Annie Vincens

Phytoliths : indicators of grassland dynamics during the late Holocene in intertropical Africa

Abstract The reconstruction of African tropical grassland history during the late Holocene can be carried out using phytolith analysis. Fossil phytolith assemblages from Lake Guiers, in the Sahelian region of Senegal, and from Lake Sinnda, in the Guineo-Congolian region of Congo were investigated. The results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previously obtained. Tall or short grass associations are discriminated by their phytolith index Iph(%) = saddle/(cross + dumbel + saddle), while the density of shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths. The phytolith data emphasize that, in the Guineo-Congolian region around Lake Sinnda, the driest phase of the late Holocene occurred between 4000 and 1200 yr B.P., commencing with the opening of the dense forest and its replacement by a short grass savanna. From ca. 1000 yr B.P., wetter climatic conditions developed, as represented by the setting up of a tall grass savanna woodland. The modern shrub and tall grass savanna was developed ca. 700 yr B.P. In the Sahelian region around Lake Guiers, the driest phase occurred after about 2000 yr B.P. and has not been followed by moister conditions. A tall grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs. A short period of development of swampy vegetation, which can be related to a lake level change, interrupted the semi-arid adaptation of the vegetation, between about 2000 yr B.P. and the present.

Pollen evidence of late Quaternary vegetation and inferred climate changes in Congo

A detailed palynological analysis of two cores recovered from a swamp in the Southern Bateke Plateaux (Congo), provides information on the botanical history of this region during the last 24,000 yr B.P. Rich and well-diversified pollen counts exhibit changes in the vegetational communities related to hydrological and climatic fluctuations. Around 24,000 yr B.P., the occurrence of hydromorphous forest elements gives evidence of a humid phase. Between 24,000 and around 13,000 yr B.P., swampy herbaceous communities expanded, indicating slightly drier climatic conditions. From approximately 13,000 yr B.P. onward, the beginning of a humid episode is reflected by the development of mesophilous forests. The decrease of these forests is documented since 3000 yr B.P. Grasslands are found to extend locally whereas Elaies guineensis (oil palm) spreads out, indicating both climatic change but also anthropogenic activities.

Quantitative estimates of full glacial temperatures in equatorial Africa from palynological data

This paper presents a new reconstruction of the mean annual temperature obtained from a high altitude pollen sequence in equatorial Africa (3°28′S, 29°34′E, 2240 m). It has been achieved with an extended modern African reference data set by adding spectra from 228 new sites and using another selection for pollen taxa. The purpose of this paper is to test how the obtained temperature value depends upon the availability of modern analogues. The results are in good agreement with those previously published, reinforcing the validity of the method. The mean standard error is reduced by 0.3°C. The mean temperature for the Holocene appears + 1.4°C warmer than the present and the last glacial maximum (25-18 kyrs BP) cooling is better specified at − 3 ± 1.9° C, a conservative value, more consistent with reconstructed sea surface temperature in the equatorial ocean.

Palynology, stratigraphy and palaeoenvironment of a pliocene hominid site (2.9-3.3 M.Y.) at Hadar, Ethiopia

Abstract The Pliocene Hadar Formation (2.9-3.3 m.y.) is well known for its abundant fossil fauna and hominid remains. Fossil pollen data obtained from 27 distinct stratigraphic layers are presented together with tectonic and sedimentary data. Comparison with modern pollen data from the Awash basin (67 surface and fluviatile samples) indicates that montane evergreen bushland, presently known from above 1500 m, occupied the Hadar basin (500 m) and was associated with a humid montane forest. A downfaulting of about 1000 m presumably happened after 2.9 m.y. whereas a shift toward more arid grasslands occurred in the middle of the Hadar Formation.

The sensitivity of a Tanzanian crater lake to catastrophic tephra input and four millennia of climate change

Diatom genera in many large East African lakes change little throughout the Holocene period suggesting relatively stable ecological conditions and some resilience to environmental change. Ecosystem stability is less common in smaller, more sensitive lakes, such as those within volcanic craters, where external impacts can cause abrupt and rapid fluctuations. A 4100-year diatom and cyanobacteria pigment record from Lake Massoko, a volcanic crater lake in southern Tanzania, is used to illustrate important switches in resource ratios following tephra deposition 1190 years ago. It is hypothesized that the tephra reduced the rate of P diffusion from the sediments and increased the Si:P ratio in the lake. A period of acute change in planktonic diatom communities resulted from the tephra impact and lasted c. 110 years. The magnitude of the change shown by the diatoms and their slow recovery from the tephra may be due in part to a coincident fall in lake level caused by a reduction in regional rainfall. The statistical significance of the tephra impact relative to that of catchment and climate change has been tested using variance partitioning and rate-of-change analysis. Multiproxy indicators show an important period of positive water balance 1700 ago and a relatively dry episode persisting between 1000 and 400 years ago. The lake ecosystem is shown to be highly sensitive to both climate change and tephra deposition.

Pollen-based vegetation changes in southern Tanzania during the last 4200 years: climate change and/or human impact

The age-constrained pollen data of a sedimentary sequence from the crater Lake Masoko, southern Tanzania (9‡20PS, 33‡45PE, 770 m), display a continuous record of vegetation for the past 4200 years. This record provides evidence that wetter Zambezian woodlands always occupied this area during the late Holocene, reaching a maximum extent between 2800 and 1650 cal yr BP related to increase in summer monsoon intensity. However, three main episodes of decline have been detected, between 3450 and 2800 cal yr BP, between 1650 and 1450 cal yr BP and from 1200 to 500 cal yr BP, for which a climatic interpretation, decrease in the summer monsoon strength, was preferentially advanced. The first is synchronous with lowstand of many tropical African lakes and, so, mainly induced by increased aridity. In contrast, the abrupt change in the pollen record at 1650^1550 cal yr BP is marked by a large extension of grasslands at the expense of arboreal cover, further by an increase in Ricinus communis and an intensification of burning. It could thus indicate local clearance of vegetation by man. However, at the same time, the decline of montane forest suggests the impact of a more regional change. During the last episode, between 1200 and 500 cal yr BP, dry climatic conditions are inferred from a combination of pollen, diatom and magnetic proxies, although the occurrence of Late Iron Age settlements in the region means that local human interference cannot be excluded. This study illustrates the difficulties in deciphering ecological and anthropological changes from pollen data in African tropical regions. A 2003 Elsevier B.V. All rights reserved.

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

Abstract 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 15,000 yr 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 12,000 yr 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 12,000 yr B.P.

Modern climate–vegetation–pollen relations in Africa and adjacent areas

Abstract The pollen–climate and pollen–vegetation relation in Africa, Madagascar and Arabia has been investigated using 1170 modern pollen samples presently available in the African Pollen Database. Data are collected from different sedimentary environments (soil, lake, river, traps, midden, etc.). In spite of an irregular spatial distribution, there are representatives in all of the major vegetation and climate zones. Accounting for the different trapping efficiency of the different sediment types, large-scale patterns can be determined from the pollen assemblages. Maps and response-surfaces depicting the distribution in geographic and climate space of the main pollen types distinguish the major phytogeographic regions. Computing dissimilarities among the African pollen spectra using squared chord distance demonstrates that pollen spectra within a particular vegetation zone are more similar to one another that they are to pollen samples from other vegetation zones. Using the method of modern analogs, we find significant correlations between modern temperature and precipitation reconstructed using the best modern analog, which suggests the potential of this database for quantitative climate reconstructions. The study of individual pollen types provides an alternative method to the biomization to characterize and quantify the land-surface conditions at a continental scale.

Study of the modern pollen rain in Western Uganda with a numerical approach

Modern soil samples from Western Uganda, from a range of ten plant communities belonging to five African phytogeographical regions and distributed along an altitudinal gradient from 650 m (grass savannas) to 4400 m (Afroalpine moorland) were analyzed for pollen content to define modern pollen/vegetation relationships. Correspondence analysis applied to the pollen counts (100 sites and 167 taxa) indicates four distinctive vegetation types arranged along an altitudinal gradient and thus a temperature one with respect to axis 1(contrast between montane and lowland vegetations), and along a physiognomical gradient (from densely structured to open vegetations) defined by axis 3. These results confirm the empirical interpretation proposed on the initial pollen data set and are in agreement with those previously obtained on modern or fossil pollen spectra from other African regions.

Pollen-rain-vegetation relationships along a forest-savanna transect in southeastern Cameroon.

Modern soil and litter samples from southeastern Cameroon, collected along a continuous forest-savanna transect were analysed for pollen content to define modern pollen-vegetation relationships. The pollen results, completed and compared with botanical inventories, leaf area index and basal area measurements performed in the same area, clearly registered the physiognomy, the main floristic composition and floral richness of the two sampled ecosystems. Distortions were observed between sampled vegetations and their pollen rain, related to important differences in pollen production and dispersal of plant species: this is a general feature in many tropical regions. The pollen data in the area studied reflected well the recent transgression of forest versus savanna. This permitted us to define inside the forest ecosystem more successional vegetation communities than the botanical surveys allowed.