Délimitation et identification des espèces végétales en Afrique centrale - apport des approches de génétique des populations et de phylogéographie

La délimitation des espèces végétales ainsi que leur identification sur le terrain est une difficulté récurrente dans les forêts tropicales humides. Bien que le choix des taxons pour nos études phylogéographiques reposait sur des espèces a priori sans problème taxonomique, il est rapidement apparu que nos données pouvaient, dans certains cas, remettre en question la taxonomie adoptée. Des éclaircissements sur la délimitation des espèces seront présentés pour les arbres des genres Carapa, Erythrophleum, Greenwayodendron et Santiria et les lianes du genre Haumania. Dans certains cas les marqueurs génétiques confirment la classification taxonomique et permettent d'identifier a posteriori des espèces difficiles à distinguer sur le terrain (Erythrophleum). D'autres cas suggèrent que la classification taxonomique ne reflète pas des espèces au sens biologique (Santiria, Greenwayodendron). Enfin, des cas d'incongruence entre génomes cytoplasmique et nucléaire mettent en évidence des phénomènes d'introgression et donc une spéciation inachevée. Ainsi, les séquences de gènes chloroplastiques ne sont pas toujours fiables pour délimiter ou identifier les espèces. Enfin, une analyse de barre-codes ADN basée sur le génome chloroplastique chez les espèces d'arbres d'un plot de 50ha à Korup (Cameroun) montre que seulement près de 60% des espèces et 80% des genres peuvent être identifiés par cette approche avec le gène rbcL. (Texte intégral

Desarrollo de nichos específicos de cacao con alta productividad y calidad sensorial: experiencia ecuatoriana

Uno de los pilares fundamentales en los que se basa hoy en día el desarrollo de la cacaocultura ecuatoriana, es el aprovechamiento de sus recursos genéticos disponibles. Con esta visión, desde el año 1995, se diseñaron nuevas estrategias de mejoramiento genético, con el objetivo de: a) obtener nuevos materiales genéticos con elevada producción y calidad organoléptica; y b) Potenciar el desarrollo de nuevos nichos de producción en áreas no tradicionales del cultivo. Para esto, un equipo multidisciplinario de trabajo ha venido combinando esfuerzos de investigación en las áreas de: mejoramiento convencional (cruzamientos), biología molecular, calidad organoléptica del grano (físico, químico y sensorial) y más recientemente también de tipo arqueológico, con el propósito de alcanzar los objetivos propuestos. El primer objetivo se empezó a cumplir desde el año 2009, con la liberación comercial de 4 clones altamente productivos: 2 para la Provincia de Manabí y 2 para la Provincia de Santa Elena, convirtiéndose esta última provincia a la época actual, en la región del país donde se evidencian los niveles más altos de producción de cacao fino por hectárea, en base al material genético liberado por INIAP. El resultado anterior, se consolidó en el año 2016, con la entrega para otra zona agroecológica de dos nuevos clones que inclusive superan en producción a nuestro referente territorial (el clon CCN-51). Al momento, se han obtenido nuevos resultados, con base en los cuales se planifica la entrega de dos nuevos clones de alta producción (2019) y que serán específicos para otra región cacaotera del Ecuador. El segundo objetivo también se está materializando con la identificación de zonas con potencial para la producción de nuevos cacaos especiales y de manera particular en la amazonia ecuatoriana, en donde se han realizado hasta el momento tres prospecciones en la región sur, con el propósito de aprovechar al máximo la diversidad natural presente en la zona. Este accionar ha permitido rescatar valiosos recursos genéticos que están siendo conservados en colecciones de campo, a partir de los cuales se están obteniendo resultados que permiten de manera preliminar avizorar en el mediano plazo, la entrega de nuevo material genético capaz de producir cacaos especiales con propiedades sensoriales diferentes al típico “Sabor Arriba” y en zonas no tradicionales del cultivo que reúnen condiciones agro-bioclimáticas y sociales con potencial para constituirse en nuevos y particulares nichos de producción

Above-ground carbon stocks, species diversity and fire dynamics in the Bateke Plateau

Savannas are heterogeneous systems characterised by a high spatial and temporal variation in ecosystem structure. Savannas dominate the tropics, with important ecological functions, and play a prominent role in the global carbon cycle, in particular responsible for much of its inter-annual variability. They are shaped by resource availability, soil characteristics and disturbance events, particularly fire. Understanding and predicting the demographic structure and woody cover of savannas remains a challenge, as it is currently poorly understood due to the complex interactions and processes that determine them. A predictive understanding of savanna ecosystems is critical in the context of land use management and global change. Fire is an essential ecological disturbance in savannas, and forest-savanna mosaics are maintained by fire-mediated positive feedbacks. Over half of the world’s savannas are found in Africa, and over a quarter Africa’s surface burns every year, with fires occurring principally in the savanna biome. These have strong environmental and social impacts. Most fires in Africa are anthropogenic and occur during the late dry season, but their dynamics and effects remain understudied. The main objective of this research is to understand the floristic composition, carbon storage, woody cover and fire regime of the mesic savannas of the Bateke Plateau. The Bateke Plateau is savanna-forest mosaic ecosystem, situated mainly in the Republic of Congo, with sandy Kalahari soils and enough precipitation for potential forest establishment (1600 mm/yr). Despite occupying 89,800 km2, its ecology and ecosystem functions are poorly understood. This study combines two approaches: firstly experimental, setting up long term field experiments where the fire regime is manipulated, and then observational, using remote sensing to estimate the carbon storage and study the past history of the fire regime in the region. I established four large (25 ha) plots at two savanna sites, measured their carbon stocks, spatial structure and floristic composition, and applied different annual fire treatments (early and late dry season burns). These treatments were applied annually during 3 years (2015, 2016 and 2017), and the plots were re-measured every year to estimate tree demographic rates and the identification of the key processes that impact them, including fire and competition. Field data were combined with satellite radar data from ALOS PALSAR, and the fire products of the MODIS satellites, to estimate carbon stocks and fire regimes for the entire Bateke Plateau. I also analyse the underlying biophysical and anthropogenic processes that influence the patterns in Above-Ground Woody Biomass (AGWB) and their spatial variability in the Bateke landscape. The total plant carbon stocks (above-ground and below-ground) were low, averaging only 6.5 ± 0.3 MgC/ha, with grass representing over half the biomass. Soil organic matter dominate the ecosystem carbon stocks, with 16.7 ± 0.9 Mg/ha found in the top 20 cm alone. We identified 49 plant species (4 trees, 13 shrubs, 4 sedges, 17 forbs and 11 grass species), with a tree hyperdominance of Hymenocardia acida, and a richer herbaceous species composition. These savannas showed evidence of tree clustering, and also indications of tree-tree competition. Trees had low growth rates (averaging 1.21 mm/yr), and mortality was relatively low (3.24 %/yr) across all plots. The experiment showed that late dry season fires significantly reduced tree growth compared to early dry season fires, but also reduced stem mortality rates. Results show that these mesic savannas had very low tree biomass, with tree cover held far below its climate potential closed-canopy maximum, likely due to nutrient poor sandy soils and frequent fires. Results from the remote sensing analysis indicated that multiple explanatory variables had a significant effect on AGWB in the Bateke Plateau. Overall, the frequency of fire had the largest impact on AGWB (with higher fire frequency resulting in lower AGWB), with sand content the next most important explanatory variable (with more sand reducing AGWB). Fires in the Bateke are very frequent, and show high seasonality. The proportion of fires that occurred in the late dry season, though smaller predictor, was also more important than other factors (including soil carbon proportion, whether or not the savanna area was in a protected area, annual rainfall, or distance to the nearest town, river or road), with a larger proportion of late dry season fires associated with a small increase in AGWB. The results give pointers for management of the savannas of the Bateke Plateau, as well as improving our understanding of vegetation dynamics in this understudied ecosystem and help orient policy and conservation

Un nouveau Psychotria (Rubiaceae) du sud Cameroun

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