Western equatorial African forest-savanna mosaics: a legacy of late Holocene climatic change?

Past vegetation and climate changes reconstructed using two pollen records from Lakes Maridor and Nguène, located in the coastal savannas and inland rainforest of Gabon, respectively, provide new insights into the environmental history of western equatorial African rainforests during the last 4500 cal yr BP. These pollen records indicate that the coastal savannas of western equatorial Africa did not exist during the mid-Holocene and instead the region was covered by evergreen rainforests. From ca. 4000 cal yr BP a progressive decline of inland evergreen rainforest, accompanied by the expansion of semi-deciduous rainforest, occurred synchronously with grassland colonisation in the coastal region of Gabon. The contraction of moist evergreen rainforest and the establishment of coastal savannas in Gabon suggest decreasing humidity from ca. 4000 cal yr BP. The marked reduction in evergreen rainforest and subsequent savanna expansion was followed from 2700 cal yr BP by the colonization of secondary forests dominated by the palm, <i>Elaeis guineensis</i>, and the shrub, <i>Alchornea cordifolia</i> (Euphorbiaceae). A return to wetter climatic conditions from about 1400 cal yr BP led to the renewed spread of evergreen rainforest inland, whereas a forest-savanna mosaic still persists in the coastal region. There is no evidence to suggest that the major environmental changes observed were driven by human impact

Caractérisation de la Biomasse sèche des mosaïques forêt-savane des plateaux Okouma et Bagombé au sud-est du Gabon

Objectif : Quantifier la biomasse sèche de savanes et du sous-bois en forêts dans la mosaïques forêtsavane au Sud-est du Gabon (Okouma et Bagombé).Méthodologie et résultats : La méthode destructive a été utilisée pour l’acquisition des données. En effet, elle a consisté à identifier, mesurer l’aide d’un pied à coulisse et peser les différents arbustes dans une parcelle de 25m2.Pour les arbustes dont les diamètres sont compris entre 1 et 5cm, ont été sectionnés, mesurés et pesés .Les aliquotes prélevés sur chaque échantillon ont permis d’estimer la biomasse en laboratoire .Ainsi, l’analyse statistique réalisée sur ces données a montré que la productivité moyenne de biomasse sèche est beaucoup plus importante en savane (35466760kg/ha) qu’en sous-bois (3442996kg/ha). Par ailleurs, la comparaison statistique «ANNOVA» entre la biomasse sèche produite dans les différents biotopes anthropisés montre que la productivité moyenne de biomasse sèche n’est pas différente d’un biotope à un autre, et afin une estimation des gains perçus par la vente des stockes de carbone dans le cadre du programme REDD a été simulée sur une aire de 3700haConclusion and application : Ce travail a permis de quantifier les stocks de carbone en savane et dans les sous-bois des mosaïque-forêt-savane. Ces travaux sont également d’intérêt capital pour les gouvernants à la fois dans la mise en oeuvre des politiques de réductions des émissions des gaz à effet des serres et d’un impact économique notamment dans le cadre du programme REDD.Mots clés: Carbone, Biomasse sèche, Gabon, savanes, forêt, changement climatiqu

Augmentation de la biomasse de la roselle (Hibiscus sabdariffa L. var. sabdariffa) par des traitements à l’acide borique, au Gabon

Pour intensifier la production de la roselle (Hibiscus sabdariffa L. var. sabdariffa), une plante à forte valeur marchande au Gabon, un essai de stimulation de la croissance végétale a été réalisé par prétraitement des graines à différentes solutions d’acide borique (H3BO3). L’effet inducteur a été mesuré par la variation de matière sèche des racines, des tiges et rameaux, des feuilles et des fruits des plantes âgées de 100 jours. Les résultats montrent que l’acide borique a induit 2 actions morphologiques opposées : la réduction de la matière sèche des organes à des concentrations élevées, supérieures à 10-10 M, et l’augmentation de celle-ci, pour des concentrations faibles inférieures à cette valeur. Les stimulations les plus importantes ont été induites par les concentrations de 10-10 M, avec des hausses de matière sèche supérieures à 30%. L’emploi de l’acide borique est donc théoriquement envisageable dans les programmes d’amélioration de la production de la roselle au Gabon.Mots clés: Acide borique, Roselle, Prétraitement, Biomasse, Augmentation, Diminutio

Selecting allometric equations to estimate forest biomass from plot- rather than individual-level predictive performance

In the context of global change, it is essential to quantify and monitor the carbon stored in forests. Allometric equations are mathematical models that predict the biomass of a tree from dendrometrical characteristics that are easier to measure, such as tree diameter, height, or wood density. Various model forms have been proposed for allometric equations. Moreover, the model choice has a critical influence on the estimate of the biomass of a forest. So far, model selection for allometric equations has been performed based on the tree-level predictive performance of the models. However, allometric equations are used to estimate the biomass of plots rather than individual trees. The distribution of trees sampled for establishing allometric equations often differs from the forest structure. Moreover, at the plot level, the residual individual errors for different trees can cancel off. Therefore, we expect the plot-level predictive performance of a model to differ from its tree-level performance. Using a dataset giving the observed biomass of 844 trees in central Africa and a null model for the size distribution of trees in the forest, we simulated forest plots between 0.1 and 50 ha in area. Then, using a Monte Carlo approach, we calculated the mean sum of squared errors (MSS) of the differences between observed and predicted plot biomass. We showed that MSS could be well approximated by a three-term formula, where the first term corresponded to bias, the second one corresponded to the tree residual error, and the third one corresponded to the uncertainty on model coefficients. For small plots (≤ 0.1 ha), the plot-level predictive performance was dominated by the tree residual error term. Model selection based on plot-level predictive performance was then consistent with that based on tree-level performance. For large plots, this term vanished. Model selection based on plot-level performance could then differ from that based on tree-level performance. In the case of large plots, chains of models that combined a general equation to predict biomass and local equations to predict some of the predictors of the biomass equation could provide a good trade-off between the bias in and the uncertainty on model coefficients. We recommend using plot-level rather than tree-level predictive performance to select allometric equations. The three-term formula that we developed provides an easy way to assess the effect of plot size on model selection and to balance the respective contributions of bias, tree residual error, and the uncertainty on model coefficients.</p

The biogeography of Gabonese savannas: evidence from termite community richness and composition

DATA AVAILABILITY STATEMENT : All biological samples collected for this study are stored at the Natural History Museum of London and remain the property of the Gabonese government. Research authorisation from Gabon's Centre National de la Recherche Scientifique et Technologique (CENAREST) will be required for use of these samples for any other research. R scripts and raw data are archived on Zenodo (DOI: 10.5281/zenodo.6475429). Sequence data have been submitted to the GenBank database under accession numbers ON952588-ON953141.SUPPLEMENTARY MATERIAL 1 : Data S1.SUPPLEMENTARY MATERIAL 2 : Table S1. Table S3. Table S4. Table S5. Table S6.SUPPLEMENTARY MATERIAL 2 : Table S2.AIM : The mosaic of savannas that persists in the forest-dominant Congo Basin is thought to be palaeoclimatic relics, but past biogeographical processes that have formed and maintained these systems are poorly understood. Here, we explored the post-Pleistocene biogeography of Gabon's savannas using termites as biological indicators to understand historical and mechanistic factors influencing present-day termite communities in the country's extant savannas. LOCATION : Gabon, Central Africa. TAXON : Blattodea: Termitoidae. METHODS : Using standardised transect methods, we sampled termite communities in four disjunct modern savanna areas of Gabon: the centre (Lopé), the southeast (Batéké) and the south (Mayombe North and South). Termites at Lopé were collected in three habitats (annually burned savannas, savannas with a depressed fire regime and forest). We used DNA barcoding of the COII region to identify termite species and compared abundance, species richness and community composition across areas and habitats. RESULTS : Community composition differed greatly between Lopé and both Batéké and Mayombe savannas with Lopé being exceptionally depauperate and lacking characteristic savanna species. Within Lopé, termite abundance and diversity was highest in forests and lowest in annually burned savannas, with a gradual change in species composition across the forest–savanna gradient associated with fire history. MAIN CONCLUSIONS : The absence of savanna typical species in Lopé savannas challenges current assumptions that these savannas were linked to the south/southeastern savannas during the Pleistocene and suggests a different evolutionary history. Lopé savannas may instead have opened as an isolated grassland and never have been contiguous with neighbouring savannas, or were isolated soon after forest expansion began and have now lost savanna-typical species. Furthermore, the patterns of termite community composition in fire suppressed savannas support a hypothesis of rapid change driven by fire frequency where either fire suppression or infrequent burning over 23 years has meant savannas have become ecologically much more forest-like.DFID and Office of the Royal Society.http://www.wileyonlinelibrary.com/journal/jbihj2024Zoology and EntomologySDG-15:Life on lan

Range Expansion Drives Dispersal Evolution In An Equatorial Three-Species Symbiosis

A-09-14International audienceBackground Recurrent climatic oscillations have produced dramatic changes in species distributions. This process has been proposed to be a major evolutionary force, shaping many life history traits of species, and to govern global patterns of biodiversity at different scales. During range expansions selection may favor the evolution of higher dispersal, and symbiotic interactions may be affected. It has been argued that a weakness of climate fluctuation-driven range dynamics at equatorial latitudes has facilitated the persistence there of more specialized species and interactions. However, how much the biology and ecology of species is changed by range dynamics has seldom been investigated, particularly in equatorial regions. Methodology/Principal Findings We studied a three-species symbiosis endemic to coastal equatorial rainforests in Cameroon, where the impact of range dynamics is supposed to be limited, comprised of two species-specific obligate mutualists –an ant-plant and its protective ant– and a species-specific ant parasite of this mutualism. We combined analyses of within-species genetic diversity and of phenotypic variation in a transect at the southern range limit of this ant-plant system. All three species present congruent genetic signatures of recent gradual southward expansion, a result compatible with available regional paleoclimatic data. As predicted, this expansion has been accompanied by the evolution of more dispersive traits in the two ant species. In contrast, we detected no evidence of change in lifetime reproductive strategy in the tree, nor in its investment in food resources provided to its symbiotic ants. Conclusions/Significance Despite the decreasing investment in protective workers and the increasing investment in dispersing females by both the mutualistic and the parasitic ant species, there was no evidence of destabilization of the symbiosis at the colonization front. To our knowledge, we provide here the first evidence at equatorial latitudes that biological traits associated with dispersal are affected by the range expansion dynamics of a set of interacting species

Long-Term Vegetation Change in Central Africa: The Need for an Integrated Management Framework for Forests and Savannas

peer reviewedTropical forests and savannas are the main biomes in sub-Saharan Africa, covering most of the continent. Collectively they offer important habitat for biodiversity and provide multiple ecosystem services. Considering their global importance and the multiple sustainability challenges they face in the era of the Anthropocene, this chapter undertakes a comprehensive analysis of the past, present, and future vegetation patterns in central African forests and savannas. Past changes in climate, vegetation, land use, and human activity have affected the distribution of forests and savannas across central Africa. Currently, forests form a continuous block across the wet and moist areas of central Africa, and are characterized by high tree cover (>90% tree cover). Savannas and woodlands have lower tree cover (<40% tree cover), are found in drier sites in the north and south of the region, and are maintained by frequent fires. Recent tree cover loss (2000–2015) has been more important for forests than for savannas, which, however, reportedly experienced woody encroachment. Future cropland expansion is expected to have a strong impact on savannas, while the extent of climatic impacts depends on the actual scenario. We finally identify some of the policy implications for restoring ecosystems, expanding protected areas, and designing sustainable ecosystem management approaches in the region

Community and ecosystem diversity: Integrating macro-ecology with evolutionary studies, community phylogenetic structure, past and present climate change and its effect on evolution

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