Vegetation Response to Temperature Change on the Rwenzori Slopes, Western Uganda: Preliminary Results from Pollen Analysis

Several climatic periods, cooler than today, were evidenced on the Rwenzori Mountains, by the accumulation of moraines deposited by glaciers that advanced to lower altitudes on the mountain slopes. This paper provides evidence from pollen analysis that between ca. 9300 and ca. 6400 yr. B.P., Hagenia abyssinica and Ericaceae, trees typical of the junction bamboo-zone-ericaceous belt (altitude 3000 m), were distributed in significant abundance in the vegetation surrounding bog 2 (altitude 2500 m). This lowering of vegetation belts suggests climatic conditions cooler than today by about 3 ⁰C. The co-existence of ericaceous vegetation and montane forest evidenced from ca. 9300 to ca. 6400 yr. B.P. is in agreement with the Omurubaho glaciation reported to have occurred on the Rwenzori from ca. 10 000 to ca. 5000 yr. B.P. Keywords: Rwenzori, Moraines, Lapse rate, Omurubaho glaciatio

Vegetation Controls on Weathering Intensity During the Last Deglacial Transition in Southeast Africa

Tropical climate is rapidly changing, but the effects of these changes on the geosphere are unknown, despite a likelihood of climatically-induced changes on weathering and erosion. The lack of long, continuous paleo-records prevents an examination of terrestrial responses to climate change with sufficient detail to answer questions about how systems behaved in the past and may alter in the future. We use high-resolution records of pollen, clay mineralogy, and particle size from a drill core from Lake Malawi, southeast Africa, to examine atmosphere-biosphere-geosphere interactions during the last deglaciation (~ 18-9 ka), a period of dramatic temperature and hydrologic changes. The results demonstrate that climatic controls on Lake Malawi vegetation are critically important to weathering processes and erosion patterns during the deglaciation. At 18 ka, afromontane forests dominated but were progressively replaced by tropical seasonal forest, as summer rainfall increased. Despite indication of decreased rainfall, drought-intolerant forest persisted through the Younger Dryas (YD) resulting from a shorter dry season. Following the YD, an intensified summer monsoon and increased rainfall seasonality were coeval with forest decline and expansion of drought-tolerant miombo woodland. Clay minerals closely track the vegetation record, with high ratios of kaolinite to smectite (K/S) indicating heavy leaching when forest predominates, despite variable rainfall. In the early Holocene, when rainfall and temperature increased (effective moisture remained low), open woodlands expansion resulted in decreased K/S, suggesting a reduction in chemical weathering intensity. Terrigenous sediment mass accumulation rates also increased, suggesting critical linkages among open vegetation and erosion during intervals of enhanced summer rainfall. This study shows a strong, direct influence of vegetation composition on weathering intensity in the tropics. As climate change will likely impact this interplay between the biosphere and geosphere, tropical landscape change could lead to deleterious effects on soil and water quality in regions with little infrastructure for mitigation

Recent pollen sedimentation in Lake Natron, Tanzania : a model for the interpretation of fossil spectra in arid region. Sédimentation pollinique actuelle dans le lac Natron, Tanzanie : un modèle pour l'interprétation de spectres fossiles en région aride

This paper presents the palynological results obtained from 12 samples of surface sediments from Lake Natron, Tanzania. The spectra are a mixture of autochtonous and allochtonous pollen which represent all the main vegetation types occurring in the basin. The abundance of herbaceous taxa, low representation of local arboreal taxa with Acacia, Commiphora dominant provide a good picture of the pollen sedimentation in an arid region. Only small variations occur within the distribution of various pollen, related to the influence of transport agents like rivers but also to variations in local edaphic conditions. A comparison with palynological data from other east african lakes Turkana and Bogoria in Kenya shows clearly that modern spectra from Lake Natron can be considered as a good model of pollen sedimentation in a lake without flat-land around it and situated in a region with a mean annual rainfall less than 500 mm.Cet article présente les résultats palynologiques obtenus sur 12 échantillons de sédiments actuels du lac Natron, Tanzanie. Les spectres sont constitués d'un mélange de pollens autochtones et allochtones qui représentent les principaux types d'associations végétales existant dans le bassin. L'abondance des herbacées, la faible représentation des arbres locaux avec comme éléments dominants Acacia et Commiphora donnent une bonne image de la sédimentation pollinique en région aride. Quelques fluctuations dans la distribution des pollens à la surface du lac ont été mises en relation avec des arrivées de rivières ou des conditions édaphiques locales particulières. La comparaison des résultats avec ceux précédemment obtenus dans d'autres lacs est-africains — Turkana et Bogoria au Kenya — montre clairement que les spectres polliniques des sédiments actuels du lac Natron peuvent être considérés comme de bons modèles de sédimentation pollinique dans un lac encaissé entre deux escarpements et situé dans une région où la pluviosité moyenne annuelle est inférieure à 500 mm.Vincens Annie. Recent pollen sedimentation in Lake Natron, Tanzania : a model for the interpretation of fossil spectra in arid region. Sédimentation pollinique actuelle dans le lac Natron, Tanzanie : un modèle pour l'interprétation de spectres fossiles en région aride. In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 155-165

Modern background of Natron-Magadi basin (Tanzania-Kenya) : physiography, climate, hydrology and vegetation. Contexte actuel du bassin Natron-Magadi (Tanzanie-Kenya) : physiographie, climat, hydrologie et végétation

Cet article présente une description détaillée du contexte actuel du bassin Natron-Magadi (Tanzanie-Kenya). Ces deux lacs sont localisés au Sud de l'équateur, dans la branche Est du rift est-africain. Ils se situent à 600 m d'altitude et sont entourés de hauts plateaux et de volcans pouvant atteindre plus de 3 000 m. Le climat autour des lacs est de type aride. La pluviosité moyenne annuelle est inférieure à 500 mm avec des pluies distribuées en une seule saison (mars-mai), les températures sont élevées et l'évaporation potentielle très importante, dépassant largement la pluviosité. De ce fait, la balance hydrologique du bassin est nettement négative, expliquant que les deux lacs sont temporaires et contiennent des dépôts évaporitiques et des saumures alcalines très concentrées. Les lacs Natron et Magadi sont alimentés par des rivières temporaires, des sources hydrothermales et, exclusivement pour le lac Natron, par des rivières pérennes. La végétation à basse altitude est une steppe sub-désertique passant graduellement en altitude à des fourrés sempervirents puis à des forêts de montagne. Dans le bassin, il existe une relation étroite entre altitude et paramètres climatiques, en particulier la pluviosité, qui permet de comprendre son hydrologie actuelle ainsi que la distribution des différents types de végétation existant.Vincens Annie, Casanova Joel. Modern background of Natron-Magadi basin (Tanzania-Kenya) : physiography, climate, hydrology and vegetation. Contexte actuel du bassin Natron-Magadi (Tanzanie-Kenya) : physiographie, climat, hydrologie et végétation. In: Sciences Géologiques. Bulletin, tome 40, n°1-2, 1987. Lac Natron. Géologie, géochimie et paléontologie. D'un bassin évaporatique du rift est-africain. pp. 9-21

Relation pollen-végétation-climat actuels en Afrique centrale (une approche numérique appliquée à la séquence quaternaire du lac barombi Mbo, Cameroun)

Ce travail présente l'analyse de 80 nouveaux échantillons polliniques actuels d'Afrique centrale atlantique, en particulier de milieu forestier. Les assemblages polliniques obtenus ont été complétés par des données déjà existantes dans cette région, soit un total de 199 assemblages. L'application d'analyses statistiques et de la méthode de biomisation a permis de tester la fiabilité du contenu pollinique de cette base de données par rapport aux données botaniques mettant en évidence que plus de 95% des sites étaient correctement reconstruits en termes de biomes (forêt tropicale humide, forêt tropicale saisonnière, savane) ou de stades de succession forestière (savane, forêt secondaire, forêt mature). Sur une base de données polliniques plus étendue à l'Afrique de l'ouest, la méthode de biomisation a été appliquée à la séquence pollinique du lac Barombi Mbo, Cameroun, afin de recontruire les différents biomes et stades forestiers qui se sont succédés au cours des derniers 33 000 ans cal BP. L'application de la technique des meilleurs analogues, parallèlement à celle des réseaux de neurones artificiels a permis de reconstruire sur cette séquence la pluviosité et l'évapotranspiration moyennes annuelles, ainsi qu'un indice bioclimatique représentatif de la physionomie de la végétationThis work presents the analysis of 80 new modern pollen samples from central Africa, mainly in forest environment. Complemented by pollen assemblages previously obtained in this region, i.e. a total of 199 assemblages, statistical analysis and the application of the biomisation method have allowed to test the reliability of the pollen content of this dataset compared to local botanical data, showing that potential biomes (tropical rain forest, tropical seasonal forest, savanna) and forest successional stages (savanna, secondary forest, mature forest) were reconstructed with more than 95% of confidence level. Based on a more extended pollen database toward drier vegetations from West Africa and using the biomisation method, the best analogues and the artificial neural networks techniques, we proposed new quantitative and more precise reconstructions of the vegetation and climate at Lake Barombi Mbo, in Cameroon, during the last 33,000 cal yrs BPMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

New Oligocene–early Miocene microflora from the southwestern Turkana Basin Palaeoenvironmental implications in the northern Kenya Rift

We report five Oligocene–early Miocene pollen assemblages from the Loperot-1 exploration well drilled in the semi-desert Lokichar Basin (latitude 02° 21′ 46.15″ N, longitude 35° 52′ 23.47″ E, ground elevation 615 m above MSL), near Lake Turkana in northern Kenya. They represent the second oldest plant microfossils so far recovered from East Africa and add significantly to the Paleogene–Neogene tropical African fossil plant record. The Loperot pollen indicate a mosaic environment of semi-deciduous forest and humid woodland whose floristic composition presents strong affinities with the vegetation occurring today in the Guinea-Congolia/Zambezia phytogeographical transition zone, with a rainfall more than 1000 mm/year and a well defined dry season. The weak representation of Poaceae and herbaceous taxa characteristic of grassland, dry bushland or savanna and the abundance of shade tolerant plants such as ferns all point to a vegetation composed to a variety of communities with closed forest formations predominant. The lack of typical temperate mountains elements, mainly Podocarpus and Juniperus today widespread on the East African highlands, indicates that the geography of the region was different from that of today. The plateaux or uplands adjacent to the Lokichar basin were probably still not high enough during this period of early rifting in East Africa to support the temperate coniferous forests characteristic of the Plio-Pleistocene

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.This work was supported by the European Community through the EEC Program Rukwa, the INSU-Variente and Eclipse (CLEHA) programs

Are modern pollen data representative of west African vegetation?

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Modern pollen-based biome reconstructions in East Africa expanded to southern Tanzania

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