25
Global aiid plane ta^ Change, 7 (1993) 25-10
Elsevier Science Publishers B.V., Amsterdam
Tropical forest changes during the Late Quaternary in African
and South American lowlands
Michel Servant
a,
Jean Maley b, Bruno Turcq a, Maria-Lucia Absy ', Patrice Brenac
hliarc Fournier a and Marie-Pierre Ledru a
a ORSTOhf, CeJitre de Boiidy, 72 Route d;?ulrta)~, 93143 Bondy C E D E 4 Frarice
des Sciences er Tec11t;iqiies du Lauguedoc, h4oo,itpellier31060, France
ORSTOh( Laboraroire de Pal~atologiedu C,VRS, U~ii~wsbe'
IA'PA, CP 478, 69000 .Vanaus, Ah( Brazil
(Received February 3, 1992; revised and accepted June 16, 1992)
ABSTRACT
Sentant, h4., htaley, J., Turcq, B., Absy, h4.-L., Brrnac, P., Fournier, h4. and Ledru, h4.-P., 1993. Tropical forest changes
during the Late Quaternary in African and South American 1ou)lands. Global Planet. Change, 7: 25-10
Arboreal pollen and montane elemenfs of Late Quaternary pollen assemblages from three lacustrine cores (M'est
Cameroon, southeastern Amazonia and central Brazil) are correlated, by the radiocarbon chronology, with other palaeoem'ironmental records in Africa and South America.
We observe in hoth continents a u9ell-de\'eloped dense forest at 30,000 and 9000 yr B.P. The succession of vegetation
types during the Late Quaternary appeared strongly related to the regional conditions: ( I ) the dense forest was more or less
degraded depending on the regions during lhe last full glacial period (20,000-15,000 yr B.P.); (2) a slow increase of tree
elements is evidenced in some areas during the Late Glacial (15,000-10,000 yr B.P.), whereas short-term fluctuations
occurred in central Brazil during the same time; (3) a strong regression of the forest during the middle Holocene (6000-5000
yr B.P.), in the southern tropical zone of South America, \vas in opposition to a full forest development in Africa.
In both continents wo main features characterize the tropical forest evolution: ( I ) h4ontane elements developed in the
lou.lands during lhe last glacial period and in some southern or northern regions during the early Holocene; and (2) the
climate seasonality was enhanced in several regions since SOO-7500 yr B.P.
For a tentative explanation, we relate the cold o r cool climate, inferred by palaeoecological evidences in the glacial
period and glacial-interglacial transition, to polar air-masses reaching more frequently the tropical zone. This interpretation
explains the apparent contradiction between the markedly low temperature of the continental lowlands opposed: (1) at
1S,OOO yr B.P., to the I-2°C lower Sea Surface Temperature of tropical oceans and (2) to the global ularming during the late
glacial. During the middle and Late Holocene, climate evolution was mainly influenced by the latitudinal shift of the ITCZ
positions in July and January and, in South America, by short-term changes of the zonal atmospheric circulation.
Introduction
In the tropical continental zones, the transition from glacial to interglacial conditions is
marked by drastic changes' in vegetation and
biomass (Adams et al., 1990) controlled by
palaeoclimate variations. The main evidences of
tropical forest changes came from geomorphological studies. Upper Quaternary eolian dunes and
gullied erosion features, today fixed by the forest,
indicate that present vegetation had regressed
during the past (Tricart, 1958, 1974; De Ploey,
1965; Ab'Saber, 1972; Journaux, 1975; Semant et
al., 1981,1989). On the other hand, the heteroge-
neous geographical distribution of rain forest tasa
and the high species diversity of rain forest
ecosystems were interpreted as a consequence of
past arid climates which have fragmented the rain
forest and isolated biological communities in some
restricted areas called "refuges" (Aubreville,
1919, 1962; Haffer, 1969; Vanzolini and Williams,
1970; Brown and Ab'Saber, 1979; Prance, 1973,
1952). Palynological evidences for a drier climate
during the last full glacial have been documented
in Ghana (Maley, 1987) and Congo (Preuss, 19901,
but a continuous record of vegetation evolution is
lacking for the last glacial period in South America lowlands (Markgraf, 1989). However, for the
'
"'Crn.
26
!
j
.
i
last 12,000 years quite a lot of results are available: the rain forest was largely degraded before
10,000 yr B.P. in some parts of Amazonia (Absy
and Van der Hammen, 1976; Van der Hammen,
1991) and Central America (Schubert, 1988,
Markgraf, 1989); short-term modifications of the
floristic composition (Absy, 1975, 1979; Liu and
Colinvaux, 1988) and regional regression of the
forest (Servant et al., 1981, 1989) occurred during
the Holocene in some areas of the present humid
tropical zone of South America.
Upper Quaternary forest fluctuations were first
explained by average rainfall variations or seasonal changes (Van der Hammen, 1974). Temperature changes were also taken into account. In
African lowlands, pollen assemblages from the
glacial 28,000-10,000 yr B.P. interval revealed the
presence of some elements today associated to
montane forest climate (Maley and Livingstone,
M. SERVANT ET AL.
1983; Maley, 1987). In South America, studies in
Ecuadorian Amazonia have also recorded the
presence of montane forest elements in pollen
assemblages dated from 30,000 yr B.P. (Bush et
al., 1990).
Moreover, during the Pleistocene-Holocene
transition, palynological records evidenced montane forest elements in some sites of Central
America (Markgraf, 1989) and Africa (west
Congo, Elenga and Vincens, 1990). These extensions to the tropical lowlands of montane elements were interpreted as an effect of lower than
today temperatures (Maley and Livingstone, 1983;
Maley, 1991; Bush et al., 1990)
Global palaeoclimate models, taking into account the variations of reconstructed parameters
such as insolation, ice-sheet area, Sea Surface
Temperature (SST). .. , explain the main stages
of climate evolution during the last 18,000 years
,
Mean positions 01 lhe ITCZ (aililude)
li;;;a~o;ilions
01 lhe meleorolosical Equalor
Polar adreclions in South Amerlca
.....,...
Trapictl humid l o r e s t s (lowlands)
Andes and Eislcrn Alricalhighlsndsl
Fig. 1. Location Map of the studied sites and other palynological records from forest ecosystems in tropical lowlands. 1 = Lake
Bosumtwi (Maley and Livingstone, 1983). 2 = Bois de Bilanko (Elenga and Vincens, 1992). 3 = Pointe Noire region (Elenga et al.,
1992). 4-6 = Guyana (Wijmstra and Van der Hammen, 1966). 7-10 = Central Amazonia (Absy 1979). II = Maraba Island (Absy,
1985). 12 = Rondonia Region (Absy and Van der Hammen, 1976). 13 = Mera (Bush et al., 1990). 14 = Lake Ayauch (Bush and
Colinvaux, 19881, Lake Kumpak (Liu and Colinvaux, 1988) ind San Juan Bosco (Bush et al., 1990). 1 5 = L a k e Valencia
(Salgado-Labouriau, 1980; Bradbury et al., 1981; Leyden, 1983). 16 = Panama Canal (Bartlett and Barghoorn, 1973). 17= El Valle
(Bush and Colinvaux, 1990). 18 = Feten (Leyden, 1985).
TROPICAL FOREST CHANGES DURING LATE QUATERNARY IN AFRICAN A“ SOUTH AMERICAN LOWLAh‘DS
by enhancement or weakening of the monsoonal
influences and by latitudinal shifts of the westerlies (COHMAP, 1988). But the low temperatures,
evidenced by the palynological records in the
tropical lowlands, are not reproduced by the
models (Rind and Peteet, 1985; COHMAF’, 1988)
and do not fit to the reconstructed tropical SST
which, excluding the upwelling zones, were only
1-2°C lower than the present values during the
last glacial maximum (18,000 yr B.P., C L I W ,
1981). Similarily the low temperatures of tropical
lowlands appear in contradiction with the global
increase of temperature during the glacial-interglacial transition between 15,000 and 9000 yr B.P.
(Jouzel et al., 1987; Broecker and Denton, 1989).
The interest of this paper is to gather new
pollen and sedimentological data from intertropical humid lowlands of Africa and South America.
A continuous pollen record has been obtained in
West Cameroon (Lake Barombi Mbo: Maley et
al., 1990; Brenac, 1988). In South America, two
different sites have been studied, one located in
southeastern Amazonia (Serra Sul dos Carajas:
Absy et al., 1991) and the second one in central
Brazil (Salitre: Ledru, 1991, 1992, 1993).
By comparison with other available data
(Markgraf, 1989; Maley, 1990; Van der Hammen,
1991) we can better describe the tropical forest
evolution since 30,000 yr B.P. in relation with
palynological data from the whole tropical zone,
particularly from the dry regions (Servant and
Servant-Vildary, 1980; Petit-Maire et al., 1991).
We will also discuss the problem of montane
element extension toward tropical lowlands and
refine a previous hypothesis explaining the low
continental temperatures by the effects of cold
polar air-mass advections into the tropical zone,
during the last glacial (Servant, 1973; ServantVildary, 1978). Moreover, we will show that low
period atmospheric circulations (magnitude order
of ten days), which are not considered by the
global climate simulations, have an important influence upon tropical environments- and especially upon the rain forests.
Studied sites
Lake Barombi Mbo (4”40’N, 9”24’S, Fig. 1) is
located in a maar crater of the West Cameroon
21
volcanic chain. It sits at 301 m elevation and is
distant of about 60 k m NNE of the 4000-m high
active Mt Cameroon stratovolcano (Maley et al.,
1990; Giresse et al., 1991). The lake Barombi
Mbo area receives a mean yearly precipitation of
2400 mm. The climate is of the monsoon type and
is linked to the seasonal migration of the Intertropical Convergence Zone (ITCZ). West
Cameroon experiences only one annual dry period (3 months) which occurs when ITCZ reaches
its southernmost position from December to
February (Maley, 1989, 1991). In that region the
rain forest is mainly evergreen with islands of
semi-deciduous type.
The Serra Sul dos Carajas (50”25’W, 6”20’S,
Fig. 1) is a narrow (1-2 km large) ferralitic
plateau, at 700-800 m altitude, where shallow
lakes or swamps are located in small depressions
of the lateritic crust forming the plateau surface.
Interest of this site is its special location in a
NW-SE “dry corridor” (RADAM, 1974) characterized by lower precipitations (1500-2000
mm/yr) than in the neighbouring Amazonian regions. The yearly dry period occurs during two to
three months between June and August.
The Serra dos Carajas is surrounded by a wet
type evergreen forest including patches of deciduous trees. Modern pollen rain in the plateau
mostly contains eIements of this surrounding rain
forest (Absy et al., 1991; Absy, unpublished data).
The Salitre depression, in central Brazil (19”S,
46“46’W, Fig. 11, is situated at a 1050 m elevation,
in the Serra do Salitre alcaline intrusion. The
depression, 1 km diameter, is occupied by a
swamp. The mean annual rainfall in that region
range between 1500 mm in Salitre and more than
2000 mm in the Atlantic coastal chains. Salitre is
influenced both by the Atlantic and Amazonian
wet air masses when the ITCZ reaches its southernmost position (south hemisphere summer).
The northward migration of polar advections enhances the precipitations in summer and reduces
the dry season in winter (3-0 months). It is also
responsible for low temperatúre minima and the
o&urrence of freezes. T h e SaIitre depression is
located in a semi-deciduous forest which is replaced by a rain-forest called “Mata Atlantica”
towards the Atlantic coast and by the Araucaria
25
forest in the South, where cold polar advections
are more intense. Modern pollen rain studies
have been used to identify the indicator taxa of
these different types of vegetation (Ledru, 1991,
1993).
M. SERVANT ET AL.
A g e 14C YR 0 P
Earombi Mbo
o Salitre
o Carajas
I
hlaterials
- 10
For the three above-described sites, the sediments were obtained through coring. In the Lake
Barombi Mbo the 23.5-m long studied core, BM-6,
loo/
was recovered with a Kullenberg corer, through a
water depth of 110 m (Maley et al., 1990). The
L
highly laminated sediments are mainly composed
of clay containing 5-10% organic carbon (Giresse
et al., 1991). In the Serra Sul dos Carajas, the .
CSS2 core was collected in a former lake, 700-m
long and 200-m wide, now occupied by a swamp.
300
The 6.55-m long core shows an alternation of
sandy layers, composed of quartz and hematite
\
tt
1 ~ ~ ' " ' ' ' ' '1~ ' '
grains, rich in siderite, and organic-rich layers,
Depth scale for
Depth scale for
with 35-60% organic carbon (Sifeddine, 1991).
Earombi Mbo I*)
Salitre ( O ) and Carajas ( O )
The clastic layers are related to high terrigenous
Fig. 2. Age data plotted against depth intervals of the samples
for the three cores. Note that the depth scale is different for
fluxes attesting an active erosion on the plateau.
Salitre (central Brazil) and Carajas (eastern Amazonia), on
The sharp bases of these layers correspond to
the left side, than for Barombi Mbo (Cameroon) on the right
sedimentation hiatuses due to overdrping of the
side of the diagram.
lake (Absy et al., 1991). In the Salitre depression,
the LC3 core is 6-ni long. The sediments are
lower for Carajas (9.8 cm/103 yr) and Salitre (6.3
organic-rich (30-50% organic carbon) with nucm/103 yr) cores. Sedimentation hiatuses were
merous vegetal remains in the upper part (Ledru,
observed be&een 22,000 and 13,000 yr B.P. in
1991; Soubibs, pers. conim.1.
Carajas (Siffedine, 1991) and between 17,000 and
As the main purpose of this paper is to com28,000 yr B.P. in Salitre (Ledru, 1993). They are
pare palaeoenvironmental changes based on the
interpreted as a drying of the shallow lacustrine
radiocarbon chronology, we will only focus our
environments during arid periods. In the Salitre
discussion on the last 30,000 years. Radiocarbon
core, quite similar ages were obtained at 73 cm
ages were mostly performed on total organic carand 85 cm. This section corresponds to low pollen
bon or on wood fragments. In the Barombi Mbo
grain concentration, to the highest organic carbon
core some measurements were done on total carcontent and to the highest C/N ratio. These
bon but ages are thought to be only 5-10%
characteristics are in good accordance with the
higher than for organic carbon samples, because
high sedimentation rate suggested by the radiocarbonate precipitation was quasi-synsedimentary
carbon data.
(Giresse et al., 1991). On Fig. 2, radiocarbon
For each site, pollen analyses have been pubages, plotted against core depth, exhibit a good
lished elsewhere or are in preparation (Barombi
internal coherence for each core. In Barombi
'Mbo: Brenac, 1988; Maley, 1989; Carajas: Absy e t
Mbo, the sedimentation rate is 72 cm/103 yr for
al. 1991; Salitre: Ledru 1991, 1992, 1993). In this
the Late Pleistocene section, increasing to 127
paper,
we use essentially the Arboreal Pollen
cm/103 yr in the Holocene section (Maley et al.,
(AP)and montane elements percentages versus
1990). The mean sedimentation rates are much
i
t
n
\
I
I
I
*
I
I
8
1
4
9
(
I
l
3
1
I
t
1"
~
TROPICAL FOREST CHANGES DURING LATE QUATERNARY IN AFRICAN AND SOUTH AMERICAN LOWLANDS
total pollen (aquatic taxa and spore; being excluded). Modern pollen rain studies confirmed
that the high (> 70%) and low ( < 20%) AP percentages are strongly correlated with dense forest
and savannas respectively (Brenac, 1988; Lezine
and Edorth, 1991; Ledru, 1991, 1993).,Thus they
offer an ubiquitous indicator of major tropical
forest changes. The intermediate values (20-70%)
correspond to transition forest types whose characterization needs a more accurate interpretation
of the pollen assemblage.
Forest changes and intertropical paleoclimate
The AP curves clearly show up differences and
similarities in the forest changes from the sites of
Barombi Mbo, Carajas and Salitre, presently submitted to different climate dynamics (Fig. 3). We
will focus our examination on the vegetation
phases which may be correlated with the main
events of the global evolution: 30,000 yr B.P. (end
of a relatively warm period during the last glacial:
Lorius et al., 1985), 20,000-15,000 yr B.P. (last
full glacial: Broecker and Denton, 1989), 9000SO00 yr B.P. (early present interglacial period)
and 6000-5000 yr B.P. (climatic optimum of the
European chronology, Huntley and Prentice,
1988).
Around 30,000 yr B.P., a forest phase, with
peculiar floristic compositions, is marked in
southeastern Amazonia (Carajas) and central
Brazil (Salitre) by high AP percentages in the
pollen assemblages (Fig. 3). It was also identified
in western Amazonia (Van der Hammen, 1991;
Bush et al., 1990) and Panama (Bush and Colinvaux, 1990). We do not dispose of such old palynological record in the African lowlands but well
dated podzolic soils in the western part of the
Congo basin (Schwartz, 1985; Preuss, 1990) indicate that a humid forest took place between
35,000 and 30,000 or 25,000 yr B.P. in this equatorial area.
The transition between the 30,000 yr B.P. forest phase and the 20,000-15,000 yr B.P. period is
not well known, still due to the lack of continental record. However, we observe that the tropical
forest remained well developed between 23,000
and 20,000 yr B.P. in West Cameroon (Fig. 3a)
29
while it was replaced by savanna or open-forest
since 28,000 yr B.P. in Ghana (Bosumtwi: Maley,
1987) and southeastern Amazonia (Fig. 31, respectively. Thus the forest regression occurred
earlier in some areas than in the other ones.
During the same time, lake levels stood high in
the presently dry tropical regions (Sahara: PetitMaire et al., 1991; Bolivian Andes: Servant and
Fontes, 1978).
During the 20,000 - 15,000 yr B.P. interval, a
phase of forest degradation took place. At 20,000
yr B.P., according to pollen data from Barombi
Mbo, humid forest began to decline, the process
having culminated at 16,000 yr B.P. (Fig. 3). Similar results were obtained from geomorphological
and palynological studies in other regions of
Africa (Congo: Lanfranchi and Schwartz, 1990;
Preuss, 1990; Cameroon: Tamura, 1990; Stoops,
1990; Ghana: Maley 1957). The forest regression
appeared more or less intense in the different
regions. For instance, the forest was completely
replaced by savanna in Ghana while an open
forest or a patchy association existed in West
Cameroon (Maley, 1991). In South America the
regression of the forest was not directly observed.
In Carajas, no data were available during the
sedimentation hyatus (22,000-13,000 yr B.P.) but
we may think that AP percentages, low at 22,000
and lower at 13,000 yr B.P. (Fig. 31, were remaining low during this period when the lake overdryed. In that case, the forest regression in Carajas should concern all the Amazonian "dry corridor" as suggested by Van der Hammen (1991).
The degraded forest between 20,000 and 15,000
yr B.P. in African lowlands and at least in some
parts of Amazonia, are in good correspondence
with a n intertropical arid phase evidenced in the
Sahara and the Sahelian zone (Servant, 1968;
Petit-Maire et al., 1991), with low Iake levels in
Africa (Street and Grove, 1979) and in some
regions of South America as the Bolivian Altiplano (Servant and Fontes, 1978; Ybert, 1992).
During the Late Glacial (15,000-10,000yr B.P.),
the open forest which is markkd by moderate AP
values in the pollen assemblages of Salitre regressed during two short phases, firstly at 14,00013,000 yr B.P., secondly and strong& around
10,500 yr B.P. (Fig. 3). Such short events did not
+
. .-
M. SERVANT ET AL.
30
tropical forest have also been documented in
other sites of Africa (Maley, 1991), South America (Van der Hammen, 1991) and Central America (Markgraf, 1989). 'During the same period,
lake levels were higher than today with relative
appear in both equatorial sites, Carajas and
Barombi Mbo. On the contrary, a progressive
increase of AP until 10,000 yr B.P. suggests a
slow development of the tree elements (Fig. 3).
Pollen evidences for a progressive increase of the
AP Oh
1O0
80
60
40
2o
o
1
1! .
O
I-
li
8
'
8
* "
'
1
I
'
'
10,000
a
"
I
"
1
' " " '
20,000
" ' 1 "
*
30,000
I
Yr B.P.
BAROMBI MBO (WEST CAMEROON)
O
10,000
20,000
30,000
Yr B.P.
CARAJAS (EASTERN AMAZONIA)
AP%
100
80
60
4Q
20
C
O
10,000
20,000
30,000
i
yr B.P.
SALITRE (CENTRAL BRAZIL)
Fig. 3. Variations of Arboreal Pollen percentages (Al'%) during the last 30,000 years' in the three studied sites. The curves are
drawn according to the age-depth relation in Fig. 2. Curve interrupts correspond to sedimentation hiatuses.
-.
--.e
TROPICAL FOREST CHANGES DURING U T E QUATERKARY IN AFRICAN AND SOUTH AMERICAN LOWLANDS
high stands between 13,000 and 11,000 yr B.P. in
equatorial Africa (Lake Bosumtwi: Talbot et al.,
1984) as well as in the Sahelian zone (Servant and
Servant-Vildary, 1980), in the Sahara (Petit-Maire
et al., 1991) and in some areas of South America
(Bolivian Altiplano: Servant and Fontes, 1978;
Colombian Andes: Van der Hammen, 1974). In
spite of these higher values of the tropical water
budget, the forest was less extended than at present during the Late Glacial.
At 9000 yr B.P., the full development of tropical forest is indicated by the high AP values in
the pollen assemblages of Barombi Mbo and
Carajas (Fig. 3). It was observed in all the other
sites which have been studied in the present
humid tropical zone in Africa (Maley, 19911,
South America (Van der Hammen, 1991) and
Central America (Markgraf, 1989). At the same
time, lake level reconstructions indicate that the
continental water-budget was better than today in
northern Africa (Street and Grove, 1979; Talbot
et al., 1984), precipitation being higher than evaporation (Servant and Servant-Vildary, 1980). Lake
levels were also higher in Venezuela (Lake Valencia: Salgado-Labouriau, 1980; Bradbury et al.,
1981) and in the Caribbean zone (Hodell et al.,
1991), on the contrary they were lower in the
northern tropical zones of Central America
(Yucatan: Covitch and Stuiver, 1974; Mexico:
Sirkin, 1985) as well as in the southern tropical
Andes where the studies of Lake Titicaca cores
indicate a lake level lower than today in the early
Holocene (Ybert, 1992).
The transition between the early and middle
Holocene is marked by an increase of deciduous
elements in central Brazil (Salitre: Ledru, 1992)
at 8000 yr B.P. This is interpreted as a mqre
prominent seasonal dry interval. Similar interpretation was obtained from palynological and palaeolimnological studies in many other regions of
South and Central America (Markgraf, 19891,
equatorial Africa (western Congo basin: Elenga
and Vincens, 1990) and northern tropical Africa
(Servant and Servant-Vildary, 1980).
At 6000-5000 yr B.P., AP percentages remained high in the pollen assemblages of Barombi
Mbo core. In Carajas, their decrease which had
begun around 8000 yr B.P. reached its maximum
.
31
at 5000 yr B.P. (Fig. 3). In Salitre, they,show very
low values at 6000 yr B.P. (Fig. 3). Therefore the
full developed forest in West Cameroon was opposed to the degraded forest in southeastern
Amazonia and central Brazil. This opposition is
ascertained by other sites in both continents. In
Ghana (Lake Bosumtwi: Maley, 1987) and western Congo basin (Bois de Bilanko: Elenga and
Vincens, 1990; Pointe Noire region: Elenga et al.,
1992) the high AP percentages indicate a well-developed forest cover. On the contrary, in eastern
Brazil, a gullied erosion phase between SO00 and
4000 yr B.P. demonstrates that the Mata Atlantica rain forest was largely degraded (Servant
et al., 1990). Such drastic changes were not observed in central and western Amazonia but
pollen assemblages indicate the regional occurrences of more dryer climate around 5000 yr B.P.
(Lago Surara: Absy, 1979). In Venezuela, the
evergreen forest was replaced by semi-deciduous
elements during the middle Holocene (Leyden,
1984). All these results suggest that the forest
regressed strongly in the present "dry corridor"
of the Amazonian basin (Carajas) and in the
southern tropical zone of South America, while it
only experienced a relative dryness in Central
Amazonia and northern South America. The low
lake levels in Venezuela (Lago Valencia: Bradbury et al., 19811, Colombian Andes (Van Geel
and Van der Hammen, 1973), Bolivian Altiplano
(Wirrmann and De Oliveira, 1987; Wirrmann et
al., 1988, Mourguiart, 1992) and a reduction of
the Amazon discharge (Shower and Bevis, 1988)
during the middle Holocene confirm this South
American dryness. On the contrary, in the equatorial and northern tropical zones of Africa, high
lake levels and the savanna shift towards the
Sahara, indicate a more positive water budget
than today during the middle Holocene (lake
Bosumtwi: Talbot et al., 1984; Chad basin: Servant, 1968; Sahara: Servant and Servant-Vildary,
1980; Petit-Maire et al., 1991; Schultz, 1987). At
the same time, high lake level was also observed
in the northern Caribbean zone (Hai'ti: Hodell et
al., 1991).
During the transition between the middle
Holocene to the Present, AP percentages strongly
increased in Carajas (Fig. 3b) and weakly de-
. .-
.
- ..- .
.
..
..
:
. .
_
&
u
, i-5
I1
32
M. SERVANT ET AL.
creased in Barombi Mbo (Fig. 3a). These data
suggest a new opposition. between southeastern
Amazonia and West Cameroon, confirmed, at a
larger scale in the tropical zone, by palynological
record and 13Canalysis of the soil organic matter
in presently forested areas of western Congo
"C
O
10,000
20,000
30,000
VOSTOK TEMPERATURE
yr BP
O
10,000
20,000
30,000
BAROMBI MBO (WEST CAMEROON)
yr BP
2 0 % - " ~ " ~ ' " ' ~ " ~ ~
.
.
.
I
I
.
*
*.....,,.
I
,
,
, . ,
-
Araucaria
3%
,,, ..
I
* * .
.
+
.
.
.
(
, , , , . , . . ,
&--
, *
SALITRE (CENTRAL BRAZIL)
I
i
i
Fig. 4. Comparison between the temperature variations (difference from the modern surface temperature value) in Vostok
Antarctic ice core (Jouzel et al., 1989) for the last 30,000 years and the percentages of Montane elements in the pollen assemblages
from Barombi Mbo (Cameroon) and Salitre (central Brazil). The dotted line in Salitre indicates the percentages of Araucaria plus
Dr)~niirplus other taxa present in Araucaria forest but also in other forest types (Symplocos, Podocuppus, Ileu).
TROPICAL FOREST CHANGES DURING LATE QUATERNARY IN AFRICAN AND SOUTH AMERICAN LOWLANDS
(Schwartz et al., 1990, Elenga et al., 19921, increased aridity in Sahara (Petit-Maire et al., 19911,
extension of forest in southeastern Brazil (Semant
et al., 1989) and an abrupt rise of the Lake
Titicaca level at 3500 yr B.P. (Wirrmann et al.,
1988; Mourguiart, 1992).
However, short-term fluctuations in both continents complicated this general trend. Despite
moderate vegetation changes in h a z o n i a since
4000 yr B.P. several periods of low river level
occurred in central Amazonia (Absy, 1979) and
Colombian Llanos (Van der Hammen, 1991). In
Ecuadorian Amazonia sedimentological and palynological records suggest drier conditions between 4300-4200 and 3800-3150 yr B.P. (Bush
and Colinvaux, 19SS; Liu and Colinvaux, 19SS)
and during the 1500-800 yr B.P. interval (Liu and
Colinvaux, 1988). In the southern limit of the
present rain forest in Bolivia, large eolian dunes
fields indicate a short regression phase of the
forest after 3600 yr B.P. (Servant et al., 1981).
Precise correlation between all these records is
not yet possible since we do not dispose of sufficiently high time scale resolution. However, the
short-term fluctuations observed in both continents reveal an important climate variability during the transition between the middle Holocene
and Present in the humid tropical zones. Another
aspect of this climate variability may be reflected
by the occurrence of fires in South America.
Charcoals, not directly related with human disturbances, were observed in the soils of northwestern Amazonia (Sadford et al., 19851, eastern
Amazonia (Soubiès, 1980), central and eastern
Brazil (Servant et al., 1989) and the Bolivian
Llanos (Servant et al., 1981). One part of these
charcoals was dated from the middle Holocene
dry episode but another part lasted to the Late
Holocene. Consequently fires have sureIy played
an important role in the forest dynamics until
very recent time.
Montane forest elements
During the last glacial period, Olea hoclzsteta montane taxon, was abundant in western
Cameroon (Barombi Mbo) (Fig. 4) and had also
colonized the Ghana lowlands (Maley, 1987). Acteri,
33
cording to the present distribution of O. liochstetferi, its past occurrences suggest a colder climate
than today and an high atmospheric humidity.
Other montane elements (Podocaipus inilaizjaiius, Olea welwitsclzii-type, Ilex initis) were also
locally present in the west Congo basin during
the Late Glacial (Elenga and Vincens, 1990). In
Salitre, an Araucaria forest developed between
13,000 and 12,000 yr B.P. (Fig. 4) in the presently
semi-deciduous forest zone of central Brazil
(Ledru, 1991, 1992, 1993). The Araucaria forest
today corresponds to a wet and cool climate with
a short dry season, low temperature minima and
frequent freezes in winter. Similarly, in Ecuadorian Amazonia, some elements as Abtus and
Weiiiinaiiia-type, today represented in the eastern
Andes, were well developed around 30,000 yr
B.P. (Bush et al., 1990) while Quercus was present in Panama (Bush and Colinvaux, 1990). At
the same time the forested phase in southeastern
Amazonia (Carajas) was characterized by high
percentages of Ilex but, depending of the species,
this taxon is not only associated with montane
forest.
During the early Holocene, the montane element percentages became very low in the pollen
assemblages of Barombi Mbo but they are still
present in other regions. In the southern tropics,
Araucaria forest elements reached their higher
frequencies between 10,000 and SO00 yr B.P. in
Salitre (Fig. 4) while the Podocaipus nzilai7janu.s
forest keeps on in Bilango (Western Congo). In
the northern tropics, the early Holocene pollen
assemblages were characterized by a mixture of
montane forest and rain forest elements in
Panama (Bartlett and Barghoorn, 1973) and
Guatemala (Leyden, 1985).
Variations in the percentages of all these montane elements were very different during the Upper Quaternary from a site to another. In Africa,
the highest percentages of O. hoclzstetteri were
dated from 22,000 to 20,000 yr B.P. and from
14,000 to 13,000 yr B.P. in Barombi Mbo (Fig. 4)
whereas they occurred around 27,000 and 10,000
yr B.P. in Bosumtwi. In South and Central America, Aruucaiia was present at 12,000-13,000 and
chiefly between 10,000 and SOO0 yr B.P.in Salitre
(Fig. 4) whereas Quercus had its maximum around
'.. .
I
.~
34
M. SERVANT ET AL.
14,000 yr B.P. in Panama (Bush and Colinvaux,
1990). This lack of correlation between montane
elements maxima suggests that their extension
toward tropical lowlands was controlled by regional climate conditions. This is particularly apparent in Congo (Bois de Bilanko: Elenga and
Vincens, 1990) and central Brazil (Salitre, Fig. 4c)
where montane elements were still abundant at
the beginning of the Holocene while temperature
was close to its present value at a global scale
inclusive in tropical mountains as in the Andes
(Servant-Vildary and Roux, 19901, and East Africa
(Bonnefille, 1990). Interpretations given in Africa
(Maley, 1991) as well as in central Brazil (Ledru,
1991, 19921, link the edension of the montane
element toward low altitude zones to lower regional temperatures. These interpretations are
confirmed by observations realized in other regions of northern tropical Africa. In the Chad
Basin, phytoplankton (diatoms) of last glacial
palaeo-lakes exhibited cold species that presently
are living in tropical mountains and/or in temperate zones (Servant-Vildary, 1975). The higher
percentages of these species occurred episodically
during the 25,000-20,000 and 12,000-10,000 yr
B.P. intervals, particularly just after 11,700 yr
B.P., but one species is abundant until 7500 yr
B.P. It is thus probable that low regional temperature is the unique explanation for the irregular
40 -
1
I
I
development of both terrestrial and aquatic vegetation in the tropical lowlands.
Discussion and paleoclimate hypothesis
The major phases of tropical forest changes
approximately correspond with the continental
water-budget variations. At ca. 30,000 yr B.P. the
forest phase, that could only be obsenjed in some
regions, is included in a period of high or moderate lake levels in present dry tropical regions.
The data, however, are still unprecise for that
time. The 20,000-15,000 yr B.P. forest degradation phase'corresponds to a dry period whereas
the well-developed forest phase at 9000 yr B.P. is
associated with a well-recognized wet period. The
same correlation is also observed in the middle
and Late Holocene despite the opposite behaviours of Africa and South America. Notwithstanding these rough correlations, regional differences clearly appear in the timing and intensity of
forest degradation during the late full glacial
period as well as in the forest development before 9000 yr B.P. These regional differences are
also well marked by the behaviour of montane
forest elements.
As exposed above, the low temperatures interpreted from palaeoecological data in the tropical
lowlands during the last glacial period did not fit
I
I
i
i
I
10 -
O
I
01-6 10-6
l
20-6
l
30-6 10-7
I
I
20-7
31-7
l
10-8
l
20-8
31-8
Date
Fig. 5. Examples of polar advection influence upon temperature maxima in Santa Cruz d e la Sierra station, Bolivia (17"47'S,
63"10'W, altitude: 437 m; in Ronchail, 1989).
TROPlCAL FOREST CHANGES DURING LATE QUATERNARY 1N AFRlCAN AND SOUTH AMERlCAN LO\VUNDS
with the only 1-2°C lower estimated tropical SST
during the Last Glacial Maximum .(CLIMAp,
1981; Rind and Peteet, 1985; C O H W , 198s)
and with the global warming during the last
Glacial-Interglacial transition (Fig. 4). For a tentative explanation of these apparent contradktions, it was proposed, in Africa, that high nebulosity, related with upwelling enhancement,
should have provoked the inland low temperatures around the Guinea Gulf (Maley, 1989,1991).
According to another complementary interpretation, enhanced polar air-mass advections toward
the equator could have provoked low temperature minima in the warm tropical environment
(Servant, 1973; Servant-Vildary, 1978; Servant and
Servant-Vildary, 1980).
The climate of tropical South America appears
as the best present equivalent of the. climatic
conditions which prevailed in the whole tropical
zone during the Last Glacial and somewhere at
the beginning of the Holocene (Servant and Villarroel, 1979). In that region, southern polar advections moving northward induce strong temperature variations (Fig. 5 ) and occasional freezes
during winter (Ronchail, 1989). Exceptionally,
they pass across the equator and reach the northern tropics (Fig. 6 , Parmenter, 1976). Rainfall
pattern is also strongly affected (Kousky, 1979).
Precipitations occurred when the cold air masses
penetrate below a wet tropical atmosphere as
observed in the southern Amazonia and Atlantic
Brazil. The present distribution of the Araucaria
forest, a mixture of montane and rain forest
elements, is directly related to these climatic conditions in Brazil. Rainfalls are weaker when the
polar advections penetrate in a dry tropical atmosphere as in the southern lowlands of Bolivia
(Servant and Villarroel, 1979). Therefore an enhancement of these polar advections is the most
probable explanation for the low temperature
observed during the glacial times and the beginning of the Holocene in central BraziI (Ledru,
1991). The same explanation can be applied in
other tropical regions. Southern polar advections
along the Guinea Gulf coast may have provoked
a decrease of temperature minima in Congo and
West Cameroon during the Last Glacial. Northern polar advections have also caused the tem-
/
35
7/21
U
Fig. 6. Example of a polar air mass advection in July 1975 in
South America. Daily frontal positions (solid lines) and low
level cold air boundaries (dashed lines) were obtained from
satellite imagery (from Parmenter, 1976).
perature lowering in the Chad basin (ServantVildary, 1978) and perhaps in some parts of Central America. This interpretation is in good accordance with recent studies upon the migration of
polar air-masses (“Mobile Polar High”) towards
the equator in the two hemispheres (Leroux,
1992).
During the early Holocene, a warm and humid
climate allows the rain forest development in
West Cameroon, Ghana and southeastern Amazonia but the montane elements in some sites of
west Congo, central Brazil and Central America
lowlands reveal the occurrences of polar advections stronger than today in the southern and
northern tropics. They frequently provoked temperature decrease and enhanced precipitations in
winter. The tropical climate was cooler and the
dry season shorter than today. The polar advections influence can explain the development of
tropical forest at 9000 yr-B.P. in southern latitudes (specially in South America) in spite of the
weakened monsoonal influences simulated by the
global models ( C O H W , 1988). On the contrary; in the Bolivian Andes, the Lake Titicaca,
located above the influence of polar advections in
!
I
'
1
I
I
I
l
I
1
I
I
1
36
atmosphere low level, presented a water level
lower than present at that time.
For the middle Holocene, the model must be
completely modified. All the available data allow
us to say that the polar advections influence was
greatly reduced in the tropical zone. Thus, the
winter season became dryer in both hemispheres.
As more humid than today conditions are observed in all the northern African and in the
northern Caribbean zones, we can believe that
the West African monsoon was stronger than at
present and that the meteorological equator
(which, on the oceans, corresponds to the ITCZ)
was moving more to the north during the northern summer.
On the other hand, in equatorial and south
tropical zones, climate evolution was different in
the western and eastern atlantic regions. Forest
regression in southeastern Amazonia and central
Brazil was opposed to a stable fully developed
forest around the Guinea Gulf. These data indicate that the latitudinal shift of the ITCZ to the
south during the austral summer was reduced in
South America. It is possible that the dry events,
which presently occurred in South America (and
in other tropical regions as Indonesia) in relation
with the SST anomalies of the Pacific Ocean
(Kousky et al., 1984; Philander, 1990), were more
frequent during the middle Holocene (Martin et
al., 1992). At the same time, the palaeoenvironmental conditions in Borneo, dryer than during
the early Holocene (Sieffermann et al., 19881, fit
well to this hypothesis. However, more detailed
studies are necessary in the western and eastern
Pacific to confirm this scenario.
During the Late Holocene, increasing dryness
in North Africa and some lowering of lake levels
in the northern Caribbean zone show that the
ITCZ position was shifted to the south. At the
same time, the forest increase in South America
and the rising of the Lake Titicaca in Bolivia
suggest that the dry events, related to Pacific
Ocean temperatures, became less frequent in
South America. However, they still episodically
occurred as indicated by regional forest changes.
Occurrences of fires may be related to these
occasional dry events as it is presently observed in
the rain forest submitted to short dry anomalies,
M.SERVANT ET AL.
for instance, in southeastern Asia (Malingreau et
al., 1985).
All these interpretations demonstrate that intensity and frequency of weather changes, at a
scale of some days (low temperature minima,
nebulosity and rainfalls associated to the polar
advections), as well as intensity and frequency of
climatic changes, at a scale of some years (continental dryness linked to SST anomalies), have
played an important role in the vegetation evolution, specially for the tropical forest which is very
sensitive to temperature minima and to rainfall
annual distribution. These short-term changes,
that, until now, cannot be taken into account in
the palaeoclimate simulations, were superimposed on the general trends of climate evolution.
Conclusions
In spite of the lack of -numerous continuous
records in the humid tropical zones and of the
still unsufficient time scale resolution, the forest
changes data for the Late Quaternary strongly
suggest that interpretations must consider the
short-term atmospheric changes (ten days to ten
years). These data allow us to propose the following palaeoclimate scenario as an implement of
the previously proposed models:
During the 30,000-10,000 yr B.P. interval of
the last glacial time, the tropical climate was
mainly related to the cold air masses reaching
more frequently the equator. Consequently,
abrupt and short temperature variations, low
temperature minima and some precipitations during what is now the dry season (winter) occurred
in the tropical lowlands and strongly affected the
forest cover and the biological communities. This
interpretation explains the low continental temperatures opposed to a 1-2°C tropical SST decrease at 18,000 yr B.P. and to the global warming of the glacial-interglacial transition.
Duiing the last 10,000 years, the polar influence decreased. The warm equatorial forest developed during the early Holocene. The disappearance of winter precipitations reinforced the
dry tropical season at 8500-7500 yr B.P. in both
hemispheres. During the middle Holocene (6000
yr B.P.) the boreal summer position of the ITCZ
I
I
l
1
I /
I
I
TROPICAL FOREST CHANGES DURING LATE QUATERNARY IS AFRICAN AND
was more to the north than today. At the same
time, a dry phase, probably related with SST of
the Pacific ocean, occurred in South America.
During the Late Holocene, the general trend of
the climate evolution is characterized by a shifting of ITCZ to the south in South America. The
regional dry events became less frequent but continued to affect occasionally the forest dynamic.
Acknowledgements
This study is sponsored by GEOCIT and
ECOFIT programs. Studies in Brazil are funded
by an ORSTOM (FRA”E>-CNPq (BRAZIL)
convention. The research in West Africa was
supported by ORSTOM, CNRS and NSF and
sponsored by the Ministry of Higher Education of
Cameroon. The cores were collected by L. Martin, J.-C. Leprun, F. Soubies and K. Suguio in
Brazil and by D.A. Livingstone and his team in
Cameroon. We appreciate assistance of J.-M.
Flexor, T. Melhem and K.Suguio in Brazil. We
thank Dr C. Lorius for the Vostok ice core temperature data.
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