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Environmental changes in the moroccan western rif mountains over the last 9,000 years

Changements environnementaux dans les montagnes de l’Ouest du rif marocain depuis 9 000 ans
Rachid Cheddadi, Ouafaa Bouaissa, Ali Rhoujjati et Laurent Dezileau
p. 15-25

Résumés

Le 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 charbons 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.

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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.

1 - Introduction

1Mediterranean 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. Mediterranean forests are also often fragmented either naturally 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.

2Environmental 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 (Benkaddour 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 mountains. 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 conservation (Zanchetta et al., 2013). Besides weak historical information, the Rif Mountains harbor a high contingent of endemic species (Benabid, 1984) that largely rationalize new paths for environmental studies in this area.

3In 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 identified 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 preservation of endangered species. However, some populations 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).

4The 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 Mediterranean.

5In 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 reduction in the annual amount of precipitation is observed in the fossil records, which leads to critical climate oscillations and an increasing ecosystem instability (Mercuri et al., 2012; Sadori et al., 2013). This has been strengthened by the more recent human activities (Sadori et al., 2013; Cheddadi et al., 2015). Throughout the Mediterranean 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 Mediterranean 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 cultivated olives probably took place much later (Ballouche & Marinval, 2003).

6The aim of the present work is to investigate the environmental changes discussed above and analyze their impacts on the ecosystems dynamics and their surrounding landscape in the western part of the Rif Mountains during the Holocene.

2 - Materials and methods

2.1 - Site description

7Bab 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 Mesozoic. 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).

8Today, 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 Tetraclinis articulata (Atlas cypress), which are more drought-tolerant species than firs and Atlas cedars.

9Around 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.

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).

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.

2.2 - Coring, Laboratory and data analysis

10In 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).

11Ten 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).

12We 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 (ZnCl2, density between 1.7 and 1.8) to separate the organic fraction from minerals, acetolysis (1 ml of sulfuric acid + 9 ml of acetic anhydride) in a water bath to destroy the remaining most 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 accurate identification). The number of pollen grains counted exceeds 200 grains for each fossil sample and 600 grains for modern surface samples. Aquatic plants and Cyperaceae were excluded from the total pollen sum.

13The 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).

14The 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 determine 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).

15In order to evaluate the geochemical composition of the sequence we used the technique of X-ray fluorescence 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 information for this study were used (fig. 6).

Tab. 1: Radiocarbon ages for the Bab El Karn core.

Tab. 1: Radiocarbon ages for the Bab El Karn core.

14C age calibration was performed using CALIB 7.0 software (Stuiver et al., 2013).

Fig. 2 : Fitted age/depth model based on the 10 calibrated 14C dates (red squares) with their corresponding depths.

Fig. 2 : Fitted age/depth model based on the 10 calibrated 14C dates (red squares) with their corresponding depths.

3 - Results

3.1 - Modern data

16The surface samples collected between Talassemtane 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 percentages increase in the samples that are further away from the national park due to the more open landscape. Abies maroccana pollen percentages are no longer recorded in those samples that are close to BEK. Quercus (evergreen and deciduous) pollen percentages are high from the fir forest to BEK but decline slightly in open landscapes.

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: 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).

3.2 - Fossil data

17BEK record spans the last 9,000 years. Twenty-nine trees, shrubs and herbs, and four aquatic taxa were identified 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)

18Atlas cedar, evergreen and deciduous oaks were dominant 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 microcharcoal remains were detected in this time period.

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

19Cedrus atlantica expanded substantially (up to 80 %) before a sharp decline. After 6,000 BP oaks expanded at the expense of Cedrus atlantica. Steppe elements (Asteraceae, 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 continuous 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)

20Cedrus atlantica and oaks make ca. 80% of the total pollen sum (20% and 60%, respectively). Olea occur throughout all this time period with a significant peak around 2,900 BP. The sediment composition 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.

Fig. 4: Pollen diagram showing the percentages of the main taxa identified in the sedimentary sequence of Bab El Karn and the micro-charcoal occurrences.

Fig. 4: Pollen diagram showing the percentages of the main taxa identified in the sedimentary sequence of Bab El Karn and the micro-charcoal occurrences.

The pollen diagram is plotted versus the age/depth model (see fig. 2). Calibrated 14C ages and depth are also provided. On the right hand side, a constrained cluster analysis provides the main pollen zones (A, B ,C and D).

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: Grain size analysis showing the percentages of the main sediment fractions (clay, silt and sand) in the Bab El Karn sequence.

Fig. 6: Percentages of the main chemical elements versus the coring age obtained by X-Ray-Fluorescence (XRF).

Fig. 6: Percentages of the main chemical elements versus the coring age obtained by X-Ray-Fluorescence (XRF).

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

21During 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.

4 - Discussion

22In 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.

23Nowadays, Bab el Karn site is located within a deciduous 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 Talassemtane 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 estimated between 50 and 150 m.yr-1 (Cheddadi et al., 2014), which, if applicable to Abies maroccana, should theoretically 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.

24Unlike 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 deciduous (Q. pyrenaica and Q. canariensis) and evergreen oaks most likely with Q. ilex.

25Besides the strong presence of Atlas cedar and both deciduous (Q. canariensis and Q. pyrenaica) and evergreen (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.

26The 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 Mediterranean 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).

27Sediment 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 Mediterranean 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.

28In 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 expansion 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 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.

29After 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).

30A strong regression of Cedrus atlantica is recorded after 2,000 BP, while the shrubby Cistus expands over the landscape. 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.

31Although 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 particularly 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 cultivation inferred from archaeological findings in the Tangier region (Ponsich, 1964) did not expand to areas located at higher altitudes.

Conclusions

32Through 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 atlantica 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.

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Table des illustrations

Titre 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).
Légende 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.
URL http://journals.openedition.org/quaternaire/docannexe/image/7517/img-1.png
Fichier image/png, 985k
Titre Tab. 1: Radiocarbon ages for the Bab El Karn core.
Légende 14C age calibration was performed using CALIB 7.0 software (Stuiver et al., 2013).
URL http://journals.openedition.org/quaternaire/docannexe/image/7517/img-2.png
Fichier image/png, 46k
Titre Fig. 2 : Fitted age/depth model based on the 10 calibrated 14C dates (red squares) with their corresponding depths.
URL http://journals.openedition.org/quaternaire/docannexe/image/7517/img-3.png
Fichier image/png, 12k
Titre 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).
URL http://journals.openedition.org/quaternaire/docannexe/image/7517/img-4.png
Fichier image/png, 57k
Titre Fig. 4: Pollen diagram showing the percentages of the main taxa identified in the sedimentary sequence of Bab El Karn and the micro-charcoal occurrences.
Légende The pollen diagram is plotted versus the age/depth model (see fig. 2). Calibrated 14C ages and depth are also provided. On the right hand side, a constrained cluster analysis provides the main pollen zones (A, B ,C and D).
URL http://journals.openedition.org/quaternaire/docannexe/image/7517/img-5.png
Fichier image/png, 68k
Titre Fig. 5: Grain size analysis showing the percentages of the main sediment fractions (clay, silt and sand) in the Bab El Karn sequence.
URL http://journals.openedition.org/quaternaire/docannexe/image/7517/img-6.png
Fichier image/png, 19k
Titre Fig. 6: Percentages of the main chemical elements versus the coring age obtained by X-Ray-Fluorescence (XRF).
URL http://journals.openedition.org/quaternaire/docannexe/image/7517/img-7.png
Fichier image/png, 31k
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Rachid Cheddadi, Ouafaa Bouaissa, Ali Rhoujjati et Laurent Dezileau, « Environmental changes in the moroccan western rif mountains over the last 9,000 years »Quaternaire, vol. 27/1 | 2016, 15-25.

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Rachid Cheddadi, Ouafaa Bouaissa, Ali Rhoujjati et Laurent Dezileau, « Environmental changes in the moroccan western rif mountains over the last 9,000 years »Quaternaire [En ligne], vol. 27/1 | 2016, mis en ligne le 01 mars 2018, consulté le 29 mars 2024. URL : http://journals.openedition.org/quaternaire/7517 ; DOI : https://doi.org/10.4000/quaternaire.7517

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Auteurs

Rachid Cheddadi

Université de Montpellier, Institut des Sciences de l’Evolution, UMR UM-CNRS-IRD 5554, place Eugene Bataillon, bâtiment 23, FR-34095 MONTPELLIER. Courriel: rachid.cheddadi@um2.fr

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Ouafaa Bouaissa

Université de Montpellier, Institut des Sciences de l’Evolution, UMR UM-CNRS-IRD 5554, place Eugène Bataillon, bâtiment 23, FR-34095 MONTPELLIER. Email: ouafaa.b@hotmail.fr

Ali Rhoujjati

Université Cadi Ayyad, Faculté des Sciences et Techniques (URAC 42), boulevard Abdelkrim Khattabi, BP 511, Gueliz Marrakech, MAROC. Courriel: a.rhoujjati@uca.ma

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Laurent Dezileau

Universite de Montpellier, Geosciences Montpellier, CNRS/UM, UMR 5243, place Eugene Bataillon, batiment 23, FR-34095 MONTPELLIER.Courriel: Laurent.Dezileau@univ-montp2.fr

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