Western equatorial African forest-savanna mosaics: a legacy of late Holocene climatic change?

Past vegetation and climate changes reconstructed using two pollen records from Lakes Maridor and Nguène, located in the coastal savannas and inland rainforest of Gabon, respectively, provide new insights into the environmental history of western equatorial African rainforests during the last 4500 cal yr BP. These pollen records indicate that the coastal savannas of western equatorial Africa did not exist during the mid-Holocene and instead the region was covered by evergreen rainforests. From ca. 4000 cal yr BP a progressive decline of inland evergreen rainforest, accompanied by the expansion of semi-deciduous rainforest, occurred synchronously with grassland colonisation in the coastal region of Gabon. The contraction of moist evergreen rainforest and the establishment of coastal savannas in Gabon suggest decreasing humidity from ca. 4000 cal yr BP. The marked reduction in evergreen rainforest and subsequent savanna expansion was followed from 2700 cal yr BP by the colonization of secondary forests dominated by the palm, <i>Elaeis guineensis</i>, and the shrub, <i>Alchornea cordifolia</i> (Euphorbiaceae). A return to wetter climatic conditions from about 1400 cal yr BP led to the renewed spread of evergreen rainforest inland, whereas a forest-savanna mosaic still persists in the coastal region. There is no evidence to suggest that the major environmental changes observed were driven by human impact

Pollen transport to southern Greenland: new evidences of a late spring long distance transport

International audienceNew observations of long distance transport of pollen to southern Greenland are recorded during two consecutive weeks, 19 May until 1 June 2003. These new results indicate again Northeastern America as the source area of the transported pollen grains. Backward trajectories studies indicate that transport occurred twice during the first week, which corresponds to the interval of maximum pollen flux emitted in the atmosphere. This is indicated by the large percentage of exotic pollen grains identified, about 11% of the total counted including the local vegetation. Conversely, the transport observed during the second week occurred during a single day, at a time of reduced pollen emission into the atmosphere in the source area, and experienced severe wash out and thus is indicated by a lower percentage of exotic grains, 1% of the total identified ones. The trajectories modeled by the HYSPLIT application show different patterns to those previously identified in 2002. Although air volumes passing over southern Greenland at 3000 m carried out the main transport, additional transport occurred at a much lower altitude leading producing a more complicated pattern of modeling than previously considered

A palaeoecological perspective for the conservation and restoration of wetland plant communities in the central French Alps, with particular emphasis on alder carr vegetation

Wetland conservation and management are generally only based on present-day studies, without integrating historical considerations. However, wetlands contain palaeoecological archives that can provide accurate records of their own history. Our study aims at reconciling this paradox in the central French Alps, by reconstructing the past wetland diversity/richness and the controls of Holocene hydroseral dynamics, and by discussing on this historical basis their conservation, management and restoration. Previously published data, complemented by the palaeoecological study of a sedge mire, reveal three main stages in the regional hydroseral succession: initial aquatic plant communities (Nymphaea alba, Nuphar cf. lutea, Menyanthes trifoliata), carrs (Alnus glutinosa/incana, Salix spp., Thelypteris palustris), then sedge meadows (Cyperaceae, Poaceae, Lythrum salicaria…). This dynamic comprises (1) a classical evolution from open water bodies to treed wet communities, controlled by the relationships between sedimentation processes and climate, and (2) an unexpected return to herbaceous wet habitats mainly triggered by Subatlantic human-induced managements. Such recent changes induced in the studied region the decline of Alnus cf. glutinosa, the disappearance of Thelypteris palustris, and the extinction of the carr communities they constituted. The historically-based assessment of community naturalness and resilience appears critical for defining conservation priorities, refining management actions, and identifying baseline conditions for restoration initiatives. The main implications of our results are to reinforce conservation measures on the less impacted habitats and to increase the diversity/richness of isolated lowlandmires, notably by restoring alder communities in some of them

Modern pollen rain in savanna and forest ecosystems of Gabon and Cameroon, Central Atlantic Africa

Eighty modern soil surface and litter samples from southern Cameroon and Gabon, Central Atlantic Africa (5 degrees N-4 degrees S. 10 degrees-15 degrees W), were analysed for pollen content. The samples are distributed among two main vegetation types: savanna (8 samples) and forest (71 samples). The aim of this study is to provide new data on the modern pollen rain in the Guineo-Congolian phytogeographical region, mainly in forest communities (secondary and mature forests on well drained soils, and hygrophilous forests) and to interpret these data using diagrams of pollen percentages and numerical analyses. The savannas are well identified by high frequencies of non-arboreal pollen with as pollen marker the Poaceae, and the forests by high frequencies of arboreal pollen with as important families the Burseraceae, Caesalpiniaceae, Mimosaceae, Euphorbiaceae and Sapindaceae. Within the forest ecosystem, secondary and mature forests on well drained soils can be differentiated on the basis of distinct assemblages of tree pollen taxa such as Zanthoxylum, Phyllanthus, Tetrorchidium, Margaritaria discoidea in secondary forest spectra and abundance of Burseraceae, Caesalpiniaceae, Sapindaceae in mature forest ones. In addition, hygrophilous forests are well identified by the presence of high pollen contributors such as Uapaca, Nauclea, Macaranga and Raphia. This work shows that the major vegetation communities occurring today in Cameroon and Gabon can be well differentiated by their pollen assemblages

Intérêt des mésocosmes « transportables » pour l'étude des impacts toxiques sur les écosystèmes aquatiques

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