Diversité Floristique Et Variation Altitudinale De La Structure Des Formations A Gnidia Glauca (Fresen) Gilg. Dans Les Forêts Communautaires De KilumIjim (Nord-Ouest Cameroun)

In order to find strategies for sustainable management of resources, a study was carried out on the analysis and management of stands of Gnidia glauca in the Kilum-Ijim region (North-West Cameroon). All individuals were identified and counted in 33 plots of 40mx40m established between 1963 and 2785 m altitude in these Gnidia glauca formations. The height and diameter of each individual were measured. A quadrat of 1m² was established around the mature trees to evaluate the regeneration. ANOVA was used to compare the average density of the species in various plots and the DUNCAN test at the 5% significance level (SPSS software version 17.0) was used to separate these means. It emerges from this study that the Thymelaeaceae (G. glauca) family is mainly represented. The Asteraceae, Rubiaceae, Poaceae and Fabaceae are the most diverse families in these formations. The distribution of individuals of G. glauca in diameter classes shows a decreasing pattern. The low density of G. glauca is observed at low altitudes (166.66 stems / hectare), and the high density is between 2350 and 2450 m (778.18 stems / ha). The average standing densities of G. glauca varies between 64.58 and 459.37 stems per hectare; these values indicate a good regeneration of this species in the site. G. glauca can therefore be rationally exploited in a sustainable way for the well-being of the surrounding human populations

Long and short-time scale climatic variability in the last 5500 years in Africa according to modern and fossil diatoms from Lake Ossa (Western Cameroon)

The reconstruction of paleoenvironmental changes from diatoms in lake sediments is based on the usual assumption that, in each studied sample, the dominant species reflect the environment that prevailed during the time of deposition. If the environment changed significantly during the period of deposition (several years) one can expect a mixture of species having different or contradictory ecological affinities. In this paper we present analyses of diatoms in surface sediment samples collected in the Lake Ossa area (3 degrees 50'N, 9 degrees 36E) and fossil diatoms from a mid-late Holocene core retrieved in the deepest part of the lake. The Lake Ossa area alternates between a short dry season centered at around the northern winter and a long rainy season during the rest of the year entailing significant changes in water level and pH. Based on multivariate analyses, we will show here that mean annual water depth is the most significant variable explaining the distribution of diatoms in the entire Lake. However, seasonal changes of water level are poorly recorded by diatom assemblages, except in some flat areas on the borders of the lakes where a mixing between species with different affinities to water depth is likely due to seasonal changes in water level. Inferred water depth based on a quantitative transfer function reflected essentially secular to millennial changes in the studied core. The relationship between pH and diatoms is not statistically significant but seasonal to multi-annual pH variations mainly observed in the central parts of the lake are reflected by a mixing of acidophilous and alkaliphilous species. Hierarchical ascending cluster analysis (HAC) considered as the most efficient mean of describing diatom mixing shows that seasonal to multi-annual changes in pH are recorded both in the modern and fossil assemblages. According to the degree of mixing between diatoms with different pH affinities we conclude that short-term pH variability was weaker than today between 5200 and 2700 cal yr BP, stronger between 2700 and 2000 cal yr BP, weaker again between 2000 and 600 cal yr BP and similar to present from 400 cal yr BP onwards. Short-term changes were thus superimposed on secular to millennium trends recorded by modifications in the abundance of alkaliphilous diatoms. All these changes are interpreted as variations in precipitation according to a previous model showing that pH is strongly controlled by acidic meteoric water. Inferred water depth slightly changed over the last 5500 years showing weak variations of precipitation minus evaporation balance at secular to millennial time scales. These results will be used to refine previously published paleoclimatic interpretations, which explained changes in precipitation and P-E balance by modifications in the vertical structure of the atmosphere and subsequently by changes in cloud cover, convective or stratiform

Relationships between algae taxa and physico-chemical characteristics of water in wetlands and waterbodies

A study of algae flora was performed on 16 samples collected in different aquatic environments in Bamenda (Cameroon) in order to evidence the relationships between algae assemblages and physico-chemical parameters of the milieu. A total of 22 algae species were identified, the most represented class being Bacillariophyceae (8 species), followed by the Cyanophyceae (5 species), Zygnemataceae (4 species), Euglenophyceae (3 species) and finally Chlorophyceae (2 species). Physicochemical analyses of water samples suggest evidences of organic pollution related to anthropogenic activities, running waters being less affected than stagnant ones. This organic pollution is inferred from highest values of TDS and conductivity. The Correspondence Analysis (CA) displayed four groups of algae on the two first axes. On axis 1 the discriminating factor is the current, because running water sites are opposed to stagnant water ones. Unfortunately measured values of water current are not available to precise that hypothesis, our interpretations in this paper are based on relative data. The second axis reveals the opposition between oligotrophic and eutrophic waters, and could be considered as a gradient of organic pollution. The species Navicula sp, Gomphonema sp, Ulothrix sp., Nostoc sp., Amphora sp., Pinnularia sp., Tabellaria sp. and Lyngbya sp. are associated to low values of TDS and conductivity while Phacus sp., Closterium sp., Euglena sp. and Trachelomonas sp. thrive better in high polluted waters. The distribution of these algae identified as indicators of high organic pollution, evidenced a spatial variability of organic pollution in accordance with the water physicochemical parameters monitored. This variability is probably modulated by the presence in the sampling sites of macrophytes that are efficient in removing pollutants. Keywords: Bamenda, algae, correspondence analyse, organic pollution, phytoremediationCaJEB Vol. 3 (2) 2007: pp. 70-7

A late Holocene palaeoenvironmental record from Lake Tizong, northern Cameroon using diatom and carbon stable isotope analyses

A late Holocene record, based on diatom and stable carbon isotopes from Lake Tizong, northern Cameroon, provides a history of environmental changes over the last 4100 years. Several coarser sediment layers among which the two younger ones are of pyroclastic origin interrupt the fine clayey sediment of the core. The detailed chronology of the core supported by 24 radiocarbon C-14 dates and proxies data results revealed an erosive phase registered in the sedimentary column from 2200 to 1500 cal BP. The diatom ecological groups suggest that between 4100 and 2800 cal yrs BP, the lake level was much higher than after corresponding to a relatively greater precipitation minus evaporation (P E) ratio, as well as increased runoff in the lake catchment. These conditions were favourable to the development of C3 plants in the lake catchment as indicated by lower delta C-13 values and higher C/N ratios than after. This hydrological phase is also characterized by eutrophic, turbid and probably circum-neutral to alkaline waters. After this episode, higher delta C-13 values between 2800 and 2500 cal BP suggest increased water use efficiency of terrestrial plants and/or potentially more C4 plant debris input into the lake: an indication of savannas patches developing, due probably to changes in the rainfall distribution. Marked lake-level declines are recorded at 2500, 2200-2100, and at 1400-1000 cal yrs BP. These low-stands are characterized by higher inputs of windblown diatoms (up to 4.2%) than before, which confirms that the NE trade-winds were strengthening. This corresponds primarily to a reduction in the P E ratio, but probably also to greater inter-annual or seasonal variability when drier periods or seasons became more prolonged and intense than previously. Consequently, savannas were maintained as suggested by relatively higher than before delta C-13 values, as well as independently supported by regional pollen data. After 1000 cal BP, the lake-level rose towards sub-modern conditions, with a deep neutral and eutrophic water column. Carbon stable isotopes suggest a reduction of organic matter input, while savannas were maintained despite the return to more humid conditions. The trends of climatic changes observed in the Lake Tizong reveal the variability in timing, magnitude and regional extent of known climatic events

Past environmental and climatic changes during the last 7200 cal yr BP in Adamawa plateau (Northern-Cameroun) based on fossil diatoms and sedimentary carbon isotopic records from Lake Mbalang

Past limnological conditions of Lake Mbalang (7 degrees 19' N, 13 degrees 44' E, altitude: 1130 m) and vegetation type were reconstructed from diatoms and sedimentary stable carbon isotope records (delta C-13) since 7200 cal yr BP. The data showed that before 3600 cal yr BP, the water column was dominantly stable except around 5000-5300 cal yr BP where diatoms evidenced a mixed upper water layer and delta C-13 data suggest more forested vegetation in the landscape. These stable conditions can be explained by a strong monsoon flux and relatively northern position of the ITCZ that entailed high or low rainfall well distributed over the year, allowing the development of mountainous forest taxa. The decreasing trend of the monsoon flux towards the mid-Holocene was affected by several abrupt centennial to millennial-scale weakening at 6700, 5800-6000, 5000-5300, 4500 and 3600 cal yr BP. However, their impact on the vegetation is not visible, probably because rainfall distribution was favourable to forest maintenance or extension. After 3600 cal yr BP, the water column became very mixed as a result of more intense NE trade winds (Harmattan) that led at similar to 3000 cal yr BP to the establishment of savannah in the vegetation landscape. At that time, rainfall was probably reduced following the southward shift of the ITCZ, and the distribution of yearly rainfall was not favourable anymore to forest development. A strong seasonality with a marked dry season was established, conditions that maintained the savannah vegetation until today. Diatom data suggest the lake did not dry up during the last 7200 cal yr BP; however, a low lake level observed at 24002100 cal yr BP is contemporaneous to a climatic event evidenced in several areas of tropical Africa and could correspond to the southernmost position of the ITCZ. Other low lake levels are observed at 1800 and 1400 cal yr BP, after which the lake rose to its present level

Last millennia of the tropical moist forest of Central Africa : forest of savanna ?

International audienc