Late quaternary vegetation history of the South-Tanganyika basin. Climatic implications in South Central Africa

Palaeogeography, Palaeoclimatology, Palaeoecology, 86 ( 1991): 207-226 Elsevier Science Publishers B.V., Amsterdam

207

Late Quaternary vegetation history of the South-Tanganyika Basin. Climatic implications in South Central Africa A. Vincens

Laboratoire de Gdologie du Quaternaire, CNRS Case 907, Luminy-13288, Marseille Cedex 9, France (Received May 9, 1990; revised and accepted February 4, 1991)

ABSTRACT Vincens, A., 1991. Late Quaternary vegetation history of the South-TanganyikaBasin. Climatic implications in South Central Africa. Palaeogeogr., Palaeoclimatol. Palaeoecol., 86: 207-226. Pollen evidence from two cores recovered from the Mpulungu basin, South lake Tanganyika, reveals a significant pattern of changes in vegetation in relation to climatic fluctuations between 25,000 and 9000 yr B.P. Prior to 15,000yr B.P., open and poorly diversified Zambezian woodlands at low and mid-altitudes, with local included patches of montane components such as Podocarpus and numerous Ericaceae are registered. This indicates cooler and drier climatic conditions than now, with a probably incidence of light frost during the night at low altitude. The coldest episode could be placed between 22,000 and 15,000 yr B.P. The period between 15,000 and 12,000yr B.P. appears to be transitional. A significant retreat of ericaceous shrublands suggests an increase in temperature. The permanence of local floristically poor woodlands and the presence of drought-tolerant montane elements on the plateaus suggest that climate was still drier than today. The development of wetter Zambezian woodlands with a composition similar to the modern vegetation and the occurrence of some arboreal taxa which have more affinities with the West and Central African flora than with the Zambezian one, imply a great increase in rainfall after 12,000yr B.P.

Introduction During the last two decades, great advances have been made concerning the evolution of African vegetation during the Late Quaternary. But, contrary to other regions such as East and West Africa (e.g. Coetzee, 1967; Livingstone, 1967; Morrison, 1968; Kendall, 1969; Hamilton, 1982; Ritchie and Haynes, 1987; Maley, 1987; Brenac, 1988; L~zine, 1988; Bonnefille and Riollet, 1988; Taylor, 1990) very few successful investigations on the vegetation history of South-Central Africa have been undertaken. Lawton (1959, 1963) has examined a number of potential sites for pollen analysis, the most important of which are the Lake Bangweulu peat deposits, in Zambia. Unfortunately, no systematic pollen diagrams have been published and there is no absolute dating of the deposits. The most complete analysis in these regions is for a 22,000 yr record 0031-0182/91/$03.50

from Ishiba Ngandu by Livingstone (1971). Nevertheless, the main feature of that long pollen record is the absence of any marked vegetation changes interpretable in terms of climatic fluctuations. These results are remarkably similar to those obtained on shorter Holocene peat bog sequences from the Ynyanga Mountains, Zimbabwe, by Tomlinson (1973, 1974) and from the Nyika Plateau, Malawi, by Meadows (1984). Some other palynological and palaeobotanical data are also available from Dundo, North-Eastern Angola (Van Zinderen Bakker and Clark, 1962), from the Kalambo Falls, South Tanzania (Van Zinderen Bakker, 1969) and from the Shaba area, SouthEastern Zai're (Roche, 1975; M'Benza-Muaka and Roche, 1980; M'Benza-Muaka et al., 1984). However, the results, extracted from outcrops, are discontinuous and not always well dated (Fig.l). Recently, palynological investigations have been undertaken on several cores from Lake Tanganyika

© 1991 - - Elsevier Science Publishers B.V.

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Fig.1. Location map of pollen sites in South-Central Africa (vegetation after White, 1983).

(GEORIFT Project, ELF-Aquitaine and EEC funds), and results from two short sequences from the Northern basin, dated between 13,000 yr B.P. and present, have recently been published (Vincens, 1989a).

The pollen records presented in this paper from the South-Tanganyika Basin, provide the longest and most thoroughly dated and continuous Late Quaternary sequence yet published for SouthCentral Africa. They show a major change in the

209

LATE QUATERNARY VEGETATION HISTORY OF THE SOUTH-TANGANYIKA BASIN

vegetation of the basin ca. 15,000 yr B.P. and suggest that climate in this region has not been stable during the last 25,000 years B.P.

Environmental setting Lake Tanganyika (3°30 ' 8°50'S, 29°-31°20'E) lies in the Western Rift Valley of Central Africa, at an altitude of 773 m (Fig.2). With a maximum depth of about 1470 m, this lake is the second deepest lake in the world. Inflows into Lake Tanganyika are relatively small. The Rusizi River, from the north (Lake Kivu) is the major contributor to its water and salt budget (Hecky, 1978). The other significant tributary is the Malagarasi from the East. Of the water input, 90% falls directly on the lake's surface or is carried to it by small mostly intermittent streams draining the escarpments. Of the water loss, 90% is by evaporation, only 10% going out by the Lukuga effluent to the Zaire River. The

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hydrology of the lake is complex. It is stratified with a thermocline at about 70 m depth (Degens et al., 1971). Lake Tanganyika lies within Walter's tropical summer-rainfall zone (Walter and Lieth, 19601967). There is a single well defined rainy season, chiefly from November to April. The average annual rainfall is about 1000-1100 mm. Mean daily maximum and minimum temperatures are, respectively, 29 ° and 19°C, with a mean annual value around 22-23°C. Southeasterly winds prevail during the dry season, creating seasonal upwelling of deep water near the southern end of the lake. Lake Tanganyika basin is located within the vegetation zone classifed by White (1983) as "wetter Zambezian miombo woodland" dominated by Brachystegia, Julbernardia and Isoberlinia which extends largely over West Tanzania, Zambia, South Zaire and Angola (Figs.1 and 2). This woodland can be divided into many types and occurs between 773 (modern lake level) and 1900m altitude

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210

(Werger and Coetzee, 1978). Many different trees occur scattered in these woodlands, such as Af-

rormosia angolensis, Erythrophleum africanum, Parinari curatellifolia, Pterocarpus angolensis, Marquesia macroura and several species of Protea, Monotes and Uapaca. Detailed analyses of the floristic composition of these communities are given by Lewalle (1972), Clutton-Brock and Gillett (1979) and Collins and McGrew (1988) from the eastern shore of Lake Tanganyika and by Cole (1963) and Lawton (1963, 1972, 1978) from the southern basin. Above 1900m altitude, the humid evergreen forest, which belongs to the Afromontane phytogeographical region defined by White (1983) occurs on the escarpments. But it is today largely destroyed by extensive farming and fires (Kerfoot, 1964; Van Zinderen Bakker, 1969; Lewalle, 1972). Above approximately 2700 m altitude, which is the upper limit of the montane forest in Central Africa (Lebrun, 1960), the Afroalpine zone, with ericaceous shrubland and thicket, occurs locally on the North Za~re highlands (White, 1983). In the southern part of the basin, the mountains (Marungu and Ufipa plateaus) are not high enough to support Ericaceous belt vegetation. Material and methods

The two cores (MPU-I1 and MPU-12) from Lake Tanganyika were collected in 1985 by an "ELF-Aquitaine" expedition (GEORIFT Project) near the southern end of the lake, in the Mpulungu Basin, in the neighbouring o f the coring site of Livingstone and Kendall (Livingstone, 1965; Haberyan and Hecky, 1987) (Fig.2). The cores consist respectively of 9,34 and 10,13 m of sediment and were recovered using a modified Kullenberg piston corer from below more than 400 m of water (Mondeguer, 1987; Mondeguer et al., 1989). The lithological and sedimentological characteristics of one of these cores (MPU-12), associated with microfloristic and isotopic data, have been recently described and published (Tiercelin et al., 1988; Vincens, 1989b, in press; Hillaire-Marcel et al., 1989; Gasse et al., 1989; Tiercelin et al., 1989). Six radiocarbon dates have been obtained using an accelerator mass spectrometer in Zurich

a. VINCENS

(Tiercelin et al., 1988) and Gif-sur-Yvette (Gasse et al., 1989) (Fig.3). All 14C dates are internally consistent and indicate that the sequence MPU-12 is continuous. Only one standard radiocarbon date was performed at the bottom of the core MPU-11 by Lafont in Marseille (Fig.4). But the presence of ash layers between 1,40 and 1,80 m similar to those found in core MPU-12 between 1,50 and 2,50 m permits to estimation of the age of these deposits to be around 12,000 yr B.P. (Mondeguer, 1987; Mondeguer et al., 1989). From the two cores, samples for pollen analysis were taken every 20 cm, implying a time resolution of less than 500 yrs between two samples. A total of 97 samples (MPU-11: 49; MPU-12; 48) were collected and analysed. Pollen was counted with 250x and 1000× magnification using a Leitz Ortholux microscope. At least 400 pollen grains were counted. Identifications were based on the reference collection of some 7000 slides present in the Laboratoire de G6ologie du Quaternaire, Marseille. In all the levels analysed, the microflora was rich and diverse. A total of 177 pollen taxa has been identified and classified in relation to the physiognomy (AP: Arboreal Pollen; NAP: Non Arboreal Pollen) and the phytogeographical affinities (Afromontane, Zambezian, peri-Guinean) of the plants which produced them (Tables 1 and 2). Classifications are based on the botanical works of White (1965) and Lewalle (1972) and on the "Flore du Congo Beige et du Ruanda-Urundi" (1948-1963), the "Flore du Congo, du Rwanda et du Burundi" (1967-1971) and the"Flore d'Afrique Centrale (Zaire, Rwanda, Burundi)" (1972-1985). The relative percentages of each taxon or group of taxa have been calculated with the ferns spores excluded from the pollen sum, as was previously done with pollen sequences from the North-Tanganyika Basin (Vincens, 1989a). Palynological results

The pollen diagrams from cores MPU-11 and MPU-12 presented here (Figs.3 and 4) only comprise 19 taxa among the 177 identified. These taxa, by their abundant pollen recovery and consistent

LATE QUATERNARY VEGETATION HISTORY OF THE SOUTH-TANGANYIKA BASIN

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TABLE 1 List of pollen taxa identified in cores MPU-11 and MPU-12, Mpulungu basin, South lake Tanganyika. ACANTHACEAE Acanthus type

Barh,ria Hypoestes type I~oglossa Justicia anselliana, type Justieia .[lava, type Justicia odora type Justicia striata, type Lepidagathis Mimulopsis type

CHENOPODIACEAE Chenopodium type COM BRETACEAE/M ELASTOMATACEAE COMMELINACEAE Commelina benghalensis type COMPOSITAE

Artemisia Hirpicium diffusunl type liguliflorae

Stoebe kilimandscharica, type tubuliflorae Vernonieae CONVOLVULACEAE Convolvulus type CORNACEAE

ALISMATACEAE

Afrocrania volkensii

Ranalisma humile

CUPRESSACEAE

AMARANTHACEAE Achyranthes aspera type Aerva lanata type Aerva javanica type

Centemopsis gracilenta Pupalia lappacea type Sericostachw9 scandens type /Chenopodiaceae ANACARDIACEAE Lannea type Rhus type

Sorendeia madagascariensis type APOCYNACEAE

Rauvolfia AQU!FOLIACEAE

llex mitis ARALIACEAE

Polyscias Julva BALSAMINACEAE

Impatiens BORAGINACEAE Cordia q/~'icana, type BURSERACEAE

Commiphora africana, type BUXACEAE

Buxus CAESALP1N1OIDEAE Al'zelia q[?icana, type

Brachystegia Cassia lsoberlinia Julbernardia Pterolobium stellatum CAMPANULACEAE

Wahlenbergia CAPPARIDACEAE Maerua type CARYOPHYLLACEAE

Cerastium Sagina abyssinica Silene burchellii type CELASTRACEAE

Juniperus procera CYPERACEAE

Ascolepsis DIPSACACEAE DIPTEROCARPACEAE EBENACEAE

Diospyros Euclea ERICACEAE EUPHORBIACEAE

Acalypha AIchornea Antidesma type Bridelia Clutya Croton type Euphorbia Euphorbia hirta, type Macaranga Olfieldia dactylophylla type Pseudolachnostylis maprouneiJolia type Rieinus communis Sapium ellipticum type Svnadenium grantii Tetrorchidium Uapaca nitida, type Uapaca kirkiana, type GENTIANACEAE GRAMINEAE GUTTIFERAE Garcinia volkensii, type

Psorospermum j~'br(fugum, type Symphonia globul(fera HALLORAGACEAE

Gunnera perpensa Laurembergia tetrandra HYMENOCARDIACEAE

Hymenocardia acida ICACINACEAE

Apodytes dimidiata LABIATAE

Ocimum type

213

214 TABLE 1 (continued)

A. VINCENS POLYGALACEAE

Polygala LEGUMINOSAE LENTIBULARIACEAE

Utricularia

POLYGONACEAE

Polygonum senegalense, type Rumex

Anthocleista

PROTEACEAE Protea type RANUNCULACEAE Clematis type

LORANTHACEAE LYTH RACEAE MALPIGHIACEAE

RHAMNACEAE ROSACEAE

LOGANIACEAE/THEACEAE

Nuxia/Ficalhoa LOGANIACEAE

Flabellaria paniculata type Flabellariopsis type MALVACEAE Abutilon type MELIACEAE

Entandrophragma type Trichilia emetica, type MIMOSOIDEAE

Acacia group I Acacia group III Albizzia Dichrostachys Entada type Mimosa pigra, type MONOCOTYLEDONAE MORACEAE

Ficus MYRICACEAE

Myrica MYRISTICACEAE

Pycnanthus angolensis MYRSINACEAE

Maesa Myrsine africana Rapanea melanophloeos MYRTACEAE Syzygium type NYMPHAEACEAE Nymphaea lotus, type OLACACEAE

Ximenia OLEACEAE

Jasminum abyssinicum, type Olea europaea subsp, africana Olea capensis PALMAE

Hyphaene type Phoenix reclinata PAPILIONOIDEAE Aeschynomene type

Indigofera Kotschya type Pterocarpus type PODOCARPACEAE

Podocarpus PODOSTEMONACEAE

Sphaerothylax

Ranunculus

Alchemilla Cliffortia nitidula Hagenia abyssinica Parinari type Prunus africana RUBIACEAE

Anthospermum Canthium gueinzii, type Canthium schimperianum, type Hallea rubrostipulata type Oligocodon type Spermacoce type Vangueria acutiloba type RUTACEAE

Teclea nobilis type Zanthoxylum type SAPINDACEAE

Allophylus Melanodiscus type Dodonaea viscosa SAPOTACEAE SIMAROUBACEAE

Brucea antidysenterica SOLANACEAE

Solanum STERCULIACEAE

Dombeya type Sterculia type THYMELEACEAE TILIACEAE TYPHACEAE

Typha ULMACEAE

Celtis Chaetacme type Holoptelea grandis Trema orientalis type UMBELLIFERAE URTICACEAE VERBENACEAE Lantana trifolia, type PTERIDOPHYTA monolete trilete BRYOPHYTA Anthocerotaceae

LATEQUATERNARYVEGETATIONHISTORYOF THE SOUTH-TANGANYIKABASIN TABLE 2

Z a m ~ z i a n arboreal taxa

List of arboreal taxa identified in cores MPU-11 and MPU-12 and classified in relation to their phytogeographical affinities.

Entandrophragma type Trichilia emetica, type Acacia group I Acacia group III Albizzia Dichrostachys Mimosa pigra, type Ficus Maesa Myrsine africana Syzygium type Ximenia Hyphaene type Phoenix reclinata Protea type

215

Lythraceae

Afromontane arboreal taxa

Ilex mitis Polyscias fulva Stoebe kilimandscharica, type Afrocrania volkensii Juniperus procera Ericaceae

Symphonia globulifera Apodytes dimidiata Myrica Rapanea melanophloeos Olea europaea subsp, africana Olea capensis Podocarpus Hagenia abyssinica Prunus africana Brucea antidysenterica Zambezian arboreal taxa

Anacardiaceae Lannea type Rhus type

Sorendeia madagascarensis type Araliaceae

Cordia africana, type Commiphora africana, type Afzelia africana, type Brachystegia Isoberlinia Julbernardia Capparidaceae Maerua type Celastraceae Combretaceae/Melastomataceae Dipterocarpaceae

Diospyros Euclea Alchornea Antidesrna type Bridelia Clutya Croton type Macaranga Olfieldia dactylophylla type Pseudolachnostylis maprouneifolia type Sapium ellipticum type Synadenium grantii Uapaca nitida, type Uapaca kirkiana, type Garcinia volkensii, type Psorospermum febrifugum, type Hymenocardia acida Loganiaceae/Theaceae

Anthocleista

Rhamnaceae

Cliffortia nitidula Parinari type Canthium gueinzii, type Canthium schimperianum, type Hallea rubrostipulata type Oligocodon type Rutaceae

Teclea nobilis type Zanthoxylum type Allophylus Melanodiscus type Dodonaea viscosa Sapotaceae Dombeya type Sterculia type

Celtis Chaetacme type Trema orientalis type Peri-Guinean arboreal taxa

Tetrorchidium Pycnanthus angolensis Holoptelea grandis

presence, are considered to be the most important ones in the interpretation of the palaeoenvironment. They are Ericaceae, Myrica, Juniperus procera, Podocarpus and Olea among the Afromontane arboreal taxa; Brachystegia, other Caesalpinioideae, Dipterocarpaceae, Uapaca type nitida and type kirkiana, Alchornea, Ulmaceae, Sapotaceae and Combretaceae among the Zambezian arboreal taxa. Among the non-arboreal taxa, Gramineae, Compositae (with Artemisia) and Cyperaceae (with Ascolepis) have been selected. The two pollen records show very similar trends. Three main zones can be distinguished, the bound-

216

aries of which were established on the basis of significant changes in the proportions of the major taxa. The numbering of the pollen zones - - from 3 to 5 - - used here, is based on the one previously defined for pollen sequences from the North-Tanganyika Basin (Vincens, 1989a). Indeed, the correlation established between the palynological data from the two basins clearly shows that the pollen zones 1 and 2 are missing in the cores MPU-11 and MPU-12. This indicates that a large part of the Holocene deposits have not been recovered. These data confirm the results previously suggested by palaeomagnetic measurements (Williamson et al., 1991).

Pollen zone 5: (MPU-11: 9,34-2,80 m; MPU-12." 10,13 3,22 m) Pollen zone 5, from 25,000 (25,650_+ 890 ya B.P. at ca. 10 m on core MPU-12) to ca.15,000 yr B.P., is dominated by herbaceous taxa such as Gramineae and Cyperaceae. Among arboreal taxa, Afromontane elements which are found today above 1900 m altitude on the escarpments around the basin, are dominant, with essentially Ericaceae and Podocarpus associated with rare Myrica, Juniperus procera and Olea. Some Zambezian woodland taxa are present, but in very low percentages. This phase is interrupted just after 21,720_+260 yr B.P. (sub-zone 5b; MPU-11: 6,605,80 m; MPU-12:5,80-5,20 m) by a slight increase of Zambezian pollen mainly due to a rise of Brachystegia and Uapaca associated with some Combretaceae and Dipterocarpaceae. At the same time, swamp elements such as Cyperaceae show their lowest percentages in the pollen zone 5.

Pollen zone 4: (MPU-11: 2,80-1,00 m; MPU-12: 3,22-1,80 m) In this pollen zone, from ca.15,000 to ca.12,000 yr B.P., the vegetation shows a significant change. Ericaceae strongly decrease and Olea (particularly in core MPU-I 1) becomes more abundant. The development of this element is contemporaneous with a slight increase of Myrica, and, among the herbaceous taxa, of Artemisia. The same feature has been found in the lower part of

A. VINCENS

one of the two pollen sequences from the NorthTanganyika Basin between ca. 13,000 and 11,000 yr B.P. (Vincens, 1989a). Podocarpus is always present. Local Zambezian arboreal taxa become progressively more abundant. Initially, the most widespread elements are Brachystegia and Uapaca type kirkiana. At the top of the pollen zone Dipterocarpaceae, Uapaca type nitida, Alchornea and Ulmaceae increase. At the same time, a significant decrease of Cyperaceae pollen is registered, particularly so in core MPU- 12. Pollen zone 4 could be considered as a transitional zone between pollen zones 5 and 3.

Pollen zone 3: (MPU-11:1,00-0 m; MPU-12: 1,80-0 m) Pollen zone 3, younger than ca.12,000 yr B.P., marks the main development and diversification of Zambezian woodland. The major components occurring today in the local vegetation of the South-Tanganyika Basin (Lawton, 1978) were found in noticeable percentages. Uapaea type kirkiana and type nitida, Braehystegia, Alchornea, Dipterocarpaceae, Ulmaceae (Celtis dominant), Sapotaceae and Combretaceae are abundant. They are associated with some Bridelia, Pseudolaehnostylis, Afzelia, Proteaceae, Allophylus and Julbernardia. Taxa more common in the lowland forest of Central and West Africa are present in this zone such as Holoptelea grandis, Pycnanthus angolensis and Tetrorchidium. At the same time, Afromontane arboreal taxa decrease considerably. Only Podoearpus and Olea are still found in noticeable percentages. Ericaceae, Myrica and Juniperus procera become very scarce. In the two pollen diagrams, herbaceous taxa are abundant. Discussion

The 25,000 to 15,000 yr B.P. cold, dry period Abundant pollen evidence and glacial geological data from East and Central Africa provide proof

LATE QUATERNARY VEGETATION HISTORY OF THE SOUTH-TANGANYIKA BASIN

of major climatic changes during the Upper Pleistocene. The period between 25,000 and 15,000 yr B.P. has been designated by Coetzee (1967) as the " M o u n t Kenya Hypothermal stadial" after the type-site. During this period, the vegetation belts on the East and Central African mountains shifted ca.1000m downslope, and moraines have been recorded 800-1000 m below modern snow lines, suggesting a climate that was ca.5 to 8°C colder and drier than at present (Coetzee, 1967; Van Zinderen Bakker, 1967; Morrison, 1968, Hamilton, 1982; Hastenrath, 1984; Mahaney, 1987a; Bonnefille, 1987; Bonnefille and Riollet, 1988). More recently, quantitative estimates of temperature and rainfall using a multivariate analysis of pollen time-series data from peat deposits in Burundi have been obtained. The results indicate a temperature decrease of 4 _ 2°C and a mean annual rainfall decrease of 30% between 30,000 and 13,000 yr B.P. (Bonnefille et al., 1990). The pollen data on environments in the East and Central African lowlands during this episode are very scanty. The pollen record of Pilkington Bay shows that the northern edge of Lake Victoria was occupied by open arid savanna ca.15,000 yr B.P. (Kendall, 1969). A similar vegetation has been recorded in the Shaba area before 12,000 yr B.P. (Roche, 1975; M'Benza-Muaka and Roche, 1980). In North-Eastern Angola, one pollen spectrum recovered in an archaeological level dated ca.14,500 yr B.P. shows that the region was most probably covered by an open vegetation and that a riverine forest rich in Podocarpus occurred in the valley at an altitude of 800m (Van Zinderen Bakker and Clark, 1962). At the same time, the Kalambo Falls Prehistoric site in Tanzania was surrounded by an open vegetation of grassland with some Proteaceae, a considerable number of Ericaceae and very few trees (Van Zinderen Bakker, 1969). The two pollen sequences clearly show that around the South-Tanganyika Basin Zambezian woodland was continuously present between 25,000 and 15,000 yr B.P. Nevertheless, it was greatly limited in distribution and not very diverse, with Brachystegia and Uapaca as the main components, indicating drier conditions than now. The

217

arboreal cover was certainly more scattered than in the modern woodlands and open herbaceous vegetation was well developed, with large swampy zones around the lake. In this region, it seems that the woodlands never completely disappeared contrary to the South-Eastern Zai're areas, where they were replaced by more xeric vegetation such as grasslands and shrublands. Ca.21,000-20,000yr B.P., a slight increase of some Zambezian elements (sub-zone 5b) could be interpreted as a short-term wet episode (about 1,000 yrs). These results are in good agreement with those previously obtained for the same time slice, on the Burundi Plateau (Bonnefille and Riollet, 1988) where it has been shown that in peat bog deposits, a brief lacustrine episode was followed immediately by forest development. On the plateaus, Afromontane vegetation seems to have been reduced and were not particularly diverse between 25,000 and 15,000 yr B.P.. Pollen data indicate that some elements such as Ericaceae, Podocarpus and Juniperus procera, extended below their present altitudinal limit, within the open woodlands.

The ericaceous shrublands Under natural conditions, the largest concentration of Ericaceae is usually found in the zones just above the montane forest. However, on mountains which are not high enough to support an ericaceous belt (Hedberg, 1951), such as in South-Central Africa, communities can occur in glades within the forest belt, on steep slopes where soil is too shallow to support forest and where competition is low. For example, on the Ufipa Plateau, near the Mbisi Forest, Ericaceae grow in great profusion at altitudes between 1500 and 3000 m. Some species, especially Agauria salicifolia and Philippia benguelensis, prefer the edges of swamp forest communities ("dambo") within the miombo, in depressions where drainage is poor and where it can be cold at night, with light frost. In these circumstances, Ericaceae can occur at lower altitudes, 1400-1650 m, in the Zambezian woodlands, such as near Abercorn and Kasama (Fig.5). But the highest abundance is generally found near the edge of the montane forest about 1900 m (Van

A, VINCENS

218

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Zinderen Bakker, 1969, 1970; Lawton, 1978; White, 1978, 1983). Owing to its tetrad morphology, pollen grains of Ericaceae are moderately well dispersed by wind or by water (Hamilton, 1972; Buchet, 1981; Vincens, 1982). High percentages of ericaceous pollen (> 20%) are only found in modern surface samples taken in areas where this element occurs locally, such as samples from the ericaceous Belt above 3000 m on the East and Central African mountains (Hedberg, 1954; Van Zinderen Bakker,

1964; Coetzee, 1967; Livingstone, 1967; Hamilton and Perrott, 1980; Bonnefille and Riollet, 1988). No data have been published on pollen rain and sedimentation from ericaceous communities which occur at lower altitude such as on the slopes of the Ufipa Plateau. However, we assume that the contribution of their pollen in modern spectra could reach similar or slightly lower values. In the two pollen sequences, Ericaceae are abundant between 25,000 and 15,000 yr B.P. They constitute, with Podocarpus, the two main taxa

LATE QUATERNARY VEGETATION HISTORY OF THE SOUTH-TANGANYIKA BASIN

among the arboreal components. Their pollen percentages, mainly above 10%, suggest a large extension of ericaceous shrubs in the basin during this period. Morphologically it is very difficult to determine, among the ericaceous family, pollen of the different genera and species. But probably Philippia and Agauria, found today at the lowest altitudes on the Ufipa and Marungu Plateaus, are concerned in the fossil spectra. An extensive colonization by ericaceous shrublands of poor drained depressions within the local open Zambezian woodlands, but probably also, of areas where there are clearances in montane forests, can be suggested. Such extension of Ericaceae below their modern limit indicates cooler climatic conditions and certainly the incidence of local light frost at lower altitudes than today. Previous pollen data from the Kalambo Falls site strongly support this view. Indeed, at the same time, pollen spectra give a clear picture of an open vegetation with abundant Ericaceae such as that found today at an altitude of up to 2000 m, i.e. 800 m above the site (Van Zinderen Bakker, 1969). The maximum development of Ericaceae in the basin seems to have occurred between 22,000 and 15,000 yr B.P., according to the highest pollen percentages recorded in core MPU-12 (20-25%). These results suggest that this episode may correspond to the coldest phase registered in the pollen diagram, with a probable temporary colonization by ericaceous shrublands of some emergent areas during the major Late Pleistocene regressive phase of Lake Tanganyika (Gasse et al., 1989; Tiercelin et al., 1989). Incidence of light frost at low altitude between 25,000 and 15,000 yr B.P. is not inconsistent with the permanence in the vicinity of the lake of some typical Zambezian components such as Brachystegia and Uapaca, whose stem frost resistance is low, respectively - 4 and - 2 ° C (Ernst, 1971). Such low absolute temperatures occur today over the southern part of the Zambezian region during the winter dry season, mostly in areas where rainfall is lower than 900 mm and where vegetation consists of dry and floristically poor miombo (Ernst, 1971; White, 1983) (Fig. l).

219

The Podocarpusforests Montane forests with Podocarpus are today very restricted in the South-Tanganyika Basin. They only occur on the Ufipa plateau above 2000 m (Mbisi forest), and above 1700 m in the ravines of the Sunzu Hill (Van Zinderen Bakker, 1969) (Fig.5). Many of these forests have been recently destroyed by fires and farming. Farther south, Podocarpus occurs around an altitude of 1700 m, as distant satellite populations (White, 1978), in the Bwinginfumu forest, in Zambia, and on the South-Eastern Zaire plateaus. But it can also be present at lower altitude, 1400 m, such as near Kasama (Lawton, 1963, 1972; Malaisse, 1967) (Fig.5). These modern disjunct populations are very small. They usually occupy specialized habitats which are either particularly favourable such as along watercourses, or in which typical Zambezian vegetation is so impoverished that competition is reduced. Under these conditions, Podocarpus associated with some other montane elements, is able to grow in riparian or swamp forest communities ("mushitu") below the lower limit of the forest belt (Lawton, 1963; White, 1978, 1981). In the two pollen sequences, pollen of Podocarpus has been recorded with mean percentages about 12-15%, indicating that Podocarpus forests never occurred directly in the vicinity of the lake (Coetzee, 1967). These low values (maximum < 20%), compared with those recorded for ericaceous pollen which is not so well dispersed, and to modern pollen sedimentation in East African lakes, suggest that coniferous forests did not largely expand in the region between 25,000 and 15,000 yr B.P. These results are concordant with those previously obtained on the plateaus between 1200 and 1400m, at Ishiba Ngandu, in Zambia (Livingstone, 1971) and at the Kalambo Falls site (Van Zinderen Bakker, 1969). These authors concluded that it was unlikely that montane forests ever occurred close to these sites during this period. At higher altitude in Central Africa, Podocarpus forests were reduced in extent and only present, in some areas such as Burundi, in local refuges, whereas they were extensive before ca.30,000 yr B.P. (Livingstone, 1967; Hamilton, 1982; Bonnefille and Riollet, 1988).

220

Together, all these pollen data indicate that between 25,000 and 15,000 yr B.P., coniferous forests had a very restricted distribution in Central Africa. They did not simply move down from mountains. They were also largely missing from the plateaus, at mid-altitude. Nevertheless, pollen data from Lake Tanganyika, show that Podocarpus was present in Northern Zambia. Presumably, they migrated after 30,000 yr B.P. along watercourses and established themselves at lower altitudes, in some local favourable habitats such as margins of well-watered depressions within the open woodlands, where competition was low. So, modern sattelite montane populations found today about 1400 m, as near Kasama, could appear as relics.

The Juniper forests In the Tanganyika basin, Juniperus procera forest is today only found on the Marungu Plateau, South-East Zaire (30°E-7°30'S). This site is located some 550 km further west than any other known station in East Africa (Kerfoot, 1961). On this plateau which is characterized by Braehytegia woodlands, Juniperus procera is limited to the area Southeast of Kasiki, at an altitude of ca.1980 m (Fig.5). It only survives in rocky clefts and similar protected habitats, on shallow black sandy soils. Wherever it occurs, it is almost invariably associated with the occurrence of fog and mist. Much of the existing forest has been recently damaged by fire, particularly on the main plateau. But traces remain to show that considerable Juniperus procera forests existed over a much wider area in recent times (Kerfoot, 1964). In the two studied pollen sequences (MPU-11 and MPU-12) from the South-Tanganyika Basin, pollen of Juniperus procera has been regularly recorded but with low percentages (2,5% maximum) between 24,000 and 15,000 yr B.P. Previous work on pollen sedimentation in East African lakes (Vincens, 1982, 1987a,b) and perennial rivers (Buchet, 1981) has shown that pollen of Juniperus procera is not well dispersed over great distances. Percentages are always less than 1% for a distance of more than 100 km. The coring sites MPU-11 and MPU-12 are located today some 130 km south of the Juniperus procera forest. Moreover, the Marungu Plaeau is

A.

VINCENS

drained by small intermittent streams which do not reach the lake directly in the Mpulungu Basin, but further north in the East-Marungu Basin (Fig.5). So, values more than 1 or 2% of Juniperus procera pollen recorded in the pollen sequences, could indicate an expansion of juniper forest on the Marungu Plateau beween 24,000 and 15,000 yr B.P., in relation to the local occurrence of fog and mist, probably at lower altitudes than at present. During this period, it is proposed that pollen of Juniperus proeera did not come from the Ufipa Plateau. Indeed, pollen of this species have not been recorded in Late Pleistocene deposits (60,00010,000 yr B.P.) from the Kalambo Falls site (Van Zinderen Bakker, 1969), indicating that this montane element did not occur in that area. This suggests that the isolated position occupied today in South-Central Africa by Juniperus procera on the Marungu Plateau largely predates the 25,00015,000 yr B.P. cool, dry period. All these data suggest that, between 25,000 and 15,000 yr B.P., except for Juniperus procera which had a very isolated location as it does today, the distribution of the modern vegetational communities, was fairly different, without well defined altitudinal belts (Fig.6). This cool and dry period was characterized by a migration from the plateaus of some montane elements and their establishment at lower altitude within open and impoverished woodlands, in favourable edaphic habitats where permanent humidity could have compensated for a deficit in rainfall, such as modern "mushitu" and "dambo". The vegetation could thus appear as a mosaic, consisting of a grassland, with patches of dry Braehystegia woodlands and distinct montane forest clumps surrounded by ericaceous shrublands in valleys and depressions with impeded drainage (Fig.6). Such a vegetation type can be found today near Sao Hill, in Tanzania (Fig.5), at an altitude of 1800 m, under rainfall less than 900 mm. In this area it forms an ecotone between the montane forest and the Brachystegia woodland (after Proctor, in Van Zinderen Bakker, 1969).

The transitional period." 15,000-12,000 yr B.P. This period is characterized on many East and Central African mountains by the retreat of the

LATE QUATERNARY VEGETATION HISTORY OF THE S O U T H - T A N G A N Y I K A BASIN

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Olea. Three species of Olea - - O. europaea subsp. africana, O. hochstetteri and O. welwitschii, occur today in Central Africa (Liben, 1973; Green and Kupicha, 1979). Recently, it has been proposed to combine the two last species in the species Olea capensis (Friis and Green, 1986). The pollen grains of O. europaea subsp, africana and O. capensis can be distinguished morphologi-

221

cally by the dimensions of the lumina of the reticulum which are larger in the first species (Bonnefille, 1971). This distinction has been employed and it appears that the majority of the pollen of Olea found in the analyses belongs to O. capensis. The two species are important constituents of upland montane forests, commonly associated with Podocarpus and Juniperus, between 1500 and 2600 m altitude. But Olea capensis can also be found at lower altitudes with Podocarpus, in remnant patches of riparian forest communities within the Zambezian woodlands, in Northern Zambia (Lawton, 1978), and near the Kalambo Falls (Van Zinderen Bakker, 1969). Some authors have considered this species as a pioneer in extent forests (Kendall, 1969; Livingstone and Van Der Hammen, 1983). Particularly in the sequence MPU-11 (Fig.4), pollen of Olea capensis has been recorded with significant percentages between 15,000 and 12,000 yr B.P., with maxima around ca.13,000 yr B.P. (5 to 10%). These values could indicate that this species, the pollen of which has a relative moderate dispersed ability (Hamilton, 1972; Vincens, 1982), expanded on the plateaus of the South-Tanganyika Basin during this period. This extension could correspond to an early seral stage in the recolonization of the plateaus by montane elements dating from 15,000 yr B.P.

Myrica. Of the four species known from East Africa, only two, M. salicifolia and M. kandtiana are widely distributed. The pollen grains of these species are very similar. M. kandtiana is a swamp tree found at relative low altitudes, usually below 2000 m. M. salicifolia is a dryland species. It is common in forest clearings, but can occur at the tree line and in the ericaceous belt. It is the first invader of grassland in revertence to temperate rain forest (Brenan and Greenway, 1949) and is also a pioneer on lava flows (Livingstone, 1967). The altitudinal range of M. salic(folia is from 2000 m to about 3000 m. Though pollen of Myrica were present all along the sequences, the most significant percentages are only found between 15,000 and 13,000yr B.P. (core MPU-12). A similar feature has been re-

222 corded in other pollen diagrams from East Africa, such as from Ruwenzori (Livingstone, 1967). It has been attributed to the widespread occurrence of open dry forest or dry montane woodland on the mountains, under generally rather arid conditions, when Podocarpus and Juniperus are not potential competitors (Hamilton, 1982).

Artem&h~. Only one species, Artemisia afra, occurs today in tropical East Africa. It is found mostly in the dry forest-savanna mosaics, between 1500 and 3000 m altitude. It does however occur up to 3000 m, in the ericaceous belt. According to Gillett (in Coetzee, 1967), Artemisia afra prefers dry conditions. The pollen of A rtemisia has a relatively moderate dispersal ability (Hamilton, 1972). Thus, percentages between 2 and 7% found in the pollen spectra probably indicate long-distance transport from the plateaus. The expansion of this element between 15,000 and 12,000yr B.P., at the same time as Olea and Myrica, which are also drought-tolerant plants, could be taken as an indicator of dryness. It should be noted that Olea, Myrica and Artemisia have also been recovered with significant pollen percentages during the same period in all the cores from the North-Tanganyika Basin (Vincens, 1989a and unpublished), from Lake Victoria (Kendall, 1969) and also, at high altitude, on the Ruwenzori Mountains (Livingstone, 1967). This would indicate that their expansion is related to a synchronous sub-continental climatic change which affected many regions of Central Africa. Between 15,000 and 12,000yr B.P., a more wooded but still not diverse vegetation began to develop around the lake and probably all over Northern Zambia. The two widespread components of the local Zambezian woodlands are Brachystegia and Uapaca associated with some others Caesalpinioideae (essentially Julbernardia). The greater abundance of their pollen in core MPU-12 than in core MPU-11, as it is also the case for many other taxa, is certainly related to the proximity of the core MPU-12 to the shore line (Fig.2). The period 15,000-12,000yr B.P. appears to have been transitional as regards temperature, but the hydrological history is difficult to establish on

A.VINCENS the basis of palynological data. The presence of some montane drought-tolerant elements and the persistance of poorly diversified woodlands only suggest that the climate was drier than today. Higher water input in the basin has been inferred from sedimentological and diatom data which have revealed a major transgressive phase of Lake Tanganyika from ca.13,000 yr B.P. (Tiercelin et al., 1988; Gasse et al., 1989). Reduction of swampy vegetation at this time is certainly related to the beginning of this high stand.

The 12,000-9000 yr B.P. humid period In East and Central Africa, the period post ca.12,000 yr B.P. is characterized by an extensive development of moister vegetation types (Hamilton, 1982; Bonnefille and Riollet, 1988; Taylor, 1990). All the lakes experienced high levels in relation to increases in rainfall and runoff (Street and Grove, 1976). Unfortunately, this period is not well documented in the two pollen diagrams. Indeed, a great part of Holocene deposits was not recovered during the coring. Palaeomagnetic measurements have shown that the top of the core MPU-12 is not younger than ca.9000yr B.P. (Williamson et al., 1991). This estimated age is confirmed on the basis of correlations established between pollen sequences from North and South Tanganyika basins (Vincens, 1989a). One taxon, Tetrorehidium, seems to be a good stratigraphic marker. In all the Holocene sequences from the northern basin, it appears and then expands ca.10,000-9000yr B.P. The same feature has been recorded, at the same time, in sediments from Lake Victoria (Kendall, 1969). In the southern basin, pollen of Tetrorchidium has been found in the upper part of the two cores (40-0 cm), but it is represented in each level by only one grain. This suggests that this element had just begun to establish itself in the basin, thus implying an age of about 10,000-9000 yr B.P. for the top of the two cores. The period between 12,000 and 9000 yr B.P. is characterized by the expansion of typical Zambezian woodlands. Except for Brachystegia and Uapaca type kirkiana which began to develop around 15,000 yr B.P., the main components occurring

LATE QUATERNARY VEGETATION HISTORY OF THE SOUTH-TANGANYIKA BASIN

today in the local vegetation, are present. The arboreal cover is well diversified with numerous representatives of the Dipterocarpaceae, Uapaca type nitida, Ulmaceae (Celtis dominant), Combretaceae, Sapotaceae and Alchornea associated with some Allophylus, Anacardiaceae, Protea, Pseudolachnostylis, Bridelia, Euclea ... The development of Dipterocarpaceae associated with Ulmaceae could characterize a wetter miombo type (White, 1983). For the first time in the two sequences, mesophilous lowland forest elements which have peri-Guinean affinities such as Tetrorchidium, Holoptelea grandis and Pycnanthus angolensis are present, indicating more humid conditions than before. The most important Zambezian component is initially Uapaca. Ecological studies in Northern Zambia have shown that the genus Uapaca belongs to an important seral stage in the regeneration of the woodlands. It is therefore considered as a basic ecological taxon. Under a Uapaca canopy, other species may become established (Lawton, 1972, 1978). These data could explain that, in the two pollen diagrams, the majority of the woodland components are only found when Uapaca reaches its maximum development. At the same time, the montane elements largely retreat indicating, contemporaneously with higher rainfall, an increase in temperature. Moreover, the expansion of Zambezian woodlands from ca.12,000 yr B.P., under more favourable climatic conditions, has certainly induced too high a degree of competition to maintain montane elements at low and mid-altitudes. Some authors such as Schmitz (in Kerfoot, 1964) have previously proposed that the beginning of the reduction in area of montane forests may have coincided with the development of the Mesolitic culture in Central Africa. Palynological data from the South-Tanganyika Basin, clearly show that forest reduction just after ca.12,000 yr B.P. in this region can be better interpreted as a result of climatic change rather than of an early deforestation by man. Conclusions The two pollen sequences from the Mpulungu Basin, South Lake Tanganyika, provide new infor-

223

mation about the environmental history of SouthCentral Africa during the period 25,000-9000 yr B.P. Contrary to suggestions of Livingstone (1971) and Meadows (1984), these data clearly show that the vegetation of this region was greatly affected by climatic changes during this time interval, such as was the case in other parts of intertropical Africa. The main results are the following. During the period 25,000-15,000 yr B.P., typical elements of the Zambezian woodlands such as Brachystegia and Uapaca consistently occurred in the vicinity of the basin. Nevertheless, the prevailing local vegetation was probably more open, with a well developed grass cover and more scattered trees than today. Lake shores were largely colonized by swampy communities. Some montane elements, such as Podocarpus, occurred at low altitude in locally favourable habitats. Others such as Juniperus, extended locally on the western plateau. Ericaceous shrublands became widespread at low and mid-altitudes within the open woodlands, and probably developed around the lake on emerged areas. Such a distribution of the vegetation suggests a regionally cool and dry climatic environment, with incidence of local light frost at lower altitudes than at present. The coldest episode could be dated to between 22,000 and 15,000 yr B.P. Migration and establishment of montane elements at low altitude between 25,000 and 11,000 yr B.P. have been demonstrated by palynological analyses in many parts of Central and West Africa: Angola (Van Zinderen Bakker and Clark, 1962), Congo (Elenga and Vincens, 1990), Cameroun (Brenac, 1988) and Ghana (Maley, 1987), but also in South Africa (Scott, 1982). At the majority of these sites, montane biotopes seem to have established themselves very locally, and preferentially in favourable edaphic habitats such as humid depressions in the Congo, along watercourses in Angola and in "mushitu" and "dambos" South of Lake Tanganyika, where permanent humidity could have compensated for a deficit of rainfall. There is thus no real evidence for a very large extension of montane populations at low altitude in Africa during the last cool and dry period and the data are at present too scattered to suggest any contact between them.

224

The p e r i o d between 15,000 a n d 12,000 y r B.P. a p p e a r s t r a n s i t i o n a l . T h e r e t r e a t o f E r i c a c e a e suggests a r e d u c t i o n o f areas where frost o c c u r r e d before, a n d t h e r e f o r e m i g h t indicate a n increase o f t e m p e r a t u r e . T h e p e r m a n e n c e o f p o o r l y diversified w o o d l a n d s at low a l t i t u d e a n d the presence o f s o m e d r o u g h t - t o l e r a n t elements o n the p l a t e a u s , indicate t h a t the climate was still drier t h a n now. The p e r i o d between 12,000 a n d 9000 yr B.P. shows the b e g i n n i n g o f a climatic a m e l i o r a t i o n . The m a j o r i t y o f the m o n t a n e c o m p o n e n t s r e t r e a t o n t o the plateaus. W e t t e r Z a m b e z i a n w o o d l a n d s begin to e x p a n d widely at l o w a n d m i d - a l t i t u d e s . The a r b o r e a l cover b e c o m e s m o r e dense a n d diversified a n d is p i o n e e r e d by Uapaca which is a n i m p o r t a n t seral stage in the r e g e n e r a t i o n o f the woodlands. The development of Dipterocarpaceae a s s o c i a t e d with s o m e elements w h i c h t o d a y have affinities with the C e n t r a l a n d W e s t A f r i c a n flora, clearly suggests m o r e h u m i d c l i m a t o l o g i c a l c o n d i tions t h a n before.

Acknowledgments T h e research p r e s e n t e d here p r o c e e d s with the G E O R I F T P r o j e c t (6174) financially s u p p o r t e d by E L F - A q u i t a i n e , the E E C ( E u r o p e a n E c o n o m i c C o m m u n i t y ) a n d the F S H ( F r e n c h H y d r o c a r b o n s F u n d s ) , T h e p a l y n o l o g i c a l w o r k was financed by the C N R S , E L F - A q u i t a i n e a n d P N E D C . T h e a u t h o r is i n d e b t e d to D r . R. Bonnefille for advice a n d e n c o u r a g e m e n t ; Dr. J. J. Tiercelin for discussions d u r i n g p r e p a r a t i o n o f the m a n u s c r i p t ; Professors J. D. C l a r k a n d E. M. Van Z i n d e r e n B a k k e r m a d e v a l u a b l e c o m m e n t s on earlier versions o f this p a p e r which also benefitted f r o m the c o m m e n t s o f two a n o n y m o u s reviewers. R a d i o c a r b o n d a t i n g was p r o v i d e d b y C. H i l l a i r e - M a r c e l , J. C. F o n t e s a n d R. L a f o n t ; technical assistance b y G. Buchet, G. Riollet a n d M. D e c o b e r t in M a r s e i l l e a n d b y technicians o f E L F - A q u i t a i n e in Boussens a n d Pau. Pollen d a t a were s t o r e d in an I B M c o m p u t e r with the assistance o f N. Buchet a n d R. S m a d j a .

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