Jean Joël Loumeto’s research while affiliated with Marien Ngouabi University and other places

Phylogeographic studies on widespread rainforest species from West and Central Africa often reveal genetic discontinuities. These discontinuities can originate from past barriers to gene flow resulting from long-lasting population fragmentation during glacial periods, according to the forest refuge hypothesis. This study 69 nuclear SNPs, 13 plastid SNPs, and 24 mitochondrial SNPs to characterized the distribution of genetic diversity in 377 individuals of the widespread tropical tree Khaya ivorensis, in western and central African evergreen forests. Two very well-differentiated nuclear genetic clusters (FST = 0.28) are located respectively in West and Central Africa. The gradual transition of allele frequencies between the clusters across a broad geographic area going from Cameroon to Nigeria accords with the recognition of a single species, although we show an incipient divergence that could eventually lead to the separation of two taxa. The two clusters have similar genetic diversity at nuclear SNPs. However, the cytoplasmic data revealed high haplotypic diversity and numerous endemic haplotypes in Central Africa, and only one widespread haplotype in West Africa, suggesting an ancient colonization of West Africa from Central Africa. The genetic diversity inside and outside putative forest refugia (Anhuf et al., Palaeogeogr Palaeoclimatol Palaeoecol 239:510–527, 2006) does not differ significantly in either genetic cluster. Hence, we cannot confirm that forest refugia played a particular role in the pattern of distribution of genetic diversity in K. ivorensis. Owing to the high haplotypic diversity of their populations, Central Africa, especially Gabon, constitutes a priority area for the conservation of the genetic diversity of K. ivorensis.

The natural regeneration of prime timber species with high economic potential is studied in the Mpoh forest, in the northern sector of the Léfini reserve. Although a source of economic and self-subsistence for local residents, the natural regeneration of these forest ecosystems has received very little attention. The floristic inventory was carried out in three one-hectare quadrats, in which (i) the d 1.30 of ligneous plants 1 cm ≤ d 1.30 < 10 cm, d 1.30 ≥ 10 cm and seedlings ≥ 30 cm in height were counted and measured; (ii) the degree of canopy openness, litter thickness and herbaceous cover. The inventory reveals 1208 individuals for 36 species and 20 families, i.e., 118 seedlings of height ≥ 30 cm; 769 individuals of 1 cm ≤ d 1,30 < 10 cm and 321 of d 1,30 ≥ 10 cm. Structural parameters favor Petersianthus macrocarpus as dominant in the 1 cm ≤ d 1.30 < 10 cm and d 1.30 ≥ 10 cm diameter classes, in density (89 trees.ha-1) and basal area (7.137 m².ha-1). In contrast, Entandrophragma sp. dominates the seedling class (h ≥ 30 cm). The bioecological indices highlight a difference in the floristic composition of the compartments. Natural regeneration, dependent on light and litter thickness, is generally good. Depending on the socioeconomic value of taxa with difficult natural regeneration, intervention may be necessary.

Une meilleure connaissance de la composition floristique, la structure et la dynamique des mosaïques forêt/savane du littoral congolais est nécessaire pour apprécier leur résilience face au changement climatique et la pression anthropique d’une part. Et d’autre part, pour la mise en place des politiques de leur gestion durable. La présente étude visait l’appréciation de la dynamique de la forêt à Okoumé (A. klaineana P.) du littoral de la république du Congo dans la période allant de 2008 à 2018. La densité, la surface terrière, la structure diamétrique et la détermination de quelques indices écologiques ont permis d’apprécier la dynamique forestière du dispositif expérimental de Bitsifa. Après une période de dix ans, la densité a diminué de 30 tiges/ha tandis que la surface terrière a augmentée de 2,78 m2 /ha. Okoumé n’est plus la seule espèce omniprésente dans les placeaux d’observation du dispositif expérimental de Bitsifa. L’indice de similarité de Sorensen a révélé l’existence de deux sous-faciès forestiers au lieu de trois, en allant de la lisière jusqu’à 300 m à l’intérieur du peuplement étudié. En somme, la diminution de la densité et l’augmentation de la surface terrière indiquent une évolution progressive de la forêt de Bitsifa vers un stade mature. English title: Dynamics of an Aucoumea klaineana P. forest on the coast of the Republic of Congo A better understanding of the floristic composition, structure and dynamics of forest/savanna mosaics along the Congolese coast is needed to assess their resilience to climate change and human pressure. And secondly, to put in place policies for their sustainable management. The aim of this study was to assess the dynamics of the Okoumé forest (A. klaineana P.) along the coast of the Republic of Congo from 2008 to 2018. Density, basal area, diameter structure and the determination of a number of ecological indices were used to assess the forest dynamics of the Bitsifa experiment. After ten years, the density has decreased by 30 stems/ha, while the basal area has increased by 2.78 m2 /ha. Okoumé is no longer the only species omnipresent in the observation plots at the Bitsifa experimental site. Sorensen's similarity index revealed the existence of two forest sub-facies instead of three, from the edge to 300 m inside the stand studied. In short, the decrease in density and the increase in basal area indicate a gradual evolution of the Bitsifa forest towards a mature stage.

Lianas are important components of tropical forest diversity and dynamics, yet little is known about the drivers of their community structure and composition. Combining extensive field and LiDAR data, we investigated the influence of local topography, forest structure and tree composition on liana community structure and composition in a moist forest in northern Republic of Congo. We inventoried all lianas ≥ 1 cm in diameter in 144 20×20 m quadrats located in four 9-ha permanent plots, where trees and giant herbs were inventoried. We characterized the functional strategies of selected representatives of the main liana taxa using a set of resource-use leaf and wood traits. Finally, we used complementary statistical tests, including multivariate and randomization schemes, to test whether forest structure, topography and tree composition influence the structure, floristic, and functional composition of liana communities. The structure of liana communities was strongly shaped by local forest structure, with higher abundances and total basal areas in relatively open-canopy forests, where lianas competed with giant herbs. Liana floristic composition exhibited a weak spatial structure over the study site, but was marginally influenced by local forest structure and topography. Only forest structure had a weak but significant effect on liana functional composition with more conservative strategies—higher stem tissue density and lower PO4 leaf concentration and SLA values—in tall and dense forests. Finally, we found evidence of host specificity with significant attraction/repulsion for 19% of the tested liana and tree species associations, suggesting that the unexplained floristic variation may be partly attributed to these host species-specific associations, although the underlying mechanisms behind remain elusive. Overall, our findings demonstrate that the structure of liana communities can be much better predicted than their composition, calling for a better understanding of the implication of the large functional diversity observed in liana communities.

Non-timber forest products (NTFPs) are important components of rural people’s livelihood, being used notably for food and medicine. Given their socio-economic importance, NTFP species are often integrated in farmers’ fields using wild seed material from adjacent forests. In sub-Saharan Africa, the evolutionary history, and more specifically, the cultivation history of these species (geographical origin, dating), is still largely unknown. This study focuses on an African medicinal tree species, originating from the Guineo-Congolian rain forests and commonly known as the bitter kola tree (Garcinia kola, Clusiaceae). We estimated species genome size and chromosome number. We developed eleven highly polymorphic nuclear microsatellite markers and used seven of them to characterize patterns of distribution of genetic diversity within populations of G. kola in the forest (161 wild individuals coming from nine different populations from Cameroon, Gabon, and the Republic of Congo) and on-farm (82 cultivated individuals from five populations from Cameroon). The species is most probably hexaploid, with a relatively large genome size (2C = 25.5 to 28.5 picogram) and ca. 170 chromosomes. Three intra-specific gene pools were observed along a north–south axis within wild populations. This genetic differentiation pattern presents some similarities with patterns already observed in other tree species from the region, suggesting that a common factor, potentially past-climatic oscillations, has affected the demography of forest trees in the same way. Cultivated populations were not spatially structured, and the genetic differentiation between wild and cultivated populations was low (GST = 0.017). The cultivation history of the species was difficult to interpret. It is actually unclear if cultivated populations from Cameroon are derived from wild Cameroonian populations or if they originate from wild populations of West African countries, notably Nigeria.

The world’s largest tropical peatland lies in the central Congo Basin. Raphia laurentii De Wild, the most abundant palm in these peatlands, forms dominant to mono-dominant stands across approximately 45% of the peatland area. R. laurentii is a trunkless palm with fronds up to 20 m long. Owing to its morphology, there is currently no allometric equation which can be applied to R. laurentii. Therefore it is currently excluded from aboveground biomass (AGB) estimates for the Congo Basin peatlands. Here we develop allometric equations for R. laurentii, by destructively sampling 90 individuals in a peat swamp forest, in the Republic of the Congo. Prior to destructive sampling, stem base diameter, petiole mean diameter, the sum of petiole diameters, total palm height, and number of palm fronds were measured. After destructive sampling, each individual was separated into stem, sheath, petiole, rachis, and leaflet categories, then dried and weighed. We found that palm fronds represented at least 77% of the total AGB in R. laurentii and that the sum of petiole diameters was the best single predictor variable of AGB. The best overall allometric equation, however, combined the sum of petiole diameters (SDp), total palm height (H), and tissue density (TD): AGB = Exp(−2.691 + 1.425 × ln(SDp) + 0.695 × ln(H) + 0.395 × ln(TD)). We applied one of our allometric equations to data from two nearby 1-hectare forest plots, one dominated by R. laurentii, where R. laurentii accounted for 41% of the total forest AGB (with hardwood tree AGB estimated using the Chave et al. 2014 allometric equation), and one dominated by hardwood species, where R. laurentii accounted for 8% of total AGB. Across the entire region we estimate that R. laurentii stores around 2 million tonnes of carbon aboveground. The inclusion of R. laurentii in AGB estimates, will drastically improve overall AGB, and therefore carbon stock estimates for the Congo Basin peatlands.

Le maraîchage urbain joue un rôle très important dans la sécurité alimentaire et la résorption du chômage. Cependant, l’intensification des pratiques culturales peuvent altérer les propriétés physicochimiques des sols et réduire les rendements. La présente étude avait pour but de caractériser les pratiques culturales maraîchères à Brazzaville et de déterminer leur influence sur les propriétés physicochimiques des sols. Dans les sites retenus, des enquêtes structurées ont été menées auprès des maraîchers et des échantillons de sol ont été prélevés suivant des transects. Les analyses physiques et chimiques des sols ont été réalisées suivant les procédures courantes de laboratoire. Les résultats obtenus ont montré que les pratiques culturales dans les sites étudiés sont intensives. Les sols étudiés sont sableux à plus de 95%, acides, pauvres en matière organique et ont une CEC relativement faibles. Cependant on relève une sursaturation en bases échangeables sur l’ensemble des sites. Ainsi, cette sursaturation serait liée à l’intensification des pratiques culturales. Urban market gardening plays a very important role in food security and the reduction of unemployment. However, intensification of soil use can alter soil properties and reduce yields. This study aims to characterize vegetable farming practices in Brazzaville and to determine their influence on soil properties. In the selected sites, structured questionnaire was conducted among market gardeners and soil samples were taken following transects. Soil physical and chemical analyses were carried out following lab common procedures. The results showed that the cultivation practices in the sites studied are intensive. The soils studied are more than 95% sandy, acidic, low in organic matter and have a relatively low CEC. However, there is an oversaturation in exchangeable bases on all sites. Thus, this oversaturation would be linked to the intensification of cultural practices.

The worlds largest tropical peatland lies in the central Congo Basin. Raphia laurentii De Wild. is a palm which forms dominant to mono-dominant stands across ca. 45 % of the peatland area. However, a lack of allometric equation for this canopy-forming trunkless palm with fronds up to 20 m long, means that this it is currently excluded from aboveground biomass (AGB) estimates for these peatland ecosystems. Here we develop an allometric equation for R. laurentii, by destructively sampling 90 R. laurentii stipes (across six mean petiole diameter size classes of: 2-4 cm, 4-5 cm, 5-6 cm, 6-7 cm, >8 cm) in a peat swamp forest, in the Likouala Department, Republic of the Congo. Prior to destructive sampling, the five parameters were measured for each stipe: diameter at stem base, mean diameter of the palm fronds, total diameter of the palm fronds (TDpf), total height and number of palm fronds. After destructive sampling and before weighing each individual was separated into the stem and the following palm frond categories: sheath, petiole, rachis and leaflets. We fitted a linear model relating AGB to each independent predictor variable separately to assess the best variable, finding that palm fronds represented at least 77 % of the total AGB in each diameter classes. We found that TDpf was the best single predictor variable for R. laurentii AGB. The best allometric equation was: Model 11. A monodominant 1 ha plot near the harvesting site had a palm AGB of 60.8 Mg ha-1, similar to the AGB of the trees in the same plot, at 86.2 Mg ha-1. Accounting for the AGB of palms is important, and can now be estimated using the allometric equations developed here.

Khaya (Meliaceae) is a widespread genus of trees distributed from the tropical forests of Africa to Madagascar and the Comoros. Khaya species are very close morphologically and some contain considerable phenotypic and ecological diversity, raising the question of their delimitation: this is the case of Khaya anthotheca s.l. (sensu lato, including K. nyasica), which is often lumped with K. grandifoliola and K. ivorensis. We tested species delimitation within K. anthotheca by combining population-genetic and morphological data. First, a visual inspection of over a thousand specimens suggested the existence of five morphogroups. Second, after genotyping 498 samples at 71 nuclear SNPs, Bayesian clustering analyses (STRUCTURE) first identified five genetic clusters (K1–K5), while further analyses subdivided K5 into two clusters. The six genetic clusters obtained are distributed in parapatry or allopatry from West Africa to East Africa, except for cluster K4, which is in sympatry with K3 in Central Africa. The partitioning of genetic clusters is also largely congruent with the morphogroups identified. The morphological differentiation of these clusters was confirmed by statistical analyses of 27 leaf characters. The clear-cut genetic differentiation between clusters, with few admixed genotypes even in contact zones, argues for the recognition of distinct species following the biological species concept. Our analyses highlight that the current taxonomic treatment of Khaya underestimates by one-half the total number of species. We identify two new species, circumscribe K. anthotheca and support the rehabilitation at the rank of species of three taxa considered to be synonyms of K. anthotheca (K. agboensis, K. euryphylla, K. nyasica). These results illustrate the strength of combining population-genetic and morphological data to delineate an African rainforest tree species complex.