Jean Maley

Vegetation dynamics, palaeoenvironments and climatic changes in the forests of western Cameroon during the last 28,000 years B.P.

The lake Barombi Mbo pollen record goes back to about 28,000 yr B.P. The pollen diagram based on 82 samples is subdivided into four main pollen zones. Zone I (ca. 28,000 to 20,000 yr B.P.) is characterized by relatively high frequencies of Caesalpiniaceae and also by a montane element with Olea capensis. The climate was cool and relatively wet. Zone II (ca. 20,000 to 10,000 yr B.P.). A sharp increase in Gramineae, the main non-arboreal land pollen taxon, began around 20,000 yr B.P. and lasted until 10,000 yr B.P. During this period the forest receded, giving way to a more open vegetation, but significant patches of forest (refuges) persisted in the area. This is confirmed by isotopic analyses (δ13C of sedimentary detritic organic matter from the catchment. Until ca. 13,000 yr B.P. Olea capensis was well represented indicating a relatively cool climate. Between 13,000 and 12,000 yr B.P. a warming trend associated with a strong increase in precipitation occurred. After this an abrupt reduction in precipitation linked to an increase in seasonality, but without temperature lowering, intervened between ca. 11,500 and 10,400 yr B.P. This last phase corresponds partly to the Younger Dryas time period. Zone III (ca. 10,000 to 2800 yr B.P.). After ca. 10,400 yr B.P. the climate became very wet until ca. 3000 yr B.P. A sharp decrease in the Gramineae intervened at ca. 10,000 yr B.P.; from ca. 9500 to 3000 yr B.P. they remained very low, between 0 and 3%, and the forest trees reached their maximum extension. Most of the trees exhibited large variations with quasi-periods of around 1000 to mainly 2000 yr (ca. 2200 calendar years), which could be related to large sylvigenetic or successional cycles. In this zone the Caesalpiniaceae were relatively well represented, with a maximum extension between 4500 and 3000 yr B.P. Podocarpus, a typical tree of the montane stratiform cloud forests, exhibited very low frequencies before 10,000 yr B.P. but their relative increase during the early and middle Holocene can only be explained by its growth on distant mountains. Its maximum extension phase was roughly synchronous with that of Caesalpiniaceae. The climate was warm and wet, but cooler on the mountains. Zone IV (ca. 2800 yr B.P. to present time). Around 2800 yr B.P. a sharp increase in the Gramineae, peaking at 30 to 40% of total pollen between ca. 2500 and 2000 yr B.P., indicates a sudden phase of vegetation opening and forest retreat, accompanied by severe erosion. Alchornea, a typical pioneer taxon, increased rapidly at the same time to large frequencies because it develops abundantly in all the openings. Elaeis guineensis, originally a pioneer palm tree, follows the same pattern. The climate was warm, relatively dry, and linked to an increase of seasonality. After 2000 yr B.P. the Gramineae returned to low frequencies, around 10%, associated with a strong increase in trees, indicating that the forest expanded again but not to the same extent as in the early and middle Holocene. The climate was warm and relatively wet, rather similar to the present-day climate.

Late Quaternary palaeoenvironments in the Lake Barombi Mbo (West Cameroon) deduced from pollen and carbon isotopes of organic matter

Abstract A sediment core, 23.5 m long, was recovered from a water depth of ca. 110 m in Lake Barombi Mbo, a maar crater of the Cameroon volcanic chain. This paper presents a carbon isotopic curve of organic matter linked to the main results of the pollen analysis. Interpretation of this carbon isotopic curve is simplified (1) because of the weak role played by the phytoplankton in the balance of the organic particle flux, and (2) because of the presumed absence of diagenetic alteration of the isotopic composition. The carbon isotopic curve exhibits an almost linear correlation with that of total grass pollen which form the main part of the C4 biomass. The major fluctuations of isotopic and pollen curves allow four main phases to be distinguished: • -from ca. 25,000 to 20,000 yr B.P., δ13C values of −25 to −30‰ (PDB) are related to a forest environment associated with a montane floral element; • -from 20,000 to 13,000 yr B.P., the δ13C values are between −23 and −28ℵ. and are linked to openings of landscape with a mosaic of forest and savanna and a fall of lake level with colonization of the shores by Cyperaceae and other aquatic plants; • -from 13,000 to 10,000 yr B.P., the forest extended again reaching a maximum density during the period from 9500 until 3000 yr B.P. and with an average δ13C value of −32‰; • -a new dry phase at around 2500-2000 yr B.P., with temporary openings in the forest, marked by a δ13C excursion above −30‰, but during the last 2 millennia, the forest developed again with δ13C values of −32‰.

The African rain forest – main characteristics of changes in vegetation and climate from the Upper Cretaceous to the Quaternary

This chapter sets out to give a historical overview of the African rain forest from its origins, towards the end of the Cretaceous period. The areas around the Gulf of Guinea, in particular from Ivory Coast to Nigeria and especially Cameroon, Gabon and Congo, appear to have been already occupied at this time by wet tropical forest formations mainly composed of Angiosperms which were then becoming established. In the course of the Tertiary period the combined effect of the equator being situated further north than now and the development of the Antarctic ice cap favoured the development of wet tropical conditions over a large part of North Africa which in turn led to the extension of tropical forest to various sites on the shores of the Tethys Sea. There were probably at this time common taxa and similar vegetation patterns stretching from the Gulf of Guinea to the Tethys Sea. Towards the end of the Tertiary, the equator reached its present position and the northern hemisphere ice caps appeared, and these phenomena resulted in the disappearance of the forest formations spread across the north of Africa, and the concentration of these formations near the equatorial zone around the Gulf of Guinea and in the Congo–Zaire basin. From 800 000 years ago onwards the marked glacial variations at middle and high latitudes in both hemispheres, with a periodicity of about 100 000 years determined by the orbital variations of the earth around the sun, lowered temperatures in equatorial areas and brought arid climates at times of maximum glacial extension. The most arid periods resulted in the fragmentation of the forest cover, and the forest biotopes and their biodiversity were preserved in a series of refugia. The lowering of temperatures also resulted in the extension of montane flora to low altitudes, with migration of montane flora and fauna between main mountain ranges. These compounded phenomena of isolation and migration, probably involving genie exchange, must have resulted in numerous speciation phenomena. Subsequently, such montane flora or fauna became isolated on mountain areas during periods of maximum warming, in the last instance in the course of the Holocene, when a vast forest cover became re-established from Guinea westwards, and to the East as far as the Lake Victoria area. The phases of maximum fragmentation, which appear to have been connected with only the coldest periods – in the last instance during the second part of isotopic stages 6 (from c. 160 to 130 000 years) and 2 (from c. 24 to 12000 years BP) – relate to less than 10% of the last 800 000 years, and the phases of maximum forest extension would likewise appear to be less than 10% of the period. The remaining 80–90% of the time relates to ‘intermediate situations’ which varied from period to period, and these intermediate extension situations seem to have been the norm over the larger part of the Quaternary, rather than the present situation which is closer to a situation of maximum extension.

Genetic diversity of the shea tree (Vitellaria paradoxa C.F. Gaertn), detected by RAPD and chloroplast microsatellite markers.

RAPDs and chloroplast microsatellites were used to quantify the genetic variation of Vitellaria paradoxa (an economically important tree species in sub-Saharan Africa, north of the equator) and to analyse the geographic distribution of diversity in relation to the refuge theory. A total of 13 locations were sampled in eight countries, covering most of the natural range from Senegal to Uganda. A total of 67 polymorphic and 15 monomorphic RAPD loci were detected in 179 individuals. No relationship was identified between diversity and longitude or latitude. An unrooted neighbour-joining tree suggested a western group and an eastern group, representing 7% (P=0.000) of the total variation. A Mantel test suggested that genetic distances between populations were correlated to geographic distances (R=0.88, P=0.001). The three-chloroplast microsatellite primers, assayed in 116 individuals, revealed 10 different alleles and seven chlorotypes. Most of the populations comprised a single haplotype. It is proposed from these results that the difference between western and eastern populations results from putative refugia separated by the current ‘Dahomey Gap’ (a semiarid zone that meets the coast around the Ghana–Togo–Benin–Nigeria borders), an area that may have been exceptionally dry during glacial periods. In addition, it is suggested that the haplotype distribution and frequency in the western populations could be due to the more recent impact of humans, particularly shea tree selection and dispersal during traditional agroforestry.

Processes of diversification and dispersion of Rice yellow mottle virus inferred from large‐scale and high‐resolution phylogeographical studies

Phylogeography of Rice yellow mottle virus (RYMV) was reconstructed from the coat protein gene sequences of a selection of 173 isolates from the 14 countries of mainland Africa where the disease occurred and from the full sequences of 16 representative isolates. Genetic variation was linked to geographical distribution and not to host species as isolates from wild rice always clustered with isolates from cultivated rice of the same region. Genetic variation was not associated to agro‐ecology, viral interference and insect vector species. Distinct RYMV lineages occurred in East, Central and West Africa, although the Central African lineage included isolates from Benin, Togo and Niger at the west, adjacent to countries of the West African lineage. Genetic subdivision at finer geographical scales was apparent within lineages of Central and West Africa, although less pronounced than in East Africa. Physical obstacles, but also habitat fragmentation, as exemplified by the small low‐lying island of Pemba offshore Tanzania mainland, explained strain localization. Three new highly divergent strains were found in eastern Tanzania. By contrast, intensive surveys in Côte d’Ivoire and Guinea at the west of Africa did not reveal any new variant. Altogether, this supported the view that the Eastern Arc Mountains biodiversity hotspot was the centre of origin of RYMV and that the virus spread subsequently from east to west across Africa. In West Africa, specific strains occurred in the Inner Niger Delta and suggested it was a secondary centre of diversification. Processes for diversification and dispersion of RYMV are proposed.

Last Glacial Maximum lacustrine and fluviatile Formations in the Tibesti and other Saharan mountains, and large-scale climatic teleconnections linked to the activity of the Subtropical Jet Stream

Abstract In the mountains of the central Sahara (lat ca. 20° to 22°N, long 16° to 19°E) and particularly in the Tibesti mountains, important lacustrine formations developed during the late Pleistocene, primarily during the Last Glacial Maximum (LGM). Two main phases, separated by a brief regression, intervened between ca. 20,000 and 15,500 BP, and between 15,000 and 12,500 BP. Pollen analyses were carried out on four samples of this formation. The high lacustrine levels were associated to both important precipitations and a reduced evaporation linked to lower temperatures. Similar lacustrine deposits were found in the Djebel Marra in the south of the Sahara. In the mountains of the central and eastern Sahara, during the same period and until the middle Holocene, the “Middle Terrace” Formation was deposited in the river valleys of the Tibesti, Hoggar, Air and the Red Sea Hills. Since the southern headwaters of the Nile were dry from ca. 20,000 to 12,500 BP, the fluviatile sediments deposited in the Nile valley in Nubia may have resulted almost entirely from the numerous wadis flowing from the Red Sea Hills. The rainfalls which fed these lacustrine and fluviatile formations were related to the Tropical Depressions which are formed in the southern part of the westerlies and are linked to the activity of the Subtropical Jet Stream (STJ), whose path remained over the central Sahara from 20,000 BP to the early Holocene. In the Rocky Mountains of the western US, the palaeolakes Lahontan and Bonneville were very large during the LGM and the main fluctuations exhibit similar chronology to that of the Saharan mountains. Broecker [Broecker, W.S., 1994. Massive iceberg discharges as triggers for global climate change. Nature 372, 421-424] estimates that these two large U.S. wet events between ca. 20,000–15,500 BP and ca. 15,000–12,500 BP may have been an indirect result of two large ice surges in the North Atlantic, related to Heinrich layers 1 and 2. We can assume, however, that the similar climatic variation of the Rocky Mountains and the central and eastern Saharan mountains was also a result of the activity of the STJ all along its path, which marks the boundary between the polar and tropical circulations. STJ activity can apparently produce long-distance climatic teleconnections. During the LGM similar teleconnections also existed in the Southern Hemisphere between South Africa and Australia. The Tropical Depressions result from the interaction of polar troughs and the influx of humid equatorial air forming transversal cloud bands. The large increase in the intensity of atmospheric circulation during the LGM was responsible for a large increase in Tropical Depressions in both hemispheres.

Middle to Late Holocene Changes in Tropical Africa and Other Continents: Paleomonsoon and Sea Surface Temperature Variations

In tropical Africa the transition from the Middle Holocene (ca. 7000 to 3800 years BP) climate to the Late Holocene (ca. 3800 BP to present) was abrupt and marked by major paleoenvironmental shifts. The sediment and lake level changes in forested equatorial belt of tropical Africa, in the Sahel and Sahara and in the tropical areas of other continents (India, Australia and South America) are interpreted in terms of paleoclimate based on comparison made between the African paleodata and some present-day rainfall anomalies. These anomalies are associated with sea surface temperature (SST) variations and are closely linked to the intensity of upwelling and trade winds. We conclude that the abrupt changes in the tropical belt at the beginning of the Late Holocene could be related to the variation of the Zonal Walker Circulation, mainly the El Nino-Southern Oscillation (ENSO. The SST opposition between northern and southern Atlantic Ocean has a major influence on climatic changes in tropical Africa. Moreover, the predicted global warming during the next century could lead in tropical Africa to climate induced large rain forest destructions.

Comment on “Intensifying Weathering and Land Use in Iron Age Central Africa”

Bayon et al. (Reports, 9 March 2012, p. 1219) interpreted unusually high aluminum-potassium ratio values in an Atlantic sediment core as indicating anthropogenic deforestation around 2500 years before the present (B.P.). We argue that there is no terrestrial evidence for forest destruction by humans and that the third millennium B.P. rainforest crisis can be clearly attributed mostly to climatic change.

Mecanisme des changements climatiques aux basses latitudes

Abstract During the last milennium, the higher levels of lake Chad were connected with cooler periods, particularly at the time of the “Little Ice Age”. This connection shows that, on the African continent, the monsoon is pushed by the Southern Polar Front from the Southern Hemisphere. It is essential to consider the part of the Polar Fronts of the two hemisphers - and especially their antagonism, to account for the climatic mechanisms in the lower latitudes.

Le cadre paléoenvironnemental des refuges forestiers africains: quelques données et hypothèses

Since at least 800,000 years, during globally cold periods, and especially when glaciers at high latitudes reached their maximum sizes, the African tropical forest was very fragmented, and only survived in a series of relatively small refuges, spread across the original forest area. The end of the last climatic cycle, that started about 25 to 30,000 years ago, is characterized by the latest large fluctuation of the African tropical forest. This last period may help in modelling cyclic changes undergone by the African forest during the last 800,000 years. The last period when the forest had retreated into refuges was between 20,000 and 15,000 BC, and thus lasted about 5,000 years. A maximum extension of the forest, which coincided with the last warm period, occurred between 9,000 and 4,000 BC, and therefore also lasted 5,000 years. It might be deduced that, during the last 800,000 years, each minimum and maximum extension phase lasted no more than 5 to 10% of the total time. This means that during about 80 to 90% of the total time, the African forest exhibited an average extension, that is about the same extension as it has nowadays. The present extension therefore is typical for the greater part of the Quaternary.