Vegetation Evolution in the Mountains of Cameroon During the Last 20 000 Years: Pollen Analysis of Lake Bambili Sediments

Tropical rainforests are the most biologically diverse ecosystems on the planet (Puig, 2001). In the highlands and mountains of western Cameroon (Central Atlantic Africa), a set of plateaus and mountains that contain large forested areas, this diversity is now subject to significant human pressure due to a large population engaged in agriculture and ranching. Momo Solefack (2009) shows that between 1978 and 2001 deforestation in Oku was 579 ha / year, with a annual rate of 4% increase. In addition to anthropogenic impacts, climate change plays a major role in influencing the distribution and composition of ecosystems (Walther et al., 2002; Thomas et al., 2004; Schroter et al., 2005; Thuiller et al., 2006).


Introduction
Tropical rainforests are the most biologically diverse ecosystems on the planet (Puig, 2001).In the highlands and mountains of western Cameroon (Central Atlantic Africa), a set of plateaus and mountains that contain large forested areas, this diversity is now subject to significant human pressure due to a large population engaged in agriculture and ranching.Momo Solefack (2009) shows that between 1978 and 2001 deforestation in Oku was 579 ha / year, with a annual rate of 4% increase.In addition to anthropogenic impacts, climate change plays a major role in influencing the distribution and composition of ecosystems (Walther et al., 2002;Thomas et al., 2004;Schröter et al., 2005;Thuiller et al., 2006).
The Bamenda Highlands have a particular forest characterized by the presence of one of the few African gymnosperms, Podocarpus latifolius.This species migrated from East to West Africa through Angola and then colonized the high mountains of Cameroon.It is currently restricted to altitudes above 1800 m, in the areas of Mount Oku and Kupé (Letouzey, 1968).Pollen data of Central Atlantic Africa have shown that this Podocarpus forest was once significantly more extensive than today during the last climatic cycle, especially during the last ice age (Dupont et al., 2000;Elenga and Vincens, 1990;Maley and Livingstone, 1983).At this time, Podocarpus was present at low and mid altitudes mixed with the dense Guineo-Congolian forest.The expansion of Podocarpus into these areas did not end until very recently, about 3000 years ago (Vincens et al., 2010).Such a distribution, involving the recent migration of the species to higher altitudes (White, 1993), suggests that such forests are likely refugia.This chapter, based on a sedimentary sequence of 14 m taken at Lake Bambili covering the last 20,000 years, presents the first palynological data from altitude in this region.Preliminary analysis of data from Bambili has been presented by Assi-Kaudjhis et al. (2008).The aim of this paper is to study the development and evolution of mountain forest in Cameroon over this interval.

Location, climate, and vegetation of bambili
Lake Bambili (05°56'11.9N, 10°14'31.6E, 2273 m asl) is a crater lake that lies in the volcanic zone of Cameroon (Figures 2 and 3) in the Bamenda Highlands and Bamboutos Mountains.Fig. 2. The crater lake Bambili (05 ° 56'11.9 N, 10 ° 14'31.6 E, 2273 m asl).The red dot indicates the core location in the lower crater lake and the yellow dot is the second core location in the swamp of the second upper crater.
Centrally located in the Guineo-Congolian region, Cameroon has a relatively humid climate due to the locationof the country to the Gulf of Guinea (Suchel, 1988) which is responsible for the long rainy season over 4 / 5 of the country (western and southern regions) and slightly drier tropical climates in the north of the Adamawa plateaus.The seasonal alternation of southwesterly moisture flux and northerly dry winds, called the Alizé, creates a wet season from March to October and a more variable drier period during the rest of the year.The influence of altitude and distance from the coast result in lower precipitation in Bambili than Douala (2280 mm in Bamenda at 1370 m asl and 2107 mm in Bafoussam at 1411 m asl) and lower average temperatures (19°C to 20°C in Bamenda and Bafoussam against 26°C in Douala).Dry season precipitation from November to February is below 50 mm (Web LocClim, FAO, 2002).During the rainy season, precipitation is as high as 400 mm per month with temperatures fluctuating between 18.1 and 21.2°C.
Cameroon vegetation (Figures 3a and 3b) was described by Letouzey (1968Letouzey ( , 1985) ) and White (1983).It is divided by latitude and altitude.At Bambili, aquatic vegetation grows in bands at the lake margin in the area of permanent open water: after Nymphaea sp.(Nymphaeaceae) on the edge of open water is a belt of Cyperaceae, ferns and aquatic plants, then, on the dry ground appear species of Poaceae.

Materials and methods
Two cores of 13.5 m and 14.01 m were taken a few meters apart at Lake Bambili in February 2007 and January 2010, respectively (Figure 4).The cores were taken using a Russian manual corer (Jowsey, 1966) in sections of 60 cm in length.The two sedimentary sequences were combined into a single sequence on the basis of benchmark levels identified in each, the depth (mcd) was calculated.The cores were sampled every 5 to 10 cm for pollen analysis.Overall, the sediments are composed of organic material and clay.From 0 to 635 cm, organic rich brown peat dominates becoming more compact and darker towards the base.Between 635 and 657 cm, sediments are mostly organic rich with centimeter-sized nodules of graygreen clay.Finally, 657 cm to the base of the sequence is an organic-rich compact black clay.A charcoal layer is observed at 1355 cm.Seventeen AMS dates were performed, which show a continuous deposition for the last 20,000 years with a sedimentation rate ranging from 0.208 cm per year between 0 and 650 cm and 0.05 cm per year from 650 cm to the base (Table II).Radiocarbon measurements were calibrated using the CALIB software version 5.0.(Stuiver et al., 2005).The samples for pollen analysis were chemically treated with hydrochloric acid (HCl) and hydrofluoric acid (HF) according to the conventional method described by Faegri and Iverson (1975) preceded by a sieving at 250 microns to remove coarse particles.Treatment was terminated by filtration at 5 microns.
A total of 141 samples were analyzed with an average time resolution of 146 years.Counts ranged from 303 to 1500 pollen grains according to the richness of each sample, and 203 pollen taxa were identified.Data are presented as a diagram drawn on the basis of percentages calculated on a sum excluding aquatic plants and ferns.
The CONISS program used in the Tilia program (Grimm, 1987) was employed for the subdivision of the pollen diagram in zones.Analysis of palynological richness (Birks and Line, 1992), in order to estimate biodiversity, was performed with the software PSIMPOLL (http://www.chrono.qub.ac.uk/psimpoll/psimpoll.html).

Results
The microflora (Table III) consists of 119 taxa of trees, lianas and palms, together with 36 herbaceous taxa and 32 undifferentiated taxa, which correspond to plants that may be trees or herbs.Finally, 10 taxa correspond to aquatics plants, 4 to ferns and one to a plant parasite.

Family
Vegetation Evolution in the Mountains of Cameroon During the Last 20 000 Years: Pollen Analysis of Lake Bambili Sediments 147

Forest cover
At Bambili, forest cover was extremely low during the late glacial period (18,000-23,000 cal yrs BP).The vegetation is dominated by herbaceous plants, as is the case in all West African sites regardless of altitude: the swamp Shum Laka (Kadomura, 1994) at 1200 m asl on the Bamenda plateau at Barombi Mbo at the foot of Mount Cameroon (Maley and Brenac, 1998) and Lake Bosumtwi, Ghana (Maley, 1987) located in low and mid altitudes.The pollen studies from the mountains of East Africa also reflect the general degradation of tropical forests during this period (Livingstone, 1967, Coetzee 1967, Hamilton, 1982).Three pollen sites located in southern hemisphere, however, show a different pattern.At Ngamakala, a forest sites in the southern Congo located 400 m asl, the environment, although degraded, remains forest during the glacial period until 13,000 yrs BP (Elenga et al., 1994).At Kisiga Rugaro, a forested part of the Eastern Arc Mountains shows a certain environmental stability during the whole glacial period (Mumbai et al. 2008).They note, however, that the herbaceous plants recorded their highest percentages between 19,000 and 14,000 cal yrs BP , which could correspond to drier and colder conditions than even the preceding LGM.Mumbai et al. (2008) suggest that the relative stability of ecosystems during the last ice age is due to the influence of the Indian Ocean that would have allowed the maintenance of a rain forest while the regional climate of East Africa was dry.At Lake Masoko (9°20'S, 33°45'E, 840 m asl), Vincens et al. (2006) also identify a uninterrupted development phase of semideciduous forest from 23,000 -11,800 cal yrs BP.

Distant contributions
The LGM is characterized by the presence of savanna and steppe taxa such as Aerva-type, Boscia-type, Capparidaceae undiff., Commiphora, Crataeva adansonii, Crudia-type, Lanneatype and Maerua-type that show the importance of long-distance aeolian input, related to an increased flow of trade winds (Sarnthein et al., 1981) and possibly the extension of Sudan-Zambezian vegetation zones due to dry conditions.A layer of charcoal was observed at 1355 cm.Dating of this layer yielded an age of 18 283 cal yrs BP (15 020 ± 60 14 C BP) suggesting the importance of fire at this time in the environment near Lake Bambili.

Chronology of colonization forest
In the global post-glacial context, recolonization of forest is observed.At Bambili, this occurs in three distinct stages (Figure 5) interrupted by two phases of regression corresponding to the Heinrich event 1 (H1) and the Younger Dryas.
The first phase of colonization starts at 18,400 cal yrs BP with the appearance of Anthospermum.This taxon is noted by Livingstone (1967) as an important element of the first stage of colonization of lava fields in the region of the Virunga volcanoes in East Africa.In Cameroon, Anthospermum camerounensis, is a dwarf grass found in Afro-alpine vegetation types (Letouzey, 1968).This taxon is followed by a large increase of Olea capensis and Schefflera, forming a first phase of increase in trees.The increase of trees stops at 17 100 cal yrs BP, with the increase of Aerva-type, which indicates the strengthening of the boreal winter northeasterly wind flow from the Sahelian steppes and large fires are indicated by charcoal layers.This increase is contemporaneous with H1, the characteristics of climate on a global scale are drought (Mix et al., 2001;Kageyama et al., 2005;Timmermann and Menvielle, 2009).
Vegetation at Bambili remains relatively stable for about 1900 years when a second stage of forest colonization begins at 14,900 cal yrs BP.This stage is initiated by the appearance of Myrica, a fire-tolerant, sun-loving shrub common in clearings of the upper montane forests (Livingstone, 1967).This taxon is followed by another sun-loving species, Ilex mitis, and finally by Olea capensis, Podocarpus and Schefflera.This phase culminates in a very short period between 13,800 and 13,700 cal yrs BP, corresponding to the beginning of the warm Alleröd in high latitudes (Roberts et al., 1993(Roberts et al., , 2010)).This increase of forest cover at Bambili is coeval with the start of the African Humid Period dated to 15,500 cal yrs BP by DeMenocal et al. (2000).This period was marked by increased flow of the Niger River (Lézine and Cazet, 2005) and the general rise in lake levels (Gasse, 2000;Shanahan et al., 2006).At Bambili, Stager and Anfang-Sutter (1999) found a positive P / E during this period.The presence of steppe taxa such as Aerva up to 11,700 cal yrs BP, however, shows that trade wind flow was still significant over this period.This is in contrast with the termination of Saharan dust transport to the ocean further north noted by DeMenocal et al. (2000).
Between 13,000 and 11,700 cal yrs BP, a second phase of forest decline is recorded.This phase does not correspond to the total destruction of the forest.It is characterized instead by a massive regression of Schefflera.All trees recorded a large decrease with the exception of Podocarpus, Rapanea and Gnidia-type.The permanence of steppe elements reflects the importance of continued northeasterly circulation.This phase corresponds to the YD (Younger Dryas) in the high latitudes (Roberts et al., 1993).As mentioned earlier, the YD is generally dry in tropical North Africa, reflected in the general lowering of lake levels (Gasse, 2000).This episode is marked by increased dust transport to the Atlantic recorded from the equator (Lézine et al., 1994) to the Saharan latitudes (DeMenocal et al., 2000).The YD signature on forest vegetation is however not very visible in the Ivory Coast as outlined Lezine and Le Thomas (1995).Despite evidence of increased sedimentological transport by the trade winds, the authors noted no major changes in the forest environment during this episode.The YD seems to be more clearly recorded in the vegetation surrounding lakes Barombi Mbo and Bosumtwi.Data from Barombi Mbo show an increase of spores (2 to over www.intechopen.comVegetation Evolution in the Mountains of Cameroon During the Last 20 000 Years: Pollen Analysis of Lake Bambili Sediments 155 15%), Poaceae (13 to 17%) and pioneers taxa such as Trema, Macaranga, Mallotus and Alchornea around 12,500 yrs BP (Maley and Brenac, 1998).At the same site, Lebamba et al. (2010) show, after ca 14,000 cal yrs BP, the decline of tropical rain forests and seasonal increases in savanna biomes.At Bosumtwi, there is an increase in grasses and sedges, while the tree taxa regress (Maley, 1991).A second phase of forest colonization starts at 14,900 cal yrs BP with the appearance of Myrica (4) monitoring and Ilex mitis Schefflera (5), Olea capensis (6) and Podocarpus (7).
Between 13,000 and 11,700 cal yrs BP, a second phase of regression is observed with decreasing forest trees.Then, from 11,500 cal yrsnBP, Schefflera (8) opens the Holocene forest expansion phase.

Example of two major forest taxa: Olea and Podocarpus
Comparison of pollen data from sites Barombi Mbo, Tilla and Bambili shows that, since 20,000 years BP, some plants have moved from low to high altitudes.This is the case of Olea (Figure 6) which shows percentages up to 35%, during the glacial period at Barombi Mbo (Maley and Brenac, 1998), indicating the presence of the plant source at low altitude near the lake.Its percentage decreased drastically from 12,000 cal yrs BP.At this time, Olea appears at Tilla, 700 m north of the volcanic line in Cameroon (Salzmann et al., 2000) where it develops until it reaches 14%.It then extends to 2200 m asl at Bambili after 10,800 cal yrs BP where it remained until the end of the Holocene forest.This tree is still present in the mountains at the edge of montane forest and sub-montane, near the village of Oku.Olea is known to be a pioneer taxon, which explains its presence at the beginning of the forest recovery at Tilla, before the development of Euphorbiaceae (Uapaca) that characterize the forest in the Holocene.The behavior of Olea during the glacial-interglacial transition could be related to temperature changes.With rising temperatures after 12,000 years at low altitude, Olea migrates toward higher elevations to it current location.
Unlike the previous taxon, Podocarpus shows a similar pattern at both sites (Figure 7).It is not represented in the LGM sediments, and pollen percentages that reached Barombi Mbo are so low that it precludes the possibility of its presence at low altitude.At Bambili, it also reduced at this time.Podocarpus at Bambili increases from 10,000 cal yrs BP, then develops during two periods centered around 7560 cal yrs BP and 3360 cal yrs BP.It is interesting to note that these two peaks are also found at Barombi Mbo; however, here values are a full order of magnitude lower than at Bambili.This could support the hypothesis of two distinct phases of expansion of Podocarpus in altitude during the Holocene.Farrera, 1995;Reynaud-Farrera et al., 1996), as well as those of Mboandong at 130 m asl (Richards, 1986), Mbalang at 1100 m asl (Vincens et al., 2009), Njupi at 1108 m asl (Zogning et al., 1997), Bafounda at 1310 m asl (Tamura, 1990), and Shum Laka (W-10) at 1355 m asl (Kadomura, 1994).

The optimum forest of the early Holocene
The optimum forest at Bambili occurs between 10,000 and 8400 cal yrs BP and is characterized by the dominance of tree taxa at values from 80-92%.Of the 118 total tree taxa determined to Bambili, this period includes more than half (67 taxa), the majority belonging to the montane and semi-deciduous forests.These are mainly Schefflera, Syzygium, Podocarpus, Olea capensis, Rapanea for the montane forest and Macaranga, Celtis, Cussonia, Antiaris for semi-deciduous forest.Elements of the open areas have disappeared (Leonotis, Leucas, Boscia, Aerva) or decreased (Impatiens, Achyrantes, Solanaceae undiff, Galium / Rubia).
In tropical Africa during the early Holocene, the percentages of pollen from woody genera are high throughout the region, indicating the expansion of forests into higher latitudes and altitudes (Lézine, 2007).In West Africa, the marine records suggest that the Guineo-congolian forests were not separated by the savanna corridor that exists today in Togo and Benin, the "Dahomey Gap" (Dupont et al ., 2000), which is confirmed by the analysis of lake sediments from Sele (Salzmann et al., 2000).Tropical plants migrated northwards along the rivers and lakes stretching across the Sahel and the Sahara (Watrin et al., 2009).The mangrove taxon Rhizophora also occupied many coastal areas northward to around 21°N (Lézine, 1997).Bonnefille et al., (1995) noted also a reduction in forest cover during the LGM and a discontinuous forest colonization at Rusaka Swamp (3°26'S, 29°37'E, 2070 m asl).Many authors link the expansion of Guineo-congolian forest and mangrove with a reinforcement of the Atlantic monsoon at the beginning of the Holocene (Marzin et Braconnot, 2009).Rainfall was significantly higher than today and seasonality was reduced as shown by the high lake levels (Gasse, 2000;Shanahan et al., 2006) and increased fluvial transport (Lézine and Cazet, 2005).

5.4
The destabilization of the forest in the mid-Holocene: The 8.2 event The forest phase at Bambili is marked by a very short but indicated episode of regression which occurs at the time of the 8.2 event in the high latitudes (Von Grafenstein et al., 1998).It is reflected in the physiognomy of the forest by lower percentages of trees and the more or less pronounced decrease in values of some taxa.The most remarkable decreases are those of Schefflera, Podocarpus, Rapanea, and to a lesser extent those of Olea capensis, Maesa, Syzygium, Ilex mitis, Nuxia, and Embelia, all standard elements of the montane forest as well as those of Cussonia, Alchornea, and the submontane forest.This degradation could be caused by dry conditions as suggested by the lower lake levels at Lake Bosumtwi (Shanahan et al., 2006) related to the slowdown of the thermohaline circulation in the North Atlantic (Pissart, 2002).

The end of the Holocene forest at Bambili
After 8400 cal yrs BP, changes in taxa suggest some forest instability.This phase of disruption occurs gradually, leading to the brutal destruction of the forest at 3300 cal yrs BP.At this period, Podocarpus opposes Schefflera, Syzygium and Alchornea.Forest degradation begins at 4500 cal yrs BP, with the decline of Schefflera followed by that of Olea capensis at 3500 cal yrs BP and Podocarpus at 3300 cal yrs BP.The drastic reduction of montane forest elements at 3300 cal yrs BP probably favored soil erosion and sediment supply from the crater rim.The latter was then increased by the return of wet conditions during the early part of this interval after logging dated between 2500 and 1300 cal yrs BP.
At 3300 cal yrs BP, lower montane forest taxa values are partially offset by increases of Syzygium, Maesa and Gnidia-type, indicating the opening of the forest.This opening peaks at 2600 cal yrs BP with the increase of Urticaceae and Poaceae undiff. .The evolution of the vegetation was organized in two stages.Between 2500 and 1300 cal yrs BP, a small forest recovery takes place.Forest vegetation is dominated by Ilex mitis associated with Schefflera and to a lesser extent Sygyzium, Rapanea, Maesa, Nuxia, and Gnidia-type.Podocarpus and Olea capensis are poorly represented.In the later part of this phase, the presence of sub-montane elements successively occurs such as Macaranga, Celtis, and Lophira.Finally, the Afromontane elements (Ericaceae undiff., Hyperycum, Isoglossa), with smaller percentages are generally better represented than in the high forest phase.This phase corresponds to wet period as suggested at Bambili by Stager and Anfang-Sutter (1999) who noted a P / E positive.The lake level rose during this period as reflected in the diatom assemblages; however, this could be artificially enhanced by the increased sediment transport from the lake margin.A humid climate is also noted in Ossa between 2700 and 1300 cal yrs BP by Nguetsop et al. (2004) confirming its regional character.At 960 cal yrs BP, the deep decline of Schefflera, partially offset by higher percentages of Olea, illustrates another phase of environmental degradation.The opening in the middle is highlighted by the dominance of Poaceae and Urticaceae.Between 960 cal yrs BP to the present, a marked upturn in forest is observed, with expansion of Schefflera (45%).The degradation of forest leads to a generalized fall in the majority of taxa, montane and submontane.The presence of Asteraceae undiff, Ilex mitis, Ficus, Embelia-type, Maesa, Rapanea, Sygyzium, Gnidia-type, Clematis-type, Nuxia-type, Hypoestes-type, Alchornea, Sapotaceae undiff., Solanaceae undiff, however, Celtis reveals nature montane forest mixed with the semi-deciduous forest.

The impact of environmental change on biodiversity
Fluctuations in biodiversity, calculated using the method developed by Birks and Line (1992) (Figure 8), show four phases of maximum biodiversity that correspond to periods of transition or disturbance of the forest.These phases are centered around 18,000 cal yrs BP, 12,000 cal yrs BP, 6500 cal yrs BP and 1500 cal yrs BP.The highest values of the record occur during the post-glacial colonization of forest following the YD (ca 12,000 cal yrs BP) and at the end of the small logging period (ca 1000 cal yrs BP).During these periods, forest biodiversity is increased due to the internal forest dynamics incorporating the gradual disappearance and appearance of certain plants resulting in greater overall species richness.
According to Birks and Line (1992), floristic richness is favored during phases of "intermediate" disruption.The ecosystem fragmentation associated with such disruption limits the domination of one single component as long as the disturbance is not sufficient to cause the complete disappearance of the ecosystem on a regional scale.At Bambili, highest biodiversity does not follow forest stabilization, in contrast, biodiversity indices are generally low during these times.As shown in Figure 8, the current biodiversity surrounding Lake Bambili is the lowest in the last 20,000 years.This confirms the considerable impoverishment of the environment related to the recent deterioration of the forest environment undoubtedly amplified by the action of man, as evidenced by the comparative analysis of satellite photos from 1998 and 2003.

Conclusion
This palynological study of the paleoenvironments surrounding Bambili reveals the history of montane Central Atlantic Africa over the last 20,000 years.It also provides unique evidence on the response of Cameroon's mountain forests to climate change.During the LGM, a highly degraded forest formation was present around Bambili dominated by lightdemanding components suggesting a dry environment.For the Holocene, the proportion of tree pollen shows continuous forest cover between 10,000 and 3300 yrs cal BP.The expansion of forest, very dynamic at the beginning of the Holocene, led to installation of a montane forest dominated by Schefflera, Podocarpus and Olea which responded individually to climate change.This forest lasted for much of the Holocene, then floristic composition changed.From this period, forest degrades very sharply during three centuries and the forest loses about 40% of its importance.This is in agreement with the general context of equatorial forest evolution as a result of drier conditions of the end of the Holocene humid period.As emphasized by Birks Line (1992) "floristic richness is maximized by the disruption and fragmentation of the landscape when it reaches a level sufficient to prevent the domination of a single species and insufficient to cause the extinction of all components of the landscape."The lowest percentages of trees on the top of the Bambili sequence dominated by a single taxon: Schefflera, indicates the disturbance of the landscape which reached a maximum level showing considerable impoverishment of the local flora.However, despite the effects of climate, intensified anthropogenic impacts have dramatically reduced forest biodiversity in recent decades.Continuation of such practices, associated largely with agriculture and ranching, in the future will likely lead to the disappearance of this ecosystem.
Palynological studies on other sites of Cameroon during LGM are needed to better assess the extent of Podocarpus forest.

Fig. 4 .
Fig. 4. Pollen diagram of Lake Bambili (Cameroon).This diagram shows, from left to right, radiocarbon ages, calibrated ages, depths, and percentages of major taxa presented in terms of large groups of regional vegetation calculated on a sum excluding aquatic plants and ferns.Poorly represented taxa are exaggerated by a factor of 10 and superimposed on the initial values (in dark).At the far right is a diagram of variations of AP / NAP.Pollen zones were delineated by applying the CONISS software (GRIMM, 1987).
From 11,500 cal yrs BP, Schefflera leads a new phase of forest expansion with a forest optimum dated between 10,000 and 8400 cal yrs BP.The development of the forest is gradual between 11,500 and 10,200 cal yrs BP and results in the expansion of montane taxa.Next, Rubus, Rapanea, Embelia-type and Syzygium appear in turn.The elements of the montane forest, in particular, Schefflera, Podocarpus, Olea and Syzygium rose steadily over t h i s p e r i o d .S u b m o n t a n e f o r e s t i s a l s o p r e s e n t t h r o u g h a n u m b e r o f t a x a t h a t a p p e a r sporadically, such as Cussonia, Macaranga-type, Antiaris-type toxicaria, and Allophyllus.

Fig. 5 .
Fig.5.Colonization of the postglacial forest.The curves are made from percentages of each taxon from 21,000 to 11,000 cal yrs BP.This colonization takes place in three phases forest interrupted by two regressions corresponding to H1 and the YD.It starts at 18,400 cal yrs BP with the appearance of Anthospermum (1) followed by Olea capensis (2) and Schefflera (3).The interruption to 17,100 cal yrs BP, marked by the increase of Aerva-type is correlated with H1.A second phase of forest colonization starts at 14,900 cal yrs BP with the appearance of Myrica (4) monitoring and Ilex mitis Schefflera (5), Olea capensis (6) and Podocarpus (7).Between 13,000 and 11,700 cal yrs BP, a second phase of regression is observed with decreasing forest trees.Then, from 11,500 cal yrsnBP, Schefflera (8) opens the Holocene forest expansion phase.

Fig. 6 .
Fig.6.The evolutionf Olea-type at Bambili, Tilla and Barombi Mbo.During the Last Glacial Maximum, Olea-type has a significant presence in Barombi Mbo until 12,000 cal yrs BP.The presence is then recorded at Tilla between 12,000 and 11,800 cal yrs BP and at Bambili at 10,800 to 3300 cal yrs BP.

Fig. 8 .
Fig. 8. Physiognomy of the vegetation and changes in biodiversity at Bambili.Left, Bambili diagram synthesis with the percentages of trees, herbs and undifferentiated.Right, rarefaction analysis showing Bambili biodiversity.Biodiversity is very high in the transition phases of the vegetation (red boxes).The dashed lines indicate the minimum of biodiversity (left line) and the value of average biodiversity (right line).

Table I .
Area (ha)of different units of land between 1988 and 2003.

Table II .
Radiocarbon dates and age model of the two cores taken in 2007 and 2010 at Bambili.

005 to 6.80 mcd (11,572-3,252 cal yrs BP)
The increase in trees initiated in the previous zone continues in zone IV with percentages reaching the maximum value of 92%.Conversely, Poaceae undiff.disappears along with all elements of non-native steppe and savanna.The changing environment consists of two phases: