Emma Gamba

Sébastien Aurelle

Clémentine Vergnaud

Brigitte Talon
email: brigitte.talon@imbe.fr

A Holocene history of forest vegetation on the northern slopes of a Mediterranean mountain: the Mont Ventoux, Southeast France

Keywords: palaeoecology, treeline, soil charcoal, altitudinal gradient, Abies alba, Fagus sylvatica, radiocarbon dating

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A Holocene history of forest vegetation on the northern slopes of a Mediterranean mountain: the Mont Ventoux, Southeast France

Histoire holocène de la végétation forestière du versant nord d'une montagne méditerranéenne : le mont Ventoux (Vaucluse, France) Emma GAMBA 1 , Sébastien AURELLE 2 , Clémentine VERGNAUD 3 , Brigitte TALON 1,* aujourd'hui absents, comme le sapin (Abies alba) qui s'élevait jusqu'au sommet et formait avec les pins la limite supérieure des arbres. Le hêtre (Fagus sylvatica) est bien représenté à basse altitude mais aucun charbon de cette essence n'a pu être trouvé au-delà de 1 340 m, alors que l'espèce atteint aujourd'hui 1 700 m d'altitude. Les datations montrent que les sols du versant nord du mont Ventoux ont conservé, malgré l'érosion des versants, plus de 4 000 ans d'histoire. La période couverte par les 6 datations 14 C AMS obtenues s'étend de l'âge du bronze à la période moderne , soit avant les reboisements de 1860 (RTM, restauration des terrains en montagne). Cette première étude pédoanthracologique se révèle novatrice et parfaitement complémentaire des études paléo environnementales préexistantes mais encore insuffisante pour reconstruire l'histoire de la végétation passée du mont Ventoux et anticiper son avenir.

Résumé

Cette étude vise à reconstituer la composition de la végétation ligneuse passée du versant nord du mont Ventoux en fonction du gradient altitudinal ainsi que l'altitude maximale atteinte par les arbres avant la déforestation des versants, qui fut intense à partir du xii e siècle, en lien avec des perturbations passées telles que le surpâturage et les feux de gestion sylvo-pastorale. Profitant de la mise en place d'un transect du suivi écologique ORCHAMP en 2019, six fosses pédologiques réparties depuis 925 m (hêtraie sapinière) jusqu'à 1 860 m d'altitude (pins à crochets prostrés) ont fait l'objet de prélèvements pédoanthracologiques (identification et datation des charbons de bois du sol). Toutes les fosses contenaient des charbons de bois. La richesse taxonomique diminue significativement à mesure que l'on s'élève le long du gradient altitudinal. Certains taxons ont été trouvés à des altitudes auxquelles ils sont and displayed more tree diversity in most of the European mountain systems (e.g. Pyrenees and the Alps) but lowered almost everywhere due to the agro-sylvo-pastoral pressure [START_REF] Cunill | Holocene high-altitude vegetation dynamics in the Pyrenees: a pedoanthracology contribution to an interdisciplinary approach[END_REF][START_REF] Finsinger | Early to late Holocene vegetation and fire dynamics at the treeline in the Maritime Alps[END_REF][START_REF] Carnelli | Past uppermost tree limit in the Central European Alps (Switzerland) based on soil and soil charcoal[END_REF][START_REF] Talon | Reconstruction of Holocene highaltitude vegetation cover in the French southern Alps: evidence from soil charcoal[END_REF]. This has particularly been demonstrated by soil charcoal analysis (pedoanthracology) in a number of sites (Carcaillet & Thinon 1996;[START_REF] Di Pasquale | The Holocene treeline in the northern Andes (Ecuador): first evidence from soil charcoal[END_REF][START_REF] Talon | Reconstruction of Holocene highaltitude vegetation cover in the French southern Alps: evidence from soil charcoal[END_REF]. Recently, the agro-pastoral abandonment and global warming have been inducing the upward movement of numerous woody taxa, which were restricted to lower elevations. [START_REF] Lenoir | La remontée de la distribution altitudinale des espèces végétales forestières tempérées en lien avec le réchauffement climatique récent[END_REF] have shown that in the case of French mountains, the altitudinal optimum of several forest trees, such as the common hornbeam (Carpinus betulus L.), or the common hazelnut (Corylus avellana L.) have elevated by approximately 30m per decade since 1980s. In the Spanish Pyrenees, the beech forest upper limit has moved 70m to higher elevations in only 50 years [START_REF] Labonne | Forêts de montagne et changement climatique : impacts et adaptations[END_REF]. A better understanding of this ecological phenomenon necessitates investigating the longterm dynamics (centennial to millennial) of the upper tree limit in response to climatic variations and human activities. The research presented here belongs to the ORCHAMP monitoring program (www.orchamp.osug. fr). This pluri-disciplinary research project is aimed at studying the links between climate change, land-use and biodiversity along altitudinal gradients in the Alpine territory. ORCHAMP sites are characterized by regularly monitored quadrats located at 200m altitudinal intervals and along an approximately 1,000m-long transect. Monitoring efforts are focused on the fauna (soil DNA), vegetation (flora, forest cover, decaying wood), climate (temperature and relative humidity), and soil (https://orchamp.osug.fr). The imprints of past human activities on the structure and dynamics of current vegetation of mountain forest ecosystems will be particularly investigated. More than 30 transects have already been established along N-S altitudinal gradients over Mont Blanc, Maritime Alps, and the Mediterranean massif of Mont Ventoux with alpine characters. The present study takes advantage of the ecological transect of ORCHAMP established by Institut méditerranéen de biodiversité et d'écologie in 2019 on the northern slopes of Mont Ventoux. In the absence of wetlands and lakes for pollen-based vegetation reconstructions, we explored the soil archives of

Abstract

This study aims at reconstructing the late Holocene vegetation dynamics and the maximum altitudinal limit of forest trees on northern slopes of Mont Ventoux before the intense deforestation of the 12th century, in relation to natural environmental changes and anthropogenic disturbances including fire history and sylvo-pastoral practices. Six soil pits were studied out along an altitudinal transect from 925m to 1,860m a.s.l. This transect crosses the beech-fir forests in the lower elevations to open woodlands containing the prostrate hooked pine (Pinus uncinata) near the summit. The study was performed within the framework of the ecological monitoring program ORCHAMP. Pedoanthracological data (identified charcoals and their 14 C ages) were further explored by numerical analyses. Charcoal was found in all pits with concentrations significantly decreasing with altitude, as does the proportion of charcoal of deciduous trees over conifers. Some woody species were found at altitudes where they are absent today, notably fir (Abies alba) at high elevations, (up to 1,860m). In contrast to fir, the beech (Fagus sylvatica) charcoals were not found beyond 1,340m, although the species currently reaches elevations as high as 1,700m a.s.l. Radiocarbon dating results suggest that the soils on northern slopes of Mont Ventoux have not been significantly eroded since at least 4,000 years ago. The period covered by the six obtained AMS 14

Introduction

During the last 10,000 years, climatic changes and human activities have deeply impacted and shaped the mountain landscapes on most of the Earth's surface. The long co-evolution of human societies and these landscapes can partly explain their current diversity and fragmentation [START_REF] Swenson | The effects of future urban development on habitat fragmentation in the Santa Monica Mountains[END_REF] and particularly the altitudinal and taxonomic variations of the treeline, one of the most important ecotones in mountain environments [START_REF] Treml | Twentieth century treeline ecotone advance in the Sudetes Mountains (Central Europe) was induced by agricultural land abandonment rather than climate change[END_REF]. From a given elevation upward, bioclimatic conditions and particularly the regional temperatures become too constraining for the optimal growth and establishment of tree populations [START_REF] Holtmeier | Ecological aspects of climatically-caused timberline fluctuations. Review and outlook[END_REF][START_REF] Körner | plant life[END_REF][START_REF] Holtmeier | Sensitivity and response of northern hemisphere altitudinal and polar treelines to environmental change at landscape and local scales[END_REF]. However, it has been shown that the treeline was higher

Forest vegetation of Mont Ventoux: past and present

Today, a mosaic of forest composed of different coniferous and deciduous species occupies the two slopes of Mont Ventoux, showing a vertical zonation mainly controlled by the altitudinal temperature gradient. The history of the vegetation of Mont Ventoux is very well documented for the Modern and Contemporary Era thanks to available historical archives, notably those of the forestry services. Before the 13th century, forest exploitation was practiced on the northern slopes while the southern flanks were cultivated and grazed. The Great Plague (1347-1348 CE) and the Hundred Years' War (1337-1453 CE) caused demographic decline and agro-pastoral abandonment that favored forest recolonization. Forest clearing was resumed from the 16th century with the increase in population and the growing need for wood (timber, firewood, charcoal) and agro-pastoral lands [START_REF] Martin-Gousset | Habitats et dynamiques de végétation en ubac du mont Ventoux -Étude de la Réserve biologique intégrale[END_REF]. The deforestation of the mountain continued until the 19th century. The consequences of the ancient deforestation, dating to as early as the Bronze Age, on the current distribution of forest vegetation on the northern slope of Mont Ventoux have been described by L. [START_REF] Martin-Gousset | Habitats et dynamiques de végétation en ubac du mont Ventoux -Étude de la Réserve biologique intégrale[END_REF] for the Mont Ventoux Biological Reserve (Réserve biologique intégrale du mont Ventoux -RBI-MV) officially created in 2010 by the French National Forestry Office (ONF) [START_REF] Terracol | La Réserve biologique intégrale du mont Ventoux, un espace d'étude des écosystèmes forestiers hors sylviculture[END_REF]. The RBI-MV (906ha), situated slightly northeast, covers an altitudinal gradient of 1200 m, from supra-Mediterranean to subalpine bioclimatic belts (680-1880 m). The current forest stands are largely the result of the mountain land reforestation program termed RTM (Restauration des Terrains de Montagne) in the 19th to early 20th centuries. Indeed, in the 1860s, the RTM service undertook the reforestation of Mont Ventoux with various coniferous and deciduous species to fight against soil erosion [START_REF] Ningre | Le reboisement du Ventoux au xix e siècle : techniques et travaux[END_REF]. Thanks to a vertical vegetation zonation scheme within the RBI-MV located to the east of our transect and elegantly drawn and described by C.F. Martins (Martins 1838, in [START_REF] Martin-Gousset | Habitats et dynamiques de végétation en ubac du mont Ventoux -Étude de la Réserve biologique intégrale[END_REF], it is possible to get an idea about the state of the forest vegetation of the northern slope before the RTM reforestations. On this sketch, while the supra-Mediterranean zone seems to be occupied by lavender and thyme heaths the experimental pedological pits and apply pedoanthracology to investigate the history of vegetation in relation to human activities. The main objectives of this study are to reconstruct (i) the composition of woody taxa in forest vegetation of Mont Ventoux and (ii) the altitudinal position of the upper limit of forest (treeline) on the northern slopes in relation to past human disturbances, such as deliberate landscape burning and tree cutting for sylvopastoral management purposes. We will specifically focus on two forest species, European silver fir (Abies alba Mill.) and beech (Fagus sylvatica L.), which are characteristic of montane forest belts and form well-established populations on the Mont Ventoux northern slopes as well as the Lure Mountain. Together, they form the southernmost mixed beech-fir populations of Provence region in Southern France. In the face of ongoing global warming, the future of these forests at their southernmost distribution area remains uncertain [START_REF] Tinner | The past ecology of Abies alba provides new perspectives on future responses of silver fir forests to global warming[END_REF][START_REF] Hendrik | Les effets de la sècheresse et de la sylviculture sur la croissance des sapins, hêtres et cèdres au mont Ventoux[END_REF][START_REF] Morales-Molino | Long-term responses of Mediterranean mountain forests to climate change, fire and human activities in the Northern Apennines (Italy)[END_REF]. Investigating their history is thus necessary to better predict their current and future ecological trajectories. This study aims at testing two working hypotheses: 1) the floristic composition of the upper forest limit was different in the past from today, 2) the treeline was located in higher altitudes, and 3) both fir and beech forests occupied more extended altitudinal extensions.

Study Site

Physical setting

Mont Ventoux (44.172912° N, 5.278177° E) culminates at an elevation of 1,910m. It is 25km long (east-west axis) and 15km wide (north-south axis) and is at the confluence of the Alpine and Mediterranean phytogeographical zones [START_REF] Rouveyrol | L'efficacité du réseau Natura 2000 terrestre en France[END_REF]. The northern slopes, steeper than the southern slopes, are subjected to very harsh ecological conditions. The average annual precipitation (from 1994 to 2007) is 1,270mm at the Mont-Serein meteorological station (1,445m altitude). Characteristic of the Mediterranean-type of climate, the annual precipitation is irregularly distributed along a year and mainly falls in autumn-winter-spring with presence of summer droughts [START_REF] Cailleret | Dendroécologie comparée du sapin pectiné et du hêtre commun le long d'un gradient altitudinal[END_REF]. The average annual temperature is 6.6°C [START_REF] Ladier | Les stations forestières des Préalpes sèches, définition, répartition, dynamique, fertilité[END_REF]).

infilling of the René Jean cave (1,650m), in particular, also rich in bear bones dated from the Neolithic to the Middle Ages, has made it possible to recognize three species: fir, Scots pine (Pinus sylvestris L.) or hooked pine and birch leaf maple (Acer opalus) [START_REF] Thinon | La végétation du mont Ventoux au cours des derniers millénaires[END_REF]. They indicate wildfires that occurred between 1,650m and the summit, which is equivalent to the topmost part of our transect located one to two kilometers to the west. Only hooked pine is still thriving in this altitudinal belt today. Finally, the last data likely to provide precise spatial information on the forest species that colonized the summit of Mont Ventoux come from an exploratory pedoanthracological study carried out by M. [START_REF] Thinon | La pédoanthracologie : une nouvelle méthode d'analyse phytochronologique depuis le Néolithique[END_REF] on the southern slope. A pit dug in a summit lithosol at 1,810m altitude, on the crest, yielded a beech charcoal, which has not been dated [START_REF] Thinon | La végétation du mont Ventoux au cours des derniers millénaires[END_REF]. All these works indicate that since at least the Middle Neolithic [START_REF] Crégut-Bonnoure | Les cavités karstiques du mont Ventoux (Vaucluse, France) : des ours bruns (Ursus arctos L.) et des milieux d'altitude à l'Holocène[END_REF], Mont Ventoux, even at its summit, has been covered by forest species such as fir, beech and Scots/hooked pine, which later disappeared mostly as a result of fires at least partly linked to pastoral management. Our study aims to complete this information, both spatially and temporally. The current forest vegetation covering the northern slopes of Mont Ventoux in which our study transect was set up is the result of both RTM reforestation and spontaneous regeneration of the original species over the last few decades, thanks to the forest cover provided by these reforestations. The species used in plantations are the Austrian black pine (Pinus nigra J.F. Arnold subsp. nigra), Scots pine (P. sylvestris) and downy oak (Quercus pubescens Willd.) in the supra-mediterranean and lower montane, Scots pine, loblolly pine (Pinus mugo subsp. uncinata), and larch instead of deciduous oak forests, the montane zone still presents in its lower part (1,000-1,250m) stands of wild maple (Acer opalus Miller) and beech, and in the upper part (up to 1,800m), fir, hooked pine (Pinus mugo Turra subsp. uncinata (Ramond ex DC.) Domin) and common juniper (Juniperus communis L. subsp. nana (Hook.) Syme). The upper treeline is depicted at 1,650m. From this sketch, the forest above 1,400m is very open. The distinctive silhouette of some of the fir and beech trees in the sketch, which are today considered as relict stands, confirms this graphic representation by C.F. Martins. Their candelabra-like aspect and well-developed low branches indicate conditions of growth without inter-individual competition and thus a certain degree of openness. The oldest stands of the RBI-MV, left to evolve naturally for more than a century, owe their survival to their hardly accessible position above the steep rock cliffs. Indeed, it is in these relic stands that the oldest known trees in Vaucluse are found (> 400-year old firs and 300-year old beech trees) [START_REF] Terracol | La Réserve biologique intégrale du mont Ventoux, un espace d'étude des écosystèmes forestiers hors sylviculture[END_REF]. Although the vegetation of the mountain slope corresponding to our transect is not as well documented prior to the 19th century RTM plantations, it is clear that the absence of steep slopes and rock cliffs allowed an easier access to forest stands compared to the RBI-MV. This fact facilitated the deforestation that was much greater there as is evident in the photographs taken at the late 19th century by the agents of the Eaux et Forêts (Collective 2004). In the northern slope of Mont Ventoux, there are numerous caves and cavities in the rock cliffs containing the accumulations of biological remains, which have been used by prehistorians for palaeoenvironmental reconstructions [START_REF] Crégut-Bonnoure | Les cavités karstiques du mont Ventoux (Vaucluse, France) : des ours bruns (Ursus arctos L.) et des milieux d'altitude à l'Holocène[END_REF]. For instance, the charcoal assemblages collected in the F. sylvatica taken from the six soil pits and transferred in 34 bags to the laboratory. The sieving protocol is described in a number of papers (e.g. Carcaillet & Thinon 1996[START_REF] Saulnier | New pedoanthracological data for the long-term history of forest species at mid-high altitudes in the Queyras Valley (Inner Alps)[END_REF]. The soil samples are dried and then weighed to obtain the dry weight in kg. Each sample is then stirred in water with the addition of a small amount of deflocculant (sodium hexametaphosphate) to facilitate the suspension of organic elements. The supernatant (including plant debris and charcoal) and then the mineral elements are poured onto a column of 4 sieves (1cm, 1.25mm, 800µm, and 400µm), rinsed, and dried in oven (40°C). Coarse material > 1cm is weighed and its weight subtracted from the total soil weight for the calculation of specific anthracomass (SA). In this study, the mineral fractions and the organic fraction between 400 and 800µm were not studied. Charcoals were picked one by one using flexible forceps under a stereomicroscope and put into micro-tubes labeled by level, fraction (organic or mineral) and size class (e.g. VTN2 20-30 Root > 800µm). For each depth, given the limited time available for this study, 20 charcoals were randomly picked from the largest size class (> 1.25mm). If the considered size class had fewer than 20 charcoals or nothing, the rest of charcoals were picked from the smaller size class (> 800µm). Charcoals were identified using an episcopic microscope (Olympus model BX41) with dark field and magnification range of x100 to x500. Identifications were based on observation of transverse, radial longitudinal, and tangential longitudinal sections and using the available (Larix decidua Mill.) in the middle montane, and finally loblolly pine in the upper montane [START_REF] Martin-Gousset | Habitats et dynamiques de végétation en ubac du mont Ventoux -Étude de la Réserve biologique intégrale[END_REF]. The main trees and shrub species currently present on our plots are summarized in Table 1.

Materiels and methods

Sampling sites

In 2019, an altitudinal transect containing six permanent plots (VTN1 to VTN6) was established on the northern slopes of Mont Ventoux, from 925 to 1,860m elevation (Figure 1). Within each plot, a 900m² (30m × 30m) quadrat was delimited for applying different monitoring protocols of the ORCHAMP project (https://orchamp.osug.fr/Protocoles). A pedological pit was dug outside each permanent plot but at the immediate proximity of one of its four corners in order to describe the physico-chemical properties of soil profile. Once the pedological properties of the six soil profiles were described, we sampled them for our soil charcoal analysis to reconstruct the fire history and woody vegetation dynamics. Wood charcoals are organic materials with high resistance to biological degradation and once buried in the soil, are preserved for very long periods of time [START_REF] Thinon | Aspects méthodologiques et applications[END_REF], Talon et al. 2005;[START_REF] Marguerie | Anthracologie et dendrologie. La dendroécologie : principes, méthodes et applications[END_REF]. After carbonization, wood anatomy is well-preserved making its taxonomic identification possible [START_REF] Chabal | Forêts et sociétés en Languedoc (Néolithique final, Antiquité tardive) : L'anthracologie, méthode et paléoécologie[END_REF]. Despite this, many charcoals are difficult to identify whether because they are covered by different deposits (clays, sands, crystals) or show a vitrified aspect [START_REF] Mcparland | Is vitrification in charcoal a result of high temperature burning of wood?[END_REF] i.e. the wood structure can be distinguished but the anatomic elements are fused together preventing the identification [START_REF] Marguerie | Anthracologie et dendrologie. La dendroécologie : principes, méthodes et applications[END_REF]). The study of charcoals coupled with their AMS radiocarbon dating allows reconstructing the dynamics and composition of vegetation with high spatial precision and on the millennial time scale (Carcaillet & Thinon, 1996;[START_REF] Touflan | Soil charcoal analysis: a reliable tool for spatially precise studies of past forest dynamics: a case study in the French southern Alps[END_REF][START_REF] Robin | Complementary use of pedoanthracology and peat macro-charcoal analysis for fire history assessment: illustration from Central Germany[END_REF].

Pedoanthracological sampling and laboratory preparation

Soil samples were taken continuously from the bottom to the top of the profile at 10cmthick levels [START_REF] Saulnier | New pedoanthracological data for the long-term history of forest species at mid-high altitudes in the Queyras Valley (Inner Alps)[END_REF]. In total, The Specific Anthracomass per Profile (SAP) which corresponds to the mass of all charcoals (identified and unidentified) in a pit (in mg), related to the total mass of soil (in kg).

The Specific Anthracomass per Level (SAL), which corresponds to the mass of all the coals contained in a level (in mg), compared to the total mass of soil in that same level (in kg).

The Taxonomic Specific Anthracomass (TSA), which corresponds to the mass of all the charcoal of a given taxon contained in a level or pit (in mg), related to the total mass of soil of this level or pit (in kg).

Statistical tools

The results of the identifications, expressed as TSA, were processed graphically using C2 data analysis software, version 1.7.7, developed by Steve Juggins, in order to present them as anthracological diagrams. A logarithmic scale was applied to the SA values in order to compare them more easily. The dataset resulting from this study is partly based on qualitative data of the "Presence/Absence" type for charcoal identifications as well as for the current vegetation surveys (carried out as part of the ORCHAMP protocol in 2019) on all plots. Two Correspondence Analysis (CA) were conducted to compare the diversity of woody species currently present on the plots with the taxonomic composition of the anthracological results. The data are expressed in "Presence/Absence". Beforehand, as regard charcoal data, taxa encountered several times on all the plots and so-called "rare" taxa, i.e. those identified only in one plot, must be distinguished. These rare taxa are included in the analyses as additional qualitative variables. The CA function of the "FactoMineR" package [START_REF] Le | FactoMineR: An R Package for Multivariate Analysis[END_REF]) was used to perform these analyses. The package was run in RStudio version 4.1.0 (R Core team 2021). Diversity measures on the surveys are summarized as measures of taxonomic richness. Specific diversity was assessed on the anthracological data. Indeed, the sum of taxa present as charcoal for each soil level in each pit was calculated. In order to test the variation of the number of taxa with altitude, a one-factor ANOVA followed by a Bonferroni test. The objective was to compare two-to-two plots and to demonstrate a significant difference or not in the number of taxa against the altitude. literature on wood and charcoal anatomy [START_REF] Jacquiot | Atlas d'anatomie des bois des conifères[END_REF][START_REF] Schweingruber | Anatomie europaïscher Hölzer[END_REF][START_REF] Vernet | Guide d'identification des charbons de bois préhistoriques et récents Sudouest de l'Europe: France, Péninsule ibérique et îles Canaries[END_REF] as well as the charcoal reference collection of pedoanthracology of the Institut méditerranéen de biodiversité et d'écologie.

The identification of charcoal can be more or less easy because it depends on the state of conservation of the extracted fragments. The worst case is that of indeterminable "vitrified" charcoal. To this should be added the problem of differential identification; some genera or species are more difficult to identify than others, especially on small charcoals, because they are anatomically very close, such as Scots pine and hooked pine or, in the case of juvenile structures, fir and juniper. In such cases, they are named Pinus sylvestris/ uncinata or Abies/Juniperus. In some cases, it may not be possible to go beyond the genus (e.g. Acer), family (e.g. Rosaceae) level and even to distinguish the deciduous from coniferous trees.

Radiocarbon dating

The selection of the charcoal pieces to achieve 14 C dating was performed according to their significance to the overall charcoal taxonomic spectrum. Six single charcoal fragments weighing at least 2mg were selected for radiocarbon dating from the set of taxonomically identified charcoals and shipped to Poznan Radiocarbon Laboratory, Poland. The radiocarbon ages were calibrated and the probability distribution curves were graphically created in R package "clam" run in RStudio version 4.1.0 (Blaauw 2010; R Core Team 2021) using the IntCal20 calibration curve [START_REF] Reimer | The IntCal20 Northern Hemisphere radiocarbon age calibration curve (0-55 cal kBP)[END_REF]. The results are reported in Appendix 2.

Calculation of specific anthracomasses

All the sorted charcoals are weighed in order to calculate the specific anthracomass (SA) and to make the quantities of charcoals comparable from one pit to another. In our study, the specific anthracomass corresponds to the ratio between the mass in mg of extracted coals (fraction > 800µm) and the total mass in kg of dry soil (fraction < 1cm), expressed in mg.kg-1 or in parts per million (ppm). Three types of specific anthracomass were calculated:

seem to result from the carbonization of very small diameter twigs, perhaps from microchamaephytes (5-15cm).

The second CA applied to modern vegetation data also helps to distinguish between plots (Figure 3). The two CA axes account for 95.27% of the variability in the dataset (68.72% for axis 1 and 26.55% for axis 2). The three lower altitude plots are spatially separated from the three higher altitude plots. Nevertheless, plots VTN3 and VTN4 are relatively close. This proximity is due to the presence of beech and fir in both plots.

According to these surveys, deciduous trees are mainly present at lower elevations and become less abundant from 1,510m upward. Species not shown on the scatter plot are those present only in plot VTN1 i.e. common juniper (Juniperus communis), false cherry (Prunus mahaleb L.), holm oak (Quercus ilex L.), downy oak (Quercus pubescens), glossy buckthorn (Rhamnus alaternus L.), alpine buckthorn (Rhamnus alpina L.), sessile-leaved laburnum (Cytisophyllum sessilifolium (L.)

Results

Anthracological diagrams

A total of 458 charcoals were identified and attributed to 15 woody taxa (Appendix 1, Figure 7): Abies, Abies/Juniperus sp., Juniperus sp., Pinus sylvestris/uncinata, Pinus sp., Taxus baccata, Fagus sylvatica, Acer sp., Prunus sp., Amelanchier sp., Sorbus sp., Rosaceae, Quercus sp., Betula sp., Fraxinus sp. Analysis of the soil charcoals revealed great diversity in the anthracological record between pits, both quantitatively and qualitatively. Charcoal was found in all pits and in all levels, with a clear decrease in taxonomic diversity towards higher elevations. Conifers were identified along the entire transect. Fir is the most represented and frequent conifer in terms of quantity: it is found in all pits. At 1,860m (VTN6), it is even the only taxon identified. Yew (Taxus baccata L.) was identified twice, in the deepest level of VTN1 (90-110cm) at 925m, and at the middle of VTN4 profile (30-40cm) located at 1,510m.

No hardwood was identified above 1,340m elevation (VTN3). In terms of quantitative values, beech remains the most represented hardwood, particularly in VTN1 where it is found throughout the profile.

Statistical analysis of qualitative data

The correspondence analysis (CA) of the soil anthracological results (Figure 2) differentiates the pits according to the taxa identified.

The CA axes 1 and 2 explain 72.45% of the variability of the data set (48.88% for axis 1 and 23.57% for axis 2). The CA scatter plot spatially separates pits located below 1,510m (VTN1, VTN2 and VTN3) from those higher than 1,510m (VTN4, VTN5 and VTN6).

Above 1,510m of altitude (VTN4), the taxa found are mostly conifers such as fir, pine, yew and juniper. Below this altitude, charcoal from deciduous trees such as beech, oak (Quercus sp.), maple (Acer sp.), ash (Fraxinus sp.), birch (Betula sp.) and Rosaceae (Rosoideae Prunoideae and Maloideae) are more abundant. In a more specific way, we were able to identify taxa such as Taxus at 925m and Pinus up to 1,340m as well as Juniperus, and Pinus sylvestris/uncinata up to 1,510m. Note also the significant proportion of vitrified charcoals present in all pits. Small in size and mostly cylindrical in shape, these charcoals

Figure 2 -Diagramme de dispersion de l'Analyse factorielle de correspondance (AFC) réalisée à partir des données d'identification de charbons par fosse (présence/absence par fosse). En noir sont représentées les placettes ; en gris, les charbons de bois identifiés ; en italique les taxons « rares ». VTN = Ventoux

Radiocarbon dates

The

Discussion

The anthracological analysis of the ORCHAMP transect plots on the northern slopes of Mont Ventoux revealed the presence of charcoal in all pits from 925m (VTN1) to 1,860m (VTN6) and at all depth levels. Our interpretation is that the erosion following the historical deforestation of the slopes of Mont Ventoux could not wash away the entire soil cover. In fact, the dates obtained in this study all predate the RTM plantations of the late 19th and early 20th centuries. The scree deporits clearly visible on RTM photographs from the

Figure 4 -Distribution temporelle des courbes de distribution de probabilité des six charbons identifiés et datés par rapport aux principales périodes historiques. (AB) Âge du bronze (4200-2750 BP) ; (MA) Moyen-Âge (1500-500 BP) ; (M) Période moderne (450-150 BP). Voir l'INRAP pour la chronologie (www.inrap.fr).

the subalpine zone (1,600 to 1,800m) on the northern slopes of Mont Ventoux (according to [START_REF] Gobert | Carte de végétation Feuille XXXI-40 -Vaison -In: Doc. Carte Vég. Alpes[END_REF]. The data also shed light on the role played by beech and fir.

Comparison of the current surveys (woody vegetation only) (Table 1) of the VTN1 and VTN2 plots with the corresponding anthracological assemblages reveals a good agreement between taxonomic diversity, despite the fact that the anthracological identifications (Appendix 1) often stop at lower taxonomic levels (genus or family e.g. Rosaceae). For plots VTN1 and VTN2, Buxus and Viburnum are the only taxa not identified at all in the charcoal assemblages. On the contrary, plot VTN3 appears to have had higher taxonomic richness in the past than present. In this plot we found Acer and Betula, which are absent from the modern vegetation. These preliminary results suggest that some species have had different altitudinal ranges than today.

Beech is currently present in the vegetation of plots VTN1 to VTN4, up to 1,510m elevation (Table 1). However, it was not found in the anthracological assemblages of VTN4, but only in those of plots VTN1 to VTN3. Considering that a larger sampling effort (> 20 charcoals) might have revealed it in VTN4, before publishing this work, we randomly picked up another 20 charcoals (> 1.25mm). But we did not find any beech. However, we did find yew. Since beech is currently present in plot VTN4, represented by a few old individuals scattered throughout the stand and formerly exploited in coppicing mode, we can assume that there has been no fire for several centuries (indeed, the dates obtained for this pit do not go back beyond the Middle Ages) or that the fire, if there was any, did not affect the beech trees. The last hypothesis is that there were no beech trees on this plot at the time of the last fire events. Only conifers provided charcoal. The absence of beech charcoal does not mean that beech was absent on the plot. In pedoanthracology, as in all paleoecological disciplines, the absence of a taxon may not mean a true absence of the taxon in the landscape and must always be interpreted with great caution [START_REF] Birks | Past forests of Europe[END_REF] The presence of yew charcoal is more surprising. Thinon 1996;[START_REF] Tinner | Uppermost limit, extent, and fluctuations of the timberline and treeline ecocline in the Swiss Central Alps during the past 11,500 years[END_REF][START_REF] Ali | Pinus cembra L. (arolla pine), a common tree in the inner French Alps since the early Holocene and above the present treeline: a synthesis based on charcoal data from soils and travertines[END_REF][START_REF] Talon | Reconstruction of Holocene highaltitude vegetation cover in the French southern Alps: evidence from soil charcoal[END_REF], Cunil et al. 2012). These results help to illustrate the important role of fire, both natural and anthropogenic in the establishment of current ecosystems and the current position of the altitudinal tree line [START_REF] Theurillat | Potential impact of climate change on vegetation in the European Alps: a review[END_REF].

To clarify the elevation at which the upper forest limit was located on this slope of Mont Ventoux prior to its deforestation, we compared the total charcoal richness of different pits, and more specifically, the decrease in general specific anthracomass (GSA) with increasing elevation [START_REF] Talon | Reconstruction of Holocene highaltitude vegetation cover in the French southern Alps: evidence from soil charcoal[END_REF][START_REF] Cunill | Holocene treeline changes on the south slope of the Pyrenees: a pedoanthracological analysis[END_REF]. The comparison of GSAs shows a significant decrease from 1,675m (VTN5) upwards. Combining qualitative (taxonomic composition) and quantitative (SA) data allows us to suggest that the upper forest limit on the north slope of Mont Ventoux was around 1,510m (VTN4) and that the tree limit reached 1,860m (VTN6). While the upper limit of the forest is higher today, largely due to RTM plantations, the upper limit of the trees is essentially the same.

Comparing the specific SA values of conifers and hardwoods, the latter becomes less abundant from 1,130m and the maximum SA value of conifers is encountered at 1,510m (VTN4). These results are consistent with the current ecological distribution pattern of hardwoods and softwoods along an altitudinal gradient. Indeed, as temperature decreases with altitude (about 0.55°C every 100m), it directly affects the physiology and biology of mountain woody species [START_REF] Ozenda | La végétation de la chaîne alpine dans l'espace montagnard européen[END_REF].

With regard to the modern vegetation of Mont Ventoux, the pedoanthracological results accurately reflect the current distribution between deciduous and coniferous trees according to altitude. Indeed, from 1,675m (VTN5) upward, the number of taxa decreases significantly: averagely, four taxa are reduced to one in the topmost pits located in the summit. This decrease corresponds to the transition from the montane zone (1,100 to 1,600m) to it is not known when fir tree occupied the highest elevation of the transect (1,860m 2000), but the geographical provenance of its pollen has never been elucidated. As Abies pollen is very heavy, it seems unlikely that it comes from the fir trees on the north face of the Ventoux, 60km away; it could rather come from the neighbouring Alpilles area.

Multiplying the pits supported by additional radiocarbon dates will allow us to complete the phytogeographical history of these two species in Provence.

We have not obtained any date posterior to the reforestation of the 19th century (RTM). The soils have not therefore been totally eroded despite successive deforestations, and still retain a lot of paleoecological information. However, this information is not chronostratified. Three of the six dates come from the same pit (VTN2): the beech and fir extracted from the same sample (20-30cm) however, are of very different ages (Modern Era and Bronze Age, respectively), while the fir and pine, both dated to the Bronze Age, come from two very different depths (20-30cm and 50-70cm, respectively). This observation common in most of pedoanthracological studies (e.g. [START_REF] Brasseur | Soil Archives: Where Soilscape History Meets Present-day Ecosystems. Historical Ecology: Learning from the Past to Understand the Present and Forecast the Future of Ecosystems[END_REF], reminds us that the existence of a chronostratigraphy for soil profiles that might spontaneously come into mind is far from obvious (Appendix 1 Figure 7). The pedological analysis carried out within the framework of the ORCHAMP protocol, in particular the granulometric analysis, could contribute to a better understanding of the burial patterns of charcoal in the soils.

in our study area, but not at this altitude. It is also found further east, in the cliffs of the RBI as well as on the southern slopes. Yew (Taxus baccata) must therefore have played a more important role in vegetation in the past than today. Palynological and archaeoanthracological data show that yew expanded rapidly in Provence around 5500-5000 BC, when the agro-pastoral economy of the Neolithic period was being established but rapidly after, it started declining perhaps because of human intervention to cut it for its toxicity [START_REF] Martin | L'if (Taxus baccata L.) : histoire et usage d'un arbre durant la Préhistoire récente. L'exemple du domaine alpin et circumalpin[END_REF]. Unfortunately, the few charcoals found in Mont Ventoux do not allow us to go further than confirming its presence on the slope in the past. These first results indicate that, in the past, beech and yew, along with fir, occupied an altitudinal range between 900 and 1,300m altitude on the northern slopes of Mont Ventoux. The absence of old radiocarbon dates makes it difficult to propose the exact timing of the establishment of this "beech-fir forest" on Mont Ventoux. On the basis of palynological data, beech was present in Provence in low-and mid-altitude areas as early as the Atlantic period with a strong expansion in the Subboreal period, as early as 4,700 years BP [START_REF] Andrieu-Ponel | Towards the reconstruction of the Holocene vegetation history of Lower Provence: two new pollen profiles from Marais des Baux[END_REF][START_REF] Azuara | The Holocene history of low altitude Mediterranean Fagus sylvatica forests in southern France[END_REF]. However, the location of these beech forests in Provence could never be accurately specified by palynologists due to the regional character of pollen data and good pollen dispersal of beech et al. (2000). However, in the absence of more radiocarbon-dated beech charcoals, it is not still possible to confirm its presence on Mont Ventoux since the Atlantic period. Fir tree has been found at all altitudes, from 925m to 1,860m. It represents about 35% of the identifications. Its presence up to 1,860m (VTN6) indicates that its upper limit was higher than today which does not exceed 1,510m (VTN4). The four dates obtained on fir fragments reveal that it was present on the northern slope of Mont Ventoux between 3,729 cal BP (Bronze Age) and 900 cal BP (Middle Ages), at an altitudinal range of 1,130m to 1,340m. The fragments found in VTN6 are too small to be dated. Therefore,

shows that fir has already colonized the upper part of the slope in the past, so it is natural that it will return there regardless of the climate warming. Beech has probably also colonized the upper part. The analysis of the pit transect on the southern slope (ORCHAMP VTS) will certainly provide answers to these questions. This work has allowed us to take a step back on the potential of the forest species to understand the future. It is necessary to continue this work, particularly with other dating methods (dendrochronology) and obtaining further radiocarbon dates, in order to correctly and more precisely retrace the history of the past vegetation of Mont Ventoux.

Conclusion

Although being exploratory and preliminary, this study contributed to improve our understanding of the long-term vegetation history on the northern slopes of Mont Ventoux. Despite heavily impacted by intensive deforestation since the 13th century and particularly in Modern Era, most of the soils developed under the former forest vegetation have survived centuries of torrential erosion. Their charcoal content has thus been preserved and can be used to depict the fire history linked to the natural and anthropogenic processes. Fir trees occupied the entire slope from 925m (VTN1) to the summit (VTN6) at 1,860m. Today, they do not exceed the altitude of 1,600m. The presence of charcoal of this taxon in the highest pit suggests that the summit cap, currently occupied by a few scattered prostrate hooked pines, would have had a more closed canopy than today. It is worth to recall that a beech charcoal was also found on the summit ridge [START_REF] Thinon | La végétation du mont Ventoux au cours des derniers millénaires[END_REF]. The upper tree line, today composed of only hooked pine, would have thus been more diverse (with fir and beech) and the treeline position was higher than today. Our results also confirm that the forest species such as beech and fir that have been growing spontaneously in the 1860s RTM plantations, particularly in the understory of the VTN4 hooked pine, are those that occupied this altitudinal zone before the deforestation. The upper altitudinal limit of the forests was higher in the past, around 1,510m (VTN4), and consisted of stands of fir and yew (VTN4). The spatial resolution of pedoanthracology thus appears to be relevant for reconstructing the altitudinal fluctuations of the upper vegetation boundary of Mont Ventoux as well as its floristic composition. Wood charcoal and its dating provide much valuable information about the important role of forest fires i n establishing present-day vegetation patterns.

Higher spatial resolution of pedoanthracological data is however needed to confirm this: use replicates at similar elevation and sample every 100m of elevation instead of 200m, but this was the grid of the Orchamp protocol.

It remains to study how the woody vegetation of Mont Ventoux, in particular beech and fir, will react to ongoing global warming. Some authors predict a progression of fir towards higher altitudes, but with a more restricted range [START_REF] Cailleret | Impact potentiel du changement climatique sur la croissance et la répartition de deux espèces en co-occurrence sur le mont Ventoux : le sapin pectiné (Abies alba Mill.) et le hêtre commun (Fagus sylvatica L.). Résumé. In: 9 e Journées d'écologie fonctionnelle, La Grande Motte[END_REF]. Our work

Figure 1 -

 1 Figure 1 -Map showing the location of the study site and the sample plots located on the ubac of Mont Ventoux (Google Maps and Google Earth).

Figure 1 -

 1 Figure 1 -Carte de localisation du site d'étude et des placettes d'échantillonnage situées à l'ubac du mont Ventoux (Google Maps et Google Earth).

Figure 2 -

 2 Figure 2 -Scatter plot of the Correspondence Analysis (CA) performed on identified charcoals data per pit (presence/absence per pit). The pits are shown in black, the identified charcoals in grey and the "rare" taxa in italic. VTN = Ventoux Nord. Inertia percentages: 48.88% (axis 1) and 23.57% (axis 2).

  Figure 2 -Diagramme de dispersion de l'Analyse factorielle de correspondance (AFC) réalisée à partir des données d'identification de charbons par fosse (présence/absence par fosse). En noir sont représentées les placettes ; en gris, les charbons de bois identifiés ; en italique les taxons « rares ». VTN = Ventoux Nord. Pourcentages d'inertie : 48,88 % (axe 1) et 23,57 % (axe 2).

  soils of the northern slope of Mont Ventoux have recorded nearly 4,000 years of vegetation history (Appendix 2). The earliest radiocarbon dates back to the Bronze Age (VTN2, 3903-4088 cal BP / 1953-2138 BCE) and the latest one to the Modern Era (VTN2, 346-460 cal BP / 1604-1490 CE). Fir charcoal was dated to the Bronze Age (VTN2 and VTN3) and the Middle Ages (VTN4), pine to the Bronze Age (VTN2), and beech to the Modern Era (VTN2). The time distribution of the ages is illustrated in Figure 4 as the probability distribution curves which represent the calibrated ages with 2-sigma errors. The total absence of radiocarbon ages dating to the Iron Age and Classical and Late Antiquity is also noted.

Figure 3 -

 3 Figure 3 -AFC performed on modern floristic data per plot (presence/absence per plot). The plots are shown in black, the species in grey and the 'rare' taxa in italic. VTN = Ventoux Nord. Inertia percentages explained by two axes are 48.88% for Axis 1 and 23.57% for Axis 2.

Figure 3 -

 3 Figure 3 -AFC réalisée à partir des données d'identification de végétation actuelle par placette (présence/absence par placette). En noir sont représentées les placettes ; en gris, les espèces identifiées ; en italique les taxons « rares ». VTN = Ventoux Nord. Pourcentages d'inertie : 68,72 % (axe 1) et 26,55 % (axe 2).

Figure 4 -

 4 Figure 4 -Temporal distribution of the probability distribution curves of the six identified and dated charcoals against the main historical periods. (AB) Bronze Age (4200-2750 BP); (MA) Middle Age (1500-500 BP); (M) Modern Era (450-150 BP). The historical periods are based on the chronology adopted by INRAP (www.inrap.fr).

Table 1 -Main tree species observed on the current plots. The star* indicates species remained from RTM plantations. Tableau 1 -Principales essences arborescentes présentes sur les placettes actuelles. L'étoile* indique les essences issues de plantations RTM.

 1

	VTN1	VTN2	VTN3	VTN4	VTN5	VTN6
	925m	1,130m	1,340m	1,510m	1,675m	1,860m
	Abies alba*	A. alba*	A. alba*	A. alba*	P. mugo subsp.	P. mugo subsp.
	Pinus sylvestris*	P. sylvestris*		Pinus mugo subsp.	uncinata*	uncinata
	Juniperus communis			uncinata*
	Quercus ilex	Fagus sylvatica	F. sylvatica
	Q. pubescens	B. sempervirens	S. aria
	Buxus sempervirens	A. opalus
	Acer opalus	S. aria
	Sorbus aria	V. lantana
	Viburnum lantana

  ). It is also not known whether it occupied this slope well before the Bronze Age although this is quite possible. Indeed, the presence of fir in southern France dates from the end of the last

	glaciation (Younger Dryas) (Terhürne-Berson
	et al. 2004). One of the charcoals found in the
	René Jean cave (but not identified), between
	an upper level dated to the Middle Ages and a
	base level dated to the Middle Neolithic, has
	been dated to 3025 ± 40 BP, i.e. the Middle
	Bronze-Final Bronze transition (Cregut-
	Bonnoure et al. 2005). The fir charcoal
	found in this same cave infill can therefore
	be attributed to this period (Thinon 2007).
	Fir pollen has been recorded in Provence as
	early as the Atlantic period at low altitude
	in the Baux marshes (Andrieu-Ponel et al.

Acknowledgments

The authors wish to thank Maryse Alvitre for her technical assistance in laboratory analyses and Dr. Emmanuel Gandouin for his thoughtful advice in statistical data analysis. They also thank the entire field team that participated in the ORCHAMP soil-anthracology sampling campaign (Lenka Brousset, Brunetto William, Raphaël Gros, Frédéric Guiter, Quentin Lambert and Yoann Pinguet). Thanks to Bruno Fady and Frederic Guibal for their advice in the reflection of this work. This work was supported by a Master thesis student grant awarded by French Government (France 2030 perspective) through "l'Initiative d'Excellence d'Aix-Marseille Université" and in the frame of the research consortium for ecological transition entitled l'Institut méditerranéen pour la transition écologique (ITEM, A*MIDEX AMX-19-IET-01). Radiocarbon dating was supported by the program "Zone Atelier Alpes" financed by CNRS-INEE.