Asymmetric response of forest and grassy biomes to climate variability across the African Humid Period : influenced by anthropogenic disturbance?

A comprehensive understanding of the relationship between land cover, climate change and disturbance dynamics is needed to inform scenarios of vegetation change on the African continent. Although significant advances have been made, large uncertainties exist in projections of future biodiversity and ecosystem change for the world's largest tropical landmass. To better illustrate the effects of climate–disturbance–ecosystem interactions on continental‐scale vegetation change, we apply a novel statistical multivariate envelope approach to subfossil pollen data and climate model outputs (TraCE‐21ka). We target paleoenvironmental records across continental Africa, from the African Humid Period (AHP: ca 14 700–5500 yr BP) – an interval of spatially and temporally variable hydroclimatic conditions – until recent times, to improve our understanding of overarching vegetation trends and to compare changes between forest and grassy biomes (savanna and grassland). Our results suggest that although climate variability was the dominant driver of change, forest and grassy biomes responded asymmetrically: 1) the climatic envelope of grassy biomes expanded, or persisted in increasingly diverse climatic conditions, during the second half of the AHP whilst that of forest did not; 2) forest retreat occurred much more slowly during the mid to late Holocene compared to the early AHP forest expansion; and 3) as forest and grassy biomes diverged during the second half of the AHP, their ecological relationship (envelope overlap) fundamentally changed. Based on these asymmetries and associated changes in human land use, we propose and discuss three hypotheses about the influence of anthropogenic disturbance on continental‐scale vegetation change

Datation absolue de dépôts organiques würmiens en Normandie

National audienc

Paléoclimatologie des 15 derniers millénaires à l'aide des méthodes nucléaires

International audienc

Etude des transgressions marines sur un atoll du Pacifique par les méthodes du carbone-14 et du rapport uranium-234/thorium-230

International audienc

India and changes coast, Karnataka, Pleistocene sea-level of Kanara on Late Palaeon1angroves their in1plications

International audiencees of K:lIlara l•oaSI. Karnataka, India and their implications on Late I'leistocelle sea•le\•el changes. III .lain, K. I'. &amp; Tiwari, R. S. (ecb)-!Jroc. ))'lIIp. 'Vislas ill Indian I)alaeobolal/y', I J alaeoho/a17i.,•/ 38 : 3 7 0. }"H.Along the K~nara coast. some welb ,ho&lt;v an organic clay layer, l,S tu -III thick, between a lavcr of 2to 6 m of coarse yellow sand abm'c and \vhite sand and pebbles belo\\' The altitude of this intermediate organic clm' fmlllatioll is more or less at thc present mean sea•level. Palynological analyses re\'eal that this sediment had been depOSited \vithin a mangrove ell\ironment. i.e. at sea•level, while the "C e1ating.s gin' ages older than -10,000 years t3.I'. No proof of vertical Illo\,cmetll, after the deposit of the intermediate organic clay formation has been put forward up to no\'\'. Therefore it can be accepted that the present ele\'mion of this formation IS mare or less the same as its original altitude. i.e.. the present sea•le•el. Comparison of thiS altitude \"ith the CUIYCS of global sea• le\'el changes indicates that the period of deposition of these organic scdiments should be around 12'&gt;.000 years t3.I' .. during the last Illlerglacial (Eemian) when the sea•Ic\•cl \y:IS ± the same that it is today The main characteristics of "I::emian" nora were the same as the present. The climatic reglllle may have been slightly different \vith less cOlltrasting ,eason,.</div

Expression of CR2 (the C3dg/EBV receptor, CD21) on normal human peripheral blood T lymphocytes.

Abstract The expression of CR2 (the C3dg/EBV receptor, CD21) on normal human T lymphocytes was investigated using purified peripheral blood T cells and indirect immunofluorescence with biotinylated anti-CR2 mAb and streptavidin-phycoerythrin. Thirty to 40% of normal peripheral blood T lymphocytes expressed CR2 Ag. The cells expressed three nonoverlapping epitopes of CR2. The specificity of the staining for CR2 epitopes was demonstrated by the ability of unlabeled anti-CR2 mAb but not of anti-CR1 mAb of the same isotype to compete for the binding of biotinylated anti-CR2 mAb to T cells. The intensity of staining of T lymphocytes with anti-CR2 mAb was approximately 10-fold lower than that of peripheral blood B cells. CR2 was immunoprecipitated from purified T lymphocytes as a single protein of apparent Mr 145,000. The presence of CR2 on normal human T lymphocytes suggests that the receptor may modulate the function of T cells in the immune response and the susceptibility of the cells to infection by lymphocytotropic viruses.</jats:p

Spectromètre à grande sensibilité destiné à la mesure du rayonnement X de faible énergie

International audienc