experimental GR platforms. Species scoring the highest belonged to the genus Sedum and are already used on GRs. Six species had an annual life form and were not present in the GRPL. Other species all had a hemicryptophytic life form.

Discussion

The results of the clustering and ordination analysis indicate that the plots, each containing their own specific (sub) Mediterranean vegetation, can clearly be separated into four vegetation types or clusters with distinct climatic, geographic and soil-related properties. The analysis of (sub) Mediterranean habitats and the assignment of species to a specific vegetation type contribute to a correct decision making regarding GR implementation. For instance, GRs in the southernmost part of Mediterranean France will benefit more from the selection of native species from clusters 1 and 2, like Aethionema saxatile, Bromus rubens and Dianthus sylvestris, as these species are most drought adapted: they are subjected to a short vegetation period during spring and autumn, extended summer drought, low AAP and high AAT (Table 2.3). In this case, the % OM and sand in the substrate should be relatively low (around 6.5 and 40% respectively) as these are the soil conditions in their natural habitats (Table 2.3). These percentages correspond with the composition range of the traditional EGR substrates [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF]. Alternatively, in higher locations with a longer vegetation period, lower AAT and higher AAP, species from clusters 3 and 4 should be preferred, and the substrate should contain higher values of OM. Some examples are Alyssum montanum, Armeria arenaria and Potentilla pusilla.

When the GRPL was compared to the MEDPL, two results need to be highlighted. First, we found that 79% of the species in the MEDPL were not yet applied on GRs and that Sedum spp., succulents with a (facultative) CAM photosynthetic pathway [START_REF] Durhman | Effect of Watering Regimen on Chlorophyll fluorescence and Growth of Selected Green Roof Plant Taxa[END_REF], are most frequently applied. Succulents are considered the best option for application on EGRs, as their shallow rooting system and efficient water use permits them to tolerate even the most extreme environmental conditions on the roof (e.g. [START_REF] Monterusso | Establishment and persistence of Sedum spp. and native taxa for green roof applications[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. However, some of these taxa could also pose problems due to slow colonization rates and frost sensitivity [e.g. Hylotelephium cauticola [START_REF] Getter | Effect of Substrate Depth and Planting Season on Sedum Plug Survival on Green Roofs[END_REF]; Sedum rupestre [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]]. Besides succulents, some herbaceous perennial plants, grasses and geophytes clearly are also capable to withstand drought [e.g. Allium cernuum, Festuca glauca (Getter & Rowe, 2008b); Armeria maritima, Prunella vulgaris, Silene uniflora, Koeleria macrantha, Trisetum flavescens (Nagase & Dunnett, 2010); Poa compressa [START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF]]. Instead of using monocultures of succulents, mixtures of diverse life forms are more interesting [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF] and have shown to optimize GR performance, by improving stormwater and cooling management [START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF]Lundholm et al, 2010). Grasses and herbaceous species are less drought tolerant compared to succulents, but are more effective for evapotranspiration and thus stormwater retention [START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF]. Alternatively, succulents have been found to facilitate growth of herbaceous species under abiotic stress [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. Despite the promising features of species mixture implementation, a (partially) shaded roof with deeper growing substrate and irrigation is often suggested when successful establishment and survival of plant species on GRs in Mediterranean climates are considered ( Getter & Rowe, 2008b;[START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]Nektarios et al., 2011). This is often not possible as most roofs are fully exposed and deeper growing substrates yield higher loads and thus special, costly building structure improvements. Furthermore, the eventual use of irrigation questions the sustainability of the whole EGR concept, especially in a region where water is very scarce during summer (cf. [START_REF] Filippi | The Dry Gardening Handbook: Plants and Practices for a Changing Climate[END_REF]. The high amount of unused, local Mediterranean species with variable life forms comprises a potential source for urban GR purposes, although further screening and testing is required to see if they can thrive in shallow substrates without irrigation.

The second result of the species list comparison shows that hemicryptophytes constitute the major proportion of currently applied GR plants, and that very little attention has gone to the possibilities of annuals. We assume that the presence of annuals in the GRPL is mainly due to spontaneous colonization, as GRs comprise a dynamic system with seed input from surrounding vegetation and [START_REF] Bornkamm | Vegetation und Vegetations-entwicklung auf Kiesdächern[END_REF] and [START_REF] Köhler | Fassaden-und Dachbegrünung[END_REF], which monitored vegetation on GRs, listed all species present on existing GRs and not only those that were introduced on them. In contrast to the low presence of annuals in the GRPL, this life form type is particularly abundant in the MEDPL, and contains promising features for EGRs. Many therophytes germinate, grow, flower and produce many seeds in a very short time period, whereupon they pe ish. Although the o t e i te esti g du i g su e o ths, the a e attractive in spring and early summer for pollinators as well as having aesthetic value, due to their colorful flowers (cf. [START_REF] Filippi | The Dry Gardening Handbook: Plants and Practices for a Changing Climate[END_REF]. Seed production is added to the seed bank, hence forming a buffer against eventual gap formation should other herbaceous plants die off. This property is a natural survival strategy which may guarantee GR performance in regions where weather conditions are very unpredictable [START_REF] Madon | The ecological significance of annuals on a Mediterranean grassland (Mt Ventoux, France)[END_REF]. The incorporation of annuals in the design of EGRs can improve the EGR performance and should certainly be considered.

People are rather skeptical towards dehydrated GRs during summer [START_REF] Dagenais | Development of Design Criteria to Improve Aesthetic Appreciation of Extensive Green Roofs[END_REF]. However, even when the GR vegetation is not very attractive in summer (except for evergreen succulents), GRs should still be the preferred option over bitumen or gravel roofs, as the substrate and other build-up layers are also effective in building cooling and stormwater retention. Outside the summer months, GRs can function optimally and offer a wide array of local drought adapted plant species, thereby increasing floral and faunal biodiversity. If people accept that non irrigated EGRs in the Mediterranean region will not be green all year round, which is in line with the general life cycles of the vegetation in this climate, more design possibilities arise which will greatly improve GR interest in the region.

Mediterranean vegetation definitely can thus inspire us as a source for development and improvement of EGR design. Many species were deleted due to a deep rooting system or lack of stress tolerance. Unfortunately also a lot of species (330 in total) had to be omitted as the plant traits were not sufficiently known (indicated by a confidential level below 50%). Therefore, possibly some interesting species have been omitted and Table 2.5 only brightens a small part of the immense possibilities that the Mediterranean vegetation offers. Alternatively, some species in the dataset a i sti ti el e des i ed as ot useful fo EGR desig ut a e i luded e ause no information was available to argue their exclusion. An example is Dittrichia viscosa, without any information in the TRY database on rooting depth or other exclusion criteria. It is therefore included in the dataset, although experts would not recommend this species because rooting depth is not expected to be shallower than 20 cm. This is a recurring problem that has also been encountered in other studies (e.g. [START_REF] Mentens | Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century?[END_REF][START_REF] Stadler | Web-based tools for data analysis and quality assurance on a lifehistory trait database of plants of Northwest Europe[END_REF], stressing the necessity to further complete and update available plant trait databases like the TRY Initiative.

Results in Table 2.5 indicated that Sedum spp. (of which some are already comprised in the GRPL) are also a good option for EGRs in the (sub) Mediterranean region. Other potential species not yet used on GRs in NW Europe include: Bromus rubens, a Mediterranean annual grass; Ceterach officinarum, a fern that likes sunny expositions and requires little or no humidity; Linum tenuifolium, a perennial that prefers sunny, dry and hot habitats on calcareous soil, and Crassula tillaea, an annual succulent that thrives in locations with strong summer drought because its short life cycle allows it to grow and flower before summer. Further plant types that need attention are mosses and lichens, as they are part of the vegetation (sometimes even exceeding 80% cover in the plots). Their value was already confirmed by certain authors also with respect to facilitation of establishment and survival of vascular plants in extreme conditions, improvement of water storage and carbon and nitrogen sequestration (e.g. [START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF][START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF].

The approach of the EGR habitat template [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF] applied in this study provided interesting outcomes. On one hand it was possible to describe the template vegetation in the study region, therefore pointing at some ecological requirements that need to be considered during GR design (like the incorporation of different life forms with special attention to annuals). On the other hand, this study highlighted the possibilities of (sub) Mediterranean vegetation for use on EGRs. However, the results do not offer scientific evidence that the species will effectively thrive on EGRs in the Mediterranean. Additional research is needed for a more thorough selection of plants in the provided species lists, for example by also using utilitarian plant traits (e.g. flower color, commercial availability) and then followed by testing them under different EGR conditions prior to commercialization.

Conclusion

GRs are an important tool to increase environmental quality in densely populated cities. The performance of GRs happens to be rather low in the (sub) Mediterranean region, as the currently applied species are more or less the same as in NW Europe and thus not very well adapted to the extreme environmental conditions in this region. This study used the habitat template concept to look at possibilities of native vegetation on shallow soils and rocky habitats in Mediterranean France. Four relevant vegetation types were identified, according to geographic, climatic and soilrelated variables. A comprehensive list of species in the Mediterranean region of southern France with a potential for use on EGRs was compiled. Special attention should go to the incorporation of annual species, as they contain special features that can guarantee GR performance in regions where weather conditions are very harsh. This study provides evidence that natural habitats can inspire and improve the design of EGRs in the (sub) Mediterranean region.

Chapter 3. Plant trait analysis delivers an extensive list of potential green roof species for Mediterranean France

Introduction

The life-quality-improving value of green roofs (GRs) in densely populated urban areas arouses the public interest for these novel urban ecosystems [START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. GRs have the potential to mitigate urban temperatures to more comfortable levels through higher albedo and evapotranspiration from vegetation and substrate (cf. [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. The evapotranspiration, together with water storage in the substrate, also leads to more effective stormwater management. Additionally, GR vegetation offers habitats and food sources that support a range of local invertebrate [START_REF] Madre | A comparison of 3 types of green roof as habitats for arthropods[END_REF] and bird communities, therefore increasing biodiversity value [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF]. Other benefits include noise reduction, aesthetic and psychological value, a longer roof membrane longevity and substitution for lost space on ground level [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF]. In an era of climate change where more intense rainfall events and extended dry periods are predicted, the buffering capacity of GRs will increase their importance even further [START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]. The focus here is on GRs of the extensive type (EGR; substrate depth < 20 cm) as these are widely applicable and their low-maintenance supports self-regulation which is an essential feature of these versatile systems

During the last decades, research and application of GRs have shown a remarkable rise in NW European and North American countries (e.g. [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]. However, knowhow and proper incentives are to some extent still lacking in a large part of the world [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Fioretti | Green roof energy and water related performance in the Mediterranean climate[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]. This is for instance the case in countries with a Mediterranean climate, where the beneficial effects of GRs would be similar or even more pronounced [START_REF] Alexandri | Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates[END_REF][START_REF] Fioretti | Green roof energy and water related performance in the Mediterranean climate[END_REF]. However, environmental conditions on rooftops are very harsh, particularly in the Mediterranean during summer, and it is necessary to have a better understanding of drought tolerant vegetation that could survive and thrive in this challenging climate [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF].

Until recently, research exploring the suitability of various plant taxa has been limited. Succulent plants are generally considered the most appropriate plants to apply on EGRs, due to their shallow root systems, (facultative) crassulacean acid metabolism (CAM) and hence efficient water use and tolerance to extreme conditions of drought [START_REF] Durhman | Effect of Watering Regimen on Chlorophyll fluorescence and Growth of Selected Green Roof Plant Taxa[END_REF][START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]. [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF] highlighted the potential of succulents by indicating that Sedum spp. a fa ilitate the pe fo a e of eigh o i g pla ts, the so alled u se effe t , reducing soil temperature during dry weather conditions, thereby decreasing abiotic stress for other life forms. While the GR industries rely mainly on the use of su ule t a d so e othe t ied-and-t ue pla ts i.e. pla ts tested a d fou d capable for use on GRs), often applied in low species numbers or limited to only one life form, a range of unexplored herbaceous perennial and annual plants exists with the necessary drought adaptations [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]Nagase & Dunnett, 2010, Chapter 2). The use of different life forms has been shown to provide better ecosystem functioning and resistance to environmental stress, mainly due to niche complementarity and facilitation (Lundholm et al., 2010). Furthermore, highly diverse GRs have a higher survival probability and are more aesthetically pleasing even under dry conditions (Nagase & Dunnett, 2010).

In order to find potential species for urban greening projects (including GRs and g ee alls , the ha itat te plate h pothesis Lu dhol , is a highly interesting concept. In the case of EGRs, this hypothesis suggests that natural habitats with similar characteristics as those on this GR type, thus scree slopes, limestone pavements and calcareous grasslands on very shallow and nutrient poor soils, can inspire EGR design. Therefore, studying the native plant species composition in these habitats is useful. As a biodiversity hotspot, representing over % of the o ld s flo a La e g e et al., a d a lot of e t e e ha itats, the Mediterranean area offers a potentially interesting species pool for EGR application (see Chapter 2). With the habitat template hypothesis in mind, we described seminatural vegetation in southern France in a previous study, and identified an elaborate list of species which do have some potential for use on EGRs (Chapter 2). Drawing attention to potential species is a starting point, but implementation of these species can still prove to be impossible, as there are also other factors besides drought adaptation to account for. For example, traits like plant size, growth potential, flowering duration, pollinator attractiveness and aesthetic value are factors that also need to be considered (cf. [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]. Detailed information on plant traits can offer insights into the typical characteristics of GR plants and therefore guide us to the most useful species for EGR application (cf. Lundholm et al., 2010). Because species selection can be a complex process when a wide range of criteria are involved [START_REF] Reubens | Tree species selection for land rehabilitation in Ethiopia: fragmented knowledge to an integrated multicriteria decision approach[END_REF], the development of a decision support tool is a welcome instrument to assist GR developers toward an appropriate choice of native plants in GR projects.

The objective of this study was to provide an overview of plant traits that are crucial for survival of plants on EGRs in areas where dry periods are prominent, in particular in the Mediterranean climate. The most important plant traits will be incorporated in an easy to handle screening tool and it will be applied on a species list of a vegetation survey in Mediterranean France.

Material and methods

Species lists

For the analysis and case study, a matrix of plant species and corresponding traits was assembled. The species lists included a set of EGR plant species (GRPL), commonly used or spontaneously occurring on EGRs in NW Europe, and a Mediterranean plant species list (MEDPL) from a previous study of an extensive vegetation survey in southern France (Chapter 2). As drought tolerance and selfregulation are always important on all EGR systems, the GRPL is used as a reference data set.

For the GRPL, seven sources were used. These included five key literature sources of internationally recognized GR specialists or scientists [START_REF] Bornkamm | Vegetation und Vegetations-entwicklung auf Kiesdächern[END_REF][START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF][START_REF] Köhler | Fassaden-und Dachbegrünung[END_REF][START_REF] Kolb | G ee a d ool oofs u a heat island mitigation potential in European climates for office buildings under free floating conditions[END_REF][START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF] and two species lists of international GR Companies (Optigreen Limited, 2012;[START_REF] Gmbh | Leaf adaptation; Seed mass; Grime strategy; Leaf phenology; Reserves and storage; Drought tolerance; Shade tolerance Valladares & Niinemets[END_REF].The number of times a species is noted in these sources is considered a proxy to its frequency of use on EGRs (FREQ), which will be used to analyze the relation between frequency and plant traits. As plant traits were only available at the species level, the GRPL comprised 483 species, after exclusion of subspecies, varieties and hybrids. By this we assume that plant traits of infraspecific taxa are the same as for the species.

The MEDPL comprised the species of (sub) Mediterranean France observed in 2011 during an elaborate vegetation survey of open habitats on calcareous, shallow stony soils (for a full habitat description we refer to Chapter 2). These extreme habitat conditions are assumed representative for the conditions on EGRs. Additionally we added four literature data sources describing similar habitats [START_REF] Molinier | La vegetation de la Crau (Basse Provence)[END_REF][START_REF] Van Den Berghen | Etude sur la végétation des Grands Causses du Massif Central de France[END_REF][START_REF] Loisel | La egetatio de l tage dite a en dans le sud-est continental Français[END_REF][START_REF] Rieux | Etude écologique et phytosociologique du Crassuletum tillaeae Molinier et Tallon 1949[END_REF]. The vegetation in these literature sources was described as Mediterranean matorral (also called garrigue), typical for thermophilic limestone pavements of southern France, with high species richness (SR) and a mosaic of different biological types. Both our vegetation descriptions and the literature data together accounted for 633 species, from which 131 species also occurred in the GRPL. Plant nomenclature follows The Plant List (2010).

Trait selection procedure

The approach was based on both functional traits and utilitarian aspects, as actual species selection for EGRs will depend on both. Drought adaptation and selfregulation (e.g. regeneration strategy, presence of a seed bank, plant longevity) were considered key factors for survival and overall success on EGRs, which also holds for more arid environments (Getter & Rowe, 2008b;[START_REF] Vinson | Green roof plant suitability analysis for northern climates[END_REF]. Relevant t ait g oups , all elated to d ought adaptatio a d/o self-regulation, were identified through an extensive literature search. For the selection of utilitarian aspects, a similar approach was used, in which four GR publications were consulted. For a summary of the literature used, we refer to Appendix 3.1. Next, the TRY Initiative (Kattge et al., 2011; list of contributed authors in Appendix 3.2), a global a hi e of pla t t aits, as s ee ed fo pla t t aits elated to the t ait g oups identified by the literature survey. A total of 67 traits (53 functional traits and 14 utilitarian aspects) were assembled for the 985 unique plant species. All species for which no information was available were omitted as they do not contribute to the analysis. This reduced the dataset to 696 plant species (316 in the GRPL and 507 in the MEDPL; 127 species occurring in both lists). Because we are only interested in Mediterranean species with a potential for EGRs, we then made a preselection based on some obvious exclusion criteria. Some traits (e.g. deep rooting system) may be considered an adaptation to drought, but since substrate depth on EGRs does not exceed 20 cm (cf. [START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF], all species having a larger rooting depth were omitted. Other exclusion criteria were: phanerophytic life form (in other words all trees); plant height exceeding 1 m (often shrubs); lack of drought tolerance and no stress tolerance according to the Grime strategy types. The final matrix for data analysis comprised 67 traits and 471 species (231 in the GRPL and 309 in the MEDPL; 69 species occurred in both lists; see Appendix 3.3 for the final species list).

Data analysis

To summarize the information in the species*trait matrix, a correlation analysis of plant traits was performed. As the goal was to highlight the most important traits for EGR plants and to use them for appropriate species selection, we first considered only the GRPL (n = 231 species). The analysis followed a hierarchy, first focusing on the ordinal variable frequency (FREQ), as we assumed plant traits associated with highly frequent species as more important and informative. Afterwards a more extensive correlation analysis was performed, to select other traits that are directly related to the traits resulting from the previous step. 31 traits (e.g. protein potential, leaf respiration, fire tolerance and moisture use) contained more than 90% missing values. To make the analysis more robust without further reduction of the amount of species, we only used the variables containing less than 75% missing values. This reduced the number of variables to 25: FREQ, 17 functional traits and 7 utilitarian aspects (Table 3.1). Exploration of the data suggested the use of non-parametric statistics, as all variables showed a non-normal distribution (p-value of Kolmogorov-Smirnov test < 0.05). Spearman rank correlation (r s ), Kruskal Wallis tests (KW) and Pearson Chi 2 a al sis χ e e applied to test fo elatio s et ee the a ia les. All analyses were performed in SPSS 20.0 (SPSS Inc, Chicago, IL). For the significant KW a d χ results, post hoc tests were performed and results were presented with bar charts (Siegel & Castella , . Fo all the sig ifi a t t aits, s o i g o ditio s were calculated for further implementation in the screening tool. These are the average values (for scale variables) and main categories (for ordinal or nominal variables) of the most frequently used EGR species, defined as occurring in five or more of the seven sources used to compile the GRPL.

Screening tool and case study

Based on the results of the correlation analysis, a screening tool was constructed which can be considered as a hierarchical multi criteria scoring system weighted by trait importance (cf. [START_REF] Reubens | Tree species selection for land rehabilitation in Ethiopia: fragmented knowledge to an integrated multicriteria decision approach[END_REF]. First before scoring, the exclusion criteria mentioned in the trait selection procedure were considered, to delete species that are obviously not suitable for EGR purposes. Then the traits significantly related to F'EQ he eafte alled p i a t aits are listed, followed by the traits that are significantly related to the primary traits. Species get a positive score for the traits (2 points for every primary trait and 1 point for every derived trait) when they meet the condition indicated in the screening tool. The final screening score (SS) is the sum of all the separate trait scores and can be considered as a measure of suitability for application on EGRs in the Mediterranean. As a case study, the screening score was applied to the MEDPL, indicating the most promising Mediterranean species for EGR purposes that merit further experimental tests.

Results

Correlation analysis

FREQ is negatively correlated with leaf area (r s = -. , p < . , spe ifi leaf a ea (r s = -. , p < . a d hole leaf d ass s = -. , p < . Ta le . i . Average values for highly frequent species (defined as occurring in 5-7 of the used GR sources) are for leaf area, specific leaf area and whole leaf dry mass lower than 368 mm 2 , 18 mm 2 /mg and 25 mg, respectively. The Kruskal Wallis test results in Table 3.2ii show a highly significant difference (p < 0.001) between FREQ and metamorphosis for storage (KW = 24.53) and plant strategy type (KW = 17.66). Leaf shape (KW = 12.86), metamorphosis for vegetative dispersal (KW = 18.71), and photosynthetic pathway (KW = 9.57) are also significantly correlated (p < 0.05) with FREQ. Fig. 3.1 illustrates the results of the post hoc tests. Frequently occurring EGR species are stress tolerant, have a CAM photosynthetic pathway, disperse through fragmentation or runners, store reserves through succulence and mostly have a scale-like or needle-like leaf shape. Most of the primary traits are correlated as well (e.g. leaf area and whole leaf dry mass; r s = 0.88, p < 0.001). The primary traits show significant correlations or associations with following traits: Table 3.3i: seed mass, typical plant height, woodiness, flowering duration and flowering period; Table 3.3ii: seed mass, leaf phenology, pollinator reward, shoot growth form, flowering duration, flowering period and life form; Table 3.3iii: life form, species reproduction, flower color, flower type, pollinator and plant longevity. o ths, espe ti el . Fo the atego ies u e a d f ag e tatio of the trait metamorphosis for vegetative dispersal, average values are respectively seed mass of 0.66 and 4.54 mg and flowering duration of 1.09 and 1.13 months. As can be seen in Fig. 3.4, most species with a needle-like leaf shape are also pollinated by insects or wind and have yellow, blue or white disk flowers. Most succulent species also happen to be evergreen, are pollinated by insects and have disk flowers. Species dispersing through fragmentation or runners are mainly evergreen and are pollinated through insects or wind. CAM plants are mostly perennial. Stress tolerant species generally have a perennial longevity, are evergreen, are pollinated by insects and have disk flowers. All the important primary plant trait categories in Fig. 3.4 (except CAM plants) reproduce through seeds or vegetatively and have a hemicryptophytic life form.

Table 3.2 The relationship between plant traits and frequency of use on EGRs (FREQ). (i) Spearman rank correlations (rs) together with their significance. (ii) Kruskal Wallis test results (KW) together with their significance. For abbreviations of plant traits see

Screening tool and case study

Table 3.4 shows the screening tool, in which all the significant results from the correlation analysis are incorporated. The traits species reproduction and pollinator type are excluded from the screening tool, as their important categories (seeds and vegetative for species reproduction; insects and wind for pollinator type) are the

LeafPH

only categories for these traits, and incorporation in the screening tool will thus not be informative for species scoring. The 309 MEDPL species that passed through the exclusion step were submitted to the screening tool. In Table 3.5, the highest scoring Medite a ea spe ies spe ies ith "" , a i u "" = a e listed. species in the list are already used on EGRs in NW Europe, and 34 species are currently unused. The average score in both groups is comparable (12 for the new species and 14 for the already used species). The highest scoring species are Sedum album and Sedum acre, both already frequently used on EGRs. We highlight that 35% of the species in the new potential species group recommended for use on EGRs in the Mediterranean region are therophytes.

Table 3.4 Decision support tool to select species which are potentially interesting for use on EGRs in Mediterranean climates, based on functional (*) plant traits and utilitarian (+) aspects related to frequently applied EGR species. Three groups of selection criteria are shown: exclusion criteria, primary traits and derived traits. Species can be deleted in the first step using exclusion criteria, or get a weighted positive score for the traits for which they meet the condition indicated in the screening tool.

Exclusion criteria

Discussion

Suitable habitats and plant traits for Mediterranean EGR plant selection

The habitat template hypothesis is a good starting point for finding potentially suitable plant species, whether this is for EGRs or other applications in urban greenery like green walls, parks and gardens [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. Environmental conditions in anthropogenic habitats such as EGRs indeed mimic circumstances also found in the field [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]. Substrates of EGRs have in common with the sampled natural habitats that they are shallow, have a high draining capacity and are relatively calcareous [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. So only these vegetation communities were used here as a starting point [START_REF] Molinier | La vegetation de la Crau (Basse Provence)[END_REF][START_REF] Van Den Berghen | Etude sur la végétation des Grands Causses du Massif Central de France[END_REF][START_REF] Loisel | La egetatio de l tage dite a en dans le sud-est continental Français[END_REF][START_REF] Rieux | Etude écologique et phytosociologique du Crassuletum tillaeae Molinier et Tallon 1949[END_REF]. Natural habitats in the Mediterranean region have an extremely high diversity of plant species [START_REF] Médail | Hot-Spots analysis for conservation of plant biodiversity in the Mediterranean Basin[END_REF]) and these offer the potential for finding new species appropriate for EGRs (cf. Chapter 2). A particular assumption is that the traits (but not the trait values) that make plant species successful on NW European EGRs are the same as in Mediterranean Europe. Indeed, drought tolerance and variables which are important in self-regulating plant communities are also expected to be important on EGRs i othe li ati egio s, although the pla t s drought tolerance properties are more essential in the Mediterranean (cf. Getter & Rowe, 2008b;[START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]. So the primary and derived traits highlighted in this study should be particularly important for survival under a Mediterranean climate. It is not advised to implement the screening tool on species not native to the Mediterranean, as other factors could also influence species survival on EGRs in other regions. For example, if Scandinavian countries would be the target, the literature study should rather focus on tolerance to low temperatures and humid conditions, which would significantly alter the screening tool (cf. [START_REF] Emilsson | Comparison of establishment methods for extensive green roofs in southern Sweden[END_REF].

Important EGR plant traits and design recommendations

Attributes characterizing frequently applied EGR species (Table 2.4) are a small leaf area, specific leaf area and whole leaf dry mass, needle-like/ensiform or scale-like leaves, CAM photosynthetic pathway (cf. [START_REF] Durhman | Effect of Watering Regimen on Chlorophyll fluorescence and Growth of Selected Green Roof Plant Taxa[END_REF], stress tolerance, dispersion through fragmentation or runners and storage of reserves through succulence. Other variables correlated to these primary traits extend the list of traits to be considered in the screening tool. An interesting result was that species with a low whole leaf dry mass were merely therophytes. These annual species contribute significantly to the vegetation of Mediterranean systems but their presence in the GR spectrum is until now limited (cf. Chapter 2), probably because of their short aboveground visibility per year, unreliable regeneration and poor ability to support perennial competition in more temperate climates. Also, as they are absent during summer months, the contribution to the cooling effect of the GR will be negligible during this period. Of the new potential Mediterranean species in Table 3.5, 35% are therophytes, indicating that many species with this life form meet the scoring conditions indicated in the screening tool. Therophytes have interesting properties, like short flowering time and production of many seeds, as explained in Chapter 2.

Chapter 3

55

Their conservation value may also be important, certainly in Mediterranean areas he e a a uals a e th eate ed La e g e et al., . The i lusio of a ual life fo as a s o i g o ditio fo spe ies sele tio shifts the t aditio al implementation of perennial and hemicryptophytic species to a more diverse and drought adapted design, which should improve the roofs value for biodiversity and ecosystem functioning. Some of the primary traits, like CAM photosynthetic pathway, stress tolerance and succulence, already proved to be important for the success on GRs [START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. The other leaf-related traits emerging from the plant trait analysis have gained much less attention in the literature (cf. [START_REF] Dunnett | Planting Green Roofs and Living Walls[END_REF][START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF]. Therefore, the presence of these attributes in the screening tool offers some novel insights that reach beyond the usual considered water storage attributes. Next to important attributes, the potential species also have specific habitat requirements (e.g. pH, organic matter, nutrient content) and the GR substrate will need customization to ensure good plant health. This is however not an objective of this chapter, but a consideration for a later phase when the potential species are tested under EGR conditions.

Like other authors (e.g. [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]Lundholm et al., 2010;[START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF], we suggest to combine different life forms for EGR design, including Sedum spp., annuals, geophytes and hemicryptophytes. Sedum and other succulent species could act as nursery plants for other herbaceous species when environmental conditions are harsh [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. By use of both novel or rarely used species (e.g. Alyssum alyssoides, Dianthus sylvestris, Erophila verna, Filago pyramidata, Linum tenuifolium) and already applied species (e.g. Allium sphaerocephalon, Euphorbia cyparissias, Helianthemum nummularium, S. album, S. acre), GR biodiversity can gradually be increased without risking failure. The EGR vegetation design recommended here should ensure EGR performance and sustainability, even under difficult climatic conditions, although further testing of the potential species is required.

Comments on the plant trait analysis

Although the plant trait based approach applied in this study offered insights in the ecological background of potential EGR vegetation, the results and the screening tool should be interpreted with caution, as some impediments could have influenced the results.

First, the most extended and complete data source currently available was consulted (TRY Initiative, Kattge et al., 2011), but still a large amount of missing values was encountered. To strengthen the analysis, species and variables with insufficient information (289 species and 43 variables, respectively) were deleted. In this way, potentially interesting species and traits could have been overlooked. Furthermore, substantial information was available for more general and common species (e.g. Aster linosyris, Galium boreale, Hieracium pilosella, Hippocrepis comosa, Veronica chamaedrys) but lacking for not well known local Mediterranean species (e.g. Argyrolobium zanonii, Carthamus carduncellus, Teucrium aureum, Lysimachia linumstellatum, Iris lutescens). This problem manifested in the results of the case study, with a high amount of common species in the final species list (Table 3.5). Also, the screening tool (Table 3.4) appears quite conservative, as the exclusion criteria are severe. For example, all species with a rooting depth greater than 20 cm were excluded, although many species have the potential to spread roots laterally, suggesting that some species that could work on EGRs were not considered. We stress that this trait only contained information for 10.87% of the species, and that the consulted sources to obtain the trait values [START_REF] Green | USDA PLANTS Compilation, Version 1[END_REF][START_REF] Paula | Fire-related traits for plant species of the Mediterranean Basin[END_REF] do not consider the possibility of lateral spread. Without this detailed information, it is better to be cautious when screening for appropriate species by leaving out all those that are doubtful. This is also why the lack of stress tolerance was added as an exclusion criterion, leaving out the species that are entirely competitive or ruderal. We believe that in our case the amount of missing traits and species is not a major issue, as the purpose was not to describe a plant community but to develop a recommendation system to screen appropriate species for EGRs in the Mediterranean. The lack of detailed information has been shown to be a common bottleneck in ecological research (e.g. [START_REF] Franzel | Choosing the Right Trees: Setting Priorities for Multipurpose Tree Improvement[END_REF]Diaz & Cabido, 1997;[START_REF] Reubens | Tree species selection for land rehabilitation in Ethiopia: fragmented knowledge to an integrated multicriteria decision approach[END_REF]. A future intent of the TRY initiative is to develop algorithms to fill gaps in the TRY trait matrix, through machine learning techniques in combination with global databases [START_REF] Shan | Gap filling in the plant kingdom -Trait prediction using Hierarchical Probabilistic Matrix Factorization[END_REF].

Secondly, during the compilation of the GRPL, varieties, hybrids and subspecies were omitted because trait data were only available at species-level. This could also have caused deletion of potential information, but as mentioned before, the research objective was to find important plant traits and the analysis was based on 231 EGR species, so we believe the results are sufficiently substantiated. Another constraint when compiling the GRPL is that no information was available on whether or not the species on the EGR received irrigation. The consulted sources only list species commonly applied on EGRs, and generally, a minimal irrigation regime during dry periods is recommended. The drought tolerant capacities of the EGR species might therefore not be completely distinct, but this does not impose a big issue as the screening tool allows the deletion of species with unsuitable specific functional traits. A third potential problem with the GRPL is that many of the frequently occurring species are horticultural (except those occurring spontaneously), hence easy to propagate and widely available in nurseries. Many of these species belong to the genus Sedum, which are phylogenetically similar and have rather comparable functional traits. The plant trait analysis is therefore somehow biased toward these horticultural species, which is reflected in the outcome of the screening tool (Table 3.5) where many species already occurring on EGRs show the highest screening scores. This does not invalidate the proposed plant trait approach, but rather demonstrates the current GR situatio he e pla ts a e ofte de oted as t ied-andt ue D o ak & Volder, 2010; Nagase & Dunnett, 2010). We suggest that, after testing of the potential species under EGR conditions, nurseries should take the successful species into account in their supply, in order to diversify and improve the EG' s performance.

A last constraint is the non-consideration of the species habitat requirements, which resulted in some odd species propositions (Table 3.5), like Gymnadenia odoratissima and Ophrys insectifera, both orchids with a high conservation value (IUCN, 2013). Because of their trait values, they are marked as potential species, but their suitability for use in Mediterranean EGR installations is doubtful because of their non-Mediterranean climate and habitat affinities. Their inclusion in the list is a side effect of the used MEDPL, which targets appropriate natural habitats but in which some species are present that are actually not representative for the selected plots. This is for example the case for Succisa pratensis, which is actually a Mediterranean wetland plant, or Peucedanum oreoselinum, originally a mountain-forest border and gravel plant [START_REF] Bernard | Flore des Causses: hautes terres, gorges, vallées et vallons (Flora of the causses: Highlands, gorges, valleys and glens)[END_REF]. This constraint does not imply that the plant trait approach is not appropriate or successful, but the lack of 1:1 correspondence between traits and niches indicates that the screening tool should be applied with caution and sufficient knowledge of the species in the plant list.

Conclusion

The ultimate purpose of this study was to reveal the most important attributes of EGR plants by use of a plant trait analysis, i.e. a correlation analysis on an extended matrix containing EGR species and a list of their functional traits and utilitarian aspects. The screening tool created based on the results of the analysis is a useful framework for making substantiated decisions for appropriate EGR design in terms of vegetation choice, when sustainability and biodiversity are of major importance.

The screening tool delivers a potential list, yet the ultimate prove will come from experimental outdoor trials. Although the adopted approach has some limitations, the plant trait based approach offers new possibilities for Mediterranean regions, and it can help GR companies in the temperate climate to adjust EGR design to cope with upcoming climate change. Parties and persons for whom this selection tool can be of interest (e.g. urban landscape designers, government, advisors, nurseries) would be surprised and inspired by the wide array of unknown species that point out to have potential for EGR purposes.

Introduction

Some consequences of the current human population growth are land use changes, a decline of natural habitats and biodiversity loss [START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF]. However, biodiversity is believed to be one of the major drivers of ecosystem services and therefore guarantees our human well-being [START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF][START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF]. Biodiversity in urban areas should hence be maintained and enhanced, which can be done through application of green infrastructure elements [START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. Because rooftops offer a substantial amount of unused space, green roofs (GRs) are a suitable option [START_REF] Dunnett | Planting Green Roofs and Living Walls[END_REF]. Greening roofs is gradually becoming a common practice in urban landscapes due to their multi-functionality, including stormwater management, urban habitat provisioning, aesthetic and amenity value and even climate change mitigation (e.g. [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]Lundholm et al., 2010). GRs that are lightweight, have a substrate depth of under 20 cm and require no or only little maintenance and irrigation are also referred to as extensive green roofs (EGRs; [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. Their biotic and abiotic characteristics are comparable to xeric natural habitats which make them particularly interesting for reconciliation ecology [START_REF] Lundholm | Mini-Review: Habitat analogues for reconciliation ecology in urban and industrial environments[END_REF][START_REF] Francis | Urban reconciliation ecology: the potential of living roofs and walls[END_REF][START_REF] Butler | Native plant enthusiasm reaches new heights: Perceptions, evidence, and the future of green roofs[END_REF]. These GRs are also examples of novel urban ecosystems, as they are self-organizing and originate from human interference (cf. [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF].

Many regions with hot and dry climates, including the Mediterranean, are still unfamiliar with GR technology [START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF], which is mainly due to the challenging climate (i.e. summer drought and elevated temperatures) and therefore limited water availability. On roof-level, these features impose severe restrictions on plant growth and survival [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF][START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF][START_REF] Farrell | Green roofs for hot and dry climates: Interacting effects of plant water use, succulence and substrate[END_REF]. Plants are assumed not to survive in semi-arid climates on unirrigated GRs with substrate depths below 5 cm, especially not during summer drought or establishment [START_REF] Getter | The Role of Extensive Green Roofs in Sustainable Development[END_REF][START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Thuring | Green Roof Plant Responses to Different Substrate Types and Depths under Various Drought Conditions[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]. Furthermore, summer water scarcity is becoming a recurring problem in the Mediterranean, and climate change will lead to even more severe water scarcity, as a decrease in summer precipitation of 5% per decade is expected [START_REF] Parry | Assessment of potential effects and adaptions for climate change in Europe: the Europe Acacia Project (a concerted action towards a comprehensive climate impacts and adaptions assessment for the European Union)[END_REF][START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF][START_REF] Blanusa | Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer ette ooli g se i e ?[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]. Also in temperate regions, water scarcity may pose an issue in a climate change context (cf. [START_REF] Baguis | Climate change scenarios for precipitation and potential evapotranspiration over central Belgium[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]). In Belgium for example, surface temperatures and winter precipitation are expected to increase while summers are predicted to become drier [START_REF] Baguis | Climate change scenarios for precipitation and potential evapotranspiration over central Belgium[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]. It is hence estimated that by 2050, summer conditions in temperate regions will in some years be relatively similar to the current dry Mediterranean summers. This could, in the event that irrigation is needed, lead for irrigation to become an unsustainable or even a regulated and limited option. It is hence necessary in both regions to adjust plant species choice in order to suit unirrigated GR conditions [START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF][START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]. Knowledge of the plants requirements, as well as their performance on site, are essential in this respect [START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]Nektarios et al., 2011;[START_REF] Butler | Native plant enthusiasm reaches new heights: Perceptions, evidence, and the future of green roofs[END_REF]. Mediterranean areas contain habitats rich in native plant species that have potential to be used on EGRs (see Chapter 2 and 3), because they are assumed to be better adapted to local climatic conditions and to require less maintenance (cf. [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]. From a biodiversity perspective one might even assume that GRs with these natives form an alternative new habitat for some Mediterranean plants whose natural habitats are endangered.

The performance of EGRs could also be improved by expanding the range of currently used plant species [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF][START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]Lundholm et al., 2010;Nagase & Dunnett, 2010;[START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. Plant diversification, in terms of plant characteristics, can indeed offer better ecosystem functions due to niche complementarity and facilitation [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF]Lundholm et al., 2010). Sedum spp. for example, can raise the survival probability of non-succulent species by buffering high soil temperatures during drought [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. This specific property has also been noted with bryophytes, which can colonize the substrate spontaneously and are appropriate to survive extreme drought [START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF][START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. Particularly in a Mediterranean climate, annual species are promising as well, as their short life cycle allows them to spend dry summer months as seeds and to germinate again after autumn rainfall, hence filling up gaps (see Chapter 2 and 3).

Without irrigation, plant selection mainly depends on the substrate depth of the GR system [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Thuring | Green Roof Plant Responses to Different Substrate Types and Depths under Various Drought Conditions[END_REF]. In semi-arid regions, roofs with deeper substrate depths may be required, ranging from Chapter 4 61 15 cm for succulents to over 30 cm for grasses and herbs [START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]. However, in practice there is a trade-off between substrate depth and weight, as building structures often cannot support excessive weight loads [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]Nektarios et al., 2011;[START_REF] Blanusa | Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer ette ooli g se i e ?[END_REF]. Inclusion of a light weight water retention layer (WRL), in which plant-available water is stored for a longer time period, could be an alternative, although the effects on plant performance are still underexplored [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF].

In this study, the performance of preselected Mediterranean plant species was monitored on unirrigated EGRs in two different climates: the Mediterranean (Avignon, southern France) and temperate climate (Heverlee, Belgium). The effects of substrate depth, additional WRL and exposition on plant performance were analyzed. The study period was two years and was assumed appropriate as it is primary the recruitment and early establishment phase, where seed germination takes place, which is critical for GR health in the long term (cf. Getter & Rowe, 2008;Nagase & Dunnett, 2013). After germination, plant performance was further monitored during two years (encompassing the growing season and period after the summer dry season) to study the GR dynamics. If plants survive during these two years and consequently also in dry seasons on these EGRs, it can be assumed that the EGR will remain functional during its lifetime. The results will help to inspire GR installation in the Mediterranean area, as well as adjust GR design in temperate regions in the light of climate change.

Material and methods

Experimental setup

The experimental EGRs were installed on existing flat roofs in September 2012, one at the U i e sit of Leu e i Belgiu He e lee, ° N, ° E a d a othe at the A ig o I stitute of Te h olog i F a e ° N, ° E . On both roofs (Fig. 4.1), one EGR was installed on the fully exposed south-side of the roof, in terms of sun and wind, and another one on the sheltered north-part of the roof. The bitumen roof was first covered with a root resistant waterproofing layer, a polyethylene foil with a thickness of 0.5 mm and density over 0.93 g/cm³ (Sopranature WF 50, SOPREMA nv). Above this layer, a drainage board with filter layer was placed (IBIC Drain 200, IBIC bvba). The drainage layer consisted of styrene and served to drain the runoff from the EGR while also protecting the roof. The filter foil of non-woven polypropylene prevented dirt accumulation in the drainage layer. Both layers together had a thickness of 12 mm, dry weight of 1.2 kg/m² and a compressive strength of 712 kN/m². On top of these layers, a wooden frame of untreated spruce (30 x 150 mm) was placed to delimit the plots of 1 m² which were surrounded by a 20 cm wide border to eliminate edge effects. Each plot represented one of the three following structure types (Fig. 4.1): (5) 5 cm substrate; (P5) 5 cm and (P10) 10 cm substrate on a WRL (Polygrow PG D1, Recticel BV). Characteristics of the EGR substrate used (LP24GREXORG1, Peltracom n.v.) are listed in Table 4.1. The 4 cm thick Polygrow WRL consisted of polyurethane foam and had flexibility, high pore content (98%) and stable structure as its main properties. Furthermore, the material had a bulk density of only 35 kg/m³, did not react with nutrients, was clean, and could absorb water even after complete drying. Depending on the type, the approximate weight of the plots ranged from 50-100 kg/m² when dry to 71.5-143 kg/m² when water-saturated. In summary, the experiment involved three factors (location, exposition and structure type) resulting in 12 treatments, which were each replicated three times. The used plants were a combination of species with different growth forms, selected after a vegetation survey and screening procedure in the French study region (cf. Chapter 2; Table 4.2). They were either novel or already used on GRs. Geophytes and succulents were pregrown in nurseries that were all located in southern France: Bulb d A ge e Fou ues , P pi i e Filippi M ze a d P pi i e 'ou Saint Etienne du Grès). Seeds were supplied by B&T World seeds (Aigues-Vives, southern France), after collection in the country of origin. No information on region of provenance of the seeds was provided, but as all selected species naturally occur in the Mediterranean study region, and their chorology covers southern Europe, we believe the seeds were sufficiently adapted to the site conditions. Sowing of seeds and sprouts and planting of bulbs and rhizomes was done in October 2012. Sowing density of the plants applied as seeds was 5 g of seed mix per plot, in which approximately 150 seeds per species were present. This sowing density is in accordance with the FLL guidelines, which advise 5-10 g/m² (FLL, 2008), and is even higher than the sowing rates of 2 and 4 g/m² used by Nagase and Dunnett (2013a). Similar to Nagase and Dunnett (2013b), the geophytes Allium sphaerocephalon and Iris lutescens were planted at a density of 3 bulbs or rhizomes per plot. Sedum spp.

were applied as sprouts at a density of 60 g/m², which is slightly higher than the sprout density of 30-50 g/m² advised by [START_REF] Kolb | G ee a d ool oofs u a heat island mitigation potential in European climates for office buildings under free floating conditions[END_REF]. Prior to sowing on the experimental plots, the viability of the seeds was examined with a tetrazolium test [START_REF] Baskin | Seeds: Ecology, Biogeography, and Evolution of Dormancy and Germination[END_REF]; see Appendix 4.1). The percentage of viable seeds ranged between 10 and 100%, but did not significantly affect the establishment, as shown by the non-significant Spearman rank correlation coefficients between % viable seeds and % germinated seeds during the first growing season (Appendix 4.1). (FAO, 2008) to calculate the reference evapotranspiration (ETo) using the FAO Penman-Monteith method. ETo and precipitation were plotted together with 0.5*ETo, which allowed determination of the summer drought period, i.e. when the precipitation bar drops below the 0.5* ETo curve (Fig. 4.2). To compare the ETo and precipitation during the study period with general local weather patterns, the average values for the period 1970-2000 from the FAO program New_LocClim were added to Fig. 4.2.

Plant performance surveys were done at seven moments: during the growing season (April, May and June 2013, and April and May 2014) and in autumn after summer drought (October 2013 and2014). The cover of the total plot (overhead projection), the seeded species (categorized into annuals and herbs), the succulents Sedum acre and S. album, and bryophyte layer was estimated. The abundance in terms of number of individuals of the annual species, herbs and geophytes was recorded in each 1 m² plot as well. Also diversity in terms of species richness (SR) and Simpson diversity index (D s ; Eq. 1), which indicates the chance that two randomly selected individual plants belong to two different species, were calculated (Gotelli and Chao, 2013).

D s = ∑ (Eq. 1)
In Eq.1, SR is the number of species present in the plot and p i is the relative abundance of the ith species.

During the experimental period, some weeds established on the roof spontaneously (Appendix 4.2), of which the diversity measures SR and D were also calculated.

Data analysis

Cover data, abundance data and diversity measures were analyzed on four moments: the growing season and autumn period of 2013 and 2014, corresponding with monitoring periods 3, 4, 5 and 7. Because the data were nonlinear in time and did not meet the assumption of normality, distributions across the two locations were compared by means of Mann-Whitney U tests (MWU). For every location separately, differences in distribution between the two expositions and the three structure types were then examined with respectively MWU and Kruskal Wallis tests (KW). Furthermore, a Spearman rank correlation test (r s ) was performed to find relations between the bryophyte cover, the SR and D s of the weeds and the other cover, abundance and diversity variables. The significance level was adjusted with a Bonferroni correction. All statistical tests were performed in SPSS 22.0 (SPSS Inc, Chicago, IL).

Finally, all the species were ranked according to their cover in the growing season and autumn period of 2013 and 2014 in both Avignon and Heverlee. Their relative cover was then calculated on a log-scale and the resulting rank-cover diagrams enable us to study the species composition development during the experimental period.

Results

Climate data

The monitored precipitation patterns were rather divergent from the expected averages derived from the FAO (Fig. 4.2). Especially in Avignon, monthly precipitation was highly variable. The expected ETo derived from FAO followed the calculated ETo rather well in Avignon, but was approximately 0.5 mm/day lower during summer months in Heverlee. In Heverlee, the spring months of 2013 were very cold and dry, but the vegetation did not face summer drought. In contrast, the growing season of 2013 was characterized by a mild spring with plenty of rain in Avignon, followed by three months of summer drought. In 2014 however, little spring rains occurred in Avignon and summer drought started in March and lasted until the end of August. Also in Heverlee the start of the spring period was relatively dry with higher temperatures than usual but in May 2014 the average precipitation increased and July and August 2014 were even characterized by an exceptionally high amount of rain.

General vegetation development

The total cover was significantly higher in Avignon compared to Heverlee in 2013, but was highest in Heverlee in 2014 (Table 4.3). Over the entire experiment, S. acre cover and the cover and abundance of annual species and herbs were significantly higher in Avignon compared to Heverlee. From the autumn of 2013 until the end of the experiment, S. album and bryophyte cover were significantly higher in Heverlee compared to Avignon. In 2014, the number of I. lutescens and A. sphaerocephalon were also highest in Heverlee.

In Avignon, the evolution in cover of the total plots, annual species, herbs, succulents and bryophytes was rather variable, with a pronounced increase in cover during the autumn periods after summer drought (monitoring times 4 and 7; Fig. 4.3a-f). The SR and D of the installed vegetation and the spontaneous colonized weeds were initially high but decreased over time and slightly increased again in autumn 2014 (Fig. 4.4ad).

In Heverlee, total cover and S. album cover increased over time, but a decrease in S. acre cover was observed during 2014, and the cover values were much lower than those of S. album at the end of the experiment (on average 3% compared to 50%; Fig. 4.3a,d,e). Bryophyte establishment started in autumn 2013 and values up to 60% cover could be observed in sheltered plots with structure type 5 (Fig. 4.3f). The SR was initially low but increased over the growing season of 2013 after which it remained rather stable (Fig. 4.4a). Values for the D s of the installed vegetation and the SR and D s of the weeds were rather variable, with peaks observed during monitoring time 3 (Fig. 4.4b-d).

Exposition effect

For the majority of the monitoring periods and measured variables, no significant differences between exposed and sheltered plots could be detected. However, whenever significant, values for cover data, abundance data and diversity measures were generally higher in the sheltered plots in both locations (Table 4.4).

In Avignon, significantly higher values were found in the sheltered plots for the cover of S. acre over the entire experimental period, for the cover of S. album from autumn 2013 until the end of the experiment, and for the total cover during 2014 (Table 4.4a;Fig. 4.3d,e). The sheltered plots also resulted in higher abundance of herbs in the growing season of 2013, higher cover of annuals in growing season 2014, and higher cover of bryophytes and number of I. lutescens in autumn 2014. Only for the D s of the weeds in growing season 2013 and the cover of the herbs in autumn 2014, the values were significant higher in the exposed plots.

In Heverlee, sheltered plots had significant higher values compared to the exposed ones for the following variables and periods (Table 4.

Structure type effect

Considering the structure types, the highest values were generally found in structure type P10, followed by P5, and lowest values were obtained in structure type 5 (Table 4.5).

In Avignon, some deviations from the general trend were observed. In the growing season of 2014 for example, the structure type P10 was significantly higher compared to both 5 and P5 for the abundance of annual species, the abundance and cover of herbs, and the SR and D s . This was also the case for the total cover in autumn 2013. Structure type P5 had the lowest values for the cover of annual species in the growing season of 2014.

For Heverlee, following deviations from the general pattern were found. Structure types P5 and P10 were both significantly higher than structure type 5 for the number of A. sphaerocephalon in the growing season of 2013 and for SR in autumn 2013. Structure type P10 was significant higher compared to both 5 and P5 for the total cover in growing season 2014 and the D s of the weeds in autumn 2014. Structure type P5 had the lowest values for S. album cover in the growing season of 2014 and both the total cover and SR of the weeds during autumn 2014. Finally, the cover and abundance of annual species were highest in structure type P5 during autumn 2013, and the bryophyte cover was highest in structure type 5 during 2014. Over the entire experimental period, 33 species colonized spontaneously in the 1 m² plots, of which 18 in Avignon and 13 in Heverlee (Appendix 4.2). In general, most abundant weeds were the phanerophyte Celtis australis and the annuals Erigeron canadensis and Veronica arvensis in Avignon. E. canadensis was also the most abundant weed in Heverlee, followed by Acer pseudoplatanus and Cerastium fontanum subsp. vulgare. When a species was present in 2013 and 2014, it was not the same individual plant, as the colonized weeds from 2013 died rather quickly when a hot-dry or cold-wet period started. The SR of weeds in both locations was positively correlated with the cover and abundance of annual species and herbs in the plots, but not with the cover of succulent species (Table 4.6). The SR of the weeds was also positively correlated with the SR and D s of the installed vegetation. In Avignon, the bryophyte cover was negatively correlated with the SR and D s of the installed vegetation. In both locations, a positive correlation was found between bryophyte cover and the total cover and cover of S. album and S. acre.

Discussion

EGR performance

As expected, the vegetation developed differently in both locations, which can be attributed to the differences in weather conditions. The effects of summer drought, spring and autumn rains are expressed in the cover data graphs of Avignon (Fig. 4.3). The growing season of 2013 was favorable for germination and plant growth in Avignon (especially the annuals and herbs), whereas in Heverlee the vegetation only started to develop in May. No summer drought occurred in Heverlee, hence allowing the vegetation to grow further during the summer months while the vegetation in Avignon dried out. The 2013 autumn rainfall in Avignon led to a germination boom of seeds originating from the wilted plants from the first growing season. The winter and subsequently dry second growing season did not allow survival of most of these plants and the vegetation started to wilt early in 2014. Also in Heverlee, the first spring months were relatively dry, but vegetation (especially S. album and bryophytes), developed further after the precipitation events in May, resulting in a higher total cover in Heverlee in 2014 compared to Avignon. However, in Avignon, there was a general decrease in SR and D s after the first growing season (Fig. 4.4), a trend which has also been observed in a study of Dunnett et al. (2008). This trend was most remarkable in Avignon, where some species (first L. maritima and in 2014 S. conica and L. ovatus) became dominant. As weather conditions vary from year to year and EGRs are dynamic systems, we expect for the following years that the vegetation will behave similar like the past two years, which were already quit contrasting. A further monitoring could of course add extra detail to the future development of the installed plant communities.

Whenever significant, the performance of the EGRs was generally better in the sheltered plots. Total cover for example was significantly higher when sheltered in both locations. Also cover of the succulents in Avignon and cover of S.album and bryophytes in Heverlee were higher in the sheltered plots. The plants take advantage of the increased humidity after summer drought and the less elevated temperatures in the sheltered exposition. In contrast, the D s of the weeds was higher in the exposed plots in the first growing season in Avignon, which could be due to the low cover of the plots together with mild weather conditions which favor the installation of ruderal species.

The factor structure type had a more pronounced effect on the EGR performance than the factor exposition, as a higher number of significant differences in distribution were found. Throughout the course of the experiment, the EGRs generally performed best under structure type P10. Structure type 5 generally had the lowest cover, abundance and diversity values, but in some cases there was a shift where the lowest values could be attributed to P5. Therefore, deeper substrates (min 10 cm) together with an additional WRL are preferred for EGRs in any climate. Using substrate depths of 5 cm or less without a WRL or provision of an irrigation system is not advised, at least not for the species tested in this study, if good performance and biodiversity value of EGRs need to be ensured. Other studies focusing on EGRs in semi-arid climates [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Thuring | Green Roof Plant Responses to Different Substrate Types and Depths under Various Drought Conditions[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF], come to the same conclusion.

Next to an additional WRL, which proved a useful addition to the EGR setup in this experiment, bryophytes have been suggested to carry out this function [START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF]. Although the bryophyte layer was considered as a whole in this experiment, it was noticed that most of them were acrocarps and that multiple species were present in the layer, e.g. Silky Forklet-moss (Dicranella heteromalla (Hedw.) Schimp.), Redshank (Ceratodon purpureus (Hedw.) Brid.), and Umbrella liverwort (Marchantia polymorpha L.). The very wet summer of 2014 in Heverlee favored the establishment and growth of bryophytes, where the highest cover was reached in structure type 5. This could be explained by a stronger competition effect in the P5 and P10 plots due to higher vascular plant cover. Bryophytes are also poikilohydric and their water content hence depends on the water potential of their environment. It could hence be possible that the WRL in P5 and P10 resulted in quicker water flow to deeper layers, whereas the water remained at the surface of the shallowest substrate where it favored bryophyte establishment. Indeed, compared to P5 and P10, the substrate surface of structure type 5 generally seemed to remain moist over a longer time period after rainfall. Furthermore, there was a positive correlation with the cover of succulents, which also benefit of superficial substrate moisture. However, in Avignon, bryophyte cover was negatively correlated with the SR and D s of the plot, hence indicating that bryophyte establishment is less likely when the biodiversity of the EGR is high. Regardless of the exposition and structure type effects, bryophytes need to be able to establish and develop spontaneously, which was counteracted on EGRs in the Mediterranean and on exposed EGRs in temperate climates mainly due to climatic conditions. Therefore, if bryophytes are not considered as a part of the vegetation composition from the beginning of an EGR project, the use of a WRL like polyurethane foam is recommended instead.

Next to bryophytes, 33 weedy species also colonized spontaneously (Appendix 4.2). For comparison, 35 colonizing species were identified by Nagase et al. ( 2008) on a GR over six growing seasons. In contrast to the bryophytes, weed establishment in the experimental plots was not correlated with the cover of succulents, but there was a positive correlation with the cover and abundance of annual species and herbs, and the SR and D s of the installed vegetation. Although the relation between bryophyte cover and SR and D s of the weeds was not significant in this experiment, [START_REF] Zamfir | Effects of bryophytes and lichens on seedling emergence of alvar plants: evidence from greenhouse experiments[END_REF] and [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF] found that the presence of a bryophyte layer may oppose weed establishment. The weeds were mainly native or exotic ruderal annuals also present in the environment surrounding the EGRs. Like the annual species installed on the EGRs, the weed development dynamics follow the weather patterns of summer drought, autumn and spring rains (cf. [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF]. E. canadensis, Epilobium hirsutum, Crepis sancta, Sonchus oleraceus and Trifolium campestris established the first growing season on the EGRs, produced seeds and reappeared the next year, hence illustrating the ecological phenomenon of the founder effect. Similar to [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF], some tree species also germinated but they all died quickly as expected when temperature and drought increased.

EGR design recommendations for Mediterranean and temperate climates

Of the species that are already used on GRs, we confirm that all, except Helianthemum nummularium and Clinopodium acinos which did not germinate or died after the first growing season, are suitable for use on EGRs (Appendix 4.3). These two species could have resulted in better germination and performance if seed quality was better or if plug plants were used. Indeed, good seed quality needs to be ensured or installation as plug plants should be considered in all cases. L. ovatus germinated and flowered abundantly in Avignon, but was not present in Heverlee. This species is therefore recommended for EGRs in the Mediterranean.

Euphorbia cyparissias can be used in temperate climates on EGRs with 5 or 10 cm substrate depth and a WRL, although this species is not expected to reach a high cover. In Avignon, this species was only noticed in structure type P10 at the end of the experiment. The succulents S. album and S. acre survived very well in the sheltered plots in Avignon. In Heverlee, the cover of S. album increases throughout the experiment, but cover values for S. acre remained rather low although still higher than many other seeded species. Similar results for S. album and S. acre were found in the experiment of [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF] in Sweden. We recommend considering Sedum spp. as part of every EGR vegetation composition because of their high cover and because it was observed that the small, annual species survived in proximity of Sedum patches. This nursing effect has also been demonstrated by [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. Other studies (e.g. [START_REF] Boivin | Effect of artificial substrate depth on freezing injury of six herbaceous perennials grown in a green roof system[END_REF][START_REF] Farrell | Green roofs for hot and dry climates: Interacting effects of plant water use, succulence and substrate[END_REF]) also point at the importance of succulent species on GRs. In general, A. sphaerocephalon grew and flowered well the first year but in 2014, only few new stems reappeared from the bulbs in substrate types P5 and P10. A. sphaerocephalon is a summer flowering accent plant with a high regeneration capacity if the substrate is deeper than 5 cm. This is in accordance with [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF], who recommend the use of geophytes when the substrate has at least 7 cm of depth.

Of the species that are not yet used on GRs, only Carthamus carduncellus is not recommended in any of the locations based on its low germination rate and bad performance. We like to advise A. alyssoides, L. maritima and S. conica for use on EGRs in both locations. In Avignon, Linum bienne is a small annual plant that can be considered for biodiversity reasons in sheltered plots with structure type P10. The other seeded species are not advised here, as they did not germinate (Erophila verna, Petrorhagia prolifera) or did not reappear in 2014 (Plantago afra, D. superbus, Sideritis hyssopifolia). In Heverlee, next to D. superbus, S. hyssopifolia and L. bienne are good performing species in structure types P5 and P10, although their cover remained rather low. E. verna did only perform well in the sheltered plots with structure type P10, and P. prolifera did only germinate at the end of the experiment. Like A. sphaerocephalon, I. lutescens is advised as an aesthetically appealing early flowering accent plant, although the combination of prolonged drought and shallow substrate on exposed EGRs can pose a regeneration problem.

In both the Avignon and Heverlee experiment, only for some variables and periods, the EGR performance appeared better in the sheltered plots. Placing the EGR in a sheltered location is hence not a major requirement to obtain a successful EGR. The structure type on the other hand appeared of much higher importance. In order to ensure a well-functioning EGR in both Mediterranean and temperate regions, one should best opt for a substrate depth of minimum 10 cm together with an additional WRL during the design phase. Opting for a shallower substrate with a WRL (structure type P5) may result in better or worse EGR performance compared to structure types 5 and P10, and is therefore a risky option. A very shallow substrate without a WRL (structure type 5) is only the best option if one wants to establish an EGR dominated by a bryophyte layer, and this is more certain to succeed in the temperate climate. In all circumstances, an irrigation system should be considered in order to keep the vegetation alive as long as possible if GRs are expected to be aesthetically pleasing (i.e. have a green appearance) throughout the major part of the year. Irrigation could also prevent the vegetation from wilting too early before having set seed, hence reducing the maintenance time and cost of reseeding the next year if no seed has been produced and in the absence of a permanent soil seed bank of the annual sowed species.

Conclusion

The discussed design and species recommendations for both Mediterranean and temperate climates shed some light on the possibilities for EGR design in regions with yearly summer drought and regions facing climate change. They can be applied immediately in the respective regions. As future summer conditions in the temperate climate are expected to be comparable to the current Mediterranean summer climate, the Avignon experiment gives an indication of how EGR vegetation could shift in the temperate climate in the next decades. As biodiversity conservation is becoming a priority on political agendas in developed countries all over the world, the promotion of urban infrastructure and especially the installation of EGRs should be supported, as it is an attainable way to counteract some of the negative effects of biodiversity loss. Even when EGRs do not appear very attractive during part of the year when the vegetation is dry, their value as a habitat for local flora and fauna cannot be neglected (cf. [START_REF] Brenneisen | Space for Urban Wildlife: Designing Green roofs as Habitats in Switzerland[END_REF][START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF], as well as their other benefits including stormwater retention and mitigation of the urban heat island effect.

Intermediate section. Plant species used in the experiment described in Chapter 4

Annual species

Alyssum alyssoides (L.) L.

Pale madwort

Introduction

Urban biodiversity is increasingly considered as an important element of sustainability, enhancing urban life quality and human health through the provision of ecosystem services (cf. [START_REF] Cilliers | Chapter 3.2 Urban Flora and Vegetation: Patterns and Processes[END_REF][START_REF] Mcdonald | Chapter 4.1 Global Effects of urbanization on Ecosystem Services[END_REF][START_REF] Robinson | Ecosystem services provided by urban spontaneous vegetation[END_REF][START_REF] Gaston | Managing urban ecosystems for goods and services[END_REF]. It should therefore be promoted and managed carefully, an objective achievable through application of urban green infrastructure elements like green roofs (GR). GRs are generally defined as roofs covered with plant species installed on top of specialized layers including substrate, drainage, filter and roof protection. Next to their role in biodiversity conservation [START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF][START_REF] Braaker | Habitat connectivity shapes urban arthropod communities: the key role of green roofs[END_REF][START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF], they mitigate both the urban heat island effect (UHI) and air pollution, reduce stormwater runoff and energy consumption and offer aesthetic, psychological and physical benefits [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. Although intensive green roofs (IGR) may accommodate more species diversity and reduce stormwater runoff more efficiently because of a deeper substrate (> 20 cm), it is the extensive green roof type (EGR) with a substrate depth of 2-20 cm that is particularly interesting as it is low-maintenance and broadly applicable. Plant species for such roofs are generally chosen for their drought tolerance and their ability to be self-sustaining. However, even for typically selected stress tolerant plants, the severe environmental conditions on EGRs remain a challenge for successful establishment, growth and reproduction. As the GR often should be as light as possible (70-170 kg/m² when water-saturated) and the substrate highly water permeable without losing stability and hampering oxygen diffusion, substrates are mainly composed of mineral aggregates with macropores [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF]. This decreases the available water for plants, and because the proportion of organic material (OM) is limited (generally 10-20 % of the weight-volume), the nutrient exchange capacity is low as well [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF]. All this may strongly hamper GR development and may result in reduced overall performance (Nagase & Dunnett, 2011).

Low resource availability could be overcome by (1) increasing species diversity to stimulate more complete resource use (cf. [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF], (2) use of different substrate materials including water retention additives (e.g. hydro-absorbent polymer gels cf. [START_REF] Sutton | Media Modifications for Native Plant Asemblages on Extensive Green Roofs[END_REF] or adding extra irrigation, (3) addition of fertilizers, (4) addition of OM or (5) inoculation with micro-organisms like arbuscular mycorrhizal fungi (AMF; [START_REF] Sutton | Media Modifications for Native Plant Asemblages on Extensive Green Roofs[END_REF][START_REF] Molineux | Characterising alternative recycled waste materials for use as green roof growing media in the U[END_REF][START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Oliveira | Effect of substrate treatments on survival and growth of Mediterranean shrubs in a revegetated quarry: An eight-year study[END_REF]. In this study, the focus will be on adding OM and AMF. For a review on the first two options, we refer to a review of Cook-Patton and Bauerle (2012) and to [START_REF] Farrell | Water-retention additives increase plant available water in green roof substrates[END_REF]. Because fertilization is a non-sustainable solution, this option is not preferred [START_REF] Sutton | Media Modifications for Native Plant Asemblages on Extensive Green Roofs[END_REF]. OM increases nutrient exchange capacity, adds nutrients to the substrate, and improves soil structure and water retention [START_REF] Friedrich | Principles for selecting the proper components for a green roof growing media[END_REF][START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF][START_REF] Molineux | Characterising alternative recycled waste materials for use as green roof growing media in the U[END_REF]. Despite the positive effects, there are also some drawbacks of this approach. OM will decay over time, causing shrinkage of the substrate volume, and again decreasing water and nutrient-holding capacity [START_REF] Friedrich | Principles for selecting the proper components for a green roof growing media[END_REF][START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF][START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. Furthermore, weed establishment becomes more likely (Nagase & Dunnett, 2011).

AMF addition could be a valuable alternative for OM enrichment. AMF are symbiotic fungi (Phylum Glomeromycota) that form mutualistic associations with plant roots of most plant species [START_REF] Parniske | Arbuscular mycorrhiza: the mother of plant root endosymbiosis[END_REF][START_REF] Bonfante | Plants, Mycorrhizal Fungi, and Bacteria: A Network of Interactions[END_REF][START_REF] Willis | The Ecology of Arbuscular Mycorrhizal Fungi[END_REF]. Intracellula st u tu es, o a us ules a e fo ed he e ut ie t a d a o exchange takes place [START_REF] Newsham | Multi-functionality and biodiversity in arbuscular mycorrhizas[END_REF][START_REF] Smith | Mycorrhizal Symbiosis. 2nd edn[END_REF]. Host plant benefits of having AMF associations include improved phosphorus and nitrogen uptake, protection against pathogens and insect herbivores, toxic metal resistance and improved water uptake [START_REF] Newsham | Multi-functionality and biodiversity in arbuscular mycorrhizas[END_REF][START_REF] Augé | Water relations, drought and vesicular-arbuscular mycorrhizal symbiosis[END_REF][START_REF] Rillig | Arbuscular mycorrhizae and terrestrial ecosystem processes[END_REF][START_REF] Veresoglou | Suppression of fungal and nematode plant pathogens through arbuscular mycorrhizal fungi[END_REF]. The association between AMF and plant roots could therefore be of major importance to increase the performance of EGRs (cf. [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Feldmann | Mycorrhiza for plant vitality: mycorrhizal fungi as factors of integrated horticultural plant production[END_REF][START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF]. The literature also points at some links between AMF and OM. It has been shown that OM is an important source of nitrogen, necessary for AMF growth [START_REF] Hodge | Substantial nitrogen acquisition by arbuscular mycorrhizal fungi from organic material has implications for N cycling[END_REF], and that it can lead to increased formation of AMF-oot asso iatio s Az ό -Aguilar & [START_REF] Azcόn-Aguilar | Applying mycorrhiza biotechnology to horticulture: significance and potentials[END_REF][START_REF] Feldmann | Mycorrhiza for plant vitality: mycorrhizal fungi as factors of integrated horticultural plant production[END_REF].

Generally, GR substrates are sterilized before use, in order to eliminate seeds of undesired species, hence also eliminating AMF [START_REF] Gormsen | The influence of collembolans and earthworms on AM fungal mycelium[END_REF][START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF][START_REF] Feldmann | Mycorrhiza for plant vitality: mycorrhizal fungi as factors of integrated horticultural plant production[END_REF]. AMF should therefore be added through inoculation, but only since the nineties, the potential of mycorrhizal inoculation applications in horticulture became evident (Azcón-Aguilar & Barea, 1997;[START_REF] Feldmann | Symbiontentechnologie in der Praxis: Arbuskuläre Mykorrhiza im Gartenbau[END_REF]. More recently, also the GR industry started promoting the use of AMF [START_REF] Streit | Mycorhizes: une stratégie de survie ancestrale redécouverte[END_REF][START_REF] Feldmann | Mycorrhiza for plant vitality: mycorrhizal fungi as factors of integrated horticultural plant production[END_REF][START_REF] John | Assessment of Arbuscular Mycorrhizal Fungi in a Green Roof System[END_REF].

Chapter 5

(*) The same roof as the Heverlee experiment in Chapter 4.

93

Evidence of the beneficial effects of AMF under GR conditions or in other shallowsoil communities remains scarce however [START_REF] Poot | Shallow-soil endemics: adaptive advantages and constraints of a specialized root-system morphology[END_REF][START_REF] John | Assessment of Arbuscular Mycorrhizal Fungi in a Green Roof System[END_REF][START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF][START_REF] Rumble | Soil microarthropod community dynamics in extensive green roofs[END_REF]. To our knowledge, only [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF] studied AMF applications on GRs, reporting that the effect of AMF can be both positive and negative, depending on the plant species used. Indeed, the interactions between AMF and the host plant are expected to depend on many factors including climate, water and OM availability, plant species and the AMF species present in the inoculum (cf. [START_REF] Van Der Heijden | Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity[END_REF][START_REF] Gange | Soil food web components affect plant community structure during early succession[END_REF][START_REF] Rillig | Arbuscular mycorrhizae and terrestrial ecosystem processes[END_REF][START_REF] Maherali | Influence of phylogeny on fungal community assembly and ecosystem functioning[END_REF][START_REF] Feldmann | Mycorrhiza for plant vitality: mycorrhizal fungi as factors of integrated horticultural plant production[END_REF][START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF]. Commercial inocula generally contain only Glomus spp. of the phylum Glomeromycota as these are known to be generalists [START_REF] Rillig | Arbuscular mycorrhizae and terrestrial ecosystem processes[END_REF][START_REF] Jansa | Are there benefits of simultaneous root colonization by different arbuscular mycorrhizal fungi?[END_REF].

The general aim of this study was to examine the effects of AMF and OM on EGR vegetation development. AMF were added to a mineral EGR substrate and to a substrate with additional OM. Experiments were conducted both under greenhouse and rooftop conditions. To improve plant performance, it is essential to establish the o hizal asso iatio s as ea l as possi le du i g seed ge i atio Az ό -Aguilar & [START_REF] Azcόn-Aguilar | Applying mycorrhiza biotechnology to horticulture: significance and potentials[END_REF]. Therefore, this study specifically focused on the initial development stages of the GR vegetation. More specifically, the following research questions were addressed:

(1)

How do AMF and OM influence species germination and early development in terms of species diversity, vegetation cover and height? (2)

For the greenhouse study in particular, is there microscopic evidence of formation of AMF-root associations and is there an effect on the vegetation biomass?

To what extent can AMF be recommended as a substitution of OM in EGR substrates, in order to enhance vegetation development on EGRs?

Material and methods

Experimental setup

The rooftop experiment was established on the horizontal roof of a building of the U i e sit of Leu e , i He e lee, Belgiu ° N, ° E i "epte e 2013 (*). Until June 2014, daily temperature and precipitation data were collected from a ea eathe statio of B ussels ° N; ° E, ele atio . Mo thl data are plotted against the yearly averages derived from New_LocClim v1.10, a Lo al Cli ate Esti ato p og a a d data ase FAO, 2005). The experimental plots of 1 m² consisted of four empty high-density polyethylene (HDPE) EGR modules (± 0.25 m²; ECCOsedum; www.eccoproducts.eu) that allow free drainage of rainwater. The trays were filled with 7 cm of FLL-approved substrate, either a mineral EGR substrate (MIN) or one with additional OM (ORG) of Peltracom NV (Table 5.1) Half of the experimental plots received 1 g/m² AMF inoculum (MYCO) consisting of Glomus spp. on a vermiculite carrier material (Peltracom n.v., personal communication), which is known to be an adequate substrate for formation of AMF spo es Az ό -Aguilar & Barea, 1997). The four experimental treatments ORG, MIN, ORG+MYCO and MIN+MYCO, were each replicated three times and installed in a randomized block design on a fully exposed bitumen roof covered with a root protection layer (Sopranature WF 50). Seeds from a commercial EGR seed mix (Optigreen Seed mix Type E for Nature roof; Optigrün International AG 2014; Table 5.2), containing 35 herbaceous plant species, were added at a rate of 2 gr/m². Seed quality is in accordance with the FLL quality requirements for the composition of wild plant seeds from reproduction stock, with a minimum seed purity and germination capacity above 75 and 70 %, respectively (Optigrün International AG, 2014). AMF dependency of the species is variable (Table 5.2). As a control, four additional HDPE modules of 0.25 m² (one for each treatment) were installed and left unsown. During two weeks after sowing, the plots were irrigated by hand two times a week to saturation, but afterwards they only received water from natural precipitation events.

Setup of the greenhouse experiment was similar to the rooftop experiment, with the four treatments ORG, MIN, ORG+MYCO and MIN+MYCO replicated three times in a randomized block design. However, experimental plots only consisted of one EGR tray (approximately 0.25 m²) instead of four. The plots were subjected to artificial lighting conditions (16h light -8h dark) and an average temperature of 22°C (maximum and minimum recorded temperatures were 35 and 10°C, respectively). Two times a week, the plots were watered from below to saturation. Two additional unsown plots (one containing MIN and one ORG substrate, ½ of the surface covered with AMF inoculum) served as control.

Plant and AMF survey

Plant survey on the rooftop took place in April 2014 (after 8 months) and included calculation of species richness (SR) and number of individual plants (abundance), total percentage cover of the plot, and the average height of the developing vegetation. Also relative frequencies of the species present in every treatment were calculated. The Shannon diversity index (H s ) and associated equitability measure E H were calculated using following formulas [START_REF] Begon | Ecology: From Individuals to Ecosystems, 4 th edn[END_REF]:

H s = ∑ (Eq. 1) E H = H/lnSR (Eq. 2)
In Eq. 1, p i is the relative abundance of the i th spe ies, so the di e sit i de H s accounts for both abundance and evenness of the species in the community. The equitability index E H in Eq. 2 takes a value between 0 and 1, with 1 representing complete evenness. (Eq. 3)

The MIE in Eq. 3 accounts for the difference in DW between a substrate with and without additional mycorrhiza. In essence, MIE is an index for the effectiveness of an AMF inoculum, with high values indicating a positive effect on biomass production of a plant community.

The root parts of four species (Cota tinctoria, Dianthus deltoides, Leucanthemum vulgare, and Prunella vulgaris) differing in AMF dependency, offered sufficient material to be analyzed microscopically for AMF presence, following [START_REF] Dalpé | Microwave-assisted technology for the clearing and staining of arbuscular mycorrhizal fungi in roots[END_REF]. Plant roots were cut into pieces of 2-4 cm and cleared in multiple successive steps: washing with H 2 O, clearing with KOH 2.5%, microwave heating (30 s, 900 W) in a domestic microwave (Primo, model MG7) and washing (three times) with distilled H 2 O. After clearing, the roots were immersed in acetic acid 5% to bring the pH below 2.5. In the staining step, the acidified roots were immersed into a staining solution consisting of lactic acid-glycerol-acetic acid 5% water solution (12-1-1) and acid fuchsin (0.01%), followed by microwave heating (30 s, 900 W). Finally, the percentage of roots colonized with AMF was evaluated by the grid-line intersection method described by Habte and Osorio (2011) using a light microscope (Wild M3, Leica Geosystems AG, Switzerland) with a magnification factor of 160.

Data analysis

For the rooftop experiment, differences between substrate type (SUBSTRATE: MIN or ORG) and AMF inoculation (MYCO: YES or NO) in terms of SR, abundance, cover, vegetation height, H s and E H were assessed by means of parametric ANOVA or a nonparametric Mann-Whitney U test (MWU), depending on the normality distribution and variance homogeneity of the variables. In the same way, height, root length, H s , E H , and DW of the greenhouse experiment were analyzed. The differences in variable values between both experiments were analyzed as well. These statistical tests were performed in SPSS 22.0 (SPSS Inc, Chicago, IL).

For the greenhouse experiment, the weekly recorded SR and cover were analyzed using Linear Mixed Models with Time, SUBSTRATE and MYCO as fixed effects and the intercept as the only random effect. If significant, two-way interaction terms were added to the final model. The weekly recorded plant abundances are count data and were analyzed using Generalized Linear Mixed Models with a Poisson distribution. The Mixed Models were performed in SAS 9.3 (SAS Institute Inc., Cary, NC).

Two-way parametric Anova or a Kruskal Wallis test (KW), depending on the normality distribution and homogeneity of variance of the data, were used to detect significant differences in the percentage of AMF colonization between the four species that were analyzed microscopically. Finally, for both the rooftop and greenhouse experiment, Spearman rank correlations (r s ) were calculated to test for correlations between a) the relative frequencies for the treatments in both experiments, and b) the measured variables. Significance levels were adjusted with a Bonferroni correction. Both tests were performed in SPSS 22.0. 2 0 0 0 * Species that can be present in the seed mix as a result of regional differences in seed mix composition

Results

Rooftop experiment

Monthly precipitation was lower than average from December to April and was higher than average during October, November, and especially May (Fig. 5.1a). The average temperature was normal during the first three months of the experimental period, but from December until May, the temperature was between 1.6 and 3.3°C higher than the average yearly temperature (Fig. 5.1b).

On the rooftop experiment, the highest SR was found in the MIN treatment (18 species), followed by MIN+MYCO (15 species), ORG and ORG+MYCO (both 14 species; Table 5 (23.42, 15.32, 14.41 and 10.81% respectively). The relative frequencies of the different treatments were highly correlated (r s between 0.72 and 0.95; Table 5.5a). Of the unexpected species, Alyssum alyssoides and Arenaria serpyllifolia were recorded in three treatments (MIN, MIN+MYCO and ORG+MYCO) and Poa annua occurred in MIN and ORG. Alyssum alyssoides probably originated from the neighboring experimental GR (Chapter 4) and the other two species are rather common weedy species that could have reached the roof as seeds or were unintentionally present in the substrate or seed mix. No germination was noticed in the control plots.

Both rooftop and greenhouse experiments differed greatly in their variable values, as indicated by the significant MWU values (Table 5.3b). For the rooftop experiment, the cover of the plots was low (between 0.25 and 3.5%) and treatments did not significantly affect SR, abundance, vegetation height, H s , E H (Table 3a), or cover (Table 5.3b). Although average values of plant abundance and SR in MIN plots (54.17 plants and 10 species, respectively) were higher compared to ORG plots (31.17 plants and 6.33 species, respectively), these differences were not significant. Plants were very small (1 cm on average) but H s and E H were rather high (approximately 1.5 and 0.8, respectively). Diversity indices H s and E H were rather high (1.5 and 0.8), indicating that no distinct dominance patterns occur in the plots (Table 5.3). SR, abundance, cover, height and H s were all strongly and positively correlated (p-alue .

; s between 0.84 and 0.96) but no significant correlations were found with E H (Table 5.5b1).

Greenhouse experiment

A total of 25 different species germinated in the experimental greenhouse plots, of which 19 to 21 species, depending on the treatment, were present in the seed mix (Table 5.2). The ORG and ORG+MYCO treatments had the highest SR (22 species), followed by MIN (21 species) and MIN+MYCO (20 species). Two unexpected species, Capsella bursa-pastoris and Sonchus oleraceus were recorded in the MIN treatment and probably reached the roof as seeds or could have been unintentionally present in the substrate or seed mix. C. tinctoria and S. officinalis were the most frequent species in the MIN plots (18.32 and 17.56 % respectively), and Leucanthemum vulgare and C. tinctoria in the MINMYC plots (16.06 and 11.65 % respectively). In the ORG treatment, P. argentea, L. vulgare, D. deltoides and C. tinctoria were most frequent (15.47, 11.7, 10.94 and 10.19 % respectively) and in the ORG+MYCO plots, this was the case for P. argentea, S. officinalis and L. vulgare (14.19, 11.29 and 10.65 % respectively). The control plots remained unvegetated during the experiment. Like the rooftop experiment, the relative frequencies of the species in the different treatments in the greenhouse were highly correlated (r s from 0.81 to 0.97; Table 5.5a). Comparing the relative frequencies between the rooftop and the greenhouse experiment, the average Spearman rank correlation was much lower (r s between 0.34 and 0.71) and only half of the correlation coefficients were significant. Both experiments hence differed in species composition and abundance.

For vegetation height (Table 5.3a) and DW (Table 5.3b), a significant difference was found between the substrate types, with MIN plots having lower average values (7.12 cm and 8.73 g, respectively) compared to the ORG plots (11.8 cm and 29.87 g, respectively). No significant differences between the plots with or without AMF were detected. Root length was around 7 cm for all plants in all treatments. Average values for H s and E H were higher than for the rooftop experiment (approximately 2.5 and 0.9, respectively).

For the dependent variables SR, cover and abundance, Time was a strongly significant fixed effect (p-alue . a d the pa a ete esti ates e e positi e; so SR, cover and abundance increased during the 15 monitoring weeks (Table 5.4). Substrate type and AMF inoculation did not significantly affect SR (Fig. 5.2a), although SR appeared slightly higher in plots with AMF inoculation. A significant effect was found for cover, which was lower in MIN plots compared to ORG plots (Fig. 5.2b). The plots with AMF had a slightly higher cover, but this effect was not significant. For cover, the interaction term Time*SUBSTRATE was significant as well, indicating that the cover in the MIN plots increased slower than in the ORG plots (Fig. 5.2b). Considering abundance, plots with ORG substrate and with AMF inoculum had a higher number of plants, although not significant. However, both interaction terms (Time*SUBSTRATE and Time*MYCO) were significant, so ORG plots and plots with AMF inoculum show a quicker increase in number of plants over time (Fig. 5.2c).

As for the rooftop experiment, the diversity indices H s and E H were high (2.5 and 0.9), so there were no dominant species in the plot. Only three significant correlations with p-alues . e e fou d Ta le .

), between cover and abundance (r s = 0.83), cover and DW (r s = 0.84) and SR and H s (r s = 0.91).

The overall mycorrhizal inoculation effect, between plots with or without AMF inoculation, was positive (MIE 1.71). When the substrate types were compared separately, a positive effect of AMF inoculation was found in the MIN plots (MIE = 8.76), but the effect in the ORG plots was slightly negative (MIE = -0.45).

The microscopic analysis of plant roots indicated the presence of AMF infected roots in the ORG+MYCO and MIN+MYCO plots except for the roots of D. deltoides in the MIN+MYCO plots (Fig. 5.3). In the ORG and MIN plots, no AMF infected roots were found. Arbuscules were only registered occasionally within the root cells and the observed mycorrhizal structures were mostly intercellular hyphae. Differences in AMF colonization between the four species seem to reflect their AMF dependency, with the highest amount of AMF colonization found in P. vulgaris (AMF dependency: 3 or frequent), followed by L. vulgare (AMF dependency: 2 or occasional) and finally D. deltoides (AMF dependency: 1 or rare). The AMF dependency of C. tinctoria was unknown, but like P. vulgaris, the amount of AMF colonization (on average 13.4 % in ORG+MYCO and 23.7 % in MIN+MYCO) is relatively high. Despite the variability in AMF colonization between the four species, the differences were not significant (ORG+MYCO: Anova F-value = 1.376; MIN+MYCO: Kruskal-Wallis Chi² = 5.18). The difference in AMF colonization between ORG+MYCO and MIN+MYCO was not significant either (MWU test statistic = 73).

Discussion

In the greenhouse study, adding OM to the substrate resulted in increased plant height, DW (Table 5.3), and vegetation cover (Table 5.4). These positive effects of OM on initial plant growth are in line with Nagase and Dunnett (2011), who concluded that, together with water availability, the most important factor for plant growth on GRs is the presence of OM. A possible explanation for the positive results of OM could be the differences in substrate composition. The water permeability for example, was 183 mm/min for MIN compared to only 27.9 mm/min for ORG (Table 5.1). The ORG plots hence had higher and longer water availability after rainfall. Moreover, OM and nutrient content were approximately five times higher (0.6 and 3.2% OM for MIN and ORG substrates respectively). In contrast to Nagase and Dunnett (2011) and [START_REF] Molineux | Characterising alternative recycled waste materials for use as green roof growing media in the U[END_REF] who found that adding respectively 10% and 25% OM was optimal for GR plant growth, the amount of OM used for this study was rather low. Nevertheless, the OM in the ORG substrate together with the higher water retaining capacity proved to be sufficient to obtain positive effects on vegetation cover, plant height and DW. Consequently, plants are competing for light, whereas competition for nutrients is going on in the MIN plots.

The results of the greenhouse study did not show a clear effect of AMF inoculation on species germination and early development. In two other studies [START_REF] Reidenbach | Einsatz van Mycorrhiza-Pilzen in der Dachbegrünung[END_REF][START_REF] Reidenbach | Einsatz von Mykorrhiza-Pilzen in der einfachen intensiven Dachbegrünung[END_REF], where the effect of AMF inoculation on plant growth on a mineral IGR substrate was tested, similar results were obtained. However, a positive MIE was found in our greenhouse experiment which suggests that biomass production is positively influenced by adding AMF. This effect appeared highly positive when only the MIN substrate plots were considered but slightly negative in the ORG plots. A possible explanation may be that the effect of OM on plant development in the ORG plots is so pronounced that AMF inoculation does not influence biomass production in this case. In MIN plots, plants have more difficult starting conditions, leading them to make optimal use of the additional AMF to increase their biomass. Next to the MIE, microscopic analysis of root material of four species in the greenhouse experiment confirmed successful AMF-root associations in the ORG+MYCO and MIN+MYCO plots. In the ORG and MIN plots, no AMF infected roots were found, so the AMF inoculum present in nearby plots had not reached and contaminated the plots without AMF. A similar result was found by [START_REF] Sutton | Media Modifications for Native Plant Asemblages on Extensive Green Roofs[END_REF], who did not detect any difference in plant growth between control and AMF inoculated plots in the establishment phase, although AMF-root associations were detected. More time is probably needed to fully establish the AMF infection and to obtain positive outcomes on vegetation development. [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF] for example, concluded that AMF inoculation resulted in overall better plant survival and drought stress on rooftops. It is hence possible that the effects of AMF inoculation become significant in later stages of vegetation development. In time, it is also possible that AMF are transported to nearby plots, as [START_REF] Reidenbach | Einsatz van Mycorrhiza-Pilzen in der Dachbegrünung[END_REF] also found AMF-root associations in their control plots and because of the easy colonization onto GR substrates by surrounding AMF ( [START_REF] Van Der Heijden | Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity[END_REF][START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF][START_REF] Rumble | Soil microarthropod community dynamics in extensive green roofs[END_REF]. Regarding the four species of which root material was used for the microscopy analysis, the AMF dependency of D.

deltoides, L. vulgare and P. vulgaris was reflected in the rate of AMF colonization. However, the AMF dependency of C. tinctoria was unknown, but the relatively high amount of AMF colonization (on average 13.4% in ORG+MYCO and 23.7% in MIN+MYCO), suggest that it is medium to highly capable of forming AMF-root associations.

On the rooftop experiment, the vegetation cover remained low, plants were small, and no significant effects of OM and AMF on early plant development were found (Table 5.3), which could be due to climatic factors. As shown in Fig. 5.1, meteorological values during the experimental period showed some deviations from the yearly average values. Except for the first two weeks after seeding, plots received only water from natural precipitation events. Sufficient water availability is a prerequisite during vegetation establishment (cf. [START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF], which is why the relatively dry winter and spring months combined with above average temperatures resulted in slow germination and vegetation development.

We believe that an additional irrigation system could have mitigated this water shortage problem. Also, as yearly changes occur in weather patterns, vegetation development will likely differ, as well as the monitored measures. Therefore, outdoor experiments should be conducted during several years in order to generalize the obtained results.

Conclusion

In conclusion, this study showed that a low amount of OM is sufficient to obtain positive effects on EGR plant growth in terms of vegetation cover, height and DW. The risk of disadvantages like substrate decomposition and decreased water retention capacity is also limited when only low amounts of OM are considered [START_REF] Friedrich | Principles for selecting the proper components for a green roof growing media[END_REF][START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF][START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. However, there is an important trade-off to account for, as bigger plants transpire more, compete for light and have more biomass to maintain, making them less resistant to drought stress, which is a disadvantage when a dry period occurs after establishment [START_REF] Rowe | Assessment of Heat-expanded Slate and Fertility Requirements in Green Roof Substrates[END_REF]. Our findings also demonstrate that GR substrates offer a suitable media where associations between AMF and plant roots can take place, but that addition of AMF is not very effective as regards initial plant development. It can hence not yet be considered as a substitution for OM. Next to encouraging studies that test the effect of AMF inoculation on plant productivity in the long term, research should also focus on species-specific inocula, as the effect of AMF inoculation is actually dependent on the used AMF and plant species [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF][START_REF] Rillig | Arbuscular mycorrhizae and terrestrial ecosystem processes[END_REF]. If AMF-and plant choice can be tuned, the GR industry is one step closer to successfully develop productive EGR with high biodiversity value.

Chapter 6. Adapting green roof irrigation practices for a sustainable future Adapted from Van Mechelen, C., Dutoit, T., Hermy, M. Adapting green roof irrigation practices for a sustainable future: a review. Building and Environment. Under review.

Introduction

Next to land use changes and habitat fragmentation, the urbanization process has a pronounced effect on the natural water cycle [START_REF] Walsh | The urban stream syndrome: current knowledge and the search for a cure[END_REF][START_REF] Schmidt | Main cause of climate change: Decline in the small water cycle[END_REF]. As urban green gets more scarce, evapotranspiration (ET) is reduced, thereby reducing precipitation which reduces ET even further and ultimately this leads to dehydration of the urban environment [START_REF] Schmidt | Main cause of climate change: Decline in the small water cycle[END_REF]. In Mediterranean areas, water is amongst the most limiting natural resources [START_REF] Kotsiris | An adaptive approach to intensive green roofs in the Mediterranean climatic region[END_REF]. The problem is becoming increasingly acknowledged and incentives arise to stimulate sustainable water management practices, which aim to balance social, economic and environmental water needs [START_REF] Schmidt | Main cause of climate change: Decline in the small water cycle[END_REF][START_REF] Bean | Advancing stormwater beneficial uses: ET mapping in urban areas[END_REF][START_REF] Blanco-Gutiérrez | Integrated assessment of policy interventions for promoting sustainable irrigation in semi-arid environments: A hydro-economic modeling approach[END_REF][START_REF] Islam | Design, Construction, and Operation of a Demonstration Rainwater Harvesting System for Greenhouse Irrigation at McGill University, Canada[END_REF]. Urban green infrastructure elements like urban trees, parks, gardens, green roofs (GRs) and green walls, are a vital part of water management [START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF][START_REF] Schmidt | Main cause of climate change: Decline in the small water cycle[END_REF]. Green infrastructure comprises by definition the entire semi-natural and artificial networks of multifunctional ecological systems in urban areas [START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF]Naumann et al., 2011). GRs provide many ecosystem processes and services, including stormwater management, cooling and insulation of buildings, mitigation of the urban heat island effect, contribution to human health and habitat provisioning [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Carter | Establishing Green Roof Infrastructure through Environmental Policy Instruments[END_REF][START_REF] Rozos | Rethinking urban areas: an example of an integrated blue-green approach[END_REF]. They can be classified into two main categories of which extensive GRs (EGRs) represent the most shallow (2-ca.20 cm substrate depth) and light weight (70-170 kg/m²) ones. The vegetation of EGRs generally consists of succulents and other drought tolerant plants [START_REF] Bousselot | Moisture content of extensive green roof substrate and growth response of 15 temperate plant species during dry down[END_REF]. Within the EGR category, semi-extensive GRs (SEGRs) have a slightly deeper substrate (12-ca. 20 cm) and the associated higher water holding capacities allow a more diverse range of plant species [START_REF] Theodosiou | Green Roofs in Buildings: Thermal and Environmental Behaviour[END_REF]. Finally, intensive GRs (IGRs) have substrate depths above 20 cm and are also denoted as roof gardens, because a more intensive management is needed [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. Classic IGRs can perform some ecosystem services (i.e. stormwater management and energy efficiency) more efficiently, but both the construction demands (e.g. higher cost, high weight) and additional management such as irrigation, make this type less interesting in terms of sustainability [START_REF] Theodosiou | Green Roofs in Buildings: Thermal and Environmental Behaviour[END_REF].

Traditionally, EGRs are designed so that natural precipitation is sufficient to ensure plant survival [START_REF] Durhman | Effect of Watering Regimen on Chlorophyll fluorescence and Growth of Selected Green Roof Plant Taxa[END_REF]. However, the harsh environmental conditions on EGRs pose severe constraints on vegetation development and plant diversity, particularly during summer when high temperatures prevail and water is scarce [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF][START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF][START_REF] Macivor | Decoupling factors affecting plant diversity and cover on extensive green roofs[END_REF]. Furthermore, climate change will lead to a shift in seasonal precipitation (IPCC, 2014) and it may be expected that irrigation on EGRs even will become a necessity in summer in order to keep the vegetation vital [START_REF] Gross | Numerical simulation of greening effects for idealised roofs with regional climate forcing[END_REF][START_REF] Macivor | Decoupling factors affecting plant diversity and cover on extensive green roofs[END_REF][START_REF] Vanuytrecht | Runoff and vegetation stress of green roofs in a climate change perspective[END_REF]. Also for ensuring ecosystem services like urban heat island (UHI) mitigation, a moist substrate and hence irrigation is required in many locations (Ascione et al., 2013;[START_REF] Coutts | Assessing practical measures to reduce urban heat: Green and cool roofs[END_REF].

Despite the eventual necessity of irrigation on GRs, there are certain drawbacks in terms of the installation of irrigation systems. The growing consensus on the urban water scarcity problem converted many cities to restrict irrigation rates on GRs [START_REF] Rowe | Effect of green roof media depth on Crassulacean plant succession over seven years[END_REF], especially in regions with frequent droughts like Southern Europe and Australia [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]Ascione et al., 2013). The additional installation and maintenance costs pose another constraint to irrigation installation [START_REF] Price | Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States[END_REF].

Looking for alternative water sources and water conservation practices are essential to obtain more sustainable GR irrigation practices (Moritani et al., 2013). Sustainable irrigation is a part of the broader sustainable water management and aims at developing policies and practices that ensure functioning and healthy irrigation systems in the long term [START_REF] Abernethy | Sustainability of Irrigation Systems[END_REF]Blanco-Gutiérez et al., 2013). In essence, a sustainable irrigation system should (i) be able to function properly and deliver benefits over its lifetime; (ii) not compromise other water needs; and (iii) be publicly supported [START_REF] Abernethy | Sustainability of Irrigation Systems[END_REF]. Knowledge on irrigation practices and specifications for GRs, especially in relation to different climates, is still limited. This chapter hence reviews the available information from scientific publications involving both GRs and irrigation. First, current irrigation techniques will be discussed and an overview of suggestions to make irrigation practices more sustainable will be given. Also public incentives regarding stormwater management will be highlighted. Thereafter, essential irrigation information from studies in which outdoor GR experiments were conducted is related to local climatic conditions in order to propose irrigation specifications for specific climates.

Methods

Publication search and selection

The review is based on scientific studies found through the electronic journal database ISI Web of Knowledge (consulted on 07/07/2014) using the specific search te s g ee oofs a d i igatio . All pu li atio ea s e e o side ed a d pape s had to be written in English. Both peer-reviewed scientific publications and conference proceedings were taken into account. The list of studies was further completed with publications, found through Google Scholar using the same search terms. In total 142 publications were found. The content of all documents was screened and not useful or irrelevant documents (not s.s. about GRs or irrigation or information also comprised in another paper) were omitted. After screening, the database for the review comprised 109 publications of which 94 peer-reviewed publications and 15 conference proceedings.

Data exploration

Essential information (study type, study location, topic, GR type, irrigation method and regime) was extracted to construct a database. The study type categorized the documents into general information, greenhouse experiments, model simulations and outdoor experiments. For outdoor GR experiment studies, the study location was linked to the respective climate using the Köppen climate classification (KCC, Table 6.1; [START_REF] Kottek | WorldMap of the Köppen-Geiger climate classification updated[END_REF]. Both warm temperate climates (KCC: C) and snow climates (KCC: D) are represented by multiple studies, as well as the arid climate (KCC: B) in Colorado and Spain. Only tropical and polar climates were not covered by the selected publications.

In Table 6.1., the P th is a threshold for dryness (in mm; Eq. 6.1) which has been introduced for the arid climate (KCC: B). It depends on the annual temperature and precipitation cycle [START_REF] Kottek | WorldMap of the Köppen-Geiger climate classification updated[END_REF]:

P th = { { } { } { } (Eq.6.1)
Irrigation methods and regimes were summarized for the outdoor experiments and greenhouse and model simulation studies (Appendix 6.1). If relevant, the minimum daily summer irrigation requirement (I day ) was calculated. When irrigation regimes were given in mm, the minimum value was converted directly to l/m², but when volumetric regimes were provided, the value was first divided by the GR dimensions (length x width) to obtain a value in l/m². Then this value was divided by the irrigation frequency (in days) to obtain the final I day . This allowed comparison of water requirements specified in different studies.

Fig. 6.1 suggests that research regarding GRs and irrigation is growing. Before 1999, o studies e e fou d ith the sea h te s g ee oofs a d i igatio . The fi st four publications, used in this review [START_REF] Argue | Catchment « Greening » using stormwater in Adelaide, South Australia[END_REF][START_REF] Velazquez | Modular green roof technology: An overview of two systems[END_REF][START_REF] Kolb | Good Reasons for Roof Planting -Green Roofs and Rainwater[END_REF][START_REF] Monterusso | Runoff Water Quantity and Quality from Green Roof Systems[END_REF], were all conference proceedings, after which many publications emerged. Although the modern GR technology was founded in Northern and Central Europe, only 21.10% of the studies used in this review originate from this continent (Table 6.2). The majority of the studies were conducted in North America (44.04%), especially in the US (40 studies, 15 from the state Michigan). In Asia and Oceania, respectively China and Australia are the countries in which GR research is progressing. In South America, only one study could be found [START_REF] Vieira | Potential of utilization of rain water excess for irrigation of green roof in Mato Grosso, Brasil[END_REF] and no studies were found in Africa.

As regards the main topics, only five publications (4.72%) just consider the subject irrigation (Fig. 6.2). The largest part of the publications involved the topics: vegetation performance (35.85%), energy and thermal performance (27.36%) and stormwater management (19.81%). Fig. 6.3 shows that most of the studies were published in scientific journals covering environmental themes like horticulture (HortScience, n = 12), urban ecology and ecosystems (Urban Forestry and Urban Greening, n = 9; Ecological Engineering, n = 8; Urban Ecosystems, n = 6), and building science and energy use (Building and Environment, N=6; Energy and Buildings, n = 5). This suggests that the subjects GRs and irrigation have a wide support base and are of particular interest to ecologists, engineers, designers and policy makers.

GR irrigation practices and stormwater management regulations

Current GR irrigation practices

A GR comprises multiple layers that protect the underlying roof and support the vegetation, but the water storage capacity is limited due to physical and economic factors [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF]. The substrate is the main water storage layer, but water can also be retained in the drainage layer [START_REF] Theodosiou | Green Roofs in Buildings: Thermal and Environmental Behaviour[END_REF] or if present, the water retention layer (WRL), which is a mat from mineral wool or polymeric fibers (1-6.5 cm thick) under or above the drainage layer [START_REF] Grant | A decision-making framework for vegetated roofing system selection[END_REF]. For EGRs, additional irrigation is often suggested during plant establishment as early drought can be very harmful for further plant establishment, performance and survival [START_REF] Getter | The Role of Extensive Green Roofs in Sustainable Development[END_REF]Nagase & Dunnett, 2010;[START_REF] Thuring | Green Roof Plant Responses to Different Substrate Types and Depths under Various Drought Conditions[END_REF]. Permanent irrigation would not be necessary after establishment if the EGR is properly designed in terms of plant selection, substrate type and depth, and if sufficient water from precipitation is available (VanWoert et al., 2005a;[START_REF] Getter | The Role of Extensive Green Roofs in Sustainable Development[END_REF]. Next to improved plant growth and survival under dry conditions [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF][START_REF] Price | Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States[END_REF], irrigation can be beneficial in other ways as well. Less drought stress implies that more variation in plant species is possible [START_REF] Köhler | Long-Term Vegetation Research on Two Extensive Green Roofs in Berlin[END_REF]. Irrigation also influences temperature control to a great extent, as moist substrates improve evaporation which mitigates air and building temperatures and hence decreases cooling costs [START_REF] Theodosiou | Green Roofs in Buildings: Thermal and Environmental Behaviour[END_REF][START_REF] Castleton | Green roofs; building energy savings and the potential for retrofit[END_REF][START_REF] Price | Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States[END_REF][START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. The model of [START_REF] Bass | The Impact of Green 'oofs o To o to s U a heat Isla d[END_REF] for example, indicated that irrigated GRs can reduce the temperature of a large part of the city of Toronto with 1°C. Furthermore, considering an irrigation system during the GR design phase is less expensive compared to the need of replanting dead plants during the annual maintenance period [START_REF] Price | Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States[END_REF][START_REF] Macivor | Decoupling factors affecting plant diversity and cover on extensive green roofs[END_REF], an action which should be added to the average ai te a e ost e.g. . €/ pe ea ; Po s he [START_REF] Porsche | Life cycle costs of green roofs -A comparison of Germany, USA, and Brasil[END_REF]Eastside Sustainability, 2004). It is also less expensive compared to an air conditioning installation. [START_REF] Sun | To irrigate or not to irrigate: Analysis of green roof performance via a vertically-resolved hygrothermal model[END_REF] calculated the costs of different irrigation strategies under the current prizes of water and electricity in Beijing (respe ti el . €/ a d . €/K H . A i igated G' of ith a i igatio o t ol li it of . / a sa e € o e a -month period compared to a non-irrigated but air conditioned building with a moderate performance coefficient [START_REF] Sun | To irrigate or not to irrigate: Analysis of green roof performance via a vertically-resolved hygrothermal model[END_REF]. Notwithstanding these positive effects, less positive effects of irrigation include a decrease in stormwater performance as moist substrates can retain less water during a precipitation event [START_REF] Spolek | Performance monitoring of three ecoroofs in Portland, Oregon[END_REF]Schroll et al., 2014b). There is also an increased probability of weed establishment on irrigated GRs, especially when the proportion of bare ground is still high (Schroll et al., 2011a). Furthermore, under saline irrigation (e.g. when secondarily treated municipal wastewater is used) salt accumulation may lead to salinity stress and reduced ET (Moritani et al., 2013). Irrigation is also considered unsustainable in regions with water scarcity or when potable water is used (Moritani et al., 2013;[START_REF] Rowe | Comparison of irrigation efficiency and plant health of overhead, drip, and sub-irrigation for extensive green roofs[END_REF]. In the US for example, almost every region has experienced water shortages over the last five years (EPA, 2008). Globally, water consumption has tripled over the last 50 years, and an increase in water demand has been registered in almost every country (WWAP, 2012). It is expected that by 2

, half of the o ld s populatio ill e living in water stressed areas (UNEP, 2007). Therefore, the government does not financially support GR installation in many countries with a semi-arid climate, hence opposing the expansion of the GR industry [START_REF] Ntoulas | Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems[END_REF].

In Northern and Ce t al Eu ope, the G' te h olog follo s the guideli es of The La ds ape De elop e t a d La ds api g 'esea h "o iet FLL, . These however, provide only little information regarding irrigation specifications. The main irrigation methods suggested in the FLL guidelines are 1) a (spray) hose or sprinkler system; 2) drip lines; 3) overhead irrigation systems and 4) automated irrigation systems with in-built reservoirs [START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF]. In the context of vegetable cultivation on GRs (i.e. lettuce, chicory and endive), [START_REF] Cho | Effects of irrigation methods on the growth, water holding capacity of substrate and nutrient uptake of Lettuce, Chicory and Endive grown in an extensive green roof system[END_REF] studied the following irrigation methods: water retained in the drainage layer, irrigation through a wick, and drip irrigation. The water holding capacity (WHC) of the substrate was highest when using drip irrigation, but wick irrigation proved to be more efficient for plant growth because the water content in the substrate remained constant for a longer time [START_REF] Cho | Effects of irrigation methods on the growth, water holding capacity of substrate and nutrient uptake of Lettuce, Chicory and Endive grown in an extensive green roof system[END_REF]. [START_REF] Rowe | Comparison of irrigation efficiency and plant health of overhead, drip, and sub-irrigation for extensive green roofs[END_REF] suggested that wick irrigation, followed by drip irrigation, are good options for GRs with plug plants, as these types of irrigation mainly provide water at the plant root zone in close proximity to the wick or drippers. However, they might not be the ideal choice for more traditional EGRs where a large vegetated surface consisting of pre-vegetated mats, seeded species or succulents, needs to be irrigated. Combining drip irrigation with an additional WRL could improve irrigation efficiency. Overhead irrigation may be a better choice, as it distributes water more uniformly and leads to higher substrate WHC, less runoff, and better plant growth and health compared to drip irrigation [START_REF] Rowe | Comparison of irrigation efficiency and plant health of overhead, drip, and sub-irrigation for extensive green roofs[END_REF].

Optimizing irrigation practices

Different water conservation strategies have been proposed that enable sustainable irrigation of green areas [START_REF] Chang | Optimal design for water conservation and energy savings using green roofs in a green building under mixed uncertainties[END_REF]. These strategies can be categorized into (1) adaptation of irrigation requirements; (2) alternative irrigation sources and (3) control and monitoring of irrigation regimes (Table 6.3). All strategies can be implemented separately or simultaneously, depending on both design and financial project considerations.

First of all, water can be conserved by targeting the GR vegetation. As indicated before, GRs are mainly designed with drought tolerant vegetation, also called xeriscaping [START_REF] Carter | Life-cycle cost-benefit analysis of extensive vegetated roof systems[END_REF]. Succulent plants are not only drought tolerant, they can also act as nurse plants for other less drought tolerant plants through facilitation, i.e. by reducing the substrate temperature and evaporation rate [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. Also many mosses can act as facilitators, and they are very well suited to survive harsh environmental conditions, can store a considerable amount of rainwater and only need a thin substrate [START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF]. Alternatively, one could think of GRs with wetland species as these species have an excellent WHC and can effectively decrease surface temperatures [START_REF] Song | Wetlands are an effective green roof system[END_REF]. Over an entire growing season, wetland plants only need 20% of the water needed to irrigate traditional EGRs, which makes them more sustainable and less costly. Wetland plants are easy to propagate and they are effective in cooling buildings because of their high transpiration rates. The two macrophyte species tested by [START_REF] Song | Wetlands are an effective green roof system[END_REF], Iris laevigata Fisch. and Iris pseudoacorus L., proved to be rather stress tolerant, as they could survive both after three weeks of flooding and two weeks of drought. The main problem that withholds wetland GRs from being implemented is their increased weight compared to traditional EGRs, together with a potential pest problem associated with standing water [START_REF] Song | Wetlands are an effective green roof system[END_REF]. Another strategy to adapt irrigation requirement is by optimal design of GR materials, by for example developing GR substrates with higher water holding capacities (Table 6.4).

Addition of sandy loam soil and use of amended soil (i.e. mix of red gravel, vermiculite and bark compost), perlite based substrates, foam sheets and fiberglass can all improve the WHC of the GR system [START_REF] Elstein | Evaluating Growing Media for a Shallow-Rooted Vegetable Crop Production System on a Green Roof[END_REF][START_REF] Harp | Preliminary evaluation of open cell foam for extensive green roof systems in Texas[END_REF]Nektarios et al., 2011;[START_REF] Kotsiris | An adaptive approach to intensive green roofs in the Mediterranean climatic region[END_REF][START_REF] Papafotiou | Growth of native aromatic xerophytes in an extensive Mediterranean green roof, as affected by substrate type and depth, and irrigation frequency[END_REF][START_REF] Kanechi | Performance of herbaceous Evolvulus pilosus on urban green roof in relation to substrate and irrigation[END_REF]. Some water holding additives, like hydrophilic gels, are also currently explored [START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF][START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF]. Alternatively, a WRL retains additional runoff and hence available water for GR vegetation (VanWoert et al., 2005a-b;Bianchini & Hewage, 2012;[START_REF] Guo | Runoff Reduction Capabilities and Irrigation Requirements of Green Roofs[END_REF]. This layer has been shown to be crucial to plant survival in semi-arid climates [START_REF] Santamouris | Investigating and analysing the energy and environmental performance of an experimental green roof system installed in a nursery school building in Athens, Greece[END_REF][START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Guo | Runoff Reduction Capabilities and Irrigation Requirements of Green Roofs[END_REF] but also in other climates, the irrigation requirements can be significantly reduced. In Vancouver (Canada), summer irrigation decreased from 54.4 to 8.6 mm by adding a WRL, and in subtropical Shanghai (China), the WRL suppressed the need of additional irrigation completely [START_REF] Roehr | Runoff Reduction Effects of Green Roofs in Vancouver, BC, Kelowna, BC, and Shanghai, P.R. China[END_REF].

A second way to conserve water is by finding alternative irrigation sources. For example gray water, which is the waste water from in and around the house (except water originating from toilet flushing, dishwashers and kitchen sinks), could be reused for irrigation purposes [START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF][START_REF] Carter | Life-cycle cost-benefit analysis of extensive vegetated roof systems[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF][START_REF] Chang | Optimal design for water conservation and energy savings using green roofs in a green building under mixed uncertainties[END_REF][START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF][START_REF] Ouldboukhitine | Impact of plants transpiration, grey and clean water irrigation on the thermal resistance of green roofs[END_REF]). In Sweden for example, gray water, together with black water, currently accounts for 58% of the yearly water discharge into wastewater treatment plants [START_REF] Semadeni-Davies | The impacts of climate change and urbanization on drainage in Helsingborg, Sweden: Combined sewer system[END_REF]. By making use of this gray water, potable water consumption of a household a e edu ed %, a d the e is less p essu e o the it s se age system [START_REF] Chang | Optimal design for water conservation and energy savings using green roofs in a green building under mixed uncertainties[END_REF]. However, if the gray water is clean enough to sustain GRs in the long term is still unclear. [START_REF] Ouldboukhitine | Impact of plants transpiration, grey and clean water irrigation on the thermal resistance of green roofs[END_REF] showed that the thermal performance of GRs decreased with approximately 30% when irrigated with gray water instead of clean water, as the chemical products in the gray water, like la ti a id, ellulose a d gl e ol, affe ted the pla t s a ti it a d he e transpiration processes. Another possibility is rainwater or stormwater harvesting (RWH), in which runoff is collected and stored [START_REF] Gill | Adapting Cities for Climate Change: The Role of the Green Infrastructure[END_REF][START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]. Runoff harvested from GRs themselves has been shown to be sufficiently clean to be reused for urban irrigation [START_REF] Argue | Catchment « Greening » using stormwater in Adelaide, South Australia[END_REF]Razzaghmanesh et al., 2014a). By passing through the substrate, the GR runoff pH increases from approximately 5.5 to 7.5, hence slightly mitigating acid rain [START_REF] Czemiel Berndtsson | Green roof performance towards management of runoff water quantity and quality: A review[END_REF]. The GR runoff also has an elevated amount of suspended solids and a higher turbidity, which will decrease over time (Morgan et al., 2011). Besides being an irrigation source, RWH helps to control floods and reduces erosion and stormwater pollution [START_REF] Chang | Optimal design for water conservation and energy savings using green roofs in a green building under mixed uncertainties[END_REF][START_REF] Price | Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States[END_REF][START_REF] Sheng | Integrated sustainable roof design[END_REF]Wittinghill & Rowe, 2011;[START_REF] Hardin | A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use[END_REF][START_REF] Islam | Design, Construction, and Operation of a Demonstration Rainwater Harvesting System for Greenhouse Irrigation at McGill University, Canada[END_REF]. There are many ways to harvest rainwater, including rainwater tanks or cisterns, constructed wetlands and treatment trains [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF][START_REF] Hardin | A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use[END_REF]. In the latter method, the rainwater passes a bio-filtration system and is stored in a tank. Sometimes, underground aquifers can be recharged with the bio-filtrated rainwater, a process also called Aquifer Storage and Recovery [START_REF] Argue | Catchment « Greening » using stormwater in Adelaide, South Australia[END_REF][START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]. By installing a rainwater tank on a GR, the total annual stormwater retention increased from 43 to 87% in Florida [START_REF] Hardin | A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use[END_REF]. In Brasil, the potential of RWH has also been acknowledged, although the installation of rainwater tanks still causes some concern as a considerable area is required, which is expensive in urban areas [START_REF] Vieira | Potential of utilization of rain water excess for irrigation of green roof in Mato Grosso, Brasil[END_REF]. A GR of 100 m² for example, would require a reservoir volume of 15.60 to 101.67 m³, depending on the region [START_REF] Vieira | Potential of utilization of rain water excess for irrigation of green roof in Mato Grosso, Brasil[END_REF].

In the third category, irrigation quantity can be minimized through monitoring and controlling of irrigation regimes [START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF][START_REF] Guo | Runoff Reduction Capabilities and Irrigation Requirements of Green Roofs[END_REF]. Meteorological factors, mainly relative humidity and number of sunshine hours as they affect the water consumption the most, are important to consider for GR irrigation systems [START_REF] Feng | A al sis o the I pa t of 'oof G ee i g s Wate Consumption Made by Meteorological Factor[END_REF]. Water requirements could be calculated by using ET data and precipitation information [START_REF] Bean | Advancing stormwater beneficial uses: ET mapping in urban areas[END_REF][START_REF] Feng | A al sis o the I pa t of 'oof G ee i g s Wate Consumption Made by Meteorological Factor[END_REF][START_REF] Stovin | A modelling study of long term green roof retention performance[END_REF], or by studying substrate moisture with sensors (Jim & Peng, 2012a-b;[START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF]. Depending on the local climate, it is often possible to use irrigation only during establishment and during prolonged drought [START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF]. Otherwise, irrigation should be turned on when the substrate moisture drops below a specified level, like the stress point (transition between readily available ate i the su st ate s la ge po es a d less a aila le ate i the s all po es; Ji & Peng, 2012b). Irrigation can be controlled using a smart controller, which turns on when necessary (at night or when soil moisture drops below the stress point) but deactivates when rainfall is registered [START_REF] Hardin | A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use[END_REF]Ascione et al., 2013;[START_REF] Sun | To irrigate or not to irrigate: Analysis of green roof performance via a vertically-resolved hygrothermal model[END_REF]. Ascione et al. (2013) are currently investigating a multivariable controller for irrigation and ET management, which takes both solar radiation and relative air humidity into account. Furthermore, best management practices for monitoring and managing irrigation are examined by [START_REF] Lea-Cox | Precision irrigation and nutrient management for nursery, greenhouse and green roof systems: Sensor networks for feedback and feedforward control[END_REF]. Their work includes the development of sensor networks and software to control irrigation at the species level. Next to water conservation strategies, many cities also aim at saving energy, as the energy required for heating and cooling buildings is expensive [START_REF] Chang | Optimal design for water conservation and energy savings using green roofs in a green building under mixed uncertainties[END_REF]. Solar shading end evaporative cooling are two ways to mitigate urban temperatures and hence the need for heating or air conditioning [START_REF] Santamouris | Investigating and analysing the energy and environmental performance of an experimental green roof system installed in a nursery school building in Athens, Greece[END_REF][START_REF] Chang | Optimal design for water conservation and energy savings using green roofs in a green building under mixed uncertainties[END_REF]. GRs play an important role in improving thermal comfort, especially during the day [START_REF] Gross | Numerical simulation of greening effects for idealised roofs with regional climate forcing[END_REF][START_REF] Kolb | G ee a d ool oofs u a heat island mitigation potential in European climates for office buildings under free floating conditions[END_REF] and when the climate is not very hot (KCC: Cfb or Dfb; [START_REF] Sailor | A green roof model for building energy simulation programs[END_REF][START_REF] Sailor | Exploring the building energy impacts of green roof design decisionsa modeling study of buildings in four distinct climates[END_REF]Ascione et al., 2013). But in order to maintain the reduced energy demand during summer, irrigation is needed to keep the substrate moist [START_REF] Sailor | Exploring the building energy impacts of green roof design decisionsa modeling study of buildings in four distinct climates[END_REF][START_REF] Kolb | G ee a d ool oofs u a heat island mitigation potential in European climates for office buildings under free floating conditions[END_REF][START_REF] Li | The effectiveness of cool and green roofs as urban heat island mitigation strategies[END_REF]. Therefore, other solutions to save energy are investigated in warm to hot climates.

Cool roofs (also referred to as reflective or white roofs) for example, are considered a good alternative (Ascione et al., 2013;[START_REF] Kolb | G ee a d ool oofs u a heat island mitigation potential in European climates for office buildings under free floating conditions[END_REF][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]. This roof type can decrease the total daily heat flux by around 80% compared to a traditional (black) bitumen roof, while GRs decrease the daily heat flux by 52% [START_REF] Scherba | Modeling impacts of roof reflectivity, integrated photovoltaic panels and green roof systems on sensible heat flux into the urban environment[END_REF]. However, the performance of cool roofs is strongly hindered by factors like dirt accumulation, acid rain and UV radiation [START_REF] Li | The effectiveness of cool and green roofs as urban heat island mitigation strategies[END_REF][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], and compared to GRs, cool roofs do not contribute to stormwater management, aesthetics or habitat value.

Chapter 6 123

Examples of stormwater management regulations

Next to the water conservation and energy saving strategies described above, public authorities can develop regulations involving stormwater management [START_REF] Carter | Life-cycle cost-benefit analysis of extensive vegetated roof systems[END_REF]. For example, in several countries, building owners need to pay a fee for the quantity of runoff that runs into the sewer system. If green infrastructure elements like GRs are installed, the owner can be exempt from paying this fee [START_REF] Carter | Life-cycle cost-benefit analysis of extensive vegetated roof systems[END_REF]. In Portland (OR), building height restrictions are attenuated if a building owner has a floor-area ratio (FAR) bonus. By definition, the FAR is the ratio of the total floor area of the building to the area of its zoning lot, which is the basic unit for zoning regulations (NOAA, 2012). The bonus can only be obtained if a G' is i stalled a d i igatio egulatio s . e e da s du i g esta lish e t;

. e e da s afte esta lish e t a e et " h oll et al., 2011a). In the US, it is also possible for building owners to obtain a Leadership in Energy and Environmental Design (LEED) certificate if GRs without irrigation or using gray water or recycled water for irrigation are installed [START_REF] Grant | A decision-making framework for vegetated roofing system selection[END_REF][START_REF] Carter | Life-cycle cost-benefit analysis of extensive vegetated roof systems[END_REF]. A University in Atlanta (Georgia, US) for example, obtained a LEED certificate by applying multiple sustainable practices, including controlled irrigation, RWH, and gray water recycling for irrigation, toilet flushing, sinks and showers. This way, the total water consumption was already reduced by 12% from 2007 to 2009 [START_REF] Lynch | Water Efficiency Measures at Emory University[END_REF]. A last incentive worth mentioning is the Water Sensitive Urban Design (WSUD), which is applied in amongst others Australia to restrict water use and retain water in the urban landscape [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]. Technologies that fall within the WSUD include RWH for irrigation, bio-retention systems, treatment wetlands and GRs and green walls [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]Razzaghmanesh et al., 2014a).

Irrigation specifications for GRs in different climatic contexts

Dry semi-arid climates

For GRs in dry semi-arid or steppe climates (KCC: Bsk) it is strongly advised to install an irrigation system. [START_REF] Coutts | Assessing practical measures to reduce urban heat: Green and cool roofs[END_REF] conducted a study in Melbourne (Australia) with unirrigated SEGRs and concluded that the ET potential of the succulent vegetation was low and those GRs only slightly offered any thermal benefit. Irrigation is hence necessary in order to mitigate both the UHI effect and surface heating during hot summer days, by stimulating evaporative cooling [START_REF] Voyde | Quantifying Evapotranspiration Rates for New Zealand Green Roofs[END_REF][START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF][START_REF] Jim | Effect of vegetation biomass structure on thermal performance of tropical green roof[END_REF][START_REF] Coutts | Assessing practical measures to reduce urban heat: Green and cool roofs[END_REF]. One condition is that GR vegetation should consist of plant species with elevated transpiration rates, like for example grasses, which constitute together with shrubs, the main vegetation in these steppe climates (Nektarios et al., 2011;[START_REF] Jim | Effect of vegetation biomass structure on thermal performance of tropical green roof[END_REF][START_REF] Coutts | Assessing practical measures to reduce urban heat: Green and cool roofs[END_REF]). If irrigation is not possible, then only succulent species should be used, as their probability of survival during and recovery after summer drought is more likely compared to herbaceous vegetation [START_REF] Bousselot | Moisture content of extensive green roof substrate and growth response of 15 temperate plant species during dry down[END_REF]. Possible irrigation techniques are drip irrigation or overhead rotator systems, which should be active during growing seasons and especially summer [START_REF] Bousselot | Extensive green roof species evaluations using digital image analysis[END_REF][START_REF] Pérez | Green roofs as passive system for energy savings when using rubber crumbs as drainage layer[END_REF]Table 6.5). When vegetation is well-established, an irrigation regime of 6.4 mm per week, or 0.91 l/m² per day for an EGR with substrate depth of 10 cm can be sufficient to support a variety of plant species, including Antennaria parvifolia and the grass Bouteloua gracilis, two species native to the Rocky Mountains region [START_REF] Bousselot | Extensive green roof species evaluations using digital image analysis[END_REF].

Warm temperate climates: subtropical and marine regions

In warm temperate regions with hot summers and sufficient annual rainfall (KCC: Cfa), GRs are very beneficial in terms of improving thermal comfort during summer [START_REF] Dvorak | Rooftop temperature reduction from unirrigated modular green roofs in south-central Texas[END_REF]. It has been shown by [START_REF] Dvorak | Rooftop temperature reduction from unirrigated modular green roofs in south-central Texas[END_REF] that this benefit can even be achieved on unirrigated GRs with succulent vegetation. The results of their study, conducted in Texas, were comparable to results of studies on irrigated GRs in the same climate [START_REF] Sonne | Evaluating green roof energy performance[END_REF][START_REF] Simmons | Green roofs are not created equal : the hydrologic and thermal performance of six different extensive green roofs and reflective and non-reflective roofs in a sub-tropical climate[END_REF]. Unirrigated GRs are the most sustainable option but the vegetation is limited to succulents. By providing a minimum irrigation regime, the range of potential plant species can be widened, while decreasing soil temperature and hence the thermal benefit [START_REF] Provenzano | Evaluation of Perennial Herbaceous Species for their Potential Use in a Green Roof under Mediterranean Climate Conditions[END_REF][START_REF] Price | Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States[END_REF]. Subsurface capillary irrigation and drip irrigation are two possible irrigation methods [START_REF] Price | Irrigation Lowers Substrate Temperature and Enhances Survival of Plants on Green Roofs in the Southeastern United States[END_REF][START_REF] Kanechi | Performance of herbaceous Evolvulus pilosus on urban green roof in relation to substrate and irrigation[END_REF]Table 6.5). During summer and after establishment, daily watering with 500 ml (or 0.62 l/m² for a 10 cm EGR) favored the growth and flowering of Evolvulus pilosus (a non-native but common ornamental groundcover plant in Japan) compared to watering every other day [START_REF] Kanechi | Performance of herbaceous Evolvulus pilosus on urban green roof in relation to substrate and irrigation[END_REF]. Other ornamental species like Thymus serphyllum and Petunia x hybrida can be used as GR plants when irrigated with 5 l of water every two days, or 6.17 l/m² daily [START_REF] Sendo | Evaluation of Growth and Green Coverage of Ten Ornamental Species for Planting as Urban Rooftop Greening[END_REF]. To prevent waste of water, irrigation should be turned off on days when more than 5 mm of rain is predicted [START_REF] Sendo | Evaluation of Growth and Green Coverage of Ten Ornamental Species for Planting as Urban Rooftop Greening[END_REF][START_REF] Aithkenhead-Peterson | Chemistry of growth medium and leachate from green roof systems in south-central Texas[END_REF][START_REF] Kanechi | Performance of herbaceous Evolvulus pilosus on urban green roof in relation to substrate and irrigation[END_REF].

When the climate is rather marine, with warm summers and fully humid (KCC: Cfb), typical for the UK, irrigation is generally unnecessary (Nagase & Dunnett, 2010). Even on unirrigated EGRs, various plant species and vegetation type options exist, like an annual meadow on 7 cm substrate depth (Nagase & Dunnett, 2013b). Also geophytes like Iris bucharica, Muscari azureum and Tulipa humilis, non-native to the UK but suitable for GR purposes, can survive without irrigation in only 5 cm substrate depth (Nagase & Dunnett, 2013a). However, plant growth can be improved by increasing substrate depth [START_REF] Fassman | Moisture Measurements as Performance Criteria for Extensive Living Roof Substrates[END_REF], or with a limited amount of irrigation once a week in summer or when substrate moisture drops below 15%. This can be done manually or by means of a drip irrigation system (Dunnett et al., 2008;Nagase et al., 2013;Nagase & Dunnett, 2013b).

Warm temperate climates: Mediterranean regions

In Mediterranean regions with dry, hot summers (KCC: Csa), GRs could provide great thermal benefits [START_REF] Jaffal | A comprehensive study of the impact of green roofs on building energy performance[END_REF], but irrigation is necessary in these regions for establishment, survival and success of EGR plants especially during dry summers [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF][START_REF] Kotsiris | An adaptive approach to intensive green roofs in the Mediterranean climatic region[END_REF][START_REF] Ntoulas | Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems[END_REF][START_REF] Schweitzer | Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate[END_REF]. Without irrigation, one should think of IGRs with substrate depths over 20 cm, or alternatively select the most drought tolerant plant species (VanWoert et al., 2005a;[START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF][START_REF] Schweitzer | Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate[END_REF].

Chapter 6 125 Succulent species are generally considered as the best vegetation option, but even these stress tolerant species have difficulties to survive the hot dry summers in Australia on EGRs if no irrigation system is provided [START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF]. "o eti es, i easi g su st ate depth is suffi ie t to ai tai the pla t s g o th and physiological status during drought [START_REF] Ntoulas | Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems[END_REF]. Nektarios et al. (2011[START_REF] Ntoulas | Semi-extensive green roof substrate type and depth effects on Zoysia matrella )eo g o th a d drought tolerance under different irrigation regimes[END_REF] found that the non-native turfgrass Zo sia at ella Zeo performed best in SEGRs with 15 cm substrate depth compared to 6 cm deep EGRs. The water requirement of turfgrass was based on the reference ET or pan evaporation (ETo), and ranged from 15% ETo every 10 days [START_REF] Ntoulas | Semi-extensive green roof substrate type and depth effects on Zoysia matrella )eo g o th a d drought tolerance under different irrigation regimes[END_REF] to 80% every 3 days (Nektarios et al., 2011;Table 6.5). Growth and physiological status of turfgrass were only slightly influenced by irrigation [START_REF] Ntoulas | Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems[END_REF], although irrigation greatly i p o ed the pla t s olo [START_REF] Ntoulas | Semi-extensive green roof substrate type and depth effects on Zoysia matrella )eo g o th a d drought tolerance under different irrigation regimes[END_REF]. Likewise for Dianthus fruticosus, native to the Mediterranean region, irrigation during drought did not result in better growth (Nektarios et al., 2011). Other studies point at a considerable water quantity needed to ensure plant survival in these Mediterranean regions. In a study of [START_REF] Schweitzer | Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate[END_REF] in Israel, the water requirement of the tested species in 5 cm deep EGRs ranged from 3-6 l/m² per day for the South-African Mediterranean succulent species Aptenia cordifolia to 7-9 l/m² per day for the tropical East-African drought tolerant grass species Pennisetum clandestinum. Although many xerophytic Mediterranean species have potential to be used on GRs in this climate [START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]; Chapter 2 and 3), more research is needed to determine minimal irrigation requirements and optimal substrate depth [START_REF] Schweitzer | Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate[END_REF].

Irrigation can be applied manually or by automated drip irrigation (Table 6.5). For EGRs, the best irrigation regime in summer is daily watering until saturation [START_REF] Pearlmutter | Performance analysis of a simple roof cooling system with irrigated soil and two shading alternatives[END_REF][START_REF] Ntoulas | Semi-extensive green roof substrate type and depth effects on Zoysia matrella )eo g o th a d drought tolerance under different irrigation regimes[END_REF][START_REF] Ntoulas | Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems[END_REF]. Alternatively, irrigation can be applied at a minimum daily quantity of 1.2 l/m² [START_REF] Santamouris | Investigating and analysing the energy and environmental performance of an experimental green roof system installed in a nursery school building in Athens, Greece[END_REF], 1 l/m² [START_REF] Ntoulas | Semi-extensive green roof substrate type and depth effects on Zoysia matrella )eo g o th a d drought tolerance under different irrigation regimes[END_REF], or 1.96 l/m² [START_REF] Papafotiou | Growth of native aromatic xerophytes in an extensive Mediterranean green roof, as affected by substrate type and depth, and irrigation frequency[END_REF]. On IGRs with a substrate depth of 30 cm, a daily quantity of 2.08 l/m² would be required [START_REF] Kotsiris | An adaptive approach to intensive green roofs in the Mediterranean climatic region[END_REF]. Another possibility relevant in Mediterranean climates during summer is to leave the GR unvegetated and only wetting the substrate, which is efficient in terms of evaporative cooling of the roof surface [START_REF] Pearlmutter | Performance analysis of a simple roof cooling system with irrigated soil and two shading alternatives[END_REF]. Although plants provide cooling through transpiration and substrate shading, they also prevent soil evaporation and hence reduce the potential of the GR to cool buildings. Without the vegetation layer, the plant-specific irrigation requirement is solved, and their shading function can be replaced by other covering materials [START_REF] Pearlmutter | Performance analysis of a simple roof cooling system with irrigated soil and two shading alternatives[END_REF]. However, other beneficial aspects of vegetated roofs, including habitat provision and aesthetic value, are eliminated.

Coastal Mediterranean areas (KCC: Csb) have dry summers but are slightly cooler than interior Mediterranean regions. This is represented in the GR irrigation requirements, as it is possible to install unirrigated GRs in this climate, especially if only succulent species are applied (Nagase & Dunnett, 2010;Razzaghmanesh et al., 2014b). Next to succulents, also geophytes and rhizomatous forbs can be used on unirrigated EGRs in Oregon (Schroll et al., 2011a). In addition, the adapted life cycle dynamics of annual species make them useful for unirrigated GR application in this climate, as they will contribute to GR ecosystem services during periods with sufficient rainfall (cf. Chapter 2 and 3). Also mosses require very little water in summer [START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF]Table 6.5). In other studies, irrigation is advised throughout the summer to ensure survival of non-succulent species with different Chapter 6 126 life forms [START_REF] Monterusso | Establishment and persistence of Sedum spp. and native taxa for green roof applications[END_REF]Schroll et al., 2011a-b;[START_REF] Rowe | Effect of green roof media depth on Crassulacean plant succession over seven years[END_REF]Razzaghmanesh et al., 2014b). The discussion whether or not to irrigate GRs in coastal Mediterranean climates depends on the main goal for which the GR was designed. A more diverse vegetation is preferred in terms of habitat-and biodiversity value (Nagase & Dunnett, 2010;MacIvor & Lundholm, 2011), but summer irrigation is e ui ed, hi h i tu edu es the G' s sto ate ete tio pote tial a d also increases weed pressure (Schroll et al., 2011a(Schroll et al., , 2011b)). If irrigation is opted for, it could be done in the early morning (4-5 AM) by use of a sprinkler system, which provides 2.3 l/m² of water per day during the growing season for a 12.5 cm EGR (Table 6.5). For Portland specifically, where the FAR bonus regime is implemented, the maximum summer irrigation is 45.7 l/m² every 5 days (Schroll et al., 2011a-b;Table 6.5). This regime is much higher than the one used by Razzaghmanesh et al. (2014b), which was only 11.1 l/m² every 10 days.

Warm temperate climates: regions with wet, hot summers and dry winters

Finally, in warm temperate climates characterized by wet, hot summers and dry winters (KCC: Cwa) like Hong Kong (China), irrigation is not essential, but it can mitigate the temperature fluctuations of the building. An EGR can be watered optimally by an automatic sprinkler system with an irrigation regime of 5 l/m² daily [START_REF] Jim | Biophysical properties and thermal performance of an intensive green roof[END_REF]Jim & Peng, 2012a-b;Table 6.5). The irrigation requirement can be reduced by selecting more drought tolerant species, by a deeper substrate or an additional WRL.

Snow climates

Multiple studies have been conducted in fully humid snow climates with warm summers (KCC: Dfb). Unirrigated EGRs are possible if succulent species are used, but geophytes (e.g. Allium cernuum) and other perennial plants, all native to Michigan, like Coreopsis lanceolata and Tradescantia ohiensis are useful as well [START_REF] Monterusso | Establishment and persistence of Sedum spp. and native taxa for green roof applications[END_REF][START_REF] Rowe | Assessment of Heat-expanded Slate and Fertility Requirements in Green Roof Substrates[END_REF]MacIvor & Lundholm, 2011). Like in the climates previously discussed, a more diverse plant community is preferred for several reasons. Furthermore, in this climate, Sedum spp. can suffer from freezing injury in winter [START_REF] Durhman | Effect of Watering Regimen on Chlorophyll fluorescence and Growth of Selected Green Roof Plant Taxa[END_REF][START_REF] Benvenuti | Initial agronomic performances of Mediterranean xerophytes in simulated dry green roofs[END_REF]. Other non-succulent drought tolerant plants can be applied if some irrigation is provided, when substrate depth is increased, organic material (OM) is added to the substrate, or when the GR is situated in the shade [START_REF] Monterusso | Establishment and persistence of Sedum spp. and native taxa for green roof applications[END_REF][START_REF] Durhman | Effect of Watering Regimen on Chlorophyll fluorescence and Growth of Selected Green Roof Plant Taxa[END_REF][START_REF] Köhler | Long-Term Vegetation Research on Two Extensive Green Roofs in Berlin[END_REF][START_REF] Rowe | Assessment of Heat-expanded Slate and Fertility Requirements in Green Roof Substrates[END_REF]Getter et al., 2009a;[START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF][START_REF] Thuring | Green Roof Plant Responses to Different Substrate Types and Depths under Various Drought Conditions[END_REF][START_REF] Getter | Seasonal heat flux properties of an extensive green roof in a Midwestern U.S. climate[END_REF][START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF]. Every year during the growing season, the vegetation on EGRs will face some drought stress, and it is normal that some plants die. This is one reason more to diversify the GR vegetation, including annual species that can fill in the bare spots in the substrate, and some succulents that help neighboring species survive drought [START_REF] Köhler | Long-Term Vegetation Research on Two Extensive Green Roofs in Berlin[END_REF][START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. [START_REF] Macivor | Decoupling factors affecting plant diversity and cover on extensive green roofs[END_REF] showed that by irrigating EGRs, plant diversity is maintained, although no effect on cover and biomass was found. So in the case that the irrigation system has to be turned off, an EGR with high plant diversity will not undergo any significant visual changes. Many irrigation methods are possible, from initial hand watering to automated overhead irrigation (Table 6.5). Except for the establishment phase and the first growing season, irrigation is not provided in most studies (VanWoert et al., 2005b;[START_REF] Monterusso | Runoff Water Quantity and Quality from Green Roof Systems[END_REF][START_REF] Monterusso | Establishment and persistence of Sedum spp. and native taxa for green roof applications[END_REF][START_REF] Rowe | Assessment of Heat-expanded Slate and Fertility Requirements in Green Roof Substrates[END_REF][START_REF] Rowe | Effect of green roof media depth on Crassulacean plant succession over seven years[END_REF]Getter & Rowe, 2008, 2009;MacIvor & Lundholm, 2011). In other studies, GRs are watered when necessary, eventually controlled with a moisture sensor [START_REF] Durhman | Effect of Substrate Depth on Initial Growth, Coverage, and Survival of 25 Succulent Green Roof Plant Taxa[END_REF]Lundholm et al., 2010;[START_REF] Carpenter | Effect of roof surface type on storm-water runoff from full-scale roofs in a temperate climate[END_REF][START_REF] Getter | Seasonal heat flux properties of an extensive green roof in a Midwestern U.S. climate[END_REF][START_REF] Macivor | Decoupling factors affecting plant diversity and cover on extensive green roofs[END_REF].

Very little information was found from studies conducted in snow climates with hot summers and dry winters (KCC: Dwa). Generally, GRs are very beneficial in this climate in terms of thermal performance [START_REF] He | Simulation of thermodynamic transmission in green roof ecosystem[END_REF]. One study tested the potential of wetland GRs in Korea [START_REF] Song | Wetlands are an effective green roof system[END_REF] and concluded that less irrigation is necessary than on traditional GRs. [START_REF] Feng | A al sis o the I pa t of 'oof G ee i g s Wate Consumption Made by Meteorological Factor[END_REF] investigated the most important meteorological factors to determine GR irrigation regimes in Beijing (China). In this context, relative humidity and number of sunshine hours appear to be more important than rainfall and temperature. Their experimental GRs received a minimum additional water quantity of 0.71 l/m² daily in July for a 10 cm EGR and 1.24 l/m² for a 20 cm IGR.

Concluding remarks

Irrigating GRs and other urban landscapes is often considered an unsustainable practice, mainly because of the installation and water cost, but also due to competition with other water needs, e.g. for agriculture or consumption [START_REF] Abernethy | Sustainability of Irrigation Systems[END_REF]. Watering urban vegetation however, also provides socio-economic and environmental benefits, by ensuring plant survival and related aesthetic aspects of the GR, as well as by improving ET and hence thermal comfort. To ensure sustainable irrigation in the long term, there should be public awareness of sustainable water consumption, and GRs should be designed carefully, without wasting any water and limiting the use of potable water. Potential solutions are gray water re-use, RWH, controlled irrigation in space and time (using for example a smart controller) and the addition of a WRL [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]Razzaghmanesh et al., 2014a;[START_REF] Sun | To irrigate or not to irrigate: Analysis of green roof performance via a vertically-resolved hygrothermal model[END_REF]. Also an appropriate selection of vegetation is needed, with a diverse mix of life forms including succulents, geophytes, mosses, annual species and droughttolerant herbs [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]. Ideally, a deeper substrate should be opted for to increase the plant water availability, but as this is not always possible due to building weight limitations and colonization of undesirable weeds, shrubs and invasive plants, efficient irrigation methods should be considered as an alternative. If irrigation is really impossible due to for example governmental restrictions, then a reflective roof could be opted for to provide thermal cooling, although this type of roof does not have the capacity to reduce stormwater runoff. By considering substrate type, depth, vegetation and irrigation regime during the design process, an adaptive approach is created in which GR establishment and sustainability can be facilitated [START_REF] Kotsiris | An adaptive approach to intensive green roofs in the Mediterranean climatic region[END_REF][START_REF] Ntoulas | Semi-extensive green roof substrate type and depth effects on Zoysia matrella )eo g o th a d drought tolerance under different irrigation regimes[END_REF][START_REF] Ntoulas | Zoysia matrella cover rate and drought tolerance in adaptive extensive green roof systems[END_REF].

Multiple irrigation methods are possible, of which drip irrigation and overhead sprinkler systems are the most commonly applied on GRs. Hand watering is possible as well, but extremely time consuming and only possible for small surfaces. The best way to irrigate is automated overhead irrigation or drip irrigation if an additional WRL is provided [START_REF] Rowe | Comparison of irrigation efficiency and plant health of overhead, drip, and sub-irrigation for extensive green roofs[END_REF].

During the establishment phase and the first growing season throughout summer, it is advised to use irrigation on all GR types and climates. Afterwards, irrigation is only necessary on EGRs in (semi)-arid climates (KCC: Bsk and Csa). Installation of EGRs with a substrate depth below 10 cm is discouraged in these climates, as both a deeper substrate and irrigation are necessary to support plant survival during summer (cf. Chapter 4). In the other climates, a minimum amount of irrigation is advised as well to ensure plant growth and survival, diversify the vegetation and improve thermal comfort during summer (Dunnett et al., 2008).

Future research should further elaborate on GR materials with increased WHC, in order to develop GRs with high biodiversity value and with no or only limited irrigation requirements, also in (semi)-arid climates [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]. Also studies selecting appropriate drought tolerant plant species should be supported, as well as studies quantifying the contribution of GRs to urban ecosystem services, weighed against the cost of adaptive GR design (Schroll et al., 2011a;[START_REF] Schweitzer | Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate[END_REF]. This review pointed at sustainable irrigation possibilities that need to be elaborated further with scientific experiments and examples of local implementations. Different irrigation regimes are needed according to climatic context, and guidelines for irrigation specifications in each climate need to be determined.

Introduction

Biodiversity is believed to be the major driver of ecosystem services through ecosystem processes and functions [START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF][START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF][START_REF] Quijas | Plant diversity and generation of ecosystem services at the landscape scale: expert knowledge assessment[END_REF][START_REF] Gamfeldt | Higher levels of multiple ecosystem services are found in forests with more tree species[END_REF][START_REF] Pinto | Linking biodiversity indicators, ecosystem functioning, provision of services and human well-being in estuarine systems : Application of a conceptual framework[END_REF]. Overall there is a positive relation between ecosystem properties (e.g. resilience, resistance) and biodiversity and there is increasing evidence that biodiversity improves the stability of ecosystems through time (for a review, see [START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF]. There is also sufficient evidence that biodiversity either directly influences or is strongly correlated with provisioning and regulating services (cf. [START_REF] Isbell | High plant diversity is needed to maintain ecosystem services[END_REF][START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF]. Furthermore, consensus is growing that functional traits, and especially the range and value of those traits, rather than species richness and evenness are important for the provision of ecosystem properties (e.g., [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF][START_REF] Lavorel | Using plant functional traits to understand the landscape distribution of multiple ecosystem services[END_REF]. Functional trait composition amongst plant communities can be compared through a series of functional diversity (FD) indices [START_REF] Petchey | How do different measures of functional diversity perform[END_REF]Mouchet et al., 2010). Next to FD, phylogenetic diversity (PD) is considered a holistic diversity measure, because genetically related species commonly show similar traits due to their shared evolutionary history [START_REF] Cadotte | Using Phylogenetic, Functional and Trait Diversity to Understand patterns of Plant Community Productivity[END_REF][START_REF] Devictor | Spatial mismatch and congruence between taxonomic, phylogenetic and functional diversity: the need for integrative conservation strategies in a changing world[END_REF][START_REF] Flynn | Functional and phylogenetic diversity as predictors of biodiversity-ecosystem function relationships[END_REF][START_REF] Srivastava | Phylogenetic diversity and the functioning of ecosystems[END_REF].

Recently, as a result of advancing habitat loss, land use changes and urbanization, the concept of novel ecosystems has gained momentum. These are, by definition, self-organizing systems originating from human interference and with actual altered biotic, abiotic and social components as compared to the prevailing historical ecosystems [START_REF] Hobbs | Novel ecosystems: intervening in the new ecological world order[END_REF]. Urban ecosystems, like green roofs, parks and gardens, fall within this definition due to their novel elements but differ from novel ecosystems in cultivated or degraded landscapes because continued management is necessary [START_REF] Hobbs | Novel ecosystems: theoretical and management aspects of the new ecological world order[END_REF][START_REF] Lundholm | Mini-Review: Habitat analogues for reconciliation ecology in urban and industrial environments[END_REF][START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. Based on evidence of previous studies, we assume that designing functional diverse urban ecosystems will improve ecosystem services (cf. [START_REF] Isbell | High plant diversity is needed to maintain ecosystem services[END_REF][START_REF] Pinto | Linking biodiversity indicators, ecosystem functioning, provision of services and human well-being in estuarine systems : Application of a conceptual framework[END_REF]Witthinghill et al., 2014) and the quality of life of urban residents (Chiesura, 2004;[START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF]Wolch et al., 2014). We analyze novel ecosystems by use of FD and PD techniques and propose two generic methods for improved design in terms of species composition. The methodology is inspired by the Noah s A k P o le f. Billio et, , e ause the goal is to a hie e opti al allocation of a limited amount of resources by maximizing the species FD. We use extensive green roofs (EGRs), which offer many benefits in urban areas such as stormwater management and mitigation of the urban heat island effect [START_REF] Carter | Vegetated roofs for stormwater management at multiple spatial scales[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Tzoulas | Promoting ecosystem and human health in urban areas using Green Infrastructure: A literature review[END_REF][START_REF] Gaston | Managing urban ecosystems for goods and services[END_REF], as a case study to explore and illustrate the methods. EGRs are semi-artificial ecosystems with vegetation on substrate (2-20 cm in depth) and specialized ate ials pla ed o a uildi g s oof O e do fe et al., . "pe ies sele tio i these novel urban ecosystems is often determined by opportunistic criteria such as commercial availability, aesthetic qualities, adaptation to the environmental conditions and substrate depth (Getter & Rowe, 2008). A practical approach that directs to plant species with improved ecosystem properties value is a meaningful addition to this criteria list, and because the approach is generic in nature, the methods will be useful for the design of other novel ecosystems as well.

The specific goals of this study were to:

(i) Analyze the initial plant species composition, as provided by commercial EGR systems, in terms of FD and PD; (ii) Offer a methodology to optimize species choice for incorporation into EGR design, making use of the FD approach.

Material and methods

System and trait selection

The European Federation of Green Roof Associations was used to select EGR systems (www.efb-greenroof.eu, consulted on 24/02/2013). The webpages of all the companies associated with this Federation were screened for EGR systems and their corresponding plant species (i.e. species that are or can be part of the original planted GR vegetation). When information was not published online, we requested the species lists from the company. In total, 57 systems originating from 19 GR companies spread over six European countries were used in the analysis (Appendix 7.1). Some companies are active in multiple countries, so the systems provided by them are assumed useful for Northern, Western and Central Europe. All systems together accounted for 199 vascular plant species (Appendix 7.2); nomenclature follows The Plant List (2013). Varieties and subspecies were not taken into account.

In order to link essential variation in GR systems with FD and PD, we collected and partly derived information on percentage of succulent species, substrate depth, installation method (cuttings, mat, seeds or plugs) and species richness (SR).

Trait selection (29 in total; Table 7.1) was determined by important GR ecosystem properties as explicitly mentioned in specific literature on GRs: [START_REF] Getter | The Role of Extensive Green Roofs in Sustainable Development[END_REF], [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF], [START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF], Lundholm et al. (2010), Cook-Patton and Bauerle (2012), [START_REF] Robinson | Ecosystem services provided by urban spontaneous vegetation[END_REF], [START_REF] Dakin | The Professional Design Guide to Green Roofs[END_REF], and [START_REF] Oudolf | Planting: A New Perspective[END_REF]. These properties were related to supporting, provisioning, regulating and cultural services based on the literature and on best professional judgment. Trait values for the plant species were obtained from multiple sources (Table 7.1).

Data analysis 2.2.1. Exploring the EGR systems

The 57 EGR systems were clustered based on their plant species composition using hie a hi al luste i g ith "ø e se dista e easu e a d fle i le eta β = -0.25) as group linkage method [START_REF] Mccune | PC-ORD[END_REF]. We used Indicator Species Analysis [START_REF] Dufrêne | Species assemblages and indicator species: the need for a flexible asymmetrical approach[END_REF] to obtain the optimal number of system types in the cluster analysis and to define species that significantly differentiate these types. Clustering and Indicator Species Analysis were performed in PCord 6.0 (MjM Software, USA). Non-parametric tests (Kruskal Wallis and Pearson Chi²) were used to explore significant differences between the system types in terms of system characteristics. Statistical tests were performed with SPSS 22.0 (SPSS Inc, Chicago, IL).

Diversity analysis

FD and PD were quantified by two independent diversity indices [START_REF] Mason | Functional richness, functional evenness and functional divergence: the primary components of functional diversity[END_REF]. The richness index is the amount of trait or phylogenetic space filled by the community and the evenness index is a measure of the uniformity of distribution of species in trait or phylogenetic space. Functional richness was quantified in this study with FDpg (referring to the FD measure as proposed by [START_REF] Petchey | Functional diversity (FD), species richness and community composition[END_REF]. FDpg is the total branch length of the functional dendrogram constructed using a dissimilarity matrix (Gower distance) based on the species functional traits. Functional evenness was quantified with FEve as proposed by [START_REF] Villéger | New multidimensional functional diversity indices for a multifaceted framework in functional ecology[END_REF].

The FD indices were calculated using the FD [START_REF] Laliberté | Measuring functional diversity (FD) from multiple traits, and other tools for functional ecology[END_REF], Ape [START_REF] Paradis | Analyses of Phylogenetics and Evolution[END_REF] and Picante package [START_REF] Kembel | Picante: R tools for integrating phylogenies and ecology[END_REF] in R2.15.3 (R Development Core Team, 2012).

The phylogenetic tree was constructed from Daphne, a dated phylogeny of a large European flora [START_REF] Durka | Daphne: a dated phylogeny of a large European flora for phylogenetically informed ecological analyses[END_REF] in Phylocom 4.2 [START_REF] Webb | Phylocom: software for the analysis of phylogenetic community structure and trait evolution[END_REF]. The few unresolved relationships were treated as polytomies and remained unresolved.

We ua tified PD ith t o diffe e t i di es: Faith s i de of PD PDf; Faith, and mean phylogenetic distance (MPD; [START_REF] Webb | Phylogenies and community ecology[END_REF]. PDf is an index for phylogenetic richness and is defined as the minimum total length of the phylogenetic branches connecting the species together on the phylogenetic tree and corresponds to FDpg [START_REF] Cadotte | Using Phylogenetic, Functional and Trait Diversity to Understand patterns of Plant Community Productivity[END_REF]. MPD is a phylogenetic evenness index and is the average distance between two random species [START_REF] Webb | Phylocom: software for the analysis of phylogenetic community structure and trait evolution[END_REF]. The branch lengths on the phylogram were used as distance measure. The indices of PD were calculated in R with the Picante package [START_REF] Kembel | Picante: R tools for integrating phylogenies and ecology[END_REF].

FDpg, FEve, PDf and MPD were calculated for every EGR system and related to SR using Spearman rank correlation (r s ). Differences in diversity indices between the system types were tested using Kruskal Wallis tests at a significance level of 0.05.

Species selection for optimization of EGR ecosystem functioning

Each species has its own habitat requirements and not all species can be applied on every GR type. In order to optimize ecosystem properties and services of EGR systems, we compiled a species list per EGR type of which species FD is maximized (FDMax1). Because the debate about which component of biodiversity influences ecosystem properties the most is still going on [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF][START_REF] Flynn | Functional and phylogenetic diversity as predictors of biodiversity-ecosystem function relationships[END_REF][START_REF] Gasc | Assessing biodiversity with sound: Do acoustic diversity indices reflect phylogenetic and funcitonal diversities of bird communities?[END_REF], the compilation of the species lists may be based on both FD and PD. However, because the resolution of the phylogenetic tree was not fine enough (many species occurring together at the same branch), evolutionary information could not be used in this analysis [START_REF] Swenson | Phylogenetic resolution and quantifying the phylogenetic diversity and dispersion of communities[END_REF]. Richness and evenness are both important factors that influence ecosystem properties, but as the calculation of FEve is very complex, we chose to maximize FDpg. We started with a species list and accompanying functional trait matrix per EGR type with the species that were present in the systems of that type. A separate species dissimilarity matrix for each of the EGR types was calculated with the Gower index as distance measure (in R2.15.3). The selection process started with the two species with the highest dissimilarity value. Next, a new species was added that maximized the average dissimilarity to the species that were already in the list. This step was repeated until the average SR of that EGR type was reached. The final result was a species list per EGR type -equal to the average SR of that EGR type -that maximized FDpg. In order to calculate the gain in FDpg of the optimized species lists, we wanted to compare the FDpg of the optimized species lists with the FDpg values of the initial EGR systems. However, values of FDpg depend on the number of species in the functional dendrogram and FDpg values that are calculated from different dendrograms can therefore not be compared. In order to be able to compare the FDpg of the optimized species lists with the FDpg values of the initial EGR system, we also calculated the FDpg of the optimized species lists based on the functional dendrogram constructed with all species.

Because not all species in the optimized species lists are readily available in every commercial assortment, the optimized species lists are not very practical. Therefore, we also provided a method to optimize the FD of EGRs that is practically applicable by GR companies (FDMax2). Per EGR type the species present in that GR type were classified in functionally similar groups based on the functional dendrogram. The number of groups corresponded again to the average SR of the EGR type. By choosing one species of each group, the maximum FDpg is approached.

The proposed methods FDMax1 and FDMax2 offer a solution to the Noah s A k Problem (Billionnet, 2013), in which FD of the respective species is maximized. Billionnet ( 2013) also addressed this problem by using nonlinear mixed-integer programming, a complex and elaborate method because transformations and logarithmic function techniques are necessary to account for the nonlinearity. Here we tackled the problem in a more simple way by using a dissimilarity matrix (FDMax1) and hierarchical clustering techniques (FDMax2).

Results

EGR type description

Clustering the sample of 57 EGR systems resulted into three types, each characterized by a number of significant indicator species (Appendix 7.2) and attributes (Table 7.2). The 22 EGR systems belonging to type 1, hereafter called Sedum t pe , o tai ed mainly succulent species, had the shallowest substrate depth (5 cm on average) and species were mainly installed by cuttings or mats. SR was significantly lower than those of type 2 and 3 (8.2 ± 2.32, Table 7.2) and eight Sedum spp. were significantly associated ith this t pe. T pe , o Dianthus-Thymus t pe afte t o highl sig ifi a tl asso iated spe ies, see Appe di 7.2), contained 27 systems with more diversified vegetation, although Sedum spp. were still present in most systems. The vegetation was mainly installed by seeding on a substrate of around 9 cm thick. The SR was high (22.3 ± 8.43), with 17 species that were significantly associated with this type. The eight systems belonging to type 3, the Linaria-Galium t pe , o tai ed 30 indicator species and were characterized by the almost absence of succulent species. Installation of plants was mainly achieved by application of plug plants. Substrate depth and SR were not significantly different from the Dianthus-Thymus type.

Diversity analysis

Kruskal Wallis test results showed significant differences between the three EGR types for all functional and phylogenetic diversity measures (Fig. 7.1). The largest differences were observed for FDpg, PDf and MPD (p-values < 0.001), where values of the Sedum type were significantly lower compared to the Dianthus-Thymus and Linaria-Galium type. Differences between EGR types were also significant for FEve (p-value < 0.05), with the Sedum type showing significant higher values than the Dianthus-Thymus type. Despite the significant differences, values for FEve ranged from 0.96 to 1, suggesting a rather uniform distribution of species in trait space. The boxplots also show considerable variation in the value range of FDpg and PDf for the Dianthus-Thymus and the Linaria-Galium type.

FDpg and PDf were strongly positively correlated with SR (r s = 0.96 and 0.91 respectively; p-values < 0.001) (Fig. 7.2). So the higher the number of species in the EGR system the higher the FD and PD and the supposed positive impact on ecosystem services. Next to the correlation with SR, the correlation between FDpg and PDf appeared highly significant too (r s = 0.95; p-value < 0.001). Although systems from the Sedum type were situated at low SR, the systems of the Dianthus-Thymus and Linaria-Galium types showed variation on the X-axis, meaning that the observed variation in Fig. 7.1 can be explained by the systems variety in SR. MPD is also correlated with SR (r s = 0.76; p-value < 0.001), but FEve and SR showed no correlation. For a given SR, the values of FDpg and PDf showed some variation, indicating that a better design in terms of species selection is possible.

Maximizing functional diversity

Table 7.3 shows the combined results of FDMax1 and FDMax2. A complete table with the results of FDMax2 is included in Appendix 7.3. Species with the highest functional dissimilarity are for the Sedum type Phedimus selskianus and Prunella grandiflora, for the Dianthus-Thymus type Sedum montanum and Tripleurospermum maritimum and for the Linaria-Galium type Papaver rhoeas and Sedum album (Table 7.3a). Phedimus selskianus could for example be replaces by Sedum album, which belongs to the same group according to FDMax2 (Table 7.3b). In the same way, Tripleurospermum maritimum could be replaced by Calendula arvensis or Arenaria serpyllifolia. These species substitutions illustrate the practical aspects of FDMax2. For all the EGR types, the attained value of FDpg by using FDMax1 was higher than the initial FDpg value of the EGR type. For a Sedum type system containing eight species FDpg increased with 22%, for a Dianthus-Thymus type system containing 22 species FDpg improved 10% and for a Linaria-Galium type system with 20 species FDpg increased with 7%.

Discussion

The FD approach and EGR systems

The objective of this study was to examine EGR systems, exemplary for novel urban ecosystems, and to offer an applicable approach to achieve better ecosystem services. The role of GRs in offering a variety of ecosystem services becomes increasingly acknowledged in scientific literature (e.g. [START_REF] Carter | Vegetated roofs for stormwater management at multiple spatial scales[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]Lundholm et al., 2010;[START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF][START_REF] Gaston | Managing urban ecosystems for goods and services[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. The specific ecosystem properties on which the trait selection was based (Table 7.1) are essential to provide those services. Plant evapotranspiration and biomass are properties influencing the regulating services climate regulation and water management. Traits like flower color, flowering time and pollinator reward help to attract pollinators or provide habitat for local flora and fauna, hence contributing to the supporting ecosystem services. The provisioning component of ecosystem services is determined by the trait provisioning value, which indicates if a species is of potential value for humans in terms of medicinal use or edibility. The cultural service is determined by the G' s esthetic and amenity value [START_REF] Dakin | The Professional Design Guide to Green Roofs[END_REF]. Drought tolerance and self-sustainability are properties necessary to cope with extreme conditions typical for GR environments, and hence ensure the resilience of the system. This is essential for the amenity value, and thus the cultural service, of the GR [START_REF] Getter | The Role of Extensive Green Roofs in Sustainable Development[END_REF][START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF]. Finally, structural diversity, determined by variation in multiple plant traits such as height, architecture and longevity, is an important aesthetic determinant, but contributes also highly to the other services (Cook-Patton & Bauerle, 2012; [START_REF] Oudolf | Planting: A New Perspective[END_REF]. Table 7.1 clearly indicates that the selected plant traits relate to different and often multiple ecosystem services. Therefore, we assumed that the ecosystem services are adequately represented by the selected traits.

Similar to [START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF], who proposed an ecological oriented GR typology, the EGR systems could be classified both on technical system properties (Table 2) and diversity values (Fig. 1). Succulent species, characteristic for the systems in the Sedum type, are the most commonly applied species on EGRs. Their specific photosynthesis pathway (CAM) makes them better suited to withstand extreme conditions on roofs [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF], and they have the greatest potential in terms of applicability on existing roofs, as they are easy to install and relatively light weight due to shallow substrate depth requirements. However, the FD of Sedum systems was significant lower compared to the other two EGR types, but we showed that FD could be improved with 22% by using FDMax1. These species-poor systems can thus become ecologically more interesting by considering alternative species. Although the FD analysis demonstrated that species-rich systems (the Dianthus-Thymus and the Linaria-Galium type) have higher FD values and hence potentially provide ette e os ste p ope ties f. Ga feldt et al., 2013), we showed that even in these systems the FD could be increased up to 10%. The higher biomass and leaf area index of plants in these EGR types result in a higher evaporative capacity which improves stormwater management and temperature regulation (Lundholm et al., 2010). The structural diversity of these systems also supports the cultural, supporting and regulating ecosystem services [START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF]. However, more diverse systems do impose constraints like a higher weight and cost, and more complexity and maintenance (cf. [START_REF] Snodgrass | The Green Roof Manual. A Professional Guide to Design, Installation, and maintenance[END_REF]. As less succulent species are present in the species-rich GR systems, the ability of these systems to tolerate drought could be questioned. In a GR context, drought tolerance is often equated to only succulence. However, drought tolerance is actually achieved by a combination of different traits, of which succulence comprises only one. For example, traits related to leaf and root morphology, and photosynthetic pathway, all contribute to a pla t s tole a e to d ought see e.g. Ma kesteij & Poorter, 2009; Chapter 3). We only considered plant traits related to GR ecosystem services, but still multiple traits related to drought tolerance were included in the FD analysis (e.g. leaf area, leaf phenology, photosynthetic pathway). So next to opting for succulents and hence drought tolerant species (trait convergence strategy), selecting plants that differ in their traits can also contribute to a drought tolerant vegetation. The tradeoff between systems with succulents and the more species-rich and diverse systems should be considered carefully and placed in the context of the main purpose and construction limitations of the respective design project.

Considerations for further research

Advantages of the FD and PD approach are that they account for trait value or evolutionary complementarity between species, that diversity is measured at all hierarchical scales and that the resulting values are continuous [START_REF] Petchey | Functional diversity (FD), species richness and community composition[END_REF]. Although being valuable diversity measures [START_REF] Petchey | How do different measures of functional diversity perform[END_REF][START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF][START_REF] Devictor | Spatial mismatch and congruence between taxonomic, phylogenetic and functional diversity: the need for integrative conservation strategies in a changing world[END_REF]Mouchet et al., 2010;[START_REF] Petchey | How do different measures of functional diversity perform[END_REF], there might also be some limitations when applied to specific examples. The used EGR systems for instance only provided information on the initial species presence/absence, but not their relative abundance and vegetation dynamics once installed on a roof. This information would have resulted in more detailed clustering results and it would have affected the calculated FD and PD, and the FD maximization procedures (FDMax1 and FDMax2). However, this limitation does not fundamentally alter the approach adopted here, as for species selection for designing urban green elements, one often starts from simple species lists. Adding species that establish spontaneously and that have potential to increase the FD of the GR system is an interesting option. Nevertheless, more research is needed in order to ensure that those additional species will be successful on GRs in the long te , as the a e ot o side ed t ied-and-t ue as the othe spe ies p ese t i the initial EGR systems. Another limitation regards the plant trait values, which were not available for species varieties and cultivars. The species lists of the EGR systems were thus reduced to the species level, therefore possibly causing some information loss as plant traits inevitably show some variability (cf. De Bello et al., 2011). Also, as indicated by the % missing values in Table 7.1, it was impossible to find specific trait information for certain species. Therefore, further trait screening of plant species and their intraspecific taxa is essential (cf. [START_REF] Pakeman | Functional trait metrics are sensitive to the completeness of the spe ies t ait data?[END_REF].

The scientific evidence that biodiversity, species functional traits and their phylogenetic relationships are linked with ecosystem properties is increasing [START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF][START_REF] Isbell | High plant diversity is needed to maintain ecosystem services[END_REF][START_REF] Lavorel | Using plant functional traits to understand the landscape distribution of multiple ecosystem services[END_REF][START_REF] Cardinale | Biodiversity loss and its impact on humanity[END_REF][START_REF] Quijas | Plant diversity and generation of ecosystem services at the landscape scale: expert knowledge assessment[END_REF][START_REF] Pinto | Linking biodiversity indicators, ecosystem functioning, provision of services and human well-being in estuarine systems : Application of a conceptual framework[END_REF]. Whether this also fully applies for novel urban Chapter 7 149 ecosystems has not yet been proven, as the literature on FD and PD and their effects on ecosystem properties of for example GRs is still very limited. Lundholm et al. (2010) focused on summer roof cooling and water capture and concluded that these services can be improved by combining different life forms. However, for some ecosystem services (e.g. roof cooling), species-poor GR systems outperformed the ones with high diversity, hence stressing the need for further investigation of biodiversity-ecosystem functioning relationships in GR ecosystems (Lundholm et al., 2010). Furthermore, our study showed that EGR systems with high FD are also the ones that require deeper substrates. This result is in line with findings of [START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF], who pointed at an important role of substrate depth on GR plant diversity.

Next to substrate depth, also substrate type (e.g. the water holding capacity or amount of organic matter) and local GR parameters (e.g. age and height) can also greatly influence the vegetation composition in the long term, hence influencing the FD [START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF]. So further research is needed to elucidate the relative contribution of substrate and GR properties to the FD of the GR system and linked to that, the provisioning of ecosystem services.

PD indices are assumed to be a valuable proxy for FD [START_REF] Flynn | Functional and phylogenetic diversity as predictors of biodiversity-ecosystem function relationships[END_REF]Srivastava et al., 2012;[START_REF] Gasc | Assessing biodiversity with sound: Do acoustic diversity indices reflect phylogenetic and funcitonal diversities of bird communities?[END_REF]. PD accounts for species evolution and hence encompasses many additional and often unmeasured traits that may be important to determine ecosystem functioning [START_REF] Cadotte | Using Phylogenetic, Functional and Trait Diversity to Understand patterns of Plant Community Productivity[END_REF][START_REF] Flynn | Functional and phylogenetic diversity as predictors of biodiversity-ecosystem function relationships[END_REF][START_REF] Srivastava | Phylogenetic diversity and the functioning of ecosystems[END_REF]. A possible problem here might be that some species were treated as polytomies because of unresolved relationships. Also, the evolutionary resolution of the phylogenetic tree was not detailed enough, which caused us to rely on FDpg for the di e sit a i izatio p o edu e. This i o t ast to the Noah s A k P o le o which our approach was inspired and in which the expected PD of the considered species is maximized (Billionnet, 2013). However, we believe that the use of FDpg is justified, as the correlation between FDpg and PDf proved to be highly significant.

Recommendations for urban ecosystem design

The two proposed methods, delivering an exact species list (FDMax1) and perhaps a more practical one that offers alternative species (FDMax2), illustrate how current GR systems can be adapted to maximize FD and presumably also the overall ecosystem properties and services. The focus of this study was on EGRs, but the methods used are generic and offer a framework for improved design of a broad range of novel ecosystems, particularly in an urban context. For a new project, whether that be for instance, abandoned industrial sites, agricultural remnants, parks or gardens, one could start from an inventory of the species, eventually supplemented with potential interesting species considered for introduction on the site and then follow the approach adopted here. Clustering can yield insight into available or desired vegetation types. Next, databases should be consulted for relevant traits of the plant species, representing the range of desired ecosystem properties, after which FD and PD analyses (eventually per vegetation type) will provide insights on the ecological properties. The application of FDMax1 and FDMax2 will then indicate the most optimal species combination in terms of ecosystem properties. It is also possible to focus the approach on plant traits that relate to a specific ecosystem service and therefore select plant species that maximize this service (cf. [START_REF] Lundholm | Mini-Review: Habitat analogues for reconciliation ecology in urban and industrial environments[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF].

The approach adopted in this study only focused on FD and PD and how we can use these measures to optimize species selection for GR purposes. We acknowledge that as more empirical research becomes available, other possibilities may arise that also offer ways to improve species selection for GRs, for example by looking at specific plant traits that are related to one or multiple ecosystem services. Also, the FD maximization methods proposed in this study should be interpreted with caution, as GRs with high FD will only offer optimal ecosystem services when the initial high plant trait diversity can be conserved. This however, is not guaranteed, as some scientific studies indicate a decline in SR of the GR over time (e.g. [START_REF] Rowe | Effect of green roof media depth on Crassulacean plant succession over seven years[END_REF][START_REF] Lundholm | Leaf and Life History Traits Predict Plant Growth in a Green Roof Ecosystem[END_REF]. Despite the number of current drawbacks of the FD and PD approach, this study offers a way to design GRs with high plant trait diversity. It also highlights that currently used EGR systems differ greatly in their FD and PD.

Conclusion

EGR systems, as model examples of novel urban ecosystems, offer a variety of ecosystem services. We assume that ecosystem properties and services are positively correlated with FD and PD. The FD and PD analyses highlighted that species-rich EGR systems offer higher FD and PD than the commonly used Sedum type, but also that the FD of the other types can be improved. Two methods are proposed that both maximize FD and thus offer a foundation for improved design of these novel ecosystems. These result in optimal spe ies lists, a e a t o e a d o e that also offers alternatives. The approach is generic and can thus also be used for other novel ecosystems, in particular urban green elements. It also offers the possibility to target specific ecosystem properties or services. By carefully considering species that maximize FD, the design of urban green infrastructure elements can become a more sustainable and vital tool to improve urban life quality.

Chapter 8. General discussion The first part of the thesis was based on the ha itat te plate h pothesis . I essence, a habitat template can be considered as a set of biotic and abiotic conditions that define a particular (semi)-natural habitat and the species that can live in it [START_REF] Southwood | Habitat, the templet for ecological strategies[END_REF][START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF]. There is a common perception that cities are unnatural and do not have any analog in nature [START_REF] Larson | The Urban Cliff Revolution: New findings on the origins and evolution of human habitats[END_REF]. However, as indicated by [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF], urban areas do harbour different habitat templates that mimic natural habitats. For example, urban lawns do contain conditions that allow many grassland species to live in them. Similarly, urban pavements and walls provide niches for many rock outcrop species, and at tree bases, mainly species originating from natural forest and riparian habitats can be found. So engineered urban ecosystems (including GRs) are novel, as they originate from human interference and the ecosystem was not present in that area before, but replicate natural habitats at the same time [START_REF] Hobbs | Novel ecosystems: theoretical and management aspects of the new ecological world order[END_REF][START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Hobbs | Novel ecosystems: implications for conservation and restoration[END_REF][START_REF] Kowarik | Novel urban ecosystems, biodiversity, and conservation[END_REF].

The G' i dust ai l elies o t ied-and-t ue pla ts ith d ought tole a t features (mainly succulents), but there is a growing interest to incorporate other native plants in GR design [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF]. According to the habitat template hypothesis, potential species for EGRs can be found in natural habitats with similar biotic and abiotic conditions as on EGRs. These natural template habitats comprise mostly rocky environments, free draining dunes, open areas on very shallow substrates, and limestone pavements [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF]. Based on the habitat template concept, [START_REF] Sutton | Prairie-based green roofs: Literature, templates, and analogs[END_REF] studied natural prairie and grassland communities and found that there are many species originating from those specific habitats that can survive on GRs (e.g. Bouteloua gracilis, Dalea purpurea, Geum trifolium, Allium cernuum, Huechera richardsonii).

The goal of Chapter 2 was to look at possibilities of native vegetation on shallow soils and rocky habitats in (sub) Mediterranean France. The Mediterranean region with its exceptional diversity of plant species [START_REF] Médail | Hot-Spots analysis for conservation of plant biodiversity in the Mediterranean Basin[END_REF][START_REF] Lavergne | Fingerprints of environmental change on the rare mediterranean flora: a 115-year study[END_REF] contains a lot of these habitats, so we hypothesized that it should be possible to find drought-adapted, native plant species that could thrive on EGRs. (Semi)-natural open habitats in Southern France were described and variation in species composition in relation to environmental factors was analyzed. In a total of 20 locations, 372 local plant species were found, and together with species lists found in literature describing specific Mediterranean habitats, the list was complemented with 261 species, resulting in a total Mediterranean plant list (MEDPL) of 633 species. Species in the MEDPL could be grouped into four major vegetation types with distinct climatic, geographic and soil-related properties. Next, the MEDPL list was compared with a list of species commonly used on GRs in NW Europe (GRPL). We found that 79% of the Mediterranean species are currently not used on GRs and hence comprise a potential source for urban greening. A first rough screening was performed based on 10 general plant traits associated with survival on EGRs (including life form and Grime strategy). 28 Mediterranean species not yet used on GRs meet at least 6 out of 10 important attributes, indicating good potential.

We pointed at some ecological recommendations that need to be considered during GR design. Sedum spp. comprise a good option for EGRs in a Mediterranean climate. Succulents have been found to facilitate growth of herbaceous species under abiotic stress [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. Besides succulents, different life forms should be incorporated. This recommendation has already been proposed by other GR researchers (e.g. Lundholm et al., 2010;MacIvor et al., 2011;[START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF]. The use of different life forms has been shown to provide better ecosystem functioning and resistance to environmental stress, mainly due to niche complementarity and facilitation (Lundholm et al., 2010). Grasses and herbaceous species are less drought tolerant compared to succulents, but are more effective for evapotranspiration and thus stormwater retention [START_REF] Wolf | Water uptake in green roof microcosms: Effects of plant species and water availability[END_REF]. Furthermore, highly diverse GRs have a higher survival probability and are more aesthetically pleasing even under dry conditions (Nagase & Dunnett, 2010). Other new potential species include Bromus rubens (annual grass), Ceterach officinalis (drought tolerant fern), Linum tenuifolium (drought tolerant perennial) and Crassula tillaea (annual succulent). The latter species has only been reported exotic and invasive in some provinces of California, USA; Invasive Plant Atlas of the US, www.invasiveplantatlas.org; consulted on December 1, 2014 ).

Annual species are currently rarely regarded for GR purposes but in the habitat template context, this life form is an important part of Mediterranean vegetation and should be considered in GR design. They contain properties that can guarantee GR performance in regions with harsh environmental conditions. More specifically, seed production is added to the seed bank, hence forming a buffer against eventual gap formation should other herbaceous plants die off [START_REF] Madon | The ecological significance of annuals on a Mediterranean grassland (Mt Ventoux, France)[END_REF]. Studies demonstrating the potential of annual species in a GR context were until recently very limited. Nagase and Dunnett (2013b) studied the establishment of an annual meadow on EGRs and concluded that in the UK, annual seed mixtures provide many benefits (e.g. long flowering, quick establishment, easy installation and little maintenance). [START_REF] Benvenuti | Wildflower green roofs for urban landscaping, ecological sustainability and biodiversity[END_REF] demonstrated that wildflower GRs in a Mediterranean context (Italy) are characterized by periodic senescence of the vegetation, but that they are nevertheless valuable for biodiversity and landscape improvement. Our study specifically pointed at the value of annual species in the context of the GR habitat template and the need for more profound research on this topic. Also in a climate change context, in which more extreme weather events are predicted, annuals comprise a valuable adaptive strategy by bridging the harsh period as seeds.

Bryophytes are also promising, as they are part of the vegetation (sometimes covering > 50% of a plot; Table 2.1). These non-vascular plants have been shown to be valuable in a GR context (e.g. [START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF][START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF]. First of all, they can colonize the substrate spontaneously through spores (sometimes vegetative propagules) and can survive extreme drought [START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF][START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF]. The drought tolerance capacity is a result of both desiccation tolerance and avoidance [START_REF] Proctor | The bryophyte paradox: tolerance of desiccation, evasion of drought[END_REF][START_REF] Glime | Michigan Technological University and the International Association of Bryologists[END_REF]. Desiccation tolerance involves dry tissue survival and restart of photosynthesis upon rehydration through a suit of interacting mechanisms (e.g. cellular protection through ABA and osmotically active substrates; accumulation of dehydrins and osmotically active sugars; [START_REF] Lakatos | Chapter 5. Lychens and Bryophytes: Habitats and Species[END_REF]. In desiccation avoidance, the plant avoids drying out through structural adaptations and by specialized life cycle stages (e.g. dormancy; dying while leaving spores behind for the next generation; altered phenology; requirement of a dry season to let the operculum burst; [START_REF] Lakatos | Chapter 5. Lychens and Bryophytes: Habitats and Species[END_REF]. Small size, thick cortices, hydrophobic surfaces, spore formation, dormancy, leaf orientation, hair points, mucilage and dense rhizoids are all properties that contribute to the drought tolerance strategy of bryophytes [START_REF] Proctor | The bryophyte paradox: tolerance of desiccation, evasion of drought[END_REF][START_REF] Glime | Michigan Technological University and the International Association of Bryologists[END_REF][START_REF] Lakatos | Chapter 5. Lychens and Bryophytes: Habitats and Species[END_REF]. They only require a thin substrate, can store a considerable amount of rainwater and facilitate the establishment and survival of vascular plants in extreme conditions (e.g. [START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF][START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF]. The facilitation property might explain the positive correlation between the bryophyte cover and the species richness (cf. Fig. 2.3). It has been suggested that bryophytes could offer an alternative for an additional water retention layer [START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF]. Furthermore, they play a significant role in ecosystem functioning in terms of carbon and nitrogen sequestration, thermal and hydrological soil regime, and provisioning of microhabitats for small organisms [START_REF] Glime | Michigan Technological University and the International Association of Bryologists[END_REF][START_REF] Lakatos | Chapter 5. Lychens and Bryophytes: Habitats and Species[END_REF]. Moss species present on the experimental EGR in Heverlee after two growing seasons include Silky Forklet-moss (Dicranella heteromalla (Hedw.) Schimp.), Redshank (Ceratodon purpureus (Hedw.) Brid.) and Umbrella liverwort (Marchantia polymorpha L.). Other potential moss species for both exposed and sheltered EGRs include Black Rock-moss (Andreaea rupestris Hedw.), Rough-stalked Feather-moss (Brachythecium rutabulum (Hedw.) Schimp.), Silky Wall Feather-moss (Homalothecium sericeum (Hedw.) Schimp.) and Frizzled Crisp-moss (Tortella tortuosa (Hedw.) Limpr.; [START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF]. The positive aspects of bryophytes should be interpreted with caution, as only few studies have assessed this topic in a GR context [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF][START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF], and bryophyte cover can have both positive and negative effects on species establishment under natural conditions [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF]. For example, the seedling germination of some short lived forbs in limestone grasslands (e.g. Carlina vulgaris, Euphrasia officinalis and Linum catharticum) was negatively correlated with the bryophyte cover, although seed mortality decreased when the bryophyte cover was high [START_REF] Keizer | Effects of bryophytes on seedling emergence and establishment of short-lived forbs in chalk grassland[END_REF]. Also when the environment is dry, seedling emergence, especially of smallseeded species (e.g. Arenaria serpyllifolia, Veronica spicata) is rather inhibited by a bryophyte cover [START_REF] Zamfir | Effects of bryophytes and lichens on seedling emergence of alvar plants: evidence from greenhouse experiments[END_REF]. Furthermore, spontaneous colonization of GR substrates is limited due to the presence of bryophytes [START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF].

In conclusion, the habitat template concept applied in Chapter 2 provided interesting outcomes. A comprehensive list of species in the Mediterranean region of southern France with a potential for use on EGRs was compiled. Evidence is provided that natural habitats can indeed inspire and improve the design of GRs in the (sub) Mediterranean region.

A plant trait approach for potential EGR species selection

Research question 2: How can the plants in the MEDPL be ranked according to their potential use for EGRs by making use of their plant traits?

In a climate change context, occasional, unpredictable drought and heat waves, heavy rainfall, flooding and cyclones are predicted [START_REF] Jentsch | Research frontiers in climate change: effects of extreme meteorological events on ecosystems[END_REF]Mooney et al., 2009). To cope with these perturbations, a GR ecosystem should be stable, which means that it should be both resilient and resistant. There is still discussion on the relationship between biological diversity and ecosystem stability (Diaz & Cabido), but evidence is growing that functional diversity (FD), and more spe ifi all the a ge a d alue of the pla t s fu tio al t aits, a ai tai ecosystem stability in the long term (Diaz & Cabido, 2001;[START_REF] Elmqvist | Response diversity, ecosystem change, and resilience[END_REF][START_REF] Hooper | Effects of biodiversity on ecosystem functioning: A consensus of current knowledge[END_REF][START_REF] Perrings | Ecosystem Services for[END_REF]. FD comprises two concepts, namely functional redundancy and functional insurance. According to the functional redundancy concept, the loss of one or several species does not affect an ecosystem process (e.g. primary productivity or nutrient cycling) significantly as there are still other species that guarantee this ecosystem process (Diaz & Cabido, 2001). In the functional insurance concept, a greater functional richness in the community implies that less species are necessary to buffer the respective ecosystem against perturbations (Diaz & Cabido, 2001).

In Chapter 3, we elaborated further on the results of Chapter 2 in which new insights on suitable native species for use on EGRs in the (sub) Mediterranean climate were explored. Both the plant species lists MEDPL and GRPL from Chapter 2 were used. The ultimate purpose of Chapter 3 was to reveal the most important attributes of GR plants by making use of a plant trait analysis, i.e. a correlation analysis of the species in the GRPL and a list of their functional traits and utilitarian aspects. The choice for functional plant traits can be explained from an ecosystem stability viewpoint, as explained in the previous paragraph. As GRs are still engineered ecosystems, more utilitarian aspects (e.g. flower color, flowering period, commercial availability) may be essential too, as these will point out the species with e.g. aesthetic appeal or that meet specific design considerations.

In a preparatory step, relevant traits related to drought adaptation and/or selfregulation (considered as key-factors essential for survival on EGRs under dry conditions) were identified through an extensive literature search. The utilitarian aspects were selected through a similar approach. This way, 67 plant traits (53 functional and 14 utilitarian) were assembled for the 985 unique plant species (MEDPL: 633 species; GRPL: 483 species of which 131 also comprised in the MEDPL). The TRY Initiative (Kattge et al., 2011) was then screened for trait values. The goal was to highlight the most important traits for GR plants in order to use these traits for appropriate species selection, so the correlation analysis was carried out on the GRPL species*trait matrix. Traits related to frequently applied GR species were efe ed to as p i a t aits = , a d t aits elated to the p i a t aits e e alled de i ed t aits = . The utilita ia t aits did ot e d up i the p i a t ait list, but 6 out of 7 were correlated to the primary traits and were hence placed in the derived trait list (Table 3.4). As the primary trait list was composed of functional traits only, we conclude that drought tolerance properties are more important for EGR species selection than aesthetic and design considerations expressed by the utilitarian traits.

Together with exclusion criteria (n = 5), both primary and derived traits were incorporated into a screening tool. As an example, this tool was illustrated on a subset of the MEDPL, which identified 34 new potential GR species. 35% of these species were annuals, which are, as was discussed in part 1.1 of the general discussion, a promising life form that has until now rarely been considered on EGRs. Also Sedum spp. (S. album, S. acre) and Mediterranean species already used on GRs (e.g. Thymus praecox, Draba aizoides, Hyppocrepis comosa, Helianthemum nummularium , esult i a high s ee i g s o e "" / .

Although the plant trait approach described here has some limitations (mainly due to the high amount of missing trait values), it is useful as it offers both new possibilities for GR design in the Mediterranean and it can help GR companies in the temperate climate to adjust GR design to cope with upcoming climate change. The screening tool is a useful framework for making substantiated decisions for appropriate GR design in terms of species choice.

Testing potential Mediterranean plants in different climates and under different experimental setup

Research question 3: Next to theoretical evidence, can the potential of some Mediterranean plants for EGRs be confirmed experimentally? Do other factors including climate, substrate depth, water retention layer (WRL) and exposition affect the suitability of these plants?

The screening tool developed in Chapter 3 delivers a potential list and should ensure EGR performance and sustainability, yet the ultimate prove should come from experimental outdoor trials. Indeed, the ultimate purpose of Chapter 3 was to reveal the most important attributes of EGR plants by use of a plant trait analysis. However, next to important attributes, the potential species also have specific habitat requirements (e.g. climate, organic matter, nutrient content, substrate depth) and the GR structure design will need customization to ensure good plant health. Because habitat requirements were not considered, some odd species have been proposed (e.g. Gymnadenia odoratissima, Ophrys insectifera, Succisa pratensis, Peucedanum oreoselinum). They have suitable trait values, but their suitability under real EGR conditions in the Mediterranean is questionable. So the traits and niches do not correspond 100%, which is why the screening tool should be interpreted with caution, and potential species need to be tested under EGR conditions. Chapter 4 discusses the results of a two-year experiment on unirrigated EGRs which were constructed in both a Mediterranean (Avignon, France) and a temperate climate (Heverlee, Belgium). In order to gradually increase GR biodiversity, the EGR vegetation should comprise different life forms, and both novel and already applied species. We tried to test as many novel potential species as possible but the supply of these species in nurseries was limited as general nurseries offer mainly horti ultu al a d t ied-and-t ue pla ts. The efo e, the u e of tested spe ies was here limited to 11 novel species and 7 already used GR species. Nevertheless, the selected species belonged to different life forms (annuals, perennial herbs, succulents and geophytes) and varied in their screening score (SS), so the experiment offered possibilities to demonstrate multiple recommendations offered in Chapter 2 and 3.

The experimental plots installed in Avignon and Heverlee differed in exposition (sheltered north-side of the roof or exposed south-side) and structure (substrate depth of 5 or 10 cm and presence/absence of a WRL). Major differences were found between the locations for both cover and abundance data, which could be attributed to the divergent climatic conditions (Fig. 8.1). Cover values in Avignon reflect the summer drought and following rainy autumn periods. Only for some variables and periods, the EGR performance appeared significantly better in the sheltered plots.

Placing the EGR in a sheltered location is hence not a major requirement to obtain a successful EGR. The structure type on the other hand appeared of much higher importance. Overall, the deepest substrate of 10 cm with a WRL resulted in the highest cover and abundance values, species richness and Simpson Index values throughout the experiment (Fig. 8.1).

Of the tested species, Lagurus ovatus is recommended on EGRs in the Mediterranean. Euphorbia cyparissias can be used in temperate climates on EGRs if a WRL is present. Sedum spp. should be considered as part of the EGR vegetation as they performed very well in both locations and because of their nursing effect (cf. [START_REF] Butler | Sedum cools soil and can improve neighboring plant performance during water deficit on a green roof[END_REF][START_REF] Boivin | Effect of artificial substrate depth on freezing injury of six herbaceous perennials grown in a green roof system[END_REF][START_REF] Farrell | Green roofs for hot and dry climates: Interacting effects of plant water use, succulence and substrate[END_REF]. We also advise Alyssum alyssoides, Lobularia maritima and Silene conica for use on EGRs in both locations. In Heverlee, next to Dianthus superbus, Sideritis hyssopifolia and Linum bienne are good performing species for EGRs with a WRL. The geophytes Allium sphaerocephalon and Iris lutescens are advised as an accent plant, as they only flower for a short time. Both bryophytes and weeds established spontaneously over the two years, especially in the sheltered plots. If one wants to establish an EGR dominated by a bryophyte layer, a very shallow substrate (± 5 cm) without a WRL should be opted for, and this kind of EGR is more certain to succeed in the te pe ate li ate. Weeds a e ostl ude al spe ies p ese t i the EG's environment. They can germinate in these novel urban habitats, especially during the establishment phase as competition with the installed vegetation is not yet going on, but when climatic conditions become too extreme (either high temperature and prolonged drought or very low temperatures), the weeds die off. However, the next growing season or autumn period, new seeds can be recruited on the EGR. The weedy species therefore represent a natural and inevitable part of the EGR vegetation dynamics. Nevertheless, the probability that weedy species will ever become a dominant part of the EGR vegetation is very low or even inexistent. In order to offer a small scale verification of the reliability of the screening method developed in Chapter 3, we compared the overall species presence in the two locations with the species respective screening score (SS), which is an indication of their potential for success on Mediterranean GRs based on their plant traits (cf.

Chapter 3). Although the species used in this study originate from natural Mediterranean habitats with similar characteristics as on EGRs, variability between the spe ies "" as ery high, ranging between 2 and 25 on a maximum of 28 (Table 8.1). Therefore, it was believed that some species would perform much better than others. Indeed, eight species performed in accordance to their screening score and six species performed even better than expected. The SS for C. carduncellus and I. lutescens was unknown due to lack of trait information, so no statement on these species could be made. H. nummularium, performed worse than expected, as both the SS and seed viability were high but germination and survival were low in both locations. Also for P. prolifera, better performance was expected, as this species only germinated in Heverlee at the end of the experiment despite its rather high SS of 14. S. acre and S. album have a very high SS and they both constitute an important part of the total cover. Other species that performed according to their SS were A. sphaerocephalon, A. alyssoides, E. verna and E. cyparissias. Their SS was medium (10-12) but they germinated and survived in one or even both locations. Two other species also performed as expected, as no germination was recorded for C. acinos, and P. afra did not reoccur after the first growing season in Avignon, which is in accordance with their very low SS of 2. The species that performed better than expected all had a low SS (2-10), but performed well in one or both locations. L. ovatus (SS = 2) performed very well in Avignon, L. bienne (SS = 2) germinated and survived in both locations, S. hyssopifolia (SS = 2) was recorded in Heverlee. Finally, S. conica (SS = 9), D. superbus (SS = 10) and L. maritima (SS = 8) did even become part of the dominant species in one or both locations. In summary, most species performed according to what was expected from their respective SS, and other species performed even better than expected. So the screening tool is a good method to indicate species potentiality for use on EGRs in climatic regions with frequent summer drought, but we stress that the tool should be used with caution as some species with a relatively high SS can also perform worse than expected. Also, the correlation between the SS and the species performance, as calculated by means of a Spearman correlation, was not significant (Avignon: r s = 0.126, p-value = 0.642; Heverlee: r s = 0.396, p-value = 0.129; N = 16).

In Chapter 4 we offered specific design and species recommendations for both the Mediterranean and temperate climate, which can be used as a base for further testing or for immediate application. An irrigation system is commonly provided on GRs in order to keep the vegetation alive and aesthetically pleasing as long as possible throughout summer. Watering the plants on critical moments (after for example 3-5 days without rain) is generally believed to prevent the maintenance time and cost of reseeding the next year. We would like to stress that the EGRs in this experiment were unirrigated and that the experiment was conducted over the course of only two years. GR vegetation development is amongst others dependent on weather conditions, which are instable and uncertain in the Mediterranean, so the GR vegetation is not expected to ever reach a stable and balanced state.

Although summer drought is a general attribute of the Mediterranean climate, it was observed that the yearly weather conditions were rather variable (mild, rainy spring and three months of summer drought in 2013; dry spring and extended summer drought in 2014; Fig. 4.2). The annual plant species applied in this study did indeed suffer from high temperatures and drought, but they were expected to wilt when environmental conditions became too harsh, as this is a distinctive feature of this particular life form. Even after the long summer drought of 2014, new seeds were able to germinate after autumn rain, which proves that these annual species are suitable for use on EGRs in the Mediterranean, even under unirrigated conditions. Their presence in the GR vegetation offers an insurance that the GR will revive after a dormant state during summer. Next to the annual species, the Sedum spp. and I. lutescens resumed their growth upon autumn rain. A. sphaerocephalon and the perennial herbs (except L. maritima) suffered highly from the elevated temperatures and drought, hence not allowing them to germinate or leading to premature wilting before the plants were able to set seed. Envisaging an irrigation system would be beneficial for these plants during the establishment phase and upon summer drought in order to provide a sufficient amount of water to allow them to germinate and grow and keep them alive until they are ready to set seed for the next year. However, as the EGR should be sustainable in the long term, irrigation practices (especially water source, time and quantity of watering) should be adapted to the local climatic conditions (see Chapter 6).

GRs have been demonstrated to provide a suitable habitat for many organisms whose natural habitats are often endangered [START_REF] Butler | Native plant enthusiasm reaches new heights: Perceptions, evidence, and the future of green roofs[END_REF], and they can function as stepping stones for many high-mobility, and eventually also low-mobility arthropod species [START_REF] Madre | A comparison of 3 types of green roof as habitats for arthropods[END_REF][START_REF] Braaker | Habitat connectivity shapes urban arthropod communities: the key role of green roofs[END_REF]. [START_REF] Colla | Can green roofs provide habitat for urban bees (Hymenoptera: Apidae)? Cities and the Environment[END_REF] showed that GRs have potential for pollinator conservation, as a variety of bee species use the GR for foraging or nesting. This is important for ecological restoration pu poses, as the p ese e of polli ato s akes the G' look o e atu al a d o t i utes to seed p odu tio a d he e the G' s sustai a ilit i the lo g te [START_REF] Colla | Can green roofs provide habitat for urban bees (Hymenoptera: Apidae)? Cities and the Environment[END_REF][START_REF] Braaker | Habitat connectivity shapes urban arthropod communities: the key role of green roofs[END_REF]. A GR with diverse and mainly native vegetation offers a variety of nesting habitats and may further attract pollinators (Tonietto et al., 2010). Our experimental EGRs attracted many animal species (Fig. 8.2). Bombus spp. and Coleoptera spp. (e.g. Coccinellidae) lingered near the flowering S. album and A. sphaerocephalon. Lepidoptera larvae, Formicidea and Caelifera spp. were also noted on the EGRs. Considering predators, lizards (Podarcis muralis spp.) did inhabit the EGRs. Birds (e.g. Pica pica and Corvus monedula) visited the EGRs regularly, as indicated by the small gaps in the substrate where they foraged. This is often considered a problem as birds can adversely affect GRs, especially during the establishment phase, as they peck at seeds and small plug plants [START_REF] Fernandez-Canero | Green roofs as a habitat for birds: A review[END_REF]. A possible solution consists of covering the newly installed GR temporarily with a specialized bird net that does not entangle the birds or the plants. In addition, this kind of protection can prevent domestic cats from damaging the GR. Although cats have difficulties to reach GRs in most cases, some GRs are accessible (including the experiment in Avignon), and cats like to use the loose GR substrate as a litter tray. Until the vegetation has covered the GR sufficiently, a net is an appropriate solution if the presence of cats is considered a problem.

People should be aware that even when EGRs do not appear very attractive during part of the year when the vegetation is dry, their value as a habitat for local flora and fauna cannot be neglected (cf. [START_REF] Brenneisen | Space for Urban Wildlife: Designing Green roofs as Habitats in Switzerland[END_REF][START_REF] Colla | Can green roofs provide habitat for urban bees (Hymenoptera: Apidae)? Cities and the Environment[END_REF][START_REF] Madre | Green roofs as habitats for wild plant species in urban landscapes: first insights from a large-scale sampling[END_REF]. Other GR benefits like stormwater retention and mitigation of the urban heat island effect are also guaranteed during summer, because of the major role that the GR substrate plays in providing those benefits.

Factors for improved green roof performance

Effects of organic matter and mycorrhiza on green roof vegetation development

Research question 4: Plant performance can also be enhanced by adding organic matter (OM) and arbuscular mycorrhizal fungi (AMF) to the growing medium. Is this also the case for plants developing under EGR conditions?

In Chapter 5, the main focus, which was the selection of appropriate GR vegetation in Chapter 2-4, is now shifted to the GR substrate. GR substrates have specific characteristics, as they have to be light weight, shallow, and should only contain a relatively low amount of nutrients [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Fll | Guidelines for the Planning, Construction and Maintenance of Green Roofing -Green Roofing Guideline[END_REF]. This is challenging for GR plant growth and survival. It has often been suggested that adding OM or AMF could overcome these limits.

OM adds nutrients to the substrate and is known to increase nutrient exchange capacity and to improve soil structure and water retention [START_REF] Friedrich | Principles for selecting the proper components for a green roof growing media[END_REF][START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF][START_REF] Molineux | Characterising alternative recycled waste materials for use as green roof growing media in the U[END_REF]. However, it comes with certain disadvantages as OM decays over time which causes shrinkage of the substrate volume, and decreases water and nutrient-holding capacity [START_REF] Friedrich | Principles for selecting the proper components for a green roof growing media[END_REF][START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF][START_REF] Rowe | Green roofs as a means of pollution abatement[END_REF]. Instead of adding OM, AMF, which are symbiotic fungi (Phylum Glomeromycota) that form mutualistic associations with plant roots of most plant species, could be considered [START_REF] Parniske | Arbuscular mycorrhiza: the mother of plant root endosymbiosis[END_REF][START_REF] Bonfante | Plants, Mycorrhizal Fungi, and Bacteria: A Network of Interactions[END_REF][START_REF] Willis | The Ecology of Arbuscular Mycorrhizal Fungi[END_REF]. AMF-root associations are known to provide multiple benefits to plants. Plant physiology is influenced, as expressed by improved mineral nutrition and plant development (cf. [START_REF] Bethlenfalvay | Mycorrhizae and Crop Productivity[END_REF]Azcón-Aguilar & Barea, 1997).

Mainly phosphorus (generally only available in low quantities in natural soils) and nitrogen uptake are improved in presence of AMF [START_REF] Smith | Roles of Arbuscular Mycorrhizas in Plant Nutrition and Growth: New Paradigms from Cellular to Ecosystem Scales[END_REF], 2012). Plant growth and development is indirectly influenced because of a stabilized soil structure (Azcón-Aguilar & Barea, 1997). AMF also protect plants against multiple biotic and abiotic stressors (e.g. nutrient deficiency, drought, salinity, pollution, pathogens; Azcón-Aguilar & Barea, 1997).

Since the nineties, the mycorrhizal technology has developed considerably and many (urban) horticultural practices are now available. For example, vegetables, fruit crops, perennial herbs, shrubs and trees can be inoculated with suitable commercial inoculum (e.g. Azcón-Aguilar & Barea, 1997;[START_REF] Feldmann | Symbiontentechnologie in der Praxis: Arbuskuläre Mykorrhiza im Gartenbau[END_REF]. This enhances seedling growth, reduces the phosphate requirement, and increases plant survival and development. Furthermore, plants become better resistant to fungal root pathogens and abiotic stresses. The inoculum also results in earlier flowering and fruiting, better rooting and fruit production (cf. Azcón-Aguilar & Barea, 1997). More recently, also the GR industry started promoting the use of AMF, as it is assumed that nutrient cycling and drought tolerance associated with AMF would be very beneficial for GR vegetation growth and survival [START_REF] Streit | Mycorhizes: une stratégie de survie ancestrale redécouverte[END_REF][START_REF] Feldmann | Mycorrhiza for plant vitality: mycorrhizal fungi as factors of integrated horticultural plant production[END_REF][START_REF] John | Colonization of green roof plants by mycorrhizal and root endophytic fungi[END_REF][START_REF] Molineux | Manipulating soil microbial communities in extensive green roof substrates[END_REF].

The benefits of AMF on plant growth and development are acknowledged in horticulture, but studies investigating the effects of AMF on vegetation in a GR context have until recently been very limited (but see [START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF][START_REF] John | Colonization of green roof plants by mycorrhizal and root endophytic fungi[END_REF][START_REF] Molineux | Manipulating soil microbial communities in extensive green roof substrates[END_REF]. Generally, GR substrates are sterilized before use in order to eliminate seeds of undesired species [START_REF] Gormsen | The influence of collembolans and earthworms on AM fungal mycelium[END_REF][START_REF] Emilsson | Vegetation development on extensive vegetated green roofs: Influence of substrate composition, establishment method and species mix[END_REF][START_REF] Feldmann | Mycorrhiza for plant vitality: mycorrhizal fungi as factors of integrated horticultural plant production[END_REF][START_REF] John | Colonization of green roof plants by mycorrhizal and root endophytic fungi[END_REF]. Succulents are known to form no or very weak mycorrhizal associations, probably as an adaptation to the harsh conditions in their natural habitats (cf. [START_REF] John | Colonization of green roof plants by mycorrhizal and root endophytic fungi[END_REF]. This makes these plants a good option for application on sterile GR substrates. However, in order for other AMF-dependent plants to obtain the benefits associated with AMF, the fungi should be added as inoculum, be present in the growing media of the plug plants, or transported by animal vectors like arthropods, birds and small mammals (cf. [START_REF] John | Colonization of green roof plants by mycorrhizal and root endophytic fungi[END_REF].

The effects of addition of OM and AMF on GR vegetation development in a greenhouse and a rooftop experiment were studied in Chapter 5. The study was conducted in Heverlee (Leuven, Belgium) on the same roof as the experiment in Chapter 4. The experimental plots of 1 m² were filled with either a mineral EGR substrate or an EGR substrate with additional OM (Peltracom NV). Half of the plots received 1 g/m² AMF inoculum consisting of Glomus spp. As vegetation, we opted for a commercial EGR seed mix (Optigrün International AG) containing 35 herbaceous plant species. In April, eight months after installation, plant surveys on the rooftop were done and included calculation of cover and abundance data, plant height, and diversity measures in terms of species richness and Shannon diversity index and evenness. Vegetation development in the greenhouse was observed weekly from October 2013 until February 2014 and included the same measurements as on the rooftop. However, at the end of the experimental period, the plant material in the greenhouse was used to determine the dry weight and to calculate the mycorrhizal inoculation effect, an index for the effectiveness of an AMF inoculum. Also the roots of four species (Cota tinctoria, Dianthus deltoides, Leucanthemum vulgare, and Prunella vulgaris) were analyzed microscopically for AMF presence.

In the greenhouse, positive effects were found of OM on plant height, dry weight and cover, and evidence of AMF-root associations was found although no significant effects of AMF on vegetation development could be demonstrated. The plots with mineral EGR substrate resulted in a positive mycorrhizal inoculation effect, indicating a positive effect of AMF on biomass production. In general, we found that AMF lead to better performance in terms of species richness and cover, but this trend was not significant. On the rooftop experiment, no significant effects of OM and AMF on vegetation development were found, which could be due to climatic factors. During two weeks after sowing, the plots were irrigated by hand two times a week to saturation, but afterwards they only received water from natural precipitation events. The EGR vegetation experienced a relatively dry winter and spring. Together with above average temperatures, this resulted in slow germination and vegetation development. Just like the experiment in Chapter 4, we believe that an additional irrigation system during the establishment phase could have mitigated the water shortage problem on the rooftop.

In conclusion, this study showed that a low amount of OM is sufficient to obtain positive effects on plant growth in terms of vegetation cover, height and dry weight. However, an important trade-off should be taken into account, as bigger plants transpire more, compete for light and have more biomass to maintain, making them less resistant to drought stress, which is a disadvantage when a dry period occurs after establishment [START_REF] Rowe | Assessment of Heat-expanded Slate and Fertility Requirements in Green Roof Substrates[END_REF]. Our findings also demonstrate that GR substrates offer a suitable media where associations between AMF and plant roots can take place. This finding is in line with a recent study of [START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF] who examined parks and GR substrates in New York and found that a considerable fungal diversity was present, of which most of the genus Glomus (Glomeromycota). This suggests that if AMF are added or when they reach the roof spontaneously, the GR substrate offers a suitable media where AMF-root associations can take place. However, in our study, addition of AMF was not very effective as regards initial plant development. This in contrast to [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF] who demonstrated that by adding AMF on a GR, plant loss could be reduced and plants became more drought tolerant. Also [START_REF] Molineux | Manipulating soil microbial communities in extensive green roof substrates[END_REF] very recently offered evidence that adding AMF inocula to GR substrates is sustainable, and that GRs with healthy AMF-root associations may be more resilient to summer drought. GRs would hence be greener and be able to provide ecosystem services (e.g. stormwater management and thermal performance) for a longer time period [START_REF] Molineux | Manipulating soil microbial communities in extensive green roof substrates[END_REF]. Although our results suggest that AMF inocula cannot yet be considered as a substitution for OM, other very recent studies [START_REF] Mcguire | Digging the New York City Skyline: soil fungal communities in green roofs and city parks[END_REF][START_REF] Molineux | Manipulating soil microbial communities in extensive green roof substrates[END_REF][START_REF] John | Colonization of green roof plants by mycorrhizal and root endophytic fungi[END_REF] demonstrate positive effects of AMF and other soil microbiota on EGR vegetation. The topic of AMF inoculation in a GR context is hence rather novel but promising, and more research is needed to sort out the knowledge gaps.

Towards sustainable green roof irrigation methods in different climatic contexts

Research question 5: What kinds of irrigation methods are commonly used on GRs? Can irrigation recommendations be made for different climates (including the Mediterranean) and are there sustainable methods that should be promoted?

As indicated in Chapter 4 and 5, vegetation development on the experimental unirrigated EGRs could have been improved if an irrigation system were available. By definition, the goal of irrigation is to maximize the net profit in the long term, by keeping soil water content between the stress point and field capacity [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF]. On GRs, which are usually non-productive systems, irrigation is generally believed to be beneficial as it improves both plant survival and temperature control.

On the other hand, irrigation of GRs and other urban green infrastructures often has a negative connotation in a water management context. Globally, the main proportion of the total available freshwater is used for irrigation of agricultural fields, reaching up to 80% in (semi)-arid regions [START_REF] Fereres | Deficit irrigation for reducing agricultural water use[END_REF][START_REF] Bjornlund | Editorial) Exploring some of the socio-economic realities of sustainable water management in irrigation: An overview[END_REF]. Extraction of water for agricultural irrigation and the excessive water use can lead to environmental problems, including water scarcity, soil salinity, soil erosion and pesticide pollution [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF]Newley & Treverrow, 2006;[START_REF] Bjornlund | Editorial) Exploring some of the socio-economic realities of sustainable water management in irrigation: An overview[END_REF]. Together with competition with other water needs (e.g. town water supplies, environmental flows, recreation), the pressure to limit irrigation in agri-and horticulture grows [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF]Newley & Treverrow, 2006).

Sustainable water management is an important global action to alleviate the growing pressure on our water resources and a way to balance both socio-economic and environmental water needs [START_REF] Pereira | Irrigation management under water scarcity[END_REF][START_REF] Bjornlund | Editorial) Exploring some of the socio-economic realities of sustainable water management in irrigation: An overview[END_REF]. To ensure sustainable irrigation in the long term, there should be public awareness of sustainable water consumption, and similar to agro-and horticulture, GRs should be designed carefully, without wasting any water and limiting the use of potable water (Newley & Treverrow, 2006).

Information on GR irrigation is still very scarce, and the review in Chapter 6 attempted to draw attention to alternative water sources and water conservation strategies in order to make GR irrigation practices more sustainable. Just like in horticulture, there are three main questions to consider for more efficient irrigation: How, when and how much [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF].

Agricultural irrigation systems include surface or flood irrigation, sprinkler systems and micro-irrigation like drip-irrigation, micro-sprinkling and sub-irrigation [START_REF] Pereira | Irrigation management under water scarcity[END_REF]. In horticulture, similar irrigation technologies are available, but surface irrigation and sprinkler systems on big scale are considered not very efficient as they do not specifically target the root zone of the plants (Newley & Treverrow, 2006). Drip irrigation and micro-sprinklers are better suited for horticultural purposes as they are better controllable [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF]. Drippers deliver water at the right locations and are less subject to wind, and micro-sprinklers do not waste water due to overlapping irrigation areas [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF][START_REF] Bjornlund | Editorial) Exploring some of the socio-economic realities of sustainable water management in irrigation: An overview[END_REF]. Also for GRs, multiple irrigation methods are possible (cf. FLL, 2008), of which drip irrigation and overhead sprinkler systems are the most commonly applied. Hand watering is possible as well, but extremely time consuming and only possible for small surfaces. The best way to irrigate is automated overhead irrigation or drip irrigation if an additional WRL is provided [START_REF] Rowe | Comparison of irrigation efficiency and plant health of overhead, drip, and sub-irrigation for extensive green roofs[END_REF].

Alternative irrigation sources are also part of more sustainable water use. In agriculture, reuse of wastewater will definitely be developed further [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF]. Suggestions for more sustainable GR irrigation showed that water can be conserved by using gray water (i.e. all untreated household effluents except those originating from toilet use) and by rainwater harvesting. Gray water is a nutrient-rich irrigation supplement with broad applicability in the urban environment (e.g. domestic gardens, small-scale food production, green infrastructure elements; [START_REF] Rodda | Development of guidance for sustainable irrigation use of greywater in gardens and small-scale agriculture in South Africa[END_REF]. The main problem that should be investigated further is the high amount of micro-organisms which could be harmful for human health, plant growth and yield [START_REF] Rodda | Development of guidance for sustainable irrigation use of greywater in gardens and small-scale agriculture in South Africa[END_REF]. Next to its usefulness in an urban context, rainwater harvesting, i.e. the process of concentrating and storing rainwater runoff, can also be applied in agriculture [START_REF] Oweis | Water harvesting and supplemental irrigation for improved water productivity of dry farming systems in West Asia and North Africa[END_REF].

Based on outdoor GR experiments, irrigation specifications for different climates were proposed. It became clear that irrigation is essential during establishment and the first growing season on all types of GR and in all climates here considered. Afterwards, irrigation is necessary on GRs in (semi)-arid climates, and advised in small amounts in the other climates. One could assume that irrigation should be applied when the soil water content reaches the stress point, because below this point, evapotranspiration (ET) will be restricted and water uptake from the roots will be difficult (Jim & Peng, 2012;[START_REF] Bjornlund | Editorial) Exploring some of the socio-economic realities of sustainable water management in irrigation: An overview[END_REF]. However, irrigation is more efficient if applied when soil water content is slightly above the stress point, and irrigation should stop before the field capacity is reached [START_REF] Bjornlund | Editorial) Exploring some of the socio-economic realities of sustainable water management in irrigation: An overview[END_REF]. Therefore, soil water content and ET should be monitored carefully [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF]Newley & Treverrow, 2006;[START_REF] Bjornlund | Editorial) Exploring some of the socio-economic realities of sustainable water management in irrigation: An overview[END_REF].

Water can also be conserved by adapting irrigation requirements, more specifically by targeting the GR vegetation or materials. An appropriate selection of vegetation is needed, with a diverse mix of life forms including succulents, geophytes, mosses, annual species and drought-tolerant herbs [START_REF] Coutts | Watering our cities: The capacity for Water Sensitive Urban Design to support urban cooling and improve human thermal comfort in the Australian context[END_REF]. Ideally, a deeper substrate should be opted for to increase the plant water availability, but as this is not always possible due to building weight limitations and colonization of undesirable weeds, shrubs and invasive plants, efficient irrigation methods should be considered as an alternative. By considering substrate type, depth, vegetation and irrigation regime during the design process, an adaptive approach is created in which GR establishment and sustainability can be facilitated [START_REF] Kotsiris | An adaptive approach to intensive green roofs in the Mediterranean climatic region[END_REF][START_REF] Ntoulas | Semi-extensive green roof substrate type and depth effects on Zoysia matrella )eo g o th a d drought tolerance under different irrigation regimes[END_REF]Ntoulas et al., , 2013)).

Another adaptive approach could be found in the concept of deficit agriculture irrigation. In (semi)-arid regions, precipitation is usually unpredictable and sporadic, and as it is quickly lost to evaporation and runoff, the contribution of rainwater to crop yield is rather low [START_REF] Oweis | Water harvesting and supplemental irrigation for improved water productivity of dry farming systems in West Asia and North Africa[END_REF]. Irrigation is used to supplement rainfall in order to achieve maximum and more stable crop yields [START_REF] Zhang | Water-yield relations and optimal irrigation scheduling of wheat in the Mediterranean region[END_REF][START_REF] Fereres | Deficit irrigation for reducing agricultural water use[END_REF][START_REF] Oweis | Water harvesting and supplemental irrigation for improved water productivity of dry farming systems in West Asia and North Africa[END_REF]. Due to water scarcity, this supplemental irrigation is often extracted from groundwater aquifers, leading to a decline of the groundwater table and further encouraging the water scarcity problem [START_REF] Zhang | Water-yield relations and optimal irrigation scheduling of wheat in the Mediterranean region[END_REF]. A better strategy would be to maximize the production per unit of water consumed (the water productivity) instead of maximizing production per unit area [START_REF] Fereres | Irrigation Water Management of Horticultural Crops[END_REF][START_REF] Oweis | Water harvesting and supplemental irrigation for improved water productivity of dry farming systems in West Asia and North Africa[END_REF][START_REF] Fereres | Deficit irrigation for reducing agricultural water use[END_REF]. In the deficit irrigation strategy, which is often applied in regions where water is scarce, irrigation is applied at quantities below the full ET requirement of the crop, thereby increasing the water potential while only marginally decreasing the crop yield [START_REF] Pereira | Irrigation management under water scarcity[END_REF][START_REF] Oweis | Water harvesting and supplemental irrigation for improved water productivity of dry farming systems in West Asia and North Africa[END_REF][START_REF] Costa | Deficit irrigation as a strategy to save water: Physiology and potential application in horticulture[END_REF][START_REF] Fereres | Deficit irrigation for reducing agricultural water use[END_REF]. It has been shown that the maximum water potential can be reached when only 1/3 to 2/3 of the full irrigation requirements are applied to the crop [START_REF] Pereira | Irrigation management under water scarcity[END_REF]. This optimization strategy could be useful in a GR context as well, as by limiting irrigation to drought-sensitive growth periods (i.e. establishment phase and growing seasons), the water productivity can be maximized, while also stabilizing yield (in the case of GRs, plant growth and survival and hence the contribution to urban ecosystem services). However, in order for the deficit irrigation strategy to be successful, the exact response of the GR vegetation to drought stress in different development stages should be known, as well as the impact of reduced GR vegetation performance caused by drought stress on ecosystem functioning. Improving ecosystem services is important for a more sustainable urban planning and a way to ensure good life quality. Therefore, examining urban ecosystems (in this case EGRs) and constructing an applicable framework to achieve better ecosystem services is essential. The role of GRs in offering multiple ecosystem services is becoming increasingly acknowledged [START_REF] Carter | Vegetated roofs for stormwater management at multiple spatial scales[END_REF][START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF][START_REF] Gaston | Managing urban ecosystems for goods and services[END_REF]Lundholm et al., 2010), but there are still many questions about the link between biodiversity and the delivery of ecosystem services in a GR context. Lundholm et al. (2010) for example, demonstrated that combining different life forms on GRs can improve ecosystem services (e.g. stormwater management) but that for the case of roof cooling, speciespoor GR systems were a better option than those with high diversity. There is hence a need for further studies on the relationship between biodiversity and ecosystem functioning. The work in Chapter 7 was based on the assumption that FD and PD are linked to ecosystem services. The goal was to analyze the initial plant species composition, as provided by commercial EGR systems, in terms of FD and PD, and to offer a methodology to optimize species choice for incorporation into EGR design, making use of the FD approach.

The EGR systems (n = 57) and their corresponding plant species (n = 199) were selected by screening the webpages of all the companies associated with the European Federation of Green Roof Associations. Information on percentage of succulent species, substrate depth, installation method and species richness (SR) were derived from the EGR systems. 29 plant traits were selected based on important GR ecosystem properties as explicitly mentioned in specific literature on GRs. Differences in FD, PD and SR between the EGR systems were considerable. Also correlations between FD, PD and SR were strongly positive, suggesting that speciesrich systems offer better ecosystem properties and services. Based on species composition, the EGR systems could be grouped in three general types that differ in SR, percentage of succulent species, substrate depth and installation method. GR systems that comprise only succulent species have significant lower FD and PD values compared to systems containing more diverse vegetation. However, results also show that the FD within the GR types could be improved. Although the speciespoor GR systems have lower FD and PD values, they contain many succulent species that are considered to be highly drought tolerant, so in these systems, species have been selected to optimize the drought tolerance capacity (trait convergence strategy). In the species-rich GR systems with higher FD and PD, fewer succulent species are present and it is questionable if these systems are sufficiently drought tolerant. In a GR context, succulence is often considered as the only way to address the drought tolerance issue. However, drought tolerance is actually achieved by a combination of multiple traits, including leaf area, size, presence of hairs or waxes, root length and biomass, and photosynthetic pathway [START_REF] Markesteijn | Seedling root morphology and biomass allocation of 62 tropical tree species in relation to drought-and shade-tolerance[END_REF][START_REF] Cook-Patton | Potential benefits of plant diversity on vegetated roofs: A literature review[END_REF]. So next to the trait convergence strategy, the diversity approach can also contribute to drought tolerant vegetation, while (assumingly) enhancing ecosystem services.

Based on a species trait dissimilarity matrix, two methods were then proposed to define species combinations (one exact and one applicable species list) that maximize the FD for every GR type, and thus the assumed overall ecosystem properties and services value.

The used species lists are the ones proposed by the GR companies, so they only provided information on the initial species presence. Vegetation dynamics and abundance data once installed on the GR were not considered. As GRs are dynamic systems, the vegetation composition will change over time, hence also the FD and PD of the system. Some scientific studies for example, have noted a decline in GR species richness over time [START_REF] Rowe | Effect of green roof media depth on Crassulacean plant succession over seven years[END_REF][START_REF] Lundholm | Leaf and Life History Traits Predict Plant Growth in a Green Roof Ecosystem[END_REF]. However, the FD and PD approach remains valid, as for GR design one often starts from simple species lists.

Despite the number of current drawbacks of the FD and PD approach, this study offers a way to design GRs with high plant trait diversity. It also highlights that currently used EGR systems differ greatly in their FD and PD. The described approaches are generic in nature and can be used for a broad range of urban ecosystems. By carefully considering species that maximize FD, the design of urban green infrastructure elements can become a more sustainable and vital tool to improve quality of life in urban areas. We acknowledge that as more empirical research becomes available, other possibilities may arise that also offer ways to improve species selection for GRs, for example by only looking at the plant traits that are highly related to one or multiple ecosystem services. Also, the optimization problem could have been approached from an operations research perspective. Mixed Integer Linear Programming for example has been used for optimal material selection in terms of design, monetary and environmental aspects of green buildings [START_REF] Castro-Lacouture | Optimization model for the selection of materials using a LEED-based Green building rating system in Colombia[END_REF]. The technique uses mathematical models to optimize a certain objective (e.g. optimal material selection) while considering multiple constraints (e.g. budget and design constraints). Tsang and Jim (2013) use a stochastic approach (demand-responsive optimal-inventory model) to model and predict the future GR demand and optimize contractor inventories of GR components under different market scenarios. To our best knowledge, such mathematical models have not yet been used for optimization of GR vegetation selection. The operations research examples mentioned above are rather timeconsuming and require mastering of specific mathematical skills. Therefore, we opted for the FD and PD analysis, which is more practical and in line with the plant trait approach of Chapter 3.

Research gaps and directions for future research

Overall, the chapters of this thesis present multiple opportunities for future studies, emerging from encountered problems, research gaps or just making use of the methodologies elaborated in this work. First, we summarize some general perspectives:

 A general problem we encountered upon working with plant traits (Chapter 2, 3 and 7) was that there were many missing values for Mediterranean species and particular traits. In Chapter 2 and 3, the most extended and complete data source currently available was consulted (Try Initiative, Kattge et al., 2011), but still a large amount of missing values was encountered. Therefore, some interesting species could possibly have been omitted. Furthermore, substantial information was available for more general and common species but lacking for not well known local Mediterranean species, which resulted in a high amount of common species in the final potential species lists. The lack of detailed information has been shown to be a common bottleneck in ecological research (e.g. Diaz & Cabido, 1997;[START_REF] Franzel | Choosing the Right Trees: Setting Priorities for Multipurpose Tree Improvement[END_REF][START_REF] Reubens | Tree species selection for land rehabilitation in Ethiopia: fragmented knowledge to an integrated multicriteria decision approach[END_REF], stressing the necessity to further complete and update available plant trait databases like the TRY Initiative.



In Chapter 3 the plant trait selection was based on the assumption that drought tolerance and self-sustainability are very important for plant survival on EGRs in the Mediterranean. We expect that drought tolerance and self-sustainability are important on EGRs in other climatic regions as well (cf. [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]Getter & Rowe, 2008;[START_REF] Williams | Green roofs for a wide brown land: Opportunities and barriers for rooftop greening in Australia[END_REF], but other factors could also influence species survival on EGRs in those regions. For example, if Scandinavian countries would be the target, the literature study should rather focus on tolerance to low temperatures and humid conditions, which would significantly alter the screening tool (cf. [START_REF] Emilsson | Comparison of establishment methods for extensive green roofs in southern Sweden[END_REF]. Therefore, the screening tool developed in Chapter 3 is particularly interesting for EGR plant selection in a Mediterranean context, but is not advised to use on species not native to the Mediterranean. The methodology however, is a useful framework for parallel research in other climates.



As demonstrated in section 1.3 of the general discussion, most species performed according to what was expected from their respective SS, and other species performed even better than expected. The screening tool is therefore a rather reliable tool to indicate species potentiality for use on EGRs in climatic regions with frequent summer drought. Knowing this, it would be interesting if some species with a high SS, like Thymus serpyllum, Silene otites, Petrorhagia prolifera and Polygala vulgaris, could be commercialized by nurseries and seed suppliers, in order to make them the subject of further EGR experiments. Also, after testing of the potential species under GR conditions, nurseries should take the successful species into account in their supply, in order to diversity and improve the GR performance.



Research should also focus on the development of species-specific AMF inocula, as the effect of AMF inoculation is actually dependent on the used AMF and plant species [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF][START_REF] Rillig | Arbuscular mycorrhizae and terrestrial ecosystem processes[END_REF]. If AMF-and plant choice can be tuned, the GR industry is one step closer to successfully develop productive and biodiverse EGRs.



In line with the suggestions on sustainable GR irrigation practices described in Chapter 6, future research should further elaborate on GR materials with increased water retention capacity. Such research is necessary in order to develop GRs with high biodiversity value and with no or only limited irrigation requirements, also in (semi)-arid climates [START_REF] Dvorak | Green roof vegetation for North American ecoregions: A literature review[END_REF]. Studies selecting appropriate drought tolerant plant species should also be supported, in order to limit irrigation requirements (Schroll et al., 2011a[START_REF] Schweitzer | Evaluation of the energy performance and irrigation requirements of extensive green roofs in a water-scarce Mediterranean climate[END_REF]. Furthermore, the potential and effects of gray water reuse and the deficit irrigation strategy should be explored further.

Upon working on the experimental EGRs in Chapter 4, following ideas for further research emerged:



The exploration of natural Mediterranean habitats as described in Chapter 2 and the subsequent screening in Chapter 3 showed that a high amount of unused, local Mediterranean species with variable life forms comprises a potential source for urban greening. However, the results do not offer scientific evidence that the species will effectively thrive on GRs in the Mediterranean or temperate climate. The research offers the ecological fundamental knowledge necessary for further testing of species and final implementation into a successful GR system. 18 species were tested in Chapter 4, but many other species with high potential could be tested in the future.



The vegetation development on the EGRs in Chapter 4 was highly affected by climatic conditions. As weather conditions vary from year to year and EGRs are dynamic systems, we expect a similar variation in behavior like in the past two years. A further monitoring could of course add extra detail to the development.



Not only climatic conditions, but also other ecological assembly rules determine the species that will eventually become part of the actual GR community. Both dispersal and (a)biotic assembly processes (i.e. habitat filtering and biotic interactions) need to be studied to evaluate the potential of species in the regional species pool to colonize and establish on EGRs (cf. [START_REF] Götzenberger | Ecological assembly rules in plant communitiesapproaches, patterns and prospects[END_REF]. Currently, little information exists on factors (situated both at landscape and local level) that determine spontaneous colonization and GR vegetation performance in the long term. Well established and novel technologies including Geographical Information System technology, and proximal and remote sensing, could be very helpful in this respect.



The colonization process can manifest in two directions. Next to investigating the potential of regional species to colonize the GR spontaneously, the species applied on the GR intentionally can also spread to the surrounding environment. It was noticed that S. album and S. acre, installed on the experimental GRs, were able to colonize and establish on other parts of the roof. This is another topic that needs consideration, for example in the context of invasive species monitoring.



The positive aspects of bryophytes as drought tolerant GR vegetation and as additional water retention function were discussed in part 1.1 of the general discussion. We also pointed at some negative effects that those species could have on the colonization and establishment of spontaneous and intentionally installed vascular plant species on GRs. Further studies should unravel all the positive and potential negative aspects of the bryophyte layer for other GR vegetation. 

In part 1.3 of the general discussion, some animal species that were observed during the experimental period were briefly mentioned to point at the habitat value that GRs offer. More attention should be paid to both above-and belowground biodiversity on GRs, as biodiversity research comprises only a small portion of the current research efforts on GRs [START_REF] Blank | Directions in green roof research: A bibliometric study[END_REF]. Bee species ensure pollination of GR plant species and are hence essential for seed production and the insurance of GR performance in the long-term [START_REF] Colla | Can green roofs provide habitat for urban bees (Hymenoptera: Apidae)? Cities and the Environment[END_REF]. A stable microarthropod community in the GR soil is critical for ecosystem functioning as well [START_REF] Rumble | Soil microarthropod community dynamics in extensive green roofs[END_REF].  Finally, next to looking at the soil biota, it is also necessary to study the seed bank composition (both transient and persistent seed bank aspects) and dynamics of the EGR. To our best knowledge, the soil seed bank has not been the main focus of any GR investigation yet. However, regarding ecosystem resilience, seed bank ecology is an important concept because soil seed banks represent a stock of regeneration potential after disturbances (cf. [START_REF] Wang | Recent Advances in soil Seed Bank Research[END_REF]. Information on the GR soil seed bank is useful as it can predict new plant recruitment and the sustainability of the GR in the long term. [START_REF] Brenneisen | Space for Urban Wildlife: Designing Green roofs as Habitats in Switzerland[END_REF] suggested, in a context of natural habitat replacement, to use top soil of natural habitats (which includes part of the seed bank and soil microbiota) as complement to the sterile GR substrate. Different GRs in Berlin have been observed by [START_REF] Köhler | Long-Term Vegetation Research on Two Extensive Green Roofs in Berlin[END_REF] over 20 years, and several annual plants and weeds were able to germinate and grow from the seed bank, although those species disappeared again after some years. Other plants (e.g. Anthyllis vulneraria and Medicago sativa), although not native to the study region, did survive over the entire study period due to regular irrigation, and complemented the seed bank for regeneration in the future [START_REF] Köhler | Long-Term Vegetation Research on Two Extensive Green Roofs in Berlin[END_REF].

The experiments in Chapter 4 and 5 were designed in order to study the aboveground vegetation and did not allow measurement of several aspects of ecosystem functioning. As already mentioned in point 2.3 of the general discussion, there is a need for more research focusing on the relationship between biodiversity and ecosystem functioning in a GR context. In line with this recommendation, following research opportunities can be considered:

 Experiments should be conducted on a bigger scale, comprising different vegetation and structure compositions, and designed in a way that allows the measurement of different GR ecosystem services including stormwater management, thermal performance, nutrient cycling and mitigation of air pollution.

Concluding part

The [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Perring | Chapter 38: Novel urban ecosystems and ecosystem services[END_REF]. Natural habitats with similar properties as EGRs should be selected and analyzed, and a species list based on fieldwork and/or literature sources should be created.

In a next step, the species list should be screened for the most potential species. The screening tool developed in Chapter 3, includes some primary traits, like CAM metabolism, stress tolerance and succulence, which already proved to be important for the success on GRs [START_REF] Oberndorfer | Green Roofs as urban e os ste s : e ologi al st u tu es, fu tio s, a d se i es[END_REF][START_REF] Snodgrass | Green Roof Plants: A Resource and Planting guide[END_REF]. The other leaf-related traits emerging from the plant trait analysis have gained much less attention in the literature. Therefore, the presence of these attributes in the screening tool offers some novel insights that reach beyond the usual considered water storage attributes. It is possible that the screening tool needs to be adapted to suit the local climatic conditions. This is for example the case when the EGR is planned to be installed in a location with very harsh winter conditions, which will cause the factor frost-tolerance to become more significant than the factor drought tolerance.

The habitat template approach and screening tool recommended in the EGR context should ensure GR performance and sustainability, even under difficult climatic conditions, although further testing of the potential species is required. We pointed out that plant species with different FD and/or PD should be considered in the EGR design, as this is also the case in natural conditions in Mediterranean habitats. Plant communities with high diversity are also increasingly believed to result in better vegetation performance and ecosystem functioning, although we pointed out that more research on the link between biodiversity and ecosystem functioning in a GR context is still necessary (cf. Lundholm et al., 2010). Until now, hemicryptophytes constitute the major proportion of currently applied GR plants, and only little attention has gone to the possibilities of annuals (also called therophytes). Many annuals germinate, grow, flower and produce many seeds in a very short time period, whereupon they perish. Although they would not be interesting during summer months, they may be attractive in spring and early summer for pollinators as well as having esthetic value, due to their colorful flowers (cf. [START_REF] Filippi | The Dry Gardening Handbook: Plants and Practices for a Changing Climate[END_REF]. Under natural conditions, seed production is added to the seed bank, hence forming a buffer against eventual gap formation should other perennial herbaceous plants die off. This property is a natural survival strategy which may guarantee GR performance in regions where weather conditions are very unpredictable, like the Mediterranean region [START_REF] Madon | The ecological significance of annuals on a Mediterranean grassland (Mt Ventoux, France)[END_REF]. As stated earlier, the capacity of annuals and other GR plants to contribute to seed bank replenishment, and hence insurance of the GR performance in the long term, needs more study. The conservation value of annuals may also be important, certainly in Mediterranean areas where many annuals of harsh environments are threatened [START_REF] Lavergne | Fingerprints of environmental change on the rare mediterranean flora: a 115-year study[END_REF]. Next to a combination of annuals and hemicryptophytes, some geophytes (e.g. Allium spp.; Iris spp.) can be incorporated on the EGR as accent plants, as they only flower for a short period. Sedum spp. should always be considered in EGR design. Also bryophytes need attention, as they are part of the natural vegetation and they can facilitate the establishment and survival of vascular plants under extreme conditions (e.g. [START_REF] Anderson | The potential value of mosses for stormwater management in urban environments[END_REF][START_REF] Krupka | Dachbegrünung. Pflanzen-und Vegetationsanwendung an Bauwerken[END_REF]. More research is needed to unravel both the positive and possibly negative effects of bryophytes on spontaneous and installed GR vegetation. In order to risk failure of the EGR, we advise to gradually increase the iodi e sit , use of oth o el o a el used spe ies a d so e t ied-and-t ue G' species. If one opts to install the EGR vegetation as seeds, then good seed quality is of very high importance. If this cannot be ensured, then plug plants should be used instead.

The potential species also have specific habitat requirements (e.g. pH, organic matter, nutrient content) and the GR substrate needs customization to ensure good plant health. In Chapter 4, we showed that for the tested species, deeper substrates togethe with an additional WRL are preferred for EGRs in any climate. Using substrate depths of 5 cm or less without a WRL or provision of an irrigation system is not advised, at least not for the species tested in this study, if good performance and biodiversity value of EGRs need to be ensured. In Chapter 5, we concluded that the most important factor for plant growth on GRs was the presence of OM. A possible explanation for the positive results of OM could be the differences in substrate composition. EGR substrates with additional OM have higher and longer water availability after rainfall, and their nutrient content is much higher. In this respect, it would be interesting to keep the dead plant material on the EGR instead of removing it, which is often done for aesthetic reasons during the yearly maintenance visit. The decomposition of this dead material allows a natural enrichment of the GR substrate with OM, hence eliminating the need of conventional fertilizers which are often polluting the runoff [START_REF] Czemiel Berndtsson | Green roof performance towards management of runoff water quantity and quality: A review[END_REF]. It also adds structure to the GR, allowing many arthropod species to find a suitable habitat [START_REF] Madre | A comparison of 3 types of green roof as habitats for arthropods[END_REF][START_REF] Rumble | Soil microarthropod community dynamics in extensive green roofs[END_REF].

No significant effects of AMF inoculation on plant development were found. More time is probably needed to fully establish the AMF infection and to obtain positive outcomes on vegetation development. AMF inocula have been very useful in many (urban) horticultural applications, but their significance in a GR context has only recently started to be discovered (cf. McGuire et al., 2013;[START_REF] John | Colonization of green roof plants by mycorrhizal and root endophytic fungi[END_REF][START_REF] Molineux | Manipulating soil microbial communities in extensive green roof substrates[END_REF]. [START_REF] Busch | Use of Endomycorrhizal Fungi for Plant Cultivation on Buildings[END_REF] concluded that AMF inoculation resulted in overall better plant survival and drought stress on rooftops. It is hence possible that the effects of AMF inoculation become significant in later stages of vegetation development.

When the effects of different substrate depths, substrate types, exposition and additional structures (e.g. WRL) on the potential plant species are still unclear, then the best option is to establish a heterogeneous GR, with varying substrate depth (within the building weight restriction boundaries), varying substrate types, expositions and in some areas an additional WRL (see also [START_REF] Lundholm | Habitat origins and microhabitat preferences of urban plant species[END_REF][START_REF] Madre | A comparison of 3 types of green roof as habitats for arthropods[END_REF][START_REF] Rumble | Soil microarthropod community dynamics in extensive green roofs[END_REF]Heim & Lundholm, 2014). This approach creates habitat variability in which the plant species can each find their preferable ones. This way, high biodiversity is encouraged, and the EGR can be used as a case study to follow the vegetation dynamics in the long term.

At the end of Chapter 6 it was concluded that irrigation is essential during establishment and the first growing season on all types of GR and in all climates that were considered. Afterwards, irrigation is necessary on GRs in (semi)-arid climates, and advised in small amounts in the other climates. To ensure sustainable irrigation in the long term, there should be public awareness of sustainable water consumption, and GRs should be designed carefully, without wasting any water and limiting the use of potable water. Potential solutions are gray water re-use, RWH, controlled irrigation in space and time (using for example a smart controller) and the addition of a WRL. An appropriate selection of vegetation, consisting of a diverse mix of life forms is essential as well. By considering substrate type, depth, vegetation and irrigation regime during the design process, an adaptive approach is created in which GR establishment and sustainability can be facilitated. Some weedy species will recruit spontaneously on the EGR. In a GR context, weeds are labeled as undesired herbaceous plants that reach the GR spontaneously (Nagase et al., 2013). They are often unattractive, annoying and some woody weeds can pose a threat for the installed GR vegetation and underlying structures (Nagase et al., 2013). Most of the weeds observed in the experiments in Chapter 4 and 5 were ruderal species also present in the EGR environment. Like the annual species installed on the EGRs, the development dynamics of the weeds follows the weather patterns of summer drought, autumn and spring rains. When climatic conditions become too extreme, the weeds die off, but the next growing season or autumn period, new seeds can be recruited on the EGR from the neighboring landscape. The weedy species therefore represent a natural and inevitable part of the EGR vegetation dynamics. As GRs are dynamic systems (mainly due to climatic variations), the vegetation composition will change over time and will most likely fluctuate around a stable vegetative state. The probability that weedy species will become a dominant part of the EGR vegetation is therefore very low. Furthermore, many weeds possess several interesting traits that add to the FD and hence performance of the GR. We suggest GR designers to reconsider the definition of GR weeds and only label them as such if they are harmful to the GR vegetation and structure. If any harmful weeds succeed to install on the EGR (e.g. Buddleia spp.) they should be removed during the maintenance period. At the same moment, it is possible to reseed the spots where the vegetation was not able to survive, but if the vegetation a d st u tu e ad i es p o ided i this EG' o ept a e follo ed, the e o t probably be any need for reseeding.

Finally, in the research part of the EGR concept, possibilities for further research are proposed, as well as adjustments that should be made in the short or long term, to make testing of potential plant species possible, or to offer adapted GR species mixes. The FD and PD analysis conducted in Chapter 7 for example, indicated that even species poor GR systems can gain ecological interest by considering alternative species. The two proposed methods, delivering an exact species list and perhaps a more practical one that offers alternative species, illustrate how current GR systems can be adapted to maximize FD and presumably also the overall ecosystem properties and services. The focus of this study was on GRs, but the methods used are generic and offer a framework for improved design of a broad range of novel ecosystems, particularly in an urban context. It is also possible to target specific ecosystem services with this approach, meeting the main goal of the respective GR project.

People are still rather skeptical towards dehydrated GRs during summer [START_REF] Dagenais | Development of Design Criteria to Improve Aesthetic Appreciation of Extensive Green Roofs[END_REF]. Nevertheless, even when the vegetation is not very attractive in summer (except for evergreen succulents), GRs should still be the preferred option over bitumen or gravel roofs, as the substrate and other structure layers are also effective in building cooling and rainwater retention. Outside the summer months, GRs can function optimally and offer a wide array of local drought adapted plant species, thereby increasing floral and faunal biodiversity. If people accept that EGRs in the Mediterranean region will not be green all year round, which is in line with the general life cycles of the natural vegetation in this climate, more design possibilities arise which will greatly improve GR interest in the region. 1 18 0 0 7 0 0 0 25 0 3 0 0 13 0 15 20 2 2 12 0 0 7 0 0 1 23 0 10 0 0 16 0 12 18 2 3 12 0 0 6 0 0 0 22 0 8 0 0 13 0 16 24 2 4 11 0 0 13 0 0 0 0 0 0 0 0 34 0 16 25 3 5 0 0 0 11 0 0 0 0 0 0 0 0 0 0 6 12 1 6 0 0 0 0 0 0 0 1 0 0 0 0 0 0 26 42 0 7 4 0 0 3 0 0 0 44 0 0 0 0 18 0 24 7 0 P5 1 19 0 0 4 0 0 1 24 0 2 0 0 24 0 14 13 3 2 15 0 0 4 0 0 1 30 0 8 0 0 23 0 8 10 3 3 13 0 0 5 0 0 1 32 0 7 0 0 20 0 10 12 3 4 13 0 0 8 0 0 0 0 0 0 0 0 68 0 6 6 2 5 0 0 0 25 0 0 0 20 0 0 0 0 0 0 25 30 1 6 0 0 0 4 0 0 0 4 0 0 0 0 0 0 58 33 0 7 4 0 0 2 0 0 0 63 0 0 0 0 27 0 5 0 1 P10 1 18 0 0 4 0 0 0 30 0 2 0 0 29 0 7 10 3 2 16 0 0 3 0 0 0 34 0 8 0 0 27 0 4 7 3 3 15 0 0 3 0 0 0 37 0 7 0 0 25 0 5 9 3 4 8 0 0 5 0 0 0 0 0 0 0 0 80 0 4 3 3 5 4 0 0 13 2 0 1 61 0 0 0 0 7 0 5 8 1 6 0 0 0 7 0 0 2 15 0 0 0 0 7 0 39 29 1 7 2 0 0 2 0 0 0 59 0 0 0 0 33 0 3 0 2 SHELTERED 5 1 17 0 0 5 0 0 0 20 0 4 0 0 20 0 17 18 2 2 19 0 0 8 0 0 1 20 0 11 0 0 24 1 7 9 2 3 10 0 0 9 0 0 1 14 0 12 0 0 12 0 19 22 2 4 3 0 0 16 0 0 0 0 0 0 0 0 35 0 14 33 3 5 0 0 0 52 0 0 0 13 0 0 0 0 0 0 8 27 1 6 0 0 0 1 0 0 0 2 0 0 0 0 0 0 33 64 0 7 0 0 0 4 0 0 0 8 0 0 0 0 3 0 37 48 0 P5 1 20 0 0 5 0 0 0 17 0 4 0 0 31 0 10 12 3 2 15 0 0 9 0 0 1 21 0 8 0 0 28 0 8 10 3 3 12 0 0 10 0 0 1 20 0 12 0 0 24 0 9 11 3 4 2 0 0 5 0 0 0 0 0 0 0 0 61 0 11 21 2 5 0 0 0 30 0 0 0 17 0 0 0 0 0 0 13 40 0 6 0 0 0 5 0 0 0 2 0 0 0 0 1 0 23 69 0 7 1 0 0 2 0 0 0 16 0 0 0 0 7 0 30 45 1 P10 1 13 0 0 3 0 0 0 23 0 2 0 0 43 0 7 9 3 2 9 0 0 6 0 0 1 33 0 7 0 0 29 1 8 8 3 3 7 0 0 6 0 0 1 36 0 10 0 0 24 0 8 8 3 4 6 0 0 10 0 0 0 0 0 3 0 0 46 0 14 22 3 5 2 0 0 18 1 0 0 41 0 0 0 0 6 0 8 24 2 6 0 0 0 17 1 0 0 19 0 0 0 0 8 0 15 41 0 7 1 0 0 13 2 0 0 12 0 0 1 0 6 0 22 43 1 HEVERLEE EXPOSED 5 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 2 0 0 0 0 0 0 0 0 0 0 0 0 5 11 82 3 3 2 0 0 0 0 0 0 0 0 0 0 0 2 4 16 76 2 4 2 0 0 0 0 0 0 0 0 0 0 0 4 0 15 79 1 5 0 0 0 0 0 0 0 2 0 0 0 0 0 0 10 87 2 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 90 1 7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 95 0 P5 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 40 0 0 0 0 0 0 4 0 2 2 0 0 0 9 45 3 3 31 0 0 0 1 0 0 4 0 3 1 0 0 1 11 46 3 4 8 0 0 0 2 0 0 0 1 2 0 0 9 0 17 59 2 5 15 0 0 0 0 0 0 10 1 2 0 0 0 1 14 57 3 6 0 0 0 0 0 0 0 8 0 3 1 0 0 1 14 74 1 7 0 0 0 0 0 0 0 0 0 3 0 0 0 0 10 87 0 P10 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 19 0 0 0 0 0 0 13 0 5 3 0 0 3 8 48 3 3 18 0 3 0 3 0 0 10 0 7 3 2 4 3 8 38 3 4 6 0 0 0 0 0 0 0 0 6 0 0 14 1 8 65 3 5 2 0 0 0 1 0 0 33 0 5 0 0 0 0 5 55 3 6 0 0 0 0 0 0 0 20 0 7 0 0 0 0 6 67 3 7 0 0 0 0 1 0 0 0 0 7 1 0 0 1 4 85 1 SHELTERED 5 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 4 0 0 0 0 0 0 4 0 4 4 0 0 2 14 70 3 3 3 0 1 0 0 0 0 3 0 1 1 0 4 0 15 72 2 4 0 0 0 0 0 0 0 0 0 0 0 0 1 0 16 83 3 5 2 0 0 0 0 0 0 2 0 0 0 0 0 0 10 86 2 6 0 0 0 0 0 0 0 2 0 0 0 0 0 0 10 87 3 7 0 0 0 1 0 0 0 0 0 0 0 0 0 0 3 96 0 P5 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 13 0 0 0 0 0 0 2 0 3 3 0 0 2 8 69 3 3 9 0 2 0 1 0 0 3 0 5 3 1 8 1 11 55 3 4 3 0 0 0 0 0 0 0 0 4 1 0 8 0 6 76 3 5 3 0 0 0 0 0 0 2 0 5 0 0 2 0 5 82 3 6 0 0 0 0 0 0 0 4 0 6 1 0 2 0 5 83 3 7 0 0 0 0 1 1 0 0 0 6 1 0 1 0 3 88 0 P10 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 2 5 0 0 0 0 0 0 17 0 3 5 0 0 2 9 61 3 3 5 0 5 0 1 0 0 15 1 7 4 2 11 3 11 36 3 4 1 0 1 0 0 0 0 0 0 8 0 0 10 0 8 70 3 5 2 0 1 0 1 0 0 11 0 11 0 0 1 0 6 67 3 6 0 0 1 0 1 0 0 6 0 13 0 0 1 1 6 70 3 7 0 0 0 0 1 1 0 0 0 16 1 0 1 1 2 76 0

  Table 4.1 Factory substrate specifications. ................................................................. Table 4.2 List of species installed on the EGR, categorized according to their longevity and installation method. Pictures of the species can be found at the end of this chapter. ............................................................................... Table 4.3 Mean values and standard errors for cover data, abundance data and diversity measures in Avignon and Heverlee in both the growing season and autumn period of 2013 and 2014. Significant differences between the locations are marked in grey and letters indicate the post-hoc results (Mann-Whitney U). ...................................................................................... Table 4.4 Mean values and standard errors for cover data, abundance data and diversity measures in the exposed and sheltered plots of a) Avignon and b) Heverlee in both the growing season and autumn period of 2013 and 2014. Significant differences between the expositions are marked in grey and letters indicate the post-hoc results (Mann-Whitney U). ..................... Table 4.5 Mean values and standard errors for cover data, abundance data and diversity measures in the structure types 5, P5 and P10 in a) Avignon and b) Heverlee in both the growing season and autumn period of 2013 and 2014. Significant differences between the structure types are marked in grey and letters indicate the post-hoc results (Kruskal Wallis).................... Table 4.6 Spearman rank correlation coefficients and significance between the bryophyte cover, the SR and D of the weeds, and the cover data, abundance data and diversity measures. (*: p < 0.004; **: p < 0.001; NS: not significant; NA: not applicable). ............................................................

  . ............................................................ Table

  Fig. 1.1 Summary of GR benefits and the relation, either direct or indirect, to different aspects of ecosystem and human health. GR benefits are classified into the four ecosystem service categories (regulating, supporting, cultural and provisioning) and others. Adapted fromTzoulas et al. (2007). ........................................................................................................ Fig. 1.2 Sketch of an EGR construction. 1: Vegetation layer; 2: Substrate or growing medium (*: an additional water retention layer can be placed under the substrate); 3: Drainage + root-permeable filter layer; 4: Waterproofing layer; 5: Roof deck, insulation. [Source: ZinCo; http://www.zincogreenroof.com/; retrieved 10-2014]. ............................................................. Fig. 1.3 An EGR installed on a carport in Flanders, Belgium. Micro variation in environmental conditions clearly influences vegetation composition [Picture: Frederik Lerouge; Division of Forest, Nature and Landscape; June 2013]. .............................................................................................................. Fig. 1.4 Average annual weather patterns for the period 1970-2000 (temperature, precipitation and potential evapotranspiration PET) for Avignon (France, Southern Europe, Mediterranean climate) and Leuven (Belgium, Western Europe, temperate climate). Summer drought occurs when 0.5xPET (dotted line) > Precipitation (black line). Derived from New_LocClim v1.10 (FAO, 2005). .............................................................................................................. Fig. 1.5 Overview of the different chapters discussed in the manuscript. .................. 14 Fig. 1.6 Pictures illustrating installed and natural EGRs in a Mediterranean context. a G ass oof o a old fa e s house o the Causse M jea i the Pa National des Cévennes. The colder and humid local climate makes this kind of EGRs possible. b) EGR installed on the Naturoptère in Sérignan-du-Comtat as part of a sustainable building project. This EGR consists mainly of succulent species and is equipped with an irrigation system (sprinklers) that water the EGR every other day during summer at night for two times 15 minutes. The water comes from an underground rainwater harvesting (RWH) well (personal communication). c) Succulent species (mainly Sedum album, S. acre and S. ochroleucum) and grass species installed spontaneously on the roof of an old Mediterranean building. The plants are able to survive on the spots where water and OM accumulates. [Pictures: Carmen Van Mechelen; June 2011]. ............................................................... 15 Fig. 2.1 Map of the study region in southern France with indication of the departments and the 20 locations. 1. Caumont-sur-Durance; 2. La Crau; 3.

  Fig. 2.2 Pictures of four of the study locations: a) Plateau de Caume; b) Plateau d Au elas; Nivoliers; d) Cuvette de Pompignan [Pictures: Carmen Van Mechelen; June 2011] .................................................................................... Fig. 2.3 Biplot of PCA-results showing spatial plot-and variable-distributions along the first two axes. Plots are indicated by symbols according to their respective group. Variable abbreviations are explained in

Fig. 3

 3 Fig. 3.1 Bar charts visualizing the significant relationship between nominal primary plant traits and the average frequency of species used on EGRs. a) Leaf shape (LeafSH); b) metamorphosis for storage (MetStor); c) metamorphosis for vegetative dispersal (MetVD); d) plant strategy (PlStrat); e) photosynthetic pathway (PSP). Significant differences between categories are indicated by different letters (Kruskal Wallis test cf. Table 3.2ii and post hoc tests). Error bars indicate the standard deviations. ................................. Fig. 3.2 Bar charts visualizing the significant relationship between primary and other traits of species used on EGRs. Average values and standard deviation for the primary trait leaf area for each category in: a) shoot growth form (ShGF); b) pollinator reward (PollRew); c) leaf phenology (LeafPH). d) The primary traits whole leaf dry mass (WLDM) and specific leaf area (SLA) for each category in life form. Significant differences between categories are indicated by different letters (Kruskal Wallis test cf. Table 3.3ii and post hoc tests). ....................................................................... Fig. 3.3 Bar charts visualizing the significant relationship between primary and other traits of species used on EGRs. Average values and standard deviation for flowering period (FlowP), flowering duration (FlowD) and/or seed mass for the important categories of the primary traits: a) plant strategy (PlStrat); b) metamorphosis for vegetative dispersal (MetVD); c) metamorphosis for storage (MetStor); d) leaf shape (LeafSH). ...................... Fig. 3.4 Bar charts visualizing the significant associations of important primary plant trait categories (see legend) and other nominal plant traits of species used on EGRs (Pearson Chi² test cf. Table 3.3iii). a) life form; b) species reproduction (SpRepr); c) maximum plant longevity (MaxPL); d) flower color (FlowC); e) pollinator (Poll); f) flower type (FlowT); g) leaf phenology (LeafPH). Trait categories with a large number of species are important for EGR species performance. ..............................................................................

Fig. 1 . 1

 11 Fig. 1.1 Summary of GR benefits and the relation, either direct or indirect, to different aspects of ecosystem and human health. GR benefits are classified into the four ecosystem service categories (regulating, supporting, cultural and provisioning) and others. Adapted from Tzoulas et al. (2007).

Fig. 1 .

 1 Fig. 1.3 An EGR installed on a carport in Flanders, Belgium. Micro variation in environmental conditions clearly influences vegetation composition [Picture: Frederik Lerouge; Division of Forest, Nature and Landscape; June 2013].

Fig

  Fig. 1.4 Average annual weather patterns for the period 1970-2000 (temperature, precipitation and potential evapotranspiration PET) for Avignon (France, Southern Europe, Mediterranean climate) and Leuven (Belgium, Western Europe, temperate climate). Summer drought occurs when 0.5xPET (dotted line) > Precipitation (black line). Derived from New_LocClim v1.10 (FAO, 2005).

Fig

  Fig. 1.5 Overview of the different chapters discussed in the manuscript.

Fig. 2 . 1

 21 Fig. 2.1 Map of the study region in southern France with indication of the departments and the 20 locations. 1. Caumont-sur-Durance; 2. La Crau; 3. Plateau de Caume; 4. Lamanon; 5. Ve gues; . Cala ues; . Mas Dieu; . Plateau d Au elas; . Plateau d A ta o ; . St. Pie e des T ipie s; . Sau e t; . C os Ga o ; . Ni olie s; . Col d Aula ; . Col Milmandre; 16. Montagne de la Chamouse; 17. Cuvette de Pompignan; 18. Causse Blandas; 19. Causse Bégon; 20. La Roche sur le Buis.

Fig. 2 . 3

 23 Fig. 2.3 Biplot of PCA results showing spatial plot-and variable distributions along the first two axes. Plots are indicated by symbols according to their respective group. Variable abbreviations are explained in Table 2.1. Axis 1 is primarily a measure of climatic variation, whereas axis 2 rather contains the soil-related measures.

Fig. 2 . 4

 24 Fig. 2.4 Life form distributions of the species in the GRPL and the METPL. Categories are those of Raunkiaer (abbreviations in Table 2.1). Portions beneath the line segment in the GRPL and MEDPL bars indicate the amount of unique GR species (not comprised in the MEDPL) and the number of species from MEDVEG, respectively.

   show that trait categories associated with significant lower leaf area values are a decumbent shoot growth form, nectar as pollinator reward and evergreen leaf phenology. In Fig.3.2d, plants with a low specific leaf area are mostly geophytes and those with a low whole leaf dry mass are generally therophytes. Fig.3.3a indicates a seed mass of 0.13 mg a d a flo e i g pe iod i ea l su e fo the pla t st ateg atego st ess tole a e . Fig. . -d show that average values of seed mass and flowering duration fo the i po ta t leaf shape atego eedle-like o e sifo a e . g a d . months, respectively. Average values of seed mass and flowering duration for the eta o phosis fo sto age atego su ule e a e . g a d .

Fig

  Fig. 3.1 Bar charts visualizing the significant relationship between nominal primary plant traits and the average frequency of species used on EGRs. a) Leaf shape (LeafSH); b) metamorphosis for storage (MetStor); c) metamorphosis for vegetative dispersal (MetVD); d) plant strategy (PlStrat); e) photosynthetic pathway (PSP). Significant differences between categories are indicated by different letters (Kruskal Wallis test cf. Table 3.2ii and post hoc tests). Error bars indicate the standard deviations.

  Fig. 3.2 Bar charts visualizing the significant relationship between primary and other traits of species used on EGRs. Average values and standard deviation for the primary trait leaf area for each category in: a) shoot growth form (ShGF); b) pollinator reward (PollRew); c) leaf phenology (LeafPH). d) The primary traits whole leaf dry mass (WLDM) and specific leaf area (SLA) for each category in life form. Significant differences between categories are indicated by different letters (Kruskal Wallis test cf. Table 3.3ii and post hoc tests).

  Fig. 3.4 Bar charts visualizing the significant associations of important primary plant trait categories (see legend) and other nominal plant traits of species used on EGRs (Pearson Chi² test cf. Table 3.3iii). a) life form; b) species reproduction (SpRepr); c) maximum plant longevity (MaxPL); d) flower color (FlowC); e) pollinator (Poll); f) flower type (FlowT); g) leaf phenology (LeafPH). Trait categories with a large number of species are important for EGR species performance.

  Fig. 4.1. Overview of the experimental setup. a: The three structure types of the experimental EGR. From left to right: (5) 5 cm substrate, no water retention layer (WRL); (P5) 5 cm substrate on a WRL; (P10) 10 cm substrate on WRL. a: vegetation layer; b: substrate layer; c: WRL; d: drainage + filter layer; e: roof deck covered with EPDM foil. b: EGR in Heverlee on exposed side of the roof, before addition of substrate and vegetation. In plots P5 and P10, the WRL (white) can be seen. c: Vegetation development on the same EGR during summer 2014 [Pictures: Carmen Van Mechelen, September 2012 and July 2014].

Fig. 4 . 2

 42 Fig. 4.2 Weather data for a) Heverlee and b) Avignon in terms of precipitation, reference evapotranspiration (ETo) and 0.5*ETo. For comparison, yearly averages for precipitation and ETo (grey bars and striped line) were derived from the FAO program New_LocClim.

  4b): SR and D s of the weeds in the growing season of 2013; S. album cover during 2013; bryophyte cover and number of I. lutescens during autumn 2013 and growing season 2014; total cover in the autumn periods of 2013 and 2014; D s in the growing season of 2014.

Fig. 4 . 3

 43 Fig. 4.3 Evolution in cover data in Avignon and Heverlee over the 7 monitoring periods. Percentage cover is given for a) the total plot; b) annual species; c) perennial herbs. Structure types 5, P5 and P10 are represented by different symbols and are either uncolored (exposed plots, dotted lines) or black (sheltered plots; full lines).

Fig. 4 .

 4 Fig. 4.3 Continued. d) Sedum album; e) Sedum acre; f) bryophytes. Structure types 5, P5 and P10 are represented by different symbols and are either uncolored (exposed plots, dotted lines) or black (sheltered plots; full lines).

Fig. 4 . 5

 45 Fig. 4.5 Rank cover diagram of the species in Avignon and Heverlee for a) 2013 and b) 2014. Expositions and structure types 5, P5 and P10 are represented by line types in different colors. Both species present during the growing season (GS) and in autumn (AUT) were considered. Relative species cover is represented on a log-scale.

[

Fig. 5 . 1

 51 Fig. 5.1 Average monthly data for A: precipitation and B: temperature. Next to the weather data recorded during the rooftop experiment, normal averages for the study site derived from New_LocClim v1.10 (FAO 2005) are plotted.

Fig. 5 . 2

 52 Fig. 5.2 Weekly recorded SR, % cover and abundance of the four treatments in the greenhouse plots, from October 2013 to January 2014. Vertical bars indicate the standard deviation.

  Fig. 5.3 Histogram showing mycorrhizal colonization (%) of roots of four selected species in treatments ORG+MYCO and MIN+MYCO. Error bars represent the standard error. Species differ in their AMF dependency (Table 5.2): D. deltoides: 1 or rare; L. vulgare: 2 or occasionally; P. vulgaris: 3 or frequent; C. tinctoria: unknown.

Fig. 6 . 1

 61 Fig. 6.1 Stacked bar chart showing the evolution in time of the number of publications on GRs and irrigation. Both publications used in the review (n = 109) and excluded publications (n = 33) after screening (because not useful or irrelevant) are shown.

Fig. 6 . 2

 62 Fig. 6.2 Diagram showing the main topics of the 109 publications used in this review.

Fig. 6 . 3

 63 Fig. 6.3 Graph showing the scientific journals in which the publications used in this review (N = 109) were published. Journals are ranked according to the number of publications they represent (in decreasing order).

a

  WHC on a dry weight (DW) basis = [(saturated weight -DW) / DW] *100 b WHC at 50 cm tension

Fig. 7 . 1

 71 Fig. 7.1 Boxplots of the FD indices FDpg and FEve and the PD indices PDf and MPD EGR types 1-3 (Sedum, Dianthus-Thymus and Linaria-Galium type respectively). Significance of the Kruskal Wallis test is indicated, with post hoc test results for every type expressed by letters above the boxes. (**: p . ; ***: p . .

  Fig. 8.1 Overhead pictures of a plot with structure type 5, P5 and P10 in both Avignon and Heverlee in autumn 2014 [Pictures: Carmen Van Mechelen, October 2014].

  Fig. 8.2 Pictures of some animal species that were observed during the monitoring periods on the EGRs. a) Lepidoptera larvae; b) Bombus sp.; c) Caelifera sp.; d) Coccinellida sp.; e) Podarcis muralis sp. [Pictures: Carmen Van Mechelen, 2013-2014].

Table 3 .

 3 1 Overview of (i) 17 functional plant traits and (ii) 7 utilitarian aspects used for the correlation analysis, as they contained less than 75% of missing values. For the other we refer to Appendix 3.4. Variable types (scale: S, ordinal: O, nominal: N) are indicated and for scale variables, units are provided as well. Categories (for ordinal variables) and codes (for nominal variables) are listed. All variables are classified in a trait group. Authors (and corresponding reference) who contributed more than 10% of the trait data are also indicated .............................................................. Table3.2 The relationship between plant traits and frequency of use on EGRs For abbreviations of plant traits see Table3.1. (*: p < 0.05; **: p . ; N: u e of alid ases; df: deg ees of f eedo . ..............

(FREQ)

. (i) Spearman rank correlations (r s ) together with their significance. (ii) Kruskal Wallis test results (KW) together with their xii significance.

Table 5

 5 Table 5.2 Plant species present in the Optigreen Seed mix Type E (Optigrün International AG, 2014), relative species frequency in the four treatments xiii and SR at the end of the study period. Plant nomenclature follows The

.1 EGR substrate specifications of LP24GREXMIN1 (MIN) and LP24GREXORG1 (ORG) (Peltracom n.v.) ......................................................

Table 6 .

 6 . ................................................. 97 Table 5.3 Results of the (a) Anova and (b) MWU test statistics of the factors substrate type (SUBSTRATE) and AMF inoculation (MYCO) for the rooftop (R) and greenhouse (GH) experiment. The differences between R and GH experiments were also tested with a MWU test. Results comprise average values together with standard deviations (Av. ± s.d.) and test statistics (F-value for Anova; MWU for the MWU test) followed by the NA: not applicable). .................................................................................................. Table 5.4 Results of the Mixed Model statistics of the factors substrate type (SUBSTRATE), AMF inoculation (MYCO), TIME and, if significantly contributing to the model, two-way interactions for the greenhouse experiment. Results comprise the test statistics (F-value from the Type III of Fixed Effects) followed by the significance level (**: p 0.001; NS: not significant) and parameter estimates together with standard error (s.e.) of the Solution of the Fixed Effects.............................................................. Table 5.5 Spearman rank correlation coefficients (n = 12) and significance level for 2 Overview of the continents and countries in which the studies used in the review were conducted. Number of publications and relative frequency indicate where research regarding GRs and irrigation is situated.

	significance level (*: p	. ; **: p	.	; N": ot significant;

a) the species relative frequencies in the treatments in both rooftop and greenhouse experiment (n = 29) and b) for the variables measured in the rooftop and greenhouse experiment (n = 12). The significance level was adjusted with a Bonferroni correction. (*: p < 0.006 for a and b1 and . fo ; **: p . ; N": not significant). .........................................

Table 6.1 Summary of Köppen Climate Classification codes (KCC) covered by the studies collected in this review. (T: temperature in °C; P: precipitation in mm) ............................................................................................................. (NA: not applicable). ..................................................................................................

Table 6 .

 6 3 Summary of water conservation strategies, categorized into (1) adaptation of irrigation requirements; (2) alternative irrigation sources and (3) control and monitoring of irrigation regimes. Most important positive and possibly negative aspects are listed per strategy. ...................Table 6.4 Summary of water holding capacity (WHC) values advised by the FLL guidelines

  .......................................................................Table 7.2 Summary of the three green roof types in terms of key variables. Significance of differences between the types was tested using: A) Kruskal Wallis tests for quantitative variables. Average values and standard deviation are listed for every type, with significant differences expressed by superscript letters. B) Pearson Chi² tests for nominal variables, with indication of number of green roof systems per category for every type. (***: p . . .................................................................. Table 7.3 Combined results of the functional diversity (FD) maximization methods

  Plateau de Caume; 4. Lamanon; 5. Vernègues; 6. Calanques; 7. Mas Dieu; 8. Plateau d Au elas; . Plateau d A ta o ; . "t. Pie e des T ipie s; . "au e t; . C os Ga o ; . Ni olie s; . Col d Aula ; . Col Mil a dre; 16. Montagne de la Chamouse; 17. Cuvette de Pompignan; 18. Causse Blandas; 19. Causse Bégon; 20. La Roche sur le Buis. .....................................

1. Caumont-sur-Durance; 2. La Crau; 3. xvi

  Table 2.1. Axis 1 is primarily a measure of climatic variation, whereas axis 2 rather contains the soil-related measures. ............................................................................. Fig. 2.4 Life form distributions of the species in the GRPL and the MEDPL. Categories are those of Raunkiaer (abbreviations in Table 2.1). Portions beneath the line segment in the GRPL and MEDPL bars indicate the amount of unique GR species (not comprised in the MEDPL) and the number of species from MEDVEG, respectively. ..............................................................

  Roussillon and Provence-Alpes-Côte d Azu Fig. . , ith a pla t spe ies richness (SR) of approximately 5880 taxa. According to the plant repartition section of Tela Botanica, this number accounts for 50% of the total amount of taxa on French territory (Association TELA BOTANICA).

	The Medite a ea egio is lassified as sh u o t eelike pseudo steppe and open fo est i less d li ates U es o-FAO, 1969). The climate is considered as
	intermediate between cold temperate and dry tropical. Summer temperatures and
	the number of sun hours are high, e.g. average of 30°C and 2835 sun hours a year in
	the Vaucluse region

Table 2 .1 Differences between four vegetation types (clusters) for all variables and Raunkiaer life forms. Significance of differences was tested using Kruskal Wallis test (KW). Average values and post hoc test results are listed for every cluster, with significant differences expressed by superscript letters. (*: p < 0.05; **: p < 0.001; NS: not significant) Variable name

 2

		Abbrev.	Unit	KW			Average values
	Environmental			significance	Cluster1Δ	Cluster2○	Cluster3 +	Cluster4•
	Altitute	A	m	**	232 a	153 a	977 c	812 b
	Heat load	HL	-	*	0.91 ab	0.90 ab	0.90 a	0.92 b
	Soil depth	D	m	*	0.09 ab	0.08 a	0.09 ab	0.1 b
	Soil pH	pH	-	NS	7.95	8.02	8.04	8.01
	Electric conductance	EC	mS	**	0.21 a	0.19 a	0.20 a	0.25 b
	Organic matter	OM	%	**	7.2 ab	5.9 a	10.5 c	7.4 b
	Sand (63-2000µm)	Sand	%	**	40.1 a	43.6 ab	52.2 b	46 b
	Silt (2-63µm)	Silt	%	**	49.8 c	49.2 bc	41.4 a	45.7 ab
	Clay (<2µm)	Clay	%	**	10.1 b	7.3 a	6.3 a	8.4 ab
	Av. annual precipitation	AAP	mm	**	55.3 b	48 a	60.8 b	72.9 c
	Av. annual temperature	AAT	°C	**	13.7 c	13.4 b	11.6 a	7.1 a
	Pot. evapotranspiration	PET	mm	**	89.7 b	100.7 c	63.7 a	67.4 a
	Aridity index	Aridity	-	**	28.8 a	25 a	34 b	56.4 b
	Dry period	Dry	days	**	130 b	161 b	49 a	55 a
	Growing degree days	GDD	GDD	**	1265 a	1190 a	1897 b	993 a
	Vegetation-related
	Species Richness	SR	#	**	17 a	28 b	18 a	16 a
	Cover (cumulative)	Cover	%	**	67 b	75 b	65 b	56 a
	Bare soil	BS	%	**	22 b	7 a	6 a	20 b
	Bryophytes + lichen	BL	%	**	8 a	51 b	7 a	4 a
	Raunkiaer life forms
	Phanerophytes	Ph	#	NS	0	3	2	1
	Chamaephytes	C	#	NS	7	3	2	3
	Hemicryptophytes	H	#	**	37 a	32 a	46 b	42 b
	Geophytes	G	#	NS	3	3	1	2
	Therophytes	Th	#	**	29 b	71 c	2 a

Table 2 .2 Plant traits related to drought tolerance and self-sustainability, two key criteria for survival on EGRs. (a) traits and criteria used to exclude unsuitable species before scoring; (b) plant traits used for scoring. Plant traits were derived from the TRY initiative (Kattge et al., 2011). (a) Plant traits for species exclusion Exclusion criteria Reference

 2

	Raunkiaer life form	P	Julve, 1998
	Typical plant height	Tall plants (> 1 m)

Table 2 .3 Detailed description of vegetation types (clusters). Explanation of location numbers and variable abbreviations can be found in Fig. 2.1 and Table 2.1.

 2

		Cluste Δ	Cluste ○	Cluster 3 +	Cluste •
	Plots from locations	1, 3, 6-9	2, 4, 5	10-13	14-20
	N° of plots	84	36	46	87
	Characteristic plant traits
	low values A, Sand, Aridity, BL	A, Aridity, OM, AAP, BS	Silt, PET, AAT, Dry, BS, BL	Silt, PET, AAT, Dry, BL, Cover,
					GDD
	high values Cover, BS, Silt, Clay, AAT, PET,	Silt, AAT, Dry, Cover, BL, PET,	A, Cover, BS, Sand, Aridity,	A, BS, Sand, EC, OM, AAP, Aridity
		Dry	SR	GDD
	ISA results
	# species	76	112	53	55
	# species significantly
	belonging to cluster	46	98	40	25
	Indicator species Brachypodium retusum,	Poa bulbosa, Vulpia ciliata,	Thymus praecox, Trinia glauca,	Bromus erectus, Thymus
		Convolvulus cantabricus, Linum	Cerastium pumilum, Saxifraga	Koeleria vallesiana, Festuca	serpyllum, Linum suffruticosum,
		strictum, Euphorbia exigua,	tridactylites, Minuartia	marginata, Galium pusillum,	Coris monspeliensis, Galium
		Dactylis glomerata, Phlomis	hybrida, Bromus madritensis,	Potentilla neumanniana,	lucidum, Helianthemum italicum
		lychnitis, Sedum sediforme	Helianthemum	Ononis striata, Anthyllis
			nummularium, Erophila	montana
			verna, Brachypodium
			distachyon
	Phytosociology	Class Thero-Brachypodietea;	Class Thero-Brachypodietea;	Class Ononido-Rosmarinetea;	Class Ononido-Rosmarinetea
	(Appendix 2.2)	Association Brachypodietum	(la Crau: association	Association Ononidion striatae
		ramosi	Asphodeletum fistulosi)
	Vegetative description	Garrigue vegetation, typical for	Very species rich, dry and	Typical for mesophilic, open	(Meso)xerophilous garrigue and
		thermophilic limestone	basophilic with many	calcareous grasslands and	dry grasslands, with a very low
		pavements, with a high amount	therophytes, mosses and	prairies with a low abundance	abundance of therophytes (Loisel,
		of therophytes and a mosaic of	lichens (Molinier & Tallon,	of thero-and geophytes (Van	1976, Van den Berghen, 1963)
		different biological types (Loisel,	1949)	den Berghen, 1963)
		1976)

Table 2 .4 List of GRPL species that were noted in five of the seven key literature sources used to compile the list. The corresponding Raunkiaer life forms are indicated (abbreviations in Table 2.1), as well as the presence/absence in the MEDPL. Species Raunkiaer life form MEDPL

 2

	Sedum album	H	x
	Sedum rupestre	H
	Sedum sexangulare	H
	Allium schoenoprasum	G
	Hieracium pilosella	H	x
	Phedimus spurius	H
	Achillea millefolium	H	x
	Antennaria dioica	H
	Cota tinctoria	H
	Campanula rotundifolia	H	x
	Erodium cicutarium	Th	x
	Festuca ovina	H	x
	Origanum vulgare	H
	Petrorhagia saxifraga	H
	Prunella grandiflora	H
	Sedum acre	H	x
	Sedum kamtschaticum	H
	Sedum lydium	H
	Thymus serpyllum	H	x

Table 2 .5 Mediterranean species with good EGR potential (adjusted S o e % . Calculations were based on 10 key criteria for survival on EGRs (cf. Table 2.2). The presence/absence in the GRPL, the original source (MEDVEG or MEDLIT) and the Raunkiaer life form (abbreviations in Table 2.1) are indicated. For species of MEDVEG which were used in the vegetation analysis, the cluster number is displayed as well. Species name Also in GRPL Species list Adjusted score (%) Raunkiaer life form

 2

	Cluster
	n°

Table 3 .1 Overview of (i) 17 functional plant traits and (ii) 7 utilitarian aspects used for the correlation analysis, as they contained less than 75% of missing values. For the other we refer to Appendix 3.4. Variable types (scale: S, ordinal: O, nominal: N) are indicated and for scale variables, units are provided as well. Categories (for ordinal variables) and codes (for nominal variables) are listed. All variables are classified in a trait group. Authors (and corresponding reference) who contributed more than 10% of the trait data are also indicated. (i) Functional traits

 3

		Var.	Unit	Categories/Codes	Trait group	Authors/
		Type				Reference
	Leaf area	S	mm²		Leaf	Cornelissen,
					adaptation	1996; Kleyer
						et al., 2008;
						Siefert*
	Leaf dry matter	S	g/g		Biomass	Garnier et
	content per leaf					al., 2007;
	water-saturated					Kleyer et al.,
	mass (LDMC)					2008;
						Siefert*
	Leaf nitrogen	S	mg/g		Nitrogen	Ogaya &
	content per dry				content	Penuelas,
	mass (LeafN)					2003;
						Garnier et
						al., 2007;
						Reich et al.,
						2009
	Leaf phenology	N		Evergreen; winter	Leaf	Kattge et al.,
	(LeafPH)			deciduous; summer	phenology	2009; Reich
				deciduous		et al., 2009
	Leaf shape	N		Broad/full/simple;	Leaf	Kühn et al.,
	(LeafSH)			pinnate; palmate; digitate;	adaptation	2004; Paula
				needle-like/ ensiform;		et al., 2009
				scale-like
	Life form	N		Chamaephyte (C);	Life form	Julve, 1998
				hemicryptophyte (H):
				geophyte (G); therophyte
				(Th)
	Maximum plant	N		Annual; annual/biennal;	Plant longevity Diaz et al.,
	longevity			biennal;		2004; Kühn
	(MaxPL)			biennal/perennial;		et al., 2004;
				perennial ; indifferent		Moles et al.,
						2004;
						Poorter **
	Metamorphoses	N		None; succulent; bulb;	Reserves and	Kühn et al.,
	for storage			pleiocorm/primary	storage	2004
	(MetStor)			storage root; rhizome;
				runner; tuft; root
				tuber/secondary storage
				root; shoot tuber
	Metamorphoses	N		Fragmentation; bulb;	Regeneration	Kühn et al.,
	for vegetative			pleiocorm; rhizome;	strategy	2004
	dispersion			runner; root/shoot; root
	(MetVD)			tuber; shoot tuber;
				rhizome/ root/shoot;
				rhizome/runner
	Photosynthetic	N		C3; C4; CAM; C3/CAM;	Photosynthesis Kattge et al.,

*: Herbaceous Leaf Traits Database Old Field New York (unpublished data) **: Categorical Plant Trait Database (unpublished data)

Table 3 .1. *: p < . ; **: p . ; N: u e of alid ases; df: degrees of freedom).

 3

	(i) Spearman rank
	Trait	N	rs
	LeafArea	128	-0.33**
	LDMC	125	0.09
	LeafN	79	-0.07
	SeedMass	176	-0.03
	SLA	139	-0.22*
	TypPH	173	-0.10
	WLDM	114	-0.28*
	Wood	173	0.07
	FlowD	156	0.05
	FlowP	124	0.12
	(ii) Kruskal Wallis test
	Trait	df	KW
	LifeForm	3	6.06
	LeafPH	2	0.86
	LeafSH	5	12.86*
	MaxPL	5	8.69
	MetStor	6	24.53**
	MetVD	8	18.71*
	PSP	3	9.57*
	SpRepr	2	0.14
	PlStrat	3	17.66**
	FlowC	5	5.73
	FlowT	6	7.90
	Poll	2	3.96
	PollRew	1	2.15
	ShGF	3	2.36

Table 3 .3 The relationship between the primary plant traits (P) and other plant traits. (i) Spearman rank correlations together with their significance. (ii) Kruskal Wallis test results together with their significance. (iii) Pearson Chi² test results of the nominal variables. For abbreviations of plant traits see Table 3.1.

 3

*: p < . ; **: p .

; N: u e

of alid ases; df: deg ees of f eedo ; NA: ot applicable).

	(ii) Continued. Kruskal Wallis Test (KW)
			LeafArea (P)	LDMC LeafN SeedMass	SLA (P)		TypPH	WLDM (P)	Wood FlowD FlowP
	Poll			2.7	10.5	5.2		0.4	0.2			4.4	2.7	2.9	18.9** 1.4
		df		2		2	2		2		2				2	2	2	2
	PollRew			4.4*		0	0		0.4	0				1.4	3.3	2.9	3.8
		df		(i) Spearman rank (rs) 1 1 1		1		1				1	1	1	1
	ShGR			12.4*		0.5	2.8	LeafArea (P) SLA (P) WLDM (P) 3.8 6.5 2.1 9.7* 26.4**	6	1.1
		df		LeafArea (P) 3 3	3		3	1	3	0.20*	3	0.88** 3	3	3
							N		128			125	114
		LDMC iii Pea so Chi² χ² LeafSH (P) MetStor (P) MetVD (P) PSP (P) PlStrat (P) -0.05 -0.11 0.02 N 120 124 113
		LeafN LifeForm	31.7*		0.18 94.7**	0.21 93.9**	0.7	0.11	138.7**
				N		164	N	153	64	115	74	132	57	162
		LeafPH	SeedMass	17.5		0.43** 29.5*	-0.06 37.1*	0.45** 3.1	49.9**
				N		152	N	135	114	121 109	102	102	152
		SLA (P) LeafSH (P)		NA		0.20* 93.9**	1 97.7**	-0.16 3.2	47.1**
				N			N	146	125	139 115	106	114	162
		MaxPL	TypPH		21.8		0.25** 16.7	-0.04 22.8	0.26** 22.2*	121.6**
				N		162	N	146	124	132 115	114	114	162
		WLDM (P) MetStor (P) 93.9**		0.88** NA	-0.16 415.7**	98**	1	115.5**
				N		146	N		114	114 109	98	114	145
		Wood MetVD (P)	97.7**		-0.24** 415.7**	-0.01 NA	-0.22* 4.9	38.0*
				N		115	N	109	118			125	74	109	115
		PSP (P)	FlowD		3.2		-0.01 98**	0.19* 4.9	-0.07 NA	43.8**
				N		106	N	98	112	74	119	105	105
		SpRep		FlowP	60.0**		-0.14 79.3**	-0.19* 57.4**	-0.08 2.9	14.2*
				N		158	N	148	105	109 110	106	99	157
		PlStrat (P)	47.1**		115.5**		38.0*	43.8**	NA
	(ii) Kruskal Wallis Test (KW) N		162		145			115		105
		FlowC	LeafArea (P) N	LDMC LeafN SeedMass 47.3* 35.5 131 121	SLA (P) 46.5 89	TypPH 5.7 92	WLDM (P)	33.5* Wood FlowD FlowP 130
	LifeForm	FlowT		6.4	0.3 71.6**	0.8	22.5** 56.2*	11.5* 25	0.4	4.6	11.8* 16.9** 14.2* 33.0*	0.2
		df		3 N		3 125	2	115	3		3 83			3	84	2	3 124	3	2
	LeafPH	Poll		11.5*	2.9 33.8**	0.8	21** 73.7**	3.4 61.9**	6.4	1.3	14.4**	7.9 17.0*	7.6*	3.4
		df		2 N		2 162	2	152	2		2 114		2	112	2	2 161	2	2
	LeafSH (P)	PollR		16.2*		2.1 4	4.6	13.3* 2.2	19.7** 7.7	1.9	0.3	15.5*	8.2 2.3	20.8** 5.3
		df		5 N		5 119	4	109	5		5 81			5	80	5	5 118	5	5
	MaxPL	ShGF		5.4		0.2 18.7	2.6	4.6 17.5	8.8 25.6		0.8	0.9	7.7	4.6	1	6.9	6.4
		df		4 N		4 149	3	137	5		4 106		5	117	4	4 148	5	4
	MetStor (P)			14.4*		12	11.3	25.2**	13.3*	3.7	14*	4.2	17.9* 10.5
		df		6		6	6		6		6				6	6	6	6	6
	MetVD (P)			20.4*	18.7	11	26.7**	9.5			11	25.4**	9.2	17.6*	11
		df		8		8	8		8		8				8	8	8	8	8
	PSP (P)			6.2		0.1	2.4		4.4	1.8			0.3	2.7	0.3	0.5	1.7
		df		3		2	1		3		3				3	2	3	2	2
	SpRepr			1.6		0.1	1.6		0.5	0.6			1.5	1.1	3.8	3.8	0.4
		df		2		2	2		2		2				2	2	2	2	2
	PlStrat (P)			16**		6.7	4.3	14.5*	8.2*		9.7	16**	1.5	0.8	9.6*
		df		3		3	3		3		3				3	3	3	3	3
	FlowC			1.1		0	3.5		8.6	2				2.1	2	4.2	2.4	7
		df		5		5	4		5		5				5	5	5	5	5
	FlowT			12		0	8.6		9.1	11.7		4	7.7	28.5**	2.5	8.2
		df		6		6	5		6		6				6	6	6	6	6

Table 3 .5 Mediterranean species for potential EGR appli atio a d thei s ee i g s o e SS ased o the s ee i g tool i Ta le . . The olu Ne i di ates Medite a ea species that are currently not used on EGRs. Raunkiaer life forms (abbreviations in Table 3.1) are given as well.

 3

	Trait	Condition	Action
	Rooting depth	> 20 cm	delete
	Tolerance to drought	None	delete
	Life form	Phanerophyte	delete
	Typical plant height	> 1m	delete
	Plant strategy type	No stress tolerance	delete
	Primary traits		Derived traits
	Trait	Scoring condition (2	Trait	Scoring condition (1 point
		points for each matching		for each matching scoring
		scoring condition)		condition)
	Low leaf area *	< 368 mm²	Low seed mass *	< 4.54 mg
	Low specific leaf area *	< 18 mm²/mg	Low typical plant height *	< 0.2 m
	Low whole leaf dry mass *	< 25 mg	High woodiness *	(Semi) woody
	Metamorphosis for	Succulence	Leaf phenology *	Evergreen
	storage *
	Plant strategy type *	Stress tolerance	Pollinator reward +	Nectar
	Metamorphosis for	Fragmentation or runners Shoot growth form +	Decumbent
	vegetative dispersal *
	Photosynthetic pathway *	CAM	Short flowering duration +	< 2 months
	Leaf shape *	Needle-like/ensiform or	Late flowering period +	Early summer or later
		scale-like
			Life form *	Geophytes, therophytes,
				hemicryptophytes
			Flower color +	Yellow, blue or white
			Flower type +	Disc flowers
			Maximum plant longevity	Perennial
			*
	Sum of Scores:	/16		/ 12
	Total screening score (SS):			/ 28

Table 4 .1 Factory substrate specifications.

 4

	Peltracom LP24GREXORG1	FLL-norm	Analysis	Unit
	Particle size distribution
	d .		9	Weight %
	d> 4mm		44
	Volume weight
	Dry	NA	1	g/cm³
	Maximum water capacity	NA	1.43	g/cm³
	Water/air distribution
	Total pore volume	35-65	62	Vol.-%
	Maximum water capacity		42	Vol.-%
	Air capacity at maximum water
	capacity		20	Vol.-%
	Water permeability	0.6-70	27.9	mm/min
	Chemical analysis
	pH (CaCl2)	6.0-8.5	7.6
	Conductance	.	0.7	g>l
	Organic material
	Organic material		32	g/l
	Nutrients
	N		33	mg/l
	P		180	mg/l
	K		700	mg/l
	M		120	mg/l
	2.2. Monitoring
	From September 2012 until November 2014, meteorological data, i.e. temperature
	(°C), precipitation (mm) and wind speed (m/s), were collected daily from nearby
	eathe statio s i B ussels site in Heverlee) and Avignon-Cau o t ° N; ° E; elevation 58 m; 10 km from the study ° N; ° E; ele atio ; . k f o
	the study site in Avignon). Temperature data and average wind speed, together with
	daily sunshine hours (derived from New_LocClim v1.10; Gommes et al., 2004; FAO,

Table 4 .2 List of species installed on the EGR, categorized according to their longevity and installation method. Pictures of the species can be found at the end of this chapter.

 4

	Scientific name	Common name	Chorology	Installed	Current
				through	GR
					species
	ANNUALS
	Alyssum alyssoides (L.) L.	Pale Madwort	Eurasia	Seeds 2	no
	Clinopodium acinos (L.) Kuntze Basil Thyme	Europe	Seeds 2	yes
	Erophila verna (L.) DC.	Whitlow Grass	Eurasia	Seeds 2	no
	Lagurus ovatus L. *	Hare's-tail	Mediterranean	Seeds 2	yes
		Grass	-Atlantic
	Linum bienne Mill.	Pale Flax	Mediterranean	Seeds 2	no
			-Atlantic
	Petrorhagia prolifera (L.)	Proliferous pink	Europe	Seeds 2	no
	P.W.Ball and Heywood
	Plantago afra L.	Black Psyllium	Mediterranean	Seeds 2	no
	Silene conica L.	Striated	Eurasia	Seeds 2	no
		Catchfly
	PERENNIALS
	Herbs
	Carthamus carduncellus L.	Distaff Thistle	Mediterranean	Seeds 2	no
	Dianthus superbus L.	Large Pink	Eurasia	Seeds 2	no
	Euphorbia cyparissias L.	Cypress Spurge	Eurasia	Seeds 2	yes
	Helianthemum nummularium	Common	Eurasia	Seeds 2	yes
	(L.) Mill. **	Rockrose
	Lobularia maritima (L.) Desv.	Sweet Alyssum Mediterranean	Seeds 2	no
	Sideritis hyssopifolia L.	Mountain Tea	S-European	Seeds 2	no
			orophyte
	Succulents
	Sedum acre L.	Biting	European	Sprouts 3	yes
		Stonecrop
	Sedum album L.	White	Eurasia	Sprouts 4	yes
		Stonecrop
	Geophytes
	Allium sphaerocephalon L.	Round-headed	Eurasia	Bulbs 1	yes
		Leek
	Iris lutescens (Lam.)	Crimean Iris	Mediterranean Rhizomes 1	no
	Sources: (1) Bulb d'Argence; (2) BandT World seeds; (3) Pépinière Rouy; (4) pépinière Filippi
	* grass species; ** evergreen shrub

Table 4 .3 Mean values and standard errors for cover data, abundance data and diversity measures in Avignon and Heverlee in both the growing season and autumn period of 2013 and 2014. Significant differences between the locations are marked in grey and letters indicate the post-hoc results (Mann-Whitney U).

 4

		Growing season 2013	Autumn 2013
		Avignon	Heverlee	Avignon	Heverlee
	Cover data
	Annuals	22.42 ± 2.9 b	3.03 ± 0.7 a	13.28 ± 1.5 b	1.50 ± 0.4 a
	Herbs	10.17 ± 1.8 b	1.06 ± 0.2 a	48.06 ± 6.7 b	3.39 ± 0.7 a
	S. acre	4.67 ± 0.4 b	1.61 ± 0.1 a	8.83 ± 1.2 b	4.06 ± 0.4 a
	S. album	5.78 ± 0.5	7.11 ± 0.5	14.78 ± 2.7 a	28.44 ± 3.3 b
	Bryophytes	0	0	0.42 ± 0.2 a	10.81 ± 3.1 b
	Total	44.17 ± 4.5 b	13.61 ± 0.9 a	78.00 ± 4.9 b	41.83 ± 4.7 a
	Abundance data
	Annuals 111.11 ± 8.0 b	12.50 ± 3.7 a	137.50 ± 14.8 b	28.39 ± 7.4 a
	Herbs	58.17 ± 5.5 b	6.94 ± 1.7 a	285.17 ± 32.2 b	53.33 ± 10.5 a
	Allium	2.61 ± 0.2	2.61 ± 0.2	2.61 ± 0.2	2.28 ± 0.2
	Iris	2.56 ± 0.1	2.67 ± 0.1	2.33 ± 0.3	2.39 ± 0.2
	Diversity measures
	SR	9.56 ± 0.2	8.83 ± 0.7	7.22 ± 0.2	7.22 ± 0.5
	Ds	0.72 ± 0.0	0.61 ± 0.1	0.46 ± 0.0	0.45 ± 0.0
	SR weeds	1.89 ± 0.3	2.22 ± 0.3	7.22 ± 0.2	7.22 ± 0.5
	Ds weeds	0.31 ± 0.1	0.38 ± 0.1	0.11 ± 0.1	0
		Growing season 2014	Autumn 2014
		Avignon	Heverlee	Avignon	Heverlee
	Cover data
	Annuals	22.03 ± 5.0 b	8.22 ± 2.5 a	17.17 ± 3.2 b	0.72 ± 0.2 a
	Herbs	1.33 ± 0.5	0.39 ± 0.2	6.56 ± 1.5 b	0.58 ± 0.2 a
	S. acre	3.56 ± 0.7	3.50 ± 0.3	11.19 ± 2.5 b	2.33 ± 0.3 a
	S. album	9.39 ± 2.5 a	34.44 ± 3.5 b	16.06 ± 5.0 a	51.67 ± 3.3 b
	Bryophytes	0.17 ± 0.1 a	9.14 ± 3.5 b	8.78 ±2.4 a	28.11 ± 5.7 b
	Total	32.50 ± 7.1 a	48.89 ± 5.5 b	61.11 ± 6.6 a	80.33 ± 3.3 b
	Abundance data
	Annuals 127.33 ± 20.8 b 66.44 ± 12.7 a 212.39 ± 30.1 b	4.11 ± 1.6 a
	Herbs	4.39 ± 1.8	6.56 ± 2.4	63.44 ± 11.4 b	6.44 ± 1.8 a
	Allium	1.17 ± 0.3 a	2.56 ± 0.1 b	0.67 ± 0.2	0.22 ± 0.1
	Iris	1.28 ± 0.3 a	2.39 ± 0.2 b	1.17 ± 0.3 a	2.22 ± 0.2 b
	Diversity measures
	SR	6.39 ± 0.5	7.28 ± 0.4	6.44 ± 0.3	6.06 ± 0.8
	Ds	0.39 ± 0.1	0.40 ± 0.1	0.48 ± 0.0	0.45 ± 0.1
	SR weeds	0.50 ± 0.2	0.61 ± 0.2	0.72 ± 0.3	0.72 ± 0.3
	Ds weeds	0.07 ± 0.1	0.11 ± 0.1	0.30 ± 0.2	0.53 ± 0.2

Table 4 .4 Mean values and standard errors for cover data, abundance data and diversity measures in the exposed and sheltered plots of a) Avignon and b) Heverlee in both the growing season and autumn period of 2013 and 2014. Significant differences between the expositions are marked in grey and letters indicate the post-hoc results (Mann-Whitney U).

 4 Fig. 4.5 shows the rank-cover distributions of the installed species in the two locations for the growing season and autumn period of 2013 and 2014. For the relative cover data for every species under the different treatments over the seven monitoring periods we refer to Appendix 4.3. S. album and S. acre were always part of the dominant species in both locations on all monitoring moments. Next to those succulent species, Silene conica and Lobularia maritima were the species with the highest cover in Avignon in the growing season of 2013, followed by Alyssum alyssoides, Dianthus superbus and Lagurus ovatus. In Heverlee, high cover values were found for I. lutescens, A. sphaerocephalon and A. alyssoides. In the autumn of 2013, the SR decreased in Avignon from maximum 11 to 10 species and in Heverlee from maximum 14 to 12 species (Fig.4.5a). In Avignon L. maritima became very dominant together with S. album, which resulted in steeper lines in Fig.4.5a. There was also a high cover compared to the other species of L. ovatus and A. alyssoides in Avignon and L. maritima, I. lutescens and A. sphaerocephalon in Heverlee. In the second growing season, the maximum SR increased to 11 in Avignon but decreased to 11 in Heverlee (Fig.4.5b). S. conica became the dominant species again in Avignon, followed by L.ovatus and the succulents. In Heverlee, besides the succulents, S. conica, A. sphaerocephalon, I. lutescens and D. superbus covered a high portion of the plots as well. In autumn 2014, the SR decreased in Avignon to a maximum of 10 species and increased in Heverlee to a maximum of 12 species.

	a) AVIGNON	Growing season 2013	Autumn 2013
		Sheltered	Exposed	Sheltered	Exposed
	Cover data
	Annuals	24.11 ± 4.3	24.72 ± 4.1	13.11 ± 2.7	13.44 ± 1.5
	Herbs	11.72 ± 2.6	8.61 ± 2.5	45.56 ± 8.2	50.55 ± 11.1
	Sedum acre 5.89 ± 0.5 b	3.44 ± 0.3 a	12.33 ± 1.6 b	5.33 ± 0.6 a
	Sedum album	6.56 ± 0.9	5.00 ± 0.5	23.33 ± 3.3 b	6.22 ± 1.5 a
	Bryophytes	0	0	0.83 ± 0.4	0
	Total	52.78 ± 7.2	35.56 ± 3.8	87.33 ± 2.7	68.67 ± 8.5
	Abundance data
	Annuals 115.56 ± 10.9	106.7 ± 12	124.8 ± 26	150.2 ± 14.4
	Herbs 70.67 ± 6.4 b	45.7 ± 6.9 a	337 ± 43.0	233.3 ± 43.3
	Allium	2.55 ± 0.3	2.67 ± 0.2	2.78 ± 0.2	2.44 ± 0.3
	Iris	2.33 ± 0.2	2.78 ± 0.2	2.22 ± 0.4	2.44 ± 0.4
	Diversity measures
	SR	9.78 ± 0.2	9.33 ± 0.2	7.33 ± 0.3	7.11 ± 0.2
	SR weeds	1.56 ± 0.2	2.22 ± 0.6	0.67 ± 0.2	0.56 ± 0.2
	Ds	0.73 ± 0.0	0.71 ± 0.0	0.38 ± 0.1	0.54 ± 0.0
	Ds weeds	0.19 ± 0.1 a	0.46 ± 0.1 b	0.1 ± 0.1	0.12 ± 0.1
		Growing season 2014	Autumn 2014
		Sheltered	Exposed	Sheltered	Exposed
	Cover data
	Annuals 32.72 ± 7.7 b	11.3 ± 3.9 a	14.22 ± 3.4	20.11 ± 5.6
	Herbs	1.94 ± 0.9	0.72 ± 0.4	3.78 ± 1.0 a	9.33 ± 2.7 b
	Sedum acre 5.11 ± 1.1 b	2.00 ± 0.3 a	20.00 ± 2.5 b	2.39 ± 0.7 a
	Sedum album 15.44 ± 4.0 b	3.33 ± 0.6 a	31.67 ± 6.6 b	0.44 ± 0.3 a
	Bryophytes	0.33 ± 0.2	0	16.44 ± 3.1 b	1.11 ± 0.7 a
	Total 51.44 ± 10.8 b	13.6 ± 2.8 a	82.78 ± 3.2 b	39.44 ± 7.5 a
	Abundance data
	Annuals 161.33 ± 26.9	93.3 ± 28.6	197.2 ± 30.3	227.6 ± 53.7
	Herbs	7.00 ± 3.4	1.78 ± 1.0	41.78 ± 7.5	85.11 ± 19.4
	Allium	1.11 ± 0.5	1.22 ± 0.2	0.44 ± 0.2	0.89 ± 0.4
	Iris	0.56 ± 0.2 a	2.00 ± 0.3 b	1.89 ± 0.4 b	0.44 ± 0.2 a
	Diversity measures
	SR	6.11 ± 0.8	6.67 ± 0.6	7.11 ± 0.5	5.78 ± 0.3
	SR weeds	0.67 ± 0.3	0.33 ± 0.2	1.11 ± 0.5	0.33 ± 0.2
	Ds	0.45 ± 0.1	0.32 ± 0.1	0.47 ± 0.0	0.48 ± 0.0
	Ds weeds	0.12 ± 0.1	0	0.53 ± 0.2	0

Together with the succulents, S. conica remained the dominant species in Avignon, followed by L. maritima. In Heverlee, D. superbus and I. lutescens had the highest average cover values next to the succulents.

Table 4 .5 Mean values and standard errors for cover data, abundance data and diversity measures in the structure types 5, P5 and P10 in a) Avignon and b) Heverlee in both the growing season and autumn period of 2013 and 2014. Significant differences between the structure types are marked in grey and letters indicate the post-hoc results (Kruskal Wallis).

 4

	a) AVIGNON	Growing season 2013		Autumn 2013
		5	P5	P10	5	P5	P10
	Cover data
	Annuals	9.83 ± 0.8 a	21.25 ± 2.6 ab	36.17 ± 3.1 b	12.42 ± 2.6	11.17 ± 3.0	16.25 ± 2.0
	Herbs	3.33 ± 0.4 a	10.25 ± 2.6 ab	16.92 ± 2.7 b	20.83 ± 4.4 a	53.33 ± 9.2 ab 70.00 ± 10.3 b
	Sedum acre	4.83 ± 0.8	4.50 ± 0.6	4.67 ± 0.9	8.67 ± 1.5	7.17 ± 1.3	10.67 ± 3.1
	Sedum album	6.17 ± 0.7	5.33 ± 1.0	5.83 ± 1.1	18.00 ± 3.3	11.50 ± 4.9	14.83 ± 6.1
	Bryophytes	0	0	0	0.67 ± 0.5	0.42 ± 0.3	0.17 ± 0.2
	Total	25.00 ± 1.3 a	48.33 ± 6.9 b	59.17 ± 5.5 b	60.00 ± 9.0 a	78.33 ± 5.7 a	95.67 ± 1.3 b
	Abundance
	data
	Annuals	77.67 ± 6.1 a	113 ± 10.1 ab 142.67 ± 9.7 b 138.50 ± 28.8	126.67 ± 30.8	147.33 ± 20.1
	Herbs	40.00 ± 8.9	62.67 ± 7.1	71.83 ± 8.4	175.17 ± 31.1 a 291.67 ± 49 ab 388.67 ± 51 b
	Allium	1.83 ± 0.4	3.00 ± 0.0	3.00 ± 0.0	3.00 ± 0.0	2.00 ± 0.5	2.83 ± 0.2
	Iris	2.17 ± 0.3	2.67 ± 0.2	2.83 ± 0.2	3.00 ± 0.0	2.00 ± 0.5	2.00 ± 0.6
	Diversity
	measures
	SR	9.17 ± 0.2	9.83 ± 0.3	9.67 ± 0.3	7.33 ± 0.3	6.83 ± 0.2	7.50 ± 0.3
	SR weeds	0.67 ± 0.2 a	2.67 ± 0.7 b	2.33 ± 0.4 b	0.50 ± 0.2	0.33 ± 0.2	1.00 ± 0.4
	Ds	0.71 ± 0.0	0.74 ± 0.0	0.71 ± 0.0	0.52 ± 0.1	0.43 ± 0.1	0.44 ± 0.1
	Ds weeds	0 a	0.44 ± 0.1 b	0.39 ± 0.1 b	0	0	0.25 ± 0.1
			Growing season 2014		Autumn 2014
		5	P5	P10	5	P5	P10
	Cover data
	Annuals 13.08 ± 5.5 ab	10.00 ± 4.4 a	43.00 ± 8.2 b	8.00 ± 0.7 a	14.08 ± 2.7 ab	29.42 ± 7.0 b
	Herbs	0.00 ± 0.0 a	0.08 ± 0.1 a	3.92 ± 0.9 b	2.67 ± 0.3	5.33 ± 1.1	11.67 ± 3.7
	Sedum acre	2.67 ± 0.4	3.17 ± 1.1	4.83 ± 1.7	13.00 ± 4.4	10.58 ± 5.0	10.00 ± 4.2
	Sedum album	7.00 ± 0.9	8.17 ± 4.5	13.00 ± 6.1	14.83 ± 6.2	15.00 ± 8.1	18.33 ± 12.0
	Bryophytes	0.33 ± 0.2	0.17 ± 0.2	0	14.00 ± 5.1	9.25 ± 4.3	3.08 ± 1.4
	Total	20.50 ± 4.6	21.17 ± 8.8	55.83 ± 15.8	50.83 ± 13.1	55.00 ± 10.9	77.50 ± 8.4
	Abundance
	data
	Annuals 79.00 ± 24.7 a	80.00 ± 28.6 a 223.00 ± 15 b 109.67 ± 5.9 a 214.8 ± 34.4 ab 312.67 ± 63 b
	Herbs	0 a	0.17 ± 0.2 a	13.00 ± 3.5 b	30.83 ± 4.7 a 58.33 ± 10.5 ab 101.17 ± 26 b
	Allium	1.33 ± 0.4	0.50 ± 0.3	1.67 ± 0.5	0	0.67 ± 0.3	1.33 ± 0.6
	Iris	1.83 ± 0.5	0.83 ± 0.4	1.17 ± 0.5	0.50 ± 0.3 a	0.83 ± 0.4 ab	2.17 ± 0.4 b
	Diversity
	measures
	SR	5.17 ± 0.3 a	5.00 ± 0.4 a	9.00 ± 0.5 b	5.67 ± 0.2 a	6.17 ± 0.5 ab	7.50 ± 0.6 b
	SR weeds	0 a	0.33 ± 0.2 ab	1.17 ± 0.3 b	0 a	0.33 ± 0.2 ab	1.83 ± 0.5 b
	Ds	0.10 ± 0.1 a	0.54 ± 0.1 b	0.52 ± 0.0 b	0.50 ± 0.0	0.42 ± 0.0	0.51 ± 0.1
	Ds weeds	NA	0	0.10 ± 0.1	NA	0	0.43 ± 0.2

Table 4 .6 Spearman rank correlation coefficients and significance between the bryophyte cover, the SR and D of the weeds, and the cover data, abundance data and diversity measures. (*: p < 0.004; **: p < 0.001; NS: not significant; NA: not applicable).

 4

			AVIGNON			HEVERLEE
		Cover bryophyte s (N=126)	SR weeds (N=126)	Ds weeds (N=76)	Cover bryophyte s (N=126)	SR weeds (N=126)	Ds weeds (N=76)
	Cover data
	Annuals	-0.45	0.55**	0.19	0.06	0.44**	-0.17
	Herbs	-0.45	0.47**	0.07	0.14	0.35**	0.21
	Sedum acre	0.37**	0.03	-0.07	0.66**	0.06	-0.11
	Sedum album	0.34**	-0.03	0.06	0.83**	0.07	-0.05
	Bryophytes	NA	-0.23	-0.16	NA	-0.08	-0.16
	Total	0.35**	0.25	-0.15	0.82**	0.13	-0.07
	Abundance
	data
	Annuals	0.19	0.30*	0.08	0.21	0.32**	-0.24
	Herbs	0.11	0.34**	-0.09	0.17	0.38**	-0.01
	Diversity
	measures
	SR	-0.27*	0.77**	0.36*	0.11	0.58**	0.16
	SR weeds	-0.23	NA	0.83**	-0.08	NA	0.99**
	Ds	-0.35**	0.62**	0.32*	-0.13	0.47**	0.35
	Ds weeds	-0.16	0.83**	NA	-0.16	0.99**	NA

Table 5

 5

	.1 EGR substrate specifications of LP24GREXMIN1 (MIN) and LP24GREXORG1 (ORG)
	(Peltracom n.v.)
			FLL-norm MIN ORG Unit
	Particle size distribution
	d .		6	9	Weight %
	d> 4mm		53	44
	Volume weight
	Dry		NA	1.1	1	g/cm³
	Maximum water capacity	NA	1.36 1.43 g/cm³
	Water/air distribution
	Total pore volume	35-65	59	62	Vol.-%
	Maximum water capacity		26	42	Vol.-%
	Air capacity at maximum water capacity	33	20	Vol.-%
	Water permeability	0.6-70	183	27.9 mm/min
	Chemical analysis
	pH (CaCl2)	6.0-8.5	7.9	7.6
	Conductance	.		0.7	g>l
	Organic material
	Organic material		7	32	g/l
	Nutrients
	N			4	33	mg/l
	P			11	180	mg/l
	K			154	700	mg/l
	M			22	120	mg/l
	Vegetation development in the greenhouse started one week after sowing. From
	October 2013 until February 2014, SR, abundance and percentage cover were
	recorded weekly (15 observations). After this experimental period, all plants were
	collected, plant height and root length were measured, and H s , E H and dry weight
	(DW) were calculated. DW was determined by drying the aboveground plant parts
	for 24 hours at 105°C and weighing on a balance with a precision of 0.1 g. This
	information allowed the calculation of the mycorrhizal inoculation effect (MIE;
	Bagyaraj, 1992):
	MIE =	[	]

Table 5 .2 Plant species present in the Optigreen Seed mix Type E (Optigrün International AG, 2014), relative species frequency in the four treatments and SR at the end of the study period. Plant nomenclature follows The Plant List (2013). The AMF dependency of the species ranges from 1 (no/rare) to 3 (frequent/obligate). [a: Fitter and Peat, 1994; b: Pánková et al., 2014; c: Gucwa-Przepióra and Blaszkwoski, 2004;d Piotrowski et al., 2004; e: Sawilska and Jendrzejczak, 2013].

 5

	a) ROOFTOP	AMF dependency	MIN	MIN+MYCO	ORG	ORG+MYCO
	Optigreen Seed Mix
	Achillea millefolium L.	3 a	1.25	1.82	1.32	2.7
	Allium schoenoprasum L.	3 a	1.25	0.61	1.32	-
	Aster amellus L.	3 a	-	-	-	-
	Campanula rotundifolia L.	3 a	0.63	-	2.63	-
	Centaurea scabiosa L.	3 a	-	-	-	-
	Clinopodium vulgare L. *	3 a	-	-	-	-
	Cota tinctoria (L.) J.Gay	unknown	23.75	20	14.47	10.81
	Dianthus armeria L. *	unknown	-	-	-	-
	Dianthus carthusianorum L.	2 c	26.88	36.36	25	15.32
	Dianthus deltoides L.	1 a	3.13	1.21	3.95	6.31
	Erodium cicutarium (L.) L'Hér. *	2 a	-	-	-	-
	Euphorbia cyparissias L. *	3 a	-	-	-	-
	Galium verum L.	3 a	1.25	-	2.63	-
	Geranium robertianum L.	2 a	-	-	-	-
	Helianthemum nummularium (L.) Mill. *	2 a	-	-	-	-
	Helichrysum arenarium (L.) Moench *	3 c	-	-	-	-
	Leucanthemum vulgare (Vaill.) Lam.	2 d	8.13	6.67	7.89	7.21
	Linaria vulgaris Mill.	2 a	1.88	0.61	-	1.8
	Linum perenne L. *	1 a	-	-	-	-
	Origanum vulgare L.	3 a	-	-	-	-
	Petrorhagia saxifraga (L.) Link	unknown	1.88	0.61	1.32	1.8
	Pilosella aurantiaca (L.) F.W.Schultz & Sch.Bip.	unknwon	-	-	-	-
	Pilosella officinarum Vaill. *	3 a	-	-	-	-
	Potentilla argentea L.	3 a	8.75	7.88	18.42	23.42
	Prunella grandiflora (L.) Scholler	unknown	3.75	1.21	-	1.8
	Prunella vulgaris L.	3 a	-	-	-	-
	Sanguisorba minor Scop.	3 a	1.88	3.03	3.95	6.31
	Saponaria ocymoides L.	unknown	-	-	-	-
	Saponaria officinalis L.	1 a	9.38	15.76	11.84	14.41
	Sedum album L.	1 a	-	-	-	-
	Sedum rupestre L.	1 a	-	-	-	-
	Silene nutans L.	2 a	-	-	-	-
	Silene otites (L.) Wibel *	2 a	-	-	-	-
	Thymus pulegioides L.	2 c	3.75	1.21	3.95	3.6
	Thymus serpyllum L.	3 a	-	-	-	-
	Unexpected species
	Alyssum alyssoides (L.) L.		0.63	0.61	-	2.7
	Arenaria serpyllifolia (L.)	1 a	1.25	2.42	-	1.8
	Capsella bursa-pastoris (L.) Medik.	1 a	-	-	-	-
	Poa annua L.	2 a	0.63	-	1.32	-
	Sonchus oleraceus (L.) L.	3 a	-	-	-	-
	Total species richness		18	15	14	14
	Germinated from Seed mix		15	13	13	12
	Unexpected species		3	2	1	2

  .2). Between 12 and 15 species of the sown species established and 3 unexpected species were observed. Most frequently occurring species (relative frequency > 10%) were Dianthus carthusianorum and Cota tinctoria in the MIN treatment (26.88 and 23.75% respectively), and D. carthusianorum, C. tinctoria and Saponaria officinalis in the MIN+MYCO plots (36.36, 20 and 15.76% respectively). In the ORG plots, D. carthusianorum, Potentilla argentea, C. tinctoria and S. officinalis showed the highest frequencies (25, 18.42, 14.47 and 11.84% respectively) and P. argentea, D. carthusianorum, S. officinalis and C. tinctoria had the highest frequencies in the ORG+MYCO plots

Table 5 .3 Results of the (a) Anova and (b) MWU test statistics of the factors substrate type (SUBSTRATE) and AMF inoculation (MYCO) for the rooftop (R) and greenhouse (GH) experiment. The differences between R and GH experiments were also tested with a MWU test. Results comprise average values together with standard deviations (Av. ± s.d.) and test statistics (F-value for Anova; MWU for the MWU test) followed by the significance level (*: p 0.05; **: p . ; NS: not significant; NA: not applicable).

 5

					Plant height			Root length
	a) ANOVA Treatment	SR	Abundance	(cm)	Hs	EH	(cm)	Cover(%)	Dry weight (g)
	ROOF	SUBSTRATE
		MIN Av. ± s.d.	10 ± 4.0	54.17 ± 42.8	1.21 ± 0.6	See b)	0.82 ± 0.1	NA	See b)	NA
		ORG Av. ± s.d. 6.33 ± 4.8 31.17 ± 33.6	1.06 ± 0.8		0.82 ± 0.1
		F-value	2.07 NS	1.07 NS	0.12 NS		0.03 NS
		MYCO
		NO Av. ± s.d.	8 ± 5.7	39.33 ± 44.8	1 ± 0.8	1.5 ± 0.75 0.84 ± 0.1	NA	See b)	NA
		YES Av. ± s.d. 8.33 ± 3.9	46±35.3	1.27 ± 0.59	1.62 ± 0.52 0.8 ± 0.1
		F-value	0.01 NS	0.08 NS	0.43 NS	0.1 NS	0.81 NS
	GH	SUBSTRATE
		MIN Av. ± s.d.	Table 5.4	Table 5.4	7.12 ± 1.8	See b)	0.9 ± 0.0	See b)	See b)	See b)
		ORG Av. ± s.d.			11.8 ± 4.0		0.91 ±0.0
		F-value			6.90*		0.37 NS
		MYCO
		NO Av. ± s.d.	Table 5.4	Table 5.4	9.35 ± 4.1	2.42 ± 0.2	0.9 ± 0.0	7.11 ± 1.3	See b)	See b)
		YES Av. ± s.d.			9.58 ± 3.9	2.59 ± 0.1 0.91 ± 0.0	7.18 ± 0.7
		F-value			0.01 NS	2.64 NS	1.76 NS	0.02 NS

Table 5 .4 Results of the Mixed Model statistics of the factors substrate type (SUBSTRATE), AMF inoculation (MYCO), TIME and, if significantly contributing to the model, two-way interactions for the greenhouse experiment. Results comprise the test statistics (F-value from the Type III of Fixed Effects) followed by the significance level (**: p 0.001; NS: not significant) and parameter estimates together with standard error (s.e.) of the Solution of the Fixed Effects Solution + Type III of Fixed Effects SR

 5

	Cover (%)	Abundance

Table 5 .5 Spearman rank correlation coefficients (n = 12) and significance level for a) the species relative frequencies in the treatments in both rooftop and greenhouse experiment (n = 29) and b) for the variables measured in the rooftop and greenhouse experiment (n = 12). The significance level was adjusted with a Bonferroni correction

 5

	. (*: p < 0.006 for a and b1 and 0.008 for b2; **: p .	; NS: not significant).
	a) Relative frequencies			ROOFTOP					GREENHOUSE
			MIN	MIN+MYCO	ORG	ORG+MYCO		MIN	MIN+MYCO	ORG	ORG+MYCO
	ROOFTOP	MIN	1
		MIN+MYCO	0.91**	1
		ORG	0.83**	0.72**		1
		ORG+MYCO	0.89**	0.95**	0.73**	1
	GREENHOUSE	MIN	0.38 NS	0.38 NS	0.58**	0.41 NS		1
		MIN+MYCO	0.42 NS	0.34 NS	0.65*	0.34 NS		0.81**	1
		ORG	0.57*	0.49 NS	0.71**	0.52*		0.83**	0.88**	1
		ORG+MYCO	0.55*	0.49 NS	0.70**	0.54*		0.84**	0.89**	0.97**	1
	b) Measured variables
	1) ROOFTOP		SR	Cover	Abundance	Height	Hs	EH	Root length	Dry weight
		SR	1
		Cover	0.96**	1
		Abundance	0.94**	0.96**		1
		Height	0.88**	0.94**	0.91**	1
		Hs	0.95**	0.94**	0.89**	0.84*	1
		EH	0.11 NS	0.04 NS	-0.09 NS	0 NS	0.3 NS	1
	2) GREENHOUSE
		SR	1
		Cover	0.15 NS	1
		Abundance	0.48 NS	0.83*		1
		Height	0.33 NS	0.6 NS	0.57 NS	1
		Hs	0.91**	0.16 NS	0.45 NS	0.15 NS	1
		EH	0.1 NS	0	0.01 NS	-0.3 NS	0.34 NS	1
		Root length	0.39 NS	-0.15 NS	-0.25 NS	0.17 NS	0.15 NS	0.31 NS	1
		Dry weight	0.16 NS	0.84*	0.68 NS	0.41 NS	0.11 NS	-0.09 NS	-0.29 NS	1

Table 6 .1 Summary of Köppen Climate Classification codes (KCC) covered by the studies collected in this review. (T: temperature in °C; P: precipitation in mm).

 6

	KCC Description				Region examples	# of
					studies
	Bsk Cold steppe climate / Dry semi-arid		US (Colorado), Spain	3
	Annual T < 18°C; annual P > 5*Pth
	Cfa Warm temperate climate with hot summer, fully	US (Texas, Alabama,	6
	humid / Subtropical				Florida), Japan, China
	-•C < Tmin < •C; Tmax	•C		(Shanghai)
	Cfb Warm temperate climate with warm summer, fully	UK, Canada	4
	humid / Marine				(Vancouver)
	A ual T < °C; -°C < Tmin < 18°C; at least 4 o ths ith T °C
	Csa Warm temperate climate with dry, hot summer /	Greece, Israel	11
	Interior Mediterranean
	-°C < Tmin < 18°C; Tmax	°C; winter Pmax >
	3*summer Pmin
	Csb Warm temperate climate with dry, warm summer /	US (Oregon), Australia	5
	Coastal Mediterranean			(Adelaide)
	-°C < Tmin < 18°C; at least	o ths ith T	°C;
	winter Pmax > 3*summer Pmin
	Cwa Warm temperate climate with wet, hot summer and	China (Hong Kong)	3
	dry winter
	-°C < Tmin < 18°C; Tmax	°C; summer Pmax >
	10*winter Pmin
	Dfb Snow climate, warm summer, fully humid /	US (Massachusetts;	17
	Continental				Michigan), Canada
	Tmin -°C; Tmax	°C; at least	o ths ith T	(Toronto, Nova Scotia;
	10°C				Kelowna), Germany
	Dwa Snow climate with hot summer, dry winter / Moist	China (Beijing), Korea	2
	continental
	Tmin -°C; Tmax	°C; summer Pmax > 10*winter
	Pmin

Table 6 .2 Overview of the continents and countries in which the studies used in the review were conducted. Number of publications and relative frequency indicate where research regarding GRs and irrigation is situated. (NA: not applicable) Continent Country Number of publications Relative frequency (%)

 6

	Oceania	8	7.34
	Australia	6
	New Zealand	2
	Asia	13	11.93
	China	7
	Japan	3
	Korea	1
	Israel	2
	Europe	23	21.10
	UK	8
	France	2
	Germany	2
	Greece	8
	Italy	2
	Spain	1
	South America	1	0.92
	Brazil	1
	North America	48	44.04
	unknown	3
	Canada	5
	US	40
	NA	16	14.68

Table 6 .3 Summary of water conservation strategies, categorized into (1) adaptation of irrigation requirements; (2) alternative irrigation sources and (3) control and monitoring of irrigation regimes. Most important positive and possibly negative aspects are listed per strategy.

 6

	1. IRRIGATION REQUIREMENTS
	GR VEGETATION
	Xeriscaping including Sedum spp. and moss	Wetland GRs
	+ Drought tolerant	+ High WHC
	+ Encourage biodiversity through facilitation
	processes	+ Good thermal performance of roof
		+ Stormwater management
		-Increased weight
		-Pest problem
	GR MATERIALS
	Substrate with higher WHC	Additional water retention layer (WRL)
	+ Many possibilities	+ Stormwater management
	-Under study
	2. IRRIGATION SOURCES
	Gray water	Rainwater harvesting (RWH)
	+ Reduction potable water consumption	+ Clean enough
	+ Less pressure on sewage system	+ Stormwater management
	-Clean enough to ensure GR performance in
	the long term?	-Cost rainwater tanks
	3. CONTROL + MONITOR IRRIGATION REGIMES
	Timed irrigation
	+ Based on meteorological factors and/or
	substrate moisture sensors
	+ Smart controller turns irrigation on when
	necessary
	-Still under study

Table 6 .4 Summary of water holding capacity (WHC) values advised by the FLL guidelines (FLL, 2008) and of alternative substrate materials tested in different GR studies. The WHC indicates the ability of a material, in compacted condition, to hold water, and is generally expressed as the water content of a substance after initial water saturation followed by a 2-hour dripping period (FLL, 2008).

 6

	Reference	Materials	WHC	Unit
	FLL, 2008 Elstein et al., 2008	General IGR substrate General EGR substrate Potting soil	a a 394 a	% vol. % vol. % DW
		Petrochemical-based hydrophilic foam		% DW
		sheets	1994 a
		Fiberglass mats	1804 a	% DW
	Harp et al., 2008	Open cell foam	56.5	% vol.
		Polyurethane foam	71.9	% vol.
		Synthetic sponge	65.9	% vol.
		Sand	36.3	% vol.
		Peat moss-bark-sand 3:1:1	51.9	% vol.
		GR substrate	30.3	% vol.
	Nektarios et al., 2011 Soilless substrate	37 b	% vol.
		Substrate with sandy loam soil	38 b	% vol.
	Kotsiris et al., 2013	Pumice-peat-zeolite 65:30:5	25 b	% vol.
		Pumice-compost-zeolite 65:30:5	30 b	% vol.
		Sandy loam soil-perlite-zeolite 30:65:5	35 b	% vol.
	Papafotiou et al.,			% vol.
	2013	Grape marc compost-soil-perlite 2:3:5	32 b
		Peat:soil:perlite 2:3:5	30 b	% vol.
	Kanechi et al., 2014	Amended soil	58.7	% vol.
		Furnace bottom ash	43.8	% vol.

Table 6 .5 Summary of irrigation methods and regimes from studies in which outdoor GR experiments were conducted in different climatic conditions. GR type: extensive (EGR), semi- extensive (SEGR) or intensive (IGR). GR dimensions (length x width) were used to calculate the minimum daily water requirements (Iday) during the growing season and summer.

 6

	KCC GR type	Irrigation method	Irrigation regime	GR dimensions	Substrate	Iday (l/m²) Reference
					(m)	depth (m)
	Bsk	EGR	Drip irrigation; Overhead	Growing season 1: 18.7 mm/week, later 8 mm/week; Growing	NA	0.10	0.91	Bousselot et al., 2010
			rotator system	season 2: 6.4 mm/week
		EGR	Drip irrigation	In summer	-		-	Pérez et al., 2012
		SEGR	No irrigation	-	-		-	Coutts et al., 2013
	Cfa	EGR	No irrigation	-	-		-	Dvorak & Volder, 2013
		EGR	Hand watering; Subsurface	After planting: every 2 days. Later: 5 l two times/day for 30	0.61 x 1.22	0.10	13.44	Price et al., 2011
			capillary irrigation	min. Winter: no irrigation
		EGR	Drip irrigation	Month 1: 500 ml daily. Later: daily or every two days (except if	0.9 x 0.9	0.10	0.31	Kanechi et al., 2014
				> 5 mm precipitation event occurred)
		EGR	unknown	Month 1: daily. Later: 5 l every two days (except if > 5 mm	0.9 x 0.45	0.10	6.17	Sendo et al., 2010
				precipitation event occurred)
		EGR	unknown	1 time/week with tap water (except if > 5 mm precipitation	-		-	Aithkenhead-Peterson et al.
				event occurred or if predicted chance of rain is > 50% next				2011
				days
		SEGR	unknown	2 times/week for 15 min with collected rainwater	-		-	Sonne, 2006
	Cfb	EGR	No irrigation	-	-		-	Nagase & Dunnett, 2013a
		EGR +	Drip irrigation	1 time/week in summer	-		-	Nagase et al., 2013
		SEGR
		EGR	Handheld hose	When substrate moisture < 15%, until saturation	-		-	Nagase & Dunnett, 2013b
		SEGR	Hand watering	1 time/week	-		-	Dunnett et al., 2008

Table 7 .1 List of 29 traits used for the FD approach, selected after screening of the literature on ecosystem services of GRs. Every trait is supplemented with the following information: ecosystem properties explaining the importance of the trait for the ecosystem service; ecosystem service category (C: cultural; R: regulating; S: supporting; P: provisioning); brief description, variable type (N: nominal; S: scale; O: ordinal) and missing values which indicate the % of species for which no trait information could be found. [Source references: Fitter and Peat, 1994; Shipley, 1995; Cornelissen, 1996; Kühn et al., 2004; Couplan, 2008; Kleyer et al., 2008; Oudolf and Kingsbury, 2013].

 7

	Plant trait	Ecosystem properties	Ecosystem	Brief description
			service
			category

Table 7 .2 Summary of the three green roof types in terms of key variables. Significance of differences between the types was tested using: a) Kruskal Wallis tests for quantitative variables. Average values and standard deviation are listed for every type, followed by a letter indicating the significant differences (Kruskal Wallis post hoc test). b) Pearson Chi² tests for nominal variables, with indication of number of green roof systems per category for every

 7

	type. (***: p .	.
	a) Quantitative variables	Kruskal	Sedum	Dianthus-Thymus	Linaria-Galium
			Wallis	type	type	type
	# Green roof systems		22	27	8
	Species richness		33.10 (***) 8.18 ± 2.3 a	22.33 ± 8.4 b	19.50 ± 7.8 b
	Substrate depth		34.50 (***) 4.91 ± 1.5 a	9.22 ± 2.6 b	9.63 ± 2.3b
	Percentage of succulent		97.73 ± 9.2
	species		42.96 (***)	b	18.87 ± 16.3 a	6.88 ± 9.7a
	b) Nominal variables		Pearson	Sedum	Dianthus-Thymus	Linaria-Galium
			Chi²	type	type	type
	Installation method		54.50 (***)
	Cuttings		9	0	0
		Mat		12	2	1
	Seeds		0	18	1
		Plugs		1	7	6

Table 7 .3 Combined results of the FD maximization methods FDMax1 and FDMax2, offering an exact and practical species list, respectively. Number of groups corresponds to the average SR of every type. Species of FDMax1 are underlined and the subsequent numbers rank these species according to decreasing functional dissimilarity. The other species (up to 6, for the complete list we refer to Appendix 7.3) can be used as alternatives. For comparison, both the system FD and the maximized FD of FDMax1 for a system with the average SR are shown.

 7

	System type	Sedum type	Dianthus-Thymus type	Linaria-Galium type
	Average SR	8		22		20
	System FDpg	0.9		2.9		2.8
	Maximized FDpg	1.1		3.2		3.3
	Group
	1	Phedimus selskianus 1	Sedum lydium	Achillea millefolium	Leucanthemum vulgare	Achillea millefolium	Leucanthemum vulgare
		Sedum album	Sedum rupestre	Anthemis tinctoria	Pimpinella saxifraga	Daucus carota	Reseda lutea
		Sedum hybridum	"edu se a gula e … Daucus carotus	Trifolium arvense	Echium vulgare
	2	portenschlagiana 6 Campanula	poscharskyana Campanula	Aurinia saxatilis 11	Achillea tomentosa	Agrimonia eupatoria 17	Hypericum perforatum
	3	Sedum spathulifolium 8		Dianthus carthusianorum Potentilla argentea	Saxifraga granulata 16	Geranium sanguineum
				Dianthus deltoides	Potentilla verna	Armeria maritima	Leontodon hispidus
				Linum perenne	P u ella g a diflo a …	Bellis perennis	P i ula e is …
	4	Hylotelephium spectabile		Allium roseum 14	Allium sphaerocephalon	Anthemis tinctoria
				Iris pumila 4	Iris tectorum
				Allium schoenoprasum	Leopolida comosa
	5	Prunella grandiflora 2		Anemone pulsatilla		Anthyllis vulneraria	Lotus corniculatus
	6	Saxifraga cotyledon 3	Sedum hispanicum	Armeria maritima	Primula veris	Carex flacca 3
				Briza media	Saxifraga granulata
				Dianthus anatolicus	"a if aga pa i ulata …
	7	Sedum cauticola 4	Sedum ewersii	Anthoxanthum odoratum Festuca glauca	Centaurea cyaneus 6	Papaver rhoeas 1
		Sedum cyaneum 7	Sedum telephium	Carex sylvatica	Festuca ovina
				Euphorbia cyparissias	Koele ia glau a …
	8	Sempervivum tectorum5 Sempervivum	Anthyllis vulneraria	Petrorhagia saxifraga	Cichorium intybus 15	Linaria vulgaris

1. Nature as a source of inspiration for extensive green roof plant species selection 1.1. Green roofs and the habitat template concept

	Research question 1: What kind of vegetation can be found in natural (sub)
	Mediterranean areas selected according to the habitat template concept of extensive
	green roofs (EGRs)? To what extent do the plants in the resulting Mediterranean
	plant list offer new possibilities for use on EGRs?

Table 8 .1 List of species tested on EGRs in Chapter 4 together with their respective screening score (SS) derived from the screening tool developed in Chapter 3. The species overall performance during the experimental period in Avignon and Heverlee is indicated on an ordinal scale (-: no germination or no reoccurrence after first growing season; +, ++, +++: species performing respectively OK, well and very well).

 8

		Screening Score (SS) AVIGNON HEVERLEE
	ANNUALS
	Alyssum alyssoides (L.) L.	12	++	++
	Clinopodium acinos (L.) Kuntze	2	-	-
	Erophila verna (L.) DC.	10	-	+
	Lagurus ovatus L.	2	+++	-
	Linum bienne Mill.	2	+	+
	Petrorhagia prolifera (L.) P. W. Ball &
	Heywood	14	-	+
	Plantago afra L.	3	+	-
	Silene conica L.	9	+++	++
	PERENNIALS
	Herbs
	Carthamus carduncellus L.	Unknown	-	-
	Dianthus superbus L.	10	+	+++
	Euphorbia cyparissias L.	10	+	++
	Helianthemum nummularium (L.) Mill.	16	-	-
	Lobularia maritima (L.) Desv.	8	+++	++
	Sideritis hyssopifolia L.	2	-	++
	Succulents
	Sedum acre L.	23	+++	++
	Sedum album L.	25	+++	+++
	Geophytes
	Allium sphaerocephalon L.	12	+	++
	Iris lutescens (Lam.)	Unknown	++	++

Ways to improve green roof functional diversity and the ecosystem services they provide

  This review pointed at sustainable irrigation possibilities that need to be elaborated further with scientific experiments and examples of local implementations. Different irrigation regimes are needed according to climatic context, and guidelines for irrigation specifications in each climate need to be determined. If public awareness of sustainable water consumption is promoted and if GRs are designed carefully, then irrigation practices can be sustainable in the long term and contribute to better urban life quality.

	2.3. Research question 6: Assuming that functional diversity (FD) and phylogenetic
	diversity (PD) of plant communities have a positive effect on ecosystem functioning,
	to what extend do different GR systems available on the market contribute to GR
	ecosystem functioning? Can FD and hence the ecosystem services of GRs be
	improved?

  studies described and discussed in this thesis indicate that the Mediterranean vegetation can definitely inspire us as a source for development and improvement of EGR design, whether this is for the current Mediterranean climate itself or for the temperate climate under prospected climate change. The elaborated concept for EGR design is summarized in Table8.2. Before the start of an EGR project, one should consider some important questions regarding the vegetation and EGR structure. For example, what is the exact location where the GR shall be installed, and what climatic conditions prevail in that location? What is the maximum accepted weight of the EGR and what is the main goal (e.g. biodiversity, thermal insulation, and aesthetics)? Not only EGRs, but also other applications in urban greenery like green walls, parks and gardens can make use of the habitat template hypothesis as a starting point for finding potentially suitable plant species

Table 8 .2 The EGR concept

 8

			EGR Concept
		1	2	3	4
		Exploratoring questions Habitat template	Screening tool	Test potential plant
			approach		species
	VEGETATION	Location? Climate?	Select + analyze natural habitats Compose native species list based on field	Score the native species according to important plant traits Eventually adapt the tool to local environmental	Incorporate different native life forms Both novel and already used GR native species
		Main goal of GR installation?	observations and/or literature	conditions (e.g. focus on frost tolerance instead of drought-tolerance)
		Design questions	Install species on	Install appropriate	Maintenance (limited)
			heterogenous EGR	irrigation system
		Building weight	Use different substrate	Adapt irrigation	Remove harmful weeds
		restrictions?	depths and types	requirements	that survived the dry
	STRUCTURE	Plant substrate requirements?	(with/without OM and/or AMF) Create spots with a WRL Use alternative irrigation sources	summer period Reseed only if no seeds have germinated after
					the dry summer period
			Make use of different	Control and monitor
			expositions if possible	irrigation regimes for a
			(shade, exposed)	sustainable use
		Stimulate research		Short term	Long term
		Further complete plant	Test effects of bryophyte	Nurseries should adjust	Offer GR species mix
		trait information	layer on GR vegetation	their offer so potential	with high FD
				plant species can be
				tested
		Study specific AMF-	Perform long-term GR		Offer GR species mixes
	RESEARCH	species relations	studies; with emphasis on spontaneous and biodiversity colonization; seed bank		that improve one or multiple urban ecosystem services
		Test different water	Study effect of
		retention materials;	functional diverse GRs
		study effects of gray	on ecosystem
		water on GR vegetation;	functioning and services
		explore potential of
		deficit irrigation

Acknowledgements

Acknowledgements

very interesting. Six months later, I was unfortunately not able to persuade the jury of the Agency for Innovation through Science and Technology (IWT) to offer me a scholarship. Lucky for me, the University

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Pictures of the tested species can be found at the end Van Mechelen, C., Baert, F., Torrez, V., Dutoit, T., Honnay, O., Hermy, M. Effect of addition of organic material and arbuscular mycorrhizal fungi on initial green roof plant development. Urban Ecosystems.