Quaternaire, 27, (1), 2016, p. 15-25

Manuscrit reçu le 25/07/2015, accepté le 06/10/2015

ENVIRONMENTAL CHANGES IN THE MOROCCAN WESTERN RIF MOUNTAINS OVER THE LAST 9,000 YEARS

n

Rachid CHEDDADI1, Ouafaa BOUAISSA1, Ali RHOUJJATI2 & Laurent DEZILEAU3

ABSTRACT

The aim of this study is to reconstruct the impact of past environmental changes in the Moroccan Rif mountainous ecosys-tems. We collected an 8.5 m sedimentary core in the western part of the Rif and analyzed its palynological content, micro-remains of fossil charcoal, particle size, and content of several chemical elements. The chronology of the record is based on ten 14C datings that indicate a continuous time span of 9,000 years. This multidisciplinary approach allowed us to reconstruct both the environmental evolution of the catchment area and the vegetation dynamics in the surrounding landscape. Besides the fossil record, we collected a series of surface samples along a transect running from Talassemtane National Park, where fir populations are currently protected, to the study site in order to determine the dispersal capacity of different plant taxa. Based on the modern observations, the fossil pollen data show that despite its closer range today to the study site than Cedrus atlantica (closest populations at ca. 20 km), Abies maroc-cana (populations at ca. 10 km) did not reach the studied site during the Holocene. At the same time, Cedrus atlantica was present and even dominated the landscape between 8,000 and 6,000 BP. After 6,000 BP, other species more tolerant to annual drought, such as evergreen oaks, expanded gradually. The early Holocene period was wet enough to promote a maximum expansion of cedar forest. The Atlas cedar disappears from the fossil record after 2,000 BP due to an increased human pressure. Particle size analysis and chemical elements are particularly relevant for reconstructing the landscape changes and for depicting the human impact. Their changes after 2,000 BP are well correlated with the forest cover decline.

Keywords: Holocene, Cedrus atlantica, Rif Mountains, Morocco

RÉSUMÉ

CHANGEMENTS ENVIRONNEMENTAUX DANS LES MONTAGNES DE L’OUEST DU RIF MAROCAIN DEPUIS 9 000 ANSLe but de cette étude est d’analyser l’impact des changements environnementaux et climatiques subis par les écosystèmes

montagnards du Rif marocain à partir d’une séquence sédimentaire de 8.5 m prélevée dans un marais. Les analyses réalisées incluent le contenu palynologique, la granulométrie, la détermination des éléments chimiques par XRF et le comptage des fragments de char-bons de bois fossiles. La chronologie de la séquence, basée sur dix datations 14C, indique qu’elle couvre les 9 000 dernières années en continu. Cette approche multidisciplinaire nous a permis de reconstituer la dynamique de la végétation ainsi que l’évolution du bassin versant du site étudié. Outre l’enregistrement fossile, nous avons collecté une série d’échantillons de surface le long d’un transect partant du parc national de Talassemtane, où les populations de sapins sont actuellement protégées, et rejoignant le site de l’étude afin de déterminer la capacité de dispersion des différents taxons présents dans l’enregistrement fossile. Tenant compte des observations actuelles, les données palynologiques fossiles suggèrent que les populations d’Abies maroccana (à environ 10 km du site d’étude), malgré leur distance plus proche que celles de Cedrus atlantica (à environ 20 km), n’ont à aucun moment au cours de l’Holocène atteint le site étudié. Inversement, des populations de cèdre ont migré de manière significative vers le site étudié et ont même dominé le paysage entre 8 000 et 6 000 BP. Après 6 000 BP, d’autres espèces plus tolérantes à la sécheresse, comme les chênes verts, se sont progressivement installés. Le début de l’Holocène était assez humide pour favoriser une expansion maximale des forêts de cèdres qui régressent progressivement pour disparaître littéralement de l’enregistrement fossile après 2 000 BP en raison à la fois d’une augmentation de l’aridification et d’une pression humaine plus accrue. L’utilisation des données granulométriques et celle des éléments chimiques sont particulièrement pertinentes pour reconstituer les changements environnementaux ayant impacté le bassin versant et notamment pour déceler l’impact humain. Leur variation après 2 000 BP est bien corrélée avec le déclin du couvert forestier.

Mots-clés : Holocène, Cedrus atlantica, Rif, Maroc

1 Corresponding author - Université de Montpellier, Institut des Sciences de l’Evolution, UMR UM-CNRS-IRD 5554, place Eugène Bataillon, bâtiment 23, FR-34095 MONTPELLIER. Courriel: rachid.cheddadi@um2.fr2 Université Cadi Ayyad, Faculté des Sciences et Techniques (URAC 42), boulevard Abdelkrim Khattabi, BP 511, Gueliz Marrakech, MAROC. Courriel: a.rhoujjati@uca.ma3 Université de Montpellier, Géosciences Montpellier, CNRS/UM, UMR 5243, place Eugène Bataillon, bâtiment 23, FR-34095 MONTPELLIER. Courriel: Laurent.Dezileau@univ-montp2.fr

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1 - INTRODUCTION

Mediterranean forests are regarded as biodiversity hotspots with an important number of endemic species (Myers et al., 2000; Bolle 2003; Blondel et al., 2010). They are highly heterogeneous due to the impact of different bioclimatic and geomorphologic factors. Medi-terranean forests are also often fragmented either natu-rally or because of human activities (Barbero et al., 2001; Quézel & Médail, 2003; Linares et al., 2011). Moroccan climate undergoes influences from the Atlantic Ocean, the Mediterranean Sea, and the southern Sahara. Such climate has shaped the ecosystems and has probably contributed to maintaining high species diversity over the past.

Environmental changes over the last few millennia in Morocco have been recovered from different proxies such as pollen (Reille 1976, 1977; Lamb et al., 1989, 1991, 1995, 1999, Cheddadi et al., 1998, 2009; Nour El Bait et al., 2014; Muller et al., 2015) and geochemistry (Benkad-dour 1993; Zeroual, 1995; Rhoujjati et al., 2010; Damnati et al., 2012; Reddad et al., 2013). These studies have focused mainly on the Middle and the High Atlas moun-tains. Two studies on the late Holocene were carried out in the Rif Mountains by Reille (1977) and Muller et al., (2015) who revisited Reille’s work. These studies provide valuable information on ecosystem changes and species dynamics that may help defining key areas for conser-vation (Zanchetta et al., 2013). Besides weak historical information, the Rif Mountains harbor a high contingent of endemic species (Benabid, 1984) that largely rationa-lize new paths for environmental studies in this area.

In fact, biodiversity hotspots are biogeographic zones with high biodiversity wealth and high endemism, which are particularly threatened by human activities. Northern Morocco is part of these biodiversity hotspot areas iden-tified all around the globe (Myers et al., 2000). The Rif Mountains fit the biodiversity hotspots criteria with two temperate conifer tree species: Abies maroccana Trabut (Moroccan fir) and Cedrus atlantica (Manetti ex Endl.) Carrière (Atlas cedar). Today, both species are considered endangered according to the IUCN Red List (IUCN, 2014). In Morocco, national parks and reserves such as Tazekka in the Middle Atlas and Talassemtane in the Rif exist, as well as in situ long term protection and preser-vation of endangered species. However, some popula-tions underwent and are still undergoing multiple stresses through the excessive exploitation of their timber and the grazing of their sprouts by cattle. This is in addition to the observed increasing aridity over the last few centuries (Till & Guiot, 1990), which is leading to a reduction of their natural range and even to the full extinction of several populations in the Rif Mountains (Benabid, 1984, 1991).

The time frame is an important element as it helps identifying and analyzing vegetation and landscape dynamics that may be related to climate changes (Sadori et al., 2011). The low number of well-dated fossil records from northern Africa prevents us from analyzing the spatial patterns and ultimately making comparisons with southern Europe and the Middle East edges of the Medi-terranean.

In the Mediterranean basin, the past environmental changes have been associated with either climatic trends or human activities (Carrión et al., 2004; Di Rita & Magri, 2009; Finné et al., 2011; Mercuri et al., 2011; Damnati et al., 2012). During the late Holocene a progressive reduc-tion in the annual amount of precipitation is observed in the fossil records, which leads to critical climate oscil-lations and an increasing ecosystem instability (Mercuri et al., 2012; Sadori et al., 2013). This has been streng-thened by the more recent human activities (Sadori et al., 2013; Cheddadi et al., 2015). Throughout the Medi-terranean basin, networks of well-dated fossil records would allow to depict potential delays and asynchronism between different areas (Carrión et al., 2010). Broad scale vegetation changes have been synthesized from approximately 200 sites located in the northern Medi-terranean at 6,000 BP (Collins et al., 2012). This spatial reconstruction showed that at 6,000 BP, Olea, Fagus and Juniperus had smaller distributions than today while Abies, Cedrus and Quercus (deciduous and evergreen) became less abundant following the mid-Holocene. After 6,000 BP, cultivated olive trees spread over the eastern and northern edges (Besnard & Bervillé, 2000; Terral et al., 2004; Breton et al., 2006; Besnard et al., 2007; Di Rita & Magri, 2009). In Morocco, the expansion of culti-vated olives probably took place much later (Ballouche & Marinval, 2003).

The aim of the present work is to investigate the envi-ronmental changes discussed above and analyze their impacts on the ecosystems dynamics and their surroun-ding landscape in the western part of the Rif Mountains during the Holocene.

2 - MATERIALS AND METHODS

2.1 - SITE DESCRIPTION

Bab El Karn (BEK) site (fig. 1) is a mire outlet located at about 10 km south-west of Talassemtane National Park and about 20 km south-east of Bouhachem reserve. The Rif Mountains is a part of the western segment of the Alpine belt formed under a convergence between the African and the Eurasian plates during the late Meso-zoic. The Rif belt includes three major paleogeographic domains: the Internal domain called “Alboran domain” (García-Dueñas et al., 1992; Negro et al., 2006); the Maghrebian flysch domain (Guerrera et al., 1993) and the External domain (Chalouan et al., 2008). The studied site is located within the Alboran domain, which is composed of calcareous complexes (Chalouan & Michard, 2004; Chalouan et al., 2008).

Today, the average annual precipitation in the Rif’s belt is about 600 mm and locally it can reach up to 2,100 mm. Such amount of precipitation allows the development and persistence of forest ecosystems with inner mires. The eastern part of the Rif that receives less rainfall is widely covered by Pinus halepensis, Pinus pinaster and Tetra-clinis articulata (Atlas cypress), which are more drought-tolerant species than firs and Atlas cedars.

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Around the BEK site, the catchment basin is covered by an oak forest with deciduous species (Quercus pyrenaica and Quercus canariensis, see picture in figure 1). The sclerophillous Q. ilex ssp. rotundifolia and Q. coccifera are much less abundant than deciduous oaks. Evergreen oaks (Quercus suber) are very abundant in the region at lower altitude. The endemic Abies maroccana grows on the limestone in Tazaout and in the national parc of Talassemtane where it may be mixed with Pinus and with Atlas cedar on the mountain summits. In BEK mire there are many aquatic plant species such as Sphagnum and Characeae.

2.2 - CORING, LABORATORY AND DATA ANALYSIS

In March 2012 we collected an 8.5 meters coring in BEK mire (fig. 1) at 35°01.364’ N, 05°12.412’ W;

1,178 m a.s.l using a Russian corer. At the same time, we collected eight surface samples along a geographical transect from Talassemtane National Park where Abies maroccana dwells towards BEK site (fig. 1).

Ten samples of 1 cm3 were extracted from the BEK coring and dated using AMS 14C (tab. 1). The obtained 14C dates were calibrated using CALIB 7.0 software (Stuiver et al., 2013). To establish a chronological framework for the record we developed an age/depth model using R software (R Core Team, 2012) (fig. 2).

We analyzed the pollen content of 92 fossil samples and 8 modern ones. The pollen extraction was performed using the following procedure: HCl (10 %) to remove carbonates, KOH (10 %) in a water bath to remove soluble humic acids, zinc chloride (ZnCl

2, density between 1.7

and 1.8) to separate the organic fraction from minerals, acetolysis (1 ml of sulfuric acid + 9 ml of acetic anhy-dride) in a water bath to destroy the remaining most

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TalassemtaneNational park

BouhachenReserve

Morocco

TA01 TA02 TA03

TA04 TA05 TA06 TA07TA08*Bab el karn (BEK coring)

20 km

10 km

BEK coring

Fig. 1: Map showing the location of surface samples collected between the Talassemtane National Park (TA 01) and the studied site of Bab El Karn (TA 08).The picture below shows the precise location of the collected 8.5 meters coring with the last 50 cm section between 8 and 8.5 m depth (upper corner). The coring material is a Russian corer and the site is a mire surrounded by a deciduous oak forest (Quercus pyrenaica). The collected modern samples are not equidistant and their elevations (in m a.s.l.) are: TA01: 1563; TA02: 1498; TA03: 1350; TA04: 1284; TA05: 1142; TA06: 1150; TA07: 1043 and TA08: 1178.Fig. 1 : Carte montrant la distribution des échantillons de surface prélevés entre le parc national de Talassemtane (TA 01) et le site étudié de Bab El Karn (TA 08). La photographie ci-dessous montre l’emplacement précis du carottage de 8,5 mètres avec la dernière section de 50 cm entre 8 et 8,5 m de profondeur (coin supérieur droit). Le matériel de sondage utilisé est un carottier russe et le site est une dayet (marécage) entourée par une forêt de chênes à feuilles caduques (Quercus pyrenaica). Les échantillons modernes collectés ne sont pas équidistants et leurs altitudes (en m a.s.l.) sont : TA01: 1563; TA02: 1498; TA03: 1350; TA04: 1284; TA05: 1142; TA06: 1150; TA07: 1043 and TA08: 1178.

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resistant humic fraction (pollen grains are resistant to acetolysis when acetolysis does not exceed 2 min), KOH and then ethanol to decrease the pH. The final residue was diluted in glycerine (usually 10 %) for mounting microscope slides. The identification and counting of pollen grains was performed under an optical microscope (Leica DM750) using x40 magnification (x63 for accu-rate identification). The number of pollen grains counted exceeds 200 grains for each fossil sample and 600 grains for modern surface samples. Aquatic plants and Cypera-ceae were excluded from the total pollen sum.

The counting of microcharcoals (10 to 180 μm) was performed on the pollen slides. Results of the modern (fig. 3) and fossil pollen analyzes including charcoal counting (fig. 4) were plotted using Tilia 1.7.16 software (Grimm, 1991-2011).

The analysis of sediment grain size was performed at the Geosciences laboratory of Montpellier with a laser particle sizer (Coulter Beckman LS13 320) at each 5 cm interval. Samples were dissolved in distilled water to homogenize the sediment and then the solution was poured into an aqueous liquid module in order to deter-mine the size of the particles using diffraction by a laser light emitted by a spindle. A total of 170 samples were analyzed. This analysis is used to classify the particles into three categories: clay (0.375-2 μm), silt (2-63 μm) and sand (63-2,000 μm) (fig. 5).

In order to evaluate the geochemical composition of the sequence we used the technique of X-ray fluores-cence spectrometry (XRF). The analysis was performed at 5 cm interval on 170 samples. XRF analysis provided the concentrations of 23 elements: Si, Al, Fe, Mn, Mg, Ca, K, Ti, P, Cl, S, Ba, Nb, Zr, Sr, Rb, As, Pb, Zn, Cu, Co, Cr and V. Only concentrations of few elements (S, Ca, K, Fe, Al and Si) that provide the most appropriate informa-tion for this study were used (fig. 6).

3 - RESULTS

3.1 - MODERN DATA

The surface samples collected between Talassem-tane National Park (fig. 1: TA01), where fir (Abies maroccana) populations grow today, and BEK site (fig. 1: TA08) show that high pollen percentages of Abies maroccana, Quercus (evergreen and deciduous) and Pinus occur in the fir forest and decrease steadily towards BEK site (fig. 3). Atlas cedars are not observed in any surface sample because they are located at slightly higher altitudes than Abies populations and then, their pollen is dispersed over rather low distances from the originating populations. Olea and Cistus pollen percen-tages increase in the samples that are further away from the national park due to the more open landscape. Abies

Sample Depth (cm) Lab. Code 14C ages (BP)Cal ages (cal BP)

Median probability (2Σ)

Calibrated

range (2Σ)

Dated

material

BEK01 19 Poz-46693 460±30 513 484 - 537 Bulk

BEK02 84,5 Poz-46694 1225±30 1154 1065- 1260 Bulk

BEK03 140 Poz-46695 2570±35 2730 2510 – 2757 Bulk

BEK04 164 Poz-46696 3320±35 3545 3475 – 3632 Bulk

BEK05 235 Poz-46699 3750±40 4113 3987 – 4229 Wood

BEK06 257 Poz-46700 4075±35 4565 4442 – 4804 Bulk

BEK07 387 Poz-46703 4240±35 4831 4654 – 4860 Bulk

BEK08 406 Poz-46704 4545±35 5157 5053 – 5315 Bulk

BEK09 516 Poz-46706 6040±40 6880 6793 – 6960 Bulk

BEK10 845 Poz-54212 8140±50 9069 9006 – 9237 Bulk

Tab. 1: Radiocarbon ages for the Bab El Karn core.14C age calibration was performed using CALIB 7.0 software (Stuiver et al., 2013).Tab. 1 : Âges radiocarbone obtenus pour la séquence de Bab El Karn. Les âges 14C ont été calibrés à l’aide du programme CALIB 7.0 (Stuiver et al., 2013).

0 200 400 600 800

020

0040

0060

0080

0010

000

Depth (cm)

Age

(ka

BP)

387406

516

845

235257

1984.5

140

164

Fig. 2 : Fitted age/depth model based on the 10 calibrated 14C dates (red squares) with their corresponding depths.Fig. 2 : Modèle âge/profondeur basé sur les 10 dates de 14C calibrées (losanges rouges) avec leurs profondeurs respectives.

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maroccana pollen percentages are no longer recorded in those samples that are close to BEK. Quercus (ever-green and deciduous) pollen percentages are high from the fir forest to BEK but decline slightly in open lands-capes.

3.2 - FOSSIL DATA

BEK record spans the last 9,000 years. Twenty-nine trees, shrubs and herbs, and four aquatic taxa were iden-tified in the fossil record. The most common tree taxa are Atlas cedar and evergreen and deciduous oaks. We performed a constrained cluster analysis (fig. 4) to define the following pollen zones.

3.2.1 - Zone A (from 848 to 620 cm / 9,200 to 7,200 BP)

Atlas cedar, evergreen and deciduous oaks were domi-nant in this period. The grain size analysis shows that the core bottom is composed essentially of sand (up to 65 %) (silts = 25 % and clay = 5 %). Most chemical elements are low except potassium (K), iron (Fe) and carbonates (Ca), which show high values at the base of the sequence

with a significant decrease after 8,000 BP. No microchar-coal remains were detected in this time period.

3.2.2 - Zone B (from 620 to 400 cm / 7,200 to 5,200 BP)

Cedrus atlantica expanded substantially (up to 80 %) before a sharp decline. After 6,000 BP oaks expanded at the expense of Cedrus atlantica. Steppe elements (Aste-raceae, Artemisia and Apiaceae) are present while aquatic taxa (Ranunculus and Myriophyllum) disappeared from the record. After 6,000 BP we observe a few occurrences of Olea. Microcharcoals were at low concentrations in the beginning of this period and then increased synchronously with the regression of Cedrus atlantica and with the conti-nuous occurrence of Olea. Particle size showed important detrital silt input and the sediment composition becomes more organic with peat deposition at 6,000 BP.

3.2.3 - Zone C (from 400 to 125 cm / 5,200 to 2,600 BP)

Cedrus atlantica and oaks make ca. 80% of the total pollen sum (20% and 60%, respectively). Olea

Fig. 3: Diagram showing the percentages of the main pollen taxa identified in the surface samples collected over a geographical transect between Talassemtane national park (TA01), from within the fir forest and the studied site of Bab El Karn (TA08).Fig. 3 : Schéma montrant les pourcentages des principaux taxons polliniques identifiés dans les échantillons de surface prélevés sur un transect entre le parc national de Talassemtane (TA01), au sein de la forêt de sapins, et le site étudié de Bab El Karn (TA08).

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0 50 100

150

200

250

300

350

400

450

500

550

600

650

700

750

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th(cm

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0

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2000

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4000

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6000

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8000

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Dates(ca

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1601-075-Mep1-2016.indd 20 29/02/16 12:16

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occur throughout all this time period with a signi-ficant peak around 2,900 BP. The sediment compo-sition is organic with a peat layer around 4,400 BP. Unlike the sandy fraction, silt and clay increased slightly. All chemical elements tend to increase after 5,000 BP. The microcharcoals were abundant during this period especially between 4,000 and 3,000 BP when Cistus expanded.

3.2.4 - Zone D (from 126 to 0 cm / 2,600 BP to present)

During this time period Cedrus atlantica disappears while oaks (evergreen and deciduous) took over. Except sulfur, all chemical elements increased and Fe, Si, Al and Ca reached their highest values of the record. After 2,000 BP, sediment composition changed abruptly from clay and silt to coarser sand. Microcharcoals occur with high concentrations until 500 years BP then they are no longer recorded.

0 20

Clay

0 20 40 60 80

Silt

0 20 40 60 80

Sand

0

2

4

6

8

Age

(ka

BP)

Fig. 5: Grain size analysis showing the percentages of the main sediment fractions (clay, silt and sand) in the Bab El Karn sequence.Fig. 5 : Analyse granulométrique montrant les pourcentages des principales fractions sédimentaires (argile, limon et sable) dans la séquence de Bab El Karn.

0

2

4

6

8

Age

(ka

BP)

0.0 3.0 6.0

Fe

5 10 15 20

Si

3.0 6.0

Al

0.08 0.16

Ca

0.5 1.0

K

0.1 0.3

S

Fig. 6: Percentages of the main chemical elements versus the coring age obtained by X-Ray-Fluorescence (XRF).Fig. 6 : Pourcentages des principaux éléments chimiques obtenus par Fluorescence à rayons-X (XRF).

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4 - DISCUSSION

In order to improve the conservation policies and provide sound scientific basis for forest management actions, we need to better understand the relationship between the dynamics of the main tree species in Morocco and their related past environments. In the present study we investigate the environmental impacts on the forests located in the western part of the Rif Mountains and their catchment area over the last 9,000 years.

Nowadays, Bab el Karn site is located within a deci-duous oak (Q. pyrenaica, see figure 1) forest and the closest Atlas cedar and fir populations may be found in the Bouhachem reserve at higher elevations and Talas-semtane national park (fig. 1), respectively. Fir pollen grains are only found during the last 1,000 years. The modern transect of surface samples shows that its pollen grains are not well wind transported as their occurrences decrease substantially within 2 km away from the forest. The migration rate of Abies alba in Europe has been esti-mated between 50 and 150 m.yr-1 (Cheddadi et al., 2014), which, if applicable to Abies maroccana, should theore-tically allow its occurrence in the vicinity of the studied site. Thus, the absence of fir pollen grains from the BEK record strongly suggests that fir did not expand, or only slightly beyond its modern range, towards the studied site during the Holocene.

Unlike fir, the high occurrences of Atlas cedar pollen grains into the fossil record shows clearly that there were forests around the site, whose ranges were probably much more extended during the Holocene than they are today. These cedar forests were mixed with both deci-duous (Q. pyrenaica and Q. canariensis) and evergreen oaks most likely with Q. ilex.

Besides the strong presence of Atlas cedar and both deciduous (Q. canariensis and Q. pyrenaica) and ever-green (Q. ilex-type) oaks throughout the Holocene, we observe substantial changes in their proportions within the succession of pollen spectra through time and in the overall presence (and/or absence) of other taxa such as the aquatic plants (Myriophyllum and Ranunculus). These taxa successions are accompanied by changes in the sediment texture and geochemical composition. Prior to 8,000 BP, the sediment was composed of rather coarse particles with a high concentration of iron (Fe), which indicates a stronger weathering of the catchment area (carbonates outcrops) through an increase of the amount of rainfall. After 8,000 BP the sediment content shifts from sand to silt and the iron concentration decreases indicating a possible reduction of the terrigenous input due to the soil fixation favored by the expansion of Cedrus atlantica around 7,500 BP.

The expansion of Cedrus atlantica, which corresponds either to a more extensive population or an increased density around the site, reaches its optimum around 6,000 BP. Afterwards, we observe changes in the forest composition between conifers, broadleaved deciduous and evergreen sclerophyllous trees, which are potentially related to a cooling trend with a decrease in the annual

amount of rainfall. The occurrences of microcharcoals (fig. 4) suggest that the dry season became potentially longer, which led to local fires. Fire can affect the forests directly by the degradation of the vegetation cover and indirectly by affecting the atmosphere through the carbon cycle perturbation once the fire is established. Roberts et al., (2001) suggested that the installation of the modern Mediterranean climate with warm and dry summers and frost free winters took place after 6,000 BP. The Medi-terranean climate, with strong seasonal hydrological contrast between summer and winter, has a strong impact on the ecosystems but also on the lakes level as is the case of the Dead Sea, which decreased steadily since 6,500 BP (Robinson et al., 2006) and lake Eski Acigol in Turkey (Roberts et al., 2001).

Sediment composition becomes more organic with peat deposition at 6,000 BP (fig. 4) and an increase of S (fig. 6) translates a strong eutrophication of the mire. This may be related either to a more favorable climate that allowed a spread of a marshy vegetation (algae) or to strong human activities. However, the BEK pollen record shows no occurrence of any cultivated plant. Cheddadi et al., (2015) suggest that human activities have probably impacted the mountain areas later than the lowlands. Bab el Karn is located at about 1,200 m asl. In the Mediter-ranean area, many complex societies emerged between 5,000 and 3,000 BP, and vegetation disturbances started to become clearly detected in pollen diagrams (Roberts et al., 2004; Sadori et al., 2004; Sadori & Giardini 2007). The lack of pollen markers of human activities in the BEK record and the absence of archaeological sites around the studied area tend to suggest that the human presence was either scarce or absent.

In the studied area Cedrus atlantica was present at as early as 9,000 BP with a huge development after 7,000 BP. In the Middle Atlas, in all sites located at 1,600 m and higher, Atlas cedar appeared in the record and started its expansion only after 6,500 BP (Lamb et al., 1995; Lamb & Van der Kaars, 1995; Cheddadi et al., 1998, 2009; Rhoujjati et al., 2010; Nour El Bait et al., 2014; Tabel et al., in press). The late presence or the lack of expan-sion of cedar forests during the early Holocene around several lakes in the Middle Atlas may be explained by the warmer and drier climate between 10,000 and 7,000 BP. This hypothesis is supported by a climate reconstruction from a Holocene pollen record from the Middle Atlas (Cheddadi et al., 1998). The mid-Holocene in the Rif is characterized by a widespread of both evergreen and deciduous oak species. This was also recorded in other Mediterranean countries such as Italy and the Balkans (Jahns & van den Bogaard, 1998; Jahns, 2005; Caroli & Caldara, 2007), Sicily (Sadori & Narcisi, 2001), Albania (Denèfle et al., 2000; Fouache et al., 2001) and Greece (Willis, 1992a,b). BEK records the first local fires after 7,000 BP with a continuous occurrence of charcoals until today. Usually, micro- and macro-charcoals reflect fire events from regional (10 to 20 km) or local (10 to 103 m) areas (Clark, 1982; Reddad et al., 2013). These fire events may explain the installation of a more arid climate that started to constrain the presence of Atlas cedar in

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the area. The expansion of Quercus forests around our studied site is coeval with the spread of Atlas cedar at higher altitudes in the Middle Atlas. This may indicate that climate became drier at lower altitudes but remained humid at higher altitudes, which forced the migration of Atlas cedars to higher elevations.

After 4,000 BP there is a noticeable increase of Si and Al indicating a high detrital input of the watershed, Ca, which originate from the dolomite erosion, and K, which is often associated with clay deposition. The high occurrence of aquatic plants suggests a more sustained water availability throughout the year, which was in favor of an expansion of the oak forest. Unlike in our study area, a decline of the deciduous oak forests is reported in southeastern Spain around 4,700 BP (Carrión et al., 2001). In southern Italy, a temporary deforestation is also observed around 4,400 BP (Sadori et al., 2008).

A strong regression of Cedrus atlantica is recorded after 2,000 BP, while the shrubby Cistus expands over the lands-cape. Cistus, which is often considered as a marker of the human disturbances or presence (López-Sáez et al., 2003), occurs along with other similar indicators such as Erica and Olea (fig. 4). The high content of Fe, Al, K, Ca and Si after 2,000 BP (fig. 6) is clearly related to the high detrital input from the watershed under a reduced forest cover.

Although the Phoenicians and the Carthaginians were present in northern Morocco, human activities became detectable in our study area only during the last two millennia under the Romans. The expansion of the Roman Empire enhanced the human impact on the Mediterranean ecosystems at all altitudes and particu-larly on the conifer forests, used for constructing houses and warships and for feeding the metal industry (Mather et al., 1998). In the Middle Atlas the main disturbances are also observed during the last two millennia and not earlier (Lamb et al., 1991; Cheddadi et al., 2015). The very reduced and hardly detectable human impact on our study area during those periods of Phoenician and Carthaginian occupations of Morocco is due to the local life style that remained traditionally based on low producing agriculture for the subsistence of local people only (Taiqui & Cantarino, 1997). In the BEK record we observe few and discontinuous occurrences of Olea during the Holocene. This suggests that the olive cultiva-tion inferred from archaeological findings in the Tangier region (Ponsich, 1964) did not expand to areas located at higher altitudes.

CONCLUSIONS

Through its multidisciplinary study, the Bab El Karn sequence provides an interesting record of vegetation and environmental changes over the Holocene in the north western part of Morocco. The two emblematic species in the Rif Mountains, Abies maroccana and Cedrus atlan-tica behaved very differently. Abies maroccana did not reach the site of Bab El Karn during the Holocene, which suggests that it has probably persisted within its current

range while Cedrus atlantica expanded tremendously during the Holocene and even dominated the landscape around the studied site between 8,000 and 6,000 BP. This time span was humid enough to promote its expansion around BEK. After 6,000 BP, the trend towards a drier climate degraded the Atlas cedar forests, which became progressively replaced by deciduous oaks. The extinction of Cedrus atlantica from the studied area occurs at about 2,000 BP and is due to a combined effect of both high aridity and anthropogenic disturbances. However, one should state that it remains difficult to identify which of the human activities or climate had the most constraining effect on the Atlas cedar forests in the Rif Mountains.

ACKNOWLEDGMENTS

RC thanks the Erasmus Mundus program for having funded the 24-month visit of O. Bouaissa at ISEM. This is an ISEM contribution, under the number 2015-200.

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