# Plant trait analysis delivers an extensive list of potential green roof species for Mediterranean France

Carmen van Mechelen, Thierry Dutoit, Jens Kattge, Martin Hermy

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Carmen van Mechelen, Thierry Dutoit, Jens Kattge, Martin Hermy. Plant trait analysis delivers an extensive list of potential green roof species for Mediterranean France. Ecological Engineering, 2014, 67, pp.48-59. 10.1016/j.ecoleng.2014.03.043. hal-01328807

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# Plant trait analysis delivers an extensive list of potential green roof species for Mediterranean France

Carmen Van Mechelen<sup>a,*</sup>, Thierry Dutoit<sup>b</sup>, Jens Kattge<sup>c</sup>, Martin Hermy<sup>a</sup>

**Keywords:**

Green roofs

Multi-criteria screening tool

Mediterranean

Biodiversity

Plant traits

## ABSTRACT

Green roofs are increasingly popular in urban areas of NW Europe and North America. However, green-roof benefits such as cooling capacity and storm water retention would be even more pronounced in this challenging climate with hot and dry summers, vegetation stress on extensive green roofs will be enhanced, causing low performance if the same species as in the temperate regions of NW Europe and North America are used. In order to support further development and application of green roofs in the Mediterranean climate, new insights on suitable native species is an essential step. Using the habitat template concept, specifically taking into account drought adaptation and self-regulation, we developed a screening procedure using both functional plant traits and utilitarian aspects. Plant traits of two species lists (one resulting from an extensive vegetation study and another one covering the successful plant species of extensive green roofs in NW Europe) were analyzed. The results were incorporated into a hierarchical multi-criteria screening tool. This tool can encourage further experimental trials and inspire and guide the green roof industry toward the most appropriate species for extensive green roof design. As an example, the key was illustrated on a subset of plant species from Mediterranean southern France, which identified 34 newly potential green roof species. Interestingly, 35% of these species were annuals, a promising life form that has until now rarely been considered for extensive green roofs.

## 1. Introduction

The life-quality-improving value of green roofs in densely populated urban areas arouses the public interest for these novel urban ecosystems (Perring et al., 2013). Green roofs have the potential to mitigate urban temperatures to more comfortable levels through higher albedo and evapotranspiration from vegetation and substrate (cf. Oberndorfer et al., 2007). The evapotranspiration, together with water storage in the substrate, also leads to more effective storm water management. Additionally, green roof vegetation offers habitats and food sources that support a range of local invertebrate (Madre et al., 2013) and bird communities, therefore increasing biodiversity value (Madre et al., 2014; Oberndorfer et al.,

2007). Other benefits include noise reduction, esthetic and psychological value, a longer roof membrane longevity and substitution for lost space on ground level (Cook-Patton and Bauerle, 2012; Oberndorfer et al., 2007). In an era of climate change, where more intense rainfall events and extended dry periods are predicted, the buffering capacity of green roofs will increase their importance even further (Vanuytrecht et al., 2014).

During the last decades, research and application of green roofs have shown a remarkable rise in NW Europe and North American countries (e.g. Dvorak and Volder, 2010). However, knowhow and proper incentives are to some extent still lacking in a large part of the world (Dvorak and Volder, 2010; Fioretti et al., 2010; Williams et al., 2010). This is for instance the case in countries with a Mediterranean climate, where the beneficial effects of green roofs would be similar or even more pronounced (Alexandri and Jones, 2008; Fioretti et al., 2010). Unfortunately, environmental conditions on rooftops are very harsh, particularly in the Mediterranean during summer, and it is necessary to have a better understanding

<footnote><sup>a</sup> Division of Forest, Nature and Landscape, KU Leuven, Celestijnenlaan 200E, 3001 Heverlee, Belgium</footnote>

<footnote><sup>b</sup> CNRS-IMBE, IUT Avignon, Site Agroparc BP 61207, 84911 Avignon Cedex 9, France</footnote>

<footnote><sup>c</sup> Max Planck Institute for Biogeochemistry, Hans Knoell Str. 10, 07745 Jena, Germany</footnote>
of drought-tolerant vegetation that could survive and thrive in this challenging climate (Benvenuti and Bacci, 2010; Van Mechelen et al., 2014; Vanuytrecht et al., 2014).

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 extensive green roofs, due to their shallow root systems, (facultative) crassulacean acid metabolism (CAM) and hence efficient water use and tolerance to extreme conditions of drought (Benvenuti and Bacci, 2010; Durhman et al., 2006). Butler and Orians (2011) highlighted the potential of succulents by indicating that Sedum species can facilitate the performance of neighboring plants, the so called 'nursery effect', by reducing soil temperature during dry weather conditions, thereby decreasing abiotic stress for other life forms. While the green roof industries rely mainly on the use of succulent and some other 'tried-and-true' plants (i.e. plants tested and found capable for use on green roofs), 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 (Dvorak and Volder, 2010; Nagase and Dunnett, 2010; Van Mechelen et al., 2014). 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 green roofs have a higher survival probability and are more esthetically pleasing even under dry conditions (Nagase and Dunnett, 2010).

In order to find potential species for urban greening projects (including green roofs and green walls), the 'habitat template hypothesis' (Lundholm, 2006) is a highly interesting concept. In the case of extensive green roofs, this hypothesis suggests that natural habitats with similar characteristics as those on this green roof type, thus scree slopes, limestone pavements and calcareous grasslands on very shallow and nutrient poor soils, can inspire green roof design. Therefore, studying the native plant species composition in these habitats is useful. As a biodiversity hotspot, representing over 10% of the world's flora (Lavergne et al., 2006) and a lot of extreme habitats, the Mediterranean area offers a potentially interesting species pool for green roof application (cf. Van Mechelen et al., 2014). With the habitat template hypothesis in mind, we described semi-natural vegetation in southern France in a previous study, and identified an elaborate list of species which do have some potential for use on extensive green roofs (Van Mechelen et al., 2014). 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 esthetic value are factors that also need to be considered (cf. Benvenuti and Bacci, 2010). Detailed information on plant traits can offer insights into the typical characteristics of green roof plants and therefore guide us to the most useful species for green roof application (cf. Lundholm et al., 2010). Because species selection can be a complex process when a wide range of criteria are involved (Reubens et al., 2011), the development of a decision support tool is a welcome instrument to assist green roof developers toward an appropriate choice of native plants in green roof projects.

The objective of this study is to provide an overview of plant traits that are crucial for survival of plants on green roofs in areas where dry periods are prominent, in particular in the Mediterranean climate. The focus is on green roofs of the extensive type as these are widely applicable and their low-maintenance supports self-regulation which is an essential feature of these versatile systems. 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 southern France.

## 2. Material and methods

### 2.1. 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 green roof species (FGR), commonly used or spontaneously occurring on extensive green roofs in NW Europe, and a Mediterranean plant species list (FMED) from a previous study of an extensive vegetation survey in southern France (Van Mechelen et al., 2014). As drought tolerance and self-regulation are always important on all extensive green roof systems, FGR is used as a reference data set.

For FGR, seven sources were used. These included five key literature sources of internationally recognized green roof specialists or scientists (Bornkamm, 1961; Köhler, 1993; Kolb and Schwarz, 1999; Krupka, 1992; Snodgrass and Snodgrass, 2006) and two species lists of international Green Roof Companies (Optigreen Limited, 2012; ZinCo Gmbh, 2012). The number of times a species is noted in these sources is considered a proxy to its frequency of use on extensive green roofs (FREQ), which will be used to analyze the relation between frequency and plant traits. As plant traits were only available at the species-level, FGR 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.

FMED 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 Van Mechelen et al. (2014)). These extreme habitat conditions are assumed representative for the conditions on extensive green roofs. Additionally we added four literature data sources describing similar habitats (Loisel, 1976; Molinier and Tallon, 1949; Rieux et al., 1977; Van den Berghen, 1963). The vegetation in these literature sources was described as Mediterranean matorral (also called garrigue), typical for thermophylous lime-stone pavements of southern France, with high species richness 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 FGR. Plant nomenclature follows The Plant List (2010).

### 2.2. Trait selection procedure

The approach was based on both functional traits and utilitarian aspects, as actual species selection for extensive green roofs will depend on both. Drought adaptation and self-regulation (e.g. regeneration strategy, presence of a seed bank, plant longevity) were considered key factors for survival and overall success on extensive green roofs, which also holds for more arid environments (Getter and Rowe, 2008; Vinson and Zheng, 2013). Relevant trait "groups", all related to drought adaptation and/or self-regulation, were identified through an extensive literature research. For the selection of utilitarian aspects, a similar approach was used, in which four green roof publications were consulted. For a summary of the literature used, we refer to Appendix A. Next, the TRY Initiative (Kattge et al., 2011; list of contributed authors in Appendix B), a global archive of plant traits, was screened for plant traits related to the trait "groups" 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 of
FGR and 507 of FMED; 127 species occurring in both species lists). Because we are only interested in Mediterranean species with a potential on extensive green roofs, we then made a pre-selection 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 extensive green roofs does not exceed 20 cm (cf. Krupka, 1992), 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 of FGR and 309 of FMED; 69 species occurred in both species lists; see Appendix C for the final species list).

## 2.3. 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 green roof plants and to use them for appropriate species selection, we first considered only the green roof species list (FGR, 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 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 (H) and Pearson Chi² analysis (χ²) were applied to test for relations between the variables. All analyses were performed in SPSS 20.0 (SPSS Inc, Chicago, IL). For the significant Kruskal-Wallis and Pearson Chi² results, post hoc tests were performed and results are presented with bar charts (Siegel and Castellan, 1988). For all the significant traits, 'scoring conditions' 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 extensive green roof species, defined as occurring in five or more of the seven sources used to compile FGR.

## 2.4. 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. Reubens et al., 2011). First before scoring, the exclusion criteria mentioned in the trait selection procedure were considered, to delete species that are obviously not suitable for extensive green roof purposes. Then the traits significantly related to FREQ (hereafter called 'primary traits') 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

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Bar charts visualizing the significant relationship between nominal primary plant traits and the average frequency of species used on extensive green roofs. (a) Leaf shape (LeafSH); (b) metamorphosis for storage (MetStor); (c) metamorphosis for vegetative dispersal (MetVD); (d) plant strategy (PIStrat); (e) photosynthetic pathway (PSP). Significant differences between categories are indicated by different letters (Kruskal-Wallis test cf. Table 2ii and post hoc tests). Error bars indicate the standard deviations.</figcaption></figure>
<caption>**Table 1**</caption>

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 D. 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.

<table><thead><tr><th></th><th>Var. type</th><th>Unit</th><th>Categories/codes</th><th>Trait group</th><th>Authors (+short reference)</th></tr></thead><tbody><tr><td colspan="6"><strong>(i) Functional traits</strong></td></tr><tr><td>Leaf area</td><td>S</td><td>mm<sup>2</sup></td><td></td><td>Leaf adaptation</td><td>Cornelissen, 1996; Kleyer et al., 2008; Siefert<sup>a</sup></td></tr><tr><td>Leaf dry matter content per leaf water-saturated mass (LDMC)</td><td>S</td><td>g/g</td><td></td><td>Biomass</td><td>Garnier et al., 2007; Kleyer et al., 2008; Siefert</td></tr><tr><td>Leaf nitrogen content per dry mass (LeafN)</td><td>S</td><td>mg/g</td><td></td><td>Nitrogen content</td><td>Garnier et al., 2007; Ogaya and Penuelas, 2003; Reich et al., 2009</td></tr><tr><td>Leaf phenology (LeafPH)</td><td>N</td><td></td><td>Evergreen; winter deciduous; summer deciduous</td><td>Leaf phenology</td><td>Kattge et al., 2009; Reich et al., 2009</td></tr><tr><td>Leaf shape (LeafSH)</td><td>N</td><td></td><td>Broad/full/simple; pinnate; palmate; digitate; needle-like, ensiform; scale-like</td><td>Leaf adaptation</td><td>Kühn et al., 2004; Paula et al., 2009</td></tr><tr><td>Life form</td><td>N</td><td></td><td>Chamaephyte; hemicryptophyte; geophyte; therophyte</td><td>Life form</td><td>Julve, 1998</td></tr><tr><td>Maximum plant longevity (MaxPL)</td><td>N</td><td></td><td>Annual; annual/biennial; biennial; biennial/pluriennial; pluriennial; indifferent</td><td>Plant longevity</td><td>Diaz et al., 2004; Kühn et al., 2004; Moles et al., 2004; Poorter<sup>b</sup></td></tr><tr><td>Metamorphoses for storage (MetStor)</td><td>N</td><td></td><td>None; succulent; bulb; pleiocorm/primary storage root; rhizome; runner; tuft; root tuber/secondary storage root; shoot tuber</td><td>Reserves and storage</td><td>Kühn et al., 2004</td></tr><tr><td>Metamorphoses for vegetative dispersion (MetVD)</td><td>N</td><td></td><td>Fragmentation; bulb; pleiocorm; rhizome; runner; root/shoot; root tuber; shoot tuber; rhizome/root/shoot; rhizome/runner</td><td>Regeneration strategy</td><td>Kühn et al., 2004</td></tr><tr><td>Photosynthetic pathway (PSP)</td><td>N</td><td></td><td>C3; C4; CAM; C3/CAM; C4/CAM</td><td>Photosynthesis</td><td>Kattge et al., 2009</td></tr><tr><td>Plant strategy type (PIStrat)</td><td>N</td><td></td><td>S (stress tolerants); cs (competitor + stress tolerants); rs (stress tolerant ruderals); crs (competitive and stress tolerant ruderals)</td><td>Grime strategy</td><td>Kühn et al., 2004</td></tr><tr><td>Seed mass</td><td>S</td><td>mg</td><td></td><td>Seed mass and density</td><td>Dickie, 2008; Moles et al., 2004</td></tr><tr><td>Species reproduction (SpRepr)</td><td>N</td><td></td><td>Seed; veg; seed/veg</td><td>Regeneration strategy</td><td>Kühn et al., 2004</td></tr><tr><td>Specific leaf area (SLA)</td><td>S</td><td>mm<sup>2</sup>/mg</td><td></td><td>Leaf adaptation</td><td>Kleyer et al., 2008; Siefert</td></tr><tr><td>Typical plant height (TypPH)</td><td>S</td><td>m</td><td></td><td>Plant height</td><td>Kleyer et al., 2008; Siefert</td></tr><tr><td>Whole leaf dry mass (WLMD)</td><td>S</td><td>mg</td><td></td><td>Biomass</td><td>Kleyer et al., 2008; Siefert</td></tr><tr><td>Woodiness (Wood)</td><td>O</td><td></td><td>0, non-woody; 1, semi-woody; 2, woody</td><td>High wood density</td><td>Kleyer et al., 2008</td></tr><tr><td colspan="6"><strong>(ii) Utilitarian aspects</strong></td></tr><tr><td>Flower color (FlowC)</td><td>N</td><td></td><td>Yellow; red; blue/purple; white/pink; brown; green</td><td>Esthetic appeal</td><td>Kühn et al., 2004</td></tr><tr><td>Flower pollinator (Poll)</td><td>N</td><td></td><td>Wind; insects; insects/wind</td><td>Ecosystem goods/service</td><td>Kühn et al., 2004</td></tr><tr><td>Flower type (FlowT)</td><td>N</td><td></td><td>Disk flowers; flower heads; lip flowers; pollen flowers; bell shaped flowers; funnel flowers; Fabaceae-type flowers</td><td>Esthetic appeal</td><td>Kühn et al., 2004</td></tr><tr><td>Flowering duration (FlowD)</td><td>O</td><td></td><td># months</td><td>Esthetic appeal</td><td>Garnier et al., 2007; Kühn et al., 2004</td></tr><tr><td>Flowering period (FlowP)</td><td>O</td><td></td><td>1, early spring; 2, spring; 3, late spring; 4, early summer; 5, summer; 6, late summer; 7, early autumn; 8, winter</td><td>Esthetic appeal</td><td>Green, 2009; Kühn et al., 2004</td></tr><tr><td>Pollinator reward (PollRew)</td><td>N</td><td></td><td>Nectar; pollen; deceit</td><td>Ecosystem goods/service</td><td>Kühn et al., 2004</td></tr><tr><td>Shoot growth form (ShGF)</td><td>N</td><td></td><td>Erect; ascending to prostrate/decumbent; prostrate: lianas/climbers</td><td>Esthetic appeal</td><td>Kleyer et al., 2008</td></tr></tbody></table>

trait) when they meet the condition indicated in the key. 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 extensive green roofs in the Mediterranean. As a case study, the key was applied to the FMED species list, indicating the most promising Mediterranean species for extensive green roof purposes that merit further experimental tests.

## 3. Results

### 3.1. Correlation analysis

Frequency of use on green roofs (FREQ) is negatively correlated with leaf area ($r_s = -0.33, p < 0.001$), specific leaf area ($r_s = -0.33, p < 0.05$) and whole leaf dry mass ($r_s = -0.28, p < 0.05$) (Table 2i).

<footnote><sup>a</sup> Herbaceous leaf traits database old field New York (unpublished data).</footnote>

<footnote><sup>b</sup> Categorical plant trait database (unpublished data).</footnote>
Average values for highly frequent species (defined as occurring in 5-7 of the used green roof sources) are for leaf area, specific leaf area and whole leaf dry mass lower than 368 mm², 18 mm²/mg and 25 mg, respectively. The Kruskal-Wallis test results in Table 2ii show a highly significant difference (p < 0.001) between FREQ and metamorphosis for storage (H = 24.53) and plant strategy type (H = 17.66). Leaf shape (H = 12.86), metamorphosis for vegetative dispersal (H = 18.71), and photosynthetic pathway (H = 9.57) are also significantly correlated (p < 0.05) with FREQ. Fig. 1 illustrates the results of the post hoc tests. Frequently occurring green roof species are stress-tolerant, have a crassulacean acid photosynthetic pathway (CAM), 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 intercorrelated as well (e.g. leaf area and whole leaf dry mass; r<sub>s</sub> = 0.88, p < 0.001). The primary traits show significant correlations or associations with following traits: Table 3i: seed mass, typical plant height, woodiness, flowering duration and flowering period; Table 3ii: seed mass, leaf phenology, pollinator reward, shoot growth form, flowering duration, flowering period and life form; Table 3iii: life form, species reproduction, flower color, flower type, pollinator and plant longevity. Fig. 2a-c shows 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. 2d, plants with a low specific leaf area are mostly geophytes and those with a low whole leaf dry mass are generally therophytes. Fig. 3a indicates a seed mass of 0.13 mg and a flowering period in early summer for the plant strategy category 'stress tolerance'. Fig. 3b-d show that average values of seed mass and flowering duration for the important leaf shape category 'needle-like or ensiform' are 2.78 mg and 1.75 months, respectively. Average values of seed mass and flowering duration for the metamorphosis for storage category 'succulence' are 0.03 mg and 1.92 months, respectively. For the categories 'runner' and 'fragmentation' of the trait metamorphosis for vegetative dispersal, average values are as follows: seed mass of 0.66 and 4.54 mg; flowering duration of 1.09 and 1.13 months, respectively. As can be seen in Fig. 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 disperse through insects or wind. CAM-plants are mostly perennial. Stress-tolerant species generally have a pluriennial longevity, are evergreen, are pollinated by insects and have disk flowers. All the important primary plant trait categories in Fig. 4 (except CAM-plants) reproduce through seeds or vegetatively and have a hemicryptophytic life form.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Bar charts visualizing the significant relationship between primary and other traits of species used on extensive green roofs. Average values and standard deviation for the primary trait leaf area for each category in (a) shoot growth form (ShGF), (b) pollinator reward (PollRew) and (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 3ii and post hoc tests).</figcaption></figure>

## 3.2. Screening tool and case study

Table 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 only categories for these traits, and incorporation in the key will thus not be informative for species scoring.

The 309 FMED species that passed through the exclusion step were submitted to the screening tool. In Table 5, the highest scoring Mediterranean species (61 species with SS ≥ 10, maximum SS = 28) are listed. 27 species in the list are already used on extensive green roofs 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 green roofs. We highlight that 35% of the species in the new

<table><caption>**Table 2**
The relationship between plant traits and frequency of use on extensive green roofs (FREQ). (i) Spearman rank correlations together with their significance. N represents number of valid cases. (ii) Kruskal-Wallis test results together with their significance. For abbreviations of plant traits see Table 1.</caption><thead><tr><th></th><th>LeafArea</th><th>LDMC</th><th>LeafN</th><th>SeedMass</th><th>SLA</th><th>TypPH</th><th>WLDM</th><th>Wood</th><th>FlowD</th><th>FlowP</th></tr></thead><tbody><tr><td colspan="11">(i) Spearman rank</td></tr><tr><td>Correlation coefficient</td><td>-0.33<sup>**</sup></td><td>0.09</td><td>-0.07</td><td>-0.03</td><td>-0.22<sup>*</sup></td><td>-0.10</td><td>-0.28<sup>*</sup></td><td>0.07</td><td>0.05</td><td>0.12</td></tr><tr><td>N</td><td>128</td><td>125</td><td>79</td><td>176</td><td>139</td><td>173</td><td>114</td><td>173</td><td>156</td><td>124</td></tr><tr><td colspan="11">LifeForm</td></tr><tr><td>LeafPH</td><td>12.86<sup>*</sup></td><td>8.69</td><td>24.53<sup>**</sup></td><td>18.71<sup>*</sup></td><td>9.57<sup>*</sup></td><td>17.66<sup>**</sup></td><td>5.73</td><td>7.90</td><td>2.15</td><td>2.36</td></tr><tr><td>MaxPL</td><td>5</td><td>5</td><td>6</td><td>8</td><td>3</td><td>3</td><td>5</td><td>6</td><td>1</td><td>3</td></tr><tr><td colspan="11">(ii) Kruskal-Wallis test</td></tr><tr><td>H</td><td>6.06</td><td>0.86</td><td>12.86<sup>*</sup></td><td>8.69</td><td>24.53<sup>**</sup></td><td>18.71<sup>*</sup></td><td>9.57<sup>*</sup></td><td>17.66<sup>**</sup></td><td>5.73</td><td>7.90</td></tr><tr><td>df</td><td>3</td><td>2</td><td>5</td><td>5</td><td>6</td><td>8</td><td>3</td><td>2</td><td>2</td><td>1</td></tr></tbody></table>

<footnote>* p < 0.05.</footnote>

<footnote>** p < 0.001.</footnote>
**Table 3**

The relationship between the primary plant traits (P) and other plant traits. (i) Spearman rank correlations together with their significance. N represents the number of valid cases. (ii) Kruskal-Wallis test results together with their significance. (iii) Pearson Chi² test results of the nominal variables. N represents number of valid cases. For abbreviations of plant traits see Table 1.

<table><thead><tr><th></th><th>LeafArea(P)</th><th>LDMC</th><th>LeafN</th><th>SeedMass</th><th>SLA(P)</th><th>TypPH</th><th>WLDM(P)</th><th>Wood</th><th>FlowD</th><th>FlowP</th></tr></thead><tbody><tr><td colspan="10">(i) Spearman rank (r<sub>s</sub>)</td></tr><tr><td>LeafArea(P)</td><td>1.00</td><td>-0.05</td><td>0.18</td><td>0.43<sup>**</sup></td><td>0.20<sup>*</sup></td><td>0.25<sup>**</sup></td><td>0.88<sup>**</sup></td><td>-0.24<sup>**</sup></td><td>-0.01</td><td>-0.14</td></tr><tr><td>N</td><td>128</td><td>120</td><td>64</td><td>114</td><td>125</td><td>124</td><td>114</td><td>118</td><td>112</td><td>105</td></tr><tr><td>SLA(P)</td><td>0.20<sup>*</sup></td><td>-0.11</td><td>0.21</td><td>-0.06</td><td>1.00</td><td>-0.04</td><td>-0.16</td><td>-0.01</td><td>0.19<sup>*</sup></td><td>-0.19<sup>*</sup></td></tr><tr><td>N</td><td>125</td><td>124</td><td>74</td><td>121</td><td>139</td><td>132</td><td>114</td><td>125</td><td>119</td><td>109</td></tr><tr><td>WLDM(P)</td><td>0.88<sup>**</sup></td><td>0.02</td><td>0.11</td><td>0.45<sup>**</sup></td><td>-0.16</td><td>0.26<sup>**</sup></td><td>1.00</td><td>-0.22<sup>*</sup></td><td>-0.07</td><td>-0.08</td></tr><tr><td>N</td><td>114</td><td>113</td><td>57</td><td>102</td><td>114</td><td>114</td><td>114</td><td>109</td><td>105</td><td>99</td></tr><tr><td colspan="10">(ii) Kruskal-Wallis Test (H)</td></tr><tr><td>LifeForm</td><td>6.43</td><td>0.25</td><td>0.83</td><td>22.48<sup>**</sup></td><td>11.53<sup>*</sup></td><td>0.42</td><td>11.77<sup>*</sup></td><td>16.85<sup>**</sup></td><td>14.18<sup>*</sup></td><td>0.19</td></tr><tr><td>df</td><td>3</td><td>3</td><td>2</td><td>3</td><td>3</td><td>3</td><td>2</td><td>3</td><td>3</td><td>2</td></tr><tr><td>LeafPH</td><td>11.50<sup>*</sup></td><td>2.88</td><td>0.75</td><td>21.01<sup>**</sup></td><td>3.44</td><td>6.42</td><td>14.42<sup>**</sup></td><td>7.89</td><td>7.59<sup>*</sup></td><td>3.41</td></tr><tr><td>df</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>LeafSH(P)</td><td>16.24<sup>*</sup></td><td>2.09</td><td>4.62</td><td>13.26<sup>*</sup></td><td>19.73<sup>**</sup></td><td>1.91</td><td>15.53<sup>*</sup></td><td>8.22</td><td>20.75<sup>**</sup></td><td>5.29</td></tr><tr><td>df</td><td>5</td><td>5</td><td>4</td><td>5</td><td>5</td><td>5</td><td>5</td><td>5</td><td>5</td><td>5</td></tr><tr><td>MaxPL</td><td>5.36</td><td>0.17</td><td>2.57</td><td>4.64</td><td>8.76</td><td>0.79</td><td>7.68</td><td>0.97</td><td>6.86</td><td>6.43</td></tr><tr><td>df</td><td>4</td><td>4</td><td>3</td><td>5</td><td>4</td><td>5</td><td>4</td><td>4</td><td>5</td><td>4</td></tr><tr><td>MetStor(P)</td><td>14.39<sup>*</sup></td><td>11.98</td><td>11.25</td><td>25.15<sup>**</sup></td><td>13.31<sup>*</sup></td><td>3.71</td><td>14.01<sup>*</sup></td><td>4.18</td><td>17.85<sup>*</sup></td><td>10.49</td></tr><tr><td>df</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td></tr><tr><td>MetVD(P)</td><td>20.40<sup>*</sup></td><td>18.73</td><td>10.98</td><td>26.73<sup>**</sup></td><td>9.53</td><td>11.10</td><td>25.43<sup>**</sup></td><td>9.19</td><td>17.55<sup>*</sup></td><td>11.00</td></tr><tr><td>df</td><td>8</td><td>8</td><td>8</td><td>8</td><td>8</td><td>8</td><td>8</td><td>8</td><td>8</td><td>8</td></tr><tr><td>PSP(P)</td><td>6.15</td><td>0.07</td><td>2.35</td><td>4.43</td><td>1.78</td><td>0.34</td><td>2.72</td><td>0.32</td><td>0.49</td><td>1.67</td></tr><tr><td>df</td><td>3</td><td>2</td><td>1</td><td>3</td><td>3</td><td>3</td><td>2</td><td>3</td><td>2</td><td>2</td></tr><tr><td>SpRepr</td><td>1.56</td><td>0.12</td><td>1.62</td><td>0.47</td><td>0.61</td><td>1.46</td><td>1.07</td><td>3.78</td><td>3.76</td><td>0.41</td></tr><tr><td>df</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>PIStrat(P)</td><td>15.99<sup>**</sup></td><td>6.65</td><td>4.28</td><td>14.48<sup>*</sup></td><td>8.19<sup>*</sup></td><td>9.67</td><td>16.04<sup>**</sup></td><td>1.51</td><td>0.77</td><td>9.64<sup>*</sup></td></tr><tr><td>df</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td></tr><tr><td>FlowC</td><td>1.09</td><td>0.00</td><td>3.46</td><td>8.55</td><td>1.95</td><td>2.12</td><td>2.01</td><td>4.20</td><td>2.36</td><td>7.06</td></tr><tr><td>df</td><td>5</td><td>5</td><td>4</td><td>5</td><td>5</td><td>5</td><td>5</td><td>5</td><td>5</td><td>5</td></tr><tr><td>FlowT</td><td>12.04</td><td>0.00</td><td>8.57</td><td>9.14</td><td>11.66</td><td>4.04</td><td>7.70</td><td>28.52<sup>**</sup></td><td>2.48</td><td>8.18</td></tr><tr><td>df</td><td>6</td><td>6</td><td>5</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td><td>6</td></tr><tr><td>Poll</td><td>2.72</td><td>10.51</td><td>5.15</td><td>0.36</td><td>0.22</td><td>4.42</td><td>2.72</td><td>2.85</td><td>18.87<sup>**</sup></td><td>1.35</td></tr><tr><td>df</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td><td>2</td></tr><tr><td>PollRew</td><td>4.35<sup>*</sup></td><td>0.00</td><td>0.00</td><td>0.40</td><td>0.02</td><td>1.41</td><td>3.26</td><td>2.86</td><td>3.81</td><td>0.00</td></tr><tr><td>df</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td><td>1</td></tr><tr><td>ShGR</td><td>12.39<sup>*</sup></td><td>0.47</td><td>2.83</td><td>3.77</td><td>6.47</td><td>2.13</td><td>9.69<sup>*</sup></td><td>26.44<sup>**</sup></td><td>6.03</td><td>1.10</td></tr><tr><td>df</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td><td>3</td></tr><tr><td colspan="10">(iii) Pearson Chi² (χ²)</td></tr><tr><td>LifeForm</td><td>31.7<sup>*</sup></td><td></td><td></td><td>94.7<sup>**</sup></td><td></td><td>93.9<sup>**</sup></td><td></td><td>0.7</td><td></td><td>138.7<sup>**</sup></td></tr><tr><td>N</td><td>164</td><td></td><td></td><td>153</td><td></td><td>115</td><td></td><td>132</td><td></td><td>162</td></tr><tr><td>LeafPH</td><td>17.5</td><td></td><td></td><td>29.5<sup>*</sup></td><td></td><td>37.1<sup>*</sup></td><td></td><td>3.1</td><td></td><td>49.9<sup>**</sup></td></tr><tr><td>N</td><td>152</td><td></td><td></td><td>135</td><td></td><td>109</td><td></td><td>102</td><td></td><td>152</td></tr><tr><td>LeafSH(P)</td><td>NA</td><td></td><td></td><td>93.9<sup>**</sup></td><td></td><td>97.7<sup>**</sup></td><td></td><td>3.2</td><td></td><td>47.1<sup>**</sup></td></tr><tr><td>N</td><td></td><td></td><td></td><td>146</td><td></td><td>115</td><td></td><td>106</td><td></td><td>162</td></tr><tr><td>MaxPL</td><td>21.8</td><td></td><td></td><td>16.7</td><td></td><td>22.8</td><td></td><td>22.2<sup>*</sup></td><td></td><td>121.6<sup>**</sup></td></tr><tr><td>N</td><td>162</td><td></td><td></td><td>146</td><td></td><td>115</td><td></td><td>114</td><td></td><td>162</td></tr><tr><td>MetStor(P)</td><td>93.9<sup>**</sup></td><td></td><td></td><td>NA</td><td></td><td>415.7<sup>**</sup></td><td></td><td>98<sup>**</sup></td><td></td><td>115.5<sup>**</sup></td></tr><tr><td>N</td><td>146</td><td></td><td></td><td></td><td></td><td>109</td><td></td><td>98</td><td></td><td>145</td></tr><tr><td>MetVD(P)</td><td>97.7<sup>**</sup></td><td></td><td></td><td>415.7<sup>**</sup></td><td></td><td>NA</td><td></td><td>4.9</td><td></td><td>38.0<sup>*</sup></td></tr><tr><td>N</td><td>115</td><td></td><td></td><td>109</td><td></td><td></td><td></td><td>74</td><td></td><td>115</td></tr><tr><td>PSP(P)</td><td>3.2</td><td></td><td></td><td>98<sup>**</sup></td><td></td><td>4.9</td><td></td><td>NA</td><td></td><td>43.8<sup>**</sup></td></tr><tr><td>N</td><td>106</td><td></td><td></td><td>98</td><td></td><td>74</td><td></td><td></td><td></td><td>105</td></tr><tr><td>SpRep</td><td>60.0<sup>**</sup></td><td></td><td></td><td>79.3<sup>**</sup></td><td></td><td>57.4<sup>**</sup></td><td></td><td>2.9</td><td></td><td>14.2<sup>*</sup></td></tr><tr><td>N</td><td>158</td><td></td><td></td><td>148</td><td></td><td>110</td><td></td><td>106</td><td></td><td>157</td></tr><tr><td>PIStrat(P)</td><td>47.1<sup>**</sup></td><td></td><td></td><td>115.5<sup>**</sup></td><td></td><td>38.0<sup>*</sup></td><td></td><td>43.8<sup>**</sup></td><td></td><td>NA</td></tr><tr><td>N</td><td>162</td><td></td><td></td><td>145</td><td></td><td>115</td><td></td><td>105</td><td></td><td></td></tr><tr><td>FlowC</td><td>47.3<sup>*</sup></td><td></td><td></td><td>35.5</td><td></td><td>46.5</td><td></td><td>5.7</td><td></td><td>33.5<sup>*</sup></td></tr><tr><td>N</td><td>131</td><td></td><td></td><td>121</td><td></td><td>89</td><td></td><td>92</td><td></td><td>130</td></tr><tr><td>FlowT</td><td>71.6<sup>**</sup></td><td></td><td></td><td>56.2<sup>*</sup></td><td></td><td>25</td><td></td><td>4.6</td><td></td><td>33.0<sup>*</sup></td></tr><tr><td>N</td><td>125</td><td></td><td></td><td>115</td><td></td><td>83</td><td></td><td>84</td><td></td><td>124</td></tr><tr><td>Poll</td><td>33.8<sup>**</sup></td><td></td><td></td><td>73.7<sup>**</sup></td><td></td><td>61.9<sup>**</sup></td><td></td><td>1.3</td><td></td><td>17.0<sup>*</sup></td></tr><tr><td>N</td><td>162</td><td></td><td></td><td>152</td><td></td><td>114</td><td></td><td>112</td><td></td><td>161</td></tr><tr><td>PollR</td><td>4</td><td></td><td></td><td>2.2</td><td></td><td>7.7</td><td></td><td>0.3</td><td></td><td>2.3</td></tr><tr><td>N</td><td>119</td><td></td><td></td><td>109</td><td></td><td>81</td><td></td><td>80</td><td></td><td>118</td></tr><tr><td>ShGF</td><td>18.7</td><td></td><td></td><td>17.5</td><td></td><td>25.6</td><td></td><td>0.9</td><td></td><td>4.6</td></tr><tr><td>N</td><td>149</td><td></td><td></td><td>137</td><td></td><td>106</td><td></td><td>117</td><td></td><td>148</td></tr></tbody></table>

NA, not applicable.

* p < 0.05.

** p < 0.001.
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** Bar charts visualizing the significant relationship between primary and other traits of species used on extensive green roofs. 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 (PIStrat), (b) metamorphosis for vegetative dispersal (MetVD), (c) metamorphosis for storage (MetStor) and (d) leaf shape (LeafSH).</figcaption></figure>

potential species group recommended for use on green roofs in the Mediterranean region are therophytes.

# 4. Discussion

## 4.1. Suitable habitats and plant traits for Mediterranean green roof plant selection

The habitat template hypothesis is a good starting point for finding potentially suitable plant species, whether this is for extensive green roofs or other applications in urban greenery like green walls, parks and gardens (Lundholm, 2006; Perring et al., 2013). Environmental conditions in anthropogenic habitats such as green roofs indeed mimic circumstances also found in the field (Benvenuti and Bacci, 2010; Lundholm, 2006). Substrates of extensive green roofs have in common with the sampled natural habitats that they are shallow, have a high draining capacity and are relatively calcareous (Lundholm, 2006; Perring et al., 2013). So only these vegetation communities were used here as a starting point (Loisel, 1976; Molinier and Tallon, 1949; Rieux et al., 1977; Van den Berghen, 1963; Van Mechelen et al., 2014). Natural habitats in the Mediterranean region have an extremely high diversity of plant species (Médail and Quézel, 1997) and these offer the potential for finding new species appropriate for extensive green roofs (cf. Van Mechelen et al., 2014). A particular assumption is that the traits (but not the trait values) that make plant species successful on NW European extensive green roofs 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 extensive green roofs in other climatic regions, although the plant's drought tolerance properties are more essential in the Mediterranean (cf. Dvorak and Volder, 2010; Graber and Rowe, 2008; Williams et al., 2010). 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 extensive green roofs 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. Emilsson, 2005).

## 4.2. Important green roof plant traits and design recommendations

Attributes characterizing frequently applied green roof species (Table 4) are a small leaf area, specific leaf area and whole leaf dry mass, needle-like/ensiform or scale-like leaves, CAM metabolism (cf. Durhman et al., 2006), 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 green roof spectrum is until now limited (Van Mechelen et al., 2014), probably because of their short above-ground 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 green roof will be negligible during this period. Of the new potential Mediterranean species in Table 5, 35% are therophytic, 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 Van Mechelen et al. (2014). Their conservation value may also be important, certainly in Mediterranean areas where many annuals are threatened (Lavergne et al., 2006). The inclusion of 'annual life form' as a scoring condition for species selection shifts the traditional implementation of perrennal and hemicryptophytic species to a more diverse and drought-adapted design, which should improve the roofs biodiversity- and ecosystem functioning value.

Some of the primary traits, like CAM metabolism, stress-tolerance and succulence, already proved to be important for the success on green roofs (Oberndorfer et al., 2007; Snodgrass and Snodgrass, 2006). The other leaf-related traits emerging from the plant trait analysis have gained much less attention in the literature (cf. Cook-Patton and Bauerle, 2012; Dunnett and Kingsbury, 2004). 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 green roof substrate will need customization to ensure good plant health. This is however not an objective of this study, but a consideration for a later phase when the potential species are tested under green roof conditions.

Like other authors (e.g. Cook-Patton and Bauerle, 2012; Dvorak and Volder, 2010; Lundholm et al., 2010), we suggest to combine different life forms for extensive green roof design, including Sedum species, annuals, geo- and hemicryptophytes. Sedum and other succulent species could act as nursery plants for other herbaceous species when environmental conditions are harsh (Butler and Orians, 2011). By use of both novel or rarely used species (e.g. Alyssum alyssoides, Dianthus sylvestris, Erophila verna, Filago pyramidata, Linum tenuifolium) and already
<figure><img src="image_5.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>c</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>d</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>e</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>f</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>g</figcaption></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

**Fig. 4.** Bar charts visualizing the significant associations of important primary plant trait categories (see legend) and other nominal plant traits of species used on extensive green roofs (Pearson Chi² test cf. Table 3iii). (a) Life form; (b) species reproduction (SpRepr); (c) maximum plant longevity (MaxPL); (d) flower color (FlowC); (e) pollinator (Poll); (f) flower type (FlowT) and (g) leaf phenology (LeafPH). Trait categories with a large number of species are important for extensive green roof species performance.

applied species (e.g. Allium sphaerocephalon, Euphorbia cyparis-sias, Helianthemum nummularium, S. album, S. acre), green roof biodiversity can gradually be increased without risking failure. The green roof design recommended here should ensure green roof performance and sustainability, even under difficult climatic conditions, although further testing of the potential species is required.

### 4.3. Comments on the plant trait analysis

Although the plant trait based approach applied in this study offered insights in the ecological background of potential green roof vegetation, the results and the screening tool should be interpreted with caution, as some impediments could have influenced the results.
<caption>**Table 4**</caption>

Decision support tool to select species which are potentially interesting for use on extensive green roofs in Mediterranean climates, based on functional (+) plant traits and utilitarian (+) aspects related to frequently applied green roof 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 key.

<table><thead><tr><th colspan="3">Exclusion criteria</th></tr><tr><th>Trait</th><th>Condition</th><th>Action</th></tr></thead><tbody><tr><td>Rooting depth</td><td>&gt;20 cm</td><td>Delete</td></tr><tr><td>Tolerance to drought</td><td>None</td><td>Delete</td></tr><tr><td>Life form</td><td>Phanerophyte</td><td>Delete</td></tr><tr><td>Typical plant height</td><td>&gt;1 m</td><td>Delete</td></tr><tr><td>Plant strategy type</td><td>No stress tolerance</td><td>Delete</td></tr><tr><th colspan="3">Primary traits</th></tr><tr><th>Trait</th><th>Scoring condition (2 points for each matching scoring condition)</th><th>Derived traits</th><th>Scoring condition (1 point for each matching scoring condition)</th></tr><tr><td>Low leaf area*</td><td>&lt;368 mm²</td><td>Low seed mass*</td><td>&lt;4.54 mg</td></tr><tr><td>Low specific leaf area*</td><td>&lt;18 mm²/mg</td><td>Low typical plant height*</td><td>&lt;0.2 m</td></tr><tr><td>Low whole leaf dry mass*</td><td>&lt;25 mg</td><td>High woodiness*</td><td>(semi) Woody</td></tr><tr><td>Metamorphosis for storage*</td><td>Succulence</td><td>Leaf phenology*</td><td>Evergreen</td></tr><tr><td>Plant strategy type*</td><td>Stress tolerance</td><td>Pollinator reward*</td><td>Nectar</td></tr><tr><td>Metamorphosis for vegetative dispersal*</td><td>Fragmentation or runners</td><td>Shoot growth form*</td><td>Decumbent</td></tr><tr><td>Photosynthetic pathway*</td><td>CAM</td><td>Short flowering duration*</td><td>&lt;2 months</td></tr><tr><td>Leaf shape*</td><td>Needle-like/ensiform or scale-like</td><td>Late flowering period*</td><td>Early summer or later</td></tr><tr><td></td><td></td><td>Life form*</td><td>Geophytes, therophytes, hemicryptophytes</td></tr><tr><td></td><td></td><td>Flower color*</td><td>Yellow, blue or white</td></tr><tr><td></td><td></td><td>Flower type*</td><td>Disk flowers</td></tr><tr><td></td><td></td><td>Maximum plant longevity*</td><td>Pluriennal</td></tr><tr><td>Sum of scores</td><td>/16</td><td></td><td>/12</td></tr><tr><td>Total screening score (SS)</td><td></td><td></td><td>/28</td></tr></tbody></table>

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 linum-stellatum*, *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 5). Also, the screening tool (Table 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 green roofs 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 (Green, 2009; Paula et al., 2009) 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 extensive green roofs in the Mediterranean. The lack of detailed information has been shown to be a common bottleneck in ecological research (e.g. Diaz and Cabido,

1997; Franzel et al., 1996; Reubens et al., 2011). 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 (Shan et al., 2012).

Secondly, during the compilation of FGR, varieties, hybrids and subspecies were omitted because trait data are 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 green roof species, so we believe the results are sufficiently substantiated. Another constraint when compiling the FGR list is that no information was available on whether or not the species on the green roof received irrigation. The consulted sources only list species commonly applied on green roofs, and generally, a minimal irrigation regime during dry periods is recommended. The drought-tolerant capacities of the green roof 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 FGR list 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 5) where many species already occurring on green roofs show the highest screening scores. This does not invalidate the proposed plant trait approach, but rather demonstrates the current green roof situation where plants are often denoted as 'tried-and-true' (Dvorak and Volder, 2010; Nagase and Dunnett, 2010). We suggest that, after testing of the potential species under green roof
<caption>**Table 5**</caption>

Mediterranean species for potential green roof application and their screening score (SS ≥ 10) based on the screening tool in Table 4. The column 'New' indicates Mediterranean species that are currently not used on green roofs. Life form (LF; C: chamaephyte; G: geophyte; H: hemicryptophyte; T: therophyte) is given as well.

<table><caption>Table 5 (Continued)</caption><thead><tr><th>Species</th><th>New</th><th>LF</th><th>SS</th></tr></thead><tbody><tr><td>Sedum album</td><td></td><td>H</td><td>25</td></tr><tr><td>Sedum acre</td><td></td><td>H</td><td>23</td></tr><tr><td>Sedum dasyphyllum</td><td></td><td>H</td><td>19</td></tr><tr><td>Thymus praecox</td><td></td><td>H</td><td>18</td></tr><tr><td>Thymus serpyllum</td><td></td><td>H</td><td>18</td></tr><tr><td>Draba aizoides</td><td></td><td>H</td><td>17</td></tr><tr><td>Hippocrepis comosa</td><td></td><td>H</td><td>17</td></tr><tr><td>Silene otites</td><td>x</td><td>H</td><td>17</td></tr><tr><td>Alyssum montanum</td><td></td><td>H</td><td>16</td></tr><tr><td>Galium verum</td><td></td><td>H</td><td>16</td></tr><tr><td>Helianthemum nummularium</td><td></td><td>H</td><td>16</td></tr><tr><td>Sedum ochroleucum</td><td></td><td>H</td><td>16</td></tr><tr><td>Aster linosyris</td><td></td><td>H</td><td>15</td></tr><tr><td>Teucrium montanum</td><td></td><td>H</td><td>15</td></tr><tr><td>Petrorhagia prolifera</td><td>x</td><td>T</td><td>14</td></tr><tr><td>Plantago coronopus</td><td>x</td><td>H</td><td>14</td></tr><tr><td>Polygala vulgaris</td><td>x</td><td>H</td><td>14</td></tr><tr><td>Bupleurum falcatum</td><td>x</td><td>H</td><td>13</td></tr><tr><td>Centaurium erythraea</td><td>x</td><td>T</td><td>13</td></tr><tr><td>Euphorbia seguieriana</td><td></td><td>H</td><td>13</td></tr><tr><td>Galium boreale</td><td>x</td><td>H</td><td>13</td></tr><tr><td>Veronica chamaedrys</td><td>x</td><td>H</td><td>13</td></tr><tr><td>Allium sphaerocephalon</td><td></td><td>G</td><td>12</td></tr><tr><td>Alyssum alyssoides</td><td>x</td><td>T</td><td>12</td></tr><tr><td>Artemisia campestris</td><td>x</td><td>H</td><td>12</td></tr><tr><td>Briza media</td><td></td><td>H</td><td>12</td></tr><tr><td>Campanula persicifolia</td><td></td><td>H</td><td>12</td></tr><tr><td>Carex alba</td><td>x</td><td>H</td><td>12</td></tr><tr><td>Genista pilosa</td><td>x</td><td>C</td><td>12</td></tr><tr><td>Helictotrichon pratense</td><td>x</td><td>H</td><td>12</td></tr><tr><td>Hieracium pilosella</td><td></td><td>H</td><td>12</td></tr><tr><td>Linum catharticum</td><td>x</td><td>T</td><td>12</td></tr><tr><td>Linum tenuifolium</td><td>x</td><td>H</td><td>12</td></tr><tr><td>Minuartia hybrida</td><td>x</td><td>T</td><td>12</td></tr><tr><td>Allium oleraceum</td><td></td><td>G</td><td>11</td></tr><tr><td>Anthericum liliago</td><td></td><td>H</td><td>11</td></tr></tbody></table>

<table><thead><tr><th>Species</th><th>New</th><th>LF</th><th>SS</th></tr></thead><tbody><tr><td>Cerastium pumilum</td><td>x</td><td>T</td><td>11</td></tr><tr><td>Chondrilla juncea</td><td>x</td><td>H</td><td>11</td></tr><tr><td>Filago pyramidata</td><td>x</td><td>T</td><td>11</td></tr><tr><td>Gymnadenia odoratissima</td><td>x</td><td>G</td><td>11</td></tr><tr><td>Scrophularia canina</td><td>x</td><td>H</td><td>11</td></tr><tr><td>Succisa pratensis</td><td>x</td><td>H</td><td>11</td></tr><tr><td>Torilis nodosa</td><td>x</td><td>T</td><td>11</td></tr><tr><td>Trifolium arvense</td><td></td><td>T</td><td>11</td></tr><tr><td>Biscutella laevigata</td><td>x</td><td>H</td><td>10</td></tr><tr><td>Campanula rotundifolia</td><td></td><td>H</td><td>10</td></tr><tr><td>Carex halleriana</td><td>x</td><td>H</td><td>10</td></tr><tr><td>Dianthus sylvestris</td><td>x</td><td>H</td><td>10</td></tr><tr><td>Erophila verna</td><td>x</td><td>T</td><td>10</td></tr><tr><td>Euphorbia cyparissias</td><td></td><td>H</td><td>10</td></tr><tr><td>Euphorbia segetalis</td><td>x</td><td>T</td><td>10</td></tr><tr><td>Hornungia petraea</td><td>x</td><td>T</td><td>10</td></tr><tr><td>Lavandula angustifolia</td><td></td><td>C</td><td>10</td></tr><tr><td>Myosotis ramosissima</td><td>x</td><td>T</td><td>10</td></tr><tr><td>Ophrys insectifera</td><td>x</td><td>G</td><td>10</td></tr><tr><td>Ornithogalum umbellatum</td><td></td><td>G</td><td>10</td></tr><tr><td>Peucedanum oreoselinum</td><td>x</td><td>H</td><td>10</td></tr><tr><td>Plantago maritima subsp serpentina</td><td>x</td><td>H</td><td>10</td></tr><tr><td>Saponaria ocymoides</td><td></td><td>H</td><td>10</td></tr><tr><td>Saxifraga tridactylites</td><td></td><td>T</td><td>10</td></tr><tr><td>Viola reichenbachiana</td><td>x</td><td>H</td><td>10</td></tr></tbody></table>

conditions, nurseries should take the successful species into account in their supply, in order to diversify and improve the green roofs performance.

A last constraint is the non-consideration of the species habitat requirements, which resulted in some odd species propositions (Table 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 green roof installations is doubtful because of their non-Mediterranean climate and habitat affinities. Their inclusion in the list is a side effect of the used FMED species list, 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 (Bernard, 2008). 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.

**5. Conclusion**

The ultimate purpose of this study was to reveal the most impor-
tant attributes of green roof plants by use of a plant trait analysis, i.e.
a correlation analysis on an extended matrix containing green roof
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 appro-
priate green roof design 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 green roof companies in the temperate climate to
adjust green roof 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 extensive green roof
purposes.

**Acknowledgements**

This work could be done thanks to a scholarship of the KU Leu-
ven (Belgium) and was supported by the TRY initiative on plant
traits (http://www trydb.org). The TRY initiative and database is
hosted, developed and maintained by J.Kattge and G. Bönisch (Max-
Planck-Institute for Biogeochemistry, Jena, Germany). TRY is/has
been supported by DIVERSITAS, IGBP, the Global Land Project, the
UK Natural Environment Research Council (NERC) through its pro-
gram QUEST (Quantifying and Understanding the Earth System),
the French Foundation for Biodiversity Research (FRB), and GIS
"Climat, Environnement et Société" France.

We would also like to thank Daniel Pavon (UMR CNRS-IRD IMBE,
Marseille, France) for his help during field excursions and plant
identification.

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