National Herbarium of Cameroon

Yaoundé, Cameroon

National Herbarium of Cameroon

Yaoundé, Cameroon
Time filter
Source Type

Schwab V.F.,Max Planck Institute For Biogeochemie | Garcin Y.,University of Potsdam | Sachse D.,University of Potsdam | Todou G.,University of Maroua | And 4 more authors.
Organic Geochemistry | Year: 2015

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. © 2014 Elsevier Ltd.

Garcin Y.,University of Potsdam | Schefuss E.,University of Bremen | Schwab V.F.,Max Planck Institute For Biogeochemie | Garreta V.,French National Institute for Agricultural Research | And 7 more authors.
Geochimica et Cosmochimica Acta | Year: 2014

Trees and shrubs in tropical Africa use the C3 cycle as a carbon fixation pathway during photosynthesis, while grasses and sedges mostly use the C4 cycle. Leaf-wax lipids from sedimentary archives such as the long-chain n-alkanes (e.g., n-C27 to n-C33) inherit carbon isotope ratios that are representative of the carbon fixation pathway. Therefore, n-alkane δ13C values are often used to reconstruct past C3/C4 composition of vegetation, assuming that the relative proportions of C3 and C4 leaf waxes reflect the relative proportions of C3 and C4 plants. We have compared the δ13C values of n-alkanes from modern C3 and C4 plants with previously published values from recent lake sediments and provide a framework for estimating the fractional contribution (areal-based) of C3 vegetation cover (fC3) represented by these sedimentary archives. Samples were collected in Cameroon, across a latitudinal transect that accommodates a wide range of climate zones and vegetation types, as reflected in the progressive northward replacement of C3-dominated rain forest by C4-dominated savanna. The C3 plants analysed were characterised by substantially higher abundances of n-C29 alkanes and by substantially lower abundances of n-C33 alkanes than the C4 plants. Furthermore, the sedimentary δ13C values of n-C29 and n-C31 alkanes from recent lake sediments in Cameroon (-37.4‰ to -26.5‰) were generally within the range of δ13C values for C3 plants, even when from sites where C4 plants dominated the catchment vegetation. In such cases simple linear mixing models fail to accurately reconstruct the relative proportions of C3 and C4 vegetation cover when using the δ13C values of sedimentary n-alkanes, overestimating the proportion of C3 vegetation, likely as a consequence of the differences in plant wax production, preservation, transport, and/or deposition between C3 and C4 plants. We therefore tested a set of non-linear binary mixing models using δ13C values from both C3 and C4 vegetation as end-members. The non-linear models included a sigmoid function (sine-squared) that describes small variations in the fC3 values as the minimum and maximum δ13C values are approached, and a hyperbolic function that takes into account the differences between C3 and C4 plants discussed above. Model fitting and the estimation of uncertainties were completed using the Monte Carlo algorithm and can be improved by future data addition. Models that provided the best fit with the observed δ13C values of sedimentary n-alkanes were either hyperbolic functions or a combination of hyperbolic and sine-squared functions. Such non-linear models may be used to convert δ13C measurements on sedimentary n-alkanes directly into reconstructions of C3 vegetation cover. © 2014 Elsevier Ltd.

Garcin Y.,University of Potsdam | Schwab V.F.,Max Planck Institute For Biogeochemie | Gleixner G.,Max Planck Institute For Biogeochemie | Kahmen A.,ETH Zurich | And 5 more authors.
Geochimica et Cosmochimica Acta | Year: 2012

Hydrogen isotope values (δD) of sedimentary aquatic and terrestrial lipid biomarkers, originating from algae, bacteria, and leaf wax, have been used to record isotopic properties of ancient source water (i.e., precipitation and/or lake water) in several mid- and high-latitude lacustrine environments. In the tropics, however, where both processes associated with isotope fractionation in the hydrologic system and vegetation strongly differ from those at higher latitudes, calibration studies for this proxy are not yet available. To close this gap of knowledge, we sampled surface sediments from 11 lakes in Cameroon to identify those hydro-climatological processes and physiological factors that determine the hydrogen isotopic composition of aquatic and terrestrial lipid biomarkers. Here we present a robust framework for the application of compound-specific hydrogen isotopes in tropical Africa. Our results show that the δD values of the aquatic lipid biomarker n-C 17 alkane were not correlated with the δD values of lake water. Carbon isotope measurements indicate that the n-C 17 alkane was derived from multiple source organisms that used different hydrogen pools for biosynthesis. We demonstrate that the δD values of the n-C 29 alkane were correlated with the δD values of surface water (i.e., river water and groundwater), which, on large spatial scales, reflect the isotopic composition of mean annual precipitation. Such a relationship has been observed at higher latitudes, supporting the robustness of the leaf-wax lipid δD proxy on a hemispheric spatial scale. In contrast, the δD values of the n-C 31 alkane did not show such a relationship but instead were correlated with the evaporative lake water δD values. This result suggests distinct water sources for both leaf-wax lipids, most likely originating from two different groups of plants. These new findings have important implications for the interpretation of long-chain n-alkane δD records from ancient lake sediments. In particular, a robust interpretation of palaeohydrological data requires knowledge of the vegetation in the catchment area as different plants may utilise different water sources. Our results also suggest that the combination of carbon and hydrogen isotopes does help to differentiate between the metabolic pathway and/or growth form of organisms and therefore, the source of hydrogen used during lipid biosynthesis. © 2011 Elsevier Ltd.

Onana J.M.,National Herbarium of Cameroon | Onana J.M.,University of Yaounde I
Rodriguesia | Year: 2015

Biodiverse Cameroon has been highlighted as the top country in tropical Africa for plant species diversity per degree square, with a higher diversity than all other West African countries added together, and including two of the top documented centres of plant diversity in Tropical Africa. Despite its reduced taxonomic capacity, with only six active taxonomists a high level of botanical activity in the country has resulted in accomplishments such as the databasing of the YA Herbarium (over 60,000 records), which has an in-country collection coverage of almost 95% of the known plant species that are recorded for Cameroon. Other accomplishments are the Red Data Book of the Flowering Plants of Cameroon, several local checklists and published volumes of the Flore du Cameroun which covers 37% of the country's species. Currently the checklist of Cameroon records 7,850 taxa at species and infraspecific level. Resources are needed to support and heighten the profile of this small botanical community. Already thanks to strong collaboration between Cameroon and renowned botanical institutes of others countries, in particular France and United Kingdom, one hundred and thirteen plant families have been published and would help this country to complete the recording of its biodiversity towards contributing to the World Flora Online 2020 project.

Fujinami R.,University of Tsukuba | Paul Ghogue J.,National Herbarium of Cameroon | Imaichi R.,Japan Women's University
International Journal of Plant Sciences | Year: 2013

Premise of research. The Podostemaceae (riverweed family) show unique morphologies and are adapted to rheophytic habitats that experience torrential floods. The fact that the Podostemaceae are sister to the Hypericaceae family in molecular phylogenetic trees suggests that the unparalleled morphologies of the Podostemaceae could have been the result of saltational evolution from the normal body construction. However, their evolutionary course remains unresolved. To deduce the evolutionary course of the Podostemaceae, Tristichoideae is a key plant taxon because it belongs to the basalmost clade of the Podostemaceae and has a unique organ termed the "ramulus." The ramulus is not so deviated compared with the aggregated leaves of Podostemoideae and was once interpreted to be the determinate shoot for Terniopsis (another genus of Tristichoideae). Methodology. A specimen of Tristicha trifaria belonging to the basalmost clade of the species and having been previously collected in Cameroon was observed. Additional Afro-Madagascan specimens with apparently different branching patterns were also observed. Observation was conducted under both LM-using a resin sectioning method-and SEM. Pivotal results. The ramulus first develops endogenously in the root tissue. It has an apical meristem and forms scaly leaves similar to a shoot apical meristem and then undergoes repeated sympodial branching. New branches arise from extra-axillary buds of the basal scaly leaves from the previous branch and replaces the latter. In Tristicha, two new branches usually arise per existing branch, resulting in a more complicated gross morphology than Terniopsis, which has only one branch. Tristicha shoots are also characterized by a welldeveloped common base, which is a product of the basal intercalary growth of ramuli. Conclusions. The ramulus of Tristicha is interpreted to be the determinate shoot portion of a sympodially branched shoot system, as in Terniopsis, not as a leaf or a leaf-stem fuzzy organ. The Tristicha shoot that bears two new branches can be compared in construction with the double sheathed (bithecous) leaf of Podostemoideae. © 2013 by The University of Chicago. All rights reserved.

Loading National Herbarium of Cameroon collaborators
Loading National Herbarium of Cameroon collaborators