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See detailOf trees and men: new insights into man-environment relationships in the moist forests of central Africa during the late Holocene
Morin, Julie ULiege

Doctoral thesis (2017)

In central Africa, vegetation history has been documented by paleoenvironmental studies (especially palynology), which mainly concerned the way climate has shaped the forest landscapes. Human impacts in ... [more ▼]

In central Africa, vegetation history has been documented by paleoenvironmental studies (especially palynology), which mainly concerned the way climate has shaped the forest landscapes. Human impacts in this region have hardly been studied so far, especially at local scale. The main objective of this PhD is to propose an approach based on archaeology and the use of charred botanical remains found in soils, either wood charcoal or seeds, in order to document the Holocene anthropogenic impacts on the forest structure and composition. When coupled with the diachronic analysis of human activities, these land-use biomarkers can allow a better understanding of the relationship between man and his environment in central Africa during this period. Thereby, the first part of this PhD introduces the conceptual framework and the materials and methods used during the research. Then, the second part constitutes the core of the work, and presents the chronology of human activities in the northern Congo Basin, the use of biomarkers to discrimate between these activities, either domestic or agricultural, and the effect of the recent anthropogenic activities in the dynamic of several light-demanding tree populations. Finally, the third part draws the main recommendations of the work, and formulates potential for additional research. [less ▲]

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See detailPresent-day central African forest is a legacy of the 19th century human history
Morin, Julie ULiege; Fayolle, Adeline ULiege; Favier, Charly et al

in eLife (2017)

The populations of light-demanding trees that dominate the canopy of central African forests are now aging. Here, we show that the lack of regeneration of these populations began ca. 165 ya (around 1850 ... [more ▼]

The populations of light-demanding trees that dominate the canopy of central African forests are now aging. Here, we show that the lack of regeneration of these populations began ca. 165 ya (around 1850) after major anthropogenic disturbances ceased. Since 1885, less itinerancy and disturbance in the forest has occurred because the colonial administrations concentrated people and villages along the primary communication axes. Local populations formerly gardened the forest by creating scattered openings, which were sufficiently large for the establishment of light-demanding trees. Currently, common logging operations do not create suitable openings for the regeneration of these species, whereas deforestation degrades landscapes. Using an interdisciplinary approach, which included paleoecological, archaeological, historical, and dendrological data, we highlight the long-term history of human activities across central African forests and assess the contribution of these activities to present-day forest structure and composition. The conclusions of this sobering analysis present challenges to current silvicultural practices and to those of the future. [less ▲]

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See detailThe influence of spatially structured soil properties on tree community assemblages at a landscape scale in the tropical forests of southern Cameroon
Vleminckx, Jason; Doucet, Jean-Louis ULiege; Morin, Julie ULiege et al

in Journal of Ecology (2016)

Species distribution within plant communities results from both the influence of deterministic processes, related to environmental conditions, and neutral processes related to dispersal limitation and ... [more ▼]

Species distribution within plant communities results from both the influence of deterministic processes, related to environmental conditions, and neutral processes related to dispersal limitation and stochastic events, the relative importance of each factor depending on the observation scale. Assessing the relative contribution of environment necessitates controlling for spatial dependences among data points. Recent methods, combining multiple regression and Moran's eigenvectors maps (MEM), have been proved successful in disentangling the influence of pure spatial processes related to dispersal limitation, pure environmental variables (not spatially structured) and spatially structured environmental properties. However, the latter influence is usually not testable when using advanced spatial models like MEM. To overcome this issue, we propose an original approach, based on torus-translations and Moran spectral randomizations, to test the fraction of species abundance variation that is jointly explained by space and seven soil variables, using three environmental and tree species abundance data sets (consisting of 120, 52 and 34 plots of 0·2 ha each, located along 101-, 66- and 35-km-long transect-like inventories, respectively) collected in tropical moist forests in southern Cameroon. The overall abundance of species represented by ≥30 individuals, and 27% of these species taken individually, were significantly explained by fine-scale (<5 km) and/or broad-scale (5–100 km) spatially structured variations in soil nutrient concentrations (essentially the concentration of available Mn, Mg and Ca) along the 120-plots area. The number of significant tests considerably decreased when investigating the two smaller data sets, which mostly resulted from low statistical power rather than weaker floristic and/or edaphic variation captured among plots. Synthesis. Our results provide evidence that tree species turnovers are partly controlled by spatially structured concentrations in soil nutrients at scales ranging from few hundreds of metres to c. 100 km, a poorly documented subject in Central African forests. We also highlight the usefulness of our testing procedure to correctly interpret the space-soil fraction of variation partitioning analyses (which always accounted here for the most important part of the soil contribution), as this fraction was sometimes relatively high (R2 values up to c. 0·3) but nearly or not significant. [less ▲]

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See detailHigh spatial resolution of late-Holocene human activities in the moist forests of central Africa using soil charcoal and charred botanical remains
Morin, Julie ULiege; Biwolé, Achille; Gorel, Anaïs ULiege et al

in Holocene (2016), 26(12), 1954-1967

Palaeoecological and archaeological studies have demonstrated that human populations have long inhabited the moist forests of central Africa. However, spatial and temporal patterns of human activities ... [more ▼]

Palaeoecological and archaeological studies have demonstrated that human populations have long inhabited the moist forests of central Africa. However, spatial and temporal patterns of human activities have hardly been investigated with satisfactory accuracy. In this study, we propose to characterize past human activities at local scale by using a systematic quantitative and qualitative methodology based on soil charcoal and charred botanical remains. A total of 88 equidistant test-pits were excavated along six transects in two contrasting forest types in southern Cameroon. Charred botanical remains were collected by water-sieving and sorted by type (wood charcoals, oil palm endocarps, and unidentified seeds). A total of 50 Accelerator Mass Spectrometry 14C dates were also obtained. Results showed that charred macroremains were found at multiple places in the forest, suggesting scattered human activities, which were distributed into two main periods (Phase A: 2300-1300 BP – Phase B: 580 BP to the present). Charred botanical remains indicated two types of land use: (i) domestic, with oil palm endocarps most often associated with potsherds (villages) and (ii) agricultural, with charcoal as probable remnant of slash-and-burn cultivation (fields). Oil palm endocarp abundance decreased with distance from the identified human settlements. Our methodology allowed documenting, at high resolution, the spatial and temporal patterns of human activities in central African moist forests and could be applied to other tropical contexts. [less ▲]

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See detailThe last 1,000 years in the Northern Congo Basin
Morin, Julie ULiege; Fayolle, Adeline ULiege; Favier, Charly et al

Conference (2015, November)

Review of the events that happened in the northern Congo basin during the last 1,000 yr. Positive impact of human disturbances on the regeneration of light-demanding trees. Negative impact of the European ... [more ▼]

Review of the events that happened in the northern Congo basin during the last 1,000 yr. Positive impact of human disturbances on the regeneration of light-demanding trees. Negative impact of the European colonization and following events on human populations and tree regeneration. [less ▲]

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See detailThe Tervuren xylarium and wood biology to decode the ecological memory of forests and trees
Morin, Julie ULiege; Bourland, Nils; De Ridder, Maaike et al

Conference (2015, October 15)

Presentation of the competences held by the Wood Biology Service members of the Royal Museum for Central Africa, relative to their potential uses in archaeology.

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See detailLate-Holocene tropical moist-forests of southeastern Cameroon: some insight from soil charcoal analysis
Morin, Julie ULiege; Biwolé, Achille; Bourland, Nils et al

Conference (2015, August)

Tropical forests of Central Africa constitute the second most important block of moist forest of the world. Little is known, however, about past vegetation in this region that remains underexplored ... [more ▼]

Tropical forests of Central Africa constitute the second most important block of moist forest of the world. Little is known, however, about past vegetation in this region that remains underexplored (Vleminckx et al. 2014; Morin-Rivat et al. 2014). Determining the past specific composition of these forests could allow bringing insights into their evolution over time and providing data about their resilience capacity facing global change. We performed a pedoanthracological analysis in the semi-deciduous forests of southeastern Cameroon. We excavated 53 test pits of 53 50 × 50 × 60 cm in plots of botanical inventory along a NS 80-km long mega-transect that followed a vegetation gradient. We sorted and quantified charred macrobotanical remains by layers of 10 cm, then identified species from wood charcoals. We used the InsideWood database, implemented with 163 new anatomical descriptions of woods present in the study area by using the reference collection of African woods of the Royal Museum for Central Africa (Belgium). Finally, we obtained 25 radiocarbon dates on charcoals and oil palm endocarps. Results showed that repeated fire events occurred across the study area during the last 2500 years, soon after the well-documented “rainforest crisis” (e.g. Lézine et al. 2013). The analyzed charcoals are likely human-induced regarding evidence of associated human settlements (e.g. potsherds). Aged were distributed into two time periods: the Early Iron Age (2300-1300 BP) and the Late Iron Age (700-100 BP) with an intermediate hiatus in human occupation (see e.g. Wotzka 2006; Morin-Rivat et al. 2014). Specific composition during both periods did not strongly differ from current composition, which is now dominated by light-demanding canopy trees belonging to old-growth semi-deciduous Celtis forests (Gond et al. 2013; Fayolle et al. 2014). This argues in favor of the maintenance of light-demanding tree species by anthropogenic activities, such as slash-and-burn shifting cultivation. We conclude that moist forests have a good resilience capacity regarding moderate and scattered disturbances. These forests can nonetheless be deeply impacted by land-use intensification (e.g. degraded forests along roads and close to cities; Gond et al. 2013). [less ▲]

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See detailContributing to wood anatomical databases to improve species identification, phylogeny and functional trait research in Central Africa
Morin, Julie ULiege; Fayolle, Adeline ULiege; De Ridder, Maaike et al

Poster (2015, May 26)

Central African rainforests shelter a high number of woody species that are anatomically very different. Knowledge of taxon-specific wood anatomical features has proven indispensable for scientific and ... [more ▼]

Central African rainforests shelter a high number of woody species that are anatomically very different. Knowledge of taxon-specific wood anatomical features has proven indispensable for scientific and non-scientific applications. The field of wood anatomy and identification has been drastically revolutionized by the development of internationally recognized lists of precisely illustrated microscopic features (e.g. IAWA Committee 1989), together with the launch of InsideWood, an online search database using these features to narrow down identification results (e.g. Wheeler 2011). However, despite these massive efforts, the anatomy of many species or even genera remains in the dark, especially in species-rich regions. Wood anatomy has been formally described for less than 25% of the Central African woody species (Hubau et al. 2012), the focus has been mainly on timber species and variations in wood anatomical structure remain to be explored. Therefore, we are assembling a wood anatomical database of about 800 species covering the Guineo-Congolian region using material from InsideWood and the Tervuren xylarium (new descriptions). As such, we present how large anatomical databases hold interesting perspectives for (i) wood and charcoal identification, (ii) exploring the phylogenetic signal of wood anatomy, and (iii) the relationship between wood anatomical features and functional traits. [less ▲]

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See detailHigh spatial resolution of late-Holocene human activities in the moist forests of Central Africa using soil charcoal and charred botanical remains
Morin, Julie ULiege; Biwolé, Achille; Gorel, Anaïs ULiege et al

Conference (2015, April 27)

Palaeoecological and archaeological studies have demonstrated that human populations have long inhabited the moist forests of central Africa. However, spatial and temporal patterns of human activities ... [more ▼]

Palaeoecological and archaeological studies have demonstrated that human populations have long inhabited the moist forests of central Africa. However, spatial and temporal patterns of human activities have hardly been investigated with satisfactory accuracy. In this study, we propose to characterize past human activities at local scale by using a systematic quantitative and qualitative methodology based on soil charcoal and charred botanical remains. A total of 88 equidistant test-pits were excavated along six transects in two contrasting forest types in southern Cameroon. Charred botanical remains were collected by water-sieving and sorted by type (wood charcoals, oil palm endocarps, and unidentified seeds). A total of 50 Accelerator Mass Spectrometry 14C dates were also obtained. Results showed that charred macroremains were found at multiple places in the forest, suggesting scattered human activities, which were distributed into two main periods (Phase A: 2300-1300 BP – Phase B: 580 BP to the present). Charred botanical remains indicated two types of land use: (i) domestic, with oil palm endocarps most often associated with potsherds (villages) and (ii) agricultural, with charcoal as probable remnant of slash-and-burn cultivation (fields). Oil palm endocarp abundance decreased with distance from the identified human settlements. Our methodology allowed documenting, at high resolution, the spatial and temporal patterns of human activities in central African moist forests and could be applied to other tropical contexts. [less ▲]

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See detailDid the savannah « flourished » 3000 years ago in the so-called Sangha River Interval of the Guineo-Congolian rainforest ? A retrospective study using stable isotopes and phytoliths
Bentaleb, Ilham; Freycon, Vincent; Gillet, Jean-François et al

Poster (2015, April)

We aim to improve our knowledge of the dynamic of the vegetation in Central Africa during the last 5 kyrs and to discuss the main hypothesis described in the literature - humans versus climatic impacts ... [more ▼]

We aim to improve our knowledge of the dynamic of the vegetation in Central Africa during the last 5 kyrs and to discuss the main hypothesis described in the literature - humans versus climatic impacts- both suggested as responsible of the Congo basin rainforest decline observed between 3 and 2.5 kyrs. We use the carbon isotopic composition of well-dated Central African soils to reconstruct the dynamic of the vegetation cover. We will discuss the carbon isotopic composition of the soil organic carbon methodology for reconstructing palaeovegetation in the light of Rayleigh distillation model. We showed that numerous sites exhibit a carbon isotopic ratios reflecting the Rayleigh distillation but few sites recorded real vegetation changes. Our study suggests that the vegetation of the Guineo-Congolian Region was disturbed between 3000 and 2000 BP (Before Present) without an extreme savannah expansion. We discussed the two hypotheses human versus climate impacts that may conduct to such new physiography of the vegetation. We suggest that the climate hypothesis is more likely than the human impact to explain the reduction of the Guineo-Congolian rainforest 3000 years ago. [less ▲]

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See detailNineteenth century human history explains the dominance of light-demanding tree species in Central African moist forests
Morin, Julie ULiege; Fayolle, Adeline ULiege; Favier, Charly et al

Poster (2015, March 21)

The canopy of central African moist forests is dominated by light-demanding trees. Most of these species show a distribution of diameters that indicates a regeneration shortage. Here we show through the ... [more ▼]

The canopy of central African moist forests is dominated by light-demanding trees. Most of these species show a distribution of diameters that indicates a regeneration shortage. Here we show through the combined analysis of botanical, palaeoecological, archaeological and historical data that most of these trees are not older than ca. 180 years. This age corresponds to the early 19th century (around 1830) when the slave-raiding, the interethnic wars and the colonization of the inlands by the Europeans disturbed the human spatial occupancy. After 1885, the spatial clumping of people and villages along the main communication axes induced less itinerancy in the forest. We believe that former activities such as shifting cultivation created scattered openings in the canopy, large enough to allow light-demanding trees to establish. Nowadays, common logging operations do not create openings sufficiently large for the regeneration of these high value timber species. Our findings emphasize the need to include considerations about the history of human spatial occupancy and activities to understand forest dynamics. We need silvicultural guidelines adapted to the autecology of the species. Population enforcements (e.g. enrichment) will be needed to ensure the sustainability of timber yields in forests dominated by long-lived light-demanding trees. [less ▲]

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See detailIdentification of charred botanical remains provides more accurate information on past history in Central Africa
Morin, Julie ULiege; Biwolé, Achille; Bourland, Nils et al

Poster (2015, January 30)

In palaeoenvironmental studies, charred botanical remains have rarely been identified to the species level before being sent to radiocarbon dating. Moreover, the age of most tropical spp. and thereby the ... [more ▼]

In palaeoenvironmental studies, charred botanical remains have rarely been identified to the species level before being sent to radiocarbon dating. Moreover, the age of most tropical spp. and thereby the age of the carbon sequestered during plant growth is not known. Dating unidentified charred wood in the tropics should be thus treated with caution because the accuracy of the dates is not guaranteed. Here we present 71 dates obtained on charred endocarps and wood charcoals sampled in soil pits in Cameroon and in the Rep. of the Congo. We taxonomically identified 43 samples then selected both identified and unidentified individual fragments for radiocarbon dating. We performed summed probability distributions of the dates calibrated in BP for the 43 identified and the 28 unidentified samples separately then for the whole dates. Results showed that the dates obtained on unidentified samples better fit the established chronology for Central Africa but that they also presented less precise standard deviations than the dates obtained on identified short-lived material, and that the dates on identified samples provide more detailed trends about the phases of human occupation in Central Africa after 2,500 BP. We can assume that dating unidentified material may introduce some blur into chronologies and that the selection of identified charred botanical remains should be systematically applied for palaeoenvironmental reconstructions in tropical contexts to refine the chronologies. [less ▲]

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See detailWood anatomical characteristics of 600 African tropical species in relationship with their ecology
Morin, Julie ULiege; Fayolle, Adeline ULiege; Doucet, Jean-Louis ULiege et al

Poster (2014, December 16)

The tropical moist forest is a biome with a high number of species that are functionally different. The question arises whether there are patterns in the spectra of wood anatomical features according to ... [more ▼]

The tropical moist forest is a biome with a high number of species that are functionally different. The question arises whether there are patterns in the spectra of wood anatomical features according to functional types. Here we propose to present the main anatomical characteristics of 600 tropical species from the Guineo-Congolian domain in relationship with their ecology. We cross-checked two databases: the anatomical database InsideWood and the CoForTraits database of functional traits produced during the CoForChange project. After characterizing the main trends of the dataset, we performed multivariate analyses between the wood traits (i.e. the anatomical features) and six groups of functional traits: leaf phenology, regeneration guild, dispersal syndrome, life form, plant maximum height, and wood specific gravity. Results showed (i) that several wood features were specific to the tropics, to Africa or only to the Guineo-Congolian region, and (ii) that phylogeny explained the main part of the variation among the traits, whereas (iii) wood structure provided nonetheless interesting functional information related to gradients in plant growth, survival, and dispersal, and (iv) that there was a functional convergence in the study species in response to similar environmental constraints. These observations suggest that certain anatomical features can be used as indicators of functional traits in species-rich biomes. Further research will enable us to increase the input of wood anatomy in explaining the functional trade-offs in African tropical species. [less ▲]

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See detailLate-Holocene moist forests of Central Africa: contribution of charcoal analysis
Morin, Julie ULiege; Bremond, Laurent; Gillet, Jean-François et al

Poster (2014, December)

Wood charcoals are often uncovered in the soils of the tropical regions. They remain little studied, however, and this observation is even truer for charcoals coming from the dense humid forests of ... [more ▼]

Wood charcoals are often uncovered in the soils of the tropical regions. They remain little studied, however, and this observation is even truer for charcoals coming from the dense humid forests of Central Africa. Here we aim at showing the interest of the analysis of soil charcoals in this region so as to understand the dynamics of past forest environments during the late-Holocene. Several examples of taxonomical identifications conducted on charcoals sampled in soil pits in Cameroon and in the Republic of the Congo are presented along with radiocarbon dates. These charcoals were hand-split then observed under an incident light microscope. The wood anatomical features that were preserved in charcoals and described according to a standard method were compared to a reference collection of woods. Results demonstrated that past burnings that spanned from 2,500 BP to Recent were human-induced and scattered in the study area. The identified species are still present in the environment today and localized changes in the vegetation occurred over the past two millennia. Charcoal analysis can thus allow a better understanding of the past history of the tropical forests in relationship with the ancient anthropogenic disturbances. [less ▲]

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See detailCaractérisation de bois utilisés dans la construction de quatre xylophones africains à résonateurs
Warneke, Nikolaus; Houssay, Anne; Morin, Julie ULiege et al

Poster (2014, November)

Nous avons besoin de collaborations avec des anatomistes pour travailler avec nous sur des méthodes d’identification des bois si possible sans ou avec le minimum de prélèvement. Notre microscope in ... [more ▼]

Nous avons besoin de collaborations avec des anatomistes pour travailler avec nous sur des méthodes d’identification des bois si possible sans ou avec le minimum de prélèvement. Notre microscope in situ facilite l’étude des collections. En cherchant à caractériser les essences de bois utilisées pour la fabrication d’instruments de musique traditionnels en Afrique, le choix du bois peut meAre en évidence la disponibilité de telle ou telle essence à un moment précis, et son apprécia1on du point de vue des musiciens. La façon de tailler les lames peut montrer des échanges culturels et techniques ou au contraire des pratiques spécifiques. L’étude des accords peut donner des échelles musicales communes ou au contraire distinctes. La richesse des timbres produits suivant ces choix peut être un élément culturel, technique et esthétique. Patrimoine naturel et musical sont ainsi mis en relation. [less ▲]

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See detailAnalyse à haute résolution spatiale et temporelle des activités humaines à l’Holocène récent dans les forêts humides d’Afrique Centrale
Morin, Julie ULiege; Gorel, Anaïs ULiege; Biwolé, Achille et al

Poster (2014, November)

Des études paléoécologiques et archéologiques ont démontré que les populations humaines ont de longue date investi les forêts humides d’Afrique Centrale. Les occupations humaines ont toutefois été peu ... [more ▼]

Des études paléoécologiques et archéologiques ont démontré que les populations humaines ont de longue date investi les forêts humides d’Afrique Centrale. Les occupations humaines ont toutefois été peu documentées en raison de difficultés relatives à l’accès au terrain. Nous présentons ici une méthodologie systématique basée sur la quantification et la datation des macrorestes botaniques carbonisés pour définir les activités humaines passées en forêt tropicale africaine. Pour cela, 53 sondages équidistants ont été creusés dans 3 sites du sud-est-du Cameroun. Dans chaque sondage ont été prélevées des quantités fixes de sol par couche de 10 cm. Les macrorestes botaniques carbonisés ont été récoltés par tamisage à l’eau directement sur le site. Les refus de tamis ont été triés (charbons de bois, endocarpes de palmier à huile, graines non identifiées), pesés sur une balance de précision et analysés statistiquement. En outre, 25 échantillons ont été datés par AMS. Les résultats montrent que les activités humaines sont réparties en deux périodes : l’âge du Fer ancien entre 2300 et 1300 BP et l’âge du Fer récent entre 670 BP et l’actuel. En outre, les charbons de bois et les endocarpes de palmier à huile ne représentent pas le même type d’activités. Deux villages âge du Fer ancien datés d’environ 2000 BP ont été identifiés par la présence concomitante d’endocarpes de palmier à huile et de tessons céramiques. Les motifs circulaires imprimés dans la céramique et inédits pour la zone d’étude rappellent les gravures et décors céramiques découverts au Gabon et dans la zone de la Sangha-Likwala-aux-Herbes, datés également de 2000 BP. L’abondance décroissante des charbons dans l’orbe d’influence de ces villages pourrait correspondre à d’anciennes zones agricoles. Les perturbations les plus récentes peuvent, quant à elles, être mises en relation avec la végétation actuelle, en particulier les arbres émergents héliophiles qui dominent la composition floristique et qui sont des recrus post-culturaux. Pour la première fois, une méthodologie quantitative basée sur les restes archéobotaniques a été appliquée en Afrique Centrale. Elle nous a permis de documenter à haute résolution la répartition spatiale et temporelle des activités humaines à l’échelle locale. Ce travail sera poursuivi par une reconstitution paléoenvironnementale à partir de l’identification des macrorestes végétaux. [less ▲]

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See detailKnowing the past to anticipate the future: soil charcoal as a proxy to model forest evolution
Morin, Julie ULiege; Biwolé, Achille; Bourland, Nils et al

Poster (2014, October)

Tropical forests of Central Africa constitute the second most important block of moist forest of the world. However little is known about their past evolution. Indeed, determining the past specific ... [more ▼]

Tropical forests of Central Africa constitute the second most important block of moist forest of the world. However little is known about their past evolution. Indeed, determining the past specific composition of these forests could allow modeling their evolution over time and providing data about their resilience capacity facing global change. To do this, we performed a pedoanthracological analysis in the semi-deciduous forests of southeastern Cameroon. We excavated test pits in 53 plots of botanical inventory along a gradient of vegetation, quantified wood charcoals by layers of 10 cm, identify the species present in charcoals, dated the charcoals by the radiocarbon method, then built up sequences including present forest composition. Results show that repeated fire events occurred across the study area during the last 2500 years. These disturbances are likely human-induced regarding evidence of anthropogenic activities (e.g. potsherds). Nonetheless the past specific composition does not strongly differ from the current one except for the oldest layers related to the major dry climatic event of 2500 BP. We conclude that moist forests have a good resilience capacity regarding moderate disturbances but were and will be deeply impacted by climate change. [less ▲]

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See detailSoil charcoal to assess the impacts of past Human disturbances on tropical forests
Vleminckx, Jason; Morin, Julie ULiege; Biwolé, Achille et al

Conference (2014, July 24)

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and ... [more ▼]

The canopy of many central African forests is dominated by light-demanding tree species that do not regenerate well under themselves. The prevalence of these species might result from ancient slash-and-burn agricultural activities that created large openings, while a decline of these activities since the colonial period could explain their deficit of regeneration. To verify this hypothesis, we compared soil charcoal abundance, used as a proxy for past slash-and-burn agriculture, and tree species composition assessed on 208 rainforest 0.2 ha plots located in three areas from Southern Cameroon. Species were classified in regeneration guilds (pioneer, non-pioneer light-demanding, shade-bearer) and characterized by their wood-specific gravity, assumed to reflect light requirement. We tested the correlation between soil charcoal abundance and: (i) the relative abundance of each guild, (ii) each species and family abundance and (iii) mean wood-specific gravity. Charcoal was found in 83% of the plots, indicating frequent past forest fires. Radiocarbon dating revealed two periods of fires: “recent” charcoal were on average 300 years old (up to 860 BP, n = 16) and occurred in the uppermost 20 cm soil layer, while “ancient” charcoal were on average 1900 years old (range: 1500 to 2800 BP, n = 43, excluding one sample dated 9400 BP), and found in all soil layers. While we expected a positive correlation between the relative abundance of light-demanding species and charcoal abundance in the upper soil layer, overall there was no evidence that the current heterogeneity in tree species composition can be explained by charcoal abundance in any soil layer. The absence of signal supporting our hypothesis might result from (i) a relatively uniform impact of past slash-and-burn activities, (ii) pedoturbation processes bringing ancient charcoal to the upper soil layer, blurring the signal of centuries-old Human disturbances, or (iii) the prevalence of other environmental factors on species composition. [less ▲]

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