Julie Chenot
email: chenot.julie@gmail.com

Renaud Jaunatre
email: renaud.jaunatre@irstea.fr

Elise Buisson
email: elise.buisson@univ-avignon.fr

Thierry Dutoit
email: thierry.dutoit@univ-avignon.fr

Long-term effects of topsoil transfer assessed thirty years after rehabilitation of dry alluvial quarries in Southeastern France

Keywords: alpha diversity, gamma diversity, quarry rehabilitation, Mediterranean ecosystem, soil analyses, steppe plant community

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The constant acceleration of urbanization and industrialization worldwide, with the ensuing demand for mineral resources, means that newly-created quarries have largely destroyed many natural ecosystems [START_REF] Wang | The ecological potential of a restored abandoned quarry ecosystem in Mt. Mufu, Nanjing, China[END_REF]. Quarrying results in significant visual and ecological impacts [START_REF] Simón-Torres | Restoration of dump deposits from quarries in a Mediterranean climate using marble industry waste[END_REF], not all of which have been identified yet [START_REF] El-Taher | Ecological impacts of Al-Jalamid phosphate mining, Saudi Arabia: Soil elemental characterization and spatial distribution with INAA[END_REF]. Quarrying causes drastic alterations. It destroys flora and fauna, thereby reducing biodiversity and disrupting fundamental ecological relationships. Moreover, it extensively damages soil by modifying the original site topography and depleting and altering soil microbial communities [START_REF] Corbett | Prairie Revegetation of a Strip Mine in Illinois: Fifteen Years after Establishment[END_REF][START_REF] Pinto | Image analysis applied to quantitative evaluation of chromatic impact generated by open-pit quarries and mines[END_REF][START_REF] Milgrom | Environmental aspects of rehabilitating abandoned quarries: Israel as a case study[END_REF][START_REF] Mouflis | Assessment of the visual impact of marble quarry expansion (1984-2000) on the landscape of Thasos island, NE Greece[END_REF][START_REF] Simón-Torres | Restoration of dump deposits from quarries in a Mediterranean climate using marble industry waste[END_REF]. Other impacts identified at regional level include nuisance to local residents, with dust, noise pollution and ground vibrations when topsoil is dug up with heavy machinery [START_REF] Mohamed | Quarry blasts assessment and their environmental impacts on the nearby oil pipelines, southeast of Helwan City, Egypt[END_REF][START_REF] Dontala | Environmental Aspects and Impacts its Mitigation Measures of Corporate Coal Mining[END_REF]. Quarrying can also cause chemical contamination of groundwater by increasingly hazardous materials [START_REF] Misra | Influence of stone quarries on groundwater quality and health in Fatehpur Sikri, India[END_REF][START_REF] Dontala | Environmental Aspects and Impacts its Mitigation Measures of Corporate Coal Mining[END_REF].

Today, many countries have legislation requiring mining and quarrying companies to implement ecological restoration after closure (DITR, 2005;UNEP et al., 2005;European Parliament, 2014). Ecological restoration sensu lato is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed (SER, Society for Ecological Restoration International, Science & Policy Working Group, 2004). The goals of restoration sensu stricto include the re-establishment of the pre-existing biotic integrity in terms of species composition and community structure.

Many quarries thus implement rehabilitation or reclamation actions of benefit to biodiversity [START_REF] Damigos | Assessing the benefits of reclaiming urban quarries: a CVM analysis[END_REF][START_REF] Carrick | Restoring degraded landscapes in lowland Namaqualand: Lessons from the mining experience and from regional ecological dynamics[END_REF]. Rehabilitation seeks to repair one or more ecosystem attributes, processes, or services. Reclamation, on the other hand, includes land stabilization, public safety guarantees, aesthetic improvement and usually a return of the ecosystem considered useful in the regional context (Society for Ecological Restoration International, Science & Policy Working Group, 2004).

Numerous experiments conducted worldwide testify to the many quarry rehabilitation techniques in use, in particular for vegetation. For example, shrub species were planted in dolomite rock quarries in China [START_REF] Wang | The ecological potential of a restored abandoned quarry ecosystem in Mt. Mufu, Nanjing, China[END_REF] or in limestone quarries in Portugal [START_REF] Oliveira | Effect of substrate treatments on survival and growth of Mediterranean shrubs in a revegetated quarry: An eight-year study[END_REF] to rehabilitate areas formerly degraded by an increased bare ground soil surface sensitive to water or wind erosion. This involved adding an improved marl substrate layer on top of the bare rock [START_REF] Oliveira | Effect of substrate treatments on survival and growth of Mediterranean shrubs in a revegetated quarry: An eight-year study[END_REF].

Other techniques using substitute substrates with fertilizers, water-holding polymers, geotextiles and mycorrhizal inoculations were used to enhance vegetation growth after marble mining in Spain. However, long-term monitoring is rarely implemented, and those results available are not encouraging [START_REF] Oliveira | Effect of substrate treatments on survival and growth of Mediterranean shrubs in a revegetated quarry: An eight-year study[END_REF]. Yet this rather limited effect of rehabilitation contrasts with the sometimes high diversity found in some abandoned quarries [START_REF] Remacle | Contribution des anciennes carrières à la biodiversité wallonne: rapport de convention (C45) [Les carrières en Région wallonne : inventaire et intérêt biologique[END_REF][START_REF] Chapel | La biodiversité dans les carrières, une réalité ? Avis des associations naturalistes[END_REF][START_REF] Prach | Spontaneous vegetation succession at different central European mining sites: a comparison across seres[END_REF][START_REF] Pitz | Etude de la flore spontanée de la carrière de Loën pour de futures restaurations -Rapport final[END_REF]. This paradox suggests a need to determine the most beneficial measures for the rehabilitation of singular flora and vegetation in disturbed areas, and for research specifically aimed at those inhabiting quarry habitats [START_REF] Oliveira | Effect of substrate treatments on survival and growth of Mediterranean shrubs in a revegetated quarry: An eight-year study[END_REF][START_REF] Ballesteros | Central role of bedding materials for gypsum-quarry restoration: An experimental planting of gypsophile species[END_REF].

One of the most frequently used techniques is to cover the post-quarrying exposed geological substrate, either with a layer of topsoil removed pre-quarrying and conserved in stockpiles during quarrying (topsoil transfer) [START_REF] Simón-Torres | Restoration of dump deposits from quarries in a Mediterranean climate using marble industry waste[END_REF], or with an artificially created soil (artificial soils) [START_REF] Frouz | Interactions between soil development, vegetation and soil fauna during spontaneous succession in post mining sites[END_REF][START_REF] Weber | Properties of soil materials derived from fly ash 11 years after revegetation of post-mining excavation[END_REF]. Topsoil transfer consists in removing the uppermost centimeters of topsoil from a donor site of ecological interest. This topsoil can be the pre-existing soil from the site itself, or can come from another site already programmed for destruction [START_REF] Bulot | Ecological restoration by soil transfer: impacts on restored soil profiles and topsoil functions[END_REF]. The soil is then re-spread on the sites to be restored [START_REF] Ghose | Management of topsoil for geo-environmental reclamation of coal mining areas[END_REF][START_REF] Sheoran | Soil Reclamation of Abandoned Mine Land by Revegetation: A Review[END_REF]. It can consist in a direct topsoil transfer from another site that begins to be exploited. In such case, there is no storage of topsoil and therefore a better chance of rehabilitation successful [START_REF] Rivera | Spreading Topsoil Encourages Ecological Restoration on Embankments: Soil Fertility, Microbial Activity and Vegetation Cover[END_REF]. In other cases, topsoil is stored for the duration of mining exploitation, the quality of soil and seed bank are then altered and this reduces the chances of rehabilitation success [START_REF] Ghose | Management of topsoil for geo-environmental reclamation of coal mining areas[END_REF][START_REF] Strohmayer | Soil Stockpiling for Reclamation and Restoration activities after Mining and Construction[END_REF]. Topsoil transfer can be used to save ecological features and their associated ecosystem services from donor sites [START_REF] Box | Critical Factors and Evaluation Criteria for Habitat Translocation[END_REF]. It has been evaluated as the best rehabilitation method to compensate for the many projects necessarily involving destruction to make way for consented or permitted development [START_REF] Box | Habitat translocation, rebuilding biodiversity and no net loss of biodiversity[END_REF]. However, success monitoring rarely exceeds the first few years [START_REF] Koch | Restoring a Jarrah Forest Understorey Vegetation after Bauxite Mining in Western Australia[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][START_REF] Muller | Temporary wetland restoration after rice cultivation: is soil transfer required for aquatic plant colonization?[END_REF][START_REF] Bulot | A comparison of different soil transfer strategies for restoring a Mediterranean steppe after a pipeline leak (La Crau plain, South-Eastern France)[END_REF]Jaunatre et al., 2014a;[START_REF] Bulot | Ecological restoration by soil transfer: impacts on restored soil profiles and topsoil functions[END_REF]. Most restoration involving soil transfer has been recent and/or has not been examined scientifically [START_REF] Fowler | Evaluating restoration potential of transferred topsoil[END_REF]. The long-term effects (over several decades) of topsoil transfer on target species survival are therefore not known.

Moreover, results obtained in the first months or years after implementation do not necessarily provide a good indication of longer-term responses [START_REF] Cooke | Ecological restoration of land with particular reference to the mining of metals and industrial minerals: A review of theory and practice[END_REF][START_REF] D'antonio | Exotic Plant Species as Problems and Solutions in Ecological Restoration: A Synthesis[END_REF][START_REF] Herrick | Monitoring ecological processes for restoration projects[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]Gaucherand et al., in press). Studies assessing medium-and long-term results are, therefore, essential for a fuller evaluation of these techniques.

In the plain of La Crau in southeastern France, many dry alluvial quarries were exploited in the 1970s for the creation of the Fos-sur-Mer port zone. These activities resulted in the destruction of nearly 300 hectares of the unique La Crau steppe ecosystem (Buisson and Dutoit, 2006). When quarrying ceased fifteen years later, the companies were not required by law to rehabilitate the environment after mining. Various rehabilitation trials, mainly using different soil transfer techniques, were thus implemented, mainly with aesthetic aims (i.e. creating a flat area like the steppe landscape) or security objectives (i.e. to avoid potholes).

Monitoring the soil and vegetation of these quarry pits today therefore provides the opportunity for 30-year long-term feedback on these techniques (i.e. involving different soil sources, composition, thickness and with or without contact with the groundwater table). Plant communities and soil physico-chemical parameters in rehabilitated pits were compared to a positive control, the steppe, generally considered as the reference ecosystem [START_REF] Dutoit | The pseudo-steppe of La Crau (South-Eastern France): origin, management and restoration of a Mediterranean rangeland[END_REF][START_REF] Hobbs | Novel Ecosystems: Intervening in the New Ecological World Order[END_REF] and to a negative control without any soil transfer treatment. The soil of this surrounding steppe was the one that existed before it was destroyed by quarrying. To evaluate the biological importance of the newly created habitats, we worked at different spatial scales. We took into account both local (alpha) and regional biodiversity (gamma diversity), as well as the originality of the plant communities in relation to a landscape repository.

Materials and methods

Study site

Our study was carried out in the plain of La Crau, located in southeastern Mediterranean France (Bouches-du-Rhône) (Fig. 1A). Characterized by a unique natural habitat, the plain was classified in 1990 as a Special Protection Area under Natura 2000, called Steppe of La Crau, the last remaining French Mediterranean steppe (Buisson and Dutoit, 2006). Its Mediterranean substeppic grassland plant community features the phytosociological association Asphodeletum fistulosii [START_REF] Molinier | La végétation de la Crau (Basse-Provence)[END_REF] dominated by species such as Brachypodium retusum (Pers.) P. Beauv, Thymus vulgaris L. and Asphodelus ayardii Jahand and Marie. This plant community is unique in terms of species richness, composition and diversity [START_REF] Cherel | Contribution à l'étude des relations végétation-mouton sur les parcours de Crau (S. E. France) : adaptation et développement de méthodes d'étude du régime alimentaire[END_REF][START_REF] Badan | Les bergeries romaines de la Crau d'Arles. Les origines de la transhumance en Provence[END_REF][START_REF] Henry | Origine et dynamique à long terme d'un écosystème herbacé pseudo steppique, le cas de la plaine de La Crau (Bouches du Rhône[END_REF]. There are on average 30 to 40 plant species per square meter, 50% of which are annuals [START_REF] Römermann | How to predict attachment potential of seeds to sheep and cattle coat from simple morphological seed traits[END_REF]. This plant community may result from a regressive or allogenic plant succession of thousands of years under the combined effects of i) the dry and windy Mediterranean climate, ii) specific soil conditions: the 40 cm deep soil is composed of 50% siliceous stones and lies on a calcareous conglomerate which cannot be penetrated by plant roots [START_REF] Molliex | Quaternary evolution of a large alluvial fan in a periglacial setting (Crau Plain, SE France) constrained by terrestrial cosmogenic nuclide (10Be)[END_REF] and iii) a recurring disturbance regime constituted by itinerant sheep grazing since the end of the Neolithic period [START_REF] Badan | Les bergeries romaines de la Crau d'Arles. Les origines de la transhumance en Provence[END_REF][START_REF] Lebaudy | Gravures et graffiti des bergers de la plaine de la Crau: un patrimoine fragile et méconnu[END_REF][START_REF] Henry | The age and history of the French Mediterranean steppe revisited by soil wood charcoal analysis[END_REF][START_REF] Tatin | Écologie et conservation d'une steppe méditerranéenne: La plaine de Crau[END_REF].

Our study site was located in the south of the plain of La Crau (43°31'36.77''N, 4°53'04.50''E). Present on the site are 296 ha of open quarries once exploited for road ballast or materials for the construction of docks in the Fos-sur-Mer port zone (Fig. 1).

All these quarries were abandoned in the 1980s, as seen from old aerial photographs (1938 -2015) (Fig. 2). A shallow Haplic Cambisol soil WRB (IUSS Working Group WRB, 2006) 40 cm deep lying above the geological conglomerate (composed of quartz pebbles in a calcium carbonate matrix forming a limestone 1 to 5 m thick), hereafter named topsoil, was removed from these quarries and exported, sold or stored. This made way for quarrying the underlying geological substrate (a 10 to 50 m thick mixture of Quaternary pebbles and sands deposited in a former large alluvial fan during the Riss-Wurm glacial period, [START_REF] Molliex | Quaternary evolution of a large alluvial fan in a periglacial setting (Crau Plain, SE France) constrained by terrestrial cosmogenic nuclide (10Be)[END_REF]) below the conglomerate.

In most quarry pits (44% of the study site area), no material was replaced after quarrying, leaving exposed the un-disturbed geological material, or the surface of the water table when quarrying went more than nine meters deep (average water table height).

We therefore considered this situation a negative control with no attempt at rehabilitating the soil or the vegetation for ecological or other reasons (Table 1). However, in some cases, quarry pits were partly filled with anthropogenic spoil materials combining various proportions of gravels, clays and sands, conglomerate or concrete blocks, etc. (11% of the study site area). This situation was considered as a rehabilitation treatment, since these new artificial soils provided conditions favorable to new primary plant successions, even though nothing was done to drive these successions towards the plant composition and dynamics of the reference steppe plant community. Contrastingly, at the initiative of certain quarry operators, other quarry pits had stored topsoil re-spread on top of the spoil materials (45 % of the study site area) to various depths, including a depth similar to the reference steppe (40 cm) and a greater depth (more than 40 cm of topsoil added).

Differing quarry operating conditions, followed by differing rehabilitation treatments in the eighties, provided us with a great diversity of artificial geomorphological and pedological situations whose impacts on the present vegetation could be compared. We compared this vegetation both to the steppe vegetation (positive control) generally considered as the reference ecosystem in the previous restoration projects realized in this area [START_REF] Dutoit | The pseudo-steppe of La Crau (South-Eastern France): origin, management and restoration of a Mediterranean rangeland[END_REF], and to quarry pits with no rehabilitation treatments (negative control) (Table 1). In total, four situations were identified along a rehabilitation gradient: (1) soil transfer (40 cm) with no contact with the water table, (2) soil transfer (40 cm) in contact with the water table, (3) more than 40 cm of soil transfer and (4) anthropogenic deposits (spoils) (Table 1). These four situations were identified by the use of ancient aerial photographs (Fig. 2), soil profiles and interviews of quarry operators. (1968( -1974( -1984( -2011 -IGN) -IGN). The extension of intensive orchard areas since 1984 can also be observed between the quarries.

Vegetation analyses

Vegetation sampling using 4 m² quadrats was carried out in May 2015. For each of the six treatments, depending on available area, 3 to 6 100-meter transects were replicated, each composed of five pseudo-replicated quadrats (Fig. 1; Table 1). Quadrats were pseudo-replicated on the same site along each transect, so as to reveal any heterogeneity in the spatial distribution of plant communities for the same treatment. The number of replicated transects differed according to type of treatment in the quarries sampled. On each transect, quadrats were spaced more than 10 m apart, to prevent spatial autocorrelation between quadrats (Buisson et al., 2006). The abundance of each plant species was visually estimated, in percent cover [START_REF] Gillet | La phytosociologie synusiale intégrée[END_REF].

Soil analyses

Soil analyses were carried out on twenty-nine 200g soil samples taken from abandoned quarries in 2015 (Table 1). Samples were extracted from the first ten centimeters of the upper soil/material layer in February 2015. Each sample was dried and sieved with a 2 mm mesh sieve. Chemical content (organic C [START_REF] Allison | Organic carbon[END_REF], total N [START_REF] Bremner | Nitrogen-Total[END_REF], C:N, CEC Metson [START_REF] Metson | Methods of chemical analysis for soil survey samples[END_REF][START_REF] Ciesielski | A comparison between three methods for the determination of cation exchange capacity and exchangeable cations in soils[END_REF], K, P2O5 [START_REF] Olsen | Estimation of available phosphorus in soils by extraction with sodium bicarbonate[END_REF], CaCO3, pH [START_REF] Sparks | Methods of soil analysis[END_REF][START_REF] Thomas | Methods of Soil Analysis Part 3 -Chemical Methods[END_REF]) and fine particle sizes (<2 mm) (percentages of clay: <0.002 mm; fine silt: 0.002-0.02 mm; coarse silt: 0.2-2 mm; fine sand: 0.05-0.2 mm; and coarse sand: 0.2-2 mm) [START_REF] Gee | Particle-size Analysis[END_REF] were measured without decarbonization by the soil analysis laboratory at INRA (Institut National de la Recherche Agronomique) in Arras.

Data analysis

To test the effects of treatments on physico-chemical parameters, soil characteristics were ordinated using principal component analysis (PCA) (6 treatments × 29 soil samples), a method appropriate for continuous variables [START_REF] Borcard | Numerical Ecology with R[END_REF].

To test the effects of the different treatments on species richness, plant community composition and structure, these variables were compared between treatments.

Community structure was measured using different indices (alpha diversity, gamma diversity, CSIInorm and HAI index). Alpha diversity, which is the number of species present in a plant community, was calculated for each 4 m² quadrat. Gamma diversity, which is the total number of species in a landscape, was calculated as the cumulated species-richness of the five quadrats for each transect in the different quarries and the reference steppe. As the number of transects used in the quarries (N =19) is higher than in the steppe (N = 6), six quarry transects were randomly selected and their cumulated species-richness was calculated. This operation was repeated 100 times. Mean cumulated species richness determined quarry gamma diversity. Two indices were used to assess the success of plant community rehabilitation by topsoil transfer: the CSIInorm index (normalized Community Structure Integrity Index) measuring the proportion of the abundance of the species of the reference community represented in the community of the restored community, and the HAI index (Higher Abundance Index) measuring the proportion of the abundance of species in the restored community which is higher than in the reference community [START_REF] Jaunatre | New synthetic indicators to assess community resilience and restoration success[END_REF]. When parametric conditions were met, we used ANOVA tests followed by post-hoc Tukey HSD tests if a significant difference was detected. When parametric conditions were not met, we used non-parametric Kruskal-Wallis tests followed by pairwise Wilcoxon comparisons with a p-value adjustment according to Bonferroni if a significant difference was detected [START_REF] Bonferroni | Teoria statistica delle classi e calcolo delle probabilità[END_REF].

Vegetation composition and abundance were ordinated using non-metric multidimensional scaling (NMDS) (6 treatments × 125 quadrats × 241 plant species) based on Bray-Curtis dissimilarity [START_REF] Borcard | Numerical Ecology with R[END_REF]. We were thus able to visualize the vegetation structure of each treatment and to compare them.

All the analyses were conducted using R 2.13.0 (R Development Core Team, 2011), multivariate analyses with the "ade4" [START_REF] Chessel | The ade4 package -I : One-table methods[END_REF]Dray and Dufour, 2007;Dray et al., 2007) and "vegan" [START_REF] Oksanen | Package "vegan[END_REF] packages.

Results

Effect of topsoil transfer on soil physico-chemical parameters

The different rehabilitation treatments showed significant differences in soil physico-chemical properties (Table 2, Fig. 3). The PCA ordination based on soil properties significantly discriminated the different treatments. Axis 1 (45.3% of inertia) separated topsoil (steppe and soil) transfer from the negative control with no spoil deposits or soil spreading. This separation is explained partly by the quantities of clay, which were significantly higher in the steppe and soil transfers. Conversely, in the absence of deposits or soil spreading, the substrate was characterized by significantly higher amounts of coarse sand, higher C:N ratios and higher pH.

Axis 2 (25.8% of inertia) clearly separated the soils of the reference steppe and of the soil transfer treatments from those of the negative control and spoil deposits. The soils of topsoil transfer treatments tended to be closer to that of the reference steppe, especially those which were not in contact with the water table.

288 289

Effect of topsoil transfer on plant communities

Alpha and gamma diversity

Vegetation inventories allowed us to identify 21 plant taxa of conservation interest out of the 241 species recorded in total (nearly 10%): 5 with high conservation priority and 16 with moderate conservation priority on the red list of the Provence-Alpes-Côted'Azur region (Table 3). 224 species were inventoried in the quarries, including 20 regional red list species and 131 in the reference steppe, including 11 regional red list species (Table 3). Only one plant species of conservation interest was identified in the steppe: Phlomis lychnitis L., against 11 in quarries. Four species were recorded in anthropogenic material deposits alone (Bupleurum semicompositum L., Limonium cuspidatum (Delort) Erben, Limonium echioides (L.) Mill., Velezia rigida L.) and one in the negative control (Lythrum hyssopifolia L.) (Table 3). All plant species of conservation interest identified in the topsoil transfer areas were also identified in the reference steppe vegetation, except for Campanula erinus L. and Paronychia capitata (L.) Lam. (Table 3). )) showed no significant difference from the vegetation of the reference steppe (42.2 ± 1.51) (Fig. 4). In contrast, the average species-richness on anthropogenic material deposits (29.88 ± 2.66 species) and on the negative control (23.25 ± 1.64) was significantly lower than in the steppe (F5, 119 = 18.98, p <0.001) (Fig. 4).

Effects of topsoil transfer on plant community composition

The NMDS ordination showed a gradient in plant community composition, from the steppe (positive control) to the no-rehabilitation negative control, and clearly discriminated plant communities (NMDS-Stress= 0.20) (Fig. 5).

In the NMDS axis1-axis2 plane, the reference steppe was characterized by its most representative species, such as perennial species (Eryngium campestre L., Stipa capillata L., Phlomis lychnitis L., etc.), especially Brachypodium retusum (Pers.) P. Beauv.,usually dominant in the steppe plant community. It was also characterized by annual species such as Aegilops ovata L., Clypeola jonthlaspi L., Cynosurus echinatus L., Hedypnois cretica Willd., Reseda phyteuma L., Stipa capillata L., Trifolium arvense L., which were recorded only in the steppe.

The reference steppe community was close to the two topsoil transfer treatments with no contact with the water table ("> 40 cm topsoil transfer" and "40 cm topsoil transfer with no contact with the water table") (Fig. 5). These two plant communities overlapped and were characterized mainly by grasses and thistles, such as Avena barbata Brot., and Galactites tomentosus Moench., as well as Euphorbia cyparissias L., Marrubium vulgare L., Medicago truncatula Gaertn., etc.. This was confirmed by the CSIInorm, which showed that only 18 to 22% of species identified in topsoil transfer plant communities were also present in the steppe community with the same composition, abundance and richness (Fig. 6A) even thirty years after these rehabilitation treatments were applied.

The topsoil transfer in contact with the water table showed a different plant community composition from that of the reference steppe. It contained Anagallis arvensis L., Catapodium rigidum (L.) C.E.Hubbs. , Hypericum perforatum L., Stellaria media (L.)

Vill., Veronica arvensis L., etc. (Fig. 5), pioneer species common to anthropogenic and wetland margin environments.

In the NMDS axis1-axis2 plane, the rehabilitated areas (the topsoil transfer and the anthropogenic material deposits treatments) and the reference steppe were also clearly separated from the no-rehabilitation ("negative control") treatment in terms of plant community composition (Fig. 5). The no-rehabilitation control was characterized by pioneer annual target species also occurring in the reference steppe, such as Centranthus calcitrapae (L.) Dufr., Valerianella locusta (L.) Laterr., Velezia rigida L., and other nontarget species such as Artemisia annua L., Pyracantha coccinea M. Roem. (Fig. 5). These last two species can be considered potential invasive species. Due to the presence of the water table, this area contained temporary wetland species such as Lythrum hyssopifolia L., a species of conservation interest.

The plant communities identified in quarries were far from similar to those of the reference steppe (F5; 119 = 227.20, p <0.001) (Fig. 6A) in terms of composition, richness and diversity. Not only did they not include all reference steppe species, but they included other species not present in the steppe community (Anagallis foemina Mill., Anagallis arvensis L., Cynodon dactylon (L.) Pers., Dittrichia viscosa (L.) Greuter, Medicago orbicularis (L.) Bartal., Rumex pulcher L., etc.). The CSIInorm index showed a significant difference between treatments (Fig. 6A). The anthropogenic material deposits treatment and the no-rehabilitation treatment showed a significantly lower CSIInorm index than the reference steppe (Fig. 6A). The HAI index was relatively high both in the different rehabilitated areas and in the no-rehabilitation treatment, and significantly different from that of the reference steppe (F5, 119 = 88.66, p <0.001) (Fig. 6B).

Discussion

Topsoil transfer has been recognized as the best method for restoring soil and vegetation when this resource has been conserved or is available elsewhere [START_REF] Box | Habitat translocation, rebuilding biodiversity and no net loss of biodiversity[END_REF], Bulot et al., 2014, in press). To date, however, the restoration success of this technique has mainly been assessed over the short term (a few years after treatment). Assessing restoration over the long term (several decades), our results show that topsoil transfer is still the best method, especially when initial soil characteristics (thickness, absence of contact with the water table) are respected. Nevertheless, in our case, soil transfer still does not compensate for the destruction of the pre-existing ecosystem: the steppe soil and plant community have not yet fully recovered even thirty years after its application. These results confirm those of Holl and Cairns, Mulligan et al., Nichols (Holl and Cairns, 1994;[START_REF] Mulligan | An assessment of the direct revegetation strategy on the tailings storage facility at Kidston gold mine[END_REF][START_REF] Nichols | Developing completion criteria for native ecosystem reconstruction: A challenge for the mining industry[END_REF], who found that plant community composition was never completely restored after rehabilitation, in quarries or elsewhere. However, it is generally acknowledged that natural successions towards steppe occur on quarries after topsoil applications and re-vegetation is positive, although species composition still differs from that of the reference steppe [START_REF] Martínez-Ruiz | Natural revegetation on topsoiled mining-spoils according to the exposure[END_REF].

The negative control, where there was no rehabilitation, no transfer of materials or soil, is the worst situation compared to the undisturbed reference steppe in terms of total number of plant species and total number of plant species of conservation interest.

In 30 years, only a fraction of the steppe vegetation has recovered. This is due to the depth of quarrying, which involves potential contact with the water table. New primary plant succession is therefore blocked by irregular changes in ground water table levels, including by the extreme conditions that develop between dry and flood periods [START_REF] Prach | Vegetation succession in restoration of disturbed sites in Central Europe: the direction of succession and species richness across 19 seres[END_REF][START_REF] Balázs | Rainfall fluctuations and vegetation patterns in alkali grasslandsusing self-organizing maps to visualise vegetation dynamics[END_REF][START_REF] Deák | Micro-topographic heterogeneity increases plant diversity in old stages of restored grasslands[END_REF][START_REF] Masson | Using Shrub Clearing, Draining, and Herbivory to Control Bramble Invasion in Mediterranean Dry Grasslands[END_REF]. The lack of rehabilitation, together with the new abiotic conditions created by quarrying, thus generated "novel ecosystems" [START_REF] Hobbs | Novel ecosystems: implications for conservation and restoration[END_REF]. The long-term dynamics of these new ecosystems are unknown, since we have no experience of similar habitat conditions, and because they are also impacted by global change (climate and land-uses) [START_REF] Doley | Adopting novel ecosystems as suitable rehabilitation alternatives for former mine sites[END_REF][START_REF] Hobbs | Novel Ecosystems: Intervening in the New Ecological World Order[END_REF]. However, we do know that these environments will not spontaneously evolve towards the reference steppe in the long-term, barring any major ecological change (significant and permanent reduction in the level of the water table ).

Usually, for these radically disturbed mining sites, it is not practicable to aim for the restoration of historical ecosystems. However, hybrid or novel ecosystems, which consist of new combinations of physical and biological components with new variables and variables common to historical ecosystems, could provide acceptable levels of stability and functionality [START_REF] Doley | Adopting novel ecosystems as suitable rehabilitation alternatives for former mine sites[END_REF]. For example, vegetation may include both native species and non-native species.

Deposition of various types of anthropogenic materials after the end of quarrying created great heterogeneity in terms of physico-chemical soil parameters, but less diversity and species-richness in terms of vegetation. However, these materials provide some very specific habitats for certain species of high conservation value, which have particular ecological requirements linked to their narrow ecological niche. At some places on our site, depositing sandy salty soils excavated closer to the Mediterranean coast has provided a habitat for Limonium cuspidatum (Delort) Erben and Limonium echioides (L.)

Mill., two halophytic species of conservation interest at the regional scale. However, deposition of anthropogenic materials is certainly not the best method to restore plant communities similar to the reference steppe in the plain of La Crau. A better method might be steppe geomorphological rehabilitation by filling the bottom of quarry pits, thus isolating the upper transferred soil layers from the water table [START_REF] Meredith | The influence of soil reconstruction methods on mineral sands mine soil properties[END_REF][START_REF] Frouz | Interactions between soil development, vegetation and soil fauna during spontaneous succession in post mining sites[END_REF][START_REF] Mchergui | Use of dredged sediments for soil creation in the Seine estuary (France): Importance of a soil functioning survey to assess the success of wetland restoration in floodplains[END_REF]. Nevertheless, our results show that spoils must be covered by at least 40 cm of topsoil from the reference steppe.

Thus, topsoil transfer appears to be the most favorable treatment for rehabilitation of the reference sub-steppe plant community, in terms of physico-chemical properties and plant community composition. Our long-term results on soil transfer restoration confirm those obtained over the short term by [START_REF] Bulot | A comparison of different soil transfer strategies for restoring a Mediterranean steppe after a pipeline leak (La Crau plain, South-Eastern France)[END_REF] and Jaunatre et al. (Jaunatre et al., 2014a(Jaunatre et al., , 2014b) ) for the same steppe area. Our study shows that these reconstituted soils present the most favorable abiotic conditions for restoring the reference steppe, particularly because they entail no significant differences in fine particle size content from the soil of the reference steppe (Buisson et al., 2006;[START_REF] Bulot | Ecological restoration by soil transfer: impacts on restored soil profiles and topsoil functions[END_REF][START_REF] Römermann | How to predict attachment potential of seeds to sheep and cattle coat from simple morphological seed traits[END_REF]. However, the plant communities identified in these treatments cannot yet be considered similar to those of the reference steppe vegetation, as measured by the various diversity indices. Soil alone does not account for the diversity of vegetation encountered here after different types of soil transfer. Other factors are involved, such as the plant composition of the different surrounding patches, i.e. steppe, abandoned quarries, fallow land, orchards, etc. [START_REF] Pärtel | Restoration of species-rich limestone grassland communities from overgrown land: the importance of propagule availability[END_REF][START_REF] Zobel | Species pool: the concept, its determination and significance for community restoration[END_REF], the reproduction capacity of their communities, the presence of dispersal vectors [START_REF] Bakker | Seed banks and seed dispersal: important topics in restoration ecology §[END_REF][START_REF] Ozinga | Dispersal potential in plant communities depends on environmental conditions[END_REF]) and between-species competition or facilitation [START_REF] Maestre | Refining the stressgradient hypothesis for competition and facilitation in plant communities[END_REF][START_REF] Teixeira | Facilitation and sand burial affect plant survival during restoration of a tropical coastal sand dune degraded by tourist cars[END_REF]. The topsoils transferred most probably had a very poor seed bank, having been conserved in large piles for several years, and the soil of the reference steppe is known to have a very poor permanent seed bank [START_REF] Römermann | How to predict attachment potential of seeds to sheep and cattle coat from simple morphological seed traits[END_REF]Buisson et al., 2006;[START_REF] Saatkamp | The seed bank longevity index revisited: limited reliability evident from a burial experiment and database analyses[END_REF][START_REF] Rivera | The fate of herbaceous seeds during topsoil stockpiling: Restoration potential of seed banks[END_REF][START_REF] Bordez | Stratégies de revégétalisation des maquis miniers nickélifères de Nouvelle-Calédonie étude sur les potentiels biologiques des Topsoils en vue de leur utilisation pour la restauration écologique des milieux dégradés aplication au massif de Koniembo pour KNS[END_REF]. Therefore there would be very little similarity between the topsoil transferred and the reference steppe seed banks. Soil transfers in contact with the water table are ruled out as a restoration treatment aiming to restore the vegetation from the reference steppe (Mediterranean xeric grasslands), since they favor species adapted to moist soils, such as Scirpus holoschoenus subsp. romanus (L.) auct., Aster squamatus (Sperng.) Hieron, etc.. The solution would be to reconstitute the conglomerate bedrock occurring in the reference steppe, or to deposit spoil materials covered by topsoil as described above, which would prevent plant roots from reaching the water table.

Compared to techniques that only impact species dispersion, such as hay transfer [START_REF] Hölzel | Restoration of a species-rich flood meadow by topsoil removal and diaspore transfer with plant material[END_REF]Coiffait-Gombault et al., 2011a, 2011b;[START_REF] Török | Fast restoration of grassland vegetation by a combination of seed mixture sowing and low-diversity hay transfer[END_REF] or seeding [START_REF] Oliveira | Suitability and limitations of native species for seed mixtures to re-vegetate degraded areas[END_REF][START_REF] Gilardelli | Restoring Limestone Quarries: Hayseed, Commercial Seed Mixture or Spontaneous Succession? Land Degrad[END_REF], topsoil transfer offers the best results,,both by dispersing target species and by at least partially recreating soil abiotic conditions [START_REF] Box | Critical Factors and Evaluation Criteria for Habitat Translocation[END_REF]Jaunatre et al., 2014b). However, real success is only possible if certain criteria are met. The topsoil's particle size fraction needs to be adequate and it should not contain blocks of conglomerate that will raise soil CaCO3 content. This could negatively impact some slightly acidophilus species which characterize the steppe vegetation, especially "tonsures" (vegetation patches composed of very small vegetation, dominated by bryophytes, lichens and vascular plants that are more acidophilus and xerophilous than other steppe species (Coiffait-Gombault, 2011)).

Confirming findings by Simón-Torrez et al. [START_REF] Simón-Torres | Restoration of dump deposits from quarries in a Mediterranean climate using marble industry waste[END_REF], our study demonstrates that in the long term, the best way to restore the reference steppe vegetation is by using the most similar soil, which has conserved its fine particle size fraction, with a thickness of at least 30 to 40 cm and without contact with the water table [START_REF] Redente | Influence of topsoil depth on plant community dynamics of a seeded site in northwest Colorado[END_REF][START_REF] Holmes | Shrubland Restoration Following Woody Alien Invasion and Mining: Effects of Topsoil Depth, Seed Source, and Fertilizer Addition[END_REF][START_REF] Bowen | Influence of Topsoil Depth on Plant and Soil Attributes of 24-Year Old Reclaimed Mined Lands[END_REF]. Schladweiler et al. [START_REF] Schladweiler | Topsoil Depth Effects on Reclaimed Coal Mine and Native Area Vegetation in Northeastern Wyoming[END_REF] found that 30 cm of topsoil produced the greatest diversity; while they found that more than 50 cm ensured the greatest biomass, this was not our goal here. Despite topsoil transfer, 30 years was not sufficient to reach the reference state, even though the traditional sheep-grazing regime was re-established after quarry abandonment. Other factors may have been involved in changing the plant succession. At soil level, fine elements such as clays could have been lost by leaching during soil transport and / or the storage phase [START_REF] Weng | Understanding the Effects of Soil Characteristics on Phytotoxicity and Bioavailability of Nickel Using Speciation Models[END_REF][START_REF] Zeng | The influence of pH and organic matter content in paddy soil on heavy metal availability and their uptake by rice plants[END_REF]. This would have modified soil physico-chemical properties and consequently the plant communities that developed there [START_REF] Citeau | Gestion durable des sols[END_REF][START_REF] Amir | Caractérisation et fonctionnement du système sol/ plante/ microorganismes dans les maquis miniers[END_REF]. On our soil transfer treatments, species richness was similar to the steppe, but the most dominant species of the steppe was still absent (Brachypodium retusum).

Conversely, species not present in the steppe were able to develop. In all cases, our monitoring should be pursued on the long-term. There is clearly room for improvement in the methods used for future restoration of quarries still in operation. Topsoil transfer in turfs could be one answer [START_REF] Good | Translocation of Herb-Rich Grassland from a Site in Wales Prior to Opencast Coal Extraction[END_REF][START_REF] Vécrin | Top-soil translocation as a technique in the re-creation of species-rich meadows[END_REF][START_REF] Bulot | A comparison of different soil transfer strategies for restoring a Mediterranean steppe after a pipeline leak (La Crau plain, South-Eastern France)[END_REF], as this method allows the conservation of the soil structure [START_REF] Bulot | A comparison of different soil transfer strategies for restoring a Mediterranean steppe after a pipeline leak (La Crau plain, South-Eastern France)[END_REF].

The transfer could also be direct, without storage, thereby limiting degradation of the soil and of the seed bank [START_REF] Granstrom | Seed Viability of Fourteen Species During Five Years of Storage in a Forest Soil[END_REF][START_REF] Jones | Relationship between soil-stored seed banks and degradation in eastern Nama Karoo rangelands (South Africa)[END_REF][START_REF] Fenner | The ecology of seeds[END_REF][START_REF] Hall | Topsoil Seed Bank of an Oak-Hickory Forest in Eastern Kentucky as a Restoration Tool on Surface Mines[END_REF][START_REF] Golos | Waterproofing Topsoil Stockpiles Minimizes Viability Decline in the Soil Seed Bank in an Arid Environment[END_REF]. This would also ensure that plant species remain established on the turfs, preventing undesired species implantation. Although direct topsoil transfer is advocated, it is often not feasible because soil in place in the steppe may not be available when required for transfer [START_REF] Bulot | A comparison of different soil transfer strategies for restoring a Mediterranean steppe after a pipeline leak (La Crau plain, South-Eastern France)[END_REF]. When only ordinary soil transfer is possible, storage time must be as short as possible: numerous studies have shown the negative impact of soil storage [START_REF] Strohmayer | Soil Stockpiling for Reclamation and Restoration activities after Mining and Construction[END_REF][START_REF] Stahl | Topsoil Stockpiling Versus Exposure to Traffic: A Case Study on In situ Uranium Wellfields[END_REF][START_REF] Boyer | Impact of soil stockpiling and mining rehabilitation on earthworm communities[END_REF][START_REF] Park | Temporal Change of Soil Animals of the Stockpiled Forest Topsoil in Relation to Soil Temperature and Soil Moisture[END_REF][START_REF] Golos | Waterproofing Topsoil Stockpiles Minimizes Viability Decline in the Soil Seed Bank in an Arid Environment[END_REF][START_REF] Bordez | Stratégies de revégétalisation des maquis miniers nickélifères de Nouvelle-Calédonie étude sur les potentiels biologiques des Topsoils en vue de leur utilisation pour la restauration écologique des milieux dégradés aplication au massif de Koniembo pour KNS[END_REF].

Storage can lead to negative impacts on the soil seed bank. Direct topsoil transfer results in loss of less than 50% of the soil seed bank, whereas stockpiling causes loss of 80-90% [START_REF] Koch | Effects of Bauxite Mine Restoration Operations on Topsoil Seed Reserves in the Jarrah Forest of Western Australia[END_REF]. In addition, stored soil piles form a barrier and can trap many nontarget anemochorous species [START_REF] Bordez | Stratégies de revégétalisation des maquis miniers nickélifères de Nouvelle-Calédonie étude sur les potentiels biologiques des Topsoils en vue de leur utilisation pour la restauration écologique des milieux dégradés aplication au massif de Koniembo pour KNS[END_REF]. Storage can also cause rotting of organic matter when piles are too high [START_REF] Stahl | Topsoil Stockpiling Versus Exposure to Traffic: A Case Study on In situ Uranium Wellfields[END_REF]. It is therefore recommended that soil be stored on low elevation piles [START_REF] Bordez | Stratégies de revégétalisation des maquis miniers nickélifères de Nouvelle-Calédonie étude sur les potentiels biologiques des Topsoils en vue de leur utilisation pour la restauration écologique des milieux dégradés aplication au massif de Koniembo pour KNS[END_REF]. Moreover, while soil transfer may go some way toward restoring the reference ecosystem, the trajectories of rehabilitated ecosystems are not those of the reference steppe ecosystem. Additional techniques should be implemented to enhance restoration, such as seeding target species or removing undesirable species [START_REF] Bordez | Stratégies de revégétalisation des maquis miniers nickélifères de Nouvelle-Calédonie étude sur les potentiels biologiques des Topsoils en vue de leur utilisation pour la restauration écologique des milieux dégradés aplication au massif de Koniembo pour KNS[END_REF]. For example, native species seeding favors the development of native species to the detriment of exotic or ruderal species [START_REF] Norman | Vegetation Succession After Bauxite Mining in Western Australia[END_REF].

In conclusion, topsoil transfer methods can be enhanced by various techniques offering prospects of improved restoration. This should encourage current quarrying operations to implement these findings when planning for rehabilitation.

Fig. 1 .

 1 Fig. 1. Location of study site and experimental design. (A) Location of the plain of La Crau area in France and location of the remnant patches of steppe (striped grey) and quarries (black); (B) experimental design of the different rehabilitation treatments realized on the abandoned quarries in the 1980s: 40 cm soil transfer (dark grey); > 40 cm soil transfer (black); anthropogenic deposits (white) and no rehabilitation (light grey).

Fig. 2 .

 2 Fig. 2. Quarry expansion in the seventies and eighties visible from old aerial photographs(1968 -1974 -

Fig. 3 .

 3 Fig. 3. PCA ordination based on 29 soil sample analyses. Samples from each rehabilitation treatment and control are identified by dashed lines: (1) soil transfer (40 cm) with no contact with the water table in white (2) soil transfer (40 cm) in contact with the water table in light grey (3) more than 40 cm of soil transfer in medium grey and (4) anthropogenic material deposits in dark grey. Both samples from controls are identified by solid lines: Steppe in white, No rehabilitation in grey. Arrows represent soil variables (chemical content (organic C, total N, C:N, CEC Metson, K, P2O5, CaCO3, pH) and fine particle sizes (<2 mm) (percentages of clay (<0.002 mm), fine silt (0.002-0.02 mm), coarse silt (0.2-2 mm), fine sand (0.05-0.2 mm) and coarse sand (0.2-2 mm))

Fig. 4 .

 4 Fig. 4. Means and standard errors of species richness (4m²) for each treatment and the two controls: steppe (white), topsoil transfer (light grey), anthropogenic material deposits and absence of soil spreading (dark grey). Within a treatment, bars sharing common letters are not significantly different according to Tukey post-hoc test.

Fig. 5 .

 5 Fig. 5. NMDS ordination based on 125 vegetation samples (4m²) and 175 species present in at least 3 samples. For clarity, only the 63 most correlated species are shown. Samples from each rehabilitation treatment are identified: (1) soil transfer (40 cm) with no contact with the water table in light green (2) soil transfer (40 cm) in contact with the water table in blue (3) more than 40 cm of soil transfer in dark green and (4) anthropogenic material deposits in red. Both samples from control are identified: steppe in yellow, absence of material or soil spreading in grey.

Fig. 6 .

 6 Fig. 6. Means and standard errors of normalized Community Structure Integrity Index (A) and higher Abundance Index (B) for each treatment: steppe (white), topsoil transfer (light grey), anthropogenic material deposits and absence of soil spreading (dark grey). Within a treatment, bars sharing common letters are not significantly different according to Tukey post-hoc test.

Table 1 .

 1 Various rehabilitation treatments and controls used in this study and analyses performed on each

	treatment
			Sample number	Transect number
		Treatments	for soil	for vegetation
			analyses	sampling
	Positive control Reference Steppe	5	6
	Topsoil	40 cm soil transfer with no contact with the water table	4	4
	transfer	40 cm soil transfer in contact with the water table	5	3
		> 40 cm soil transfer	5	3
	Anthropogenic soil	Anthropogenic deposits (spoils)	5	5
	Negative control	No rehabilitation: absence of deposits or soil spreading in the quarried geological substratum	5	4

Table 2 290

 2 Soil analyses for each rehabilitation treatment realized in May 2015. The given values are means ± standard 291 errors. The F value and p value are from ANOVA and the X² value is from Kruskal-Wallis tests on the 292 effects of treatments on each variable. Two values in the same row with a different letter are significantly 293 different according to Tukey post-hoc tests or Mann-Whitney-Wilcoxon tests[START_REF] Bonferroni | Teoria statistica delle classi e calcolo delle probabilità[END_REF].

	294
		Statistic parameter	p-value	Reference steppe (Positive control)	Topsoil transfer (40 cm) with no contact with the water table	Topsoil transfer (40 cm) in contact with the water table	Topsoil transfer (More than 40 cm)	Anthropogenic material deposits	No rehabilitation Absence of soil spreading/material deposits (Negative control)
		X 2 =19.38,
	Clay (g kg -1 )	Df=6	0.004**	231.2±6.6 b	199.5±33.9 b	223.4±4 b	204±17.5 b	109.8±28.1 a	98.8±8.9 a
	Fine silt (g kg -1 )	F6,27= 1.7	0.16	210.8±5.5	173.5±15.3	177.4±2.7	176.8±6.5	150.8±46.3	131.6±13
	Coarse silt (g
	kg -1 )	F6,27= 0.56	0.76	143.0±3.7	116.5±11.6	128.4±2.6	123.6±5.9	123.8±66.1	81.2±6.9
	Fine sand (g
	kg -1 )	F6,27= 0.5	0.801	202.8±5.1	239.5±34.9	218.0±6.2	205.8±5.3	224.0±47.1	234.6±16.1
	Coarse sand (g
	kg -1 )	F6,27= 2.91	0.026*	212.2±13.6 a	271.0±26.9 ab	252.8±3.4 ab	289.8±29.1 ab 391.6±118.7 ab	453.8±35.5 b
	Total carbon (g
	kg -1 )	F6,27= 1.1	0.385	17.4±1.4	12.4±2.1	15.4±1.9	15.9±1.6	31.7±15.6	11.4±3.2
	Total nitrogen
	(g kg -1 )	F6,27= 0.72	0.637	1.6±0.1	1.1±0.2	1.4±0.1	1.5±0.2	2.2±1.3	0.8±0.3
	C:N	F6,27= 2.72	0.034*	10.7±0.3 a	11.8±1.8 ab	10.7±0.5 ab	10.9±0.6 ab	22.1±6.2 b	15.1±0.9 ab
	pH	F6,27= 4.31	0.004**	7.6±0.3 a	8.2±0.2 ab	8.3±0.1 ab	8.2±0.1 ab	8.7±0.2 b	8.6±0.1 b
		X 2 = 19.38, Df=
	CaCO3 (g kg -1 )	6	0.004***	5.8±4.8 a	70.8±62.3 a	20.8±5.7 a	58±46.1 a	216.6±20.4 b	223.4±19.1 b
		X 2 = 10.59, Df=
	P2O5 (g kg -1 )	6	0.102	0.184±0.173	0.009±0.001	0.014±0.003	0.030±0.0162	0.016±0.010	0.026±0.008
	CEC Metson
	(cmol + kg -1 )	F6,27= 1.37	0.261	9.1±0.4	7.1±1.3	8.4±0.2	7.8±0.4	8.7±4	3.4±0.6
	Ca (g kg -1 )	F6,27= 2.89	0.026*	2.8±1 a	4.1±1.1 ab	4.9±0.6 ab	4.3±1 ab	6.7±0.4 b	6.4±0.1 b
	K (g kg -1 )	F6,27= 1.32	0.282	0.1±0	0.1±0	0.2±0	0.1±0	0.1±0.1	0.1±0

Table 3

 3 Species of conservation interest identified for each treatment in May 2015. The species' regional conservation status was determined from the regional red list of the Provence-Alpes-Côte-d'Azur region[START_REF] Noble | Liste Rouge de la flore vasculaire de Provence-Alpes-Côte d'Azur[END_REF]

	Species	Regional conservation status	Reference steppe (Positive control)	Topsoil transfer (40 cm) with no contact with the water table	Topsoil transfer (40 cm) in contact with the water table	Topsoil transfer (More than 40 cm)	Anthropogenic material deposits	No rehabilitation Absence of soil spreading /material deposits (Negative control)
	Bupleurum semicompositum L.	Moderate					x
	Campanula erinus L.	Moderate		x	x		x	x
	Centaurea melitensis L.	Moderate	x		x		x
	Gastridium ventricosum (Gouan) Schinz & Thell. Moderate	x		x	x
	Limonium cuspidatum (Delort) Erben	Strong					x
	Limonium echioides (L.) Mill.	Moderate					x
	Linaria arvensis (L.) Desf.	Moderate	x	x	x		x
	Lythrum hyssopifolia L.	Moderate						x
	Melilotus indicus (L.) All.	Moderate					x
	Parapholis incurva (L.) C.E.Hubb.	Moderate	x		x	x	x	x
	Paronychia capitata (L.) Lam.	Moderate		x			x
	Phlomis lychnitis L.	Moderate	x
	Polygala monspeliaca L.	Moderate	x			x		x
	Polypogon maritimus Willd.	Moderate					x	x
	Psilurus incurvus (Gouan) Schinz & Thell.	Moderate	x	x	x	x	x	x
	Ruta Montana (L.) L.	Moderate	x	x
	Taeniatherum caput-medusae (L.) Nevski	Strong	x	x	x	x	x	x
	Trifolium glomeratum L.	Strong	x	x	x		x
	Trifolium subterraneum L.	Moderate	x			x
	Valerianella microcarpa Loisel.	Strong				x	x
	Velezia rigida L.	Strong					x

At the regional scale (gamma diversity), the mean species richness of the plant communities of the different rehabilitation treatments (156.78±0.77) was higher than the mean species richness of the reference steppe vegetation (130±0) (F1, 198 = 1219, p <0.001). However at the scale of the plant community, the species richness (alpha diversity) of the different topsoil transfer treatments (contact with water table (47.6 ± 2.02) ; no contact with water table (36 ± 1.20); more than 40 cm soil transfer

(35.55 ± 1.46

Acknowledgements

This study was supported by Gagneraud construction funding. We would like to thank the Réserve Naturelle Nationale des Coussous de Crau and the Grand Port Maritime de Marseille in Fos-sur-Mer for permission to access the study site. We thank Pierre Bourguet and Jean-Marc Arnal (Société des Carrières de la Ménudelle) for their collaboration. We are also grateful to students, technicians, research assistants, field ecologists for help with field work: Anne Aurière, Cannelle Moinardeau, Hervé Ramone, Manon Hess, Elie Gaget and Daniel Pavon.


Di Gao

Edzang Judicaelle Andeme

Abdou Karim Diallo

Thibault Dutronc

Teodor Silviu Balaban

Christine Videlot-Ackermann

Emmanuel Terazzi
email: emmanuel.terazzi@unige.ch

Gabriel Canard
email: gabriel.canard@univ-amu.fr

Light absorption and hole-transport properties of copper corroles: from aggregates to a liquid crystal mesophase

The synthesis of the corrole-based liquid crystal phase CorLC is described together with its full characterization by a combination of Polarised Optical Microscopy (POM), ThermoGravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) measurements and Small-Angle X-Ray Diffraction (SA-XRD) which shows that CorLC has an hexagonal columnar organization at room temperature. The light absorption and hole-transport properties of the mesophase CorLC is compared with the ones featured by assemblies of its parent meso triaryl (Cor1) and per-aryl (Cor2) derivatives. J-Aggregates are changed into H-aggregates when the intermolecular distance is increasing with the peripheral steric hindrance producing lower holetransport properties.

Introduction

The highly versatile optical, electronic and photophysical properties of porphyrin and phthalocyanine derivatives has led to their large application in the fields of organic electronics and optoelectronics. [START_REF]): 6417.5 (Maximum iso); calcd for[END_REF] In this regard, their ability to produce large assemblies or aggregates is particularly employed to construct organized and high light-absorbing materials displaying efficient charge and/or exciton transport properties that are essential in the preparation of organic solar cells 2 or photoconductors. 3 Among the different supramolecular tools that are used to build self-organized porphyrin and phthalocyanine systems, their incorporation in liquid crystal (LC) phases 4 is attractive since the morphology and orientation of LC thin films can be controlled to obtain large area and defect free devices. 5 For example, an exciton diffusion length longer than 12 nm has been measured in a nematically organized porphyrin layer 6 while a field-effect mobility of 0.7 cm 2 V -1 s -1 was exhibited by an Organic Field Effect Transistor (OFET) incorporating a liquid crystalline copper phthalocyanine. 7 Corrole and its metal complexes are currently attractive members of the porphyrinoid family since their syntheses 8 and functionalizations [START_REF]M+Na] + ): 6440.4; calcd for C426H649CuN4O33Na +[END_REF] have been extensively developed during the last fifteen years and are thus amenable to gram-scale synthesis, sometimes essential for developing devices. Consequently, corrole derivatives are employed in many research areas including catalysis, 10 sensors 11 or medicine. 12 Nevertheless, very few reports concern the preparation and characterization of large corrole-based assemblies 13 and their incorporation into electronic devices. 14 We report here the synthesis and full characterization of a copper corrole-based liquid crystal mesophase CorLC (Scheme 1). Its supramolecular organization was studied by UV/Visible spectroscopy and was compared to the ones produced by aggregates of its parent meso triaryl (Cor1) and per-aryl (Cor2) derivatives (Scheme 1). The hole-transport properties of these three compounds were additionally measured through their incorporation in OFETs.

Results and discussion

Synthesis of the mesogen CorLC.

The liquid crystal CorLC was prepared in four synthetic steps starting from the copper(III) meso-tris(pmethoxyphenyl)corrole Cor1 (Schemes 1 and S1, ESI †). 15 The full bromination of -positions followed by a Suzuki crosscoupling procedure afforded the per-aryl substituted copper corrole Cor2 as previously reported. 16 A demethylation using standard conditions afforded the per-aryl complex substituted by eleven p-hydroxyphenyl groups in 44% yield without any noticeable removal of the copper ion. It has to be noted that this latter derivative is enough soluble in sufficiently polar media in order to be purified and characterized. The expected compound CorLC was prepared in a yield of 23% by the subsequent condensation of an excess of the benzyl chloride derivative 1 in basic conditions. Its purity was confirmed by its 1 H NMR spectrum and by its MALDI-TOF spectrum showing only its molecular peaks and the corresponding sodium adduct (ESI †). We note here that if a reaction has to be performed more than tenfold on the same substrate, without allosteric effects, it is remarkable that the overall yield is in the two digit region.

Scheme 1 Synthesis of the liquid crystal corrole CorLC. i) a) Br 2 , pyridine, CHCl 3 ; b) NaS 2 O 5 , H 2 O (45%). ii) Pd 2 (dba) 3 , K 2 CO 3 , p-H 3 CO-C 6 H 4 -B(OH) 2 , toluene (71%). iii) BBr 3 , CH 2 Cl 2 (44%). iv) K 2 CO 3 , DMF (23%).

Thermal behaviour of CorLC.

The thermal behavior of CorLC was studied by a combination of Polarised Optical Microscopy (POM), ThermoGravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC) measurements and Small-Angle X-Ray Diffraction (SA-XRD) (Fig. 1, Fig. S1, S2 and S3, ESI †). The TGA measurement indicates that the compound is stable up to 300 °C (Fig. S1, ESI †). Temperature dependent POM observations showed a fluid and birefringent texture that is typical of the occurrence of a liquid-crystalline organization. This texture does not show any significant changes until the isotropisation occuring at 103 °C i.e. a transition from the liquid crystalline phase to the standard liquid state (I). Upon cooling, this phase transition is reversible and occurs at 95 °C (i.e. with a small hysteresis). Notably, the liquid crystal phase remains birefringent and fluid at room temperature. Cooling down the isotropic liquid sample at very low speed (0.1 °C•min -1 ) forms the liquid crystalline phase characterized by a fan texture that strongly suggests that the compound self-organizes in a columnar hexagonal (Col h ) liquid crystalline phase (Fig. 1a). This preliminary result was confirmed by DSC (Fig. 1b). The solid complex CorLC was transformed to the Col h mesophase through a broad first order transition (-30 °C, ΔH = 80.4 kJ•mol -1 , ΔS = 330.5 J•mol -1 •K -1 ) that preceded a second first order transition (103.4 °C, ΔH = 22.1 kJ•mol -1 , ΔS = 58.8 J•mol -1 •K -1 ) of the Col h phase into the liquid isotropic state I. 10 min per scan) allowed to detect a very weakly temperaturedependent sharp and intense signal at low angle in the 20-100 °C temperature range. A longer acquisition time at 30 °C (i.e. 24 hours) revealed the presence of three additional very weak reflections in the low angle region (Fig. 1c)). These four sharp reflections were indexed as (hk) = ( 10), ( 11), ( 20), ( 21) (squared spacing ratios h 2 + k 2 + hk = 1, 3, 4, 7; see Fig. S2, ESI †) confirming the formation of a Col h arrangement suggested by the POM observation (plane group p6 or p6mm). 17 A unit cell parameter of a = 41.1 Å and a cross-sectional area of S = 1462 Å 2 were thus deduced from this last measurement.

The pattern recorded at 30 °C shows a second broad reflection (h Col ) located at about 7 Å partially overlapping the first one corresponding to the molten aliphatic chains (i.e. h ch ). The separation of these two broad signals by fitting them with Gaussian functions (Fig. S3, ESI †) allowed to accurately determine the value of h Col = 6.86 Å. Making the assumption that h Coh corresponds to the separation between the complexes within a column, we thus calculated a volume of V = S•h Col = 10029 Å 3 for an hexagonal columnar unit cell. Taking into account a realistic density of d = 1.0 g•cm -3 in the mesophase, 17d we estimated that the number of complexes CorLC per unit cell Z amounts to 1 according to equation ( 1) where N av is the Avogadro's number, MM m is the molecular weight of the molecule in g•mol -1 , and V is the volume of the unit cell in Å 3 . 17d The liquid crystalline supramolecular organization of complex CorLC should thus be considered as a bidimensional long-ranged network with a rather disorganized third direction (i.e. along the column axis) that is typical of Col h organizations. This kind of organization is commonly observed in mesophases produced by porphyrin-and phthalocyanine-based discotic derivatives bearing multiple long alkyl chains. 4 For example, a Col h organization was also displayed by a porphyrin bearing four peripheral mesogenic bis-dodecyloxy aryl groups which are similar to those born by CorLC. 4b

(1) 𝑍 = (𝑑. 𝑁 . 𝑉. 10 ) ÷ 𝑀𝑀 = 0.94~1

Aggregation properties of Cor1, Cor2 and CorLC.

Copper meso-triarylcorroles are known to show hyper electronic spectra that are strongly sensitive to the nature of the meso-substituents that are involved in phenyl-to-Cu(𝑑 ) charge transfers and contribute to the main Soret band peak. 18 This orbital interaction is due to and increases with the saddling of the macrocycle which is reinforced by the introduction of bulky -aryl groups. 16,19 This effect is here illustrated by the UV/Visible spectra of diluted dichloromethane solutions of Cor1, Cor2 and CorLC (Fig. 2). The per-aryl substitution produces the broadening of the Soret band which is red-shifted from 433 nm in the spectrum of Cor1 to 463 nm and 464 nm in the respective electronic spectra of Cor2 and CorLC. On the other side, the replacement of the methyl groups of Cor2 by benzyl substituents in CorLC does not induce any significant change in the location of the B and Q bands and on the shape of the UV/visible spectrum. This similarity is due to the equivalent electron-donating properties of these two groups and shows that the concomitant introduction of 11 additional aryl groups and 21 peripheral alkyl chains does not produce any supplementary distortion of the aromatic corrole ring.

Thin solid films of Cor1, Cor2 and CorLC were prepared by the slow evaporation of their respective dichloromethane solutions. The formation of aggregates is illustrated by the broadening of the resulting electronic spectra that gives, in addition, insights of the relationship between the corroles substitutions and their supramolecular organizations (Fig. 2). aggregates. This supramolecular architecture is commonly observed in the single crystal X-ray structures of copper mesotriarylcorrole derivatives where corrole assemblies are formed thanks to, among others, an intermolecular interaction between the meso-aryl group of one complex with the metallic centre of the adjacent one producing short inter-macrocycle distances. 20 On the other side, the per-aryl substitution of Cor2 and CorLC induces the formation of H-aggregates as evidenced by the ca 20 nm blue-shift of their Soret band maxima (Fig. 2). The peripheral sterical hindrance of Cor2 combined with the multiple intermolecular interactions between alkyl chains in the LC phase CorLC, inhibits the direct participation of the copper ion in the supramolecular organization and thereby allows the formation of a different intermolecular packing. The formation of H-assemblies along the column axis is in accordance with the Col h organization of the CorLC mesophase (see below).

Hole-transport properties of Cor1 and Cor2.

To further study the physico-chemical properties of Cor1, Cor2 and assemblies, their charge-transport properties were investigated through their incorporation in OFETs.

Bottom-gate thin-film transistors with bottom electrode contacts were fabricated to realize so-called BGBC (Bottom-Gate Bottom-Contact) devices. Linear channel lengths (L = 25 or 50 µm) were realized by vacuum evaporation of gold through a shadow mask directly on the gate. To complete the device structure, the depositions of the active layers were achieved by drop-casting of dichloromethane solutions (or liquid CorLC) onto the surface of linear BGBC devices. All measurements were performed in air and at room temperature.

Even if numerous OFETs were prepared for each derivative, only the two best results obtained among the operating transistors are presented herein.

A field effect transistor activity was observed for Cor1 and Cor2-based assemblies by applying a negative drain and negative gate voltages in order to operate in the accumulation mode. These results demonstrate that Cor1 and Cor2 behave as p-type semiconductors in air as shown by the typical output characteristics of a Cor1-based OFET on Si/SiO 2 substrate under negative and positive voltages (Fig. 3a and Fig. 3b). Under negative gate voltages, the successive increase of the drain current indicates that the BGBC device based on Cor1 operates as an accumulation mode p-channel device (Fig. 3a). The transport of positive carriers is confirmed by the depletion mode under positive voltages (Fig. 3b). In the depletion mode, carriers are driven away from the semiconductor interface, which leads to a decrease of the drain current. The field effect mobility (μ), threshold voltage (V T ) and on/off ratio (I on /I off ) of the most efficient OFETs incorporating Cor1 and Cor2 are summarized in Table 1 (All results are presented in Table S1, ESI †). The highest field effect mobility in the saturation regime at V D = -90 V was extracted for Cor1 and gives a value of 1.3610 -6 cm 2 /V.s -1 together with a threshold voltage V T of +14.8 V and an on/off ratio of 24.5. The increasing sterical hindrance of the peripheral substitution of Cor2 probably produces a lower mobility of 1.3210 -7 cm 2 /V.s -1 together with a threshold voltage V T of +38.5 V and an on/off ratio of 10. All devices incorporating CorLC failed to give charge-transport properties. Identical results were obtained when an hydrophobic substrate Si/SiO 2 modified by a grafted self-assembly monolayer of long alkyl chains was used to induce the homeotropic alignment of the CorLC discotic molecules. 21 Although thin films were cast as such, or with an annealing step, these failed to improve the microstructural order for achieving more efficient charge transport properties in Cor1, Cor2 and CorLC thin films. However, these functional transistors are the first examples of OFETs using corrole derivatives as active layers and the variation of their low efficiencies are still illustrating the impact of the peripheral substitution on the corroles supramolecular organization.

Conclusions

We described herein the synthesis and full characterization of the first example of a corrole-based liquid crystal mesophase (CorLC). This mesophase features, at room temperature, an hexagonal columnar organization combined with an Harrangement of the discotic chromophores producing a blue shifted light absorption. This study shows that the introduction of numerous peripheral groups on copper corroles tunes their aggregation modes and their hole-transport properties. Since the copper ion can be removed using mild experimental conditions, 22 CorLC can be used to prepare liquid crystal phases incorporating other metallic centers such as Co(III) 11a and Mn(III) 14a that are already applied in the fields of sensors.

(-I D ) (A) V D (V) V G (V) -100 -60 -10 (-I D ) (A) V D (V) V G (V) -10 10 60 0 a) b)

Experimental section

General Remarks

All reagents were purchased from Acros, Alfa Aesar or Aldrich and were used without further purification unless otherwise stated. Toluene was distilled from sodium and benzophenone. Dichloromethane, hexane and nheptane were distilled from calcium hydride. Dichloromethane stabilized by methanol was used during the preparation of OFETs. Silica gel plates Merck 60 F254 were used for Thin Layer Chromatography (TLC). Silica gel 60 (230-400 mesh) was used for preparative column chromatography. Flash column chromatography was performed on silica gel 60 (230-400 mesh). (3,5-bis(dodecyloxy)phenyl)methanol, 23 , Cor1 15 and Cor2 16a were prepared and purified according to literature procedure. UV/Vis absorption spectra were measured with a Shimadzu UV-2401 (PC) instrument or with a Varian Cary 1E spectrophotometer.

Spectroscopic and analytical measurements

Pneumatically-assisted electrospray (ESI-MS) mass spectra were recorded from 10-4 M solutions on an Applied Biosystems API 150EX LC/MS System equipped with a Turbo Ionspray source. High Resolution Mass Spectrometry (HRMS-ESI) analyses were performed on a QStar Elite (Applied Biosystems SCIEX) spectrometer or on a SYNAPT G2 HDMS (Waters) spectrometer. These two instruments are equipped with an electrospray ionization source and were used within the positive ion mode and with a capillary voltage set at +5500 V. MALDI-TOFF mass spectra were obtained on a Voyager Instrument from Applied Biosystems either with anhydrous glycerol, HABA [2'-(4-hydroxyphenylazo)benzoic acid], or 1,8,9-anthracenetriol (dithranol) matrices.

Liquid crystals analyses.

TGA was performed with a Seiko TG/DTA 320 thermogravimetric balance (under N2). DSC traces were recorded with a Mettler Toledo DSC1 Star Systems differential scanning calorimeter from 3 to 5 mg samples (5 °C min -1 , under N2). Characterization of the mesophases was performed with a Leitz OrthoplanPol polarizing microscope with a Leitz LL 20x/0.40 polarizing objective and equipped with a Linkam THMS 600 variable temperature stage. The SA-XRD patterns were obtained with three different experimental setups, and in all cases, the crude powder was filled in Lindemann capillaries of 1 mm diameter. (1) A STOE STADI P transmission powder diffractometer system using a focused monochromatic Cu-Kα1 beam obtained from a curved germanium monochromator (Johann-type) and collected on a curved image-plate position sensitive detector. A calibration with silicon and copper laurate standards, for high-and low-angle domains, respectively, was preliminarily performed. Sample capillaries were placed in the high-temperature attachment for measurements in the range of desired temperatures (from 20 to 160 °C), for which the sample temperature was controlled within 0.05 °C. The exposure times were varied from 1 to 6 h, depending on the specific reflections being sought (weaker reflections obviously taking longer exposure times).

OFET preparations and studies

Cor1, Cor2 and CorLC were dissolved in CH2Cl2 to a concentration of ca 5.10 -3 M. The Bottom-Gate Bottom-Contact (BGBC) configuration was used for the OTFT devices. Highly n-doped silicon wafers (gate), covered with thermally grown silicon oxide SiO2 (3000 Å, insulator layer), were from Vegatec (France) and used as bottom-gate substrates. The capacitance per unit area of silicon dioxide dielectric layers was 1.210 -8 F/cm 2 . The gold (Au) source and drain electrodes (channel length L = 25 or 50 m, channel width W = 1 mm) were evaporated directly on SiO2 layer prior the organic deposition through a shadow mask. Solutions of Cor1, Cor2 and CorLC (or pure liquid CorLC) were deposited by dropcasting to form the organic active layers in OTFT devices through a dropwise addition of 20-40 µL volumes onto the devices surfaces. Such volume covered entirely the channel of one OTFT and beyond. After the organic solution deposition, the devices were left overnight in ambient condition in order to remove residual solvent prior electrical measurements. A post-annealing step from 50 to 80°C was applied to all devices in order to increase the microstructural order of thin films. Currentvoltage characteristics were recorded at room temperature under ambient conditions with Hewlett-Packard 4140B, or 4145A, pico-amperemeter-DC voltage sources. The source-drain current (ID) in the saturation regime is governed by the equation ( 2):

(

2) (ID)sat = (W/2L) Ciµ(VG-Vt) 2
Ci is the capacitance per unit area of the gate insulator layer, VG is the gate voltage, Vt is the threshold voltage, and µ is the field-effect mobility. The on/off ratio values were determined from the I-V transfer plots under a constant drain-source voltage VD = -100 V.

Organic syntheses.

Compound 1 : (3,5-bis(dodecyloxy)phenyl)methanol (5.00 g, 10.5•mmol) was dissolved in dry CH2Cl2 (50 mL) and DMF (50 l) was added under a nitrogen atmosphere. A solution of SOCl2 (1.87 g, 15.7•mmol) in dry CH2Cl2 (50 mL) was added dropwise over a period of 1 hour. The reaction mixture was stirred at room temperature for 24 hours, evaporated to dryness and dried under vacuum to provide 1 as a white solid (5.15 g, 10.4 mmol, 99%). 1H NMR (400 MHz, CDCl3): δ=0.91 (t, 3 J(H, H) = 6.6 Hz, 6H), 1.29-1.50 (m, 36H); 1.75-1.80 (m, 4H), 3.95 (t, 3 J(H, H) = 6.5 Hz, 6H), 4.52 (s, 2H), 6.42 (t, 4 J(H, H) = 2.5 Hz, 1H), 6.53 (d, 4 J(H, H) = 2.5 Hz, 2H). ESI-MS (CH2Cl2/MeOH 9:1): m/z: (M+.): 495.5; calcd for C31H56ClO2 + : 495.4.

Cu III undeca-(4-hydroxyphenyl)corrole: A 1M solution of BBr3 in CH2Cl2 (10 mL, 10 mmol) was added to a solution of Cor2 (45 mg, 29.6 mol) in CH2Cl2 (6 mL) which was stirred at 0 °C for 15 min. The mixture was kept at 0 °C for 1 h and then stirred at room temperature for 3 days before CH2Cl2 (20 mL) and CH3OH (10 mL) were added together with an aqueous saturated solution of NaHCO3 (35 mL). The crude compound was precipitated by pouring the reaction mixture into water (100 mL). The precipitates were filtered and purified by chromatography on silica gel (CH2Cl2/CH3OH, 5:1) to afford 3 as a brown powder (18 mg, 13.1 mol, 44%). Rf = 0.11 (CH2Cl2/CH3OH, 4:1). [START_REF]): 6417.5 (Maximum iso); calcd for[END_REF] H NMR (400 MHz, THF-d8, 25 °C): = 7.85 (s, 2H, Ar-OH), 7.83 (s, 3H, Ar-OH), 7.63 (s, 2H, Ar-OH), 7.53 (s, 2H, Ar-OH), 7.47 (s, 2H, Ar-OH), 6.94 (d, 3 J(H, H) = 8.4 Hz, 2H, Ar-H), 6.76 (d, 3 J(H, H) = 7.9 Hz, 4H, Ar-H), 6.66 (d, 3 J(H, H) = 8.4 Hz, 4H, Ar-H), 6.40 (d, 3 J(H, H) = 7.6 Hz, 8H, Ar-H), 6.40 (d, 3 J(H, H) = 8.0 Hz, 4H, Ar-H), 6.27 (d, 3 J(H, H) = 8.5 Hz, 4H, Ar-H), 6.09 (m, 8H, Ar-H), 6.03 (d, 3 J(H, H) = 8.6 Hz, 4H, Ar-H), 5.96 (d, 3 J(H, H) = 7.5 Hz, 4H, Ar-H), 5.93 ppm (d, 3 J(H, H) = 8.5 Hz, 2H, Ar-H). UV-Vis (THF): λmax (ε х 10 -3 )= 468 (83.6), 550 (18.7), 643 nm (12.2). MALDI-TOF (M +. ): 1370.2; calcd for C85H55CuN4O11 + : 1370.3.

CorLC : Cu III undeca-(4-hydroxyphenyl)corrole (7.2 μmol, 10 mg), compound 1 (22 equiv., 79.2 μmol, 39 mg) and K2CO3 (44 equiv., 158.4 μmol, 22 mg) were added to a round-bottomed flask which was oven dried overnight at 111 °C. The flask was evacuated and backfilled with argon three consecutive times. Dry DMF (5 mL) was then charged by a syringe. The mixture was subsequently stirred at 90 °C for 24 h. After being cooled to room temperature, the solvent was evaporated under reduced pressure. The crude product was purified by chromatography on silica gel

Fig. 1

 1 Fig. 1 (a) Optical texture observed by the POM of CorLC in the Col h mesophase at 90 °C after cooling from the isotropic liquid. (b) DSC trace of CorLC recorded at 5 °C/min (second heating and cooling). (c) SA-XRD profile in the liquid crystalline phase and associated indexation of CorLC at 30 °C. Structure of the CorLC mesophase. Temperature-dependent SA-XRD patterns of CorLC were recorded in the 20-140 °C temperature range. The presence of a large and diffuse signal at approximately 4.5 Å (h ch ), which was associated with the liquid-like molten chains confirmed the fluid-like nature of the mesophase. Short acquisition times (i.e.

Fig. 2

 2 Fig. 2 Normalized UV-Visible absorption of diluted dichloromethane solutions (dashed lines) and thin solid films (solid lines) of Cor1, Cor2 and CorLC.

Fig. 3

 3 Fig. 3 Output characteristics of an OFET incorporating Cor1 under a) negative and b) positive voltages.

1 H

 1 Nuclear Magnetic (NMR) spectra were recorded on a Bruker Avance 400 Ultrashield NMR spectrometer. Chemical shifts are given in ppm relative to residual peaks of chloroform ( = 7.26 ppm), DMSO-d6 ( = 2.50 ppm) or THF-d8 ( = 1.73 and 3.58 ppm).

Table 1 :

 1 Field effect mobilities (μ), threshold voltages (VT) and on/off ratio (Ion/Ioff) of corrole-based OTFT devices.

		µ (cm 2 .V -1 .s -1 )	VT (V)	Ion/Ioff
	Cor1	1,3610 -6	+14,8	24,5
	Cor2	1,3210 -7	+38,5	10

Acknowledgements

Dr. Laure Guénée is acknowledged for SA-XRD measurements. Dr. Di Gao gratefully thanks the China Scholarship Council for granting him his PHD fellowship in Marseille.

Notes and references


Jad Nassar

Karen Miranda

Nicolas Gouvy

Nathalie Mitton

Prédiction de données différenciée pour les Smart Grids †

Keywords: Smart Grids, WSN, Time Series Estimation, Least Mean Square, Qualité de Service

La transformation des réseaux électriques existants en Smart Grids (SGs) ambitionne d'en faciliter l'automatisation pour une meilleure qualité de service tout en y facilitant l'intégration de sources d'énergies renouvelables. Cette évolution vers un réseau électrique plus intelligent nécessite de pouvoir transmettre en temps réel un maximum de données sur l'usage du réseau. Un réseau de capteurs sans fil (WSN) disséminés à travers le réseau électrique est une solution prometteuse vu les coûts réduits et la facilité du déploiement de tels réseaux. Ces avantages se heurtent avec les liens radios instables et les ressources limitées des WSNs. Afin de réduire la quantité de données envoyée sur le réseau, et donc de réduire la consommation énergétique, la prédiction des données est une solution efficace. Cette dernière consiste en une estimation des valeurs mesurées permettant de ne pas envoyer les données brutes lorsque l'estimation s'avère correcte. Ce papier présente un travail en cours qui consiste à utiliser les time series estimation avec l'algorithme Least Mean Square pour la prédiction des données dans un WSN appliqué aux SGs, tout en considérant les différents types de données et trafics des SGs. Les premiers résultats de simulation numérique présentent une meilleure prédiction de données tout en minimisant l'erreur quadratique moyenne en se comparant avec une solution de l'état de l'art.

Introduction

Wireless sensor networks controlling and exchanging messages is a promising solution in the shift from the traditional electric grid into a "smarter" one in which the integration of renewable energy resources, energy reduction and auto-adaptation are the main benefits. This is done by equipping the electric grid with wireless sensors located at strategic measuring points to achieve remote monitoring, data collection and control of the grid. In a SG, electricity and energy do exist, but connecting sensors to such high voltage with intermittent and ill-adapted energy levels is sometimes inappropriate. For that, battery-powered sensors must be deployed all over the grid alongside with the main-powered ones. Thus, reporting data measurements at specific intervals has a direct effect on the sensors battery lifetime since the communication task consumes most of their available energy. In such contexts of continuous data reporting, where data changes are limited, will cause redundant information at the destination. To mitigate these energy loss, data reduction strategies aim at reducing the amount of data sent by each sensor by predicting the measured values both at the source and the sink node using specific algorithms, which will require sending the predicted information only if it is shifted from the sensed one by a certain threshold. We focus on a time series forecasting technique, called Least Mean Squares (LMS), which is an adaptive algorithm with very low computational overhead and memory consumption, that despite its simplicity, provides satisfactory performances in terms of computational speed, robustness and precision [START_REF] Hassibi | H-infinity optimality of the lms algorithm[END_REF]. In this paper, we propose a modification for the LMS filter adaptation used for prediction in WSN which was introduced in [START_REF] Santini | An adaptive strategy for quality-based data reduction in wireless sensor networks[END_REF], in order to adapt it to the different data types. We applied the algorithm to photo-voltaic cells monitoring data set. This is done by tuning the parameters of the filter by training it for a certain time with the real data values of every data set and choosing the values that minimizes the Mean Square Error (MSE).

A sensor node generates a data stream u[n] which consists of the previous N readings, which will form the input of the filter. The predicted values at the output of the filter y[n] = w T [n]u[n], is a linear combination of the previous N samples of the data stream weighted by a weight vector w[n] ; where w(n) = [w 1 , w 2 , ..., w M ] T and u(n) = [u(n-1), u(n-2), ..., u(n-M)] T . We note that both w and u are of length M, where M is an integer corresponding to the memory of the filter (how many previous samples it will use). The error between the output and the desired signal d[n] that the filter tries to adapt to is computed by : e

[n] = y[n] -d[n].
This error is given as an input for the adaptation algorithm, which will update the weight coefficients at the next instant n + 1 by : w

[n + 1] = w[n] + µu[n]e[n]
, where µ is the step size parameter. The weight vector is modified at each step in order to minimize the MSE. With a simple modification for the LMS filter structure, the LMS algorithm can be used for prediction by delaying the input signal by one step, using it as a the reference desired signal d[n]. The filter computes the estimated value û[n] of the input signal at time instance n, as a linear combination of the M previous readings. The step-size µ and the filter length M are two important parameters that need to be defined in order to ensure the convergence and robustness of the algorithm. The former will tune the convergence of the algorithm and the latter impacts directly on the computational load and memory consumption by considering more or less samples. A detailed explanation of the LMS filter can be found in [START_REF] Haykin | Least-mean-square adaptive filters[END_REF]. The implementation of the LMS algorithm for data prediction in WSN is first presented in [START_REF] Santini | An adaptive strategy for quality-based data reduction in wireless sensor networks[END_REF]. Here, identical filters are introduced at both the source and the sink referred as LMS-DPS (dual prediction scheme). The algorithm consists of three modes of operation : Initialization, normal and stand-alone mode. In the initialization mode the data samples are collected and reported to the sink without prediction. In this phase the step-size µ must be determined. Both the node and the sink compute the value of µ. It must satisfy the following condition [START_REF] Butterweck | A steady-state analysis of the lms adaptive algorithm without use of the independence assumption[END_REF] : 0 ≤ µ ≤ 2 λ max ; where λ max is the greatest eigenvalue of the auto-correlation matrix R. In normal mode, both the sink and the node use the last M samples to compute the prediction for the upcoming measurement, and update the filter coefficients. When the error drops below e max for M consecutive iterations, the node switches to stand-alone mode. We note that the default start values for the filter weights are assumed to be zero. In the stand alone mode, the node still collects data and makes predictions, but as long as the error is below e max , the filter is fed with the prediction y[n] instead of the reading value u[n], and the sink receiving no reading from the node assumes that the predicted readings are below the error threshold. If the error exceeds e max , the filter switches back to normal mode and reports the readings.

Related Work

In literature, we may find extensive work on time series forecasting techniques for WSNs [START_REF] Miranda | Using efficiently autoregressive estimation in wireless sensor networks[END_REF] [BBJK17]. In [START_REF] Santini | An adaptive strategy for quality-based data reduction in wireless sensor networks[END_REF], an implementation of LMS algorithm for prediction in WSN is presented. The LMS algorithm uses a dual prediction scheme by running the instance of the filter on both the sink and the node. In [START_REF] Stojkoska | Data prediction in wsn using variable step size lms algorithm[END_REF], a variable step-size is proposed to improve the initial adaptation to the data by switching to a new step-size stable value after µ has sufficiently learned what kind of data the filter receives. Many other works have addressed the variable step-size of LMS [START_REF] Bismor | Review and comparison of variable step-size lms algorithms[END_REF]. However, these proposals mostly require many adjustments of several parameters in order to optimize µ which is not suitable for a WSN with limited computation capabilities.

Problem Statement and Proposed Solution

LMS adaptive algorithm is proved to be robust and accurate with a very low computation [START_REF] Hassibi | H-infinity optimality of the lms algorithm[END_REF]. However, the choice of the step-size and the filter length are essential in the convergence of the algorithm. Starting with a large step size gives a fast convergence of the filter but results in a larger MSE, and a too small step size degrades the capabilities of the algorithm. Varying the step size to a smaller value after a certain number of iterations is then beneficial. Concerning the filter length, the choice will indicate the computation load of the algorithm (how many samples we will consider on every iteration). We note that increasing the filter length does not necessarily improve the performance of the filter. Choosing the right parameters is then crucial. Many propositions to adapt and adjust these variables were proposed in literature, but having a direct mathematical analysis of the stability and steady-state performance is a very complicated task in LMS [START_REF] Haykin | Least-mean-square adaptive filters[END_REF]. These adaptations may seem adequate for one application and kind of data-set, but less efficient to other ones. Our contribution, that we will denote by LMS_MOD (for modified) for the rest of the paper, consists of adding another step to the initialization phase for the LMS prediction algorithm in [START_REF] Santini | An adaptive strategy for quality-based data reduction in wireless sensor networks[END_REF] by training the filter with enough data. We vary the step-size and filter length in order to optimize these values for every specific application. We start by the upper bound of µ ( 2 λ max ). In order to minimize the MSE, we compute : the optimal time to switch to a smaller value, the new value of µ, and the appropriate filter length M. After that, we run our prediction algorithm with these parameters for the rest of the data. Concerning the energy load resulting from this adaptation, we run our adaptation script offload using numerical simulations in order to obtain the coefficients before running it on a WSN.

Simulation Setup

In order to validate our proposition, we use real value traces from the solar technology acceleration center ‡ for voltaic cells. We considered the irradiance, current, average wind speed and air temperature between 01/07/2018 and 31/08/2018 with data collected every minute. A description of the traces characteristics is presented in Table 1. It is worth to mention that each data type has different characteristics and range which will make the prediction task more challenging. We considered a fixed threshold for each data type (can be adjusted for specific needs). We considered a one hop communication environment with one source and one destination and no loss in order to prove its efficiency in an optimal case scenario as a first step. We test our algorithm by means of numeric simulation on Matlab. For the adaptation in the initialization phase we execute a Matlab script for one day of collected data chosen randomly. We varied three parameters i, j and k corresponding to the filter length, the factor by which we will divide the old µ and after how many iterations simultaneously (the time we will switch to the new computed µ value). We vary i between 1 and 10, and j, k between 1 and 100 with a step of 5, and we chose the value that minimizes the MSE. We note that the choice of these intervals can be changed, but we realized after several tests that the optimal values fall within these ranges. The obtained values are shown in Table 1 are then fed for the filter in order to predict the data for the whole previously mentioned duration. We compare LMS_MOD to LMS_VSS proposed in [START_REF] Stojkoska | Data prediction in wsn using variable step size lms algorithm[END_REF]. LMS_VSS respects the prediction phases as in [START_REF] Santini | An adaptive strategy for quality-based data reduction in wireless sensor networks[END_REF] (Initialization, normal and stand-alone) but with a variable step-size like our proposition. µ starts with the value of µ old =2λ -1 max .10 -2 , and switches to a stable value µ new = µ old /M after n iterations, where M is the filter length and n is the number of consecutive readings in stand-alone mode. They chose n = M 3/2 . We note that we chose the same filter length used for LMS_MOD for every data set. Figure 1b shows the Root Mean Square Error (RMSE) which corresponds to the root of the MSE that is given by : MSE

= 1 n ∑ i=1 n (y[n] -u[n]
) 2 for LMS_MOD and LMS_VSS. We observe that our proposal reduced the RMSE for the current, average wind speed and air temperature. The only value where LMS_VSS performs better is for the irradiance data set and this is probably due to the high deviation of the collected data as we observe from the high value of the standard deviation, 350.07, in Table 1. For that, training the filter for one day to estimate the parameters may not be enough to minimize the RMSE. On the other hand, as we see in Figure 1a, the percentage of data reduction in stand-alone mode where the error is below the given threshold) for the same data set for LMS_MOD is higher than the percentage for LMS_VSS. Same for the other data sets, LMS_MOD performs better than LMS_VSS.

Conclusion and Future Work

In this paper, we presented a work in progress that consists of a modification of the LMS prediction algorithm for WSN in order to adapt it to different applications with different characteristics, by training the filter for one day traces in order to optimize the parameters that minimize the MSE. We tested our approach with real value traces for photo-voltaic cells, and performed simulations considering one hop communication networks. Our first numerical results shows a better performance than LMS_VSS in terms of RMSE and percentage of data economy. As future work, we will continue investigating our approach with more tests on different data sets and considering more metrics. Later on, we will implement our algorithm on a sensor platform § , to evaluate its performance in a real scenario with interference and losses.

† This work was partially funded by a grant from the MEL (Métropole Européenne de Lille), the SoMel SoConnected project (Ademe, PIA2), Yncréa Haut-de-France, Collège Doctoral Lille Nord de France and CPER Data. Part of this work was performed when Jad Nassar was a visiting student at the Metropolitan Autonomous University, Mexico.