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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 (Revi et al., 2014). 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 (Giorgi & Coppola, 2009). In Belgium (North-Western Europe), surface temperatures and winter precipitation are expected to increase while summers are predicted to be drier (Baguis et al., 2010; Vanuytrecht et al., 2014). In contrast, Southern Europe will experience a decrease in annual precipitation with 1% per decade, and even decreases of 5% are expected in summer (Parry, 2000).
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.
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).
In the study of Vanuytrecht et al. (2014), 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 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 (1981-2010) 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 (Sutton et al., 2012). The concept is very promising and is now increasingly adopted when looking for new potential EGR species (Williams et al., 2010; Butler & Orians, 2011; Sutton et al., 2012; Caneva et al., 2013). 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 (Sutton, 2008; Farrell et al., 2013).
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 (Friedrich, 2005; Emilsson, 2008; Molineux et al., 2009). AMF are symbiotic fungi (Phylum Glomeromycota) that form mutualistic associations with plant roots of most plant species (Bonfante & Anca, 2009; Willis et al., 2013). Benefits for the plant include better phosphorus and nitrogen uptake and improved water uptake (Rillig, 2004; Veresoglou & Rillig, 2012). Both substrate additions could be of importance to increase the performance of EGR (cf. Busch & Lelley, 1997; Oberndorfer et al., 2007; McGuire et al., 2013), 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 (MacIvor et al., 2013; Vanuytrecht et al., 2014). However, in many cities, water scarcity is a yearly reoccurring problem, especially in regions with frequent droughts like Southern Europe and Australia (Coutts et al., 2012; Ascione et al., 2013), which has led to irrigation restrictions for GRs (Rowe et al., 2012). 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 (Lavergne et al., 2006), 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.
Fig. 1.6 Pictures illustrating installed and natural EGRs in a Mediterranean context. a) Grass oof o a old fa e s house o the Causse M jea i the Pa Natio al des C e es. 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].
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. Oberndorfer et al., 2007). 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 (Oberndorfer et al., 2007). 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 (Oberndorfer et al., 2007; Francis & Lorimer, 2011). 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 (Köhler
& Keeley, 2005; Dvorak & Volder, 2010; Williams et al., 2010). In 2007, GR coverage in Germany increased on average 13.5 million m² per year (cf. Oberndorfer et al., 2007). In France, EGR surface reached 1 million m² in 2011, and is expected to rise to ca. 1.5 million m² in 2015 (Lassalle, 2012). Modern GR technology is relatively recent and its origins lie in North-and Central Europe, with Germany as leading country.
Table of contents :
Chapter 1. Introduction
1. Cities in the 21st century
2. A brief introduction to green roof technology
3. Problem statement
4. Research questions and chapter outline
Chapter 2. Mediterranean open habitat vegetation offers great potential for extensive green roof design
1. Introduction
2. Material and Methods
2.1. Vegetation description
2.1.1. Study area
2.1.2. Data collection
2.1.3. Data analysis
2.2. Species list comparison
2.3. Potential of Mediterranean vegetation
3. Results
3.1. Vegetation description
3.2. Species list comparison
3.3. Potential of Mediterranean vegetation
4. Discussion
5. Conclusion
Chapter 3. Plant trait analysis delivers an extensive list of potential green roof species for Mediterranean France
1. Introduction
2. Material and Methods
2.1. Species lists
2.2. Trait selection procedure
2.3. Data analysis
2.4. Screening tool and case study
3. Results
3.1. Correlation analysis
3.2. Screening tool and case study
4. Discussion
4.1. Suitable habitats and plant traits for Mediterranean extensive green roof plant selection
4.2. Important extensive green roof plant traits and design recommendations
4.3. Comments on the plant trait analysis
5. Conclusion
Chapter 4. Vegetation and structure recommendations for extensive green roofs in Mediterranean and temperate climates
1. Introduction
2. Material and methods
2.1. Experimental setup
2.2. Monitoring
2.3. Data analysis
3. Results
3.1. Climate data
3.2. General vegetation development
3.3. Exposition effect
3.4. Structure type effect
3.5. Species-specific performance
4. Discussion
4.1. Extensive green roof performance
4.2. Extensive green roof design recommendations for Mediterranean and temperate climates
5. Conclusion
Chapter 4. Intermediate section. Plant species used in the experiment described in on initial green roof plant development
Chapter 5. Effects of addition of organic material and arbuscular mycorrhizal fungi
1. Introduction
2. Material and methods
2.1. Experimental setup
2.2. Plant and AMF survey
2.3. Data analysis
3. Results
3.1. Rooftop experiment
3.2. Greenhouse experiment
4. Discussion
5. Conclusion
Chapter 6. Adapting green roof irrigation practices for a sustainable future
1. Introduction
2. Methods
2.1. Publication search and selection
2.2. Data exploration
3. Green roof irrigation practices and stormwater management regulations
3.1. Current green roof irrigation practices
3.2. Optimizing irrigation practices
3.3. Examples of stormwater management regulations
4. Irrigation specification for green roofs in different climatic contexts
4.1. Dry semi-arid climates
4.2. Warm temperate climates: subtropical and marine regions
4.3. Warm temperate climates: Mediterranean regions
4.4. Warm temperate climates: regions with wet, hot summers and dry winters
4.5. Snow climates
5. Concluding remarks
Chapter 7. Functional and phylogenetic diversity as a framework for novel ecosystem design, the example of extensive green roofs
1. Introduction
2. Material and methods
2.1. System and trait selection
2.2. Data analysis
2.2.1. Exploring the extensive green roof systems
2.2.2. Diversity analysis
2.3. Species selection for optimization of green roof ecosystem functioning
3. Results
3.1. Extensive green roof type description
3.2. Diversity analysis
3.3. Maximizing functional diversity
4. Discussion
4.1. The functional diversity approach and extensive green roof systems
4.2. Considerations for further research
4.3. Recommendations for urban ecosystem design
5. Conclusion
Chapter 8. General discussion
1. Nature as a source of inspiration for extensive green roof vegetation selection
1.1. Green roofs and the habitat template concept
1.2. A plant trait approach for potential extensive green roof species selection
1.3. Testing potential Mediterranean plants in different climates and under different experimental setup
2. Factors for improved green roof performance
2.1. Effects of organic matter and mycorrhiza on green roof vegetation development
2.2. Towards sustainable green roof irrigation methods in different climatic contexts
2.3. Ways to improve green roof functional diversity and the ecosystem services they provide
3. Research gaps and directions for future research
4. Concluding part
References
