Holocene paleoecological changes and agro-pastoral impact on the La Narce du Béage mire (Massif Central,France)

Abstract

A paleoecological study (macrofossils, pollen, cryptogam spores, non-pollen palynomorphs) was performed to investigate environmental changes recorded on the peat deposits of the La Narce du Béage mire (Massif Central, France). We reconstructed the development of a limnogenous mire ecosystem during the Holocene, consequently to the infilling of a small Late Glacial lake. Successions from aquatic flora (Isoëtes, Nitella opaca/syncarpa, Botryococcus, Alisma plantago-aquatica, Potamogeton, Sparganium) to mire plant species (Alnus glutinosa, Betula nana, Betula pubescens, Cyperaceae, Ericaceae, Sphagnum) underlined a gradual eutrophication and acidification during the lowering of the water table. We demonstrated a clear link between these local hydro-ecological changes and the early Holocene climatic warming. Also dealing with archeology, we provided key issues for the identification of human-induced environmental changes. Three phases of ecological disturbances with clearings of the mixed-oak forest and agro-pastoral activities were evidenced at 7700–7300, 6800–6420, and 5500–4250 cal. BP. Our results confirmed early Neolithic (Cardial influences), middle Neolithic (Chassean), and late Neolithic (Ferrières group) frequentations of the Béage Plateau, probably included in large-scale socio-cultural changes (SE France, NW Mediterranean Basin). Later, forest clearings (beech and fir), cultivation, and pastoralism were outlined since the Iron Age (after 2300 cal. BP, that is, 350 BC). Increasing human pressure was especially recorded during the last 1700 years, with rye (Secale cereale) cultivation and extensive livestock grazing around and on the mire, which is nowadays a protected ‘Natura 2000’ area.

Keywords

fire history lake-level changes Neolithic to Medieval human impact paleoecology terrestrialization

Introduction

Within the context of global climatic change and human impact, the evolution of peatland ecosystems over time is one of the major concerns related to water supply and quality, biodiversity, or carbon storage (Erwin, 2009; Moor et al., 2015). In Western European Mountains, this human/environment issue was addressed by numerous studies combining environmental data – sediment records and palynology – and archeology (e.g. Ejarque et al., 2009; Lozny, 2013; Miras and Surmely, 2006). These studies discussed (1) forest openings and agro-pastoral activities since the Mesolithic (Behre, 2007; Innes et al., 2013; Richard, 2004; Tinner et al., 2007), (2) correlations between Neolithic occupations and climatic changes (Berger and Guilaine, 2009; Magny, 2013; Sánchez Goñi et al., 2016), and also (3) long-term landscape management (Miras et al., 2015; Surmely et al., 2009). However, understanding ecosystem and biodiversity trajectories also need local ecological investigations (Birks and Birks, 2000).

Paleoecology, performed on macrofossils from organic sediments (e.g. seeds, leaves, bud- or catkin-scales, stems, roots, charcoal, fungi, algae, and zoological remains), does provide significant environmental proxies, thanks to (1) a limited dispersal of these remains, involving the local presence of the taxa; (2) reliable taxonomic identifications, often to genus or species level; and (3) a variety of remains from a same species (seeds, fruits, leaves, etc.) increasing the chances to identify past taxa (Birks, 2014; Mauquoy and van Geel, 2007). In addition, macrofossil data can be acquired on the same sediment samples as palynological data, providing comprehensive paleoenvironmental reconstructions from the local to the regional scale (Payette and Rochefort, 2001). Such studies were commonly performed in the French and Swiss Alps, in particular to reconstruct Holocene tree line fluctuations and fire history (Blarquez et al., 2010; Genries et al., 2009; Guiter et al., 2005; Reasoner and Tinner, 2009; Schwörer et al., 2014; Stähli et al., 2006; Tinner and Kaltenrieder, 2005), as well as on the British Isles, in Germany, and in Denmark to evaluate wetness trends on bogs (Barber et al., 2003, 2004). As regards the Eastern French Massif Central, recent approaches were developed in the Morvan (Jouffroy-Bapicot et al., 2013) and near the Mont Lozère (Baron et al., 2005; Servera Vives et al., 2014). However, combined pollen and macrofossil studies existed only for the Mont d’Or and the Aubrac Plateau (Lang and Trautmann, 1961) or for the Livradois-Forez Mountains (Argant and Cubizolle, 2005; Cubizolle et al., 2013, 2014).

Our multidisciplinary approach, combining macrofossils, paly-nology, and archeology, is quite unique in the French Massif Central. The main study site is the La Narce du Béage mire, on the Mézenc Plateau (Figure 1), where analyses of sediment and volcanic deposits highlighted a promising record for the Late Glacial and the Holocene (Dendievel et al., 2015). Here, our focus is devoted to decrypt vegetation dynamics and geomorphological evolutions related to hydro-climatic and anthropogenic changes affecting the mire over the last 10,200 years. The paper will elucidate (1) the development of a terrestrial mire ecosystem and (2) the onset of local to regional agro-pastoralism practices, discussing short- and long-term modifications of mountain ecosystems and landscapes.

Figure 1.

(a) The study area in the South-Eastern French Massif Central (black square). (b) Topographical map showing the La Narce du Béage mire (red star No. 1) and other locations of regional palaeoenvironmental research cited in the text: (2) Peyrebeille bog; (3) Ribains marsh; (4) Lac du Bouchet; (5) Limagne marsh; (6) Saint-Front Lake; and (7) Narces de Chaudeyrolles. (c) Photographic overview of the La Narce du Béage mire (NW view, picture: A.-M. Dendievel, 2017). The white dots symbolize cores A (this paper) and D (Dendievel et al., 2015). Former agricultural terraces are represented by ‘at’ abbreviation and black lines.

Regional and local settings

The site of La Narce du Béage is one of the mires of the Mézenc volcanic Plateau, on the South-Eastern French Massif Central (Figure 1). This area is influenced by an oceanic climate tempered by the mountains with strong seasonal contrasts: the annual average temperature is ca. 5°C, with cold winters (January mean: –6°C) and rather warm summers (July mean: 20°C). The annual rainfall is 900–1200 mm/yr with Mediterranean (‘Cevenol’) events in spring and autumn. Snow frequently occurs from October to April. The La Narce du Béage mire (44°49′53″ N; 4°05′57″ E; 1200 m a.s.l.) is situated in a small catchment area (11 ha), bounded to the north and to the east by two basaltic outcrops (dated back to 8–9 million years according to Defive, 1996), and to the south by the Cherchemus volcano (25,000 ± 9000 years old after Nomade et al., 2014). Late Pleistocene block-streams and screes are also located next to the peatland (Figure 1). Regional geological data indicate the presence of a granitic substratum (Defive et al., 2011).

The mire is a ‘Natura 2000’ protected area. This wetland is supplied by a spring (located at the granite/basalt contact zone) and by groundwater. Its outlet directs water toward the Veyradeyre River, which is one of the first tributaries of the Loire River (Figure 1). The mire vegetation is divided into three areas: (1) a hygrophilous tall herbs fringe with belts of Gentiana lutea (great yellow gentian), Narcissus pseudonarcissus (jonquil), Caltha palustris (kingcup), and Cirsium palustre (marsh thistle); (2) the middle of the fen is occupied by Molinia caerulea (moor grass); and (3) the outlet bears Equisetum palustre (marsh horsetail), Comarum palustre (marsh cinquefoil), Carex vesicaria (inflated sedge), and Sphagnum hummocks. Around the fen, old beech woods (Fagus sylvatica) are present and conifer stands are mainly the result of recent plantations (since AD 1860, including Picea abies and Pinus uncinata). Open meadows represent 54% of the catchment area and are mainly devoted to livestock, but agricultural soils also indicate past cultivation in front of the mire (Figure 1).

Archeological background

A micro-regional synthesis has listed 334 archeological entities from the Mesolithic to the Middle Ages on the Mézenc Massif (Dendievel, 2017; Figure 2(a)). First post-glacial occupations occurred during the Mesolithic (Figure 2(b)). Seasonal hunter camps are known at the Clarion pass, on the Les Trémoulèdes outcrop, and at the Longetraye rock-shelter, 4 km north of the La Narce du Béage mire (Crémillieux, 1999; Debard and Philibert, 1979).

Figure 2.

(a) Number of archeological sites during the Holocene on the Mézenc Massif, including the Béage Plateau (after Dendievel, 2017). Chronological maps: (b) Mesolithic and Neolithic; (c) Protohistory, including Bronze and Iron Ages (the first Iron Age is the Hallstatt period, and the second is the La Tène period), Roman Times; and (d) Middle Ages. A red star symbolizes the coring site of La Narce du Béage.

Early Neolithic occupations were also documented at Longe-traye and at Les Trémoulèdes, where archeological tools suggest coeval agro-pastoral and hunting activities (Chomette, 1989; Philibert, 1982). Cardial – early Neolithic – pottery dated to 7600–6700 cal. BP was also found near the Mount Mézenc (Figure 2(b); Daugas and Raynal, 1989). Middle Neolithic sites (Chassean culture, 6000–5550 cal. BP) were identified at Longetraye, Goudet, and La Baume d’Arlempdes (Daugas and Raynal, 1989; Philibert, 1982). Recent and final Neolithic (5500–4100 cal. BP) are characterized by collective burials: Rond-du-Lévier and Echamps dolmen (Figure 2(b)).

Protohistorical data are scanty and far from the Béage Plateau, except for the mid-second Iron Age occupation of Le Crouzet-La Veysse located next to La Narce du Béage (Figure 2(c)). Coeval findings of wheel-thrown pottery with to-and-fro millstones illustrate technological evolutions, agricultural activities, and domestic activities ca. 2400 cal. BP (Dendievel et al., 2016). The sites of the late second Iron Age and of Roman Times are mainly distributed along the Loire River and along other communication roads (Figure 2(c)). The trade with the Mediterranean world was locally controlled by the Camp d’Antoune, a fortified place located 13 km NW of the studied mire (Kurzaj, 2012).

Only few occupations are known for the late Antiquity and the early Middle Ages (Figure 2(a)). However, pottery fragments, found at Pérot, confirmed a local presence until the 5th–7th centuries AD (Figure 2(d)). After AD 1000, a significant increase of sites was recorded by medieval texts (Figure 2(d)). Numerous castles were built, at Le Béage, Géorand, or in front of the Mount Mézenc for instance, and a large number of churches and monasteries were founded, such as the Carthusian Abbey of Bonnefoy on the Béage Plateau itself, suggesting an economic and demographic growth (Laffont, 2009; Lemaître, 1990). Laws on fishing, flour milling, clearings, and livestock grazing revealed an intensive landscape management since the 12th century AD (Bréchon, 1998, 2000). Finally, a rural exodus occurred since the end of the 19th century AD, whereas national reforestation programs started in AD 1860–1904.

Materials and methods

Coring and sampling

In September 2012, a 550 cm long sediment core (core A) was extracted from the La Narce du Béage mire using a Russian-type peat corer. Here, we focus on the upper 237 cm of the sediment core. The stratigraphy was described using the Munsell Soil Color Charts (Munsell Color, 1994), and the ‘von Post test’ was used to estimate peat decomposition (Payette and Rochefort, 2001). Core segments were placed in PVC tubes, wrapped, and stored in a cold room (4°C). The sediment sampling was done at the Institute of Botany of the University of Innsbruck (Austria). The resolution interval for macrofossil and palynological analyses was between 1 and 4 cm, depending on the peat accumulation rate.

Chronology

The chronological control was based on eight Accelerator Mass Spectrometry (AMS) radiocarbon dates obtained on macrofossils from terrestrial plants (Table 1). The ages of two samples were rejected because of a recent contamination (ANB 48, Alopecurus hull) or an ‘old wood’ effect (ANB 38, mixed wood remains). Radiocarbon ages were calibrated with a 95% confidence interval and expressed as cal. BP (years before AD 1950), by using CALIB 7.0 and the IntCal13 curve (Reimer et al., 2013; Stuiver and Reimer, 1993). A linear age-depth model was calculated using the clam package (Blaauw, 2010) in R (R Core Team, 2015). The presence of pollen from varied conifer species, including P. abies at 12 cm in depth, was used as a chronological marker for national reforestation (see details on Table 1).

Table 1.

Radiocarbon dates obtained on the core A from the La Narce du Béage (SE Massif Central, France). The sample with an asterisk refers to an additional age based on pollen data. The samples ANB 38 and ANB 48 (italics) were rejected because of possible ‘old wood’ effect and recent contamination, respectively. All ages were calibrated with the CALIB 7.0 software, using the ‘IntCal13’ calibration curve (Reimer et al., 2013; Stuiver and Reimer, 1993).

Sample	Depth (cm)	Details	Laboratory code	Measured Age (1 σ)	Calibrated Age (2 σ)		δ¹³C (‰)
Sample	Depth (cm)	Details	Laboratory code	Measured Age (1 σ)	cal. BP	cal. BC/AD	δ¹³C (‰)
ANB 12*	12	Reforestation evidence: Picea abies pollen curve	–	–	90–46	AD 1860–1904	–
ANB 24	23.5–24.5	Macro-charcoals	Lyon-11089 (SacA38119)	1390 ± 30 BP	1346–1278	AD 604–672	−25.2
ANB 38	37.5–38.5	Wood	Lyon-12775 (GrA)	4135 ± 35 BP	4822–4535	2872–2585 BC	–27.1
ANB 44	43.5–44.5	Wood (Alnus sp.)	ETH-85059	3862 ± 65 BP	4495–4088	2545–2138 BC	−23.8
ANB 48	47.5–48.5	Alopecurus sp. hull	ETH-55536	–468 ± 124 BP	–	–	–30.9
ANB 61	60.5–61.5	Wood	Lyon-11088 (SacA38118)	5490 ± 35 BP	6395–6210	4445–4260 BC	−26.5
ANB 82	81.5–82.5	Wood	ETH-50454	6008 ± 32 BP	6940–6753	4990–4803 BC	−29.6
ANB 103	101.5–104.5	Wood	Lyon-12776 (GrA)	6545 ± 40 BP	7562–7338	5612–5388 BC	–
ANB 144	143.5–144.5	Wood	Lyon-11090 (SacA38120)	8135 ± 40 BP	9245–8998	7295–7048 BC	−28.7
ANB 182	181.5–182.5	Betula sect. Alba seeds	ETH-50455	8628 ± 44 BP	9684–9530	7734–7580 BC	−24.3
ANB 237	236.5–237.5	Betula sp.: leaves, budscales, and seeds	Lyon-12777 (GrA)	9025 ± 45 BP	10255–9948	8305–7998 BC	–

Macrofossil analysis

Sixty-seven sediment samples (mean sediment volume = 15 mL) were extracted and sieved with water (meshes: 1 mm, 500 µm, 250 µm, and 125 µm, the latter fraction was not further studied). Placed in distilled water, macrofossils were identified and quantified with a Leica MZ-6 stereomicroscope (magnification ×7.5 to ×75), and with a Nikon eclipse 80i light microscope for specific identifications (×10 to ×300). Macrofossils were identified using seed and fruit collections of the Institute of Botany of the University of Innsbruck and by using determination keys and literature compilations (Berggren, 1969; Cappers et al., 2006; Dombrovskaja et al., 1959; Haas, 1994; Lévesque et al., 1988; Mauquoy and van Geel, 2007; Tomlinson, 1985). All plant, zoological, and minerogenic macrofossils were documented and quantified. The main peat components (Sphagnum stems, Cyperaceae, and Ericaceae roots), homogenously distributed, were counted on a quarter of a Petri box and numerically extrapolated to the whole sample. Macrocharcoals were counted during routine analysis. The following figures contain a selection of macrofossils, but all macrofossil data are available upon request and will be stored in the Neotoma macrofossil database in due times.

Palynological analysis

For the palynological study, 69 samples (1 cm³ each) were chemically prepared. A known quantity of Lycopodium spores was added for concentration calculations (Maher, 1981). The samples were diluted in water and sieved. Then, the fraction 7–150 μm was processed following standard chemical treatments, including chlorification and acetolysis (Moore et al., 1991; Seiwald, 1980). Mounted in glycerin and stained with fuchsine, the palynological finds were observed with an Olympus BX50 light microscope (×400 to ×1000) and counted using PolyCounter (Nakagawa, 2012). Identifications were achieved, thanks to the modern pollen reference collection of the Institute of Botany of the University of Innsbruck and with identification manuals (Punt and Blackmore, 1991; Punt and Clarke, 1976, 1980, 1981, 1984; Punt et al.,1988, 1996, 2003; Reille, 1999). At least 500 arboreal pollen by sample were identified. Cerealia-type pollen were identified following size (Poaceae pollen larger than 40 µm), pore diameter (>8 µm), and annulus thickness characteristics (Beug, 2004; Colombaroli et al., 2013). Black, sharp, and opaque particles were identified as micro-charcoals. Non-pollen palynomorphs (NPPs) were identified using reference studies (Cugny et al., 2010; Dietre et al., 2012; Van Geel and Andersen, 1988; Van Geel and Aptroot, 2006; Van Geel et al., 2003), and NPP-types were named according to the HdV-type listing (Hugo de Vries-Laboratory Amsterdam; see Miola, 2012). The following figures contain a selection of pollen, cryptogam spores, and NPPs, but all palynological data are available upon request, and will be deposited in the European Pollen Database (EPD) in due times. The total curve of coprophilous fungi contains spores of Cercophora, Delitschia, Neurospora, Podospora, Sordaria-type, and Sporormiella.

Data presentation and numerical methods

Paleoecological diagrams for selected taxa were drawn using Tilia (Grimm, 2011; v.1.7.16). Macrofossils are expressed as concentration values (n/15 mL). Taking into account the diversified nature of macrofossils, we performed multivariate regression trees (MRT) in order to detect ecological groups along the stratigraphy (Birks, 2014). The MRT partitioning was realized using mvpart (v.1.6-2; Therneau et al., 2014) and MVPARTwrap packages (v.0.9-1; Ouellette and Legendre, 2013) available for R via GitHub (https://github.com/cran). Pollen, spores, NPPs, and micro-charcoal are represented as percentages relatively to the pollen sum of terrestrial vascular plants (excluding Cyperaceae, mire, and aquatic taxa). Influxes (n/cm²/yr) were calculated for micro-charcoals. Local pollen assemblage zones were defined by a hierarchical clustering stratigraphically constrained based on the terrestrial pollen taxa (CONISS; Grimm, 1987). The number of zones was determined using the ‘broken stick’ model (Bennett, 1996), available in the rioja package in R (Juggins, 2013). Finally, after summarization (to avoid redundancies among the taxa represented by multiple macrofossils), we rarefied macrofossils and pollen data to estimate the taxonomic diversity. The rarefaction was based on at least 106 macrofossils and 545 pollen grains. The expected number of taxa, E(T), and standard errors were computed with the vegan package in R for each sample (Oksanen et al., 2017; v.2.4-3). E(T) is representative of the local to regional past biodiversity (Berglund et al., 2008; Birks and Line, 1992).

Results

Chronostratigraphy and sediment accumulation

According to the depth-age model, the transition from lacustrine gyttja to peat occurred during the early Holocene, ca. 10,200–9800 cal. BP (units 1–2 on Figure 3). During the mire inception process, the sedimentation rate reached 0.9 mm/yr in very few decomposed – fibric – peat layers (unit 3 on Figure 3). During the middle Holocene (Atlantic chronozone), the sediment accumulation rate decreased to 0.25 mm/yr (units 4–5 on Figure 3). It dropped and remained very low after 6000 cal. BP (second half of the middle Holocene and late Holocene). Finally, a sharp sediment accumulation was observed in the upper part of a minerogenic-rich peat layer, that is, during the last 150 years.

Figure 3.

Lithology, radiocarbon age-depth model, and accumulation rates for the core A from the La Narce du Béage mire (SE Massif Central, France). The model was calculated on the basis of nine accepted dates (Table 1), by using a linear regression within the clam package (Blaauw, 2010) in R (R Core Team, 2015). Each radiocarbon sample is plotted next to its probability densities (2 σ confidence interval). Lithological units are (1) organic gyttja and decomposed (sapric) peat (277–207 cm), (2) dark brown sapric to hemic peat (207–190 cm), (3) brown fibric peat (190–160 cm); (4) hemic peat (160–120 cm); (5) black brownish-red sapric peat (120–43 cm), and (6) black and mineralized peat (43–0 cm). Regional chronozones are defined after Cubizolle et al. (2014). Prebor.: Preboreal.

Macrofossils

MRT partitioning helped to divide the macrofossil data into six local macrofossil assemblage zones (LMAZ), from 237 cm to the current surface (Figure 4).

Figure 4.

Selected macrofossils (n/15 mL) from the La Narce du Béage core A (SE Massif Central, France). The local macrofossil assemblage zones (LMAZ) were defined according to the MRT partitioning (Birks, 2014). For lithological units, see Figure 3. On the right, the taxonomic richness E(T) and standard error bars were computed in R with the vegan package (Oksanen et al., 2017; v.2.4-3). Br: bracts, BS: budscales, C: capsules, CS: catkin-scales, F: fruits, L: leaves, MP: mouthparts, MS: macrospores, N: needles, OO: oospores, R: roots, S: seeds, SL: Substantia lignosa (wood), Spg: Sphagnum capsules, St: stems, VR: vegetative remains.

LMAZ 1 (237–190 cm; 10,200–9680 cal. BP)

The first LMAZ offered numerous macrofossils from aquatic plants. Some are typical of oligo- to eutrophic conditions: macrospores of Isoëtes echinospora and Isoëtes lacustris (quillworts), oospores of Nitella opaca/syncarpa (charophytes), seeds of Alisma plantago-aquatica (water plantain), Potamogeton sp. (pondweed), and Sparganium sp. (bur-reed). The aquatic fauna consisted of Daphnia (micro-crustaceans), Plumatella (Bryozoans), Sialis (insects), and Trichoptera (caddisflies). In parallel, pioneering floating mats of bryophytes were composed of Calliergonella cuspidata (pointed spear-moss), Meesia longiseta (long seta hump moss), and Sphagnum sp. (Figure 4). Betula macrofossils were also abundant, possibly growing on or immediately around the wetland such as Betula nana (dwarf birch), Betula pubescens (downy birch), and Betula pendula (silver birch). Such great taxonomic richness characterized this terrestrialization step (Figure 4). Relatively small macro-charcoals were recorded during this phase (250–500 µm). Minerogenic particles (angular quartz fragments) and sclerotia of soil fungi (Coenococcum geophilum) eroded from the slopes were present, but only in very low concentrations (Figure 4).

LMAZ 2 (190–157 cm; 9680–9260 cal. BP)

During the LMAZ 2, the number of taxa showed a quick drop linked with the vanishing of many lacustrine plants. If Typha seeds still indicated a consistent water level, the maximum concentration of Sphagnum, Comarum palustre, and Carex vesicaria suggested a global increase of peatland and waterside communities. Alnus glutinosa (black alder), B. nana, and B. pubescens characterized riparian environments. Decreasing trends of aquatic fauna were also noticed, while numerous remains of hygrophilous Coleopteran fauna, such as Limnobaris sp. (data not shown), arrived. Macro-charcoals, minerogenic particles, and fungi remained at very low levels.

LMAZ 3 (157–131 cm; 9260–8570 cal. BP)

The LMAZ 3 showed a significant change: the main peat components were vegetative parts and seeds/fruits of Cyperaceae (including C. vesicaria). Ericaceae (heather) and Equisetum/Pteridophyta (horsetails/pteridophytes) showed a gradual increase at the end of this zone (Figure 4). Aquatic plants were absent. Macrofossils from aquatic fauna were also very scarce, whereas Coleoptera were continuously present. Local upland forests were represented by Gymnosperm macrofossils. The taxonomic richness is low (from 5 to 13 taxa) but punctuated by two peaks at 9000 and 8600 cal. BP.

LMAZ 4 (131–51 cm; 8570–5100 cal. BP)

A high concentration of Ericaceae and Equisetum/Pteridophyta remains defined the LMAZ 4 (Figure 4). Cyperaceae showed a decreasing trend and locally disappeared ca. 7250 cal. BP. Pinus sylvestris and Gymnosperm remains vanished as well at 7300 cal. BP. The upper part of the LMAZ 4 showed two major events with rather high concentrations of macro-charcoal; minerogenic particles, including flint; and soil fungi (C. geophilum). These two abrupt changes happened at 7500–7200 cal. BP (5550–5250 BC) and at 6750–6550 cal. BP (4800–4600 BC). The second episode was followed by high amounts of Substantia lignosa (wood), and Betula macrofossils were regularly found (Figure 4).

LMAZ 5 (51–17 cm; 5100–600 cal. BP)

The first part of this zone, LMAZ 5.1 (51–35 cm; 5100–3000 cal. BP), presented a very low diversity of taxa (6–10 taxa). A heather-sedge peat developed, including some Pteridophyta/Equisetum remains. Eriophorum vaginatum (cotton grass) seeds were present in the initial stage. Regular but isolated small macro-charcoals (250–500 µm) were found between 38 and 36 cm, that is, 3400–3100 cal. BP (1450–1150 BC). In the second sub-zone, LMAZ 5.2 (35–17 cm; 3000–600 cal. BP), high minerogenic and macro-charcoal contents were recorded. The highest charcoal phase was from 2500 to 1300 cal. BP (550 BC–AD 650) and some were determined as cf. Fagus sylvatica (beech) and cf. Abies alba (silver fir). Other macrofossils derived from moist and open environments, such as Juncus effuses/conglomeratus (common/compact), were regularly found since 1600 cal. BP (AD 350), and the biodiversity index increased to 16 and 18 taxa (Figure 4).

LMAZ 6 (17–8 cm; 600–20 cal. BP)

During the LMAZ 6, Cyperaceae and Sphagnum remains were the main peat components (Figure 4). Macrofossils derived from moist to wet open environments regularly occurred: J. effusus/conglomeratus, Potentilla cf. sterilis (cinquefoil), and Viola sp. (pansies). The latter was especially present since 250 cal. BP (AD 1700). Low concentrations of minerogenic particles and macro-charcoals were finally observed.

Palynology

Four local pollen assemblage zones (LPAZ) were recorded based on the CONISS analysis (Figure 5).

Figure 5.

Selected pollen, cryptogam spores, and NPPs from the La Narce du Béage core A (SE Massif Central, France). Values are expressed as percentages of the defined 100% pollen sum. Micro-charcoal values are expressed as influx (n/cm²/yr). Pale curves refer to a 10 times exaggeration. The taxonomic richness and standard error bars were computed in R with the vegan package (Oksanen et al., 2017; v.2.4-3). Local pollen assemblage zones (LPAZ) were determined by using the CONISS clustering method (Grimm, 1987). HdV: Hugo de Vries Laboratory (Amsterdam, The Netherlands) defining NPP types; Ms: microspores; Oo: oocytes; Sp: spores; Spm: spermatophores.

LPAZ 1 (237–72 cm; 10,200–6580 cal. BP)

The first zone was characterized by a high percentage of Corylus avellana (hazel, up to 60%), Quercus (oak, up to 22%), and Ulmus (elm, up to 12%), followed by the arrival of Tilia (lime) between 9100 and 8100 cal. BP (Figure 5). Few non-arboreal pollen (NAP) were generally observed (NAP < 10%). However, important micro-charcoal influxes occurred simultaneously with Artemisia (sagewort), Poaceae (grasses), Ranunculaceae, and Rumex acetosa-type (sorrel) ca. 10,100–8900 cal. BP. Coprophilous fungi spores were also recorded (Cercophora and Sporormiella). Aquatic taxa, such as Potamogeton/Triglochin-type, Sparganium-type, Isoëtes, Botryococcus (green algae), and Microdalyellia armigera (flatworm), were prevalent until 9500 cal. BP. Then, Cyperaceae and Sphagnum mosses successively developed between 9800 and 9100 cal. BP. Finally, Botryococcus increased again (highest percentages from 8500 to 7300 cal. BP).

LPAZ 2 (72–43 cm; 6580–4200 cal. BP)

This zone recorded a very high arboreal pollen sum (up to 95%). C. avellana, Quercus, Ulmus, and Tilia were the main timber species. Ericaceae shrubs, including Calluna vulgaris (heather) and Vaccinium-type, exhibit two maxima at 6500 cal. BP and then from 5500 to 4400 cal. BP (Figure 5). Micro-charcoal, Cerealia-type pollen, anthropogenic pollen indicators (API) such as Artemisia and Plantago lanceolata-type (buckhorn), Pteridium aquilinum (bracken), and coprophilous spores (Cercophora, Neurospora, and Sordaria) occurred during the same intervals, increasing the biodiversity (Figure 5). Botryococcus disappeared ca. 6700 cal. BP, whereas Sphagnum and swamp forest species culminated with a maximum of Betula ca. 6300 cal. BP (55%), followed by Vitis (wild grape), Rhamnus frangula (buckthorn), and Fraxinus excelsior (ash). Finally, Fagus sylvatica established between 5300 and 4600 cal. BP.

LPAZ 3 (43–32 cm; 4200–2500 cal. BP)

The arboreal pollen was dominated by very high values of F. sylvatica (up to 82%). Other arboreal taxa showed low percentages. This low taxonomic richness (<20 taxa) also indicated a dense and local forest. At the end of this zone, Abies alba increased (after 2800 cal. BP). Only few herbs (mainly Poaceae, Ranunculaceae, and Artemisia) were present because of the tree cover indicated by the high AP values (>95%).

LPAZ 4 (32–0 cm; 2500 cal. BP–the present)

This zone was characterized by low arboreal pollen values (50–65%) where Abies alba and then P. sylvestris-type were prevalent. Poaceae (>20%) and all API increased at the beginning of this zone. A regular presence of Cerealia-type pollen, Artemisia, P. lanceolata-type, Cichorioideae (aster family), Caryophyllaceae (pink family), Chenopodiaceae (chenopods), and Rumex was highlighted after 2300 cal. BP. Maximum values were reached after 1600 cal. BP (AD 350), with the presence of Secale cereale (rye). Centaurea cyanus (cornflower) was recorded for the last 900 years. Shrubs, from pastured lands, were well represented during the whole zone (C. vulgaris, Vaccinium-type, Juniperus-type). Spores of coprophilous fungi were found for the last 500 years. High micro-charcoal influxes were also evidenced from 2300 to 1200 cal. BP (Figure 5). Pollen from cultivated trees such as Juglans regia (walnut) and Castanea sativa (chestnut) were recorded continuously since ca. 1900 and 1500 cal. BP, respectively. Wetland species were dominated by Cyperaceae and by acidophilus taxa such as Sphagnum and P. aquilinum. Botryococcus and M. armigera worms returned during two episodes at 900 cal. BP (AD 1050) and at 550–300 cal. BP (AD 1400–1650).

Discussion

Early Holocene terrestrialization

The transition from lacustrine gyttja deposits to peat occurred between 10,200 and 9800 cal. BP at the La Narce du Béage mire, with a high accumulation rate (Figure 3). This age is posterior to the end of the lake infilling estimated ca. 10,800–10,300 cal. BP on the core D from the same site (Dendievel et al., 2015). This difference is very likely because of a more central location of the core A in the former lake basin (Figure 1(c)). Such gradual infilling is part of typical terrestrialization processes (Payette and Rochefort, 2001). Former Late Glacial lakes are frequent over the French Massif Central, such as Le Suc near La Godivelle in the Chaîne des Puys (Pons et al., 1987), La Taphanel in the Cantal (Ponel et al., 1991), Ribains and Limagne marshes (De Beaulieu et al., 1984; Reille and de Beaulieu, 1988; Tourman, 2007), or the Les Roustières paleolake in Aubrac (Gandouin et al., 2016; Ponel et al., 2016). These post-glacial ecosystems generally emerged between 11,000 and 8500 cal. BP but, conversely, some sites showed opposite trends like the Lac du Bouchet (Haute-Loire, France) or the Saillant Lake (Cantal, France), where the aquatic flora, Isoëtids included, became more prevalent (De Beaulieu et al., 1984; Reille et al., 1985). So, we propose that precise timing and vegetation dynamics are strongly affected by local conditions on each site. In order to reconstruct and discuss hydro-ecological changes at La Narce du Béage, we combined and compared the macrofossil and palynological data (Figure 6).

Figure 6.

Early to middle Holocene combined diagram of selected macrofossils (bars, n/15 mL) and palynological taxa (dark gray curves, %) from the La Narce du Béage core A (SE Massif Central, France). Light gray curves refer to a 10 times exaggeration of palynological percentages. The lithological data come from Figure 3: (1) organic gyttja and sapric peat, (2) sapric to mesic peat, (3) fibric peat, (4) mesic peat, (5) sapric peat. Local macrofossil and pollen assemblage zones (LMAZ and LPAZ) and taxonomic richness curves refer to Figures 4 and 5. The final infilling of the Late Glacial lake (lowering and terrestrialization) is highlighted by a gray bar. BR: bracts; BS: bud-scales; F: fruits; L: leaves; S: seeds; Spg: sporangium; St: stems.

From the lake to the mire (10,200–9200 cal. BP)

During the early Holocene, the oligotrophic ecosystem of La Narce du Béage hosted a very high taxonomic diversity (Figures 6 and 7(a)). Among the major lacustrine plants, Isoëtes species were highlighted by both palynological and macrofossil approaches. The latter was really efficient to determine the presence of I. echinospora and I. lacustris, growing in a water depth of 1–2 m in the sublittoral zone (Hannon and Gaillard, 1997). An Isoëtes phase also existed during the early Holocene in other West-European mountain lakes (Guenet and Reille, 1988; Guiter et al., 2005; Reille and de Beaulieu, 1988), but, at La Narce du Béage, this short event ended with the terrestrialization of the lake. Also found, Nitella opaca/syncarpa requires deeper lake conditions as they are growing on fine silts and muds generally with Myriophyllum alterniflorum (Haas, 1994; Murphy et al., 1994). Micro-algal colonies of Botryococcus were abundant. Aquatic animals such as Cladocera (Daphnia sp. in particular), Sialis sp., or Trichoptera were common (Figures 4 and 6) and Turbellarian worms (M. armigera) were also present in this lacustrine ecosystem (Figure 5).

Figure 7.

Reconstructed vegetation during the terrestrialization process at La Narce du Béage (SE Massif Central, France) from 10,200 to 9200 cal. BP. The E-W profile is extracted from Dendievel et al. (2015). The taxonomic richness refers to E(T) computed on the basis of the macrofossil and pollen record (see Figures 4 and 5). (a) Mesotrophic to eutrophic lake with littoral mire: (1) Swamp forest (Alnus glutinosa, Betula pendula, B. pubescens); (2) understory mire species: B. nana; (3) peat mosses (Calliergonella cuspidata, Meesia longiseta, Sphagnum); (4) mire and waterside flora: (4a) Carex vesicaria, Comarum palustre; (4b) Alisma plantago-aquatica, Sparganium sp.; submerged vegetation of (5) Ranunculaceae, (6) Potamogetonion p.p., Myriophyllum alterniflorum, and Nitella; (7) micro-algal communities (mainly Botryococcus). (b) Sphagnum peatland: (1) Alnus glutinosa; (2) Sphagnum hummocks; 3) Acidophilus communities: (3a) B. nana; (3b) C. palustre; (4) Cyperaceae (sedges) vegetation; (5) remaining water lenses with Potamogeton sp. (c) Heathland: (1) A. glutinosa; (2) Pteridophyta/Equisetum communities; (3) Ericaceae.

Since 10,200 cal. BP, frequent findings of Potamogeton sp. (endo- and pericarps) validated high percentages of Potamogeton/Triglochin-type pollen, suggesting meso- to eutrophic conditions (Figure 6). The local abundance of Ranunculaceae pollen certainly referred to extended water crowfoot populations as demonstrated by macrofossils. A reed vegetation, made of Sparganium sp., Typha sp., and Alisma plantago-aquatica, formed an ecotone in front of the littoral mire (Figure 7(a)). Peat mats were probably floating on the water surface as revealed by moss remains from Meesia longiseta and Calliergonella cuspidata (Figures 4 and 6), usually found in more or less eutrophic fens (Hugonnot et al., 2012). Sphagnum sp. was documented by both palynological (spores) and macrofossil findings (stems, leaves, and capsules). Fruits of Betula pendula, B. pubescens, and B. nana were frequent, suggesting a forested swamp along the lake. Here, birch macrofossils were systematically associated to more than 7% of Betula pollen, so this limit implied the local presence of birches (Figure 6). From 9600 cal. BP onward, the ecosystem progressed toward a Sphagnum peatland. This phase ended the infilling of the former lake of La Narce du Béage (Figures 6 and 7(b)). Alnus glutinosa and B. pubescens macrofossils suggest a forested mire (Alnion glutinosae), surrounded by a belt of Carex vesicaria and Typha sp. (Figures 4, 6, and 7(b)).

Thus, despite the presence of some peat components in the lithology since 10,200 cal. BP, our paleoecological analysis describes a lake water shift from oligo- to eutrophic conditions with a littoral mire during the early Holocene (Figures 7(a)). Aquatic and terrestrial taxa could be considered as an integral part of the terrestrialization at La Narce du Béage. This infilling process seems to be closely linked to low water tables and followed by high erosion on the margins, as revealed in core D (Dendievel et al., 2015; Figure 8). Interestingly, a warming is evidenced by the appearance of Alisma plantago-aquatica seeds ca. 10,200 cal. BP, revealing a temperature increasing to, at least, 13°C in July (Gaillard and Birks, 2007). It fits well with the rise in temperature recorded by the chironomid content of Les Roustières (Aubrac) ca. 10,000 cal. BP at 1200 m a.s.l. (Gandouin et al., 2016). Our results fit also very well with a major change from a cold/wet event (10,400–10,200 cal. BP) to more temperate conditions in Northern and Central Europe after 10,200 cal. BP (Björck et al., 2001).

Figure 8.

Synthesis of hydro-ecological changes recorded between 10,200 and 7500 cal. BP on the cores A and D from La Narce du Béage (SE Massif Central, France). The final infilling of the Late Glacial lake (lowering and terrestrialization) is highlighted by a gray bar. The dotted lines and the gray bar represent periods of water table changes. Early Holocene Massif Central lakes: BBE: Bonnecombe; BLP: Brameloup; BOR: La Borie; BOU: Lac du Bouchet; COL: Collanges lake; FRE: Freycinet lake; GIM: Gimberte lake; LIM: Limagne marsh; PEC: Le Pechay; SAI: Saillant lake.

Another important point of the discussion is the local presence of Betula nana during the Preboreal and the Boreal. We did not try to differentiate the pollen from Betula species because of the difficulties remaining concerning unambiguous criteria and as hybridization between species can occur (Birks, 1968; Clegg et al., 2005; Wang et al., 2014), but the B. nana presence is validated by typical fruits (Figure 9). This is very remarkable because this shrub is typical of circum-arctic or alpine environments and often considered as a post-glacial relict in Western and Central Europe (Ashburner and McAllister, 2016). Its distribution is now mainly limited to the North of Europe, above 55°N, with a secondary refuge zone in Switzerland, where the plant is very rare (De Groot et al., 1997). In France, only two locations of B. nana are assured: (1) in the Jura and (2) north of the Cevennes National Park (Southern Massif Central) from Lajo to Chanaleilles (Coste and Soulié, 1919; as well as own field observations in 2017). The presence of B. nana at Le Béage until 9350 cal. BP highlights a more southern distribution of this species than today during the early Holocene, which is an important result to better understand the biogeographical evolution of this species (Wang et al., 2014).

Figure 9.

Photographs of (a) Betula nana and (b) Betula pendula extracted from the La Narce du Béage core A (Massif Central, France). The scale is the same for both pictures.

Finally, based on our lithological and paleoecological data, we propose to date the terrestrialization of the middle of the La Narce du Béage basin to 9800–9200 cal. BP (Figure 8). At the same period, evidence with comparable trends also existed at several sites on the French Massif Central (Cubizolle et al., 2012). A very close parallel timing was proposed for the Limagne and the Brameloup infillings (Figures 1 and 8). La Narce du Béage seems therefore included into a regional wave of terrestrialization. At a larger scale, this event fits also very well to a major terrestrialization phase in Northern and Central Europe (Figure 8). According to Jones and Yu (2010) and to Ruppel et al. (2013), such widespread change clearly emanates from the global warming of the early Holocene, producing major hydro-ecological changes at local scales.

Heathland development (9200–6500 cal. BP)

After 9600 cal. BP, the presence of Sphagnum sp., together with other low-pH tolerant plants (B. nana, C. palustre), implies an acidification of the wetland. It progressed toward a local Ericaceous heathland, with Equisetum and/or Pteridophyta, after 9200 cal. BP (Figures 7(c)). Roots and woody fragments frequently occurred, together with high arboreal pollen values (Figures 4 and 5). We assume that the size of the catchment area was critical for the local development of a heathland. Indeed, when the catchment soils were stabilized, an acidification of the internal water is often noticed in similar cases (Birks et al., 2000). At the same time, increasing pollen percentages of Tilia were observed (Figure 6). Such chronological link between the local acidophilous and lowering water table with the establishment of lime may reflect hydro-ecological consequences of a global climatic change (Giesecke et al., 2017). This succession also happened in parallel to low lake levels in the Jura Mountains (Magny, 2013), to the retreat of Alpine glaciers (Joerin et al., 2006), and to a weak hydro-sedimentary phase in the Rhône and the Loire river basins (Berger et al., 2008; Castanet et al., 2014).

Thereafter, the return of Botryococcus algae together with a major change in the minerogenic accumulation in core D announced a more humid period around 8300 cal. BP (Figures 6 and 8). On the basis of a close time-based correlation with the so-called ‘8.2 ky climatic deterioration’ recorded in Europe and worldwide (Alley and Ágústsdóttir, 2005; Castanet et al., 2014; Haas et al., 1998; Kofler et al., 2005; Magny, 2013), we propose to relate this humid and erosional event on the Ardèche Plateau with local consequences of this widespread humid and cold event.

Detecting human impact and vegetation changes on the Mézenc Plateau

Early human impact on the early to middle-Holocene environments may be evidenced by comparison of our paleoecological results with archeological evidence. Local-to-regional human activities were highlighted according to the interpretation of macrofossil, pollen, and NPP taxa deriving from cultivated and pastured lands (cf. Behre, 1981; Brun, 2011). Prehistorical increases in macro-charcoal abundances (>250 μm, and up to charcoal > 1 mm) are also highly significant, as these may be the result of local anthropogenically induced fires (Rius et al., 2009). However, the attribution of charcoal to fire incidents produced by human action and climatic change (drought) or derived from lightning events remains difficult. For the core A, a graphical correlation between macro-charcoal (>250 μm) and micro-charcoal (7–150 μm) data implies the record of major local fires (Figure 10). The Spearman-rank correlation test provided a relative low ρ (ρ = 0.4, highly significant p value = 0.99) because of the difference of size and scarcity between macro-charcoals and micro-charcoals even if they are synchronously recorded. The presence of pollen from pioneer species (e.g. Betula sp.) could also indicate the afforestation of recently abandoned lands. In addition, erosion markers (soil fungi remains or minerogenic particles) may be highly relevant for the reconstruction of landscape disturbances.

Figure 10.

Late Mesolithic to Bronze Age (8600–3000 cal. BP) combined diagram with selected macrofossils (bars, n/15 mL) and palynological taxa (dark gray curves, %) from the La Narce du Béage core A (SE Massif Central, France). Micro-charcoal (7–150 µm) values are expressed as influx (n/cm²/yr). Macro-charcoal concentrations are represented by curves (250–500 µm) and ‘+’ indicates large macro-charcoal occurrences (500 µm to >1 mm). The local macrofossil and pollen assemblage zones (LMAZ and LPAZ) and taxonomic richness curves refer to Figures 4 and 5. Vertical bars highlight phases of agro-pastoral impact during the middle Holocene. L-R: late to recent (at the end of the Mesolithic and of the Neolithic).

Neolithic – 7450–4150 cal. BP (5500–2200 BC)

Pollen grains of Cerealia-type (0.4%) and other human impact indicators (1.8%) – such as Artemisia and Plantago lanceolata-type – were observed at 7600 cal. BP and related to a high pollen taxa diversity (Figures 5 and 9). High minerogenic inputs and sclerotia of soil fungi (Coenococcum geophilum) evocated soil erosion. These findings were also synchronous with a major fire period characterized by increases in micro- and macro-charcoal data between 7700 and 7300 cal. BP (Figure 10). In Western and Central Europe, north of 44°N, this period is usually reconstructed as wet and cold (Berger et al., 2008; Haas et al., 1998; Joerin et al., 2006; Magny, 2013) with few natural fires (Vannière et al., 2011). On the contrary, such paleoecological succession could be because of an early Neolithic human impact, with short-lived forest burning and local cereal cultivation. Indeed, human frequentation is highly probable on the studied area: archeological material suggested coeval agro-pastoral and hunting activities (Figure 2(a); Beeching, 2013; Chomette, 1989; Philibert, 1982). In addition, Cardial pottery, dated to 7600–6700 cal. BP, was also found near the Mount Mézenc (Figure 2(b); Daugas and Raynal, 1989). This first agro-pastoral impact, dated to 7700–7300 cal. BP (5750–5350 BC), is also consistent with other agricultural initiatives in Central France (review by Miras et al., 2004) and, at a broader scale, in South-Eastern France (Shennan et al., 2013). The additional discovery of allochthonous flint shards at La Narce could be interpreted as an evidence for the frequentation of the mire (Figure 10; see also Dendievel, 2017).

Another charcoal-rich layer comprising, among others, high percentages of Cerealia-type, Plantago lanceolata-type, Artemisia, and Ranunculaceae (buttercup) pollen was recorded from 6800 to 6420 cal. BP (Figure 10). Soil fungi sclerotia (C. geophilum) indicated watershed erosion. Spores from coprophilous fungi (Sordaria-type) were also present in small percentages suggesting the passage of livestock or wild animals. Short-lived increases of Calluna vulgaris and Vaccinium-type pollen also support possible pastoral practices on the wetland and its catchment. These changes coincide with a sharp decrease of the arboreal pollen, in particular Quercus (27–9%; Figure 10). Around 6300 cal. BP, high concentrations of wood (S. lignosa) and a synchronous high proportion of Betula sp. pollen (55%) point to a local pioneering stage after this event. Actually, this typical succession may reveal a brief phase of Neolithic disturbances (4850–4470 BC) with local agro-pastoral frequentation, probably followed by an early successive vegetation phase. These paleoecological evidence of Neolithic activities (end of early Neolithic and/or beginning of the middle Neolithic 1) are intriguing because regional archeological sites seem to have been rare at that time (Jallet and Georjon, in press). However, numerous paleoecological studies present anthropogenic impacts at the same period on other parts of the French Massif Central (Cantal, Chaîne des Puys, Lozère Mount). In Central Europe as well, this period is marked by human impact (pastoralism) which could be defined as one of the main factors impacting environment (Schwörer et al., 2014). Interestingly, a warming climatic trend was synchronously detected in the Alpine Hinterburgsee, Krummgampen, and Brunnboden sediment cores (Heiri et al., 2004; Kofler et al., 2005) and might suggest a mild climate favoring agro-pastoral developments in mountain areas.

Another human impact is palynologically recorded from 5500 to 4250 cal. BP (3950–2300 BC), given the presence of Cerealia-type pollen together with Artemisia, P. lanceolata-type, Ranunculaceae, and NAP increases (Figures 5 and 10). Micro-charcoal particles (all sizes) frequently occurred during the whole period, suggesting local-to-regional fires (Figure 10). In parallel, a sharp decrease of Quercus (26% to less than 10%) might underline some clearings affecting the mixed-oak forest. These vegetation changes took place from 5500 to 4250 cal. BP, showing a higher taxa diversity and jagged pollen curves (Figure 5). Thus, it is likely that several short-lived phases of human impact existed. However, our resolution does not allow to further discuss it, so we prefer to interpret these data as general agro-pastoral disturbances during the late and the recent Neolithic, a period also well documented by archeology (Figure 2(b); see also Daugas and Raynal, 1989). It matches with the transition from late Chassean to new Neolithic cultures, such as the ‘Ferrières group’, involved in late Neolithic trade routes between the Pyrenees, the Southern part of the Massif Central, the Alps, and the Jura (e.g. Vaquer et al., 2015). From a regional point of view, P. lanceolata-type, Rumex, and Artemisia pollen were also found at Saint-Front Lake and at Chaudeyrolles during the Late Atlantic chronozone and might support these wide-ranged human disturbances (Andrieu-Ponel et al., 1995; Couteaux, 1984). Landscape management was also well documented in the Puy-en-Velay Basin from the middle Neolithic 2 to the recent Neolithic transition, where einkorn (Triticum monococcum), emmer wheat (Triticum dicoccum), and flax (Linum usitatissimum) were cultivated (Daugas and Raynal, 1989; Rialland and Letterlé, 2013). Other synchronous human impacts were also documented from the north of the Massif Central (l’Etui bog: Cubizolle et al., 2014) to the south (Countrasts bog: Servera Vives et al., 2014).

Finally, there is almost no paleoecological evidence of human impact during the Bronze Age (Figures 10 and 11). This lack of human disturbance could be due to the establishment of a local and dense beech forest (Fagus sylvatica; Figure 5). Such beech forest constitutes one of the best markers for the beginning of the Subboreal chronozone in the Velay region according to M. Reille and J.-L. de Beaulieu (1988). Indeed, the local establishment of F. sylvatica occurred from 5300 cal. BP (frequent occurrences) to 4800 cal. BP (continuous curve) and fits well with the expansion of Fagus in Western Europe (Giesecke et al., 2017). In addition of this dense forest cover, we also noticed a sharp decrease of the number of archeological sites, eventually suggesting a demographic collapse. Many similar cases are also known in Southern France, in the Languedoc and in the Western Alps (Shennan et al., 2013; Walsh et al., 2014). Therefore, a correlation between this large population migration and new environmental impacts reported from the Western Pyrenees to the Massif Central and to the Central Alps (Dietre et al., 2017; Tinner et al., 1996; Wick et al., 2003) is suspected by Carozza et al. (2015).

Protohistory, Roman Times, and Middle Ages – 4150–450 cal. BP (2200 BC–AD 500)

The last 4200 years were characterized by the massive expansion of F. sylvatica (up to 82% between 3700 and 2900 cal. BP) and a consecutive low taxonomic diversity (Figure 11). The rise of Abies alba occurred also during this period but its local establishment at La Narce du Béage occurred very late compared with other places in the French Massif Central (Cubizolle et al., 2014; Giesecke et al., 2017).

Figure 11.

Bronze Age to the present (4000 cal. BP–current time) combined diagram with selected macrofossils (bars, n/15 mL) and palynological taxa (dark gray curves, %) from the La Narce du Béage core A (SE Massif Central, France). Micro-charcoal (7–150 µm) values are expressed as influx (n/cm²/yr). Macro-charcoal concentrations are represented by curves (250–500 µm) and ‘+’ indicates large macro-charcoal occurrences (500 µm to >1 mm). The local macrofossil and pollen assemblage zones (LMAZ and LPAZ) and taxonomic richness curves refer to Figures 4 and 5. C: capsules; F: fruits; MC: modern and contemporaneous periods; RT: Roman Times; S: seeds.

A sharp decrease of F. sylvatica (80–20%) happened then from 2650 to 2050 cal. BP, while NAP values massively rose (Figure 10). An increase in micro-charcoal (7–500 µm) and minerogenic inputs argues for coeval clearings and erosion. This picture is a good overview for the landscape opening of the Iron Age. Spots of Cerealia-type pollen were frequent during this phase and occurred as a continuous curve. Accompanying plants (Artemisia, Caryophyllaceae) report local cereal cultivation, whereas Calluna vulgaris, Plantago lanceolata-type, and Vaccinium-type pollen indirectly suggests livestock grazing (Figure 11). This human impact coincides with a regional increase of the number of archeological sites (Figure 2(b)). If first Iron Age settlements are well known in the Upper Loire and in the Middle Rhône Valleys (Delrieu et al., 2015; Voruz and Treffort, 2014), local Gallic presence around the La Narce du Béage mire is also evidenced by the recent discovery of a nearby archeological site dated to 2360–2200 cal. BP at Le Crouzet-La Veysse (Dendievel et al., 2016). The discovery of basaltic millstones clearly implies flour production. Other recent archeological excavations, 27 km to the east on the Aric Plateau, have also highlighted a landscape opening during the middle second Iron Age (Maza and Collombet, 2015). Examples of middle La Tène occupations are very rare in Europe, probably because of the current state of the art. However, such anthropization stages on middle mountains were inevitable within the development of Gallic territories during the late Protohistory (Bradley et al., 2016).

During Roman Times, a loss of upland forests is attested by high macro-charcoal concentrations and fire events affecting both beech and fir stands (Figure 11). After 1600 cal. BP (AD 350), increases of Cerealia-type, Secale cereale, and other anthropogenic pollen indicators connote local winter crop cultivation, in particular rye. Marked minerogenic concentrations (up to 1200 particles/15 mL) also hint at erosion of bare soils, such as fields. Reconstructed biodiversity continue to rise. We consider Juncus effusus-conglomeratus occurrence as a record for trampled soil. Its coeval presence with C. vulgaris and Vaccinium-type pollen suggests pastured lands. Walnuts (Juglans regia) and chestnuts (Castanea sativa) were probably present at a regional scale (Figure 11). Little is generally known about the Late Antiquity/Early Middle Ages transition in middle mountain contexts, but fortunately, this period is documented at Pérot, next to the mire (Figure 2(c); see Dendievel et al., 2016). Major human-induced environmental changes are also observed in other regional sites, such as at nearby Saint-Front Lake where massive minerogenic and Cinnamyl inputs derived from deforestation and agrarian erosion (Rhoujjati, 1995).

Evidence of agro-pastoral practices increased after 950 cal. BP. Stable anthropogenic pollen indicators (>10%) and total NAP percentages (>35%) confirm ongoing agriculture and livestock grazing. The assemblage of Cerealia-type, S. cereale, and Centaurea cyanus pollen characterizes agricultural strategies based on rye cultivation (Figure 11). Moreover, twigs of broom (Cytisus scoparius) and rye straw were commonly associated in the roof of the so-called ‘paillisse’ farms. Juniperus stands and monks’ archives also underlined extensive sheep and cattle breeding (Bréchon, 1998, 2000).

Finally, Cyperaceae, Sparganium, green algae (Botryococcus), and M. armigera indicated two very humid steps: (1) after 900 cal. BP (AD 1050) and (2) from 550 to 300 cal. BP (AD 1400–1650). The latest one could be related to the direct and local consequences of the ‘Little Ice Age’, particularly well addressed by geomorphological studies but rarely highlighted in terms of hydro-ecological consequences on headwater wetlands (Defive et al., 2017, 2019). Numerous terrestrial macroremains, such as Juncus effusus-conglomeratus, Viola sp., and Potentilla cf. sterilis, are recorded during the last centuries and decades. This major increase in local biodiversity was probably because of direct disturbances on the mire and its surroundings.

Conclusion

The paleoecological study (macrofossils and palynology) of the peat deposits of La Narce du Béage (SE Massif Central, France) provided an overview of a mire ecosystem evolution during the Holocene. We proposed a reconstruction of the hydro-ecological changes (aquatic and mire vegetation, minerogenic inputs, accumulation rate) involved in the terrestrialization steps of a former Late Glacial lake. Such phenomena, implying a water-level lowering, erosion, and acidification, helped to understand similar infilling trends in European wetlands during the early Holocene warming (Ruppel et al., 2013). Local lake-level changes inferred from these paleoecological data were also set into a broader context of Western European and worldwide climatic phases.

This work integrated recent archeological results to provide evidence on the timing of Neolithic frequentations on the Béage Plateau. As a major result, human impact is clearly identified by the early presence of typical macrofossil data (erosion, flints, local fires, pioneering vegetation), anthropogenic pollen indicators, and NPPs (coprophilous fungi spores mainly). We reconstructed three short-lived fire events and forest clearings followed by agro-pastoral activities at 7700–7300 cal. BP (early Neolithic), at 6800–6420 cal. BP (middle Neolithic 1), and from 5500 to 4250 cal. BP (late and recent Neolithic). Forest regenerations involving birches (Betula sp.) characterized post-human impact phases. It looks like that these human disturbances on wetlands and landscape management could be related to widespread Neolithic demographic phenomena, affecting also a large range of NW Mediterranean ecosystems.

The combination of macrofossils, pollen, NPPs, and archeology was also useful to highlight long-term environmental changes at both local and regional scales since the Iron Age, and especially during the middle La Tène period (ca. 2300 cal. BP, that is, 350 BC). We demonstrated extensive forest clearings, cultivation, livestock grazing, and a rising taxa diversity. Increasing anthropogenic pressure until the 7th century AD and then during the Middle Ages produced an extremely open environment, devoted to rye cultivation and extensive livestock grazing. Global climatic and human impact on the hydro-ecology of the La Narce du Béage peatland rise the question of the future of limnogenous mires, which are relatively frequent but intensively used for pastoral or agricultural purposes in Southern and Western Europe (Joosten et al., 2017).

Footnotes

Acknowledgements

The authors express their gratitude to the Teyssier family for allowing us to core the mire. We thank the colleagues working on the Massif Central for their relevant help during fieldwork and discussions (J. Argant, E. Defive, F. Delrieu, Y. Miras, J.-P. Raynal, D. Rius). Special thanks go to V. Delvigne (Liège University, Belgium) and A. Lafarge (Montpellier University, France) for the determination of the flint shards and to A.S. Schwarz (University of Innsbruck, Austria) for the identification of some macro-charcoal particles. We also thank the two anonymous reviewers for their valuable comments that have improved our research paper.

Funding

This study was funded by a MESR PhD-scholarship for A.-M. Dendievel and by the AGES program (2013–2016), supervised by C. Castanet and co-founded by the European Union. Material support and facilities were provided by the Universities of Innsbruck (Institute of Botany) and by the University Jean Monnet of Saint-Etienne (UMR CNRS 5600 EVS-ISTHME).

ORCID iD

Benjamin Dietre

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