Clara Therville
email: clara.therville@cefe.cnrs.fr

Thomas Mangenet

Christelle Hinnewinkel

Sylvie Guillerme

Hubert De Foresta

Is truffle-growing a response to sustainable development and heritage issues in Mediterranean territories? The case of Uzès, southern France

Keywords: Tuber melanosporum, natural heritage, cultural heritage, biodiversity, rural areas, landscape

The northern part of the Mediterranean Basin recently experienced major transformations.

Landscapes evolved along two contrasting trends: agricultural intensification and urbanization in the plains, and disuse of marginal areas. This led to a gradual disappearance of the natural and cultural heritages associated with Mediterranean scrublands. To revitalize rural areas, promote sustainable development, preserve social-ecological heritages and cultural landscapes, decision-makers in Mediterranean France often support "traditional iconic activities". Based on a case-study in Uzès (Languedoc-Roussillon Région, southern France), this paper attempts to characterize the potential of truffle (Tuber melanosporum Vitt.) production, a symbolic but agriculturally marginal production, to respond to decision-makers expectations. The multidisciplinary approach developed includes landscape analysis, social surveys and plant inventories. Beyond the truffle growers and local decision-makers discourses and strategies, this paper shows that truffle-growing has limited implications in maintaining farmers and ensuring a sustainable development of the study area. Trufflegrowing reflects Uzès' society transformation, now more influenced by recent urban needs and values than by rural aspirations and traditions. However, our study brings in favorable arguments regarding the roles of truffle-growing ecosystems for biodiversity and landscape heterogeneity conservation.

Introduction

Truffles (Tuber spp.) are hypogeous (below-ground) fungi, generally restricted to calcareous soils, where they form ectomycorrhizal associations with a number of tree species, particularly with oak trees (Quercus spp.). The fruiting bodies of some Tuber species, actively collected for their special taste, are among the most expensive non-timber forest products [START_REF] Bonet | Cultivation methods of the black truffle, the most profitable mediterranean non-wood forest product; a state of the art review[END_REF]. In particular T. melanosporum Vittad., known as the "black diamond", the "Périgord truffle", or the "winter black truffle" (hereafter simply called "truffle"), has a considerable commercial value and can reach up to 1,500 US$/kg1 . T. melanosporum is naturally distributed in the northern part of the Mediterranean Basin, where it is nowadays mostly harvested from "truffle orchards", tree plantations2 managed for the production of truffle [START_REF] Callot | La truffe, la terre, la vie[END_REF].

In France, truffle orchards are reported since the mid-19th century both in the south-west (Périgord), and in the south-east (mainly Vaucluse and Drôme). These two regions still harbor the main truffle markets and are still the main truffle producing areas in France [START_REF] Callot | La truffe, la terre, la vie[END_REF].

However, the country production suffered a dramatic drop, from about 1,000 tons/year in the early 20th century to about 50 to 100 tons/year in the 1950's, and to between 10 and 20 tons/year since the 1970's [START_REF] Callot | La truffe, la terre, la vie[END_REF]. The latter drop in truffle production has been observed in the three main producing countries (France, Italy and Spain) and is generally attributed to climatic changes [START_REF] Callot | La truffe, la terre, la vie[END_REF]. The initial drop, from 1900's to 1950's, is generally attributed to the mortality associated to the two world wars and to the rural exodus, which have led to considerable changes in both rural landscapes and rural societies during the first half of the 20th century in western Europe [START_REF] Callot | La truffe, la terre, la vie[END_REF].

Mediterranean landscapes, shaped by an age-old history of human presence, were until recently characterized by high levels of complexity and diversity, and by an important biological and aesthetical interest [START_REF] Farina | The cultural landscape as a model for the integration of ecology and economics[END_REF][START_REF] Myers | Biodiversity hotspots for conservation priorities[END_REF][START_REF] Pasquale | Forest history in the Mediterranean region[END_REF]).

During the second half of the 20th century, the evolution of socio-economic conditions in the northern part of the Mediterranean Basin completed the drastic conversion of traditional rural landscapes. As everywhere in western Europe, the most favorable and fertile areas were reserved to, and strongly impacted by, agricultural intensification, while marginal lands were neglected and abandoned [START_REF] Debussche | Mediterranean landscape changes: evidence from old postcards[END_REF][START_REF] Aranzabal | Modelling of landscapes changes derived from the dynamics of socio-ecological systems. A case of study in a semiarid Mediterranean landscape[END_REF][START_REF] Hill | Mediterranean desertification and land degradation: mapping related land use change syndromes based on satellite observations[END_REF].

These transformations of the Mediterranean landscapes led to decreasing patchiness and to an important biodiversity erosion, directly related to the gradual disappearance of low forestcover habitats [START_REF] Etienne | Abandoned lands and land use conflicts in Southern France[END_REF][START_REF] Debussche | Mediterranean landscape changes: evidence from old postcards[END_REF]) -which were the habitats of T.

melanosporum.

This evolution also led to changes in the values, practices and perceptions [START_REF] Antrop | Why landscape of the past are important for the future[END_REF].

With the urban sprawl, new functions were assigned to rural areas, from recreation in a healthy environment to biodiversity preservation and the maintenance of cultural and identity values. These roles were reinforced by the integration of the 'sustainable development' concept in public policies [START_REF] Austad | The future of traditional agriculture landscapes: retaining desirable qualities[END_REF]. In parallel, territories marginalized by the economic Cedrus atlantica (Manetti ex Endl.) Carrière. The production of fruiting bodies in T. melanosporum. being associated to a relatively open environment, truffle orchards should be maintained with a low canopy cover [START_REF] Callot | La truffe, la terre, la vie[END_REF]. globalization appropriated the 'heritage' concept3 , using it to promote their local specificities and improve their cohesion [START_REF] François | Territoire et patrimoine: la co-construction d'une dynamique et de ses ressources [Territory and heritage: the co-construction of one dynamic and its resources[END_REF][START_REF] Méo | Processus de patrimonialisation et construction des territoires [Heritage and territories construction process[END_REF].

Marginal rural areas have to cope with the recent social-ecological transformations and integrate the principles of multi-functionality and sustainability [START_REF] Vos | Trends in European cultural landscape development: perspectives for a sustainable future[END_REF].

Decision-makers are requested to preserve cultural landscapes that are no longer maintained by traditional uses, to integrate biodiversity conservation issues in land-use planning, to promote the recognition of local natural and cultural heritage, and to encourage local development mainly through tourism and agriculture. One of the most popular pathways to integrate these issues is to support local iconic agricultural activities such as truffle-growing.

According to the national professional union of truffle growers -Fédération Française des Trufficulteurs-, truffle-growing in France has a sizeable economic impact in marginal areas [START_REF] Escafre | Rapport relatif au développement de la trufficulture française[END_REF]. It contributes to retain the rural population through the set-up of agro-tourism activities related to truffle and truffle-growing. It allows farmers to use marginal lands that cannot be cultivated for other crops and todiversify their agricultural activities and sources of income. Truffle-growing is also often currently featured as a factor of social cohesion, local culture upholding and as a fire-preventing tool in Mediterranean areas [START_REF] Escafre | Rapport relatif au développement de la trufficulture française[END_REF]. Those arguments, equally highlighted by truffle growers at local scale, have also captured the attention of local politicians, as they respond to their concerns about the maintenance and revitalization of neglected rural areas (Région Languedoc-

Roussillon 2007).

What reality underpins the highlighting in such discourses of the heritage character and of the social-ecological benefits of truffle-growing? This question is addressed here through a case study undertaken in the city of Uzès (Languedoc-Roussillon Région, Gard Département, southern France) and in its surroundings. Although Uzès is located outside of the two main historical regions of French truffle production (Périgord and Vaucluse-Drôme), it was selected because (i) truffle-growing is an obviously important land-use clearly visible in the Uzès landscape for any initiated visitor, (ii) there is a very active local professional union of truffle growers, -Syndicat des Producteurs de Truffes du Gard-hereafter called the "Syndicat", and (iii) Uzès is often portrayed as the capital of truffle-growing in the Languedoc-Roussillon Région. In addition, the Syndicat recently echoed back the sustainable development and heritage discourses by adding to its range of activities the promotion of truffle-growing as a natural and cultural heritage and as a tool to promote an harmonious economic, social and environmental development of the territory (Aumeeruddy-Thomas et al.

2012).

In this paper, we focus on truffle-growing as a potential natural and cultural heritage that could play a role in both social and ecological local issues: biodiversity conservation, local development, land-use planning and identity construction. Our specific objectives were (i) to assess the level of appropriation of truffle and truffle-growing as a local socio-cultural heritage, and (ii) to assess plant biodiversity in truffle-growing ecosystems as an indicator of their potential as natural heritage.

Material and methods

Study site

The study area included three adjacent municipalities: Uzès (northern part), Montaren et St Médiers, and St Quentin la Poterie (see Figure 1). The landscape is characterized by a fertile plain surrounded by hills covered with "garrigues", a Mediterranean scrubland with a more or less open tree canopy. Garrigues are the result of a long history of coupled socialecological processes that converted Mediterranean forests into open savanna-like treeformations [START_REF] Quézel | French. Région Languedoc-Roussillon 2007. La Région Languedoc-Roussillon soutient le développement de la trufficulture [The Languedoc-Roussillon Region supports trufflegrowing development[END_REF]. Black truffles were traditionally, and are still occasionally, gathered in the garrigues surrounding Uzès [START_REF] Prat | État des lieux de la trufficulture gardoise et perspectives d'avenir [Situation of truffle-growing in the Gard Département and prospects for the future[END_REF]. The first truffle orchards, inspired by modern arboriculture practices, appeared in the 1950's, more than one century later than in the two main truffle producing regions of France. The Syndicat was established in 1971 and has since been active in disseminating technical information on truffle-growing and in looking for funds to support the plantation of truffle orchards. It also supported the development of truffle silviculture and the conversion of closed canopy garrigues into truffle-growing systems, known as "truffle-renovation systems" [START_REF] Diette | La sylviculture truffière: Aperçus historiques, apports techniques et enjeux pour la région méditerranéenne [Truffle silviculture : historical overview, technical input and issues in the Mediterranean region][END_REF]).

[insert Fig1 here]

Methods

We used a multidisciplinary approach (ecology and geography) including landscape analysis, semi-directive interviews, closed questionnaires and plant inventories.

The first phase of the study consisted in reconstructing the transformations of the landscape that accompanied the social-ecological transformations of the Uzès region, to check that the land-use changes in the study area matched the dynamics of emergence of a dichotomous landscape as described in the introduction. We analyzed a temporal series of aerial photographs (1946, 1954, 1963, 1970, 1996 and 2001) and satellite images (2009).

Field observations allowed the identification of all the land-uses found on the most recent image, which in turn allowed mapping the various land-uses making up the current landscape.

The evolution of the social context that came with the landscape changes was then assessed through interviews with truffle growers (see below).

In a second phase, we conducted semi-directive interviews with twenty five (25) truffle growers and resource stakeholders (mainly truffle professional organizations officials), and we analyzed their socio-economic profiles and their expectations. This allowed us to describe the evolution over time of the main truffle growers' social characteristics. Data collected were also used to build a typology of truffle growers in the study area (using a Multiple Correspondence Analysis -MCA, and a Hierarchical Clustering Analysis).

The third phase of the study consisted in the assessment of truffle and truffle ecosystem as elements of the Uzès region's cultural identity. A questionnaire with closed questions was prepared and tested before being implemented on a sample of eighty one (81) people, randomly encountered in the study area. The aim of this questionnaire was to understand how people consider the contribution of truffle-growing ecosystems and truffle to the local natural and cultural heritage. Interviews were conducted between May and July 2009, in the streets of Uzès and Saint-Quentin, mostly during the weekly markets.

Our research hypothesis was that the perception of truffle and truffle ecosystem varies according to the historical link of the interviewee with the region. The sample thus included three (3) equally represented categories:

-local people, born and grown in the study area, -recent immigrants, established in the area for less than 15 years, -tourists.

[insert Fig2 here]

Finally, to assess the importance of truffle-growing for the plant biodiversity of the Uzès region, we carried out plant inventories in truffle ecosystems and in the various other landuses. We identified sixty seven (67) representative plots distributed over the whole study area. We used a plant inventory protocol based on the point-centered-quarter method [START_REF] Cottam | The use of distance measures in phytosociological sampling[END_REF]; see Figure 2), chosen because it allowed a fast characterization of plant diversity at plot scale.

Results of the plant census were first analyzed by a Non-Symmetric Correspondence Analysis (NSCA; [START_REF] Kroonenberg | Nonsymmetric correspondence analysis: a tool for analysing contingency tables with a dependence structure[END_REF], which gives more weight to the most abundant species. We then explored the variation of specific composition across the various land-uses using a Between Class Analysis which allows quantifying the proportion of the NSCA inertia explained by a qualitative instrumental variable (in our case: the various landuses) [START_REF] Lebreton | Principal component and correspondence analyses with respect to instrumental variables: an overview of their role in studies of structure-activity and species-environment relationships[END_REF]. Finally, we compared the observed biodiversity of the various land-uses by using three diversity indices (species richness, Shannon index and Gini-Simpson index). All statistical analysis were made using the R free software (http://www.rproject.org/) and the ade4 package (http://cran.r-project.org/web/packages/ade4/index.html).

Results

From agro-sylvo-pasture to urbanization

At the beginning of the 20 th century, the livelihood of most inhabitants in the Uzès region was based on a traditional agro-sylvo-pastoral model [START_REF] Chabaud | L'Uzège et la région bagnolaise[END_REF]. The plain was the domain of cereal crop fields, vegetable gardens, orchards, and vineyards. The garrigues were mainly used as grazing ground for sheep and goats flocks, and for collecting fodder for the animals in winter. They were also used for firewood and charcoal production [START_REF] Prat | État des lieux de la trufficulture gardoise et perspectives d'avenir [Situation of truffle-growing in the Gard Département and prospects for the future[END_REF]. The semi-open environment associated to these garrigues, coupled with the Uzès sandy and shallow soils, rich in limestone, provided optimal conditions for the development of the winter black truffle [START_REF] Callot | La truffe, la terre, la vie[END_REF]. Truffles provided the mainly rural population a significant supplementary income in winter [START_REF] Byé | Production, protection et profession truffières [Truffle production, protection and profession[END_REF][START_REF] Callot | La truffe, la terre, la vie[END_REF].

As in other marginal Mediterranean areas during the second half of the 20 th century, the rural society in Uzès was affected by important changes (Figure 1). In the 1970's, tourism gradually became a prominent economic activity; in parallel, a new population, mainly composed of wealthy retired persons originating from outside the region, settled in the city and in the neighboring rural areas. This immigration process led to marked changes in the landscape, with the extension of urban areas and the disappearance of many agricultural plots, replaced by settlements. In the meantime, agriculture modernized and the contrast became more and more obvious between the plain, characterized by agricultural intensification and by urbanization, and the drier surrounding hills, gradually abandoned to garrigue. Traditional uses of the garrigues gradually vanished. Marginal fields were abandoned to fallow and replaced by garrigue, which itself gradually evolved from an open scrubland to a small forest with a closed canopy cover, incompatible with truffle production.

The landscape changed as did the identity of the region, from a declining agricultural region to a prized tourist landmark area. These changes had an impact on truffle-growing, on truffle growers' socio-economic profiles, on the areas dedicated to truffle, and on the place of truffle in the making of the region's identity.

The advent of the "rurban" truffle grower

Based on our qualitative and quantitative data (Table 1), the Multiple Correspondence Analysis and the Hierarchical Clustering Analysis show that truffle growers are distributed into 4 distinct groups in our sample (Figure 3).

[insert Tab1 here] [insert Fig 3 here]

Through the analysis of these 4 groups, it is possible to retrace the evolution of the truffle growers' realm in Uzès, as well as the way it adapted to the changes of the local natural and cultural environment. Confronted with the closing of the canopy in the garrigues and the resulting truffle production drop-off, some passionate people established the first truffle plantations in the 1970's (group 1, 8 persons). Mostly native from the Uzès region, they all own inherited lands that they tried to valorize. Their passion for truffle is most often a family inheritance and they have been actively involved in the Syndicat since its creation. Most of them (2/3) are farmers. They currently own the largest plantation areas (between 4 and 20 ha, generally distributed into a few plots), and their truffle production provides them with a complementary income which can be significant in favorable years4 .

The second group (group 2, 7 persons) is mostly composed of farmers who planted truffle orchards on the fringe of their fertile lands, where soil was less fertile and could not sustain cereal cropping. The wine producing crisis deterring them to invest and plant grapevine, they chose to plant truffle orchards. They most often own few truffle plantations, with a total area ranging between 1 and 4 ha. Their membership to the Syndicat is sometimes motivated only by the possibility to get financial support. Most of them have planted in anticipation of their retirement, expecting truffle to provide them a leisure activity and a small additional income.

Young farmers investing in truffle orchards are rare5 , as they usually cannot afford to use their lands for a so uncertain production with such a long unproductive period (10 to 15 years in the best cases).

The two previous categories have strong rural and farming roots. This is not the case with the third group (group 3, 6 persons) composed of « rurbans », wealthy people either working in the tertiary sector of the economy or retired. They own inherited or purchased properties they like to improve through olive trees and truffle orchards. Some of them, native of the region, started truffle-growing after discussion with their relatives. Others, recently settled, have been seduced by different facets of truffle-growing, such as the connection with nature, or the link to perceived local traditions and heritages. They have, often recently, planted small areas -less than 1 ha -close to their home, and when they already harvest, they keep truffles for their own consumption. Only half of them received financial support for planting, and they mostly perceive the Syndicat as a source of information. For people in this « rurban » category, truffle is first of all a leisure activity and a way to show off their attachment to what they perceive as local identity and cultural heritage.

The last group (group 4, 4 persons) is composed of working people whose main economic activity is directly related to truffle (catering, selling local products, truffle trading, truffle tree seedling production). They all own small plantations for their pleasure, but the bulk of their income comes from truffle products and services, in relation with the enthusiasm for truffle in the Uzès region and with the intense tourist activity in the city.

To summarize, almost two third of the truffle growers in our sample are retired (60%), mostly native from the Gard Département (64%) where Uzès is located. Forty-four (44%) percent of them are or have been farmers, the others either work or have worked in the tertiary sector. None of them makes a living only from truffle production. For the majority, truffle provides a relatively small income complement (84%), or is only a leisure activity.

Truffle as a cultural heritage?

As exposed earlier, the Syndicat highlights truffle-growing as an integral part of the local cultural identity. The truffle growers interviewed confirmed this perception: for them, truffle is representative of Uzès heritage. However, the results of our survey among randomly encountered people lead to moderate the importance of truffle and truffle related activities in making the local cultural identity.

[insert Fig4 here]

We first dealt with the place of truffle-growing in the landscape (Figure 4A and4B). Our results show that 96% of the natives, 78% of the recently settled, but only 26% of the tourists, know about truffle tree plantations in the Uzès region. Among tourists, some even ignore that "truffle trees" can be cultivated. The survey thus reveals a straight distinction between the Uzès region inhabitants, familiar with truffle-growing ecosystems, and the tourists who, for most of them, have at best a very vague idea of what a truffle production landscape could look like. Figure 4B shows the ranking of the various land-use components of the local landscape, in terms of their representativeness of the cultural identity of the Uzès region. For the three respondent categories in our sample, truffle plantation ranks fourth, behind garrigue, vineyard, and olive orchards. In addition (data not shown), the reputation of Uzès and its touristic attractiveness are attributed to its architecture, its mild and sunny climate, its landscapes, and its proximity to famous tourist spots such as the "Pont du Gard".

We then tried to understand how truffle itself is perceived. Among a list comprising 14 terms, respondents had first to choose 3 terms that they associate with truffle, and then to rank the 3 terms : 3 points were attributed to the term having the closest link to truffle, 2 points for the second and 1 point for the third. Figure 4C shows that the perception of truffle is relatively homogenous among the three respondent categories, with the three most cited terms being « luxury product », « mushroom », and « gastronomy ». At the opposite, the terms « heritage », « family tradition », or « pleasure of harvesting » have very few citations, whatever the respondent category.

To summarize, it seems that, contrary to the situation in famous truffle-growing areas such as Périgord, truffle represents a marginal component in the cultural identity of Uzès, although collecting wild truffles in the garrigues and, more recently, cultivating oak trees for truffle production have let obvious traces in the history and in the landscapes of the region. Truffle is not -yet? -a specificity commonly associated with Uzès reputation, as are wine and olives.

Truffle-growing and biodiversity

Floristic specificities of truffle-growing ecosystems

In the last facet of this study, we assessed the ecological heritage value of truffle-growing at the landscape scale, through an analysis of the contribution of truffle-growing areas to the plant biodiversity of the Uzès region. We encountered a total of 372 species across the 67 sample plots, representative of the eight major land-uses encountered in the study area (cereal crop, fallow, garrigue, meadow, olive orchard, road edge, truffle orchard, and vineyard). 60 species could not be identified, the individual plants being either too young or damaged.

[insert Fig5 here]

The two first component axis resume almost 65% of the NSCA inertia (Figure 5A). It appears that, in spite of the numerous species encountered across the different plots, the global vegetation is mainly dominated by about 15 abundant species and that there are The Between Class Analysis shows that the different land-uses have significantly different species compositions (Figure 5B). Cereal crops (Cc) were excluded from the analysis because they were highly dominated by distinctive cultivated cereals and too poor in terms of total species richness (see Table 2). Vineyards (V) were also excluded because they had a too

[insert Table 2 here]

This specificity in the floristic composition of truffle-growing ecosystems goes hand in hand with the highest species richness of our samples (Table 2). On average, 39 to 47 species per inventory (thus for 250 sampled individual plants) have been recorded in the trufflegrowing plots, which is quite remarkable when compared to the average species richness of the other major land-uses, such as cereal crop (8 species), vineyard (24 species), and garrigue (24 species). Similar trends are observed for the other diversity indexes (Table 2). In all cases, cereal crop appears as the poorest ecosystem, followed by vineyard and garrigue. Trufflegrowing areas, be they plantations or renovations, systematically have the highest biodiversity values, although differences with some other land-uses are rarely significant. With their intermediate level of canopy closure and their intermediate level of management -and thus of disturbance -truffle-growing areas represent, in the agriculture realm, an interesting example of the "intermediate disturbance hypothesis", which postulates that the highest species diversity occurs for intermediate disturbance regimes [START_REF] Blondel | The "design" of Mediterranean landscapes: a millennial story of humans and ecological systems during the historic period[END_REF].

Truffle-growing ecosystems and the landscape scale [insert Fig6 here]

The above results take an additional significance when they are interpreted at the landscape scale (Figure 6). The plain itself is a fertile area, mainly covered by urban areas, cereal crops and vineyards. Surrounding hills are mainly covered by a closed canopy garrigue.

These two different landscapes both harbor a number of -generally small -areas of trufflegrowing ecosystems. What do these truffle-growing "spots" represent in terms of sustainability and biodiversity of each landscape?

In the closed canopy garrigue, the presence of a truffle-growing ecosystem allows the maintenance of an open environment with trees, which in turn allows enrichment in species, plants and animals, commonly associated with slightly disturbed open environments. To this biodiversity enrichment, and to the economic valorization of lands that otherwise would be abandoned, should be added another important service: the presence, in such fire prone areas, of a relatively large open area reachable by car or truck, more or less regularly maintained and often equipped with irrigation facility, is of utmost importance for fire prevention and control.

In the plain and its matrix of intensive crops marked by a strong urban pressure, trufflegrowing ecosystems bring in species commonly associated with slightly disturbed or undisturbed environments, both open and closed -species that are lacking in the other landuses. They also allow the establishment and the maintenance of open woods which can serve as habitat for birds and other animals.

All the above is true as far as the truffle-growing plantations and renovations do not bear any chemical treatment, are maintained through simple mowing down and a very superficial tillage. Then, and in both landscapes, the truffle-growing ecosystems confer heterogeneity to otherwise very uniform landscapes, making up "islands" that differ from the landscape matrix by their richness in species and by the specificity of their species composition.

Discussion

Synergism of truffle growers adaptation strategies with local authorities development strategies

Rural areas in the northern part of the Mediterranean Basin have been confronted with major social-ecological changes during the 20th century. The traditional agro-sylvo-pastoral system collapsed after the second world war, with the traditional agricultural landscape turned into either intensive agriculture or closed canopy systems [START_REF] Debussche | Mediterranean landscape changes: evidence from old postcards[END_REF][START_REF] Aranzabal | Modelling of landscapes changes derived from the dynamics of socio-ecological systems. A case of study in a semiarid Mediterranean landscape[END_REF]. In a context of globalization and competition between territories, the construction and adaptation of heritages is a widespread strategy to promote local specificities and identity, and to support sustainable development [START_REF] François | Territoire et patrimoine: la co-construction d'une dynamique et de ses ressources [Territory and heritage: the co-construction of one dynamic and its resources[END_REF][START_REF] Méo | Processus de patrimonialisation et construction des territoires [Heritage and territories construction process[END_REF]. In particular, economically unsustainable societies based on traditional agriculture could find a resurgence through agro-tourism, quality labels and the preservation of their cultural landscapes and natural heritages [START_REF] Bessière | Local development and heritage: traditional food and cuisine as tourist attractions in rural areas[END_REF][START_REF] Frayssignes | Démarches de qualité et développement territorial: quels apports pour la géographie rurale? L'exemple des AOC fromagères françaises [Quality approach and territorial development: what contributions for the rural geography? The example of the French cheese AOC[END_REF]. Reyna-Domenech and Garcia-Barreda ( 2008), and Aumeeruddy-Thomas et al. ( 2012) describe how Mediterranean rural forests and their associated production (such as truffle) faced a major crisis but currently contribute to revive the territories they fall within.

In Uzès, the promotion of truffle-growing is one of the answers of the local and regional decision-makers concerning their territories' challenges. The regional government actively supports this "typical traditional culture of Languedoc-Roussillon" which "responds to the double challenge of economic development and land-use planning", combines "culture and gastronomy", allows to "bring back to life neglected and fallow lands", and "restore biodiversity […] while limiting fire propagation" (Région Languedoc-Roussillon 2007).

This political discourse was inspired by the network of professional truffle growers.

Several authors have underlined the serious difficulties encountered by the truffle sector since the 1950's: fall in production, abuses connected to the opacity of the markets, competition with the China truffle6 , recently introduced on the European markets [START_REF] Byé | Production, protection et profession truffières [Truffle production, protection and profession[END_REF][START_REF] Callot | La truffe, la terre, la vie[END_REF][START_REF] Escafre | Rapport relatif au développement de la trufficulture française[END_REF]. Encouraged by the national professional union, many efforts have been made to respond to this crisis, such as improving the openness of the sector and developing sustainable development and heritage related arguments for a new positioning of truffle-growing [START_REF] Escafre | Rapport relatif au développement de la trufficulture française[END_REF]. The mobilization of these central issues allowed truffle growers to be acknowledged for their good practices and their participation in public programs. The truffle growers' professional network in turn identified these opportunities to increase its visibility: visibility towards public authorities, to obtain the allocation of funds for new truffle plantations and for reviving production; visibility as professionals, fighting against theft and fraudulent utilization of China truffle. This strategy was especially effective in the Languedoc-Roussillon Région, where truffle-growing got significant financial supports (Région Languedoc-Roussillon 2007).

The alliance between truffle growers and public authorities highlights a mutual interest for collaboration. Whereas the former find a financial support to revive a declining activity, the later see truffle-growing as a potential solution to the environmental, social and economic issues facing neglected garrigue areas. Beyond political discourse and collective strategies, our study allows clarifying, in the case of the Uzès region, the roles of truffle-growing in economic development and agricultural activity maintaining, cultural heritage construction, natural heritage and cultural landscape preservation.

Truffle-growing in Uzès: a social-ecological heritage for a sustainable development?

Truffles were initially a non-timber forest product collected from the wild in the garrigues by a rural population of farmers, for which truffles could represent a significant source of income in winter. The current situation is completely different, reflecting the transformation of both the landscape and the population in the Uzès region. In the abandoned garrigues the canopy gradually closed, preventing truffle production. With the urban sprawl, the development of tourism and the concentration of farming in a few hands, the population is now mainly urban or "rurban", with a high proportion of newcomers either retired or working mostly in the tertiary and leisure sectors. Truffles are now produced in specialized truffle orchards, owned and managed by truffle growers, whose profiles reflect this transformation of the society. Compared to the portrait of truffle growers in the Gard Département proposed by [START_REF] Dalonis | État de la trufficulture française en 1996 illustré au travers de l'espace de production gardois [Condition of French truffle-growing in 1996[END_REF]Prat in 2002, our study reveals rapid changes. The proportion of native truffle growers considerably declined, whereas the proportion of retired people increased.

Truffle-growing, initially adopted by local farmers whose lifetime had been marked by truffle, is more and more practiced by newcomers to truffle and to the area. While motivations of the former were essentially financial, today, almost all of the truffle growers consider trufflegrowing more as a leisure activity than as an economic activity. We cannot confirm that truffle-growing is an effective solution for maintaining agricultural activities. Truffle-growing can hardly be the main agricultural activity in a farm, because of the length of the preproductive period, and of the unpredictability of production. In contrast, truffle-growing fulfills the expectations of rurbans and retired people: for most, it is a leisure activity in connection with nature, while for newcomers to the region, it also represents a link with the perceived local traditions, heritage and identity. This situation however cannot be generalized: for instance, [START_REF] Samils | The Socioeconomic Impact of Truffle Cultivation in Rural Spain[END_REF] highlighted the economic importance for farmers of truffle cultivation in a non-tourist rural area in Spain. Truffle growers' claim, that truffle and trufflegrowing represent an important local cultural heritage for the Uzès region, is not supported by our general population survey. As social construction, heritage doesn't exist a priori: the construction process of a heritage requires its adoption by a social group, that identify itself through it [START_REF] Leniaud | L'utopie française. Essai sur le patrimoine [The French utopia[END_REF]). To characterize truffle, items like "heritage" and "tradition" were rarely chosen by local inhabitants, who mostly associated truffle with its luxury image. Until recently, truffle related activities were indeed very much discreet, with most of the production sold outside the Gard Département7 . The situation is very different in Périgord and Drôme-Vaucluse, where truffle orchards and truffle business are attested for more than 150 years. In Uzès, the Syndicat made great efforts to «create a culture of truffle». This quotation, extracted from one of our interview with a member of the Syndicat, clearly demonstrates their will to construct a cultural heritage around truffle, which may never have existed in Uzès. The construction of truffle and truffle-growing as a local heritage is well illustrated by the very popular "Journée de la Truffe d'Uzès" (i.e. the Uzes truffle day), organized by the Syndicat since 1994, which nowadays welcomes more than 15,000 visitors every year. Despite this annual event, truffle has not been adopted -yet -by the inhabitants as a component of their identity, and cannot be considered as a common heritage in Uzès.

The study of truffle-growing ecosystems has confirmed their importance for the maintenance of a high plant biodiversity at Uzès landscape scale. Truffle-growing ecosystems are indeed characterized by high species richness and by an original species composition,

Conclusion

To enhance their visibility and promote their activities, the network of professional truffle growers used arguments focusing on local development, natural and cultural heritage and sustainability of marginal territories. This discourse was taken up by decision-makers to respond to social-ecological issues related to the management of neglected garrigue areas. In Uzès, our study confirms the favorable role of truffle-growing ecosystems for biodiversity and landscape heterogeneity conservation. But it leads to question the potential of truffle-growing for local sustainable development. Although it allows the maintenance of savanna-like treeformations and can be used as a fire-preventing tool in the garrigues, truffle-growing can hardly be adopted as their main agricultural activity by farmers, and thus can hardly be promoted as a stand-alone solution to maintain farmers and ensure sustainable development in rural areas. Our study also leads to moderate the cultural heritage character of truffle-growing.

Although the winter tourism activity is boosted and the social link favored by the Uzès truffleday, truffle is not -yet?-part of the local cultural heritage Truffle-growing in Uzès reflects Uzès' society: it essentially responds to urban and "rurban" needs and values such as leisure, landscape quality and biodiversity preservation. Richness 7.6±2.5 a 24.3±7.3 b 24.5±4.3 abc 31.1±8.8 bcd 34.4±14.2 bcde 35.5±8.

  important variations in species composition across the different plots. The first axis explains 43.5% of the total inertia and discriminates plots characterized by a herbaceous and open canopy vegetation, dominated by Bromus diandrus Roth and Cynodon dactylon (L.) Pers., from plots characterized by a closed canopy garrigue vegetation dominated by species such as Rubia peregrina L., Quercus coccifera L. and Quercus ilex L. The second axis explains 21.1% of the inertia and discriminates plots dominated by Medicago minima (L.) Bartal. and Arenaria serpyllifolia L. from other plots.

  distinctive specific composition, largely dominated by three species (Solanum nigrum L., Lolium strictum C. Presl. and Tragus racemosus (L.) All.) that were almost lacking in the other land-uses. Across the remaining land-uses, garrigue plots (G) have a distinctive species composition, dominated by R. peregrina, Q. coccifera and Q. ilex. Plots of the other land-uses are distributed on a relatively regular vegetation transition. Truffle orchards (P) and truffle renovations (RA and RB) contain species that are characteristics of closed canopy vegetation similar to garrigue, whereas permanent meadows (M) or road edges (R) contain only species of open canopy vegetations. Unmaintained and maintained truffle renovations (RB and RA) and truffle orchards (P) are also characterized by a relative richness in M. minima, A. serpyllifolia and Vulpia ciliata Dumort., and a relative poorness in more typical garrigue species. Then, from olive orchards (O) to road edges, passing by permanent meadows (M) and fallow lands (F), the species composition changes with a decrease in M. minima, A. serpyllifolia and V. ciliata and an increase in the two Poaceae C. dactylon and B. diandrus , marking the transition towards a more herbaceous vegetation. Interestingly, the three types of truffle-growing ecosystems (RA, RB and P) are characterized by species commonly associated to both open and closed canopy environments.

  including species usually associated to closed canopy forests as well as species associated to open canopy forests. The numerous truffle orchards planted in the plain since the 1950's represent something new in the history of truffle production in the Uzès region, where truffle harvesting was until then restricted to the garrigues. A few truffle orchards and renovation plots have also been set-up in the garrigues, reviving their initial truffle production role, although under a very different format -private individual vs. community ownership and management. Whether located in the plain or in the garrigues, truffle-growing plots increase landscape heterogeneity, and represent spots of high biodiversity, in an otherwise intensive agriculture landscape in the plain, and in an otherwise very homogeneous closed canopy forest in the garrigues area.

Figure 1 :

 1 Figure 1: Land-use changes in Uzès between 1970 and 2001

Figure 3 :

 3 Figure 3: Results of the Multiple Correspondence Analysis. Each number corresponds to one truffle-grower respondent. The four groups of truffle-growers have been distinguished by a hierarchical clustering analysis.

Figure 4 :

 4 Figure 4: Results of the survey on the perceptions of truffle and truffle-growing in the Uzès region. A: knowledge about the presence of truffle plantations (Question: Do you know that there are truffle orchards in the Uzès region?). B: heritage value of the different land-uses types in the Uzès region (Question: in the following list, could you identify and rank the three land-uses that best represent the Uzès region in terms of local identity and heritage?). C: values associated to truffle (Question: in the following list, could you identify and rank the three terms that best represent truffle?).

Figure 6 :

 6 Figure 6: Distribution of plant species richness at the Uzès landscape scale in 2009. Average species richness per 250 plants estimated for each land-use on the basis of our 67 inventories (crops: 3; fallow: 7; garrigue: 6; vineyard: 13; road edge: 5; olive orchard: 8; truffle orchard: 8; meadow: 10, truffle renovation:7)

  .63±0.54 a 2.13±0.49 c 2.43±0.16 bc 2.57±0.41 bc 2.63±0.51 bcd 2.77±0.28 bd 2.81±0.34 bd 3.18±0.26 bd 2.93±0.15 bd 3.14±0.24 d PIE 0.30±0.32 a 0.79±0.11 c 0.86±0.02 bc 0.86±0.06 bc 0.88±0.05 bc 0.89±0.03 bc 0.89±0.07 bc 0.93±0.03 bc 0.90±0.04 bc 0.93±0.02 b

	1 cde 37.5±6 de	47.5±5.8 e	39±4 bcde	44.1±6.9 e
	H'

0

T. magnatum Pico, the Italian white truffle, fetches even higher prices (ca. 5,000 US$/kg), but it is has a much limited distribution area (Italy)

The main tree species used for truffle orchards are various Quercus spp. (especially Q. ilex L., Q. pubescens Willd., and Q. pedunculata Hoffm.), although other species are sometimes planted such as Corylus avelana L. or

We define 'heritage' here as a « complex and willing social construction » (Di Méo 2008), recognized for a number of more or less normative values, protected and managed in the interest of all.

We could not obtain detailed figures: profitability of truffle growing being a highly sensitive subject, our interviews did not include any related detailed question.

Only 2 among the 7 farmers in group 2.

Truffle species (T. himalayense B.C. Zhang and Minte, and T. indicum Cooke and Massee) produced in China, with a morphology very similar to T. melanosporum, but with very different organoleptic properties (Garcia-Montero et al. 2010).

Interestingly, there had never been any truffle cannery in Uzès (Tournayre M. personnal communication).

Acknowledgements:

We are thankful to the Agence Nationale de la Recherche (ANR) for funds provided through the POPULAR project (ANR-05-PADD-014, Programme Agriculture et Développement Durable). We thank for their help and time the Syndicat des Trufficulteurs du Gard, and especially its President, Mr. Michel Tournayre, the CRPF Languedoc-Roussillon, and all the persons who have given of their time, knowledge and passion for truffles.


Yohan Dall'agnese

Pierre-Louis Taberna

Yury Gogotsi

Patrice Simon

Two-Dimensional Vanadium Carbide (MXene) as Positive Electrode for Sodium-Ion Capacitors

Ion capacitors store energy through intercalation of cations into an electrode at a faster rate than in batteries and within a larger potential window. These devices reach a higher energy density compared to electrochemical double layer capacitor. Li-ion capacitors are already produced commercially, but the development of Na-ion capacitors is hindered by lack of materials that would allow fast intercalation of Na-ions. Here we investigated the electrochemical behavior of 2D vanadium carbide, V 2 C, from the MXene family. We investigated the mechanism of Na intercalation by XRD and achieved capacitance of ∼100 F/g at 0.2 mV/s. We assembled a full cell with hard carbon as negative electrode, a known anode material for Na ion batteries, and achieved capacity of 50 mAh/g with a maximum cell voltage of 3.5 V.

L ithium-ion capacitors (Li-IC) are new promising energy storage devices that bridge the gap between batteries and supercapacitors. [START_REF] Dubal | Hybrid Energy Storage: The Merging of Battery and Supercapacitor Chemistries[END_REF][START_REF] Armand | Building Better Batteries[END_REF][START_REF] Cericola | Hybridization of Rechargeable Batteries and Electrochemical Capacitors: Principles and Limits[END_REF] Because of their high energy and power densities, they are intended for use in a wide variety of applications, such as transportation (electric and hybrid cars), electronics (telephones and laptops), and storage of renewable energy. Ion capacitor materials combine the high energy density from the intercalation mechanism of batteries and the high power of supercapacitor. [START_REF] Naoi | Second Generation 'Nanohybrid Supercapacitor': Evolution of Capacitive Energy Storage Devices[END_REF][START_REF] Amatucci | An Asymmetric Hybrid Nonaqueous Energy Storage Cell[END_REF][START_REF] Du Pasquier | A Comparative Study of Li-Ion Battery, Supercapacitor and Nonaqueous Asymmetric Hybrid Devices for Automotive Applications[END_REF][START_REF] Khomenko | High-Energy Density Graphite/AC Capacitor in Organic Electrolyte[END_REF][START_REF] Naoi | New Materials and New Configurations for Advanced Electrochemical Capacitors[END_REF] JM Energy and JSR Micro have commercialized a graphite/activated carbon Li-IC; however, the limited supply of lithium and quickly widening use of energy storage devices justify replacement of lithium with cheap and abundant sodium. Sodium ion batteries emerged as an alternative to Li-ion batteries in specific applications, such as large-scale stationary storage, where lower cost can compensate for the lower energy density. [START_REF] Slater | Sodium-Ion Batteries[END_REF] Na-ion batteries use hard carbon (HC) as negative electrode with a capacity up to 320 mAh/g at C/10 when cycled in the 2 V window. [START_REF] Ponrouch | High Capacity Hard Carbon Anodes for Sodium Ion Batteries in Additive Free Electrolyte[END_REF] Similarly to Li-ion batteries, layered metal oxides, such as NaMnO 2 , NaCoO 2 , or V 2 O 5 , have been proposed as positive electrodes, with capacities of 160, 95, and 250 mAh/g, respectively. [START_REF] Aravindan | Insertion-Type Electrodes for Nonaqueous Li-Ion Capacitors[END_REF][START_REF] Zhang | An Aqueous Capacitor Battery Hybrid Device Based on Na-Ion Insertion-Deinsertion in λ-MnO 2 Positive Electrode[END_REF][START_REF] Kuratani | Na-Ion Capacitor Using Sodium Pre-Doped Hard Carbon and Activated Carbon[END_REF] Recently, sodium-ion capacitors (Na-IC), where the Li intercalation electrode is replaced with a low-cost Na ion electrode, [START_REF] Ding | An Investigation of Spinel NiCo 2 O 4 as Anode for Na-Ion Capacitors[END_REF] were demonstrated. Most of the ongoing work on these systems is focused on the development of negative electrodes, and several anodes have been proposed, such as carbon nanotubes, NiCo 2 O 4 , and sodium titanate nanotubes. [START_REF] Aravindan | Insertion-Type Electrodes for Nonaqueous Li-Ion Capacitors[END_REF][START_REF] Zhang | An Aqueous Capacitor Battery Hybrid Device Based on Na-Ion Insertion-Deinsertion in λ-MnO 2 Positive Electrode[END_REF][START_REF] Kuratani | Na-Ion Capacitor Using Sodium Pre-Doped Hard Carbon and Activated Carbon[END_REF][START_REF] Ding | An Investigation of Spinel NiCo 2 O 4 as Anode for Na-Ion Capacitors[END_REF] In 2012, Kuratani et al. investigated HC/activated carbon Na-IC and showed that HC could be used as negative electrode. Despite Chen et al. proposing V 2 O 5 , all other studies included activated carbons as positive electrodes, [START_REF] Chen | High-Performance Sodium-Ion Pseudocapacitors Based on Hierarchically Porous Nanowire Composites[END_REF][START_REF] Yin | Sodium Titanate Nanotubes as Negative Electrode Materials for Sodium-Ion Capacitors[END_REF] and little was done to develop alternative cathodes.

We report the electrochemical characterization of a new positive electrode material, namely, V 2 C, that belongs to a large family of 2D transition metal carbides called MXenes. [START_REF] Naguib | New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries[END_REF] MXenes are synthesized by selective etching of the A layer from MAX phases. [START_REF] Naguib | Synthesis of Two-Dimensional Materials by Selective Extraction[END_REF] Since discovery of the first MXene (Ti 3 C 2 ) in 2011, [START_REF] Naguib | Two-Dimensional Nanocrystals Produced by Exfoliation of Ti 3 AlC 2[END_REF] more than 10 new members have been successfully synthesized and many more predicted. Ti 2 C, Ti 3 C 2 , Nb 2 C, and V 2 C have quickly drawn the attention as candidates for energy storage due to the possibility of spontaneous intercalation of a variety of cations between their layers. [START_REF] Naguib | New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries[END_REF][START_REF] Naguib | Synthesis of Two-Dimensional Materials by Selective Extraction[END_REF][START_REF] Naguib | Two-Dimensional Nanocrystals Produced by Exfoliation of Ti 3 AlC 2[END_REF][START_REF] Come | A Non-Aqueous Asymmetric Cell with a Ti 2 C-Based Two-Dimensional Negative Electrode[END_REF][START_REF] Hu | MXene: A New Family of Promising Hydrogen Storage Medium[END_REF][START_REF] Dall'agnese | High Capacitance of Surface-Modified 2D Titanium Carbide in Acidic Electrolyte[END_REF][START_REF] Yan | Enhanced Supercapacitive Performance of Delaminated Two-Dimensional Titanium Carbide/Carbon Nanotube Composites in Alkaline Electrolyte[END_REF][START_REF] Liang | Sulfur Cathodes Based on Conductive Mxene Nanosheets for High-Performance Lithium-Sulfur Batteries[END_REF][START_REF] Mashtalir | Intercalation and Delamination of Layered Carbides and Carbonitrides[END_REF][START_REF] Lukatskaya | Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide[END_REF][START_REF] Xie | Prediction and Characterization of Mxene Nanosheet Anodes for Non-Lithium-Ion Batteries[END_REF] For example, Ti 3 C 2 has been used in aqueous supercapacitors and as electrode material of Li-, Na-, and K-ion batteries. [START_REF] Mashtalir | Intercalation and Delamination of Layered Carbides and Carbonitrides[END_REF][START_REF] Lukatskaya | Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide[END_REF][START_REF] Xie | Prediction and Characterization of Mxene Nanosheet Anodes for Non-Lithium-Ion Batteries[END_REF] Two layers of Li or Na ions were predicted to intercalate between MXene layers, [START_REF] Xie | Prediction and Characterization of Mxene Nanosheet Anodes for Non-Lithium-Ion Batteries[END_REF] and the formation of a double-layer of Na has been experimentally shown. [START_REF] Wang | Pseudocapacitance of MXene Nanosheets for High-Power Sodium-Ion Hybrid Capacitors[END_REF] From density functional theory (DFT) predictions, [START_REF] Xie | Prediction and Characterization of Mxene Nanosheet Anodes for Non-Lithium-Ion Batteries[END_REF] V 2 C is one of the most promising electrode materials for Li-ion batteries, but experimental results show a very wide working potential window and a sloping charge-discharge; [START_REF] Naguib | New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries[END_REF][START_REF] Xie | Prediction and Characterization of Mxene Nanosheet Anodes for Non-Lithium-Ion Batteries[END_REF][START_REF] Ji | Different Charge-Storage Mechanisms in Disulfide Vanadium and Vanadium Carbide Monolayer[END_REF] however, these materials show a potential for assembling hybrid devices, and we previously proposed Ti 2 C for Li-ion capacitors. [START_REF] Come | A Non-Aqueous Asymmetric Cell with a Ti 2 C-Based Two-Dimensional Negative Electrode[END_REF] In early 2015, the first experimental investigation of Na-IC using MXenes was published. [START_REF] Wang | Pseudocapacitance of MXene Nanosheets for High-Power Sodium-Ion Hybrid Capacitors[END_REF] To create a Na-IC, we took into account the previous experimental and theoretical research on MXene and selected V 2 C as a promising material for Na-IC. In this work, we investigate for the first time the sodiation of V 2 C in a half cell. The energy-storage mechanism is studied by X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS). To assemble a full cell, we selected HC as negative electrode.

Figure 1 shows a schematic view of the synthesis and structure of V 2 C and its Na-intercalation mechanism during cycling. From previous studies, [START_REF] Naguib | New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries[END_REF][START_REF] Dall'agnese | High Capacitance of Surface-Modified 2D Titanium Carbide in Acidic Electrolyte[END_REF] it is known that MXenes synthesized using HF contain fluorinated and oxygenated surface functional groups, such as -OH, -O, and -F, [START_REF] Naguib | New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries[END_REF] and their presence was noted by adding "T x " to the chemical formula, V 2 CT x . Note that the effect of the surface chemistry has not been studied here.

Figure 2a shows CVs at different scan rates, while the change of the capacitance with the scan rate is described in Figure 2b. High capacitance of 100 F/g or 170 F/cm 3 was obtained at slow scanning, and 50 F/g was still measured at 50 mV/s, evidencing a good power performance of V 2 CT x for Na intercalation. At low scan rates, two different regions can be seen in the CV, corresponding to two different electrochemical processes. From 1 to 2.2 V, the rectangular shape of the CV describes pseudocapacitive behavior. A similar storage mechanism has been previously demonstrated in other MXenes. For example, Ti 3 C 2 cycled in aqueous electrolyte exhibits a rectangular-shaped CV attributed to redox reactions and intercalation. [START_REF] Lukatskaya | Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide[END_REF][START_REF] Lukatskaya | Probing the Mechanism of High Capacitance in 2D Titanium Carbide Using in Situ X-Ray Absorption Spectroscopy[END_REF] Redox peaks are identified at low scan rates, with an oxidation peak at 3 V (peak A) and a reduction peak at 2.5 V versus Na + /Na, (peak B). As the scan rate increases, the redox peaks tend to disappear, thus suggesting a diffusion limitation at scan rate beyond 2 mV/s. The large potential range and the absence of any 2-phase system plateau make V 2 C less suitable for sodium ion battery electrodes, but such features are attractive for sodium ion capacitors.

Characterization by EIS was made at different potentials (Figure 3a). The constant charge-transfer resistance, as well as the improvement in the capacitive region at low frequencies, between 1 to 2.5 V correlates well with a pseudocapacitive intercalation mechanism. The charge-transfer resistance (200 Ohm/cm 2 ) associated with the Na + pseudointercalation reaction explains the resistive behavior observed in the CV curves. The increase in the charge-transfer resistance and the semi-infinite diffusion limitation visible in the low-frequency region at 3.2 V is associated with the full desodiation of V 2 CTx, in agreement with the redox peaks observed in the CV.

Figure 3b shows ex situ XRD patterns of V 2 CT x recorded at different voltages, where it can be observed that the (002) peak shifts continuously and reversibly from 9 to 12°during cycling between 1 and 3.5 V versus Na + /Na. In this potential range, the change is perfectly reversible, thus demonstrating that there are no undesired side reactions. During sodiation, c-lattice parameter increases with the amount of Na + stored. This demonstrates that V 2 CT x stores energy through intercalation of Na ions in between layers in a similar way as that previously demonstrated for both intercalation of Li + into Ti 2 C 20 or Ti 3 C 2 [START_REF] Xie | Role of Surface Structure on Li-Ion Energy Storage Capacity of Two-Dimensional Transition-Metal Carbides[END_REF] and Na + into Ti 3 C 2 . [START_REF] Wang | Atomic-Scale Recognition of Surface Structure and Intercalation Mechanism of Ti 3 C 2 X[END_REF] There is no new phase appearing at 3.5 V versus Na + /Na, and thus the redox process identified by peaks A and B in the CV does not modify the crystallographic structure of the material. A 4.6 Å change in c-lattice parameter was observed, as calculated from Bragg's law. Taking into account the fact that there are two interlayer gaps in a lattice unit, there is a 2.3 Å expansion or shrinkage during sodiation and desodiation, respectively. This is a larger change than expected for a single layer of Na + ions, which indicates that a second layer of Na + could be intercalated, as shown for Ti 3 C 2 intercalated by Na + . [START_REF] Wang | Atomic-Scale Recognition of Surface Structure and Intercalation Mechanism of Ti 3 C 2 X[END_REF] The peak at 13°corresponds to V 2 AlC from incomplete synthesis reaction. This peak does not move during cycling, demonstrating that the MAX phase is not electrochemically active and that the capacity could be increased by increasing V 2 CT x yield. Nevertheless, the presence of this peak is useful as a reference for the other peaks.

Differently from Ti 2 C and Ti 3 C 2 , which can only be used as negative electrodes because of their operating potential window, V 2 CT x shows a potential window ranging from 1 to 3.5 V versus Na + /Na, being attractive as a positive electrode in Na sodium ion capacitors. V 2 CT x was cycled at current density from 30 mA/g to 1 A/g, corresponding to the rate from C/3 (3 h discharge) to 20C (3 min discharge), as shown in Figure 4b. The objective was to assemble a full cell using V 2 CT x as positive electrode and HC as negative electrode for sodium intercalation. Galvanostatic charge-discharge cycling of HC electrodes was done at the same C rate. The expected key features of a carbon intercalation electrode were observed, with an intercalation potential below 1 V versus Na + /Na and a capacity beyond 200 mAh/g at low rates (C/3, Figure 4b).

Figure 4b shows the cycling stability at different C-rates. In this example, the positive electrode/negative electrode mass ratio was 1:2, to keep each electrode potential in their working potential window. At low charge/discharge rates, the faradic efficiency decreases, thus leading to a capacity fade with cycling. At low rate (C/3), the capacity fading is more pronounced in the first 40 cycles. Good capacity up to 70 mAh/g was obtained, with remarkable stability at discharge rates beyond 3C. The performance achieved experimentally is lower than that predicted by first-principles simulation corresponding to the maximum theoretical capacity for a bare V 2 C monolayer (335 mAh/g). [START_REF] Xie | Prediction and Characterization of Mxene Nanosheet Anodes for Non-Lithium-Ion Batteries[END_REF] Experimentally, the capacity is limited by the presence of MAX phase residue, functionalized layers, and stacked layers. Thus, there is much room for further increase in capacitance of this material.

On the basis of half-cell results, hybrid Na-ion capacitor cells were assembled. By anticipating the capacity decrease in V 2 CT x during the first cycles, we calculated a HC/V 2 CT x weight ratio of 1:2. In such conditions, the overcapacitive HC electrode allows a better potential stability for the negative electrode. Before assembling a full cell, each electrode was pretreated as described in the Experimental Methods. The full cells were tested from C/3 to 20C rate. Figure 5 shows the electrochemical performance obtained in a full cell configuration. All gravimetric capacities are calculated based on the total weight of both positive and negative electrodes to focus on the performances of the device. Because the mass ratio of positive to negative electrode is 1:2, the equivalent capacities based on the mass of V 2 CT x are three times higher than those presented in Figure 5.

The charge/discharge galvanostatic profiles are presented in Figure 5a. During discharge, a sharp potential drop occurred from 3.5 V down to 2.6 V, followed by a small plateau at 2.5 V due to the redox reaction peaks observed in Figure 2a, so that the practical cell voltage was 2.6 V. Figure 5b shows that highpower performance could be achieved, with 40% of the total capacity obtained at 20C, despite the use of a Na-ion intercalation HC negative electrode. The capacity decrease during the first cycles at the low rate (C/3) is associated with a decrease in the Coulombic efficiency due to redox reaction beyond 3 V.

Figure 5c shows the cycle life of the full cell at a high rate (20C). After 300 cycles, the capacity retention was 70%. Interestingly, the capacity slightly increased during the first 70 cycles. Afterward, the capacity decrease was associated with the decrease in the faradic efficiency down to 98%. To better understand this decrease, both electrodes were studied using a three-electrode cell. Figure 5d shows the 2nd and 200th cycles. First, we can notice that the negative electrode potential range is smaller than 1 V because HC is in excess. As previously explained, a slightly overcapacitive negative electrode is necessary to ensure a good stability. Although the positive electrode has low irreversibility, it is sufficient to drive a shift of the negative electrode toward higher potentials after a large number of cycles. The potential of the V 2 CT x electrode goes slightly beyond the optimum operating potential range of 1 to 3.5 V, which leads to irreversible redox reactions, thus explaining the capacity decrease upon cycling. An optimization of the electrode mass ratio should prevent the observed drift of the positive electrode potential, leading to an improvement in cyclability.

It is important to note that all results shown here were obtained on samples containing residual MAX phase and therefore decreased the overall performances. Recently, another synthesis route of MXene, via a fluoride salt and HCl etching, was proposed, [START_REF] Ghidiu | Conductive Two-Dimensional Titanium Carbide / ̀Clay/' with High Volumetric Capacitance[END_REF] and samples produced by that method showed improved capacitance. It may offer material of higher purity, which will also show improved performance in Na-ion capacitors. Finally, with just two MXenes studied in Na-ion capacitors to date and already showing promise for use as both negative and positive electrodes, there is clear opportunity to create devices with both electrodes made of 2D carbides, but further studies of electrochemical behavior of those new materials are needed.

This work shows that at least one representative of the large family of MXenes, V 2 CT x , can serve as the positive electrode for a sodium ion capacitor. Investigation of the mechanism of sodiation and desodiation of V 2 CT x by XRD shows continuous intercalation of sodium ions between the V 2 CT x layers in a wide range of potentials. Electrochemical testing in half cells demonstrated that both capacitive (pseudocapacitive) and diffusion-limited redox processes take place. An asymmetric full cell was assembled using hard carbon, a known anode material for sodium ion battery. V 2 CT x /HC sodium-ion capacitor showed promising results, with a maximum cell voltage of 3.5 V and a capacity of 50 mAh/g.

■ EXPERIMENTAL METHODS

V 2 C was synthesized by selectively etching the aluminum layer out of the MAX phase V 2 AlC with a 50% concentrated HF solution for 8h. Hard carbon was synthesized by pyrolysis of sugar under argon flow, as described elsewhere. [START_REF] Ponrouch | High Capacity Hard Carbon Anodes for Sodium Ion Batteries in Additive Free Electrolyte[END_REF] XRD patterns of the V 2 CT x electrodes were collected using a Bruker D4 diffractometer using a Cu Kα radiation (λ = 1.5406 Å) in the range 2θ = 5-50°with a step of 0.016°. The samples were polarized and maintained at a given potential versus metallic sodium in a Swagelok cell prior to testing.

The electrodes were rolled and cut into ∼30 μm thick disks with average mass loading of 5 mg•cm -2 . V 2 CT x electrodes were prepared with 5 wt % polytetrafluoroethylene binder (60 wt % in H 2 O, Aldrich) and 15 wt % carbon black (Alfa Aesar) to secure a sufficient electronic conductivity. Hard carbon electrodes were prepared with 5 wt % polytetrafluoroethylene binder (60 wt % in H 2 O, Aldrich).

A two-electrode Swagelok cell was used for half-cells with metallic sodium as counter and reference electrode separated by a glass fiber separator (GFA). Full cells were assembled in three-electrode Swagelok cells using an Ag wire as pseudoreference electrode. The electrodes were presodiated by immersing them in the electrolyte and short-circuited with metallic sodium. The electrolyte used was 1 M NaPF 6 in EC/ DMC (1:1). The cells were assembled in an argon-filled glovebox.

A VMP3 potentiostat (Biologic SA, France) was used for electrochemical testing by cyclic voltammetry (CV), galvano- static charge-discharge, and electrochemical impedance spectroscopy (EIS). In full cells, capacities were calculated based on the total masses of negative and positive electrodes.

■ AUTHOR INFORMATION

Corresponding Author *E-mail: simon@chimie.ups-tlse.fr. Tel: +33(0)5.61.55.68.02.

Notes

The authors declare no competing financial interest.

  Wang et al. investigated Ti 2 C as negative electrode and alluaudite Na 2 Fe 2 (SO 4 ) 3 as positive electrode. It showed a good rate capability and high specific power of 1.4 kW/kg with specific energy of 260 Wh per kg of Ti 2 C. Although promising results were shown, previous sodium ion battery studies demonstrated that Ti 2 C is not the best MXene in terms of performance.

Figure 1 .

 1 Figure 1. Schematic illustration of the synthesis of V 2 CT x and its sodium intercalation.

Figure 2 .

 2 Figure 2. (a) Cyclic voltammetry of V 2 CT x at different scan rate and (b) summary of rate performance.

Figure 3 .

 3 Figure 3. (a) Nyquist plot from EIS. (b) XRD patterns at different potentials. (*) Peak of unreacted V 2 AlC.Figure 4. (a) Charge-discharge profiles of V 2 CT x (2 mg) and hard carbon (2 mg) and (b) cycle life from galvanostatic charge-discharge at different rates.

Figure 4 .

 4 Figure 3. (a) Nyquist plot from EIS. (b) XRD patterns at different potentials. (*) Peak of unreacted V 2 AlC.Figure 4. (a) Charge-discharge profiles of V 2 CT x (2 mg) and hard carbon (2 mg) and (b) cycle life from galvanostatic charge-discharge at different rates.

Figure 5 .

 5 Figure 5. (a) Charge-discharge profiles at various rates. (b) Change of the capacity during galvanostatic charge-discharge at different rates. (c) Capacity versus cycle number. (d) Details of the potential range of positive and negative electrodes at 1A/g. Capacities were calculated for the total mass of both positive and negative active material, taken in the weight ratio of 1:2.

■ ACKNOWLEDGMENTS We thank B. Anasori for help with V 2 AlC synthesis. This work was supported by the Partner University Fund (PUF) of French Embassy. Y.D.A. was supported by the European Research Council (ERC, Advanced Grant, ERC-2011-AdG, Project 291543 -IONACES). P.S. acknowledges funding from the Chair of Excellence of the Airbus group foundation "Embedded multi-functional materials". Y.G. was partially supported by the Competitive Research Grant from King Abdullah University for Science & Technology (KAUST).