late quaternary and south american...

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 CEDE4 Frarice ORSTOh( Laboraroire de Pal~atologie du C,VRS, U~ii~wsbe' des Sciences er Tec11t;iqiies du Lauguedoc, h4oo,itpellier 31060, France

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'i- ronmental 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 Amer- ica lowlands (Markgraf, 1989). However, for the

' " ' C r n .

!

j . i

,

26

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). Temper- ature 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 exten- sions 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 pos i t ions 01 l h e ITCZ (a i l i lude)

li;;;a~o;ilions 01 l h e m e l e o r o l o s i c a l Equalor

Polar a d r e c l i o n s in South Amerlca

. ... .,... Trapict l humid l o r e s t s ( lowlands)

Andes and Eislcrn Alr ica lhighlsnds l

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). 15=Lake 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 21

by enhancement or weakening of the monsoonal influences and by latitudinal shifts of the wester- lies (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; Servant- Vildary, 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 espe- cially 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

volcanic chain. It sits a t 301 m elevation and is distant of about 60 km 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 In- tertropical 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 en- hances 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. The 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

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, was recovered with a Kullenberg corer, through a water depth of 110 m (Maley et al., 1990). The 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. The 6.55-m long core shows an alternation of sandy layers, composed of quartz and hematite grains, rich in siderite, and organic-rich layers, with 35-60% organic carbon (Sifeddine, 1991). The clastic layers are related to high terrigenous fluxes attesting an active erosion on the plateau. The sharp bases of these layers correspond to sedimentation hiatuses due to overdrping of the lake (Absy et al., 1991). In the Salitre depression, the LC3 core is 6-ni long. The sediments are organic-rich (30-50% organic carbon) with numerous vegetal remains in the upper part (Ledru, 1991; Soubibs, pers. conim.1.

As the main purpose of this paper is to com- pare palaeoenvironmental changes based on the radiocarbon chronology, we will only focus our discussion on the last 30,000 years. Radiocarbon ages were mostly performed on total organic carbon or on wood fragments. In the Barombi Mbo core some measurements were done on total carbon but ages are thought to be only 5-10% higher than for organic carbon samples, because carbonate precipitation was quasi-synsedimentary (Giresse et al., 1991). On Fig. 2, radiocarbon ages, plotted against core depth, exhibit a good internal coherence for each core. In Barombi Mbo, the sedimentation rate is 72 cm/103 yr for the Late Pleistocene section, increasing to 127 cm/103 yr in the Holocene section (Maley et al., 1990). The mean sedimentation rates are much

l o o /

- 10

L

300 i t \ 1" t t n I I I * I I 8 1 4 9 ( I l 3 1 I t 1 ~ ~ ' " ' ' ' ' ~ ' ' ~ ' 1 \

Depth scale for Salitre (O) and Carajas ( O )

Depth scale for Earombi Mbo I*)

Fig. 2. Age data plotted against depth intervals of the samples for the three cores. Note that the depth scale is different for Salitre (central Brazil) and Carajas (eastern Amazonia), on the left side, than for Barombi Mbo (Cameroon) on the right side of the diagram.

lower for Carajas (9.8 cm/103 yr) and Salitre (6.3 cm/103 yr) cores. Sedimentation hiatuses were observed be&een 22,000 and 13,000 yr B.P. in Carajas (Siffedine, 1991) and between 17,000 and 28,000 yr B.P. in Salitre (Ledru, 1993). They are interpreted as a drying of the shallow lacustrine environments during arid periods. In the Salitre core, quite similar ages were obtained at 73 cm and 85 cm. This section corresponds to low pollen grain concentration, to the highest organic carbon content and to the highest C/N ratio. These characteristics are in good accordance with the high sedimentation rate suggested by the radiocarbon data.

For each site, pollen analyses have been pub- lished elsewhere or are in preparation (Barombi

'Mbo: Brenac, 1988; Maley, 1989; Carajas: Absy e t al. 1991; Salitre: Ledru 1991, 1992, 1993). In this paper, we use essentially the Arboreal Pollen (AP) and montane elements percentages versus

TROPICAL FOREST CHANGES DURING LATE QUATERNARY IN AFRICAN AND SOUTH AMERICAN LOWLANDS 29

total pollen (aquatic taxa and spore; being ex- cluded). 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 char- acterization 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), 9000- SO00 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 Colin- vaux, 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)

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: Petit- Maire 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). Simi- lar 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 remain- ing low during this period when the lake over- dryed. In that case, the forest regression in Cara- jas 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 an 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 Alti- plano (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 14,000- 13,000 yr B.P., secondly and strong& around 10,500 yr B.P. (Fig. 3). Such short events did not

- +

. .-

M. SERVANT ET AL. 30

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

tropical forest have also been documented in other sites of Africa (Maley, 1991), South Amer- ica (Van der Hammen, 1991) and Central Amer- ica (Markgraf, 1989). 'During the same period, lake levels were higher than today with relative

AP Oh 1 O0

80

60

40 1 li I-

2o 1 o ! . 8 ' 8 * " ' 1 I ' ' a " I " 1 ' " " ' " ' 1 " * I

O 10,000 20 ,000 30,000 Yr B.P.

BAROMBI MBO (WEST CAMEROON)

A P % 100

80

60

4Q

20

C

O 10 ,000 2 0 , 0 0 0 3 0 , 0 0 0 Yr B.P.

CARAJAS (EASTERN AMAZONIA)

i O 10,000 20,000 30,000 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 . 31

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 evap- oration (Servant and Servant-Vildary, 1980). Lake levels were also higher in Venezuela (Lake Va- lencia: 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) a t 8000 yr B.P. This is interpreted as a mqre prominent seasonal dry interval. Similar interpretation was obtained from palynological and palae- olimnological 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

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 At- lantica 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: Brad- bury 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: Ser- vant, 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 . . , 5 i--

2 0 % - " ~ " ~ ' " ' ~ " ~ ~ . . . I . I * . . . . . , , . I , , , . ,

32 M. SERVANT ET AL. I 1

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 13C analysis of the soil organic matter in presently forested areas of western Congo

* Araucaria

3% , . , . , . I * * . . + . . . ( , , , , . , . . , , *

& - -

"C

-

O 10 ,000 2 0 , 0 0 0 3 0 , 0 0 0 yr BP VOSTOK TEMPERATURE

10,000 2 0 , 0 0 0 30,000 yr BP O BAROMBI MBO (WEST CAMEROON)

SALITRE (CENTRAL BRAZIL)

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)~niir plus other taxa present in Araucaria forest but also in other forest types (Symplocos, Podocuppus, Ileu).

i i


(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 suffi- ciently 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 distur- bances, were observed in the soils of northwest- ern 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 hoclzstetteri, a montane taxon, was abundant in western Cameroon (Barombi Mbo) (Fig. 4) and had also colonized the Ghana lowlands (Maley, 1987). Ac-

cording to the present distribution of O. liochstet- feri, its past occurrences suggest a colder climate than today and an high atmospheric humidity. Other montane elements (Podocaipus inilaiz- jaiius, 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 Ecuado- rian 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 Up- per 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 Amer- ica, 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

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 tem- perate 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

M. SERVANT ET AL.

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 be- haviours of Africa and South America. Notwith- standing 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

40 - 1 I I I I i i I

10 -

O I l l I I l l

01-6 10-6 20-6 30-6 10-7 20-7 31-7 10-8 20-8 31-8 D a t e

Fig. 5. Examples of polar advection influence upon temperature maxima in Santa Cruz de 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 35

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 contradk- tions, 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 Vil- larroel, 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. North- ern polar advections have also caused the tem-

/ 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 (Servant- Vildary, 1978) and perhaps in some parts of Cen- tral 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 Ama- zonia 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 lati- tudes (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

! 36 M. SERVANT ET AL.

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

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

I 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,

I

'

1

I

I

l I frequent during the middle Holocene (Martin et I

1

I

1

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 superim- posed 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 follow- ing 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 disap- pearance 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

37 I TROPICAL FOREST CHANGES DURING LATE QUATERNARY IS AFRICAN AND soum AMERICAN LO\VLANDS l

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 shift- ing of ITCZ to the south in South America. The

tinued to affect occasionally the forest dynamic.

1

I / I

I regional dry events became less frequent but con-

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. Mar- tin, 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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