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Central Africa includes the world's second largest rainforest block. The ecology of the region remains poorly understood, as does its vegetation and archaeological history. However, over the past 20 years, multidisciplinary scientific programmes have enhanced knowledge of old human presence and palaeoenvironments in the forestry block of Central Africa. This first regional synthesis documents significant cultural changes over the past five millennia and describes how they are linked to climate. It is now well documented that climatic conditions in the African tropics underwent significant changes throughout this period and here we demonstrate that corresponding shifts in human demography have had a strong influence on the forests. The most influential event was the decline of the strong African monsoon in the Late Holocene, resulting in serious disturbance of the forest block around 3500 BP. During the same period, populations from the north settled in the forest zone; they mastered new technologies such as pottery and fabrication of polished stone tools, and seem to have practised agriculture. The opening up of forests from 2500 BP favoured the arrival of metallurgist populations that impacted the forest. During this long period (2500-1400 BP), a remarkable increase of archaeological sites is an indication of a demographic explosion of metallurgist populations. Paradoxically, we have found evidence of pearl millet (Pennisetum glaucum) cultivation in the forest around 2200 BP, implying a more arid context. While Early Iron Age sites (prior to 1400 BP) and recent pre-colonial sites (two to eight centuries BP) are abundant, the period between 1600 and 1000 BP is characterized by a sharp decrease in human settlements, with a population crash between 1300 and 1000 BP over a large part of Central Africa. It is only in the eleventh century that new populations of metallurgists settled into the forest block. In this paper, we analyse the spatial and temporal distribution of 328 archaeological sites that have been reliably radiocarbon dated. The results allow us to piece together changes in the relationships between human populations and the environments in which they lived. On this basis, we discuss interactions between humans, climate and vegetation during the past five millennia and the implications of the absence of people from the landscape over three centuries. We go on to discuss modern vegetation patterns and African forest conservation in the light of these events.
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rstb.royalsocietypublishing.org
Research
Cite this article: Oslisly R, White L, Bentaleb
I, Favier C, Fontugne M, Gillet J-F, Sebag D.
2013 Climatic and cultural changes in the
west Congo Basin forests over the past 5000
years. Phil Trans R Soc B 368: 20120304.
http://dx.doi.org/10.1098/rstb.2012.0304
One contribution of 18 to a Theme Issue
‘Change in African rainforests: past, present
and future’.
Subject Areas:
environmental science, health and disease and
epidemiology, ecology
Keywords:
Holocene, archaeology, CongoOgooue
´
basin,
palaeoenvironment, climatic change,
vegetation
Author for correspondence:
Richard Oslisly
e-mail: richard.oslisly@ird.fr
Climatic and cultural changes in the
west Congo Basin forests over the
past 5000 years
Richard Oslisly
1
, Lee White
2,3,4
, Ilham Bentaleb
5
, Charly Favier
5
,
Michel Fontugne
6
, Jean-Franc¸ois Gillet
7
and David Sebag
8
1
Institut de Recherche pour le De
´
veloppement, UMR 208 IRD/MNHN, Patrimoines Locaux,
Agence Nationale des Parcs Nationaux, BP 20379 Libreville, Gabon
2
Agence Nationale des Parcs Nationaux, BP 20379 Libreville, Gabon
3
Institut de Recherche en Ecologie Tropicale, BP 13354 Libreville, Gabon
4
University of Stirling, Stirling FK9 4LA, UK
5
Universite
´
de Montpellier 2, CNRS, ISEM, Place Eugene Bataillon, 34095 Montpellier, France
6
Laboratoire des Sciences du Climat et de l’Environnement, UMR 8212 CNRS/CEA/UVSQ, Domaine du CNRS,
91198 Gif-sur-Yvette Cedex, France
7
Universite
´
de Lie
`
ge, Gembloux Agro-Bio Tech, Laboratoire de foresterie des re
´
gions tropicales et subtropicales,
Passage des De
´
porte
´
s 2, 5030 Gembloux, Belgium
8
Institut de Recherche pour le De
´
veloppement, HydroSciences Montpellier, Universite
´
de Ngaounde
´
re
´
,
BP 454, Cameroon
Central Africa includes the world’s second largest rainforest block. The ecol-
ogy of the region remains poorly understood, as does its vegetation and
archaeological history. However, over the past 20 years, multidisciplinary
scientific programmes have enhanced knowledge of old human presence
and palaeoenvironments in the forestry block of Central Africa. This first
regional synthesis documents significant cultural changes over the past
five millennia and describes how they are linked to climate. It is now well
documented that climatic conditions in the African tropics underwent sig-
nificant changes throughout this period and here we demonstrate that
corresponding shifts in human demography have had a strong influence
on the forests. The most influential event was the decline of the strong Afri-
can monsoon in the Late Holocene, resulting in serious disturbance of the
forest block around 3500 BP. During the same period, populations from
the north settled in the forest zone; they mastered new technologies such as
pottery and fabrication of polished stone tools, and seem to have practised
agriculture. The opening up of forests from 2500 BP favoured the arrival of
metallurgist populations that impacted the forest. During this long period
(25001400 BP), a remarkable increase of archaeological sites is an indication
of a demographic explosion of metallurgist populations. Paradoxically, we
have found evidence of pearl millet (Pennisetum glaucum) cultivation in the
forest around 2200 BP, implying a more arid context. While Early Iron Age
sites (prior to 1400 BP) and recent pre-colonial sites (two to eight centuries
BP) are abundant, the period between 1600 and 1000 BP is characterized by
a sharp decrease in human settlements, with a population crash between
1300 and 1000 BP over a large part of Central Africa. It is only in the eleventh
century that new populations of metallurgists settled into the forest block. In
this paper, we analyse the spatial and temporal distribution of 328 archaeolo-
gical sites that have been reliably radiocarbon dated. The results allow us to
piece together changes in the relationships between human populations and
the environments in which they lived. On this basis, we discuss interactions
between humans, climate and vegetation during the past five millennia and
the implications of the absence of people from the landscape over three centu-
ries. We go on to discuss modern vegetation patterns and African forest
conservation in the light of these events.
&
2013 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution
License http://creativecommons.org/licenses/by/3.0/, which permits unrestricted use, provided the original
author and source are credited.
1. Introduction
Geological and biological studies carried out over the past
40 years have dispelled the illusion of the ‘eternal rainforest’
in equatorial Africa. Indeed, it is clear that during the last
glacial maximum severe climatic conditions restricted dense
equatorial forests in Africa into a small number of larger
refuge areas and a complex mosaic of ‘micro-refugia’,
where favourable conditions persisted [15].
The Holocene marked the return to milder conditions in
Central and West Africa, and forests quickly regained lost
ground, as evidenced by increasing levels of rainforest
pollen in lake sediments across the region [6,7]. Several
records from marine [810] and terrestrial climate archives
[4,11] also suggest humid conditions interspersed by numer-
ous climate fluctuations [12] during the Early and Middle
Holocene followed by a dry Late Holocene.
In Central Africa, there is strong evidence for a significant
forest regression event between 3000 and 2000 years BP. The
Warm African monsoon declined from about 3500 years BP,
causing a serious disturbance of the forest massif, as attested
by many palynological and geological studies undertaken on
sedimentary deposits of lakes (figure 1) in the Congo basin
[3,1327].
Pollen profiles show that humid forest trees were replaced
by light-demanding pioneer and herbaceous species, charac-
teristic of degraded forests and savannas between 3000 and
2500 BP [21].
The fragmentation of the Congolese forests, at a time
when there were variations in the levels of many lakes in
the region and corresponding changes of average surface
water temperatures in the Gulf of Guinea [10], has been inter-
preted as a response to a generalized arid period in Central
Africa [14,28], related to a weakening of the Atlantic mon-
soon. The palynological and geochemical data and diatoms
at the site of Mbalang in Cameroon [22,29] demonstrate
significant environmental change from 3200 years BP,
linked to different hydrological conditions.
The consensus that has developed is that climatic con-
ditions with a long dry season combined with more severe
storm events resulted in severe erosion. A recent study [30]
suggested that this can be explained by anthropogenic soil ero-
sion, but others [31,32] argue that climate was the main driver.
A large-scale spread of Bantu iron workers occurred only
around 19001700 BP. It seems likely therefore that the first
Bantu farmers opportunistically followed forest fragmentation
to penetrate the forest zone.
These studies also demonstrate that the medieval period in
the Northern Hemisphere (1100800 years BP) is characterized
by decadal fluctuations in the lake levels in Atlantic Central
Africa [33], coinciding with the opening up of the canopy of
mature forests in peripheral areas adjoining the forest block
[14,20] as well as forest recovery in the central Gabon [34].
During the Little Ice Age (500200 years BP), lake levels
were low [16,33], cover of rainforests decreased and there
was a change in the type of vegetation from evergreen to
deciduous forests [16].
Archaeological studies have demonstrated that cultural
and technological evolution occurred in parallel to these
regional environmental changes. Here, we consider the distri-
bution of archaeological sites discovered over the past 30
years in Cameroon and especially Gabon to evaluate the inter-
actions between humans and the environment. Following a
number of pioneering studies in the 1960s [35,36], systematic
surveys were undertaken between 1980 and 1990. These early
studies identified the major stages of cultural change in the
region [37]. Subsequent studies in the period 19902000
focused on a number of major sites, providing a regional
chronological reference for cultural changes [3840]. More
recently, the focus has been on interactions between man and
the environment, focusing particularly on the impact of man
on his habitat and the impacts of climate change on human
societies [4143]. In this synthesis, we assemble a database
of 328 archaeological sites known in West Central Africa and
analyse their distribution, in an attempt to better understand
the relationships between climate, human demography and
forest distribution through the Holocene.
2. Study area and methods
This is the first study to compile data on all known archaeo-
logical sites in the Central African rainforest region. The area
under study includes six countries of Atlantic Central Africa
(the southern half of Cameroon, mainland Equatorial Guinea
(the island of Bioko has a different history), Gabon, Republic
of Congo, the western part of the Democratic Republic of
Congo and the southwest of the Republic of Central
Africa). We divide the region into two generic biomes:
forest and savanna/savannaforest mosaic (figure 1).
We identified 328 archaeological sites with at least one
reliable carbon date from the last 5000 years. Of these, 32%
(106 of 328) were sites where one of the authors had been
involved in research, including 20 sites for which no data
have previously been published; the remainder were from
the literature [38,40,4391]. Oslisly et al. [92] provide additional
details of dating methods, materials and precision.
We compiled all known published and unpublished radio-
carbon dates for the 328 archaeological sites [92]. Of a total of
4
1
2
3
7
8
6
9
11
12
13
10
GABON
CONGO
D.R.C
dense forest
savanna and woodlands
N
300 km
Atlantic
Ocean
EQ.
GUINEA
CAMEROON
5
C.A.R
equator
16
14
15
Figure 1. The study area (Central Atla ntic Africa) showing the location of 328
archaeological sites and 16 lake coring sites in the dense forest and forest/
savanna mosaic (references are cited in the main text): 1, Barombi Mbo;
2, Ossa; 3, Nyabesan; 4, Bambili; 5, Mbalang; 6, Kamale
´
te
´
; 7, Ngue
`
ne; 8,
Maridor; 9, Bilanko; 10, Ngamaka; 11, Sinnda; 12, Kitina; 13, Coraf/Songolo;
14, Mopo Bai; 15, Goualougo; 16, Bodingue
´
.
rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120304
2
733 dates, 16 were rejected by researchers as being spurious,
either owing to contamination or sampling error, 25 were
modern, 94 were considered duplicates (very similar dates in
the same site) and 12 were from Bioko island [93] (Equatorial
Guinea), where there is an unbroken culture of pottery and
polished stone tools from 700 AD until the arrival of Europeans
in the eighteenth century. Dates on the same site a century or
more apart were retained. A total of 586 dates were retained
for this analysis, from a total of 328 sites.
Our method assumes that relative population numbers are
related to the number of radiocarbon dates. However, caution
is necessary, because sampling remains patchy reflecting the
concentration of research activity and possible impacts of
prevailing environmental conditions on charcoal preservation.
It is also possible that the economic pursuits of pre-historic
populations leave differential amounts of dateable material
(i.e. iron-working versus mobile huntergatherers).
3. Results
Figure 2 shows the distribution of archaeological sites across
the region divided between four cultural sequences: Late
Stone Age; Neolithic stage; Early Iron Age and Late Iron Age.
Two-thirds of the sites are located in the modern forest/
savanna mosaic. These areas are easier to prospect and
archaeological sites often show up as sites with active erosion.
Forest sites were generally discovered in places where artificial
openings were made during construction of roads, railways,
pipelines, dams and power plants, or during mining and for-
estry exploitation. Given the state of research today, it is not
possible to say whether distribution of sites reflects the actual
spatial distribution of archaeological sites or is simply an
effect of patchy sampling.
Of 586 radiocarbon dates analysed, 33 were associated with
Late Stone Age sites, 97 with Neolithic sites and 456 were Iron
Age remains. The distribution of 586 carbon dates from 328
archaeological sites across the region is plotted by number of
sites per century in figure 3. Figure 3 shows that few sites
have been found dating prior to 3000 years BP. The Late
Stone Age in the region ends around 3500 years BP. Neolithic
sites increase in numbers from about 3000 years BP onwards,
peaking at 2300 BP and petering out at 1900 years BP (except
in Bioko island, which never had an Iron Age).
Map (a) shows the disparate nature of sites at the end of
the Late Stone Age.
Map (b) shows the migration of the Malongo peoples
from Cameroon to Congo over a period of 700 years.
<3500 BP 3500–1900 BP
2800–1000 BP 1000–100 BP
Late Stone Age
Early Iron Age
Late Iron Age
Neolithic stage
2800 BP
2400 BP
2800 BP
2300 BP
1900 BP
2500 BP
Lopé area
2300 BP
2300 BP
(a) (b)
(c) (d)
Figure 2. Location of 328 archaeological sites in Atlantic Central Africa for (a) the Late Stone Age, (b) the Neolithic stage, (c) the Early and (d) the Late Iron Ages.
rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120304
3
Map (c) shows a northsouth spread of iron smelting with
older dates in the north and more recent dates in the south. The
oval represents the region with the most iron-rich deposits.
Map (d) shows a gradual repopulation after the crash.
3500 years ago, a radical change is observed as groups of
stone working huntergatherers give way across the region
to new migrants, who settle the land and begin the first
forms of rudimentary slash and burn agriculture [94]. Probably
originating from northern Sahelian areas, these populations
settled firstly along the edge of the forest block and benefited
from forest fragmentation to penetrate further. They created
small villages on hilltops and dug rubbish pits close to their
houses, unlike Late Stone Age peoples who left their waste
on the surface [39]. They mastered new technologies such as
pottery and stone polishing and seem to have practised early
forms of agriculture, as shown by the presence of stone hoes.
This period, known as the ‘Neolithic stage’, occurs in the
forest zone between 3500 and 2000 BP. People settle on hilltops
and in dominant positions close to rivers.
Beginning approximately 3200 years BP, a migratory
wave travelled south from Cameroon along the Atlantic
coast, taking advantage of the presence of a continuous
string of narrow savannas that range from Equatorial
Guinea to the mouth of the Congo. This tradition, defined
by a common ‘Malongo’ pottery style, lasts from 2700 to
2000 years BP [75]. Their pottery containers were decorated
with large zigzag patterns produced by a series of pivoting
combs (figure 4). The edges of the vases are generally
straight, thickened externally with a fluted lip with settings
spanning from the neck to the base. The first Malongo pottery
was dated at 2700 years BP at Bissiang [63] (figure 2). Homo-
geneous pottery remains found in the coastal areas of Kribi in
Cameroon (2600 2000 BP) [75], near Kogo in Equatorial
Guinea [95], at Libreville (24002000 years BP) [43], at
Iguela and Mayumba in Gabon (23002100 years BP) and
at Tchissanga in the Republic of Congo (2500 2000 years
BP) [53,96] indicate that this was one cultural group that
migrated southwards from Cameroon.
A second much more important current begins appro-
ximately 28002500 years BP in the hinterlands, and
corresponds to the Early Iron Age. The first signs of iron smelt-
ing date back to 2800 years BP, at Oliga, in southern Cameroon
[61]. The number of Iron Age sites rises rapidly from 2600 years
BP onwards, peaking 1900 years BP. From 1600 to 1000 years
BP, there is a drastic drop, suggesting that Atlantic Central
Africa was almost devoid of people during this period.
Figure 2c shows the spread of iron smelting to the south
over a period of 900 years. Note that the dates on the map
indicate an initial spread from north to south in inland
areas where geological formations rich in iron occur; and a
second phase along the coast, indicating it was principally
a result of commercial exchanges, although some iron
furnaces have been found.
These metal workers demonstrated a very good knowledge
of geological formations and established their settlements on
hilltops [41].
They made a range of forms and decorative styles of cer-
amics that differ markedly from those of the Neolithic
cultures, suggesting that they replaced or displaced peoples.
Bi-lobed and careened pots decorated using comb swivels dis-
appear in favour of generalized closed vessels with open edges,
with more intricate decorations: concentric circles and bands of
incised parallel lines mainly in the upper part of the body,
where handles and gripping appendices were fitted.
In the middle valley of the Ogooue
´
, corresponding rock
engravings are found that include comparable geometric
shapes with those found on the pottery as well as animal figures
[39]. Oslisly [41] has mapped the southwards spread of charac-
teristic pottery styles associated with these populations,
demonstrating their gradual migration to the south. The distinct
lack of any hybrid forms and decorations suggests that the
populations that moved in displaced those already there.
Increasing densities of archaeological sites around 2000
1900 years BP demonstrate a demographic explosion of metal
working populations (figures 2c and 3). Using iron tools,
these peoples had the potential to profoundly affect the
forest by slash and burn forms of agriculture and it is likely
that they also managed forestsa vanna mosaics using fire as
a hunting str ategy. The y also produced great quantities of
charcoal during iron reduction operations and would have
maintained savannas through the lighting of bush fires [74,97].
In addition to evidence of gathering of rainforest fruits and
seeds, such as Antrocaryon klaineanum, Canarium schweinfurthii
and Coula edulis [41,42], there is also evidence of extensive use
of oil palm beginning from 3000 years BP in the Yaounde
´
area
[94] and at 2800 years BP at Otoumbi in central Gabon [38].
There is also an isolated record around 2200 BP of cultivation
of pearl millet, P. glaucum, in the modern forest zone of
southern Cameroon [98]. This observation is consistent with
suggestions that it was generally drier at this time, with
longer, more pronounced dry seasons.
Figure 5 presents four maps showing the distribution of
archaeological sites in four periods:
Map (a): the period from 2100 to 1700 years BP indicates
that there were many sites of Iron Age and Neolithic stage in
Cameroon and Gabon.
Map (b): the period from 1600 to 1400 years BP shows a
slight iron age population deflation.
Map (
c): the period from 1300 to 900 years BP reveals a total
absence of dates in the forest hinterland South Cameroon/
Centre and North of Gabon/North of Congo and little
human presence in the peripheral areas of the forestry block.
There was a clearly defined ‘population crash’ from 1300 to
900 years BP in the central Gabon [41].
Map (d): the period from 800 to 400 years BP shows that the
forest block has been recolonized by Late Iron Age populations.
From 1000 years BP, new populations of metalworkers
settled in the region, achieving their greatest numbers around
500 BP, in the pre-colonial period. The division in terms of abun-
dance of sites is corroborated in some regions by cultural
differences. For example, in central Gabon, iron ore reduction
40
Late Stone Age
Neolithic stage
Early Iron Age
Late Iron Age
30
20
no. sites per century
10
0
5000 4000 3000
year BP
2000 1000 100
1
1
2
2
Figure 3. Graph of 586 radiocarbon dates for Atlantic Central Africa, distin-
guished on the basis of the character of associated archaeological remains.
rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120304
4
structur es built abo v e ground with clay in the Early Iron Age
disappear in favour of pits, and the ceramic traditions are totally
different, later styles being chara ct erized by frequent use of
recurrent patterns created using wooden roulettes, with no
evidence of cultural exchange [41].
The metallurgists of the Late Iron Age resettled along
hilltops and practised shifting cultivation [39]. Ceramics of
this period include large and small pots of flattened spherical
shape with out-curved apexes and pots of uneven curvature,
as well as clay pipes indicating smoking of tobacco. The
decoration of these pots is unique, with small circular
motifs made with knotted strips of plant material forming
herringbone patterns. The designs on pots form a band of
variable width high on the sphere, and similar patterns are
found on the clay bowls of pipes.
The maximum expansion of the new metallurgists peaks
around 500 years BP (figure 3) and then decreases to around
100 years BP. While the end of this last phase coincides with
the end of the Little Ice Age, other factors are probably
important. This period was marked by the first contacts
with Europeans, which caused a radical shift in Aboriginal
culture: the traditional pottery was supplanted by colonial
pots. The ‘cultural shift’ is not the only impact: these contacts
also saw the development of the slave trade, which had a pro-
found impact on populations. It is very difficult to assess the
impacts of the slave trade, because data are scarce and diffi-
cult to verify. Some data exist for Sao Tome
´
and for the
Kongo Empire but few are available for Gabon. Picard-
Tortorici & Franc¸ois [99] put forwards a figure of 18 000
slaves leaving the Gabonese coast, which had an estimated
population of 100 000. This suggests that the consequences
on human population densities may have been significant.
Slave trails penetrated far into the interior, reaching Lope
´
in
the Central Gabon [100] and further southeast. Data on veg-
etation composition and structure in Lope
´
have allowed for
modelling of vegetation change over the past 2000 years, indi-
cating that a spurt of savanna colonization corresponds to the
period covered by the slave trade [101]. It seems likely that
depopulation in the Lope
´
region resulted in reduced frequen-
cies of savanna fires, allowing forest expansion, and reduced
intensity of agriculture, resulting in forest succession from
young to more mature formations.
However, the subject has received little attention and
deserves further study by archaeologists, historians and geo-
graphers, although, from about 500 years BP onwards, the
archaeological data are of limited value owing to the fact
that archaeologists rarely radiocarbon date sites that they
know on the basis of stratigraphy and pottery to belong to
the relatively recent past. Hence, for this period, we have to
rely on historical documents or undertake new detailed
archaeological surveys.
4. Discussion and conclusion
In the early part of the Holocene Stone Age, human settle-
ments seem to be few and far between in the forest belt of
West Central Africa and there is little to suggest that man
played an overly determinant role in the ecosystem. The
Late Stone Age, which started at about 40 000 years BP
[102], ended approximately 3500 years BP, when Neolithic
cultures migrated from Cameroon into Gabon and Congo
along the coast. These peoples were the first to make pottery,
which would have greatly increased their ability to store and
preserve foodstuffs, and their polished stone hoes indicate an
early form of agriculture.
With the arrival of iron smelting and working, which
moved into the region from the Sahel and central Sahara
around 2800 years BP, human impacts on the rainforest
would have increased significantly. During the same period,
1
2
3
4
5
6
Figure 4. Typical Malongo tradition pottery: profile and vase fragments of fluted edges decorated with collars and swivel impressions. 1 and 2, Malongo
(Cameroon); 3, Kogo (Equatorial Guinea); 4 and 5, Okala (Libreville) and 6, Tassi (Loango National Park) in Gabon (scale bar, 3 cm).
rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120304
5
approximately 30002000 years BP, Central Africa was also
affected by a worsening climate that contributed to the frag-
mentation of the forest block and probably also to the
accelerated expansion of Bantu peoples towards the south.
The increase in numbers of archaeological sites from 2000
years BP through to about 1600 years BP (figure 3) suggests
that human population numbers increased greatly through
this period. Despite somewhat fragmented evidence it seems
safe to conclude that the reduction of forest cover at this time,
through a combination of changes in the hydrological cycle
and increased anthropogenic pressure, may have resulted in
a deforestation peak on a scale similar to that which has
affected West Central Africa over the last century or so [103].
To date, there is little evidence of what was being culti-
vated. De Maret records oil palm, Elaeis guineensis, for the
first time in the region 3000 years BP in the Yaounde
´
area
[94] and Oslisly [38] and Fay [104] record it, respectively,
2850 years BP in the Lope
´
region of Central Gabon and
from 2400 years BP in the Nouabale
´
Ndoki region of northern
Congo. In some areas of coastal Cameroon, there is evidence
of extensive oil palm groves [75], and extensive forest fires in
the valley of the Le
´
le
´
di and Offoue
´
in the Central Gabon at
around 1800 years BP [97] are consistent with agriculture
on a comparable scale with that recorded in Okomu in south-
west Nigeria around 700 years BP, continuous over at least
1000 km
2
[105107].
It will require further archaeological exploration across
Central Africa in order to fully document and understand the
implications of pre-historic man on vegetation patterns across
the African rainforest zone, but recent studies in Central
Africa as well as other studies further afield [108] suggest
that humanity’s impact in the recent past may have been sig-
nificant. Indeed, the region known as the Sangha interval, a
low biodiversity corridor separating richer forests of the
Ogooue
´
and Congo basins [109], has been revealed in recent
years to contain numerous archaeological sites, as well as
vast deposits of oil palm seeds dating to the period between
2400 and 1000 BP (peaking at 1700 BP). The forests in this
region remain dominated by species indicative of old second-
ary vegetation (Entandrophragma, Triplochiton, etc.) as well as
extensive formations with a dense understorey dominated by
species of Marantaceae, indicative of cultivation, savanna colo-
nization or forest fires [104,110,111], and it is possible that their
low diversity is a result of severe disturbance by humans over
the past three millennia, particularly considering that their
actions have been coupled with phases of climate stress.
2100–1700 BP 1600–1400 BP
800–400 BP1300–900 BP
?
(a)
(b)
(c) (d)
Figure 5. Maps showing distribution of dated archaeological sites in the periods: (a) 21001700 years BP; (b) 16001400 years BP; (c) 1300 900 years BP;
(d) 800400 years BP. The relations between these populations are described in detail in [39].
rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120304
6
The decrease in abundance between 1600 and 900 years
BP and possible disappearance of humans from large parts
of the landscape between 1350 and 900 years BP would
have had an equally marked, if different, impact on veg-
etation (figure 5c). Palaeoenvironmental data reveal that
from 1400 years BP a stronger monsoon would have favoured
forest regeneration [21], so reduced frequencies of anthropic
fires at this time would have created conditions favourable
to wide-scale forest regeneration [74,97].
It is interesting to hypothesize about the cause of the
human population decline. If the population rise was built on
cultivation of pearl millet (P. glaucum) [98] during a period of
climatic stress for rainforest vegetation and from 1400 years
BP conditions became too humid for this species, then it
would have been difficult to maintain high populations, and
people may have migrated away in search of a climate appro-
priate for millet cultivation. If Mbida’s report of banana
cultivation [112] in the Yaounde
´
area proves reliable this
hypothesis would seem less likely, because bananas would
have thrived in the new climate, but if no alternative was
available, then this hypothesis seems plausible.
An alternative hypothesis, which remains impossible to
test due to the fact that acidic soils preclude the discovery
of human bodies dating to this time, is that the risk of epi-
demics would have increased as the human population
density increased. Old literature on epidemics in Atlantic
Central Africa shows that sleeping sickness seems to have
been the disease that most affected the populations of the
forest, owing to its high mortality rate. At the end of the nine-
teenth century, the French colonial administration reported a
major outbreak of trypanosomiasis, which resulted in the dis-
appearance of populations in eastern Gabon and northern
Congo [113]. Around the year 1920, when Dr Eugene Jamot
fought against human African trypanosomiasis in South
Cameroon, he found that 116 000 persons out of 664 000
examined (17%, with peaks at 30% in some places) were
infected with sleeping sickness, a deadly disease with no
traditional cure [114].
Recently, apes that had risen to unusually high densities
in old secondary vegetation in northeast Gabon and north-
west Congo following displacements of human populations
died of Ebola in density-dependent epidemics [115] that
might mirror what happened to people some 1400 years BP.
Another possibility is that the extreme weather events of
AD 535536, known to be the most severe and protracted
short-term episodes of cooling in the Northern Hemisphere
in the past 2000 years [116], had an impact in Central
Africa. The event is thought to have been caused by an exten-
sive atmospheric dust veil, possibly resulting from a large
volcanic eruption in the tropics [117]. Its effects were wide-
spread, causing unseasonal weather, crop failures and
famines worldwide.
These hypotheses can be confirmed or refuted only by
further research. While we attempt to demonstrate that the
number of radiocarbon dates is related to demographic
changes, other questions related to shifts in human activities
across the landscape require further studies.
Irrespective of the cause, the evidence for the population
decline seems sound, and the implications of the rise and
subsequent fall of human populations for vegetation are
likely to be significant. In more recent times, the impacts of
the slave trade and the subsequent forced relocation of
rural populations to roads and urban centres by colonial
authorities, leaving vast areas in Gabon and Congo devoid
of people [118], also had significant impacts on vegetation,
such as the distribution of Okoume
´
trees (Aucoumea klaineana)
in Gabon [101].
The pattern that emerges over the past 5000 years is a
complex interaction of variations in climate and in human
population density, distribution and ability to impact on
forest vegetation. While it is clear from botanical work that
these changes have not overwhelmed the signature of
longer-term climate change resulting in the cyclical retraction
of forests into refuges and expansion across the Congo basin
[5,119], no scientific study of vegetation, including work on
the dynamics of carbon stocks, should ignore the possibility
of disturbance linked to human activities over the past two
millennia having a significant bearing on the results. Archae-
ologists have developed methods of mapping past human
activity using trees such as A. klaineana, Lophira alata and Bail-
lonella toxisperma, all of which are associated with old village
sites [120]. Furthermore, many of the forests with the highest
densities of large mammals, which tend to be prioritized for
conservation, are in areas that have been significantly
impacted by humans over the past 1000 years or so [103].
This synthesis of 30 years of archaeological studies allows
us to describe the main stages in cultural development, as
well as key changes in climatic conditions through the past
5000 years. The picture we have painted is not without ana-
logy to the current situation, where human populations are
growing in the context of increasing climatic stress. Currently,
it seems unlikely that we will avoid the large-scale deforesta-
tion that such circumstances have caused repeatedly in the
past. The combination of climate change and increased log-
ging pressure is likely to result in increased frequencies of
forest fires [110], whereas the global appetite for productive
agricultural land is likely to see more and more of the Central
African forests converted to large-scale oil palm plantations
and other commercial crops [121]. The REDDþ process
seemed for a while to offer some potential for preservation
of extensive tracts of forests, but interest in this process
seems to be stagnating, with very few concrete success stories
to bolster flagging enthusiasm of political leaders.
The archaeological data presented in this paper suggest
that several waves of forest disturbance and oil palm cultiva-
tion have affected the Congo Basin over the past three
millennia and that the forest has been relatively resilient.
Were the conditions to be put in place to favour forest
growth, it is likely that regeneration would be rapid. In the
meantime, conservationists should look to mirror the pat-
terns of forest survival that have made it possible for
forests to recover quickly once given the chance.
Acknowledgements.
We want to thank C. Moreau and J-P. Dumoulin
of LMC14 ARTEMIS Facility (Saclay, France), and three anonymous
referees for commenting on the manuscript.
Funding statement. This research was funded by the ERA-Net Biodi-
vERsA (CoForChange project), with the national funders ANR
(France) and NERC (UK), part of the 2008 BiodivERsA call for
research proposals. Other funding was also provided by the EU pro-
gramme ECOFAC (1992 1998), IRD Projects (ECOFIT 19921996,
PALEOFORGA 19971999, ADHENTRO 20002005, PALOC since
2008, PPR-FTH since 2011) and CNRS projects (PNEDC PRIMUS
20022004 and ECLIPSE REGAB 2005 2009). L.W. was supported
by the Wildlife Conservation Society and the Centre International
de Recherches Me
´
dicales de Franceville.
rstb.royalsocietypublishing.org Phil Trans R Soc B 368: 20120304
7
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... Those sites that did not have such features but that had been assigned to either the "Neolithic," "Early Iron Age," or "Recent Iron Age" by their original authors were also considered to represent farming populations and hence excluded from our analyses. In addition, we checked the agreement of our chronocultural affiliation and assessment of dating reliability with that reported in recent summaries of the archaeological findings in Central Africa by Garcin et al. (36), Oslisly et al. (60), Morin-Rivat et al. (61), Cornelissen (24), and Seidensticker et al. (35). The remaining dates, that is, those with a chronocultural affiliation within the Stone Age (Middle or Later), no evidence of farming population occupation and reliable 14 C dates (according to the criteria outlined above) were assumed to represent huntergatherer sites in the past. ...
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Full-text available
Significance We combined ethnographic, archaeological, genetic, and paleoclimatic data to model the dynamics of Central African hunter-gatherer populations over the past 120,000 years. We show, against common assumptions, that their distribution and density are explained by changing environments rather than by a displacement following recent farming expansions, and that they have maintained large population sizes and genetic diversity, despite fluctuations in niche availability. Our results provide insights into the evolution of genetic and cultural diversity in Homo sapiens .
... The first phase of expansion of the Bantu populations has been estimated between 2300 and 1400 BP and corresponds to a massive arrival of human populations from the northwest of Cameroon. A second phase of expansion took place between 670 and 20 BP, separated by a phase of depopulation (Wotzka, 2006;Oslisly et al., 2013b;Morin-Rivat et al., 2016). The Bantu people roamed the forests and practised shifting cultivation on bushland, then left former agricultural land fallow. ...
Article
Despite the implementation of management plans, commercial tree species densities are declining in the forests of Central Africa. In the region, Cylicodiscus gabunensis Harms (Fabaceae-Caesalpinioideae; common name ‘okan’), is one such species most exploited, but its ecology remains poorly understood. The rarity of its regeneration in evergreen forest suggests that, like other commercial light-demanding species, the conditions that allowed populations to become established are no longer present. Using a combined archaeobotanical and pedological approach, the aim of this study is to identify the factors explaining the current distribution of C. gabunensis individuals at local scale. Within a plot of 1050 ha in a forest concession in south-eastern Gabon, we installed 40 archaeological pits equally divided between sites with and without C. gabunensis. The artefacts encountered were collected and analysed. Charcoal masses were quantified and 18 charcoals were dated. These ages were compared with the average age of the tree population, using growth data from 50 individuals and heartwood dating from 4 individuals. An analysis of the physico-chemical properties of the soil was carried out on composite samples from each archaeological pit. Pottery sherds were found in two pits while charcoal was present in all pits, suggesting widespread human occupation and fire throughout the study area. Human occupation occurred in two phases: between 2480 and 1010 BP and from 590 to 80 BP. The abandonment of agricultural land at the end of this second phase could coincide with the establishment of the C. gabunensis cohort whose average age has been estimated at between 90 and 148 years. Soil analyses showed that C. gabunensis individuals were located on soils that were comparatively richer in element potentially toxic (Fe) and in some plant nutrients (K, P) and total nitrogen. The current scarcity of young trees argues for the implementation of a silviculture that integrates the light requirements of the species as well as the chemical fertility of the soil.
... The conversation around human-induced changes in TWS of the Congo basin is important and requires further details. This is because as home to the world's second largest rainforest block (e.g., Oslisly et al., 2013), it is critical to advance knowledge on long term effects of intense human-actions such as deforestation on TWS dynamics. This will build on existing compendium of knowledge highlighting the sensitivity of climate to the loss of the Congo basin rainforest and other ecological disturbance in the region (e.g., Bell et al., 2015, Malhi et al., 2013, Verhegghen et al., 2012. ...
Chapter
Il est nécessaire de comprendre les impacts du climat sur l'hydrologie des eaux de surface pour prévoir les conséquences et les implications sur les habitats d'eau douce, les actifs écologiques et les fonctions des zones humides. Bien que le Bassin du Congo soit considéré comme une région riche en eau douce, largement caractérisée par de nombreuses ressources en eau à l'instar du Bassin de l'Amazone, les récits récents de sécheresses dans le bassin indiquent que même les régions les plus humides du monde peuvent être affectées par les sécheresses et leurs impacts. Compte tenu de la rareté et de la disponibilité limitée des données hydrologiques dans la région, les observations de GRACE (Gravity Recovery and Climate Experiment) sont combinées avec des modèles et des données SPEI (standardized precipitation evapotranspiration index) pour étudier la probabilité de tels impacts sur l'hydrologie des eaux de surface du Bassin du Congo. En intégrant l'analyse multivariée avec la régression par machine à vecteur de support (SVMR), cette étude fournit quelques points saillants sur les caractéristiques (intensité et variabilité) des événements de sécheresse et le stockage d'eau terrestre (TWS) dérivé de GRACE et l'influence du climat global sur le débit du fleuve Congo. La section sud du bassin montre une variabilité considérable dans les modèles spatiaux et temporels du SPEI et les sécheresses extrêmes sur le Bassin du Congo semblent avoir persisté avec une couverture de plus de 40 % en 1994. Cependant, on observe une baisse considérable de l'intensité des sécheresses depuis 2007, ce qui coïncide avec des périodes de fortes anomalies positives du débit (c'est‐à‐dire 2007‐010). La TWS dérivé par GRACE sur le Bassin du Congo est déterminé par les fluctuations annuelles des précipitations ( r = 0.81 avec un décalage de phase de trois mois) et par les fortes variations interannuelles du débit des rivières ( r = 0.88, α = 0.05). D'une manière générale, les résultats montrent que les variations des eaux de surface (issues des jauges et des modèles) du Bassin du Congo sont une composante clé de la colonne d'eau de GRACE. Les sorties du schéma SVMR indiquent que le climat global à travers les anomalies de température de surface de la mer des océans Atlantique ( r = 0.79, α = 0.05), Pacifique ( r = 0.79, α = 0.05) et Indien ( r = 0.74, α = 0.05) sont associés aux fluctuations du débit du fleuve Congo, et ils confirment l'importance de l'influence climatique sur l'hydrologie des eaux de surface du Bassin du Congo.
... The conversation around human-induced changes in TWS of the Congo Basin is important and requires further details. This is because as home to the world's second largest rainforest block (e.g., Oslisly et al., 2013), it is critical to advance knowledge on long-term effects of intense human actions, such as deforestation, on TWS dynamics. This will build on existing compendium of knowledge highlighting the sensitivity of climate to the loss of the Congo Basin rainforest and other ecological disturbance in the region (e.g., Bell et al., 2015;Malhi et al., 2013;Verhegghen et al., 2012). ...
Chapter
Understanding the impacts of climate on surface water hydrology is required to predict consequences and implications on freshwater habitats, ecological assets, and wetland functions. Although the Congo basin is considered a freshwater‐rich region, largely characterized by numerous water resources after the similitude of the Amazon basin, recent accounts of droughts in the basin are indications that even the most humid regions of the world can be affected by droughts and its impacts. Given the scarcity and limited availability of hydrological data in the region, GRACE (Gravity Recovery and Climate Experiment) observations are combined with model and SPEI (standardized precipitation evapotranspiration index) data to investigate the likelihood of such impacts on the Congo Basin's surface water hydrology. By integrating multivariate analysis with support vector machine regression (SVMR), this study provides some highlights on the characteristics (intensity and variability) of drought events and GRACE‐derived terrestrial water storage (TWS) and the influence of global climate on the Congo river discharge. The southern section of the basin shows considerable variability in the spatial and temporal patterns of SPEI and extreme droughts over the Congo Basin appear to have persisted with more than 40% coverage in 1994. However, there has been a considerable fall in drought intensities since 2007 and this coincides with periods of strong positive anomalies in discharge (i.e., 2007‐010). GRACE‐derived TWS over the Congo basin is driven by annual fluctuations in rainfall ( r = 0.81 at three months phase lag) and strong interannual variations of river discharge ( r = 0 .88, α= 0.05). Generally, results show that changes in the surface water variations (from gauge and model output) of the Congo basin is a key component of the GRACE water column. The outputs of the SVMR scheme indicate that global climate through sea surface temperature anomalies of the Atlantic ( r = 0.79, α= 0.05), Pacific ( r = 0.79, α= 0.05), and Indian ( r = 0.74, α= 0.05) oceans are associated with fluctuations in the Congo river discharge, and confirm the importance of climatic influence on surface water hydrology in the Congo basin.
Article
The vast tropical rainforests of West and Central Africa were impacted by marked climatic deterioration, which mainly developed between 2500 and 2000 cal yr BP. However, the incidence of human intervention in these rainforests appears all the more speculative, although this was the period when the first stages of Bantu migration occurred towards the south. On the scale of the last millennium, which has seen increasing penetration and colonization of these Bantu peoples, one can envisage a priori a more accentuated anthropogenic pressure on the forest landscape. The combined observations of the environmental processes during the last 1000 years, as preserved in sediments from lakes, swamps, flooded forests and even oceanic environments, are presented on the basis of 24 records. The dominant number of sites in the humid forest or wooded savannas recorded only processes of relatively small size and local effects influencing vegetation cover almost simultaneously around 1000 and 500 cal yr BP; nothing is comparable with the devastating effect on a very large scale of the 2600 cal yr BP forest crisis. Conversely, other sites located in more perennial forest or savanna did not reveal any discontinuity. In all the examples of sedimentary archives considered, no out-of-phase or unusual event, possibly linked to human population and its impact, was observed. However, in more recent times, in intensive iron-ore mining/smelting areas, changes in the vegetal cover could be a legacy of charcoal production impacting forest biomass. Nevertheless, despite the clear cultural and socio-political importance of iron, its role in shaping vegetation communities in the Central African forest is generally presumed to be negligible. The oceanic causes of these climatic oscillations are more widely accepted. On a larger scale, the Medieval Warm Period and Little Ice Age are climatic anomalies visible in a number of palaeoenvironmental records worldwide and date to ~1600–1000 cal yr BP and ~ 700–100 cal yr BP, respectively. These events are less visible or sometimes not present in some palynological records of tropical Africa, whereas reconstructions from lacustrine records in Atlantic equatorial Africa show overall reductions in lake levels and increases in pollen belonging to light-demanding and pioneer vegetation formations in relation to these two oscillations. To reply to what was more important «the potential influence of human activities on regional climate, or the vulnerability of societies to environmental variability», it appears clearly to date, that the second paradigm is most likely. However, today, the ongoing combined efffects of climate change and human impact, including the decimation of the diversity of seed dispersers necessary to regenerate the rainforests of West and Central Africa, may have irreparable and unforeseen consequences.
Article
The dispersal of Bantu-speaking people from their ancestral homeland in the borderland between current-day Nigeria and Cameroon across most of Central, Eastern, and Southern Africa had a significant impact on the languages, cultures, and demography of autochthonous populations. Inversely, foragers and pastoralists also considerably contributed to the gene pool of Bantu-speaking communities, the speciation of their languages, and the evolution of their cultures. In this chapter, the impact of indigenous languages on Bantu language variation is assessed by comparing the language contact situations in Southern and Central Africa. Southern Africa is much better documented, because the much shallower time depth of contact between Bantu-speaking newcomers and autochthonous populations allowed the latter to survive as separate populations, often maintaining a language unrelated to Bantu. In Central Africa, the dispersal of Bantu languages is much older. Together with the success of other families, such as Ubangi and Central-Sudanic, it led to the death of all languages previously spoken by rainforest hunter-gatherers. Still little is therefore known about prehistoric language contact between indigenous forest foragers and immigrant communities. Nonetheless, Southern Africa provides us with useful insights to be tested in Central Africa.
Article
With a drastic decrease in their populations over the last decades, forest elephants Loxodonta cyclotis are facing increasing human pressure. Their decline will have serious ecological consequences, as they are key actors in shaping ecosystems. Whilst timber concessions host many mammal species, the interactions between selective logging and forest elephants remain unclear. Through an extensive literature review, we discussed the following: 1) the ecological and human factors that drive the distribution of forest elephants on a large scale as well as in the specific context of logged forests; 2) the contribution of forest elephants to the regeneration of timber species; and 3) the damage caused by forest elephants to timber species. Although human activities have the greatest impact on forest elephant distribution, it is the availability of food, water, and minerals that locally determines their use of the habitat. Under specific conditions, timber concessions may host large populations of forest elephants. As effective seed dispersers, forest elephants contribute to the regeneration of at least 41 timber species, such as Bobgunnia fistuloides (pao rosa), one of the most expensive woods on the market. Damage caused by forest elephants is diverse and affects a wide range of species. From branch breaking to bark stripping, at least 61 timber species are used by forest elephants, and little is known about the consequences for the tree's vitality and wood quality. The interactions between forest elephants and logging are complex and involve many variables, requiring additional research. Nevertheless, this review suggests that timber concessions constitute key areas for forest elephant conservation, provided that low‐impact logging and wildlife management are implemented. Avec une diminution drastique de leurs populations au cours des dernières décennies, les éléphants de forêt Loxodonta cyclotis font face à une pression humaine croissante. Jouant un rôle clé dans le façonnement des écosystèmes, leur déclin aura d'importantes conséquences écologiques. Alors que les concessions forestières abritent de nombreuses espèces de mammifères, les interactions entre l'exploitation sélective du bois d'œuvre et les éléphants de forêt restent floues. Par une revue approfondie de la littérature, nous avons examiné: 1) les facteurs écologiques et humains qui déterminent la distribution des éléphants de forêt à grande échelle ainsi que dans le contexte spécifique des forêts exploitées; 2) la contribution des éléphants de forêt à la régénération des espèces de bois de d'œuvre; et 3) les dommages causés par les éléphants de forêt aux espèces de bois d'œuvre. Bien que les activités humaines soient le facteur le plus important dans la répartition géographique des éléphants de forêt, c'est la disponibilité en nourriture, en eau et en minéraux qui conditionne localement leur utilisation de l'habitat. Dans certaines conditions, les concessions forestières peuvent abriter de grandes populations d'éléphants de forêt. En tant qu'important disperseur de graines, les éléphants de forêt contribuent à la régénération d'au moins 41 espèces de bois d'œuvre, telles que Bobgunnia fistuloides (pao rosa), l'un des bois les plus chers du marché. Les dommages causés par les éléphants de forêt sont divers et touchent un large éventail d'espèces. Du cassage des branches à l'écorçage des arbres, au moins 61 espèces de bois d'œuvre sont concernées, et peu de choses sont connues quant aux conséquences sur la vitalité de l'arbre et la qualité du bois. Les interactions entre les éléphants de forêt et l'exploitation forestière sont complexes et font intervenir de nombreuses variables, ce qui nécessite des recherches supplémentaires. Néanmoins, cette revue de la littérature suggère que les concessions forestières constituent des zones clés pour la conservation des éléphants de forêt, à condition qu'une exploitation à faible impact et une gestion de la faune soient mises en œuvre. Forest elephant populations have been seriously declining for decades. Covering 27% of the central African rainforest, timber concessions are key areas for the conservation of this emblematic species. Through an extensive literature review, we described the interactions between logging and forest elephants. Under specific conditions, timber concessions can host large populations of forest elephants, which contribute to the regeneration of many timber species. Conversely, foraging activities by forest elephants may also cause severe damage to trees, but the consequences to the tree's vitality and wood quality are unknown. We suggested additional research to identify the most appropriate management strategies and promote forest elephant conservation within timber concessions.
Chapter
Apart from inducing unusually high atmospheric CO2 growth rates in productive regions, and its other far-reaching long-term impacts on ecophysiology and ecosystem dynamics, drought events are the world’s most costly natural disasters. The United States, for example, suffered agricultural losses estimated at 40 and 30 billion due to the 1988 and 2012 extreme drought events, respectively. The overarching complexities of drought make it difficult to predict and accurately assess its causes, intensity, duration, magnitude and impacts (e.g., extents) on several spatio-temporal scales. This chapter explores a satellite remote sensing monitoring perspective and a range of multivariate framework and geospatial tools that underpin accurate assessment of the role of global climate teleconnection patterns.
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First book using archaeological data to explain the past 100,000 years of the history of Gabon, Central Africa.
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A new pollen sequence from the Lake Mbalang (7°19´ N, 13°44´ E, 1110 m a.s.l.) located on the eastern Adamawa plateau, in Central Cameroon, is presented in this paper to analyze the Holocene African Humid Period (AHP) termination and related vegetation changes at 7° N in tropical Africa, completing an important transect for exploring shifts in the northern margin of the African Monsoon. This sequence, spanning the last 7000 cal yr BP, shows that the vegetation response to this transitional climatic period was marked by significant successional changes within the broad context of long-term aridification. Semi-deciduous/sub-montane forest retreat in this area is initially registered as early as ca. 6100 cal yr BP and modern savannah was definitely established at ca. 3000 cal yr BP and stabilized at ca. 2400 cal yr BP; but a slight forest regeneration episode is observed between ca. 5200 and ca. 4200 cal yr BP. In this area with modern high rainfall, increasing in the length of the dry season during the AHP termination linked to a contraction of the northern margin of the Intertropical Convergence Zone (ITCZ) from ca. 6100 cal yr BP onward, probably associated with decreasing in cloud cover and/or fog frequency, has primarily controlled vegetation dynamics and above all the disappearance of the forested environment on the Adamawa plateau. Compared to previous studies undertaken in northern tropical and Central Africa, this work clearly shows that the response of vegetation to transitional periods between climatic extremes such as the AHP termination might be different in timing, mode and amplitude according to the regional climate of the study sites, but also according to the stability of vegetation before and during these climatic transitions.
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Past limnological conditions of Lake Mbalang (7°19′ N, 13°44′ E, altitude: 1130 m) and vegetation type were reconstructed from diatoms and sedimentary stable carbon isotope records (δ<sup>13</sup>C) since 7200 cal yr BP. The data showed that before 3600 cal yr BP, the water column was dominantly stable except around 5000–5300 cal yr BP where diatoms evidenced a mixed upper water layer and δ<sup>13</sup>C data suggest more forested vegetation in the landscape. These stable conditions can be explained by a strong monsoon flux and relatively northern position of the ITCZ that entailed high or low rainfall well distributed over the year, allowing the development of mountainous forest taxa. The decreasing trend of the monsoon flux towards the mid-Holocene was affected by several abrupt centennial to millennial-scale weakening at 6700, 5800–6000, 5000–5300, 4500 and 3600 cal yr BP. However, their impact on the vegetation is not visible, probably because rainfall distribution was favourable to forest maintenance or extension. After 3600 cal yr BP, the water column became very mixed as a result of more intense NE trade winds (Harmattan) that led at ~3000 cal yr BP to the establishment of savannah in the vegetation landscape. At that time, rainfall was probably reduced following the southward shift of the ITCZ, and the distribution of yearly rainfall was not favourable anymore to forest development. A strong seasonality with a marked dry season was established, conditions that maintained the savannah vegetation until today. Diatom data suggest the lake did not dry up during the last 7200cal yr BP; however, a low lake level observed at 2400–2100 cal yr BP is contemporaneous to a climatic event evidenced in several areas of tropical Africa and could correspond to the southernmost position of the ITCZ. Other low lake levels are observed at 1800 and 1400 cal yr BP, after which the lake rose to its present level.