Deux nouvelles espèces de Rinorea, série Ilicifoliae (Violaceae) du Cameroun = Two new species of Rinorea, series Ilicifoliae (Violaceae) from Cameroon

Two new species, Rinorea mezilii sp. nov. and R. letouzeyi sp. nov., are described and illustrated from Cameroon littoral forest. Both belong to the series Ilicifoliae Engl. of subgenus Tubulosae Brandt. Within the group Ilicifoliae the two species hereby described differ from others by their stamen enlarged at the base and their rudimentary connective appendage aduate to the anther summit. The first species differs from the second by its habit densely branched and its alternate leaves with cuneate base, not cordate. Rinorea letouzeyi is characterised by its slender stem, much branched, subopposite leaves, clustered at nodes and at the end of twigs. The lamina base is cordate to amplexicaul. Rinorea meziIii is relatively well widespread in the entire southern part of the plain while R. letouzeyi appears to be a narrow endemic of its littoral fring

Distribution, ecology and management of Chromolaena odorata : proceedings of the third international Chromolaena workshop = Répartition, écologie et gestion de Chromolaena odorata : comptes rendus du troisième atelier international sur Chromolaena

Cette étude préliminaire des contacts forêt-savane porte sur une zone de l'est du Cameroun, au sud de la ville de Bertoua. Sous climat équatorial de transition, la savane de Kandara est caractérisée par une avancée rapide de la forêt sur les formations graminéennes malgré les feux de brousse - souvent tardifs - annuels. Le rôle passif et actif de #Chromolaena odorata$ est analysé. Dans le contexte général de conquête forestière, cette plante semble accélérer le phénomène. (Résumé d'auteur

Dinosterol deltaD values in stratified tropical lakes (Cameroon) are affected by eutrophication

In freshwater settings, dinosterol (4a,23,24-trimethyl-5a-cholest-22E-en-3b-ol) is produced primarily by dinoflagellates, which encompass various species including autotrophs, mixotrophs and heterotrophs. Due to its source specificity and occurrence in lake and marine sediments, its presence and hydrogen isotopic composition (dD) should be valuable proxies for paleohydrological reconstruction. However, because the purity required for hydrogen isotope measurements is difficult to achieve using standard wet chemical purification methods, their potential as a paleohydrological proxy is rarely exploited. In this study, we tested dD values of dinosterol in both particulate organic matter (POM) and sediments of stratified tropical freshwater lakes (from Cameroon) as a paleohydrological proxy, the lakes being characterized by variable degrees of eutrophication. In POM and sediment samples, the dD values of dinosterol correlated with lake water dD values, confirming a first order influence of source water dD values. However, we observed that sedimentary dinosterol was D enriched from ca. 19 to 54‰ compared with POM dinosterol. The enrichment correlated with lake water column conditions, mainly the redox potential at the oxic–anoxic interface (Eh OAI). The observations suggest that paleohydrologic reconstruction from dD values of dinosterol in the sediments of stratified tropical lakes ought to be sensitive to the depositional environment, in addition to lake water dD values, with more positive dinosterol dD values potentially reflecting increasing lake eutrophication. Furthermore, in lake sediments, the concentration of partially reduced vs. non-reduced C34 botryococcenes, stanols vs. stenols, and bacterial (diploptene, diplopterol and bb-bishomohopanol) vs. planktonic/terrestrial lipids (cholesterol, campesterol and dinosterol) correlated with Eh OAI. We suggest using such molecular proxies for lake redox conditions in combination with dinosterol dD values to evaluate the effect of lake trophic status on sedimentary dinosterol dD values, as a basis for accurately reconstructing tropical lake water dD values

Effect of aridity on delta ¹³C and delta D values of C₃ plant- and C₄ graminoid-derived leaf wax lipids from soils along an environmental gradient in Cameroon (Western Central Africa)

The observation that the hydrogen isotope composition (δD) of leaf wax lipids is determined mainly by precipitation δD values, has resulted in the application of these biomarkers to reconstruct paleoclimate from geological records. However, because the δD values of leaf wax lipids are additionally affected by vegetation type and ecosystem evapotranspiration, paleoclimatic reconstruction remains at best semi-quantitative. Here, we used published results for the carbon isotope composition (δ13C) of n-alkanes in common plants along a latitudinal gradient in C3/C4 vegetation and relative humidity in Cameroon and demonstrated that pentacyclic triterpene methyl ethers (PTMEs) and n-C29 and n-C31 in the same soil, derived mainly from C4 graminoids (e.g. grass) and C3 plants (e.g. trees and shrubs), respectively. We found that the δD values of soil n-C27, n-C29 and n-C31, and PTMEs correlated significantly with surface water δD values, supporting previous observations that leaf wax lipid δD values are an effective proxy for reconstructing precipitation δD values even if plant types changed significantly. The apparent fractionation (εapp) between leaf wax lipid and precipitation δD values remained relatively constant for C3-derived long chain n-alkanes, whereas εapp of C4-derived PTMEs decreased by 20‰ along the latitudinal gradient encompassing a relative humidity range from 80% to 45%. Our results indicate that PTME δD values derived from C4 graminoids may be a more reliable paleo-ecohydrological proxy for ecosystem evapotranspiration within tropical and sub-tropical Africa than n-alkane δD values, the latter being a better proxy for surface water δD values. We suggest that vegetation changes associated with different plant water sources and/or difference in timing of leaf wax synthesis between C3 trees of the transitional class and C3 shrubs of the savanna resulted in a D depletion in soil long chain n-alkanes, thereby counteracting the effect of evapotranspiration D enrichment along the gradient. In contrast, evaporative D enrichment of leaf and soil water was significant enough to be recorded in the δD values of PTMEs derived from C4 graminoids, likely because PTMEs recorded the hydrogen isotopic composition of the same vegetation type