Aleman, J., Saint-Jean, A., Leys, B., Carcaillet, C., Favier, C., and Bremond, L. (2013). Estimating phytolith influx in lake sediments. Quaternary Research, 80 341-347.

• Résumé : “So far, no phytolith extraction protocols have been tested for accuracy and repeatability. Here we aim to display a phytolith extraction method combining the strengths of two widely used protocols, supplemented with silica microspheres as exogenous markers for quantifying phytolith concentrations. Phytolith concentrations were estimated for samples from two sedimentary sequences in which numerical age–depth models make it possible to calculate phytolith influxes (phytolith numbers per cm2 per yr). Analysis of replicates confirmed the statistical robustness, the repeatability and the very few biases of our extraction technique for small phytoliths, since the relationship between grass silica short cells and microspheres was kept stable. Furthermore, we demonstrated that silica microspheres are robust exogenous markers for estimating phytolith concentrations. The minimum number of items (i.e., phytoliths plus silica microspheres) that must be counted to estimate phytolith concentrations and thus influxes depends on the ratio of phytoliths to microspheres (R) and is minimized when R = 1. Nevertheless, we recommend using ratios R ≤ 1 in order to avoid having the counting process become excessively time-consuming, because microspheres are easier to identify and count than phytoliths.”

• DOI: http://dx.doi.org/10.1016/j.yqres.2013.05.008

Aleman, J., Blarquez, O., Bentaleb, I., Bonté, P., Brossier, B., Carcaillet, C., Gond, V., Gourlet-Fleury, S., Lefèvre, I., Oslisly, R., Power, M.J., Yongo, O., Bremond, L., Favier, C. (2013). Tracking land-cover changes with sedimentary charcoal in the Afrotropics. The Holocene, 23(12) 1853-1862.

• Résumé : “Fires have probably played an important role in creating and maintaining savannas over the centuries and are also one of the main natural disturbances in forests. The functional role of fires in savannas and forests can be investigated through examining sedimentary charcoal in order to reconstruct long-term fire history. However, the relationship between charcoal and vegetation structure in tropical ecosystems characterized by grass as the fuel remains to be elucidated. Here, we compared recent charcoal records from lake sediments in three tropical ecosystems (forest, savanna, forest–savanna mosaic) with land-cover inferred from remote sensing images. Charcoal width/length ratio (W/L) is a good proxy for changes in fuel type. A significant W/L change was recorded simultaneously with changes in fuel type due to land-cover transformation, from W/L > 0.5 representing mainly wood to W/L < 0.5, which corresponds to grass. The results also suggest that the presence of a riparian forestaround a lake could potentially act as a physical filter for charcoal accumulation; we used the mean charcoal size as a proxy to evaluate this process. Charcoal Accumulation Rates (CHAR), a burned biomass proxy, were combined with W/L ratio and the mean charcoal size to investigate the land-use history of the watersheds. This combined approach allowed us to distinguish between episodic slash-and-burn practices in the forest andmanaged fields and/or pastures burning frequently.”

• DOI: http://dx.doi.org/10.1177/0959683613508159

Aleman, J., Leys, B., Apema, R., Bentaleb, I., Dubois, M.A., Lamba, B., Lebamba, J., Martin, C., Ngomanda, A., and Truc, L. (2012). Reconstructing savanna tree cover from pollen, phytoliths and stable carbon isotopes. Journal of Vegetation Science 23 , 187–197.

• Résumé : “Aim: To calibrate a model of the relationship between bio-proxies (pollen, phytoliths and delta13C of soil organic matter) and woody cover, measured as the leaf area index (LAI). This relationship, applied in palaeosequences, enables reconstruction of past savanna tree cover. Location: The samples are from tropical Africa. Modern soil samples are from the Central African Republic and past samples are from sediments of lakes in Senegal and Congo. Methods: We analysed the pollen and phytolith content and stable carbon isotope values of 17 soil samples taken from three short transects in the Central African Republic; LAI was measured on the same transects. The indices used were the AP/ NAP ratio of arboreal (AP) to non-arboreal (NAP) pollen, the D/ P ratio of ligneous dicotyledons (D) to Poaceae (P) phytoliths, and the delta13C of soil organic matter, i.e. the 13C/ 12C ratio. Results: A multi-proxy model was calibrated. The best model included only a combination of pollen and phytolith as proxies, excluding organic matter delta13C because of its long mean residence time in the soil. The model was then applied to two palaeosequences in Africa, and a time series of relative LAI changes was obtained, providing new information about vegetation changes. Conclusion: This model can be applied in palaeosequences to reconstruct relative time series of LAI in African savannas and can help interpret vegetation changes quantitatively. This approach is complementary to the description of pollen and phytolith assemblages.”

• DOI: http://dx.doi.org/10.1111/j.1654-1103.2011.01335.x

Bourbier, L., Cornu, G., Pennec, A., Brognoli, C., and Gond, V. (2013). Large-scale estimation of forest canopy opening using remote sensing in Central Africa. Bois et Forets Des Tropiques 3–9.

• Résumé : “Human activities in tropical rainforests cause disturbances and degradation by opening up the forest canopy. Capacities for measuring the extent of the damage are essential to the calculation of carbon emissions under REDD+ programmes (Reducing Emissions from Deforestation and Forest Degradation). Remote sensing is a powerful tool to provide this type of information (e.g. on logging, mining or infrastructure projects). Various techniques have been used to identify and quantify canopy opening. A further step is to cross-reference past and present canopy opening in order to document the recovery of ecosystems after logging. The method proposed in this article involves a semi-automatic processing chain adapted for LANDSAT imagery. During post-processing, canopy opening information is extracted by means of specific algorithms. A spatial index, calibrated to a low-resolution radiometer, indicates present and past degrees of opening. This process produces estimations of forest degradation that can describe low-resolution remotely-sensed data (such as MODIS) used in continental mapping. These estimations are then cross-referenced with continental land cover maps to gain insights into present-day forest categories. This tool was developed under the CoForChange project, whose overall objective is to predict changes in forests and in tree species distribution in the Congo Basin as a result of global change, and to develop decision-making tools. The article focuses on a full-scale example in real time, in the rainforest located at the borders of the Central African Republic, the Republic of the Congo and Cameroon, which was analysed on a year-by-year basis with thirty years of LANDSAT image archives.”

Bry, X., Trottier, C., Verron, T., and Mortier, F. (2013). Supervised Component Generalized Linear Regression using a PLS-extension of the Fisher scoring algorithm. Journal of Multivariate Analysis.

• Résumé : “In the current estimation of a GLM model, the correlation structure of regressors is not used as the basis on which to lean strong predictive dimensions. Looking for linear combinations of regressors that merely maximize the likelihood of the GLM has two major consequences: (1) collinearity of regressors is a factor of estimation instability, and (2) as predictive dimensions may lean on noise, both predictive and explanatory powers of the model are jeopardized. For a single dependent variable, attempts have been made to adapt PLS regression, which solves this problem in the classical Linear Model, to GLM estimation. In this paper, we first discuss the methods thus developed, and then propose a technique, Supervised Component Generalized Linear Regression (SCGLR), that combines PLS regression with GLM estimation in the multivariate context. SCGLR is tested on both simulated and real data.”

• DOI: http://dx.doi.org/10.1016/j.jmva.2013.03.013

Favier, C., Bremond, L., Oslisly, R., Ngomanda, A., Nguetsop, F., Maley, J., Bentaleb, I., Fayolle, A., Doucet, J.-L., and Gourlet-Fleury, S. (submitted). Climate drove forest and human occupation during the last 4000 years in Atlantic Central Africa. Nature.

Fayolle, A., Swaine, M. D., Bastin, J.-F., Bourland, N., Comiskey, J. A., Dauby, G., Doucet, J.-L., Gillet, J.-F., Gourlet-Fleury, S., Hardy, O. J., Kirunda, B., Kouamé, F. N., Plumptre, A. J. (2014) Patterns of tree species composition across tropical African forests. Journal of Biogeography.

• Résumé : “Aim. In this study we identified large-scale variation in tree species composition across tropical African forests and determined the underlying environmental and historical factors.
  Location. Tropical forests from Senegal to Mozambique.
  Methods. Distribution data were gathered for 1175 tree species in 455 sample sites scattered across tropical Africa, including all types of tropical forests (wet, moist, dry, and lowland to moderate elevation montane forests). The value of elevation and 19 climatic variables extracted from the BIOCLIM data set were assigned to each sample site. We determined the variation in species composition using correspondence analysis and identified the environmental correlates. We defined floristic clusters according to species composition and identified the characteristic species using indicator analysis.
  Results. We identified a major floristic discontinuity located at the Albertine rift that separated the dry, moist and wet forests of West and Central Africa (the entire Guineo-Congolian Region) from the upland and coastal forests of East Africa. Except for the Albertine Rift, we found no evidence to support the other proposed floristic discontinuities (Dahomey Gap etc.). We detected two main environmental gradients across tropical African forests. The rainfall gradient was strongly correlated with the variation in tree species composition in West and Central Africa. The elevation/temperature gradient highlighted the major floristic differences within East Africa and between East Africa and the Guineo-Congolian Region, the latter being most probably due to the geological disruption and associated climatic history of the East African uplift.
  Main conclusions. We found floristic evidence for three main biogeographical regions across the tropical African forests, and described six floristic clusters with particular environmental conditions within these regions: Coastal and Upland for East Africa, Dry and Wet-Moist for West Africa, and Moist and Wet for Central Africa.”

• DOI: http://dx.doi.org/10.1111/jbi.12382

Fayolle, A., Picard, N., Doucet, J.-L., Swaine, M., Bayol, N., Bénédet, F., and Gourlet-Fleury, S. (2014). A new insight in the structure, composition and functioning of African moist forests. Forest Ecology and Management 329 , 195-205.

• Résumé : “The greater part of the semi-deciduous moist forests of the Congo basin has been given to logging companies for exploitation. In the next decades, very few of these forests will remain intact. In this paper, we aimed to identify large-scale variations in the structure, composition and functioning of African moist forests that could serve as a baseline for both management and conservation purposes. Commercial forest inventory data were assembled for 49,711 0.5-ha plots, covering an area of more than six million hectares, crossing the borders of Cameroon, Central African Republic and Republic of Congo. Floristic composition was analyzed for a subset of 176 genera reliably identified in the field. Three key functional traits of tropical trees: regeneration guild, leaf phenology, and wood specific gravity, were collected at the species level from various sources, and assigned at the genus level. We first investigated the main variations in forest structure and composition, and identified seven forest types based on these variations. Differences in the percentage of pioneer and deciduous stems, and mean wood specific gravity were tested between forest types. Most of the study area was composed of a mosaic of the structural variations of the forests characterized by the occurrence of Celtis (Ulmaceae) species, which are mostly composed of frequent and abundant genera that formed the common floristic pool of the region. Secondary Musanga (Moraceae) forest is located in repeatedly disturbed areas, along roads and around main cities; mixed Manilkara (Sapotaceae) forest covers a huge area in the southern Central African Republic and in the northern Republic of Congo; and monodominant Gilbertiodendron (Fabaceae) forest is sparsely distributed along rivers. The contrasted structure, composition, and functioning of the forest types imply pronounced differences in population and ecosystem processes, and call for adapted management and conservation strategies.”

• DOI: http://dx.doi.org/10.1016/j.foreco.2014.06.014

Fayolle, A., Engelbrecht, B., Freycon, V., Mortier, F., Swaine, M., Réjou-Méchain, M., Doucet, J.-L., Fauvet, N., Cornu, G., and Gourlet-Fleury, S. (2012). Geological Substrates Shape Tree Species and Trait Distributions in African Moist Forests. PLoS ONE 7 , e42381.

• Résumé : “BackgroundUnderstanding the factors that shape the distribution of tropical tree species at large scales is a central issue in ecology, conservation and forest management. The aims of this study were to (i) assess the importance of environmental factors relative to historical factors for tree species distributions in the semi-evergreen forests of the northern Congo basin; and to (ii) identify potential mechanisms explaining distribution patterns through a trait-based approach.Methodology/Principal FindingsWe analyzed the distribution patterns of 31 common tree species in an area of more than 700,000 km2 spanning the borders of Cameroon, the Central African Republic, and the Republic of Congo using forest inventory data from 56,445 0.5-ha plots. Spatial variation of environmental (climate, topography and geology) and historical factors (human disturbance) were quantified from maps and satellite records. Four key functional traits (leaf phenology, shade tolerance, wood density, and maximum growth rate) were extracted from the literature. The geological substrate was of major importance for the distribution of the focal species, while climate and past human disturbances had a significant but lesser impact. Species distribution patterns were significantly related to functional traits. Species associated with sandy soils typical of sandstone and alluvium were characterized by slow growth rates, shade tolerance, evergreen leaves, and high wood density, traits allowing persistence on resource-poor soils. In contrast, fast-growing pioneer species rarely occurred on sandy soils, except for Lophira alata.Conclusions/SignificanceThe results indicate strong environmental filtering due to differential soil resource availability across geological substrates. Additionally, long-term human disturbances in resource-rich areas may have accentuated the observed patterns of species and trait distributions. Trait differences across geological substrates imply pronounced differences in population and ecosystem processes, and call for different conservation and management strategies.”

• DOI: http://dx.doi.org/10.1371/journal.pone.0042381

Freycon, V., Wonkam, C., Fayolle, A., Laclau, J.-P., Lucot, E., Jourdan, C., Cornu, G. and Gourlet-Fleury, S. (2015). Tree roots can penetrate deeply in African semi-deciduous rain forests: evidence from two common soil types. Journal of Tropical Ecology 31 , 13-23.

• Résumé : “Despite the important functional role of deep roots in withdrawing water during drought, direct measurements of root distribution are very rare in tropical rain forests. The aim of this study was to investigate the root distribution of Entandrophragma cylindricum, a common tree species in the Central African semi-deciduous rain forest, in Ferralsols and Arenosols. We dug two pits to a depth of 6 m in Ferralsols and two pits to a depth of 3 m in Arenosols, close to E. cylindricum trees. The vertical soil profiles were divided into 10 × 10-cm grid cells and the roots counted were distributed in three diameter classes. We fitted a root distribution model to our dataset. We found that vertical root distribution was shallower in Arenosols than in Ferralsols. Root penetration was not stopped even by a Ferralsol with high gravel content in its subsoil. Overall, our measurements showed that 95% of all roots were distributed to depths of between 258 and 564 cm from the soil surface, which is much deeper than the 95 cm depth previously reported in the literature for tropical rain forests. As sampling depth could explain this discrepancy, we recommend a sampling depth of at least 3–5 m to accurately estimate root distribution. The drier the dry season, the deeper the sampling depth should be. Our results are consistent with global models of root distribution in forest ecosystems, which are driven by climate variables.We thus suggest that deep rooting could be common in rain forests with a marked dry season.”

• DOI: http://dx.doi.org/10.1017/S0266467414000595

Gillet, J.-F., and Doucet, J.-L. (2012). A commented checklist of woody plants in the Northern Republic of Congo. Plant Ecology and Evolution 145 , 258–271.

• Résumé : “Aim – This paper aims to provide the first exhaustive list of woody species present in the northern part of the Republic of Congo, describing the vegetation types, characterizing the ligneous forest stands and explaining the distribution range of characteristic species.Location – The 1.23 million ha of the logging concession of ‘Congolaise Industrielle des Bois’, situated in the North of the Republic of Congo (1.41°N 16.32°E), were prospected during six years. The study area is bordered by the Sangha River to the west and by the Likwala-aux-Herbes swamps to the east.Methods – The checklist, given as an electronic appendix, documents 702 woody species. Each species is annotated with the following headings when the information is known: scientific, pilot and local names, lifeform, preferential habitat, distribution, seed dispersal type, human use, abundance and foliage phenology. Field observations have been augmented with existing bibliography.Results – A total of 392 genera and 79 families are presented. Ficus is the largest genus with seventeen species. 73% of taxa are endemic to the Guineo-Congolian Region centre. The sarcochorous functional diaspore type represents 71%. Eleven percent of the species are anemochorous and are mostly emergent trees. Forty-four species newly recorded for the Republic of Congo are especially commented.Main conclusions – Terra firma forests are characterized by deciduous, light-demanding and winddispersed emergent tree species. Evergreen stands are mainly found at the confluence of the Sangha River and Likwala-aux-Herbes swamps. This forest could be regarded as equivalent to the other interfluvial forest blocks of the Major Fluvial Refuge in the Congo basin.”

• DOI: http://dx.doi.org/10.5091/plecevo.2012.648

Gond, V., Fayolle, A., Pennec, A., Cornu, G., Mayaux, P., Camberlin, P., Doumenge, C., Fauvet, N., and Gourlet-Fleury, S. (2013). Vegetation structure and greenness in Central Africa from Modis multi-temporal data. Phil. Trans. R. Soc. B 368 .

• Résumé : “African forests within the Congo Basin are generally mapped at a regional scale as broad-leaved evergreen forests, with the main distinction being between terra-firme and swamp forest types. At the same time, commercial forest inventories, as well as national maps, have highlighted a strong spatial heterogeneity of forest types. A detailed vegetation map generated using consistent methods is needed to inform decision makers about spatial forest organization and their relationships with environmental drivers in the context of global change. We propose a multi-temporal remotely sensed data approach to characterize vegetation types using vegetation index annual profiles. The classifications identified 22 vegetation types (six savannas, two swamp forests, 14 forest types) improving existing vegetation maps. Among forest types, we showed strong variations in stand structure and deciduousness, identifying (i) two blocks of dense evergreen forests located in the western part of the study area and in the central part on sandy soils; (ii) semi-deciduous forests are located in the Sangha River interval which has experienced past fragmentation and human activities. For all vegetation types enhanced vegetation index profiles were highly seasonal and strongly correlated to rainfall and to a lesser extent, to light regimes. These results are of importance to predict spatial variations of carbon stocks and fluxes, because evergreen/deciduous forests (i) have contrasted annual dynamics of photosynthetic activity and foliar water content and (ii) differ in community dynamics and ecosystem processes.”

• DOI: http://dx.doi.org/10.1098/rstb.2012.0309

Gourlet-Fleury, S., Rossi, V., Rejou‐Mechain, M., Freycon, V., Fayolle, A., Saint‐André, L., Cornu, G., Gérard, J., Sarrailh, J., Flores, O., et al. (2011). Environmental filtering of dense‐wooded species controls above‐ground biomass stored in African moist forests. Journal of Ecology 99 , 981–990.

• Résumé : “ 1. Regional above-ground biomass estimates for tropical moist forests remain highly inaccurate mostly because they are based on extrapolations from a few plots scattered across a limited range of soils and other environmental conditions. When such conditions impact biomass, the estimation is biased. The effect of soil types on biomass has especially yielded controversial results. 2. We investigated the relationship between above-ground biomass and soil type in undisturbed moist forests in the Central African Republic. We tested the effects of soil texture, as a surrogate for soil resources availability and physical constraints (soil depth and hydromorphy) on biomass. Forest inventory data were collected for trees ≥ 20 cm stem diameter in 2754 0.5 ha plots scattered over 4888 km2. The plots contained 224 taxons, of which 209 were identified to species. Soil types were characterized from a 1:1 000 000 scale soil map. Species-specific values for wood density were extracted from the CIRAD’s data base of wood technological properties. 3. We found that basal area and biomass differ in their responses to soil type, ranging from 17.8 m2 ha-1 (217.5 t ha1) to 22.3 m2 ha-1 (273.3 t ha-1). While shallow and hydromorphic soils support forests with both low stem basal area and low biomass, forests on deep resource-poor soils are typically low in basal area but as high in biomass as forests on deep resource-rich soils. We demonstrated that the environmental filtering of slow growing dense-wooded species on resource-poor soils compensates for the low basal area, and we discuss whether this filtering effect is due to low fertility or to low water reserve. 4. Synthesis. We showed that soil physical conditions constrained the amount of biomass stored in tropical moist forests. Contrary to previous reports, our results suggest that biomass is similar on resource-poor and resource-rich soils. This finding highlights both the importance of taking into account soil characteristics and species wood density when trying to predict regional patterns of biomass. Our findings have implications for the evaluation of biomass stocks in tropical forests, in the context of the international negotiations on climate change.”

• DOI: http://dx.doi.org/10.1111/j.1365-2745.2011.01829.x

Gourlet-Fleury, S., Beina, D., Fayolle, A., Ouédraogo, D., Mortier, F., Bénédet, F., Closset-Kopp, D., and Decocq, G. (2013). Silvicultural disturbance has little impact on tree species diversity in a Central African moist forest. Forest Ecology and Management 304 , 322–332.

• Résumé : “Timber production is an important economic sector in most forested countries of Central Africa, where about 14 million hectares of lowland moist forests are now planned for management. This production is expected to be sustainable, but the actual impact of logging on biodiversity is still questioned. To answer this question, we used a unique long-term controlled experiment implemented more than 20 years ago in an old-growth semi-deciduous moist forest of the Central African Republic (CAR). We tested whether (i) anthropogenic disturbances associated with silvicultural operations had an effect on the composition and diversity of tree communities, and (ii) there is a relationship between diversity and disturbance intensity in those forests. For this, we botanically identified all trees ≥ 10 cm DBH in 28 1-ha plots where no treatment (controls), logging and logging + thinning operations were implemented 24 years ago and created a strong gradient of disturbance. We investigated the relationships between five diversity indices and a disturbance index calculated for each 1-ha plot, for all species and separately for three regeneration guilds. We found a strong positive monotonic relationship between the intensity of disturbance and the percentage of pioneer species in the tree communities, which proved to be equally detrimental, in terms of relative abundance, to the non-pioneer light-demanding and the shade-bearing species.Overall, disturbance appeared to have a weak monotonous negative effect on diversity, irrespective to the index considered. The diversity of shade-bearers slightly decreased along the disturbance gradient without significant decrease in species density; disturbance had no effect on non-pioneer light demanders, but a clear significant negative effect on the diversity of pioneers, with a significant decrease in speciesdensity. This negative effect was associated with the massive recruitment of the early-successional, fast-growing Musanga cecropioides R. Br. (Urticaceae), which rapidly preempted space and resources in the most disturbed plots. Despite this effect, disturbance did not significantly affect the local heterogeneity in species distribution.These results suggest that the semi-deciduous moist forests of CAR are locally resilient to small-scale disturbances associated with silvicultural operations. This may be a consequence of the past anthropogenic and/or climatic disturbances, which have been stronger and more long-lasting than elsewhere within the tropical forest biome, and would have removed the most vulnerable species. Because logging intensity in these forests is usually low, we do not expect any direct major impact on tree species diversity,at least after the first felling cycle.”

• DOI: http://dx.doi.org/10.1016/j.foreco.2013.05.021

Morin-Rivat, J., Fayolle, A., Gillet, J.-F., Bourland, N., Gourlet-Fleury, S., Oslisly, R., Bremond, L., Bentaleb, I., Beeckman, H., and Doucet, J.-L. (2014). New evidence of human activities during the Holocene in the lowland forests of the northern Congo Basin. Radiocarbon 56(1), 209–20.

• Résumé : “In the last decade, the myth of the pristine tropical forest has been seriously challenged. In central Africa, there is a growing body of evidence for past human settlements along the Atlantic forests, but very little information is available about human activities further inland. Therefore, this study aimed at determining the temporal and spatial patterns of human activities in an archaeologically unexplored area of 110,000 km2 located in the northern Congo Basin and currently covered by dense forest. Fieldwork involving archaeology as well as archaeobotany was undertaken in 36 sites located in southeastern Cameroon and in the northern Republic of Congo. Evidence of past human activities through either artifacts or charred botanical remains was observed in all excavated test pits across the study area. The set of 43 radiocarbon dates extending from 15,000 BP to the present time showed a bimodal distribution in the Late Holocene, which was interpreted as two phases of human expansion with an intermediate phase of depopulation. The 2300–1300 BP phase is correlated with the migrations of supposed farming populations from northwestern Cameroon. Between 1300 and 670 BP, less material could be dated. This is in agreement with the population collapse already reported for central Africa. Following this, the 670–20 BP phase corresponds to a new period of human expansion known as the Late Iron Age. These results bring new and extensive evidence of human activities in the northern Congo Basin and support the established chronology for human history in central Africa.”

• DOI: http://dx.doi.org/10.2458/56.16485

Mortier, F., Ouédraogo, D.Y., Claeys, F., Tadesse, M.G., Cornu, G., Baya, F., Bénédet, F., Freycon, V., Gourlet-Fleury, S. and Picard, N. (2014). Mixture of inhomogeneous matrix models for species-rich ecosystems. Environmetrics 26(1), 39-51.

• Résumé: “Understanding how environmental factors could impact population dynamics is of primary importance for species conservation. Matrix population models are widely used to predict population dynamics. However, in species-rich ecosystems with many rare species, the small population sizes hinder a good fit of species-specific models. In addition, classical matrix models do not take into account environmental variability. We propose a mixture of regression models with variable selection allowing the simultaneous clustering of species into groups according to vital rate information (recruitment, growth and mortality) and the identification of group-specific explicative environmental variables. We develop an inference method coupling the R packages flexmix and glmnet. We first highlight the effectiveness of the method on simulated datasets. Next, we apply it to data from a tropical rain forest in the Central African Republic. We demonstrate the accuracy of the inhomogeneous mixture matrix model in successfully reproducing stand dynamics and classifying tree species into well-differentiated groups with clear ecological interpretations. Copyright © 2014 John Wiley & Sons, Ltd.”

• DOI: http://dx.doi.org/10.1002/env.2320

Oslisly, R., Bentaleb, I., Favier, C., Fontugne, M., Gillet, J.-F., and Morin-Rivat, J. (2013). West Central African peoples: survey of radiocarbon dates over the past 5000 years. Radiocarbon, Vol. 55, (2-3), 1377-1382.

• Résumé : “Tracing human history in west central Africa suffers from a scarcity of historical data and archaeological remains. In order to provide new insight into this problem, we reviewed 733 radiocarbon dates of archaeological sites from the end of the Late Stone Age, Neolithic Stage, and Early and Late Iron Age in Cameroon, Gabon, Central African Republic, Equatorial Guinea, Republic of Congo, and the western Democratic Republic of Congo. This review provides a spatiotemporal framework of human settlement in the forest biome. Beyond the well-known initial spread of Iron Age populations through central African forests from 2500 cal BP, it depicts the geographical patterns and links with the cultural evolution of the successive phases of human expansion from 5000 to 3000 cal BP and then from 3000 to 1600 cal BP, of the hinterland depopulation from 1350 to 860 cal BP, and of recolonization up to 500 cal BP.”

• Lien

Oslisly, R., White, L., Bentaleb, I., Favier, C., Fontugne, M., Gillet, J.-F., and Sebag, D. (2013). Climatic and cultural changes in the west Congo Basin forests over the past 5000 years. Philosophical Transactions of the Royal Society B: Biological Sciences 368 .

• Résumé : “Central Africa includes the world’s second largest rainforest block. The ecologyof the region remains poorly understood, as does its vegetation andarchaeological history. However, over the past 20 years, multidisciplinaryscientific programmes have enhanced knowledge of old human presenceand palaeoenvironments in the forestry block of Central Africa. This firstregional synthesis documents significant cultural changes over the pastfive millennia and describes how they are linked to climate. It is now welldocumented that climatic conditions in the African tropics underwent significantchanges throughout this period and here we demonstrate thatcorresponding shifts in human demography have had a strong influenceon the forests. The most influential event was the decline of the strong Africanmonsoon in the Late Holocene, resulting in serious disturbance of theforest block around 3500 BP. During the same period, populations fromthe north settled in the forest zone; they mastered new technologies such aspottery and fabrication of polished stone tools, and seem to have practisedagriculture. The opening up of forests from 2500 BP favoured the arrival ofmetallurgist populations that impacted the forest. During this long period(2500–1400 BP), a remarkable increase of archaeological sites is an indicationof a demographic explosion of metallurgist populations. Paradoxically, wehave found evidence of pearl millet (Pennisetum glaucum) cultivation in theforest around 2200 BP, implying a more arid context. While Early Iron Agesites (prior to 1400 BP) and recent pre-colonial sites (two to eight centuriesBP) are abundant, the period between 1600 and 1000 BP is characterized bya sharp decrease in human settlements, with a population crash between1300 and 1000 BP over a large part of Central Africa. It is only in the eleventhcentury that new populations of metallurgists settled into the forest block. Inthis paper, we analyse the spatial and temporal distribution of 328 archaeologicalsites that have been reliably radiocarbon dated. The results allow us topiece together changes in the relationships between human populations andthe environments in which they lived. On this basis, we discuss interactionsbetween humans, climate and vegetation during the past five millennia andthe implications of the absence of people from the landscape over three centuries.We go on to discuss modern vegetation patterns and African forestconservation in the light of these events.”

• DOI: http://dx.doi.org/10.1098/rstb.2012.0304

Ouédraogo, D., Mortier, F., Gourlet-Fleury, S., Freycon, V., and Picard, N. (2013). Slow-growing species cope the best with drought: evidence from long-term measurements in a tropical semi-deciduous moist forest of Central Africa. Journal of Ecology 101, 1459–1470.

• Résumé : “1. Understanding how drought affects annual tree growth in tropical forests is of crucial importance to predict their response to climate change. Previous studies, mainly led in the Neotropics and in South-East Asia, have yielded contradictory results which might be explained by differences in species studied, in the tree development stages considered, or by differences in other environmental factors than water availability.2. Here we described the growth responses of functional groups of tree species to drought in a Central African semi-deciduous moist forest. Species groups were automatically defined using a finite mixture model, that grouped species according to their growth model parameters. The growth model considered the variation in species response to drought, and the effect of competition for resources and of tree development stage on growth. Groups were further characterized by species functional traits. Nine species groups were identified. They differed in their ability to acquire, use, and conserve resources, as suggested by their differences in maximum growth rate, regeneration guild, maximum dbh, wood density and leaf habit. The species were organized along a light-requirement gradient that here closely matched a broader continuum of plant strategies for resource use, from slow-growing shade-tolerant conservative species to fast-growing pioneer acquisitive species.3. Tree growth decreased with drought intensity, and species drought tolerance was found to be related to resource use strategy: slow-growing species using a conservative strategy were the least sensitive to variations in water availability, while fast-growing species using an acquisitive strategy were the most sensitive.4. Synthesis. Shade-tolerant species, characterized by a low potential growth rate and thus a conservative strategy of resource use were found to be the least sensitive to drought. This support the hypothesis of a single axis summarizing multiple traits that represents a general trade-off between the conservation and rapid acquisition of resources.”

• DOI : http://dx.doi.org/10.1111/1365-2745.12165

Ouédraogo, D., Fayolle, A., Gourlet-Fleury, S., Mortier, F., Freycon, V., Fauvet, N., Rabaud, S., Cornu, G., Bénédet, F., Gillet, J.-F., Oslisly, R., Doucet, J.-L., Lejeune, P. and Favier, C. (2016). The determinants of tropical forest deciduousness: disentangling the effects of rainfall and geology in central Africa. Journal of Ecology.

• Résumé : “1. Understanding the environmental determinants of forests deciduousness i.e. proportion of deciduous trees in a forest stand, is of great importance when predicting the impact of ongoing global climate change on forests. In this study, we examine (i) how forest deciduousness varies in relation to rainfall and geology, and (ii) whether the influence of geology on deciduousness could be related to differences in soil fertility and water content between geological substrates. 2. The study was conducted in mixed moist semi-deciduous forests in the northern part of the Congo basin. We modelled the response of forest deciduousness to the severity of the dry season across four contrasting geological substrates (sandstone, alluvium, metamorphic and basic rocks). For this, we combined information on forest composition at genus level based on commercial forest inventories (62 624 0.5 ha plots scattered over 6 million of ha), leaf habit, and rainfall and geological maps. We further examined whether substrates differ in soil fertility and water-holding capacity using soil data from 37 pits in an area that was, at the time, relatively unexplored. 3. Forest deciduousness increased with the severity of the dry season, and this increase strongly varied with the geological substrate. Geology was found to be three times more important than the rainfall regime in explaining the total variation in deciduousness. The four substrates differed in soil properties, with higher fertility and water-holding capacity on metamorphic and basic rocks than on sandstone and alluvium. The increase in forest deciduousness was stronger on the substrates that formed resource-rich clay soils (metamorphic and basic rocks) than on substrates that formed resource-poor sandy soils (sandstone and alluvium). 4. Synthesis. We found evidence that tropical forest deciduousness is the result of both the competitive advantage of deciduous species in climates with high rainfall seasonality, and the persistence of evergreen species on resource-poor soils. Our findings offer a clear illustration of well-known theoretical leaf carbon economy models, explaining the patterns in the dominance of evergreen versus deciduous species. And, this large-scale assessment of the interaction between climate and geology in determining forest deciduousness may help to improve future predictions of vegetation distribution under climate change scenarios. In central Africa, forest is likely to respond differently to variation in rainfall and/or evapotranspiration depending on the geological substrate.”

• DOI : http://dx.doi.org/10.1111/1365-2745.12589